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Lesson 11 Weather


Especially the wind is of great importance for sailors. Wind is air that flows from a high pressure area to a low pressure area.


The rotation of the earth deflects winds to the right in the northern hemisphere. This is called the Coriolis effect.

Because of the coriolis effect wind do not flow directly from high to low pressure, but circles clockwise out of the high pressure area and circle counter clockwise into the low pressure area.

The wind makes an angel of 15 degrees with the isobars, depending on the roughness or the surface.


Coriolis effect

The deflection of winds to the right by the Coriolis effect is the direct result of the rotation of the earth. If we look at the earth from the top and so we look at the North pole, the earth turns counter clockwise. If we look at the earth the usual way, the earth turns to the right. The speed to the right on the parallel of 45°N is much higher than on the parallel of 50°N. Both go around in 24 hour, but the length of the parallel of 50°N is less than the 45°N parallel. Air that flows from a high pressure area on 45°N to low pressure area on 50°N, keeps the high speed to the right, causing the deflection to the right.

Trade winds

In the image below we can see the trade winds on earth.


When we look at a surface pressure chart we should be able to understand what is the direction and force of the wind. For example on the location of the green cross below, the wind direction is like the red arrow, from high to low deflecting to the right. Note that the map is not a mercator projection. North is as the green arrow shows, not always the top of the chart. The wind direction is more of less South West. If the isobars are close to each other the wind is strong. If the isobars are far from each other the wind is not strong at all.


Exercise reading weather maps:

Download the surface pressure chart and make forecast of the wind direction and force in Nederland.
check your forecast here.

Buys Ballot's law

This means that if you are facing the wind, you will have the low behind you on the right hand and the high pressure in front of you on your left hand.

wet van buys ballot

Beaufort scale

Warnings are given from force 6.

Force Name km/h knots m/sec Sea state
0 Calm 0-1 0-1 0-0,2 Sea like a mirror
1 Light air 1-5 2-3 0,3-1,5 Ripples with appearance of scales are formed, without foam crests
2 Light   breeze 6-11 4-6 1,6-3,3 Small wavelets still short but more pronounced; crests have a glassy appearance but do not break
3 Gentle breeze 12-19 7-10 3,4-5,4 Large wavelets; crests begin to break; foam of glassy appearance; perhaps scattered white horses
4 Moderate   breeze 20-28 11-16 5,5-7,9 Small waves becoming longer; fairly frequent white horses
5 Fresh   breeze 29-38 17-21 8,0-10,7 Moderate waves taking a more pronounced long form; many white horses are formed; chance of some spray
6 Strong   breeze 39-49 22-27 10,8-13,8 Large waves begin to form; the white foam crests are more extensive everywhere; probably some spray
7 Near gale 50-61 28-33 13,9-17,1 Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind; spindrift begins to be seen
8 Gale 62-74 34-40 17,2-20,7 Moderately high waves of greater length; edges of crests break into spindrift; foam is blown in well-marked streaks along the direction of the wind
9 Severe gale   75-88 41-47 20,8-24,4 High waves; dense streaks of foam along the direction of the wind; sea begins to roll; spray affects visibility
10 Storm 89-102 48-55 24,5-28,4 Very high waves with long overhanging crests; resulting foam in great patches is blown in dense white streaks along the direction of the wind; on the whole the surface of the sea takes on a white appearance; rolling of the sea becomes heavy; visibility affected
11 Violent storm 103-117 56-63 28,5-32,6 Exceptionally high waves; small- and medium-sized ships might be for a long time lost to view behind the waves; sea is covered with long white patches of foam; everywhere the edges of the wave crests are blown into foam; visibility affected
12 Hurricane >117 >63 >32,6 The air is filled with foam and spray; sea is completely white with driving spray; visibility very seriously affected

We can calculate from beaufort to meter per second and back with this formula:

From knots to beaufort:
(knots/5)+1 bft
+1 bft till 40 knots.

From m/s to knots:
m/s x 2 = knots

Veering - backing

Veering wind means that the wind direction changes clockwise, for example from North to East. Backing means that the wind direction changes counter clockwise, for example from West to South.

ruimen krimpen

surface pressure charts

On surface pressure charts we see lines connecting points of equal atmospheric pressure, called isobars. If these isobars are close to each other, the difference in pressure is on a small distance is large. As a result the wind will be strong. If the lines are far away from each other the difference in pressure is small and the wind will not be strong at all.

Air mass

We can distinguish Arctic, Polar and Tropic air. If the air comes from the continent it is called continental. Continental air is dry and very hot in the summer and very cold in the winter. Air from sea is called maritime air. Maritime air is moist and not the temperature is moderate. Combinations are: Maritime Polar air masses, Continental polar air masses, Maritime Tropical air masses, Continental Tropical Air masses.


When we have less then 1000m sight there is fog. Fog is the result of condensation as moist air cools down. Typical situations are when a South Western wind brings warm and moist wind over the cold North Sea in the spring. Another example is when a Northern wind brings cold to the warm North sea in the fall.



Thunderstorms occur in cumulonimbus clouds. Possibly thunder comes in your direction even when you would expect that the surface wind would push the thunder away from you.

Exercise surface pressure chart part 2

Download surface pressure chart of the Metoffice determine the wind direction and force in some districts with the help of the "Geostrophic wind scale" in the left top corner and the table below.

  backing relative to geowind percentage van geowind
Sea 10 - 15 degrees 70 - 80 %
land during daytime unstable 20 - 30 degrees 50 - 60 %
land daytime stabile 30 - 40 degrees 30 - 40 %
land night 40 - 50 degrees 10 - 20 %

Check with these forcasts.

Development of a frontal depression

On the Atlantic Ocean, the "kitchen" of the European weather, the relatively warm air in the South floats over the cold air in the North. This is called a front. The cold air has a higher density and so a wave arises in this front. This wave develops into a depression or low pressure area with a warm front (recognizable by a line with balls), cold front (recognizable by a line with triangles) and in between a warm sector. Because the cold air is heavier, it passes the warm air in it creates an occluded front (recognizable by a line with balls and triangles). Because of the prevailing winds in Europe, the direction is always in easterly.


Before the warm front

A warm front always announces itself with cirrus clouds, these are ice crystals at high altitudes. Then there is ever lower gray clouds with moderate rain. Just before the front passage, the wind will shrink for a while.

Warm air mass

During the passage of the warm front the wind will veer from south east to south west. The temperature will rise, because after the warm front you are in the warm air mass. The air pressure drops because you get closer to the center of the low-pressure area.

Before cold front

After the warm sector there is the cold front. After the passage the wind will veer from south west to north west. The temperature will drop, because you will return to relatively cold air mass. Cumulonimbus clouds are situated in the cold front. This results in heavy rain and hailstorms, thunderstorms and gusts of wind. After the passage of the cold front, the weather is cloudy.

CB cloud

Sea wind

Sea winds and land winds are caused by differences in temperature between land and sea. During the day the air rises above land because the land warms up faster then the sea. The thermal low pressure is filled up with air from the sea, resulting in the sea breeze. At night the land cools down faster than the sea and as a result the air descends over land and the air rises above the warm sea. The "vacuum" at sea is supplemented with air from land and that therefore causes the land wind.

The rising air over land condenses resulting in a sea wind front.
sea wind front

Land effects

As wind or water flows around a cape, the rate or windspeed will increase enormously. We call this cape effect. If wind or water is squeezed through a narrowing, we call it the funnel effect.


Gribfiles are useful on board because they are small files that have to be downloaded. These can be read in a grib reader such as www.zygrib.org. If you have a good internet connection via telephone or WIFI in the port, then www.windy.com is a very good alternative.


Factors for wave height

The following factors determine the wave height:

  • Duration certain wind
  • Wind speed
  • Depth of water
  • Fetch


Swell are the waves caused by a previous wind. Sea are waves that are created by the current wind. Waves are the swell and the sea. A ground sea is created when waves reach a sudden shallow area, such as sandbanks. Breaking waves are the result. We can find the surf even closer to the shore. Cross swell are patterns that run through each other like after the passage of a frontal depression. If the tidal current is against the wind direction, the sea becomes choppy.



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Questions & explanations

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Question 1: How does the wind blow around a low pressure area?


a: counter clockwise

b: clockwise

c: parallel to the isobars

Question 2: Between which air masses is the Arctic front?


a: arctic- and tropical

b: tropical- and polar

c: arctic- and polar

Question 3: Where does the wind blows the hardest?


a: in the cold front

b: where the isobars are close to each other

c: in the warm front

Question 4: Facing the wind the high pressure area is located:


a: in front on your left hand

b: behind on your right hand

c: in front on your right hand

Question 5: Veering wind is for example a change of direction from:


a: east to north

b: south to east

c: south to west

Question 6: Gale is wind force:


a: 7

b: 8

c: 9

Question 7: What does the wind after the cold front?


a: veering north west

b: backing south

c: backing north

Question 8: What is the name of the cloud that has the shape of an anvil?


a: Cumulus

b: Nimbostratus

c: Cumulonimbus

Question 9: In North West Europe a wind from the east is...


a: maritime continental

b: polar continental

c: polar tropical

Question 10: A south west wind in springs at North Sea means?


a: cold air mass

b: warm air mass

c: none

About Elmar Teeuwen

Elmar is a CWO sailing instructor and RYA Yachtmaster® Offshore, Commercial Endorsement with STCW95 and certified coastal navigation teacher.
He has more than a decade of experience with teaching sailors on the North Sea, Atlantic, Med and Caribbean from beginners to instructor level, as well as regatta teams.
In addition to his native language Dutch, Elmar speaks English, German and Italian.
His knowledge and teaching skills could be off great value to anyone who wants to realise his sailing dreams.