A storm system swirls moisture into the West Coast. (WeatherBell)
When you think of the atmosphere, you probably think of gases like nitrogen and oxygen—the air we breathe. But the atmosphere can hold an entire river of water too.
Atmospheric rivers are long, flowing regions of the atmosphere that carry water vapor through the sky. They are about 250 to 375 miles wide and can be more than 1,000 miles long. Rivers on land generally flow downhill; atmospheric rivers flow in the direction of moving air created by weather systems.
In general, 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 ranked on a 1-through-5 scale based on how much moisture they transport from the tropics to the mid-latitudes. The Center for Western Weather and Water Extremes has ranked the ongoing atmospheric river event as a Level 4 out of 5 — that corresponds to “extreme” moisture transport.
The atmospheric river could carry more than 220 million pounds of water through the skies over the Pacific Coast every second — providing copious fuel for downpours.
Atmospheric rivers begin as rising moist air in the tropics and result in heavy rain and snow over land. Credit: NOAA
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.
One well-known atmospheric river called the “Pineapple Express” picks up warm, moist air near Hawaii. When the Pineapple Express hits land in the Western United States and Canada, it can cause heavy rain and snow. In California, it can cause up to 5 inches of rain in a day.