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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.
During a cloudburst, we are usually thinking more about keeping dry or navigating our car safely through traffic. But have you ever watched the approach of a thunderstorm darken the sky and then been surprised at how bright the day became during the heaviest downpour?
We logically expect that, given the pre-rain darkness of a thunderstorm, that when heavy rainfall begins, the darkness will deepen. But very often, a sudden downpour comes with an unexpected brightening of the surroundings. Here's why.
Each raindrop can act as a reflector of any light falling on it (reflection and refraction of light through distant raindrops form bright, beautiful rainbows). During a heavy downpour, a large number of raindrops encircle us, and we are, in effect, caught in the rain of a large number of little mirrors. As any interior decorator knows, you can brighten a room with a few lights by putting mirrors on the surrounding walls or ceilings. Thus, each light ray passing through the downpour undergoes multiple reflections so that the light reaching our eyes comes from all directions rather than just directly from the source.
The most important factors in producing the rainfall-brightening around us are the raindrop quantity and the size distribution. In a cloudburst, the number of large drops is much greater than in a light rainfall. Since the surface area of a drop increases as the square of its radius, a drop twice as large has four times the potential reflecting surface area. Thus, as these liquid mirrors get bigger, we become more effectively surrounded by reflecting surfaces, and this makes the scene appear brighter.
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