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. 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. 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] 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. 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. 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  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.  9-6 Thermal Shape and Structure Thermal Soaring Weather Figure 9-7.  The bubble or vortex ring model of a thermal.