If a unit of air starts to descend in a region of the atmosphere characterized by a lepse rate between dry and saturated adiabatic, it becomes warmer than its surroundings and tends to return to its original position. Yet, in air where such a vertical temperature distribution exists, violent downdrafts are observed after the onset of a thunderstorm. Continuous cooling of the descending units of air by water and ice hydrometeors keeps them colder than the surrounding air. Thus a downdraft is maintained.
The cooling can be divided into several factors. The most important is the heat of vaporization of the heavy precipitation falling with or through the descending units. The precipitation keeps the downdraft practically saturated; so that a plot of the path of a descending unit of air on an adiabatic chart will follow a saturation adiabat.
The factor of next importance is the specific heat of the tremendous quantities of water and ice accompanying or passing through each unit of descending air. Although the ratio of heat of evaporation to specific heat of water is about five hundred to one, the specific heat factor is a more important cooling agent than this indicates. For, in estimating the relative cooling effects of these two factors one must remember that in descending from one level to another, an air parcel can only evaporate a limited amount of water. On the other hand the quantity of liquid and solid water involved in the specific heat transfer has no limitations.
The third factor, the one which does not have, the continuous cooling properties of the other two but is of tremendous importance in the regions where the downdraft acquires above freezing temperatures, is the melting of ice hydrometeors. Large quantities of melting ice tend to give the unit isothermal descent characteristics, rapidly increasing the area on the adiabatic chart between a plotting of this descent and the plotting of the original atmospheric sounding. In other words, this melting causes a large acceleration in descent.
Falling raindrops and ice hydrometeors, by compressing air ahead of them and rarefying air behind them, contribute to the downward motion of air. This factor, although not great, may be instrumental in initiating downdrafts.
The sum of these effects, each one having an importance depending on particular conditions, makes up the downdraft characteristics.
Downdrafts having a greater horizontal component than the accompanying precipitation, may leave the precipitation zone and continue to descend dry adiabatically until they acquire the same temperature (or better, the same density characteristics) of the surrounding air. Those leaving the precipitation at a low level may reach the earth as a cool wind unaccompanied by rain.
The understanding of the necessarily close relationship between thunderstorm downdrafts and precipitation should be of valuable aid to pilots flying in thunderstorm areas.