When preparing forecasts, meteorologists consider atmospheric circulation on many scales. To aid the forecasting of short- and long-term weather, various weather events have been organized into three broad categories called the scales of circulations. The size and lifespan of the phenomena in each scale are roughly proportional, so that larger size scales coincide with longer lifetimes. The term “microscale” refers to features with spatial dimensions of .10 to 1 NM and lasting for seconds to minutes. An example is an individual thermal. The term “mesoscale” refers to the horizontal dimensions of 1 to 1,000 NM and lasting for many minutes to weeks. Examples include mountain waves, sea breeze fronts, thunderstorms, and fronts. Research scientists break down the mesoscale into further subdivisions to better classify various phenomena. The term “macroscale” refers to the horizontal dimensions greater than 1,000 NM and lasting for weeks to months. These include the long waves in the general global circulation and the jet streams embedded within those waves. [Figure 9-6]
Smaller scale features are embedded in larger scale features. For instance, a microscale thermal may be just one of many in a mesoscale convergence line, like a sea breeze front. The sea breeze front may occur only under certain synoptic (i.e., simultaneous) conditions, which is controlled by the macroscale circulations. The scales interact, with feedback from smaller to larger scales and vice versa, in ways that are not yet fully understood by atmospheric scientists. Generally, the behavior and evolution of macroscale features are more predictable, with forecast skill decreasing as scale diminishes. For instance, forecasts of up to a few days for major events, such as a trough with an associated cold front, have become increasingly accurate. However, nobody would attempt to forecast the exact time and location of an individual thermal an hour ahead of time. Since most of the features of interest to soaring pilots lie in the smaller mesoscale and microscale range, prediction of soaring weather is a challenge.
Soaring forecasts should begin with the macroscale, which identifies large-scale patterns that produce good soaring conditions. This varies from site to site and depends, for instance, on whether the goal is thermal, ridge, or wave soaring. Then, mesoscale features should be considered. This may include items such as the cloudiness and temperature structure of the air mass behind a cold front, as well as the amount of rain produced by the front. Understanding lift types, and environments in which they form, is the first step to understanding how to forecast soaring weather.