How Hard of a Rain is Torrential Rain?

It's an Extremely Hard or Pouring Rain

heavy rainstorm umbrella
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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 (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).

Chances are you've used or heard one of these idioms, or sayings, about heavy rain before:

  • It's raining cats and dogs.
  • The rain is coming down in sheets.
  • It's coming down in buckets.
  • It's raining pitchforks.

What Causes Heavy Rain?

Rain occurs when the water vapor "held" in warm, moist air condenses into liquid water and falls. For heavy rain, then, 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 and hurricanes, 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.

Torrential Rain Dangers

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 the 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 actually 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.

More: 5 of the Weirdest Things to Ever "Rain" from the Sky

Torrential Rain on Radar

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

More: How to Easily Identify Severe Storms on Radar

Edited by Tiffany Means