Microbursts are made of wind rushing down to the ground. Once the wind hits the ground, it spreads in all directions. Wind speeds in a microburst can be 60-100mph, damaging roofs, snapping trees and knocking over power poles.
In the early part of the monsoon, dry microbursts (without rainfall) are common. Read about them here.
Wet microbursts have their own, unique characteristics. Like the dry microburst, wind rushes out of the storm, hits the ground and spreads in all directions. But unlike the dry microburst, a rush of heavy rainfall accompanies the damaging wind.
So, how does a wet microburst form?
First, here's what one looks like.
Figure 1: A wet microburst.
And, here's the damage that can be left behind.
Figure 2: Wet microburst damage to trees on the grounds of Oro Valley Country Club.
It all starts with a typical thunderstorm. So, let's take a look at them.
A thunderstorm is like an engine. It pulls air and moisture in, converts it to rainfall (or hail) and pushes rain and wind out. Here's the key: in order to not suffocate itself, the thunderstorm must be titled. The top of the storm can't be directly over the bottom of the storm.
Figure 4: Upper-level wind tilts a thunderstorm, so that the top is not directly over the bottom. This allows the thunderstorm to act like an engine, pulling moist air in, converting it to rain, and pushing rain and wind out.
During the monsoon, especially in late July and August, the steering flow in the upper-level of the atmosphere (see the white arrow in the image) is very weak. That upper-level wind is what tilts the storm, by pushing its top over a bit.
When this flow is too weak, the storm will form and then quickly lose its tilt.
Now, it's raining on top of the inflow of the storm. That suffocates the storm, stopping the engine's process, resulting in a quick collapse of wind and rain down to the surface. Voilà: a wet microburst.
Figure 5: Upper level wind is weak, resulting in the thunderstorm's inflow and outflow being on top on one another. In other words, the thunderstorm has lost its tilt. This results in a quick collapse of the storm.
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