The most important application of vertical motion is the prediction of rainfall probability and rainfall amount. In addition, vertical motion affects practically all meteorological properties, such as temperature, humidity, wind distribution, and particularly stability. In the following section the distribution of large-scale and small-scale vertical motions are considered.

Since cold air has a tendency to sink, subsidence is likely to be found to the west of upper tropospheric troughs, and rising air to the east of the troughs. Thus, there is a good relation between upper air meridional flow and vertical flow.

In the neighborhood of a straight Northern Hemisphere jet stream, convergence is found to the north of the stream behind centers of maximum speed as well as to the south and ahead of such centers. Divergence exists in the other two quadrants. Below the regions of divergence the air rises; below those of convergence there is subsidence.

These general rules of thumb are not perfect, and only yield a very crude idea about distribution of vertical motion in the horizontal. Particularly over land in summer, there exists little relation between large-scrale weather patterns and vertical motion. Rather, vertical motion is influenced by local features and shows strong diurnal variations. Large-scale vertical motion is of small magnitude at the ground (zero if the ground is flat).

Above ground level, it increases in magnitude to at least 500 hPa and decreases in the neighborhood of the tropopause. There have been several studies of the relation between frontal precipitation and large-scale vertical velocities, computed by various techniques. In all cases, the probability of precipitation is considerably higher in the 6 hours following an updraft than following subsidence. Clear skies are most likely with downdrafts. On the other hand, it is not obvious that large-scale vertical motion is related to showers and thunderstorms caused during the daytime by heating.

However, squall lines, which are formed along lines of horizontal convergence, show that large-scale vertical motion may also play an important part in convective precipitation.

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