Have you ever gone out for a long walk, run, or bike ride on a gorgeous summer day and been caught in a torrential cloudburst? Bad enough to be caught in the rain, but cloudbursts can leave you drenched to the bone. And in some cases, concerned for your life.
Cloudbursts or downpours have no strict meteorological definition. The term usually signifies a sudden, heavy fall of rain over a short period of time. Some observers suggest a rainfall rate in excess of 25 millimetres per hour (1 inch per hour) constitutes a downpour, but when you're drenched, the amount does not matter all that much.
We do know that most cloudbursts come from convective, cumulonimbus clouds that form thunderstorms and that the air is generally rather warm in order to contain the amount of moisture needed for a heavy downpour. Besides providing the proper conditions to spawn large quantities of liquid water drops, cumulonimbus clouds have regions of strong updrafts which hold raindrops aloft en masse and can produce the largest raindrops (those greater than 3.5 mm, (0.14 inches)in diameter). These updrafts are filled with turbulent wind pockets that toss small raindrops around with surprising force. Within the turmoil of the randomly moving drops, there are more collisions among the drops than a bumper car ride, and many of those close encounters result in their conglomeration into new drops larger in size.
Eventually all updrafts collapse, and when they do, the upheld raindrops descend unimpeded toward the surface, often forming a strong downdraft - such as a downburst or microburst - in the process, an event that appears as if the cloud has burst open like a soggy paper bag. So, not only are the larger drops falling with a terminal velocity of around 12 km/h (20 mph), but they have the added giddy-up of the downdraft speed, which can easily exceed 80 km/h (50 mph).
The resulting rainfall is a torrent of water, large raindrops falling at high speed, over a small area. The force and quantity of such downpours can be damaging to vegetation, small animals, and property. When the speed of water accumulation on the ground exceeds the surface's ability to absorb it, localized flooding will occur in low-lying terrain. In hilly or mountainous terrain, the runoff of water can congregate in stream beds or canyons and cause deadly and damaging flash flooding.
Here are some world record cloudbursts:
1 minute: 1.5 inches / 38.1 mm at Barot, Guadeloupe, 26 November 1970.
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