What Causes Microbust Storms?

Note: This entry was initially written for Level 2 cadets at the 540 Golden Hawks Squadron. See other posts in this series.


An interesting weather phenomenon known as a “wet microburst”. which is when a lot of rain falls in a very small area, which causes a lot of winds as a result. These can look very cool, but are very dangerous to fly through!

Wet microbursts are a type of “downburst”, which happen during thunderstorms. A downburst is anytime cool air is forced out of a storm in a smallish area; if that air also has rain in it, and if the downburst is smaller than 4 kilometers in diameter, it becomes a “wet microburst”.

One way to think about thunderstorms is that they first need to “breathe in” by taking in warm air from the ground; and then as the warm air is cooled at high altitudes, the storm “breathes out” the cool air.

If the “breathed in” air is humid enough, and the high altitude air is cold enough, precipitation forms—which means the “breathed out” air can be either rain or hail. (This is why hail mostly happens in the spring.)

Thunderstorms — ANAC National Civil Aviation Agency - Brazil
Clouds first “breathe in” warm air, get bigger, then “breathe out” cold air and precipitation. Picture from the National Civil Aviation Agency of Brazil: https://www.anac.gov.br/en/safety/aeronautical-meteorology/conditions/thunderstorms

Most of the time, storms take in a “deep breath”, forcing the air to very high altitudes which also causes it to spread out, and then “breathe out” over large areas.

But sometimes, especially after a hail storm where the melting ice forms an area of much cooler air, the breathing out process starts much earlier—at both a lower altitude and before the air has had a chance to spread out over the sky.

This is what causes a downburst, and in effect it’s like dumping a huge bucket of water (or heavy air) from the heavens unto the earth. With gusts of wind that can travel up to 240 km per hour, you want to be very far away from one!