energy in a rising column of air is translated as lift. This  chapter refers to lift as the rising air within an updraft and  sink as the descending air in downdrafts. There are two favored theories of thermal characteristics. One  is that thermals are continuous updrafts like a plume of smoke  from a campfire rising up and sometimes twisting depending  on the wind currents. This theory requires the glider pilot to  locate the rising thermal current and estimate the amount of  slant caused by the winds shifting the rising updraft downwind  from the heat source. Just like a campfire, the winds may tend  to twist and oscillate around the hotter origin.  When locating and utilizing thermals for soaring flight, called  thermaling, glider pilots must constantly be aware of any  nearby lift indicators. Successful thermaling requires several  steps: locating the thermal, entering the thermal, centering the  thermal, and, finally, leaving the thermal. Keep in mind that  every thermal is unique in terms of size, shape, and strength.  The other theory is that thermals can be more like a hot water  bubble rising in a pan on a stove. If you can stay in the bubble,  you can climb. If you spill out of the bubble, there is no lift  even just under where you were climbing because the lift is  not a continuous vertical stream. The glider pilot must search  for the next rising bubble to obtain lift.  According to the last chapter, if the air is moist enough  and thermals rise high enough, cumulus clouds, or Cu  (pronounced like the word “cue”) form. Glider pilots seek Cu  in the developing stage, while the cloud is still being built by  a thermal underneath it. The base of the Cu should be sharp  and well defined. Clouds that have a fuzzy appearance are  likely to be well past their prime and probably have little  lift left or even sink as the cloud dissipates.  [Figure 10-1] In the practical world, the nature of thermals is probably a  blend of these theories. The sun produces heat, and often that  heating is unequal on the surface of the earth, so winds occur  as natural forces that work to equalize the atmosphere. Some  of these winds are vertical currents. We call them updrafts  and downdrafts. Updrafts provide lift and downdrafts provide  sink. Since the atmosphere always seeks equality, if there  is an updraft, there must be a downdraft or drafts close to  replace the upward flowing air. The goal of soaring pilots is  to maximize time in lifting currents and minimize their time  in sinking currents.  Judging which clouds have the best chance for a good thermal  takes practice. On any given day, the lifetime of an individual  Cu can differ from previous days, so it becomes important to  observe Cu lifecycle on a particular day. A good looking Cu  may already be dissipating by the time it is reached. Soaring  pilots refer to such Cu as rapid or quick cycling, which means  the Cu forms, matures, and dissipates in a short time. The  lifetime of Cu often varies during a given day as well; quick  cycling Cu early in the day often become well formed and  longer lived as the day develops. The contemporary models and experience seem to support  the theory of a central updraft surrounded by compensating  downdrafts. The skill involves staying in the lift as long as  possible and exiting or passing through the sink as quickly  as possible.  As a note, glider pilots refer to rising air as lift. This is not  the lift generated by the wings as discussed in Chapter 3,  Aerodynamics of Flight. The use of this term may confuse  new pilots, but when used in the context of updrafts, the  Sometimes Cu cover enough of the sky that seeing the cloud  tops becomes difficult. Hence, glider pilots should learn  to read the bases of Cu. Generally, a dark area under the  Figure 10-1.  Photographs of (A) mature cumulus probably producing good lift, and (B) dissipating cumulus. 10-2 A B Thermal Soaring Locating Thermals Cumulus Clouds 