What on Earth is an atmospheric river?

Even though you can't see them, atmospheric rivers are a very important phenomenon.
Christopher McFadden

Atmospheric rivers might be one of the most important natural phenomena you've never heard of. However, they do appear in the news from time to time. 

For example, in late October 2021, an atmospheric river brought record-setting rain to Northern California, and in mid-November, a storm caused devastating flooding in Washington. 

All very interesting, but what on Earth are they? Let's find out. 

Where are atmospheric rivers?

Atmospheric rivers are long, relatively narrow, column-like regions of the atmosphere that move most of the water vapor outside of the tropics. In a manner of speaking, you can consider them to be like rivers in the sky.

atmospheric rivers explanation
Infographic explaining the basic principles behind atmospheric rivers (NOAA). Source: NOAA

The weather moves these columns of vapor, which carry about as much water vapor as the average flow of water at the mouth of the Mississippi River. When the atmospheric rivers hit land, this water vapor is often released as rain or snow.

Even though there are many different kinds of atmospheric rivers, the ones with the most water vapor and the strongest winds can cause heavy rain and flooding, often stalling over watersheds prone to flooding. These events can severely impact travel, cause mudslides, and do a lot of damage to the environment, people, and property.

Researchers at the Massachusetts Institute of Technology, Reginald Newell and Yong Zhu, came up with the term in the early 1990s to describe the narrow regions of water vapor. Most atmospheric rivers are thousands of kilometers long but only a few hundred kilometers wide. A single one can carry more water than the Amazon River, the largest river on Earth by volume.

On average, the Earth has four to five active atmospheric rivers at any time. Despite the impact of global warming, atmospheric rivers have likely remained stable. However, researchers predict that increases in greenhouse gas will lead to an intensification of atmospheric river-induced precipitation.

In the field of atmospheric river research, as it stands now, the length and width described above, along with an integrated water vapor depth of more than 25/32ths of an inch (2 cm), are used as standards for putting atmospheric river events into groups.

The so-called "Pineapple Express" is one well-known example of an atmospheric river. It is a strong atmospheric river that can bring moisture from the tropics near Hawaii to the U.S. West Coast.

Are atmospheric rivers dangerous?

In short, yes and no, but the vast majority of atmospheric rivers are completely benign. In fact, most of them are weak systems that often bring rain or snow, which is important to the water supply of many regions of the world. Atmospheric rivers are a key part of the global water cycle and have a lot to do with how much water is available and how likely an area is to flood, especially in the western United States.

example of an atmospheric river
A storm at the mouth of the Russian River, north of Bodega Bay, California. The storm was driven largely by an "atmospheric river" over California. Source: NOAA

Even though atmospheric rivers can cause a lot of rain, which can lead to flooding, they also help increase the snowpack - which is a good thing.

For example, the strong winter storms that hit the U.S. West Coast from western Washington to southern California from December the 10th to the 22nd of 2010 were caused by a series of atmospheric rivers. Some places got between 11 inches (27.94 cm) to 25 inches (63.5 cm) of rain. At the same time, they also contributed to the snowpack in the Sierras, which received 75% of its annual snow by December the 22nd of 2010, the first full day of winter.

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NOAA researchers (like the NOAA Hydrometeorological Testbed and CalWater) use satellite, radar, aircraft, and other observations, as well as numerical weather models, to learn more about atmospheric rivers and how they affect weather and climate.

Scientific research gives us important information that helps NOAA's National Weather Service forecasters warn about heavy rain and flooding up to five to seven days ahead of time in places where atmospheric rivers are likely to cause problems.

How many atmospheric river categories are there?

To help characterize atmospheric rivers, the Center for Western Weather and Water Extremes (CW3E) at the Scripps Institution of Oceanography released a five-level scale for describing atmospheric rivers in February of 2019. The scale ranges from "weak" to "exceptional" in terms of strength, or "beneficial" to "hazardous" in terms of impact.

atmospheric river from space
NASA Earth Observatory Image tracking the development of an atmospheric river over the  Pacific Ocean between the 14th and 17th of October 2017. Source: NASA/Wikimedia Commons

F. Martin Ralph, the director of CW3E, worked with Jonathan Rutz from the National Weather Service and other experts to devise the scale. The scale takes into account both how much water vapor is moved and how long an event lasts.

A preliminary rank is given to atmospheric rivers based on the average maximum vertically integrated water vapor transported over 3 hours. Those that last less than 24 hours drop one rank, while those that last more than 48 hours move up one rank. 

The following storms from history are examples of different types of atmospheric rivers (shortened to AR):

  • AR Cat 1 (Weak): Primarily beneficial most of the time. For example, an AR hit California on February 2, 2017, lasted 24 hours at the coast, and brought only light rain.
  • AR Cat 2 (Moderate): Mostly beneficial most of the time, but can also be dangerous. On November 19 and 20, 2016, Northern California was hit by a moderate atmospheric river. It lasted 42 hours at the coast and dropped several inches of rain, which helped fill up low reservoirs after a drought.
  • AR Cat 3 (Strong): A mix of good and bad. On Oct. 14 and 15, 2016, an atmospheric river lasted 36 hours at the coast. It brought 5 to 10 inches of rain, which helped fill reservoirs after a drought but also caused some rivers to rise just below the flood stage.
satellite imagery atmospheric rivers
Layered precipitable water imagery of particularly strong atmospheric rivers on 5 December 2015. Source: NWS OPC/Wikimedia Commons
  • AR Cat 4 (Extreme): Most of the time dangerous, but sometimes beneficial. For example, on January 8 and 9, 2017, a 36-hour-long atmospheric river dumped up to 14 inches of rain in the Sierra Nevada range and caused at least a dozen rivers to flood.
  • AR Cat 5 (Extraordinary): Mainly dangerous. For example, an atmospheric river that ran along the Central California coast from December 29, 1996, to January 2, 1997, lasted more than 100 hours. Heavy rain and runoff from the storms caused more than $1 billion in damage.

On average, one Cat 4 atmospheric river happens every year on the Oregon coast, every two years in Washington state, every three years in the San Francisco Bay Area, and every ten years in Southern California, which usually has one Cat 2 or Cat 3 atmospheric river every year.

Facts about atmospheric rivers

We've already covered a lot of ground above, but here are some major takeaways about these incredible natural processes. Here are six things you should know about atmospheric rivers:

1. Atmospheric rivers are like giant atmospheric conveyor belts

atmospheric river converyor
Atmospheric rivers work like giant conveyor belts in the sky. Source: romaset/iStock

Atmospheric rivers tend to form near the equator. Since the Sun heats the Earth most directly there, this causes water to evaporate and rise into the sky.

Some of that water vapor is drawn away from the equator by air circulation, generating a narrow band that, like a conveyer belt, transfers the water vapor to other parts of the Earth's atmosphere. Atmospheric rivers flow at the lowest level of the atmosphere, only a half-mile to a mile above the ground.

When atmospheric rivers reach the coasts and flow inland across land, the air is pushed upwards and cools, forcing much of the water vapor to condense and fall to the ground as rain or snow, resulting in an atmospheric river-driven storm.

2. The largest "rivers" of fresh water on Earth are in the atmosphere

atmospheric rivers water
Atmospheric rivers put ground-based rivers to shame in their scale and size. Source: Toltek/iStock

Even though atmospheric rivers are not like rivers made of liquid water on the ground, they do carry enough moisture to earn the name "rivers." Researchers have found that atmospheric rivers over the Pacific carry water vapor at a rate that is 7–15 times higher than the average daily flow of the Mississippi River.

They can be anywhere from hundreds to thousands of miles long, and when it comes to weather systems, "narrow" can mean up to 300 miles (483km) wide!

There are always atmospheric rivers flowing somewhere on Earth, but they don't always stay in the same place in the way that rivers on the ground tend to. At any given time, 90% of the water vapor moving toward the poles is concentrated in about 4-5 atmospheric rivers around the world. Together, these thin moving bands of water vapor cover less than 10% of the planet's circumference.

Scientists think that atmospheric rivers are responsible for more than half of the average annual runoff on the east and west coasts of North America, France, Northern Spain and Portugal, the United Kingdom, Southeastern South America, Southern Chile, Southeast Asia, and New Zealand.

3. Just like with hurricanes, atmospheric rivers are also rated

atmospheric rivers ratings
Just like hurricanes or earthquakes, atmospheric rivers are rated too. Source: marchmeena29/iStock

Like the scales for hurricanes and other dangers, the scale for atmospheric rivers is based on both its physical characteristics (the speed of the wind for hurricanes and the amount of water vapor for atmospheric rivers) and the amount of damage it causes.

Other rating systems only look at how dangerous an event is, but the atmospheric river system takes into account that events can be both dangerous and helpful. At the low end of the scale, AR Cat 1 events are seen as mostly 'good', and at the high end, AR Cat 5 events are seen as mostly 'bad'.

The chart shows a rating scale from 1 to 5, with higher numbers indicating mostly negative outcomes and lower numbers indicating mostly good outcomes. When there is more water vapor being transported, and the river lasts longer, the numbers go up.

Storms brought by atmospheric rivers can be very helpful in places like California, which is suffering from an extended drought.

Up to 50 percent of California's annual rain can come from atmospheric rivers, which can bring enough water to end a drought. United States Geological Survey (USGS) research shows that between 1950 and 2010, atmospheric river storms broke 33–74% of the droughts on the West Coast.

The October 2021 atmospheric river helped ease California's current drought but did not end it. On the other hand, high-intensity atmospheric rivers can be just as destructive as hurricanes and can cause widespread flooding, landslides, and debris flows.

Both the one that hit Northern California on October 24 and the one that hit the Northwest on November 15 of that year were rated 5, AR Cat 5 (Exceptional): Mostly dangerous.

4. Atmospheric rivers may be both good and bad for forest fires

atmospheric rivers fires
Source: Scott L/Wikimedia Commons

When a big wildfire burns on a hillside, it kills most of the plants and makes the hillside more likely to be hit by flash floods or debris flows. This is partly because there are fewer plants to hold onto the soils, leading to reduced moisture retention and capacity, and also because fires can make the top layer of soil more impermeable to water infiltration for a time afterward, leading more water to just run off the land. 

Because of this, the rain brought by atmospheric rivers, along with more localized weather patterns, can make conditions near burned areas especially dangerous.

Scientists from the USGS do post-fire debris-flow hazard assessments on a regular basis for some fires in the Western U.S., usually not long after the fire burns. During these assessments, hazard maps are made to help officials find potentially dangerous situations so they can take steps to protect people and property before and during extreme weather events.

For example, USGS hazard maps of the 2020 Bond Fire helped people know what to do during storms in early 2021 that were caused by atmospheric rivers.

But, atmospheric rivers are thought to also affect future fires.

In 2017, USGS researchers found that atmospheric rivers could lead to fires burning more land the year after an event, especially in the driest parts of the Southwest. Wet winters can increase soil moisture in the short term and drop more snow in the mountains, but they also cause a lot of plant growth at lower elevations.

A lot of that growth is made up of invasive grasses that dry out quickly in the summer and become fuel for fast-moving wildfires.

5. A mega-storm caused by an atmospheric river could be California's other "Big One"

atmospheric rivers big one
California's next "big one" might not be what most expect. Source: SteveCollender/iStock

If you live on the West Coast of the continental United States, you've probably heard of "the big one" or even "the really big one." These phrases refer to possible large earthquakes along the faults of California and the Pacific Northwest.

But there's another "big one" you might not have heard of.

According to USGS natural hazards scientists, California could be hit by an atmospheric river-driven mega-storm that could do a great deal of damage. Such a storm could cause widespread flooding across the state, which would be bad for the environment and public health. It could also cause thousands of landslides, shut down important infrastructure for days or weeks, and potentially lead to an estimated $350 billion dollars in damage and $290 billion dollars in lost business.

USGS scientists modeled ARkStorm, a hypothetical but scientifically plausible future winter storm scenario, to figure out the details of what such an event would look like. ARkStorm, which stands for "Atmospheric River 1,000," was modeled to be as strong as the California winter storms of 1861 and 1862, which were the biggest and longest storms in California's history and led to the Great Flood of 1862, as well as data from large storms in 1969 and 1986. However, this kind of storm is only expected to happen once every 500 to 1,000 years, on average.

6. Atmospheric rivers in California are only likely to get stronger over time

atmospheric rivers coast
Source: USGS

Climate change is widely believed to be the main driving force for making many extreme weather events worse, and in California, atmospheric rivers are no exception. Scientists from the USGS and their partners have found that as the ocean surface has warmed over the past 70 years, there has been a pattern of more water vapor moving to the West Coast.

It's not expected that atmospheric rivers will happen more often, but California's rain will become more unpredictable, with more water falling in a smaller number of stronger events.

Powerful atmospheric river storms can do a lot of damage, and we are likely to see more of them in the future. But scientists from the USGS give us the information and tools we need to be ready, even for a "big one." Scientists can predict when and how strong an atmospheric river will be several days ahead of time. This means that people can stock up on emergency food and water, make plans for shelter, and avoid high-risk areas.

Long-term studies like the ARkStorm Scenario can help people become more aware of the possibility of a big storm in the future. They can also help with major logistical planning and infrastructure development, which can help people get ready for big atmospheric river storms and limit the damage they cause.

And that, atmospheric enthusiasts, is your lot for today. 

Atmospheric rivers are mightily impressive natural phenomena that have many wide-ranging effects on various aspects of our planet. Normally invisible to the eye, it is fascinating to think just how complex the Earth's natural systems really are.

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