What Is an "Atmospheric River"?

A flooded playground in San Jose, California, on February 22. Blame the high water on an atmospheric river. Image Credit: Noah Berger/AFP/Getty Images

Storms make for attention-grabbing headlines, and almost every disaster has a meteorological term that makes a hazardous situation sound 10 times as terrifying. A derecho tore through the Mid-Atlantic back in 2012 and had such a profound psychological impact on the affected residents that you could cause mass hysteria by just mentioning the term. Then came the dreaded polar vortex, an ever-present large-scale wind pattern that encircles the North Pole and occasionally gets wavy and injects bitterly cold air into southern Canada and the United States. It was nothing new—but it sounded scary, so the name took off.

The recent deluges in California highlighted the latest captivating weather-y buzzword: an “atmospheric river.” Like its counterparts, this scary-sounding phenomenon is not as uncommon as it seems. It's responsible for almost all of the rain on the West Coast this winter.

Clouds outline the atmospheric river stretching from Hawaii to California in the storm that affected the West Coast on February 17, 2017. Image Credit: Dennis Mersereau

So what is it? An atmospheric river is a long, narrow band of deep tropical moisture that can span thousands of miles in length. They occur from the tropics to the mid-latitudes. Atmospheric rivers aren’t actual rivers, of course, but it’s a pretty good description of a feature that resembles a river on satellite imagery and can bring torrents of water to the unlucky communities caught beneath one as the system comes ashore.

These ribbons of tropical moisture are the result of sprawling low-pressure systems that form just far enough south that their counter-clockwise circulation scoops up water vapor from the tropics and transports it northward. The storms that cause atmospheric rivers to form in the first place are usually able to generate enough upward lift to create precipitation. Mountains can play a role—they're very effective at wringing moisture out of the atmosphere as wind travels up the side of their terrain. Whether it’s rain or snow, any precipitation that forms within that band of elevated moisture levels can be quite heavy, producing steep rainfall totals and many feet of snow in extreme cases.

NASA’s Global Precipitation Measurement (GPM) mission captured three weeks of heavy rainfall slamming into the West Coast between February 1 and February 20, 2017. Watch it happen in the video below.

California has a reputation for calm, sunny weather, but the state never really has it easy when it comes to dealing with nature’s temper tantrums. For the past couple of years, the state has been mired in a devastating drought, a cycle of dryness that was finally broken this winter as one storm after another roared in from the Pacific and drenched the state with unmanageable amounts of heavy rain. The driving force that gave each storm its bulk was an atmospheric river. Without it, there wouldn’t have been much moisture for the storm systems to work with.

An atmospheric river that affects the West Coast—and California in particular—is usually nicknamed the “Pineapple Express” since the source of the tropical moisture is the area around Hawaii. Though they go without a popular nickname, these features are also common over the eastern half of the United States during the warm season. Many of the major flash floods that occur in the eastern U.S. during the summer months are the result of intense thunderstorms tapping into the bountiful moisture present in an atmospheric river flowing over the region.

All weather is the result of nature trying to balance out inequality, and atmospheric rivers, just like every other weather condition, serve this purpose. Wind blows from areas of high air pressure to areas of lower pressure in an attempt to erase the inequality of more air molecules over one spot than another. The jet stream is the direct result of sharp temperature differences between the tropics and the poles. Hurricanes exist to transport heat from the tropics to the poles. Atmospheric rivers exist to take moisture out of the tropics and spread it around the world. Though they can seem difficult to enjoy, we’d be in some pretty big trouble without them. In 1998, a study by MIT scientists reported that 90 percent of all the moisture transfer between the tropics and the rest of the world each year occurs within these narrow bands of evaporated paradise.

What is a Polar Vortex?

Edward Stojakovic, Flickr // CC BY 2.0
Edward Stojakovic, Flickr // CC BY 2.0

If you’ve turned on the news or stepped outside lately, you're familiar with the record-breaking cold that is blanketing a lot of North America. According to The Washington Post, a mass of bone-chilling air over Canada—a polar vortex—split into three parts at the beginning of 2019, and one is making its way to the eastern U.S. Polar vortexes can push frigid air straight from the arctic tundra into more temperate regions. But just what is this weather phenomenon?

How does a polar vortex form?

Polar vortexes are basically arctic hurricanes or cyclones. NASA defines them as “a whirling and persistent large area of low pressure, found typically over both North and South poles.” A winter phenomenon, vortexes develop as the sun sets over the pole and temperatures cool, and occur in the middle and upper troposphere and the stratosphere (roughly, between six and 31 miles above the Earth’s surface).

Where will a polar vortex hit?

In the Northern Hemisphere, the vortexes move in a counterclockwise direction. Typically, they dip down over Canada, but according to NBC News, polar vortexes can move into the contiguous U.S. due to warm weather over Greenland or Alaska—which forces denser cold air south—or other weather patterns.

Polar vortexes aren't rare—in fact, arctic winds do sometimes dip down into the eastern U.S.—but sometimes the sheer size of the area affected is much greater than normal.

How cold is a polar vortex?

So cold that frozen sharks have been known to wash up on Cape Cod beaches. So cold that animal keepers at the Calgary Zoo in Alberta, Canada once decided to bring its group of king penguins indoors for warmth (the species lives on islands north of Antarctica and the birds aren't used to extreme cold.) Even parts of Alabama and other regions in the Deep South have seen single-digit temperatures and wind chills below zero.

But thankfully, this type of arctic freeze doesn't stick around forever: Temperatures will gradually warm up.

A Simple Trick for Defrosting Your Windshield in Less Than 60 Seconds


As beautiful as a winter snowfall can be, the white stuff is certainly not without its irritations—especially if you have to get into your car and go somewhere. As if shoveling a path to the driver’s door wasn’t enough, then you’ve got a frozen windshield with which to contend. Everyone has his or her own tricks for warming up a car in record time—including appropriately-named meteorologist Ken Weathers, who works at WATE in Knoxville, Tennessee.

A while back, Weathers shared a homemade trick for defrosting your windshield in less than 60 seconds: spray the glass with a simple solution of one part water and two parts rubbing alcohol. “The reason why this works,” according to Weathers, “is [that] rubbing alcohol has a freezing point of 128 degrees below freezing.”

Watch the spray in action below.

[h/t: Travel + Leisure]