Why is it so rainy?
Rain needs moist air and something to lift it. As air rises it cools, cool air can't hold as much water, and the extra condenses into clouds and falls out. A rainy spell means both keep happening day after day.
Show me why1 min
Four ways air gets lifted
- Fronts: warm air slides up over a wedge of cold air, giving long, steady rain.
- Mountains: wind runs into slopes and is forced up. Windward sides get drenched.
- Heating: the sun warms the ground, bubbles of air rise, and afternoon showers and thunderstorms pop up.
- Low pressure: air flows into a low from all sides and has nowhere to go but up.
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How a raindrop is made
Cloud droplets are tiny and float. To fall, millions of them have to join into one raindrop. In warm clouds they collide and merge. In most clouds outside the tropics, ice crystals grow by stealing water from the droplets around them, get heavy, and fall, melting into rain on the way down. Much of the rain at mid-latitudes starts as snow.
Why rainy spells stick around
A stalled front, a slow low-pressure system or a steady feed of tropical moisture (an atmospheric river) can keep lifting moist air over the same place for days. Air Story's radar shows where it's raining now, and the trail shows where your moist air came from.
Questions people ask
- Why does it rain more on one side of a mountain?
- Air rising up the windward side cools and drops its water. By the time it sinks down the other side it's dry, creating a rain shadow.
- Why does rain often come in the afternoon in summer?
- The sun heats the ground through the day until warm, moist air rises fast enough to build showers and storms.