How Forceful Is Torrential Rain?

Two people under an umbrella during a rainstorm.
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Torrential rain, or a torrential downpour, is any amount of rain that is considered especially heavy. It isn't a technical weather term as there is no formal definition of torrential rains as recognized by the National Weather Service (NWS), but NWS does define heavy rainfall as rain that accumulates at a rate of 3 tenths of an inch (0.3 inches), or more, per hour.

While the word may sound like another severe weather type--tornadoes--this isn't where the name comes from.

A "torrent," rather, is a sudden, violent outpouring of something (in this case, rain).

What Causes Heavy Rain?

Rain occurs when the water vapor "held" in warm, moist air condenses into liquid water and falls. For heavy rain, the amount of moisture in the air mass must be disproportionately large compared to its size. There are several weather events where this is typical, such as in cold fronts, tropical storms, hurricanes, and monsoons. Rainy weather patterns like El Niño and the Pacific coast's "Pineapple Express" are also moisture trains. Global warming, too, is thought to contribute to heavier precipitation events, since in a warmer world, the air will be able to hold more moisture to feed soaking rains.

The Dangers of Torrential Rain

Heavy rain can trigger any one or more of the following deadly events:

  • Runoff: If heavy rains arrive more quickly than the ground can absorb water, you get runoff--stormwater that "runs off" the land instead of seeping into the ground. Runoff can carry pollutants (like pesticides, oil, and yard waste) into nearby creeks, rivers, and lakes.
  • Flooding & flash flooding: If enough rain falls into rivers and other bodies of water it can cause their water levels to rise and overflow onto normally dry land.
  • Mudslides: If rain is record-breaking (typically more rain in a few days than is normal over a month or year) the ground and soil can liquify and carry unsecured objects, people, and even buildings away in debris flows. This is exacerbated along hillsides and slopes since the ground there is more easily eroded away. Here in the U.S., mudslides are common in Southern California. They're also common in Europe and Asia, especially India, Bangladesh, and Pakistan where they often lead to death tolls in the thousands.

    Torrential Rain on Weather Radar

    Radar images are color-coded to indicate precipitation intensity. When looking at weather radar, you can easily spot the heaviest rain by the red, purple, and white colors that symbolize the heaviest precipitation.

    Edited by Tiffany Means

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    Oblack, Rachelle. "How Forceful Is Torrential Rain?" ThoughtCo, Apr. 19, 2018, thoughtco.com/torrential-rain-basics-3444237. Oblack, Rachelle. (2018, April 19). How Forceful Is Torrential Rain? Retrieved from https://www.thoughtco.com/torrential-rain-basics-3444237 Oblack, Rachelle. "How Forceful Is Torrential Rain?" ThoughtCo. https://www.thoughtco.com/torrential-rain-basics-3444237 (accessed April 23, 2018).