Atmospheric Rivers

Caitlin Dempsey

Updated:

Atmospheric Rivers are trails of moisture in the atmosphere composed of condensed water vapor. Atmospheric rivers are typically found on the boundaries of different weather zones like cyclones that form in regions other than the tropics.

Atmospheric rivers are airborne rivers of water vapor that flow from the tropics to the poles.

Atmospheric rivers originate at the Equator where warm, moisture laden air starts moving towards away from the Equator. The moist air current forms a narrow band that travels in the lowest part of the atmosphere as it travels over the ocean.

When the band of air reaches land, the air is pushed upwards and cools. The water moisture in the now cooler air condenses and falls as rain and/or snow in an atmospheric river-driven storm.

At any given time there are a few (between 3-5) atmospheric rivers in Earth’s hemisphere; each can be thousands of miles long and hundreds of miles wide.

Atmospheric Rivers Carry A Lot of Water

The moisture contained within an atmospheric river can be equivalent to the amount of water in the Amazon River, although only 10% of the Earth’s circumference is covered by an atmospheric river band.

90% of Earth’s north to south water vapor transport is done through atmospheric rivers.

Rivers in the Sky

These “rivers in the sky” are typically 250 to 375 miles (400 to 600 kilometers) broad and convey as much water vapor as around 25 Mississippi Rivers.

An atmospheric river occured between 9th and 12th of Dec. 2014 over the Pacific Ocean and Southwest US. Image: NASA.
An atmospheric river occured between 9th and 12th of Dec. 2014 over the Pacific Ocean and Southwest US. Image: NASA.

Where Do Atmospheric Rivers Happen?

Extreme precipitation events (which can produce severe floods) are mostly caused by atmospheric rivers in numerous mid-latitude, westerly coastal locations around the world, notably the west coasts of North America, Western Europe, and North Africa.

Since the 1990s researchers have tracked and studied atmospheric rivers and analyzed their incredible importance to the water cycles of Earth .

Video of an Atmospheric River

These atmospheric rivers are made up of the water droplets that evaporate from rainstorms, puddles, rivers, lakes, streams and the oceans and come back town to Earth as rain, snow, and other precipitation.

This animation, which was developed by NASA using satellite imagery and remote sensing data, highlights the water pathway of atmospheric rivers, including one particularly powerful occurrence that drenched regions of Washington, Oregon, and Northern California between January 11 and 18, 2021.

The dark blue areas in the video are regions of intense water vapor.

As atmospheric rivers often are caused by extraordinary weather fronts they are also capable of dumping massive amounts of precipitation back to Earth causing storms, floods, and other disasters in many parts of the world.

How Atmospheric Rivers are Rated

Similar to hurricanes, atmospheric rivers are rated based on the intensity of water being carried and the level of destruction. In 2019, a scale for rating atmospheric rivers was developed.

From the USGS:

  • AR Cat 1 (Weak):  Primarily beneficial. For example, a Feb. 2, 2017 AR hit California, lasted 24 hours at the coast, and produced modest rainfall.
  • AR Cat 2 (Moderate): Mostly beneficial, but also somewhat hazardous. An atmospheric river on Nov. 19-20, 2016 hit Northern California, lasted 42 hours at the coast, and produced several inches of rain that helped replenish low reservoirs after a drought.
  • AR Cat 3 (Strong): Balance of beneficial and hazardous. An atmospheric river on Oct. 14-15, 2016 lasted 36 hours at the coast, produced 5-10 inches of rain that helped refill reservoirs after a drought, but also caused some rivers to rise to just below flood stage.
  • AR Cat 4 (Extreme): Mostly hazardous, but also beneficial. For example, an atmospheric river on Jan. 8-9, 2017 that persisted for 36 hours produced up to 14 inches of rain in the Sierra Nevada and caused at least a dozen rivers to reach flood stage.
  • AR Cat 5 (Exceptional): Primarily hazardous. For example, a Dec. 29 1996 to Jan. 2, 1997 atmospheric river lasted over 100 hours at the Central California coast. The associated heavy precipitation and runoff caused more than $1 billion in damages.
Color chart showing how atmospheric rivers are rated.
Atmospheric river rating system chart. Source: USGS, adapted from Ralph et al. 2019. public domain.

Pineapple Express

The most common or well-known atmospheric river might be the Pineapple Express, which affects the western coast of the United States.

Water vapor imagery of the eastern Pacific Ocean from the GOES 11 satellite, showing a large atmospheric river aimed across California. Image from United States Naval Research Laboratory, Monterey. (Captured December 20, 2010).
Water vapor imagery of the eastern Pacific Ocean from the GOES 11 satellite, showing a large atmospheric river aimed across California. Image from United States Naval Research Laboratory, Monterey. (Acquired December 20, 2010).

Most of California’s Yearly Rainfall Arrives Via Atmospheric Rivers

A study that looked at two-decades worth of rainfall data has found that the majority of California’s rain arrives due to atmospheric rivers.

Furthermore, about 10-30% of the rainfall measured at locations across the state comes from one major storm.  

The study analyzed rainfall rates at 176 weather stations across California for this study.

Along the West Coast, warm moist air from Hawaii is the source of atmospheric rivers during the rainy season.

An atmospheric river occured between 9th and 12th of Dec. 2014 over the Pacific Ocean and Southwest US. Image: NASA.
An atmospheric river occured between 9th and 12th of Dec. 2014 over the Pacific Ocean and Southwest US. Image: NASA.

Rainfall is Different Between Northern and Southern California

Rainfall is distinctly different between northern and southern California, with the line demarcated at 37.5°N.  

Northern California averages about twice as many rainfall events as southern California.  Researchers found that, on average, there are 25 – 45 rainfall events each year in Northern California.   Of those, about 40% – 50% are atmospheric rivers which contribute an average of 79% of extreme-rainfall along the northern coast and 76% in the norther Sierras.

Median characteristics of rainfall events including (A) annual numbers, (B) event-total rainfall, (C) event-duration. Figure: Lamjiri et al., 2018
Median characteristics of rainfall events including (A) annual numbers, (B) event-total rainfall, (C) event-duration. Figure: Lamjiri et al., 2018

Southern California tends to also experience lower median rainfall totals than northern California (10–14 mm versus 10–22 mm per event) as well as shorter rainfall durations (<5–11 hours versus 10-14 hours for northern California). Atmospheric rivers contribute to 68% of extreme-rainfall accumulations in southern California.

Studying Atmospheric Rivers is Important

Aside from their incredible importance in relation to the water cycle of the Earth, studying atmospheric rivers can help atmospheric scientists and others determine the quantity, quality and health of the water cycle in addition to flow and volume of this vital water transportation entity.

References

Earth System Research Laboratory. 2014. Atmospheric River Information Page. Web access 5 January, 2015. http://www.esrl.noaa.gov/psd/atmrivers/

Earth System Research Laboratory. 2014. What are Atmospheric Rivers? Web access 5 January, 2015. http://www.esrl.noaa.gov/psd/atmrivers/questions/

Lamjiri, M. A., Dettinger, M. D., Ralph, F. M., Oakley, N. S., & Rutz, J. J. (2018). Hourly Analyses of the Large Storms and Atmospheric Rivers that Provide Most of California’s Precipitation in Only 10 to 100 Hours per YearSan Francisco Estuary and Watershed Science16(4).

New scale to characterize strength and impacts of atmospheric river storms | U.S. geological survey. (2019, February 11). USGS.gov | Science for a changing world. https://www.usgs.gov/news/new-scale-characterize-strength-and-impacts-atmospheric-river-storms

Weill, A. (2021, December 10). Rivers in the sky: 6 facts you should know about atmospheric rivers | U.S. geological survey. https://www.usgs.gov/news/featured-story/rivers-sky-6-facts-you-should-know-about-atmospheric-rivers

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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.

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