The visual appearance of the updraft can tell a lot about its strength and potential to produce a strong thunderstorm. As the updraft rises going through various phases of Cumulus and potentially Cumulonimbus , it cools and eventually looses buoyancy. In addition to the speed at which the updraft rises, the appearance of its edge is indicative of its strength.
Sharp, crisp updraft on a severe thunderstorm over north Italy. This is a strong updraft. Photo: Marko Korošec.
A sharp, crisp edge, that may resemble an explosion, indicates a strong updraft. It may, sometimes, form a pileus cloud cap on top of it. On the other hand, a soft, fuzzy edge is indicative of a weak updraft, that has lost its strength and upward speed and is likely not rising anymore.
Upward speed of the updraft
Fair weather Cumulus humilis and Cumulus mediocris clouds typically have gentle updrafts, with vertical speeds of about 2-3 m/s. Weak thunderstorms may have vertical updraft speeds of 6-12 m/s. At such a speed, a thunderstorm will vertically develop by 10 km in about 15 minutes.
On the other side of the spectrum are severe thunderstorms. A severe thunderstorm may have a vertical updraft going up at 30-35 m/s. In the most extreme cases, severe thunderstorm updrafts grow at up to 75-80 m/s! That means the updraft rises by a full kilometer in less than 15 seconds and can reach 10 km in 2-3 minutes – this is called explosive thunderstorm development.
Explosive updrafts on a severe thunderstorm over central Po plain, north Italy. July 17, 2009. This storm had an updraft strong enough to hold aloft tennis ball sized hail. Photo: Marko Korošec.
Explosive updraft on a severe thunderstorm over central Slovenia. Note the crisp edges of the updraft, indicating its strength. June 28, 2008. Photo: Marko Korošec.