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.

Wayfair’s Fourth of July Clearance Sale Takes Up to 60 Percent Off Grills and Outdoor Furniture

Wayfair/Weber
Wayfair/Weber

This Fourth of July, Wayfair is making sure you can turn your backyard into an oasis while keeping your bank account intact with a clearance sale that features savings of up to 60 percent on essentials like chairs, hammocks, games, and grills. Take a look at some of the highlights below.

Outdoor Furniture

Brisbane bench from Wayfair
Brisbane/Wayfair

- Jericho 9-Foot Market Umbrella $92 (Save 15 percent)
- Woodstock Patio Chairs (Set of Two) $310 (Save 54 percent)
- Brisbane Wooden Storage Bench $243 (Save 62 percent)
- Kordell Nine-Piece Rattan Sectional Seating Group with Cushions $1800 (Save 27 percent)
- Nelsonville 12-Piece Multiple Chairs Seating Group $1860 (Save 56 percent)
- Collingswood Three-Piece Seating Group with Cushions $410 (Save 33 percent)

Grills and Accessories

Dyna-Glo electric smoker.
Dyna-Glo/Wayfair

- Spirit® II E-310 Gas Grill $479 (Save 17 percent)
- Portable Three-Burner Propane Gas Grill $104 (Save 20 percent)
- Digital Bluetooth Electric Smoker $224 (Save 25 percent)
- Cuisinart Grilling Tool Set $38 (Save 5 percent)

Outdoor games

American flag cornhole game.
GoSports

- American Flag Cornhole Board $57 (Save 19 percent)
- Giant Four in a Row Game $30 (Save 6 percent)
- Giant Jenga Game $119 (Save 30 percent)

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Expeditions Gather Climate Change Clues on Mount Everest in Two New Documentaries

Team members climb up a slope during the expedition to find Sandy Irvine's remains on Mount Everest.
Team members climb up a slope during the expedition to find Sandy Irvine's remains on Mount Everest.
Matt Irving/National Geographic

Two one-hour documentaries premiering tonight reveal what Mount Everest is really like—and what scientists can learn from studying it.

Both docs are produced by and airing on National Geographic. In Lost on Everest, premiering at 9 p.m. EDT, climber Mark Synnott and Nat Geo photographer Renan Ozturk lead a team of seasoned mountaineers on a mission to discover what happened to Andrew “Sandy” Irvine, who vanished with fellow explorer George Mallory during the first Everest climb in June 1924. While Mallory’s body was located by a BBC-sponsored operation in 1999, Irvine’s exact fate has remained a mystery for nearly a century since his disappearance. As Synnott and his companions search for evidence, they encounter their own harrowing set of obstacles, from hurricane-force winds to medical emergencies.

Climbers on Mount Everest
Climbers ascend the Khumbu Icefall, a notoriously dangerous section of the summit route.
Mark Fisher/National Geographic Society

But Mount Everest isn’t only a challenge for adventure-seekers and intrepid investigators—it also holds thousands of years’ worth of information about how climate change has altered the environment, which can help scientists predict its future effects. In Expedition Everest, airing at 10 p.m. EDT, actor Tate Donovan narrates the journey of an international group of scientists and climbers with an ambitious set of data-collecting objectives.

One task is to use drones, laser scanners, and cameras to capture footage of every inch of the ascent, so researchers can create a 360-degree portrait of the mountain and track how glacial melt alters the landscape in the coming years. Since the Himalayas contain the water supply for roughly one-fourth of the world’s population, the increase in glacial melt—which has already doubled since 2000—could threaten the futures of billions of people living in the region.

Scientists drill ice cores on Mount Everest
Mariusz Potocki and members of the National Geographic and Rolex Perpetual Planet Everest Expedition team collect the highest-ever ice core at 8020 meters (26,312 feet) near the South Col of Everest.
Dirk Collins/National Geographic Society

Even more immediate is the risk of flash floods, which are difficult to predict without a constant feed of weather data from high altitudes. Another goal of the expedition is to install weather stations at five locations along the climbing route, which will monitor temperature, humidity, air pressure, wind speed, and other factors that help alert meteorologists to an impending flood.

Some researchers have joined the expedition to drill deep into the ice at an altitude above 8000 meters (26,000 feet)—Mount Everest's "death zone"—and collect ice cores. These long tubes of ice reveal how the atmosphere has changed over thousands of years. Others are collecting similar cores of sediment at the bottom of a lake, as well as examining how plant and animal life has adapted to the warming temperatures and rising water levels.

Overall, Expedition Everest illustrates how the Himalayas function as an early indicator of what climate change will do to other places.

As climate scientist Anton Seimon explains in the documentary, “We’re getting a window into what the rest of the world is starting to experience—and likely to experience in growing proportions.”

You can watch the double feature tonight, June 30, at 9 p.m. EDT on National Geographic.