Wind is a powerful forecasting tool, especially for temperature forecasts. Wind contributes to thermal advection and moisture advection. The stronger the mid-latitude wind, the greater the probability of having temperature advection (either warm air or cold air advection). When beginning a forecast analysis, note the PBL wind speed and direction. Next, look upstream to see what type of thermal and moisture advection will take place over your location during the course of the day. If temperatures are warmer upstream, expect warm air advection. If temperatures are colder, expect cold air advection. Weight the warm air and cold air advection against the warming caused by the sun during the day and the cooling at night. The same is done for moisture advection (relative humidity and dewpoint). The wind speed determines how much vertical mixing will occur in the PBL and the depth of the PBL. The PBL tends to be shallower when wind speeds are light and deeper when wind speeds increase. Temperatures will be cooler at night and warmer during the day in situations when the wind is light, all else being equal. Why? At night the surface gives off longwave radiation and cools. Therefore the coldest temperatures are right at the earth's surface. If wind speed is light, there will be minimal mixing between the colder surface air and warmer air above the surface. Cooler air will "build" at the surface. The opposite effect occurs during the day. The sun heats the earth's surface. Therefore, warmest daytime temperatures are right at the earth's surface. If the wind speed is light, heat will "build up" at the earth's surface and will not be mixed with cooler air aloft. If wind speed is stronger, the mixing of warm surface air and cooler air aloft will cause temperatures near the surface to be cooler than the otherwise would have been if the wind was light. In summary, strong wind tends to reduce the difference between the high and the low temperature while light winds increase the difference between the high and the low temperature, all else being equal. Along with wind speed it is important to analyze wind direction to look for temperature changes caused by advection. Temperature changes by the reduction or increase of thermal energy, reduction or increase of latent heat, and horizontal thermal advection. |