What are atmospheric rivers and lakes

If someone told you that high up in the sky there are rivers comparable to the Amazon, would you believe them? No? Well, you should! In this new lesson of the Windy.app Meteorological Textbook (WMT) and newsletter for better weather forecasting you will learn what are atmospheric rivers and lakes and how they work.

What’s the Amazon doing in the sky?

Just 30 years ago, nobody knew what atmospheric rivers were. That term only appeared in 1994, only when we began to actively study them.

It’s referring to long thin streams of water vapor that stretch from the Tropics to more moderate latitudes, thousands of kilometers long and just a couple hundred wide. Since these streams are made of vapor, they don’t look like liquid river, but like a very thinly stretched strip of clouds.

Atmospheric river stretching from Hawaii to the West Coast of the US in February 2017. Photo: GPM / Suomi NPP — VIIRS / NASA

But atmospheric rivers have earned their name: a stream like that can contain 25 times more water than the entire Mississippi river, and even more than the Amazon itself! Considering that one Earth hemisphere constantly has three to five atmospheric rivers ‘flowing’, calling them just streams or creeks would be an understatement.

Atmospheric rivers can more commonly be found on the borders of regions with different climates — usually near a cold front or cyclone. Apparently, the front’s cold air ‘raises’ the warm moisture-rich air near the ocean surface up to the sky, where the river then ‘flows’ at an altitude of up to three kilometers.

This phenomenon plays an important role in our planet’s water cycle: atmospheric rivers are only found on 10% of the Earth’s surface, but they account for 90% of water vapor transfer from North to South and back.

Atmospheric river near Africa’s East Coast. Photo: Brian Mapes / NOAA ERA-Interim

And in 2021, the first atmospheric lakes were discovered — clusters of water vapor formed by the atmospheric rivers over the West part of the Indian Ocean which contain so much moisture it could fill a puddle about 1000 kilometers wide if it was just several centimeters deep.

When atmospheric lakes and rivers reach land, a collision with mountains can force them to rise even higher, where they condense because of lower temperatures — the vapor turns back into liquid, forcing the rivers to pour down on as rain or snow.

And here we find possibly the trait of atmospheric rivers that matters the most to people: according to current data, up to half of all hurricanes, floods, landslides and other extreme weather conditions on Earth’s coasts happen when atmospheric rivers meet land.

What the Pineapple Express brings

Atmospheric rivers often benefit humanity — for instance, they bring rains to arid regions. Lack of atmospheric rivers even caused droughts in South Africa, Spain, and Portugal.

But the phenomenon is often destructive, too. A special classification was invented to determine the risk an atmospheric river poses — from ‘mild, mostly favorable’ (category 1) to ‘exceptional, mostly dangerous’ (category 5).

Probably, the most famous atmospheric river is the Pineapple Express. It’s a stream that begins near Hawaii and then follows the jet stream towards the Pacific coast of North America — towards the shores of California, Canada, and Alaska.

Aftermath of the 1862 Sacramento flood as pictured by the artist. Picture: UC Berkeley Bancroft Library / Courtesy

The Pineapple Express can be caused by temperature fluctuations in the ocean, including El Niño episodes, and led to destructive hurricanes in California more than once. The heaviest flood recorded in all of the state’s history happened in January 1862, when the Pineapple Express poured down on the coast over 45 days in a row.

And the biggest flood in California after the 19th century happened in 1955 when the Pineapple Express was branded the Hawaiian Storms, before the phenomenon was studied.

Other Pacific states also have no luck with atmospheric rivers: Oregon, for instance, has run-ins with a category 4 river every year (‘extreme, mostly dangerous, but also favorable’), and Washington experiences it every two years. In general, atmospheric rivers are responsible for 30%-40% of all precipitation on the US West Coast, and in California — for up to 50%.

Atmospheric rivers also lead to strong rains and snowfalls in Western and Northern Europe, the West coast of Africa, in the South of South America, Iran, and New Zealand.

According to meteorologists, globally, atmospheric rivers can cause 40% to 75% cases of hurricane winds and hard precipitation on 40% of the world’s coasts, and the streams themselves are getting stronger due to climate change.

Text: Windy.app

Cover photo: Roya Ann Miller / Unsplash