A powerful atmospheric river fueled the deadly early April flooding disaster in the central and southern United States.

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Atmospheric rivers

are an important part of the Earth’s climate. It is a stream of water vapor moving in the sky. Atmospheric rivers are large, narrow sections of the Earth’s atmosphere. It carry moisture from the Earth’s tropics near the equator to the poles. They carry massive amounts of moisture. When they reach land, atmospheric rivers release this moisture, producing heavy snow and rain. They are responsible for 90 percent of the movement of moisture from the tropics toward the poles. Atmospheric rivers flow in the direction of moving air created by weather systems. Other names for this phenomenon are tropical plume, tropical connection, moisture plume, water vapor surge, and cloud band.

They pick up water vapor from the warm, moist air of tropical regions. And they drop the water over land in cooler regions as rain or snow.

Atmospheric rivers are commonly seen in the extratropics, a region between the latitudes of 30 and 50 degrees in both hemispheres.

It’s a well-known example of the “Pineapple Express”. It transports moisture from the tropics near Hawaii to the US West Coast.

Atmospheric rivers usually begin over tropical regions. Warm temperatures there cause ocean water to evaporate and rise into the atmosphere. Strong winds help to carry the water vapor through the atmosphere. As atmospheric rivers move over land, the water vapor rises up farther into the atmosphere. It then cools into water droplets, which fall as precipitation. Atmospheric rivers are the largest “rivers” of fresh water on Earth. It transports on average more than double the flow of the Amazon River.

• Category 1 (Weak): A Category 1 atmospheric river would be a milder and briefer weather event
• Category 2 (Moderate): A Category 2 atmospheric river is a moderate storm
• Category 3 (Strong): A Category 3 atmospheric river is more powerful and longer lasting,
• Category 4 (Extreme): A Category 4 atmospheric river is mostly hazardous,
• Category 5 (Exceptional): A Category 5 atmospheric river is primarily hazardous