In the lower troposphere, above the surface, horizontal advection is usually the dominant factor affecting local temperature changes. In most precipitation situations, particularly in borderline situations, warm air advection and upward motion are occurring simultaneously, giving rise to the fact that warming generally accompanies precipitation. However, this effect is frequently offset when there is weak warm advection, or even cold advection, in the cold air mass in the lower layers.

In situations where precipitation is occurring in association with a cold upper low, upward motion is accompanied by little, if any, warm advection. In such borderline cases, precipitation may persist as snow, or tend to turn to snow, due to cooling, as a result of upward motion or advection.

The most important of the nonadiabatic effects taking place during the precipitation process is the cooling, which takes place due to evaporation as the precipitation falls through unsaturated air between the clouds and the surface. This effect is especially pronounced when very dry air is present in the lower levels, with wet-bulb temperatures at or below freezing. Then, even if the dry-bulb temperature is above freezing in a layer deeper than 1,200 feet in the lower levels, the precipitation may still fall as snow, since the evaporation of the snow will lower the temperatures in the layer between the cloud and the surface until the below-freezing wet-bulb temperatures are approached. The actual cooling that occurs during the period when evaporation is taking place may often be on the order of 5 to 10F within an hour. After the low-level stratum becomes saturated, evaporation practically ceases, and advection brings a rise in temperature in the low levels. However, reheating often comes too late to bring a quick change to rain since the temperatures may have dropped several degrees below freezing, and much snow may have already fallen. The lower levels may be kept cool through the transfer of any horizontally transported heat to the colder, snow-covered surface.

Melting snow descending through layers that are above freezing is another process which cools a layer. To obtain substantial temperature changes due to melting, it is necessary to have heavy amounts of precipitation falling, and very little warm air advection. As cooling proceeds, the temperature of the entire stratum will reach freezing, so that a heavy rainstorm could transform into a heavy snowstorm, Incidents of substantial lowering of the freezing level due to melting are relatively rare. The combination of heavy rain, and little, if any, warm advection is an infrequent occurrence.

The combined effects of horizontal temperature advection, vertical motion, and cooling due to evaporation are well summarized by observations of the behavior of the bright band on radar (approximately 3,000 ft). Observers have found that within the first 1 1/2 hours after the onset of precipitation, the bright band lowers by about 500 to 1,000 feet. This is attributable primarily to evaporational cooling, and probably secondary to melting. Since evaporational cooling ceases as saturation is reached, warm air advection, partially offset by upward motion, again becomes dominant, and the bright band ascends to near its original level. The bright band will ascend to its original level approximately 3 hours after the onset of precipitation, and may ascend a few thousand additional feet.

Other nonadiabatic effects, such as radiation and heat exchange with the surface, probably play a relatively smaller role in the snow-rain problem. However, it is likely that the state of the underlying surface (snow-covered land versus open water) may determine whether the lower layers would be above or below freezing. Occasionally, along a seacoast in winter, heat from the open water keeps temperatures offshore above freezing in the lower levels. Along the east coast of the United States, for example, coastal areas may have rain, while a few miles inland snow predominates. This situation is associated with low-level onshore flow, which is typical of the flow associated with many east coast cyclones. Actually, this situation cannot be classified as a purely nonadiabatic effect since the warmer ocean air is being advected on shore.

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