Microbursts are common in southern Arizona during the Monsoon. They often take down power poles, leaving us in the dark and without air conditioning during the hottest part of the summer!
First, it's important to understand the structure of our atmosphere. Think of it as a sandwich: it has three layers. They are called the "lower", "middle", and "upper" layers.
Early in the Monsoon, moisture makes its way northward from the tropics into Arizona, but resides ONLY in the middle and upper layers. That leaves the lower layer, where we live, dry as a bone.
With daytime heating causing rising air, thunderstorms form, feeding off of the moisture in the middle and upper layers.
As it begins to rain, the rain falls into the very dry air at the surface (in the lower layer.)
Figure 1: Thunderstorms form aloft, but the air at the ground is still very dry.
When rain drops evaporate in the very dry air, the air is cooled. Cooler air is heavier and more dense. Thus, the air begins to sink toward the ground.
This process sets off a chain reaction of sinking air that results in the thunderstorm collapsing in a matter of minutes, with all of its energy (in the form of wind) rushing straight down to the ground.
When the sinking air hits the ground, it spreads out in all directions. The wind speeds in this "microburst" of air can reach 60-100 mph.
Figure 2: As the cold air sinks from the thunderstorm, it hits the ground and spreads out in all directions.
A microburst in Tucson can do damage here, but can also begin the formation of a mammoth Haboob headed for Phoenix. Read about haboobs here.
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