Updrafts are upward-moving air currents that can be the result of several causes. The heating of the ground can sometimes increase surface air temperature, which becomes much warmer than the air above, and, because warmer air is less dense, it rises easily and gets replaced by descending cooler air.
This causes a vertical ascending current - known as a thermal - which is capable of reaching altitudes of 3 km (2 miles) or more. The greater the thermal’s radius, the higher it will rise.
Updrafts and downdrafts can also be a result of thunderstorms, with updrafts usually present early on in the storm’s development, during which warm air rises to the level at which condensation begins and precipitation is formed.
Updrafts can be dangerous for planes, as when the updraft comes into contact with the aircraft it can push it upwards, and in some cases will cause it to reach dangerous altitudes.
Because updrafts have the potential to put a plane’s safety in jeopardy, usually flights are canceled if very bad thunderstorms occur.