Microbursts occur during thunderstorms. A storm can cause rain clouds full of water droplets or hail to mix with patches of dry air. When that happens, the dry air sucks moisture from the wet air. That makes the wet air cool down, and as it cools, it starts to sink. If the whole process occurs fast enough and over a large enough area, you get a large column of cool air sinking rapidly and spreading out over the ground with great force–a microburst.