What’s an ‘Atmospheric River?’ Terms From the New Vocabulary of Climate Change

The term “hurricane” is quite familiar to Americans. “Typhoon” and “cyclone” are probably somewhat less so, but still far from esoteric. But what people may not know is the difference between the three.

That’s because there isn’t one.

The three terms refer to a type of storm that meteorologists call a “tropical cyclone.” That’s the term for a low-pressure system that forms over tropical waters, with thunderstorms and “cyclonic,” i.e. swirling, winds. In the northern hemisphere, the winds swirl counterclockwise. In the southern hemisphere, they spin in the other direction.

The difference among the three types of storms is only geographical. A tropical cyclone is called a “hurricane” only when it occurs in the north Atlantic Ocean, the northeast Pacific east of the international dateline, and the south Pacific east of the 160th eastern meridian (roughly 300 miles off the coast of New South Wales, Australia).

The same type of storm goes by the name “typhoon” in the northwest Pacific west of the dateline. The term “cyclone” is used in the southwest Pacific west of the 160th east meridian, and in the Indian Ocean.

To qualify as a hurricane, cyclone, or typhoon, a tropical cyclone must have winds of over 74 miles per hour. Under that speed but above 39 mph, it’s a “tropical storm.” Slower than 39 mph, it’s a “tropical depression.”

Whatever they’re called, tropical cyclones have been getting worse. The number of Category 3, 4 and 5 tropical cyclones—storms with winds of at least 111 miles per hour—has been increasing by about 5 percent per decade since the end of the 1970s, according to Princeton University's High Meadows Environmental Institute.

The “bomb cyclone” is aptly named. Unlike hurricanes, which take some time to get up to speed, bomb cyclones grow very strong very quickly. They form when warm air rises from the surface into the atmosphere, causing a sudden, steep drop in barometric pressure.

A plain old tropical cyclone is caused by warm ocean water evaporating and forming a cloud that releases a large amount of heat, stirring up a storm. The bomb cyclone forms via a process known as “explosive cyclogenesis” or sometimes “bombogenesis.” This does not require warm ocean water—or even water. Bomb cyclones can form over dry land, as happened with a 2019 storm that pummelled Colorado and parts of the Midwest with heavy snow and rain.

A bomb cyclone that slammed into the Bay Area in October of 2021 quickly became the strongest storm to hit that region in 26 years, according to Golden Gate Weather Services. The storm set off flooding in most Bay Area counties and doubled the level of the Russian River in Sonoma County, from six feet to 12 feet, in a matter of hours. Sacramento was drenched by torrential rainfall that broke a 160-year-old record. The bomb cyclone produced 60-foot waves that battered the coastline in parts of Northern California.

The name “bomb cyclone” was concocted in 1980 by MIT meteorology professor Fred Sanders, whose intention was to come up with a dramatic name for the phenomenon in order to alert people to the dangers of sudden atmospheric pressure drops and the destructive storms they spawn.

Like something out of a fantasy novel, or a really weird dream, the weather phenomenon known as an “atmospheric river” turns out to be pretty much what it sounds like—a river in the sky. The difference, of course, is that water moves through the air in the form of vapor. 

Atmospheric rivers are also considerably bigger than their terrestrial counterparts. Or at least they’re a lot wider. The widest river on Earth is the Amazon in South America, which during the rainy season can expand to a width of almost 25 miles from its normal, still impressive width of 6.8 miles. An atmospheric river can measure up to 375 miles across and 1,000 miles long. An AR, as they’re known to meteorologists, can carry up to 15 times the amount of water as the quantity that flows through the mouth of the Mississippi River.

Atmospheric rivers mostly flow over oceans, the source of the evaporated water that forms a river in the sky. When they hit land, the water vapor rises higher into the atmosphere, where it cools down and creates significant precipitation. AR storms can be so severe that in 2019 meteorological scientists began giving them category numbers, much like hurricanes. An “AR1” is considered “weak,” while an “AR4” would be “extreme.” The highest category, AR5, is “exceptional.”

Some atmospheric rivers have even been given names. The “Pineapple Express,” for example, runs from Hawaii to the West Coast. When it hits California it can cause up to five inches of rainfall. In the Pacific Northwest, it delivers un-Hawaii-like heavy snows.

More vividly referred to as the atmosphere’s “thirst,” evaporative demand is a measure of how much water on the ground could potentially evaporate. The amount of evaporative demand is not necessarily the same as the amount of water that actually evaporates, because in drought-ridden areas where there isn’t much surface water, the atmosphere “wants” to suck up more water than it can actually get. Hence, it gets “thirsty.”

Why is this an important concept in the era of climate change? First of all, let’s look at what “evaporation” actually means. The word describes the process by which a liquid, such as water, turns into a vapor—something that happens when the liquid is affected by heat. The water vapor then rises into the atmosphere. In fact, 90 percent of all the water in the atmosphere gets there through evaporation.

The remaining 10 percent is generated through transpiration. That’s when plants release the water they contain into the air, in the form of vapor.

When the amount of water that reaches the atmosphere through these two combined processes (a combination called, appropriately enough, evapotranspiration) is less than the amount that the atmosphere could potentially consume—the “evaporative demand”—the atmosphere can be described as “thirsty.”

 Researchers generally determine evaporative demand using a method called “pan evaporation,” which is pretty much exactly what the name implies. A certain amount of water is placed in a specially designed pan and placed on the ground. Scientists then measure how fast and how much water evaporates out of the pan. They use those figures to figure out how much water would evaporate into the air under normal circumstances.

When evaporative demand is high, which happens in warmer areas that get lots of sun, the ground dries out quicker. Without enough water, the ground heats up, releasing more heat into the environment and causing an even thirstier atmosphere. In other words, greater evaporative demand, resulting in a vicious cycle of drought.

Meteorologists now use a measure called the Evaporative Demand Drought Index (EDDI) to monitor droughts. The EDDI is still in the experimental phases, but if it proves to work the way researchers believe it should, it could be used to predict future droughts and even the risk of fires.

The name sounds like an iPhone battery, or maybe a type of terrorist organization, but in fact it’s a relatively uncommon type of thunderstorm that is generally considered the most destructive and unpredictable of all thunderstorms. Supercell storms, like the one in the photo at the top of this page, are most common over the flat plains of the central United States, but can crop up almost anywhere under the right conditions.

For a supercell thunderstorm to form, the most important factor is wind shear. That means, between the surface and 20,000 feet, winds must be rapidly changing direction in the atmosphere.

What sets a supercell apart from a typical thunderstorm is something called a mesocyclone—powerful, rotating winds that results from heavy wind shear. A mesocyclone is similar to a tornado, but it’s not a tornado, which is a funnel-shaped cloud of violently whirling winds. Mesocyclones within a supercell storm frequently cause tornadoes, however. And extremely violent tornadoes at that. A 2021 MIT study found that more than 20 percent of mesocyclone-generated tornadoes are capable of causing damage equivalent to a EF4 or EF5 tornado, the most destructive categories on the Enhanced Fujita Damage Intensity Scale. They come with wind speeds ranging from 166 mph to more than 200 mph.

Tornadoes are just one of the weapons supercell storms can wield against us lowly surface dwellers. High non-tornado winds, heavy rain and hail the size of baseballs are among the other frightening hazards of a supercell.

Winters are getting warmer, meaning snow falls less often even in cold-weather climates. But in the era of climate change, when snow does fall, it can be extreme. According to a study at Indiana’s Ball State University, the number of annual blizzard-level snowstorms has more than doubled over the past six decades. From 1960 to 1994, the U.S. experienced an average of nine blizzards per year, the Ball State study found. From 1995 to 2016 the country was hit with 19 blizzards per year, on average.

To qualify as a “blizzard,” a snowstorm must produce heavy snowfall with visibility of no more than a quarter-mile, with winds of at least 35 mph—and all of those conditions must persist for three straight hours or more.

So when does a blizzard become “Snowpocalypse?” Well, that’s not entirely clear because the word and its synonym, “Snowmageddon,” are not exactly technical terminology used by meteorologists. They appear to have been created by the media, or possibly by users of social media. 

Though the terms may crop up more often in the Canadian press, they seem to have first arisen in the American media back in 2010. No less a personage than President Barack Obama referred to what was the largest snowstorm to rake the Mid-Atlantic region in 90 years as “Snowmageddon.” Apparently, however, “Snowmaggedon” does not have to meet the exact qualifications of a blizzard. It just has to dump a huge amount of snow. The 2010 storm, which was actually two back-to-back storms, buried Baltimore and Philadelphia under totals upwards of 50 inches of snow. Atlantic City, New Jersey, got almost 37 inches and Dulles Airport in Washington, D.C., was paralyzed by 46.1 inches of snow.