Weather for Sailors

Weather for Sailors

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00:03
the agenda that I'm going to use for today you see deck level observations talk about the wind the wave the current the sky because that's where all the weather takes place fog weather data data sources that you have available to you storm tracks and I'll just use the example of these US East Coast for that and then a few notes on a further study that you can take with this so let's talk first about deck level observations and just picture yourself standing on
00:35
the deck of your boat you're your underway and what are the things that you can actually see and what are some of the meanings of these things so first off we win the direction of the strength of the wind and and that's a primary item of interest but the more important issue really is the wave direction height and period or the speed of the waves they're the currents that you have to deal with the water currents the
01:07
color of the sky and the progress of the colors in the sky and the clouds and and how these sky images are moving and what do they mean to you lightning that you see in the distance and the approach of lightning and I know it so many times at sea we we see lightning and you say to yourself well that's a way long way off but surprisingly it's going to be on top of you before you know it squalls which are generated by local weather systems
01:40
visibility fog miss rain air temperature and dew point which produces a fog sea water color and currents and temperature the the barometer what's the barometer doing rising falling steady radar observations and the condition of the sea surface that is is there Sargasso weed leaves branches logs these are all considerations that you take into account when you're sailing and some of the things that you observe from the deck level so let's take these one at a
02:12
time and talk about them in in a bit more detail are interested in changes that occur to each of these things in other words these are not static the weather is never static like some someone said years ago if you don't like the weather just wait a few minutes and it will change so you want to be aware of changes to wetter weather conditions and to correlate these changes with each other of these various factors so let's talk
02:43
about wind as I'm sure most of you know it's caused by pressure gradients and you have low pressure systems high pressure systems you have ridges and troughs you have temperature and density differences of the air itself these things cause wind when wind flows basically from high pressure to low pressure and the wind of course produces sea surface waves these ways are the locally generated waves where the wind is taking place but also the wind will
03:17
generate swells that travel a long distance if you have storm force winds that you're dealing with you'll you'll end up with very steep confused seas locally where the storm is and then the swells will flow out from that point and really flow entirely across the ocean and affect other areas of the world so let's look at this first off for the Beaufort wind scale which is commonly used and this had this the Beaufort wind
03:48
scale has actually had quite a history that initially it was based on the the condition of the sails on the ships with the wind of a certain strength a Beaufort 3 wind strength they could have certain sails up and a Beaufort 7 strength they could have fewer sails up with time this has evolved into really sea state more than anything but we also
04:18
correlate wind strength with a wind velocity with the Beaufort number so you see the Beaufort 3 signifies winds in the neighborhood of 7 ten knots and Beaufort 12 beginning of hurricane-force winds over 64 knots so Beaufort five is a nice comfortable sailing breeze fresh breeze you really go charging along very nicely in 17 to 21 knot winds but these are the wind
04:49
scales just for reference and we'll be referring to Beaufort wind scales later on so you might keep these numbers in mind wind direction high pressure towards low pressure so that's the direction of flow generally but also there's the inward circular motion towards the center of a low-pressure system and the outward circular motion away from the center of a high-pressure system and I'll show some diagrams that show this and there's
05:21
also the Coriolis effect which very significantly affects the wind direction and causes really the circular patterns that produce low-pressure systems let's talk about Coriolis effect for just a moment it causes the rotation of the air around the center of the low and you see as noted here counterclockwise in the Northern Hemisphere clockwise in the southern hemisphere now in the Northern Hemisphere any flow to the north or
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south will bend to the right of its path relative to the earth I'll show you a diagram of this in a second and the effect of this will increase towards the poles as you get towards the poles and as you see here in the southern hemisphere it bends to the left of the path and so here's a diagram of Coriolis effect and how it actually works and this really gets down to your frame of reference that you're that you're
06:23
observing the wind from now if you look at this diagram of the earth here here's North Pole South Pole here's the equator across the middle the equator is moving to the right and since the earth is roughly twenty-five thousand miles sir conference at the equator and since it rotates one revolution per day it's going to rotate at roughly a thousand
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miles an hour at the equator now the air is travelling with the earth rotation and thank goodness for that because otherwise we would have a thousand mile wind blowing in our face standing at the equator but the wind the the atmosphere basically is rotating with the earth in that direction so a little parcel of air and I have a little block here showing a parcel of air at the equator block a and it's rotating from left to right along
07:27
with the equator okay but that's a parcel of air now if that parcel of air a decides to move north for some reason towards the pole it's actually going to be traveling to the east faster than the earth below it now let that sink in for a minute this air once is going to travel north
07:57
to point B but it when it's at the equator it's traveling at a thousand miles to the east thousand miles an hour to the east but as we get further to the north it's still traveling at a thousand miles to the east but the earth may be traveling at only 600 miles to the east at that point because there's a smaller circumference so this parcel of air will actually get ahead of the Earth's
08:29
rotation in the easterly direction so therefore as it flows from the equator towards the pole it will turn to it to the right of that northerly path okay in the southern hemisphere the same thing will happen it will turn to the left of its path in in this direction so this flow to the right of its path is actually what causes the circulation
09:00
of some of the low-pressure systems and by the way we'll see in a little while it also affects the rotation of the currents at sea so let's look at a low pressure system and a low pressure system has warm air rising due to lower density than the cool air surrounding it okay so we have an area where we have warm air for some reason and that air since it's warm it's a lower density and
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it therefore rises well when that air rises in the center this low pressure system other air has to flow in to fill the void from from the from a surrounding air when this air flows in it bends to the right of its path as the Coriolis effect showed us so when it bends to the right it actually produces a circulation within this the center of this low and this entire system begins
10:03
to rotate due to the Coriolis effect and as mentioned here it's count in a low pressure system it's counterclockwise in Northern Hemisphere clockwise and southern and in a low pressure system the wind speed increases the closer you get to the center so when you're way out here you have the circulation but it's a slower speed then as you get towards the middle it accelerates to as towards the middle a high pressure system does the
10:35
opposite in the northern hemisphere you have cooler air than the surrounding air so that cooler air is more dense it sinks and therefore flows out of that region as it flows out it also bends to the right of its path and therefore circulates in a clockwise direction northern hemisphere okay so that's low pressure and high pressure systems and and how they develop their spin now
11:07
here's a there's more of the structure of a low pressure system Northern Hemisphere and low-pressure systems are generally formed by the collision of a warm air mass and a cold air mass and where these two come together it produces eventually develops into a circular pattern and this circular pattern is shown here and the construction of a of a low-pressure system basically this line here with the
11:39
with the triangular Barb's on it is a cold front will talk more about cold fronts in a little while and here with the round Barb's on it is a warm air system so here you have cold air and warm air weather come together we begin to develop a circular pattern and these circles they're shown here are isobars that is lines of equal barometric
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pressure so 12 indicates a thousand and twelve Millett millibars so a thousand being a thousand millibars so they don't print the one zero one two they just print the 12 so 12 signifies a barometric pressure of 10 12 10 8 10 4 and then if it goes below a thousand
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you'll see 9 6 9 4 9 2 and so forth and so these are the pressures and here these straight lines are continuation of these isobars but they become straightened out in this region and so this first straight line is 1004 1008 1012 and the direction of these isobars
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indicates basically the direction that this entire system is moving so here's an arrow here that's parallel to these isobars and that's the direction that this entire system is moving okay now let's look at the wind the blue arrows indicate the wind direction the wind will Bend it won't go exactly parallel to the isobars you'll actually bend in towards the middle a little bit as the arrows show
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here so as you come around this this low-pressure system the wind direction is changing always circulating around and always pointing somewhat in towards towards the center the air in these areas first off let's look here cool air cloudy steady rain warm air partly cloudy now here's a here's cooler air heavy clouds driving rain up here but here's
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where it's starting to clear and here in back of the cold front and you have cold air brisk winds clearing with clearing skies and you notice the significant change in wind direction from this zone to this zone and this is something that's very significant to sailors when a on a low pressure system comes barreling through this change in wind direction when the cold front passes so that's the basic structure of a low-pressure system of all the things in
14:50
weather the thing that you should best understand is the as a low pressure system the structure the movement and the the functions if you will of a low pressure system it's vitally important to a sailor offshore to a sailor anywhere let's look at another aspect of this low pressure system and that is the dangerous semicircle now you notice with the red arrow here that the path of this
15:22
low-pressure system is as shown and to the right of the path to the right of the low center is what's called the dangerous sense Circle it's stronger winds for for a couple of reasons one the isobars are closer together and therefore the wind pressure gradient is greater therefore the wind strength is stronger but secondly the the velocity of movement of
15:53
the low-pressure system itself the speed that the system is moving adds to the wind speed so the speed of this red arrow is going to add to the speed of this blue arrow and you're going to have stronger winds here for that reason as well and here on this left side of the of the low-pressure system this wind will subtract from the speed of movement so you'll have a canceling somewhat of the wind on the left side so the right
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side is usually the more hazardous part of a low-pressure system now in the bullseye as low approaches you may you may be directly on its track if the wind direction remains fairly constant and the barometer continues to fall okay so let's look at that in in this diagram here's a your boat here's your little red boat here and here's a low-pressure
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system approaching and moving in the direction of this arrow and as you see by this blue arrow that the wind that you're experiencing is roughly from the east it's on your port bow in this turret in this picture okay now the wind the low-pressure system has now traveled over you and the wind is still basically going to be from the east it may be starting to shift to the northeast somewhat and as we go one
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further step as the wind passes you it shifts to the west to the northwest on your starboard side so let's look at this again the wind has gone as the low crossed over you the wind has gone from your port bow to Northeast and maybe North to Northwest okay so the wind has we call this backing the wind has backed if you
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stand on the deck of your boat and you face forward the wind in this picture is on your left side your port side the port side of the boat in this picture it's coming from astern in this picture is coming from your right side so the wind has backed around the circle and you're now experiencing these strong northwesterly 's that come with the
18:36
movement of the cold front itself so that's in the bull's eye and what happened here is that the prep I didn't draw all the all the all of the isobars in this picture but what has happened here is when the low-pressure is here the low-pressure system is here your barometric pressure is a certain level let's say it's 1004 and as the weather
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the system gets here you may be a thousand so the pressure has reduced on this path from here to here but the wind direction has remained fairly constant okay now let's go on to if the lowest passing north of you again northern hemisphere the wind will veer that is move clockwise so let's look at this example here again is your boat here's a
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low-pressure system and it's passing north of you it's moving in the direction of the red arrow and here the wind is blowing our southeast on your on your port bow okay as you travel as it travels further it has shifted it has veered to your starboard bow and further it has veered a to your starboard quarter so what has happened is the wind
20:12
has from southeast to Southwest to Northwest ok the wind is the the lowest passing north of you but also in that scenario the barometric pressure has changed because as you go from here let's say you're 1004 and the low is close to you you now might be a thousand and here you're back up to 1004 again okay you see that pressure difference as
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well as wind difference so that's a low passing north of you now let's go a low passing to yourself the wind will back or move counterclockwise so here again you are here is a low south of you and the wind is blowing from the east or northeast the low has moved to this point and the wind has backed to north
21:18
okay and the wind has backed further to Northwest the wind the the low has passed south of you and here again the pressure is gone from 1004 a little lower here and higher again here so you see it so the two factors the change in wind in in barometric pressure as well as wind direction helps to tell you where that low is and and what you
21:49
should be expecting and this all relates to byes ballets law the Uys ballets was actually named of a person who who discovered and codified the idea of changing wind direction and the location of the low and basically let me go back a few slides here oops if you if you face the wind the low is
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going to be roughly to your right side now picture yourself here facing the wind and the wind is going to be roughly to your right side so here then is a is a an extreme low-pressure system which we all recognize as a hurricane offshore coming across the Caribbean here and heading on towards Florida and the Georgia coast and it's huge and you can see the circular pattern and the circular pattern as we mentioned before
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is produced by the Coriolis effect I'll show you more about the hurricane here in a few minutes the hurricanes are at very intense and destructive low pressure systems now they usually develop in the North Atlantic along the inter inter tropical convergence zone i TC Z as the zone moves north during late summer they also sometimes develop outside of the tropics and if you go
23:31
you'll have the printed handout so if you go to this to this website you'll find a guide that gives a lot of good information on on hurricanes and dealing with hurricanes but let's look at the Intertropical Convergence zone first itcz it's basically the zone between the northern and the southern hemisphere we have the equator here and as this diagram shows the ITCZ lies along the equator but here's a picture of how that
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itcz moves with the seasons of the year this dotted line is where it is in January and the solid line is where it is in the summer and you notice on the west coast of Africa that it has moved quite a bit north from the equator and you recall in discussing correo the fact that we said that the Coriolis effect increases as you go towards the poles from the equator so what happens
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is that the ITCZ is an area of a great instability weather instability that is you have a lot of warm air warm moist air it's rising and therefore we have the inflow and we have the Coriolis effect and the fact that this zone has moved further north in summer means that the Coriolis effect will be greater and therefore there's a greater probability of developing an intense low and this is
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why as we go from July August September we get the the peak of hurricane season it's because of the movement of the ITCZ and you can discover more of it by looking at this website this is a summary of north atlantic storm data that i extracted from the pilot charts and I'll show you pilot charts in a little bit but basically this is a statistical summary historical summary of tropical cyclones hurricanes and
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gales and if you're going to sail let's say from Norfolk to Bermuda you say to yourself well when's the best time to go well let's look at this in the month of April May June and so forth the number of tropical cyclones now remember this is on average so this says in April you have very very practically none but as the season progresses into the summer you see the number of tropical cyclones increasing reaching a peak of five and
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September now this resulted from the ITCZ moving north and a certain number of these trop cyclones will will intensify to hurricanes and you see the height of hurricane season September and there are gales these are gales reported from ships and gales meaning 34 knots or greater percent of reports that are reporting gales and most of these are coming from
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northern weather systems as opposed to tropical weather systems and you see these reducing as we get towards summer so if we're going to choose the best time to sail to Bermuda well it's not surprisingly June is the month that most people choose and it has some possibility of hurricanes and tropical cyclones it has some possibility of northern system gales but it's kind of the optimum balance between the both of
27:20
those and this information as they say is extracted from the pilot charts for the for the Atlantic Ocean here's an image of the various this is for the year 2009 and it shows the tracks of the various hurricanes that took place that year here's Bermuda here here's Cape Hatteras here's Norfolk and you see the track of the various hurricanes that are shown here and there
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most of them are developing here of the west coast of Africa near the Canaries and traveling west and then doing something here some of them go inland some of them Bend and go over to England okay now hurricane track avoidance again Northern Hemisphere and here's a picture of a hurricane a low pressure system it's a hurricane and traveling this way and you remember the right hand side
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that is the dangerous semicircle the right hand side of the track but in order to avoid the worst of that hurricane the recommended maneuvering actions for a boat is if you are on the right side of the track put the wind on your starboard bow you see this if you keep the wind on your starboard bow you'll be sailing in this direction as the as the low approaches
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if you are on the left side of the track and with the rules I showed you before about wind direction wind direction changes in barometric changes you can judge whether you're right or left to track so if you're left or track you want to put it on your starboard quarter northern hemisphere okay and it's the opposite for southern hemisphere and if you look at this website this is the nga
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website and you can actually download Bowditch in its entirety about each which is like the Bible for for sailing navigation weather and so forth and from chapter 36 you'll see a lot of discussion on hurricanes you can also download from this website Mariners guide PDF this guide it's a multi-page guide that gives you a lot of information on hurricane awareness and
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how to deal with hurricanes how to recognize some of the pitfalls and how to deal with them shown here the two basic options one if you have access to Internet images the Konev avoidance is actually shown on the predicted track image that no publishes so in this case here's the storm here in northern Florida and you can see the forecast track and this cone that they've drawn around this track and that's the cone that you want to avoid you want to
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maneuver your boat to stay out of that cone if you don't have internet images and you have only the voice forecast over single sideband radio or the nav text the predicted track is given in a narrative and you need to apply the Mariners one-two-three rule to determine the cone of avoidance now let me show you all of that here's a forecast of a wind track let's come over voice radio
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or the nav text and it says tropical storm winds of 30 4 knots gale extend out to 120 nautical miles from the center then it says our initial prediction is that on the eighth that is the date of eight let's say that June 8th at o 600 the storm center will be located at 30 point-0 north 68 point to west and the maximum winds are 40
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knots 12 hours later at 1800 it's going to be at this location 45 knots max 24 hours from the initial forecast at o 600 on the 9th it will be here 48 hours two days after the initial it's forecast to be here 72 hours three days is forecast to be here now recognize that these are forecasts these are not actuality so at
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that time when they gave this report this is what they expected but this may change as as time progresses and they gather more information but you take this information and in order to apply the Mariners 1 2 3 rule which will develop a cone of avoidance for you to avoid the first thing you do is plot the positions so here I've plotted the initial position 12 hours 24 hours 48 72
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hours so this is where the storm is predicted to be in those time periods now you recall that from this it said the gale force conditions extend out 120 miles from the center so we'll take 120 miles and draw circles of 120 mile radius around these points and I've drawn those in here so these are the gale force wind circles with a radius of
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120 120 miles the next step is to apply the Mariners one-two-three rule which basically is one day out that is 24 hours you add 100 miles to the radius two days out you add 200 miles three days out you had 300 miles and basically that is to deal with the unpredictability of the hurricane
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track so here's what you would do one day out you would apply this diet this radius so it would be 120 miles plus 100 to 200 20 mile radius at 24 hours from now and two days from now 48 hours we would apply 120 plus 200 320 and three days from now you would apply that circle 120 Plus 300 is 420 so these blue
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circles establish the cone of avoidance that you you would like to avoid if at all possible and in order to stay away from the possible track changes of the of the storm because what they predict here on this date may not be the case when they get later information from now but this is felt to be a conservative estimate of where the storm might be and
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it's used in in planning your route of avoidance now let's talk a little bit about local winds we're still on winds there are thermal winds and these again are due to temperature density differences and also the topography of the land as we talked about before warm air rises cool air sinks we have sea breezes land breezes valley breezes mountain breezes downslope breezes but all of these relate to changes in
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density you also have Geographic wind effects the wind can twist the follow the long axis of a river it may be blowing partly across the river but if it as it comes to the river it's going to basically flow more towards the centerline of the river not necessarily on the centerline but in that direction when refraction flowing offshore bends toward the perpendicular
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to the shoreline and will it win will accelerate in narrow valleys okay let's look at cumulus clouds cumulus clouds indicate rising air and thus they indicate a small low-pressure area now the cloud does not cause the low-pressure area the low-pressure area causes the cloud okay basically we're taking warm moist air and it's rising to
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a higher altitude cooling off and causing a puffy cloud but these clouds indicate local changes in wind direction and basically when did this area will be back shifted counterclockwise if you're sailing close-hauled towards the cloud you'll be lifted on port tack and headed on starboard tack okay you got to draw yourself some pictures to get your mind
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around that let's talk about water spouts now there's two times a water spouts that we basically see it see one is from tornadoes which are exactly the same as tornadoes on land they develop from up above from the clouds above and they touch down and the people down in the south eastern United States so it just experienced terrible tornadoes as we know and these can be also destructive at sea and we also have
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Fairweather waterspout develop from the surface level and develop up and here are tornadic water spouts these are very very destructive they develop from the cloud layer and they touch down um you can have wind speeds of well over 100 miles an hour I think in these recent tornadoes in the south I thought I heard that they said up to 150 knot winds so these can be very destructive
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here's a fair-weather waterspout and it developed from the from the surface up and it can also be destructive so don't minimize the potential danger of Fairweather water spouts and what I'd like to shift to now are waves because the thing that really gives us significant concern at sea are the waves produced by the wind and we have two terminologies here two terms that we use
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one or seas commonly referred to as waves but the seas are local there wind driven from nearby winds and their prime interest of Mariners and what happens is that they're they're very confused they're very steep because waves are being generated by the local winds and they bunch up on one another and become steep and and hard to handle swells on the other hand our long distance their
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storm driven from a distance away and you can use a swell to imply or infer the difference that the location of a distant storm and the height diminishes the further they travel also that all the speed changes tsunamis are caused by geologic disturbances then we've seen a lot of those here are here's another website that talks about a lot about waves here are here's a picture from
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Wikipedia on locally generated waves you can see a very confused see you can see steep steep waves interfering with one another the ship is having a significant time maneuvering these waves let's look at local storms they cause confused and violent wave action steep closely packed waves travel on out of the area mentioned and you can have a storm here on the on the west coast of Canada near
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Alaska let's say and here you have waves traveling all the way out to Hawaii and beyond and I'll show you some other pictures of this in a minute now let's look at wave terminology first off the length of a wave as you see from the picture is the distance between crests in feet here's the still water level and the height of the wave is from the peak to the trough okay and the period that
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is is the time in seconds for successive wave crests to pass a stationary object so the period is the time for two wave crests to pass the steepness of a wave is the ratio of height to length and here's wave steepness and here's the height and here's half the length and that gives you a steepness of a wave we're going to be using that terminology
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in a few minutes now wave steepness will increase when the wind blows against the current okay when blowing with the current will decrease the wave Steve steepness this is the actual steepness of the way this next bullet refers to the effective steepness let's say there is no wind at the moment but you have waves the effective least wave steepness will
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increase depending on the direction that the vessel is traveling so a vessel traveling in the direction of the waves the effective steepness will reduce the boat is going in the opposite direction it will increase what a boat is going in this direction of the waves it will decrease and here again as a website related to this waive circulation now the thing you have to keep in mind is that the water does
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not flow horizontally with the wave the wave is actually a a pile of energy that's travelling across the surface of the water and let's look at this diagram for a minute here are waves traveling from left to right and here's a ball floating in the water now here's a wave crest here to the left of the ball as this wave crest moves to the right it picks the ball up but the ball does not
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flow to the right the ball basically stays in the same location it moves a little bit to the right and left but not much and when the wave passes the ball is back down here so the way the the water basically rises and falls with the passage of the wave but it does not move significantly in a horizontal direction okay let's look again at this diagram so the surface water since it's standing
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still and the wave is moving the surface water actually flows up the incline as the wave moves from left to right with respect to the wave the water is going up okay and on the backside of the way the water is coming down and there's actually an acceleration here of this water because it's traveling it's covering a greater distance in a in a given period of time so it accelerates
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up the hill it decelerates down the hill okay and let's look at wave trains we have different waves generated by different sources and they may be at different periods that is different wave lengths and different speeds but as two different wave trains combine they'll produce a net effect as shown in
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this example so the dotted law the dotted wave and the solid wave are traveling at different speeds and you see that they're their frequencies are different and therefore one wave catches up to the other the dotted wave is going faster than the solid wave well back here the waves reinforce each other so so the combined effect is is higher but here they cancel each other so the combined effect is lower and this is why
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you will see at any given time a significant difference in the waves that you're experiencing and I'll show you more on that in a few minutes wave speed measure the time between successive crests as I mentioned before is period in seconds is between successive crests short periods locally generated waves for example one second long periods indicate waves well
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generated by a distant storm might be 16 seconds between crests wave speed in knots is three times the wave speed in period so if you have a 10 second wave it's traveling at about 30 knots here's a download from the NOAA website on weather and this is showing wave period
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that is the speed of the waves and the colors indicate different speeds if you look on the right here you see the color code and down at the bottom you see the blue the dark blue is a one-second wave and up top the pink is a 21 second wave so over here in the dark blue off the coast of Delaware is a storm generating waves and here there are 1 second waves but as we
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come across the Atlantic Ocean the wave period gets longer so that here as we as we approach Spain we have looks like 13 second waves okay so we've had this outflow of storm swells going all out from this locally this gent the storm that generating these waves and sweat becoming swells as a cross as fully as it flows fully across the ocean to all parts of the
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ocean okay now keep in mind that this wave this tan-colored wave 14 seconds approaching the coast of Spain is traveling at 12 13 or 14 3 times 13 is 39 39 knots as it as it crashes on the shore of Portugal here okay wave length
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and speed the length in feet is five times the period squared at 10 second wave is 500 feet long regardless of its height so length equals 5 P squared 5 times 10 squared 10 second wave 5 times 10 squared equals 500 feet long speed and knots as we said before 3 times a period for that wave 3 times 10 is 30 knots
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ok swells that are traveling across the ocean like a surge in that previous diagram they'll reduce in height about 50% a day and they'll increase in period about 2 seconds per day wave energy is proportional to the wave height squared so energy of the wave is a constant
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times the height squared so a 15-foot wave is five times the height of a three-foot wave therefore five squared becomes twenty five times the energy of a three-foot wave a 15-foot wave is twenty five times the energy of a three-foot wave okay here's a reference for that as well now
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wave height drivers as we talked about before in open ocean we have the wind speed wind fetch wind duration and currents near land there's also shallow waters narrows tides that affect wave height on breaking waves taller steeper waves will break sooner long swells like tsunamis generally don't break until tripping on shallows they're not they're not a problem to boats at sea but there
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are big problems of boats near the shore and inland but it's not just a matter of height because other factors include as we talked about before the current in opposition to the wind the suddenness of wind increase how suddenly has this occurred an air temperature colder air attacks a surface at a greater angle thus imparting more energy to the water okay
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now ground swells swells begin to feel the bottom when the water depth is about half of the swell length so a 500 foot wave will begin to feel the bottom at about 250 foot depth this well becomes taller and slower as it drags the bottom and it will break when the water depth is equal to one to two times the wave
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height so here's a breaking wave this is a ground swell it's being built built up higher and and steeper by shallower water and it breaks like this similar to us occurs on the north coast of Oahu wave steepness and the wave movement is an issue for a boat pitchpoling here we
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have an image of a boat sailing down a wave front but recall that the way the surface water relative to the wave is moving this direction so as this boat plunges into the trough it not only has to arrest its speed but it's also resisting the water flow in this direction which will depress the bow further so sailing off in stormy
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conditions is not an advisable thing to do here's a ship encountering a 50 foot wave at sea now Nowell wave height forecasts the waves come in many sizes as we saw because we have trains mixing we have swells traveling we have locally generated waves so you have a whole combination of wave heights at a given moment and a given location NOAA reports
52:07
wave height as significant wave height and don't be fooled by this term because it's very deceiving term and I'll explain in detail why it refers to the highest one-third of the waves at a given time and location but it's not the highest waves significant significant wave height is the average of the highest one-third of the waves in a spectrum average or the highest one
52:40
third now one-tenth of the waves will be 25% higher than that then the HS a hundred two the waves that is one out of a hundred waves we'll be 65 percent higher than the average now let's just think about that if you have a six second wave train that means there are ten waves per minute
53:14
that means that every 10 minutes you can expect a wave 65 percent higher than the significant wave height forecast and the maximum height can be a hundred percent higher it can be double the significant wave height so let's look at this this is a Brettschneider distribution it's a bell-shaped curve and basically it plots a number of waves on the vertical scale
53:48
and height of the waves on the horizontal scale okay and this says that at any given moment statistically here's your most likely wave height here because you have most ways of that height traveling but here's the NOAA forecast high of significant wave height HS in other words this
54:20
height along the scale here is HS significant wave height and this gray area here is one-third of all the waves in this distribution and the significant wave height is the average of all of these wave heights okay so here is HS as
54:49
reported by NOAA but here is the distributor the remainder of this wave spectrum here's the height of one tenth of the waves so let's look a little further on this now if you again go to Bowditch at this website this chapter xxxiii you'll find this table and this table will give
55:23
you headaches but there's a lot of information and I'll just go over it briefly and then I'll extract some information from it and show it to you on the left-hand column is the fetch in miles that is the distance of open water that the wind is blowing 10 miles 50 miles on up to a thousand miles fetch across the top is the Beaufort number three four five here's 17 to 21 knots the five here's just below hurricane the
55:54
11 okay then th P T is the time that is the period the I'm sorry said they're wrong the duration of the wind blowing that is four point four hours duration of the wind in hours here is a height significant wave height in in feet here is the period in seconds and then
56:25
eventually this wave can reach maturity at a fetch of 180 miles okay now I'm going to take this data and and show you some curves that I extracted from it here's wind duration and significant wave height for a fetch of 10 miles 10 miles of open water and the left scale is feet significant wave height the
56:57
right scale is ours that is duration at the wind is blowing and I can interpret this curve by saying okay a 5 Beaufort 5 wind blowing for about three and a half hours little over three hours will produce a mature significant wave height of about three and a half feet so a fetch of ten
57:30
miles Beaufort five blowing for a little over three hours produces mature waves of about three and a half feet significant wave height okay same thing a fetch your 500 miles feet height duration hours on the right
58:00
Beaufort five blowing four it takes almost 60 hours at Beaufort five to produce a nine foot wave but Beauford 11 blowing for 33 hours will produce a fifty eight foot wave so this
58:35
is extracted from this table okay and as I say the table will give you headaches a few other things extracted from the table just to give you a perspective of it here is height for different wind strengths - feet for different wind strength the yellow is Beaufort five seven nine eleven and you see four different fetches how the wave heights
59:07
will change no matter what you do about a Beaufort five will level out at about five at about nine feet regardless of the fetch and these others will approximately level out but you see the Beaufort 11 continues to build even with a thousand-mile fetch here is further here is wind speed I'm sorry wave speed in knots versus fetch
59:40
for different Beaufort numbers alright so the bottom is Beauford 5 the speed will increase with fetch as the fetch increases up to 25 knots okay and likewise the others will build the next thing I just want to mention our currents
01:00:12
yeah wind driven currents which we've been talking about we have tidal currents that is the swing in the in the tide elevation produced by the moon we have gravity currents that is flowing down a hill like a river we have the Coriolis effect that we talked about before and of course you have the tide tables and the current tables that have all the predictions for basically tides and tidal currents but on a worldwide
01:00:46
basis let's look at this diagram we show these great circulations within the oceans of the world and let's look at the ocean at the Atlantic Ocean the Atlantic Basin and we see these circulations and you see these clockwise circulations in the ocean it's the Coriolis that's producing the circulation and part of this major ocean Geyer are all these smaller currents that we refer to here is the Gulf Stream
01:01:18
current the Antilles current canary current North Atlantic Current so these are all part of this major ocean Geyer but it is it is the temperature differences within the ocean also salinity differences and and or and then also the the Coriolis effect that produces this major ocean currents and these are things that we have to deal with at sea they can be
01:01:51
quite significant I'll talk a little bit more about currents in a few slides down down the road here but I want to go to the sky because this is where the weather takes place and it gives us many signals as to what's what's happening and you want to be cognizant of changes to these signals that it's giving first off let's talk air masses and we have different designations of air masses continental polar that means it's air
01:02:22
that's flowing over land and it's coming from a polar region is going to be dry and cold and has given this designation continental CP continental polar continental tropical is going to be dry and warm warm for the tropical now maritime air air flowing over the oceans is going to be moist and again from the polar to be cold maritime tropical moist
01:02:52
and warm ok so there's four basic categories of air masses air mass movement is what creates weather system and the important point here is a moving cold air mass will push under a warm air mass raising the warm air to colder addict colder altitudes and causing storms a moving warm air mass will overlay a cold air mass causing
01:03:22
precipitation and it's the collision of cold and warm air masses that produce lightning thunder squalls low pressure systems and so forth now let's look at a cold air mass movement into a warm air mass and here on the left of this diagram is a cold air advancing cold air mass and it's pushing in under this warm air mass that's here and it's raising the warm air mass to higher altitudes and
01:03:54
therefore the the moisture in that warm air mass is cooled and produces cumulonimbus clouds and storm conditions that occur with that we know commonly that we get severe pre-fund of pre-frontal squalls that occur in advance of this condition and we can have lightning thunder hard driving rain and so forth as you see here this could be 50 or 300 miles ahead of the cold
01:04:26
front and this rain within the front of the cold front could be in the neighborhood of 50 miles or so so that's the advance of a cold airmass these are airmass symbols we've already used these symbols so this is no new news to you but let's look at warm air mass movement here's a warm advancing warm air mass and here's a cold air mass sitting here on the ground and the warm air mass will
01:04:59
ramp up over the cold airmass that the air mass will still hug the ground and the advancing signs of this are the high-altitude cirrus clouds and I'll show you a picture of these but as we come down the ramp towards the rain part we go from Cirrus and cirrostratus to altostratus to Alta Q and so forth and then the nimbostratus which are the dark rain clouds if you are out on a nice
01:05:33
bright clear day puffy puffy clouds in the sky and you see these real high-altitude cirrus clouds which are up here well these are the are the precursors of the movement of a warm air mass over your position and usually within 12 to 24 hours you're going to have rain from seeing these I'm talking in the temperate so you'll go from cirrus to cirrostratus
01:06:03
altostratus Alto Coombes or puffy nimbostratus the dark black rainy clouds and then as that clears out you have stratus clouds you may you have the rain tapering off and here's a here's a website for cloud classification cloud types this is if you're entering in your log you're writing the percentage of sky covered by the clouds and the type clouds that are up there here's are the
01:06:34
different codes that you can use to make sense out of these and and other people understand what you're what you're writing okay now cloud heights low clouds are up to 6,500 feet you see mid level clouds high clouds go up to 43,000 feet now lightning thunder squalls as we saw already nimbostratus clouds and lightning that you see in the distance these results from rising warm
01:07:06
air to colder altitudes more memories rising to colder altitudes occurs by two methods one is the cold air mass movement that we showed already and the other is what air mass heating on a clear hot day as occurs in summer afternoons on Chesapeake Bay and this is basically not because of an air mass movement it's because of air mass heating on that hot sultry day and
01:07:37
lightning thunder schools can be very hazardous can produce strong winds and I have commonly seen these up to 50 knots up to 60 knots so your method for dealing with a lightning squall these are things you have to think about and plan on how you're going to deal with a lightning squall and basically what I do is I want to button up the boat from the stand point to put my hatches in I take down my head sail or ifer my head sail and I reef my mainsail to the to the
01:08:10
deepest reef that I have and I do not use the engine unless I'm near a an obstruction that I have to avoid but if I have enough open water on a sail through this and basically maintain the wind as a squall hits you maintain the wind at roughly 45 degrees apparent wind to your bow and simply follow the wind around as it as it moves past and deal with deal with that
01:08:42
lightning swallow Thunder squall in that manner thunderstorm development developing stage as warm air is rising to higher altitudes it gets so high altitude it is cooled off if this cool there now plunges down and we have this cold downdraft when you feel the cold downdraft coming at you you know that you're very very close to strong driving rain because you're going to be hit by
01:09:12
that by that storm at that point but the cool air or the shift from warm air to cool air is the key sign that it's imminent okay let's talk about fog dew point is the temperature at which full the air is fully saturated with moisture and we use dry bulb temperature wet bulb temperature as methods of determining dew point and basically the closer these
01:09:46
two temperatures the closer we are to a foggy condition one when the spread between dry and wet bulb and I'll explain these in a minute when the spread between dry and wet bulb is down five four five degrees or so spread you are in fog territory if you will and it's imminent at that point fog is basically condensation of moisture in the air and by either lowering the air
01:10:19
temperature when you're in this four or five degree temperature spread if the air temperature is lowered by some mechanism or if the moisture in the air is raised you'll have fog and we would measure dry bulb wet bulb by what's called a sling psychrometer and this goes way back and basically is two thermometers and you can take this hold by the handle and
01:10:50
twirl it to create a wind over the over the thermometers and one thermometer has a cloth sock on the bulb and just think it is that if the weather is very dry and you put a wet cloth sock on this bulb then a lot of moisture will evaporate from that sock and it will cool this bulb off but if the weather if
01:11:21
the weather is very humid there will be very little evaporation so when the weather is dry you'll have a large spread because the drive the wet bulb has evaporated a lot of moisture and therefore cool the wet bulb so you have a large spread if it's humid the evaporation is low and therefore the temperature of wet and dry bulb are close so when they're close you're
01:11:53
you're ready for flop for for fog and here's a website related to that this is a modern psychrometer which I've never used I've used these but I've never used a modern one now fog there are several different terms used for fog but basically it still gets back to this issue of when you're close to fog conditions if you either lower the air temperature or increase the moisture content of the air
01:12:27
you'll get fog radiation fog and advection fog are very very are similar from the standpoint that the surface that is the surface of the earth or the water cools the moist air and and the and the air condense cools the moisture in the air in the air and the moisture condenses radiation fog occurs on clear calm nights when the land is radiating its heat to the atmosphere to the
01:12:59
universe really and it cools off quicker than the air and it cools the air below the dew point so we've cooled the air in this case this is similar except we've had warm moist air flows over a cold surface like cold water and the same thing happens we cool the moisture in the air and it condenses see smoke is different in that wisps of water vapor
01:13:31
vapor coming off the surface of the water are condensed in the cold air haze fine dust and salt particles suspended in air dam haze has water vapor in it ms2 fine drivel drizzle smog a mixture of smoke and fog okay let's look at weather data now there are lots of weather data that we have available on websites and and all of that but what do
01:14:02
we have available on our boat at when it's underway well first of all if you have the ocean pilot charts which can be an education for you as far as what to expect we have radar in which we can make observations of moving storms we do have the internet before we go to sea or if you have a really big boat you might have internet on board but for the most part you can prep ahead of time near sure you have VHF radio and cell phone
01:14:35
weather reports and now our cell phones our smart phones have all kinds of apps to give you great weather information near coastal is a device called nav text and this is transmitted from shore stations and it includes weather forecasts frequencies times of forecasts and so forth as well as the forecasts themselves for different areas the navi-x is a very valuable device to have
01:15:07
you can get a few Oh No a Navtech for neighborhood of four or five five hundred dollars in I feel for any coastal sailor it's an excellent thing to have the Navitus reaches out about 200 miles to see but coastal it's it's great and then offshore you have single sideband radio for voice forecasts you have single sideband fax and you have satellite all kinds of satellite weather forecasts are available at this point
01:15:37
you have DMA pub one one seven which is the radio navigation aids and I urge that you download this or buy it and hardcopy downloaded from the NGA website n G a website and it has a description of all the different radio aids that are that are available and the content of those broadcasts ocean Pilon charts as I
01:16:09
said that these are a historical record of past weather in in in all the oceans of the world and here you go you can go to this nga website and you can download this this ocean pilot chart in its entirety or you can buy them in printed copy from US chart agents now here's a typical page if you squint your eyes and
01:16:41
look in the upper left hand corner this is for November remember this is a historical summary so there's a page like this for each month of the year for each ocean of the world and also the Med and also the Caribbean places like that and it includes a a historical summary of weather data big red lines on here are the boundaries of wave heights
01:17:11
these are wave high probability circles and I'll show you some details on that and these little circles here with the arrows are the wind roses for each location on it on this chart for each five degree rectangle on the chart they also have descriptions and you can't read this very well but I just want to point out that these descriptions describe for example wave heights that you should expect in this area they describe gales that you should expect
01:17:43
extra tropical cyclones that you can expect here are the wind rose explanations what does this mean and here's examples of it yeah here's a blow up a part of this chart and as xxx refers to 30 percent of the time within that boundary there are waves of significant wave height of 12-foot and greater okay 30 percent of the time 20 percent 10 percent 40 percent okay
01:18:15
this five degree rectangle shows this wind rose and the length of the arrows indicates to you the distribution of wind from the different directions the number of feathers indicate the the Beaufort number so right here you have five feathers that means Beaufort five and the length as I say indicates the amount of time from each direction so as you scan this chart you can see the different wind rows as you can see here
01:18:47
the wind is here the wind is more balanced in direction than here it's more predominant from the north and the west okay here's icebergs here's a triangle an iceberg that was in a certain at it these green arrows indicate currents of that those ocean gyres that I showed you before there are also lines on here these black lines going in this direction are the
01:19:19
magnetic variation lines of magnetic variation if you trace the lines down somewhere you'll find a number on those lines that will tell you that it's 10 or 11 degrees east or west whatever it is okay then of course there are cruising guides there the coast pilots there's the the sailing directions the sailing directions and route the different locations local cruising guides and these all give you strategies and
01:19:52
weather expectations for this time of the year for this location radar collision avoidance you can use radar to identify and track other vessels of course you can also use it to identify land masses ports inlets bridges nav aids and so forth ray cons which are the radar repeater nav aids that will show
01:20:23
up on your radar you can use it for measuring bearings and distances you can also in talking about weather use it for tracking storms walls and you'll see a storm squall very clearly showing up on your radar screen and you can use collision avoidance techniques to track those like this so here is a plotting sheet called a maneuvering board but basically if I take my radar and I'm
01:20:54
located at the center of this plot on my radar screen if I plot the location of this storm school here and then let's say six minutes later I plotted here I can then draw a line between those two positions these this is the relative position of this storm compared to me and this is the relative motion of the storm compared to me and therefore this is the closest point of approach of that
01:21:27
storm this is the exact same way that we use radar to plot collision avoidance tracking of other vessels there's also internet weather data and these websites I include for your use and these are websites that I frequently use when I'm reviewing weather for boats at sea and this this we looked at before this is this shows on a surface level
01:21:58
forecast either a fax forecast that you can receive on your boat but in black and white or from the internet in color like this but basically this is a surface forecast for a particular date and time and here are the weather systems the low pressure systems here are the highs here the isobars this says developing storm here off the coast of Delaware here's a developing gale over
01:22:30
here here's a cold front here's a warm front okay so all the different attributes that we were talking about these numbers in the black squares a black and white these are the barometric pressure you see 90 it means nine nine zero as a barometric pressure this means a thousand and eight pressure okay so the different pressure is nine ninety four nine eight zero and then this would
01:23:05
be an example of a portion small portion of a NOAA offshore text forecast that you can download off the internet and in a certain date and time it gives a synopsis high pressure will build into the area pass east of the waters a warm front will occur Baltimore Canyon to Hatteras Canyon out to certain location expect gale force winds tonight expect these wind strengths and directions
01:23:36
these seas six to twelve feet these our significant wave heights so you have to use that Brettschneider distribution to estimate what you're going to see actually here's a 7-day here's one page of a seven-day forecast this shows the East Coast the US here's the West Coast the US and it shows the lows the highs the cold fronts loaf fronts this is day three it's a surface
01:24:09
forecast for day three of the seven days and there's a series like this so you can look at these and see what's going to happen out to seven days this again is internet download now storm tracks I'd like to just give an example of the east coast of the United States this is an area where the prevalent tracks flow is shown by these red arrows if I have a low coming out here off of Georgia will have lows
01:24:41
travel like this and usually turn northeast they may go out to sea at this point or they may continue up towards New England and off over to over to Britain but this is a typical track it follows in this area that pretty much follows the Gulf Stream in this area and then departs from the Gulf Stream up here and this follows pretty much the continental shelf right here here's a thousand fathoms here's thirty-nine fathoms so the Shelf
01:25:13
drops off right here and that's what the Gulf Stream does here is again the Gulf Stream flowing and if I'm planning to sail from Norfolk to Bermuda and the black line is my run line this is my desired course but I know I have the Gulf Stream flowing like this the blue line so I want to compensate for that so before I get to the Gulf Stream on the head south of my rum line as shown by
01:25:45
the Green Line and I want to intersect the Gulf Stream maybe 30 miles south west of my rum line okay because the stream as I entered it's going to push me roughly 30 miles to the northeast by the time I get get across it it's flowing it two knots or so so I pick a point aim towards it here I know I'm going to be pushed up in that direction my strategy is to use my my
01:26:17
GPS and set a waypoint for my destination safely say Bermuda and use the vmg function of my GN of my GPS to advise me on how I should be steering and how I should be trimming sails I find the best thing to do in crossing the Gulf Stream is to maximize your vmg to destination and not be as concerned
01:26:47
with your track over ground now let's look at another issue sailing in this area if I'm doing exactly this sailing in this direction and I have a low develop at this point or proceed to this point I'm going to be getting northeast winds from this low I'm only showing the low here but remember it's a big big set of circles around here I'm going to be getting northeast winds blowing right down the pipe in the Gulf Stream and
01:27:20
that's bad I have wind against the tide went against the current I mean strong wind strong current big steep waves in the Gulf Stream not a pretty sight so I'd like to avoid this if at all possible if one the other hand I'm crossing and I have a low passing north of my rum line now I'm going to be getting wind's blowing from the Northwest as the
01:27:51
trailing cold front comes through and that's beautiful this is when you leave you have a storm passing north of you a low passing north of you you want to be ready to leave on the tail of that storm just as it's beginning to clear in the wind pipes up to the northwest because that gives you a tremendous driving force to get through this Gulf Stream very very neatly so this is the time to go on on the heels of that storm
01:28:23
here's a picture of the Gulf Stream offshore and you see the Wiggles in it and you also see these Eddie's that are that are pictured here these circles and this is a warm Eddy here's a warm Atty here's a cold Eddy now the way oh and Eddy forms is the Gulf Stream will get these meanders in it like shown here and eventually this beyond or remember it's
01:28:54
flowing around this meander in this direction because the Gulf Stream is flowing from Southwest to Northeast and it's flowing around this meander well as this meander if it deepens and gets longer it will eventually teardrop and then break off as an eddy and it's circulating this way okay and it traps cold water now it doesn't show well on all of these but you notice that this
01:29:25
Eddy this darker green is is warm is colder water the lighter colors are warmer water you notice that this Headey is a warm Eddy it has it has trapped warm water in it you notice this Eddy is a cold Eddy because it trapped Northern Water in it so it's colder and it shows darker green all right Gulf Stream Eddy's look something like
01:29:57
this they they usually circulate inward in the direction shown and the way I'm showing the arrows if you look at these this is circulating counterclockwise circulating clockwise the warm 80 is clockwise the cold 8 is counterclockwise now I'm going to say with all of this I'm going to say usually because there are exceptions to these but the other thing is at the these themselves uh I've been in Eddie's or I had currents of eddy current of
01:30:30
four and a half knots within the eddy and if you have the electronics the electronics will measure that for you but the eddy itself is traveling in the southwesterly direction it's not traveling in the direction of the streamflow okay so that concludes my presentation and the important thing here is understand the diamond dynamics of weather systems mainly in order to
01:31:04
minimize surprises if I know what's coming I can prepare for it and I can be better prepared for it when it hits for example a storm spoil lightning squalls going to hit you don't be complacent prepare the boat and be ready for it when it hits wave action is the most destructive aspect of severe weather the wind we can usually deal with by by by taking by reducing sail but the wave action is is the the real destructive
01:31:37
force out there decide in advance what you will do in survival conditions will you heave - or will you run off we use a sea anchor so decide in advance what your strategy will be have the equipment prepared have the procedures prepared instruct your crew and practice you

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