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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. 

## Thermal Soaring Weather 

A thermal is a rising mass of buoyant air. Thermals are the most common updraft used to sustain soaring flight. In the next sections, several topics related to thermal soaring weather are explored, including thermal structure, atmospheric stability, the use of atmospheric soundings, and air masses conducive to thermal soaring. 

Convection refers to an energy transfer involving mass motions. Thermals are convective currents and are one means by which the atmosphere transfers heat energy vertically. Advection is the term meteorologists use to describe horizontal transfer; for instance, cold air advection after the passage of a cold front. As a note of caution, meteorologists use the word “convection” to mean deep convection, that is, thunderstorms. Unfortunately, there is often a fine meteorological line between a warm, sunny day with plenty of thermals, and a warm, sunny day that is stable and produces no thermals. To the earthbound general public, it matters little—either is a nice day. Glider pilots, however, need a better understanding of these conditions and must often rely on their own forecasting skills. 

## Thermal Shape and Structure 

Two primary conceptual models exist for the structure of thermals: bubble model and column or plume model. Which model best represents thermals encountered by glider pilots is a topic of ongoing debate among atmospheric scientists. In reality, thermals fitting both conceptual models likely exist. A blend of the models, such as individual strong bubbles rising 

within one plume, may be what occurs in many situations. These models attempt to simplify a complex and often turbulent phenomenon, so many exceptions and variations are to be expected while actually flying in thermals. Many books, articles, and Internet resources are available for further reading on this subject. 

The bubble model describes an individual thermal resembling a vortex ring, with rising air in the middle and descending air on the sides. The air in the middle of the vortex ring rises faster than the entire thermal bubble. The model fits occasional reports from glider pilots. At times, one glider may find no lift, when only 200 feet below another glider climbs away. At other times, one glider may be at the top of the bubble climbing only slowly, while a lower glider climbs rapidly in the stronger part of the bubble below. [Figure 9-7] More often, a glider flying below another glider circling in a thermal is able to contact the same thermal and climb, even if the gliders are displaced vertically by 1,000 feet or more. This suggests the column or plume model of thermals is more common. [Figure 9-8] 

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Which of the two models best describes thermals depends on the source or reservoir of warm air near the surface. If the heated area is rather small, one single bubble may rise and take with it all the warmed surface air. On the other hand, if a large area is heated and one spot acts as the initial trigger, surrounding warm air flows into the relative void left by the initial thermal. The in-rushing warm air follows the same path, creating a thermal column or plume. Since all the warmed air near the surface is not likely to have the exact same temperature, it is easy to envision a column with a few or several imbedded bubbles. Individual bubbles within a thermal plume may merge, while at other times, two adjacent and distinct bubbles seem to exist side by side. 

