When a thermal rises abruptly air from around the area rushes in to replace the thermal air. If this air is also heated it will be entrained by the thermal and rise into it. A vast source of heated air as in case III mentioned earlier will feed the thermal for several minutes creating a thermal column that stretches for thousands of feet (1000m). If the supply of warm air is limited, then cool air will replace the thermal which will then be of limited size. The cool air will take time to heat then will release as another thermal. The time for a thermal to form in this repetitive process may be from several minutes to an hour or more depending on the strength of the heating.
The figure below shows how the air rushing in below the thermal can come from all directions in a light general wind. This in-rushing air can be quite vigorous in strong thermal conditions and can make landing in mid-day thermals a tricky affair. Switching winds called "light and variable" on weather reports are a sign of thermals. In a stronger general wind the direction won't change as much but the gustiness will increase.
When a thermal rises in the first 1,000 feet (300m) or so it may have an inflow of air from all sides. This general "convergence" tends to pull a soaring aircraft towards the center so that less bank angle is needed to produce a given diameter of circle. Up higher the bank angle may have to be increased to maintain the circling diameter. In general, thermals tend to be more turbulent close to the ground until they become more uniform up higher. However, thermals often rise into inversion layers that break up the thermal or contain shear turbulence. In windy conditions thermals may be so broken up that there exists a layer of mixed and turbulent heated air near the surface as shown in the figure below. This air may send off turbulent thermals at trigger points which will continue up as rowdy lift.
