The Atmosphere
Movement in the Atmosphere
Atmospheric Stability
Localised Winds
Clouds
Fronts
Visibility and Fog
Aircraft Icing
Thunderstorms
Altimetry
Weather Reports and Forecasts
Practice Exam

Wind

Wind Direction

Previously we learnt about the gradient wind (the wind that follows curved isobars). However, the wind doesn’t always follow the isobars perfectly and we will learn about many different factors that alter wind direction. First, we need a way of describing any changes to the wind:

Veering and Backing

When wind direction changes it is said to be either backing or veering, depending on the direction of the change.

When the wind swings counter-clockwise it is called backing and when the wind swings clockwise it is called veering.

Surface Friction

Close to the ground, the surface wind is slower because of friction between the wind and the ground. This gives the Pressure Gradient Force more time to influence the wind direction, pulling it slightly across the isobars.

This typically means that the wind at 3000ft will roughly follow the isobars but the surface wind will slow down and back.

Since there are more obstacles over the land than over the sea, the surface friction is stronger and the change in wind velocity is greater over the land.

You might find it easier to remember the saying “the surface wind backs and slacks

The rule of thumb is:
Over the land, the surface wind speed drops by two thirds and backs by 30°
Over the sea, the surface wind speed drops by one third and backs by 10°

Turbulence

As the air flows over the surface, it has to navigate any obstacles – buildings, trees, hills etc. As the wind flows around obstacles it creates some turbulence. Bigger obstacles and stronger winds will create greater turbulence.