If conditions are right, the thermals may become aligned in rows extending for tens of kilometers, creating long bands of lift with parallel bands of sink on either side. These highly organized convection systems are called cloud streets and, if aligned in the appropriate direction, make it possible to soar in a straight line underneath the clouds for tens of kilometers. However, if the pilot attempts to cross from one cloud street to another, the strong sink is likely to force an out-landing. Of course the butterflies can avoid this by switching from soaring to flapping flight. The other pattern of convective development can be much more exciting.
Cumulus clouds can continue to build into towering cumulonimbus , or thunderstorms.
Unlike the rather explosive beginning of convection earlier in the day, the decline in thermal activity at the end of the day is a more gradual process. Towards the end of the day, as the setting sun heats the air near the ground less and less, the distance between thermals continues to increase and the velocity of the upward moving air slows and eventually quits. . Eventually, except perhaps for a few thermal puffs, all convective activity stops and the clouds dissipate. This diurnal cycle of thermal development is the reason that the sky is usually clear of clouds in the morning hours and the hour or so before sunset. If is also the reason that migrating monarch butterflies, having apparently spent hours aloft soaring, begin to descend from the sky in the late afternoon or, on particularly good days, just prior to sunset.