Wind is a flow of air. It moves globally from high pressure to low pressure. The greater the difference (gradient) in pressure (for a given distance), the stronger the wind.
The wind moves parallel to the isobars (without taking friction into account): this is the geostrophic wind: balance between the pressure forces and the Coriolis force.
The friction is due to the earth surface. The geostrophic wind is therefore a (theoretical) wind from above.
At the surface, the friction will slow down the wind. The Coriolis force applied to the air masses will be weaker and the wind will deviate slightly (to the left in the northern hemisphere) and strengthen as it rises in altitude.
The wind “exits” the highs and “enters” the lows. The deviation due to
The deviation due to friction is of the order of 15° on the sea and 30° on land.
Consequence: for a person placed with his back to the wind, the high pressures are to his right and the low pressures to his left (Buys-Ballot law).
Buys-Ballot law
The legal unit of wind is the m/s. However, in sailing we use the knot (kt).
1 kt = 1 nm/h = 1.852 km/h.
It is measured with an anemometer placed in a volume as clear as possible at a height of 10m.
It is averaged over 10 minutes. The average wind does not take into account the gusts.
On the map it is materialized with the help of barbules that allow to estimate the wind speed:
a calm wind (less than 1 knot or about 0.5 m/s) is indicated by a circle instead of a staff and barbules ;
a wind speed of 1 to 3 knots (or between 0.5 and 1.5 m/s) is indicated by a staff without a line;
each half-dash represents 5 knots (about 2.5 m/s);
each full line represents 10 knots (about 5 m/s);
each flag (triangular symbol) represents 50 knots (about 25 m/s).
The barbules are always on the lower pressure side of the staff.
Wind barb
Make a barbule diagram of a wind staff to describe a wind coming from the northeast and 25 kts in speed:
