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Dear Dot,
I’ve been reading some headlines about atmospheric rivers. What exactly are they?
– Jas
The Short Answer: Atmospheric rivers are water vapor picked up in warmer areas (typically on western coasts) that cools as it rises and hits mountain ranges. It then falls in the form of sudden, heavy rain or snow. While climate change isn’t increasing the frequency of this weather phenomenon, it is increasing the severity.
Dear Jas,
Perhaps you’re referring to this headline? Or maybe this more recent one, suggesting that the incidence of atmospheric rivers could double by 2100 due to climate change? Whatever headline piqued your curiosity about atmospheric rivers, Dot is eager to elucidate.
Of course, I am no expert on weather phenomena, though I have long been fascinated by the topic. Young Dot even developed a penpal relationship with my local news station’s weather man. Alas, Bill Hutchins died in 1999, so I cannot write to him for an explanation of atmospheric rivers.
So, without a favorite meteorologist to consult, Dot turned to Char, a student intern at Bluedot who shares Dot’s deep curiosity about weather, just like you, Jas.
For a start, Char reported, an atmospheric river is not really a river at all, in the way we think of rivers. NOAA calls them “rivers of the sky” because they are, essentially, an accumulation of water vapor, picked up in the tropics and carried inland over mountains, where it cools and rises, producing condensation. NASA describes them as “long, narrow jets of air,” carrying water vapor, that range from 250 to 375 miles wide and average 1,200 miles in length. The condensation they carry is released as rain or snow.
“Atmospheric river” is a relatively new term (coined in 1994) for a phenomenon that was more typically called the Pineapple Express (if it originated in Hawaii) or the Rum Runner Express (Caribbean).
Atmospheric rivers are neither necessarily dangerous nor rare (NASA says there are roughly 11 in the world at any given time). The average atmospheric river can be beneficial, providing valuable precipitation and replenishing freshwater reserves. Char tells me that the amount of water in an “average” atmospheric river – in the form of vapor — is as much as 25 Mississippi Rivers, which kinda sounds like it would be a lot. Some back-of-the-napkin math by a student at Loyola University in New Orleans indicates that an atmospheric river’s rate of water flow could fill the state of New Orleans with 10 feet of water in a year.
The largest atmospheric rivers (that can carry 15x the water flow at the mouth of the Mississippi River) can create catastrophes, such as what happened in Montecito a couple of years ago, or this once-in-a-generation atmospheric river flooding in British Columbia in 2021. (No coincidence, according to Char, that both of these historic weather events occurred on the North American West Coast — 80% of the flooding on that coast is a consequence of atmospheric rivers.)
The reason we’re hearing more about atmospheric rivers, Jas, is the same reason we’re hearing more about a lot of weird and extreme weather events — climate change. NASA has determined that, with a warming planet creating air that can hold more water vapor, we can expect to see less frequent but more extreme atmospheric events.
Dot appreciates your question, Jas. But let us both hope that our experience with atmospheric rivers remains strictly theoretical.
Condensationally,
Dot
