When listening to a weather forecast you may hear that a wind is ‘veering’ or ‘backing’. But what does it mean?
When a wind veers it changes direction in a clockwise direction. In other words, a westerly wind would become a north westerly wind. A backing wind would change from a northerly wind to a north westerly wind.
A veering wind is generally associated with warm air advection. A backing wind is associated with cold air advection.
In the northern hemisphere (as opposed to the southern hemisphere) low pressure, relatively warm air systems, known as depressions, revolve in an anti-clockwise direction. Cooler air, high pressure, systems revolve in a clockwise direction. Therefore, if you know where the centre of a high or low pressure system is located, relative to your position, and you know its direction, you can establish whether a wind direction will veer or back.
For example, if a low pressure system passes north of you (in a west-to-east direction) in the northern hemisphere then the wind will veer, or change clockwise. If you are north of the path of the same system then the wind will back as the depression passes through. To establish the centre of a depression, use Buys Ballot’s Law.
During the passage of a low pressure system in the northern hemisphere, the passage of a warm front, warm sector and cold front will result in the wind veering (at the warm front), stabilising through the warm sector and then veering again at the cold front.