Veering and Backing Wind

Veering and Backing Wind 

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. 

Source: 
(http://www.theweatherprediction.com/habyhints/48/)

 

The terms veering and backing originally referred to the shift of surface wind direction with time but meteorologists now use the term when referring to the shift in wind direction with height. Winds shifting anti-clockwise around the compass are ‘backing’, those shifting clockwise are ‘veering’. At night, surface friction decreases as surface cooling reduces the eddy motion of the air. Surface winds will back and decrease. During the day, as surface friction intensifies, the surface winds will veer and increase. 

Source: 
(http://www.pilotfriend.com/av_weather/meteo/prv_wnd.htm)

 

Diurnal Effect 

– During the day, the wind veers and increases due to daytime heating and mixing with the 3000ft winds 

– At night, the wind backs and decreased through the loss of daytime heating 

Wind Shift as a Front Passes 

– With the passage of a front, the surface wind changes from that in the cold air to warm air for a warm front and warm to cold air in a cold front. This change will always be a veer. 

– The wind shift is such that alteration of heading to the right is required to stay on course, no matter which way you fly through it. 

– Cold front passage: Surface winds will back in advance of the front and veer abruptly upon passage. 

– Warm front passage: Surface winds will veer more gradually upon the passage of the front. 

Source: 
(http://vateud.org/index.php?option=com_content&view=article&id=188&Itemid=250)

Wind Direction with an Approaching Depression 

 

In the N (S) hemisphere, if a depression is approaching from the W and passing to the N (S) of the ship, clouds appear on the W horizon, the wind shifts to a SW (NW) or S (N) direction and freshens, the cloud layer gradually lowers and finally drizzle, rain or snow begins. If the depression is not occluded, after a period of continuous rain or snow there is a veer (backing) of the wind at the warm front. In the warm sector, the temperature rises, the rain or snow eases or stops, visibility is usually moderate and the sky overcast with low cloud. 

The passage of the cold front is marked by the approach from the W of a thick bank of cloud (which however cannot usually be seen because of the customary low overcast sky in the warm sector), a further veer (backing) of wind to W or NW (SW) sometimes with a sudden squall, rising pressure, fall of temperature, squally showers of rain, hail or snow, and improved visibility except during showers. 

The squally, showery weather with a further veer (backing) of wind and a drop in temperature may recur while the depression recedes owing to the passage of another cold front or occlusion. 

If the depression is occluded, the occlusion is preceded by the cloud of the warm front; there may be a period of continuous rain mainly in front of and at the line of the occlusion, or a shorter period of heavy rain mainly behind the occlusion, according as the air in front of the occlusion is colder or warmer than the air behind it. There may be a sudden veer (backing) of wind at the occlusion. 

Often another depression follows 12 to 24 hours later, in which event the barometer begins to fall again and the wind backs towards SW (NW), or even S (N). 

If a depression travelling E or NE (SE) is passing S (N) of the ship, the winds in front of it are E and they back (veer) though NE (SE) to N (S) or NW (SW); changes of direction are not likely to be so sudden as on the S (N) side of the depression. 

Source: 
(http://thenauticalsite.com/NauticalNotes/Met/MyMet-Lesson07-StructDepressions.htm)

 

Summary 

Arrow Due to friction the surface wind is slower than the geostrophic (freestream) wind. 

In NH the surface wind Backs. 

In SH the surface wind Veers. 

That means going up from the surface the wind will: 

In NH: Veer and Increase. 

In SH: Back and Increase. 

Arrow On the passage of a front the wind: 

In NH: Veers 

In SH: Backs 

Arrow In the day wind increases and at night it decreases. 

Arrow In the day it Veers in the N.H and Backs in the S.H. 

Arrow At night it Backs in the N.H and Veers in the S.H.