The change in wind direction and wind speed with height gives clues to the synoptic temperature advection. A clockwise turning of the wind with height is termed veering. Winds turn from southeasterly at the surface to westerly aloft in a veering case. A veering wind is associated with warm air advection. The strength of the warm air advection will depend on the strength of the wind and the amount of veering with height. If winds are strong and southerly at the surface and from the west at 700 mb, through time the low levels of the atmosphere will warm while the upper levels may stay near the same temperature. This will cause instability. The amount of instability in the low levels will depend on the amount of thermal advection and the amount of veering from the surface to the mid-levels. A veering profile is common in the warm sector of a mid-latitude cyclone. The wind will veer with height in the vicinity of a warm front. Before warm front passage it is common for winds to be light northerly, shift to the east, then finally shift to a southerly direction. Winds that turn counterclockwise with height are termed a backing wind. A backing wind is associated with cold fronts. Behind a cold front, wind will be from a northerly direction, then shift counter-clockwise to a westerly direction with height. Keep in mind that the winds in the mid and upper levels usually have a more westerly component than an easterly component due to the prevailing planetary scale westerlies. A backing wind is associated with cold air advection. A backing wind in the low levels of the atmosphere is favorable for synoptic scale sinking motion. Most rain and thunderstorms are out ahead of cold fronts. Precipitation behind cold fronts is generally lighter or lacking all together in most situations. A hodograph displays the wind speed and direction with height. Veering and backing of wind can be figured very easily through the diagram. A hodograph can be used to determine most likely thunderstorm type. The low level of the atmosphere is from the surface to 850 mb, the mid-levels from 850 to 500 mb, and the upper levels 500 to 150 mb. These hodograph types are described below: SUPERCELL *Strong veering of wind in low levels, extending into mid-levels *Wind speed greater than 20 knots in low levels and preferably greater than 100 knots from 500 to 300 millibars *The stronger the wind, generally the more favorable *Wind speed very high in upper levels, greater than 100 knots, the higher the better *Hodograph curved sharply with height MULTICELLS *Wind veers with height, but not as pronounced as supercell *Wind direction remains fairly unidirectional from lower mid-levels into upper levels (850 to 300 mb) *Speed shear is present (increase of wind speed with height) AIR MASS STORMS *Wind speed change with height is relatively small *Wind direction is fairly constant with height or unorganized *Upper level winds are much weaker than supercell or multicell case The link below has current online hodographs CURRENT HODOGRAPHS WITH SKEW-T |