Meteorology for sailors!

Gilleleje Sailing Club Tuesday 11 February 2020 Beddingskajen 4, 3250 Gilleleje, from 19 to 21.

How do we predict the weather? This is the question sailors have tried to answer for centuries. In recent years satellites and systematic collection of weather data have enabled us to process raw data from theoretical models and through them, we can today predict the weather with relatively higher accuracy!

We spend an evening in Gilleleje Sailing Club telling about how we as sailors can get hold of weather data, so we can quickly assess the development of the weather and compare the data with observations from the cockpit. In the cockpit we can assess the development in the cloud deck and based on it, we can say something about the development of the wind and the lake. In certain situations we have the opportunity to change our sailing plan, we can seek shelter or lie down in an area of high pressure where sea and wind are less, the better and the further ahead in time we can predict the development of the weather, the greater the chance we have to take action that brings us into a more favourable situation.

The global weather:

There are many powers that affect the air masses that surround the ground and create the wind and travel the lake. The temperature on the ground is created by the sun, but the sun heats the world mostly around the equator and the air rises to the weather and seeks to the poles, at the poles, cool air and fall to the surface. But the planet also changes the declination from plus minus 23.5 ⁰ in winter solutions over equal 24 to plus 23.5 ⁰ in summer solutions, the declination is the earth axis slope to the sun. The change in the earth of declination over seasons also affects the weather. If the world didn't rotate around the north south axis, we'd have a relatively simple system. However, the world is rushing around its north-south axis, at a speed at the equator (which has an peripheral circumference of approximately 40,000 km) is given a peripheral speed of 40,000 / 24, i.e. almost 1,700 km/h! The longer we move north, the lower the periphery rate, so we here in Gilleleje at 56⁰ North, have a peripheral rate of approximately 900 km/hour. (Cos56⁰x40,000/24). The difference in peripheral speed makes an air mass moving from the equator to the north will be defeated to the right. That power was discovered by the French Corioli in 1835 and he's got his name:

The illustration can see what happens if you throw a ball from the North Pole against the equator, the recipient who is just south of getting the ball because it will be deflected to the right, corresponding to the larger peripheral speed at the equator. On the southern hemisphere, the other way is to be defeated to the left.

The power of Corioli has an influence on the winds that blow the globe, but the power of Corioli also affects our oceans, so, for example, the Gulf stream also deflected to the right when it starts in the Caribbean and moves north.

The illustration can see how the warm sea flows such as the Gulf stream are deflected to the right of the North Atlantic and former are circulating power that turns right on the North Atlantic but south-south. The illustration can continue to see the cold sea flows marked with blue arrows, in the north, the Labradorstrim and the East Greenland power are the most famous.

Global climate zones:

The globe is divided into some belts which are within certain latitudes. Around the equator we have the Tropical Zone where a low pressure is formed due to the warm air masses that rise into the air and form a negative pressure. We move north to about 30⁰ North latitude, we have the Subtropical High Pressure (or south to 30⁰ South latitude), the high pressure is formed via the tropical winds that cool and move towards the sea surface and in the Atlantic the high pressure forms the Passat over the Atlantic Ocean, which blows quite constantly from east and west and this wind is since Columbus used to sail across the Atlantic. Today, ARC sailors and sailors crossing the Atlantic also use the passage through the Canary Islands. If we move further north to about 60⁰ we come to the Temperate zone with the low pressures we know (or south to about 60⁰ South ready). The low pressures are formed at Newfoundland in the zone where the Temperate air masses meet the Polar Front, in the boundary layer between the two air masses the Temperate Cyclones are formed which move eastwards and give the westerly wind and the road we know all too well.

If we move further north we come to the Arctic zone (or south to the Antarctic zone) here we find the polar high pressure which is formed via air masses from the Temperate zone, flows to the north cools sinks to the sea surface and moves south, on the route it turns to the right and forms the polar east wind.

As can be seen from the illustration above, the movement of air masses around the entire globe is affected by Coriolis' force and in the higher air layers there are a number of "cells" that circulate ascending and descending air masses, which help to form belts with high and low pressure.

Temperate Cyclones:

At our latitudes, which are in the Temperate zone from 30⁰ to 60⁰ north latitude, the weather is characterized by temperate cyclones, not to be confused with tropical cyclones as we will discuss later. Temperate cyclones are created at the polar front, as previously mentioned, where the temperate air masses meet the polar air masses and in the boundary layer some waves occur on the polar front which develop into temperate cyclone, or low pressure.

The low pressures move from the western part of the North Atlantic and eastwards and form coherent systems of hot and cold fronts which on average move by about 27 kn in an easterly direction.

The illustration shows a Temperate Cyclone about the donor of isobars which represent different pressures. From the Temperate Cyclone there is an occlusion marked with a red line and dents and triangles. If we follow the occlusion we meet the warm front, marked with red dents and to the left we meet cold front marked with the blue triangle.

The occlusion is a fusion of the hot and cold fronts. The lines that surround the low pressure are isobars, ie curves that represent the same pressure eg 950 mb. The closer the isobars are, the greater the pressure difference over a given distance and the stronger winds are formed. If there was no Coriolis force, the wind would blow from a higher pressure directly towards a lower one and thus towards the center of the low pressure. However, the Coriolis force deflects the movement to the right and this causes the winds around a low pressure to move around the low pressure, or the temperate cyclone, and follow the isobars.

As can be seen in the illustration, the masses of air move counterclockwise around the low pressure in the northern hemisphere and clockwise in the southern hemisphere.

Warm and cold fronts:

Picture showing a warm front on the way, the clouds gathering and the front getting closer.

The hot front moves at low pressure and at a speed of about 20 to 30 kn. A hot front has an extension of about 500 to 800 cm and a passage lasts in the order of 24 hours. The hot front has a relatively flat slope of 1: 200 and the first signs that a warm front is under sail is a cloud cover of Cirrus Ci, which becomes denser and forms Cirrusstratus Cs, which changes to medium-high clouds AltoStratus As, which in turn changes to vertical clouds of the type Nimbusstratus Ns. The pressure will drop and the rain we start and will be persistent and last typically for 10 to 12 hours before it clears up the temperature starts to rise and the clouds will change to StratusCumulus and the barometer will rise. When the hot front meets cold air masses the warm air will be on top of the cold air with the the mentioned slope of 1: 200, so the front is relatively flat and has a large extent.

During a passage of a warm front the wind will turn to the right and after the passage the wind will be decreasing.

After a hot front usually comes a cold front which has a somewhat different course the cold air mass will wedge under the hot air mass and be relatively steep 1:50 in relation to the hot front. When the hot air meets the cold air we condense the moisture in the air mass, clouds will form with precipitation to luv.The wind will typically be stronger at a cold front and the wind will again turn to the right, but it will be a greater wind turn than at a warm front.

Profile of a cold front where the cold air cools down under the hot air, the hot air cools, condenses and rain is formed, the clouds in the boundary layer can form vertical clouds of the type Cumulus-Nimbus. The cloud cover will change from Cumulus Cu to StratusCumulus Sc and subsequently vertical clouds CumulusNimbus Cb are formed - the ugly Armbolt Clouds, which give strong winds and can develop violently. See below the table of the most common cloud shapes from DMI.

Example of vertical Cumulus-Nimbus under construction, if you have the opportunity to stay in port, it is probably preferable, alternatively you must prepare for strong winds and sea!

Another image of a Cumulus-Nimbus cloud under construction as you can see in the table of clouds, these clouds can get very high up in the atmosphere and the forces that develop when water vapor changes to liquid phase, correspond to the energy used when you reach a boiler dries, just the opposite way! From the first main theory of thermodynamics, we know that energy is constant and it gives rise to the very large forces that are released when a Cumulus-Nimbus is allowed to develop.

Also a picture of a Cumulus-Nimbus rising very high in the atmosphere and generating thunderstorms and thunderstorms and strong winds!

Weather map downloaded from the app Windy which you can find in the Appstore or GooglePlay. A tool that is good to have on the voyage, where you can continuously check the weather and the development of the weather over time so you have time to prepare for bad weather, strong winds and high seas.

A current weather map of the weather over Europe with Low Pressure, Hot and Cold Fronts, Occlusions and isobars (lines with the same pressure), as can be seen, the isobars are very close over Denmark and it gives strong winds. The fronts that move from west to east at a speed of about 25 kn, gives the heavy rain we are experiencing these days.

DMI’s overview of the most common cloud shapes and at the bottom the dreaded Cumulus-Nimbus which we as sailors must take care of and guard against.

Passage of temperate cyclones

As a sailor, you will usually sail with the temperate cyclones, ie east, for example from Bermuda to the Azores, where you follow the low pressures across the Atlantic. For a sailboat, it will not be possible to follow the low pressures that certain high-speed merchant ships can. For a sailboat, you will be overtaken by the low pressures which, as previously mentioned, move by 20 - 25 kn.

The problem here is how to try to position yourself when you are overtaken by a low pressure?

If we are in front of the low pressure, i.e., to the right of the low pressure which is moving to the east and we can see, for example via the program "Windy", how we will be overtaken by a relatively strong low pressure, where should we look?

The worst thing will be to lie north of the low pressure, in that sector we will experience headwinds and the wind strength we can assess based on how close the isobars are and based on the plotted wind arrows.

If we can place ourselves in front of the warm front, we will find that the wind blows perpendicular to the direction of low pressure and the wind will not be strong, the wave height will also be moderate due to the relatively short wind grip. We return to the wind grip. Once the hot front has passed, it will be an advantage to adjust the course to port, the waves in this sector will also be relatively moderate, again due to the wind grip now lying between the cold and hot front and thus is relatively short.

When passing the cold front, you will usually experience the strongest wind and the highest lake and here it is important that the course is adjusted to starboard and that sailing is reduced. When the cold front is passed, we experience Backside weather, good weather with few clouds as shown in the picture below.


The wind forms waves on the sea, but there are various factors that determine how big waves we experience. The wind strength is of course important for how big waves we experience, but also the duration, i.e., how long it has blown from the same direction and finally there is the wind grip. The wind grip is the distance the wind has to build up waves. Here in Gilleleje, a north north west wind gives the highest waves and the wind grip is like the distance from around Skagen and to Gilleleje.

However, water depths also play a role and on Lægtvand the waves break earlier. Lægtvands waves are talked about when the water depth is 1/20 x The wavelength. For Tsunamis, waves with a wavelength of 100 km above water depths of about 4,000 meters were experienced. The speed of such a wave can be calculated from the formula.C=√(2&g x D) = √(2&9,81 m/s2 x 4,000) = 198 m/sec = 384 km = 711 km/h.

Tropical Cyclones:

Tropical cyclones are a rotating wind system where air masses rotate around a very deep low pressure, the rotation is counterclockwise in the northern hemisphere and opposite in the southern hemisphere. The rotating air masses are hot and humid and the wind speed is over 34 kn. If the wind speed is above 64 it is called a tropical hurricanes.

If the wind speed is between 34 and 64 kN is called a tropical storm.

The Condition for the formation of tropical cyclones is that the sea temperature over a large area is over 27⁰ and that the warm sea is heated to a depth of 30-50 meters. The width must be higher than 5⁰N or S, ie on the equator, no tropical cyclones will occur.

Conversely, Tropical Cyclones do not occur north of 30⁰ or south of 30⁰ as the sea temperature here is too low. Since the sea temperature only reaches its maximum in late summer, summer is thus the time for the start of the period for Tropical Cyclones.

Tropical Cyclones form in a belt from 5 to 30 degrees on each hemisphere and start in summer and end in autumn.87% of Tropical Cyclones form on latitudes less than 20 degrees and 67% in a belt between 10 and 20 degrees.

Tropical cyclones around the globe and below a plot of the times they occur, ie from June all the way to autumn.

There are theories on how to sail a Tropical Cyclones. If a tropical cyclone moves from east to west and if the orbit is 290 degrees and we are in the northern hemisphere, it is most dangerous to lie north of the cyclone where the wind blows east and the cyclone often turns clockwise on its trajectory. It is best to lie down on the south side, where you can get the cyclone to help get away from the cyclone and in that way follow a course that turns clockwise, ie the same direction as the cyclone.

Hans Tørsleff

Instructor for Yachtskipper 3 and 1 in Gilleleje Sailing Club 11 February 202011. februar 2020


Clouds we like, "Good weather Clouds" Cumulus clouds!