Weather Words: Rainfall Rate

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Not all rain is created equal. Sometimes it’s a gentle sprinkle, sometimes it’s a torrential downpour that turns streets into rivers in minutes. Understanding rainfall rate is key to knowing when all you need is an umbrella and when to head for higher ground.

Rainfall rate is all about how much rain falls in a specific amount of time. For example, a shower that has a rainfall rate of 0.1 inches per hour, will describe a dreary day with mist and light rain. However, if the rainfall rate is three inches per hour, then you know that’s serious business. This is when the rain is falling so quickly that it can overwhelm storm drains, flood streets and create dangerous conditions for drivers and property.

In the desert, mountainous areas and urban locations, two inches of rain can cause enormous problems. In the desert, the ground can’t absorb the water as quickly, so much of the rain runs off and flash flooding can occur even with relatively small amounts of rainfall. In mountainous regions, the water rushes down the mountainsides and into the valleys below which can become flooded very quickly. And urban areas can flood with high rainfall rates because the water can’t be absorbed by all the cement, and many times storm drains can’t keep up.

That’s why it’s important to pay attention to flash flood watches and warnings when rain is expected for your area, because an ordinary afternoon shower can become life threatening in a matter of minutes if the right conditions are in place.

J​ennifer Gray is a weather and climate writer for weather.com. She has been covering some of the world's biggest weather and climate stories for the last two decades.