A veering wind is a wind that turns clockwise with height. An example of a veering wind would be a southeast wind at the surface and a west wind at 700 millibars. The wind turns in the same direction as a clock from the surface to 700 millibars. A veering wind is associated with warm air advection and dynamic lifting (primarily because a south wind in the PBL transports warmer air to the north). The magnitude of warm air advection is a function of wind speed and the pre-existing thermal gradient. Weak winds will result in weak advection. Winds often veer ahead of cold fronts (in the warm sector of a mid-latitude cyclone). A backing wind is a wind that turns counter-clockwise with height. An example of a backing wind would be a north wind at the surface with a west wind at 700 millibars. A backing wind is associated with cold air advection and dynamic sinking. Winds back behind cold fronts. A way to remember the difference between the two is the memorize the set of letters CVW and CCBC, where CVW stands for (Clockwise, Veering, Warm air advection) and CCBC stands for (CounterClockwise, Backing, Cold air advection) You may wonder why a veering wind is associated with WAA and a backing wind is associated with CAA. A veering wind turns clockwise with height. Since warmer air is in the southern latitudes, a south wind will promote the bringing of warmer air into the forecast region. Winds having a southerly component at the surface and a westerly component aloft is a veering wind. The middle and upper level winds in the mid-latitudes will generally have a westerly component to them. The upper level winds blow from a much more persistent direction than the surface winds. It is primarily the surface wind direction that determines whether the wind is backing or veering with height. A backing wind turns counterclockwise with height. A north wind at the surface and a westerly wind aloft is an example of a wind that is backing. A north wind generally brings in cooler air. The VEERING associates with WAA and BACKING associates with CAA dictum is a synoptic scale mid-latitude rule of thumb. There is no 100% guarantee that veering produces WAA and backing produces CAA, especially when winds are light or the temperature gradient is barotropic. In the warm sector of a mid-latitude cyclone the wind will almost always veer with height (generally a WAA pattern will be present). In the cold sector of a mid-latitude cyclone the wind will almost always back with height (generally a CAA pattern will be present). If you are in the tropics, in a light wind environment (such as under a high), or in a barotropic environment the backing / veering rule of thumb has very little relevance. The backing / veering rule of thumb works well in the mid-latitudes near mid-latitude cyclones. Thermal advection is most significant in the mid-latitudes and in the vicinity of low pressure or a tight pressure gradient. |