RESEARCH STARTER

Atmospheric river

An atmospheric river is a long, narrow band of condensed water vapor in the lower atmosphere that transports vast amounts of moisture, often significantly exceeding the volume of major rivers like the Mississippi. These phenomena typically originate in tropical regions, where they gather water vapor and then carry it across the globe. When they encounter mountain ranges or cold fronts, the moisture is forced upward, resulting in substantial precipitation in the form of rain or snow. Atmospheric rivers play a crucial role in the Earth's climate by delivering essential precipitation, especially in areas like California, where they account for about half of annual rainfall. However, they can also lead to severe weather events, causing floods, mudslides, and substantial property damage. The intensity of atmospheric river storms can vary, with higher-rated events often resulting in hazardous conditions. Monitoring and predicting these systems is complex, as they can change rapidly, but advancements in technology, such as satellite observations and measuring devices, have improved forecasting capabilities.

Full Article

An atmospheric river is a long, narrow band, or filament, of concentrated water vapor that moves through the lower portion of Earth’s atmosphere. Research indicates these formations can transport fifteen times the amount of water contained in the Mississippi River. Atmospheric rivers collect water in tropical and subtropical regions and transport it through the atmosphere to other parts of the planet. When these sky rivers collide with weather fronts or are forced upward by mountains, they drop heavy amounts of precipitation. Atmospheric rivers are a normal part of Earth’s climate and can provide necessary precipitation or deposit enough rain or snow to cause major storms.

Background

The term atmospheric river was first used in the 1990s by Massachusetts Institute of Technology (MIT) researchers Reginald Newell and Yong Zhu. They were studying the bands of rain that travel across the Pacific Ocean from Hawaii to California. These bands produce significant storms, which were once called “the Pineapple Express” or simply “Hawaii storms.” The term atmospheric river was later applied to similar bands of concentrated moisture in the sky, regardless of where they occurred.

Overview

Atmospheric rivers can occur in both the Northern and Southern hemispheres and are commonly found in latitudes between 30 and 60 degrees. The rivers form over oceans in the tropics and subtropical regions within the troposphere, where they transport large amounts of water vapor through the lower atmosphere. Once there, this water vapor is driven by weather forces to other parts of the world.

These rivers of the sky are usually found with a low-level jet stream, which is a narrow band of moving air currents. They are almost always associated with extratropical cyclones, or low-pressure regions that produce rapid changes in temperature and dew point, which is the temperature to which the air must cool in order for evaporated water to condense to form liquid water.

Atmospheric rivers are generally between 250 and 375 miles wide (402 and 603.5 kilometers) and carry varying amounts of water. The rate at which they carry water vapor is about the same as that of the Amazon River—about 176,000 tons per second. The water vapor travels through these rivers in the sky until it runs into a cold front or land formation such as a mountain range. Then, the river is forced upward, where the vapor cools and turns to liquid rain or snow and falls to the ground.

The phenomenon occurs most often in the winter months; atmospheric rivers are rare during the summer. In the United States, most atmospheric river-related events occur between September and March. In general, more northern latitudes will experience storms from atmospheric rivers in the fall, while more southern latitudes see these events during the winter months of December, January, and February.

Atmospheric rivers create weather events of varying intensity. Many produce relatively gentle storms that are an important part of an area’s climate. For example, California receives about half of its annual precipitation from events associated with atmospheric rivers. Without them, the area would experience extreme drought. However, atmospheric rivers can also produce damaging storms with excessive precipitation. Scientists have determined that the ten strongest atmospheric rivers to affect the Western United States caused nearly half of all flood damage there between 1978 and 2017.

Atmospheric river storms are a common occurrence in the US state of California, and are increasingly more violent and fatal.  For example, twenty-five atmospheric river storms struck the state in the first three months of 2023. In February 2024, California received five consecutive days of severe atmospheric river events, which left nine people dead. In early 2024, an atmospheric river dropped a month’s worth of rain on San Diego, California. Los Angeles received over 9 inches of rain, with over 562 mudslides being reported. In one day, an estimated $11 billion in damages were sustained throughout Southern California. Fifty-six atmospheric rivers impacted the western US during the late 2024 and early 2025. By April 2025, Northern California had experienced nine atmospheric river events.

A high level of damage is most likely to occur when an atmospheric river is slow-moving or stalls, bringing intense precipitation and winds that result in flooding, mudslides, and significant loss of property and possibly even life.

To help minimize the effects of storms caused by atmospheric rivers, scientists have developed ways to measure their strength and potential. They measure the integrated water vapor (IWV) and integrated vapor transport (IVT). IWV refers to how deep the rain or snow would be if it fell from the river. IVT is the total amount of water vapor carried in the atmospheric river. Scientists have established a scale of 1 to 5 for rating atmospheric rivers. Rivers with a score of 1 or 2 generally result in beneficial storms that help replenish water tables, while higher scores of 4 and 5 tend to produce flooding, extreme snowstorms, and other hazardous conditions.

To help protect property and lives, scientists have developed a number of ways to monitor and measure atmospheric rivers. They use satellites, measuring devices called radiosondes attached to weather balloons, and dropsondes, measuring devices dropped from an aircraft. Despite the available technology, however, it can be difficult to predict the result of a particular atmospheric river. This is because it can strengthen, move faster, slower, stall completely, change its angle, interact with other weather formations, including other atmospheric rivers, and change where it makes landfall.

Forecasters can usually predict the outcome of a storm caused by an atmospheric river up to five days before it occurs. Being able to predict the potential precipitation associated with an atmospheric river is also important because it helps in planning flood control and water reservoir operations. When heavy rain is forecasted, officials often open dams or release stored water to make room for the anticipated precipitation. If this does not occur, the released water will have been wasted and could potentially lead to drought conditions for the area.

While the term atmospheric river is generally used, not all scientists agree that the term is accurate. Some believe that the water vapor attributed to atmospheric rivers is actually the result of a continuous cycle of evaporation and condensation within an extratropical cyclone. Instead of a ribbon of water vapor moving from the tropics, these scientists say that the vapor forms as the weather event moves. They suggest that the phenomenon known as atmospheric rivers are actually the footprint of these cyclones as they move away from the tropical region.


Bibliography

“Atmospheric Rivers.” Water Education Foundation, 1 Dec. 2017, www.watereducation.org/aquapedia-background/atmospheric-rivers. Accessed 16 Apr. 2026.

Barmann, Jay. “Atmospheric Rivers, Explained.” SFist, 26 Jan. 2021, sfist.com/2021/01/26/atmospheric-rivers-explained/. Accessed 16 Apr. 2026.

Berwyn, Bob. “A Surge from an Atmospheric River Drove California’s Latest Climate Extremes.” Inside Climate News, 2 Feb. 2021, insideclimatenews.org/news/02022021/a-surge-from-an-atmospheric-river-drove-californias-latest-climate-extremes. Accessed 16 Apr. 2026.

“California Storm: After 5 Days, the Rain Has Stopped. In Its Wake, 9 Dead, a Trail of Destruction.” The Los Angeles Times, 8 Feb. 2024, www.latimes.com/california/story/2024-02-05/california-storm-weather-los-angeles-2024. Accessed 16 Apr. 2026.

Dacre, H. F., et al. “How Do Atmospheric Rivers Form?” American Meteorological Society, 1 Aug. 2015, journals.ametsoc.org/view/journals/bams/96/8/bams-d-14-00031.1.xml. Accessed 16 Apr. 2026.

Davis, William B., et al. “Track the Latest Atmospheric River to Hit the West Coast.” The New York Times, 16 Apr. 2026, www.nytimes.com/interactive/2024/us/atmospheric-river-california-west-tracker.html. Accessed 16 Apr. 2026.

Erdman, Jonathan, and Rob Shackelford. “What is an Atmospheric River? Explaining the Blessing and Curse.” The Weather Channel, 1 Nov. 2025, weather.com/science/weather-explainers/news/atmospheric-river-explained. Accessed 16 Apr. 2026.

“General Information About Atmospheric Rivers.” Center for Western Weather and Water Extremes, cw3e.ucsd.edu/wp-content/uploads/AR_FAQ’s.pdf. Accessed 16 Apr. 2026.

Hill, Alice C., and Tess Turner. “How Big a Climate Threat Are Atmospheric Rivers?” Council on Foreign Relations, 22 Mar. 2024, www.cfr.org/article/how-big-climate-threat-are-atmospheric-rivers. Accessed 16 Apr. 2026.

Mendez, Emely. “A Climate Expert Explains Why Atmospheric Rivers Are Causing Historic Rainfall in California.” Columbia Climate School, 5 Feb. 2024, news.climate.columbia.edu/2024/02/05/a-climate-expert-explains-why-atmospheric-rivers-are-causing-historic-rainfall-in-california. Accessed 16 Apr. 2026.

Steenburgh, Jim. “Atmospheric Rivers.” University of Utah, 31 Mar, 2017, www.inscc.utah.edu/~steenburgh/classes/5210/lecture_notes/AtmosphericRivers.pdf. Accessed 16 Apr. 2026.

“What Are Atmospheric Rivers?” National Oceanic and Atmospheric Administration, 21 Feb. 2025, www.noaa.gov/stories/what-are-atmospheric-rivers. Accessed 16 Apr. 2026.

Wulfeck, Andrew. “Mapped: What a Barrage of 56 West Coast Atmospheric River Events Looks Like.” Fox Weather, 23 Apr. 2025, www.foxweather.com/weather-news/atmospheric-river-recap-2024-2025-california-northwest. Accessed 17 Apr. 2026.

Zhou, Yang, et al. “Atmospheric River Frequency‐Category Characteristics Shape US West Coast Runoff.” Journal of Geophysical Research: Atmospheres, vol. 130, no. 2, 16 Jan. 2025, doi:10.1029/2024jd041805. Accessed 16 Apr. 2026.

Full Article

An atmospheric river is a long, narrow band, or filament, of concentrated water vapor that moves through the lower portion of Earth’s atmosphere. Research indicates these formations can transport fifteen times the amount of water contained in the Mississippi River. Atmospheric rivers collect water in tropical and subtropical regions and transport it through the atmosphere to other parts of the planet. When these sky rivers collide with weather fronts or are forced upward by mountains, they drop heavy amounts of precipitation. Atmospheric rivers are a normal part of Earth’s climate and can provide necessary precipitation or deposit enough rain or snow to cause major storms.

Background

The term atmospheric river was first used in the 1990s by Massachusetts Institute of Technology (MIT) researchers Reginald Newell and Yong Zhu. They were studying the bands of rain that travel across the Pacific Ocean from Hawaii to California. These bands produce significant storms, which were once called “the Pineapple Express” or simply “Hawaii storms.” The term atmospheric river was later applied to similar bands of concentrated moisture in the sky, regardless of where they occurred.

Overview

Atmospheric rivers can occur in both the Northern and Southern hemispheres and are commonly found in latitudes between 30 and 60 degrees. The rivers form over oceans in the tropics and subtropical regions within the troposphere, where they transport large amounts of water vapor through the lower atmosphere. Once there, this water vapor is driven by weather forces to other parts of the world.

These rivers of the sky are usually found with a low-level jet stream, which is a narrow band of moving air currents. They are almost always associated with extratropical cyclones, or low-pressure regions that produce rapid changes in temperature and dew point, which is the temperature to which the air must cool in order for evaporated water to condense to form liquid water.

Atmospheric rivers are generally between 250 and 375 miles wide (402 and 603.5 kilometers) and carry varying amounts of water. The rate at which they carry water vapor is about the same as that of the Amazon River—about 176,000 tons per second. The water vapor travels through these rivers in the sky until it runs into a cold front or land formation such as a mountain range. Then, the river is forced upward, where the vapor cools and turns to liquid rain or snow and falls to the ground.

The phenomenon occurs most often in the winter months; atmospheric rivers are rare during the summer. In the United States, most atmospheric river-related events occur between September and March. In general, more northern latitudes will experience storms from atmospheric rivers in the fall, while more southern latitudes see these events during the winter months of December, January, and February.

Atmospheric rivers create weather events of varying intensity. Many produce relatively gentle storms that are an important part of an area’s climate. For example, California receives about half of its annual precipitation from events associated with atmospheric rivers. Without them, the area would experience extreme drought. However, atmospheric rivers can also produce damaging storms with excessive precipitation. Scientists have determined that the ten strongest atmospheric rivers to affect the Western United States caused nearly half of all flood damage there between 1978 and 2017.

Atmospheric river storms are a common occurrence in the US state of California, and are increasingly more violent and fatal.  For example, twenty-five atmospheric river storms struck the state in the first three months of 2023. In February 2024, California received five consecutive days of severe atmospheric river events, which left nine people dead. In early 2024, an atmospheric river dropped a month’s worth of rain on San Diego, California. Los Angeles received over 9 inches of rain, with over 562 mudslides being reported. In one day, an estimated $11 billion in damages were sustained throughout Southern California. Fifty-six atmospheric rivers impacted the western US during the late 2024 and early 2025. By April 2025, Northern California had experienced nine atmospheric river events.

A high level of damage is most likely to occur when an atmospheric river is slow-moving or stalls, bringing intense precipitation and winds that result in flooding, mudslides, and significant loss of property and possibly even life.

To help minimize the effects of storms caused by atmospheric rivers, scientists have developed ways to measure their strength and potential. They measure the integrated water vapor (IWV) and integrated vapor transport (IVT). IWV refers to how deep the rain or snow would be if it fell from the river. IVT is the total amount of water vapor carried in the atmospheric river. Scientists have established a scale of 1 to 5 for rating atmospheric rivers. Rivers with a score of 1 or 2 generally result in beneficial storms that help replenish water tables, while higher scores of 4 and 5 tend to produce flooding, extreme snowstorms, and other hazardous conditions.

To help protect property and lives, scientists have developed a number of ways to monitor and measure atmospheric rivers. They use satellites, measuring devices called radiosondes attached to weather balloons, and dropsondes, measuring devices dropped from an aircraft. Despite the available technology, however, it can be difficult to predict the result of a particular atmospheric river. This is because it can strengthen, move faster, slower, stall completely, change its angle, interact with other weather formations, including other atmospheric rivers, and change where it makes landfall.

Forecasters can usually predict the outcome of a storm caused by an atmospheric river up to five days before it occurs. Being able to predict the potential precipitation associated with an atmospheric river is also important because it helps in planning flood control and water reservoir operations. When heavy rain is forecasted, officials often open dams or release stored water to make room for the anticipated precipitation. If this does not occur, the released water will have been wasted and could potentially lead to drought conditions for the area.

While the term atmospheric river is generally used, not all scientists agree that the term is accurate. Some believe that the water vapor attributed to atmospheric rivers is actually the result of a continuous cycle of evaporation and condensation within an extratropical cyclone. Instead of a ribbon of water vapor moving from the tropics, these scientists say that the vapor forms as the weather event moves. They suggest that the phenomenon known as atmospheric rivers are actually the footprint of these cyclones as they move away from the tropical region.


Bibliography

“Atmospheric Rivers.” Water Education Foundation, 1 Dec. 2017, www.watereducation.org/aquapedia-background/atmospheric-rivers. Accessed 16 Apr. 2026.

Barmann, Jay. “Atmospheric Rivers, Explained.” SFist, 26 Jan. 2021, sfist.com/2021/01/26/atmospheric-rivers-explained/. Accessed 16 Apr. 2026.

Berwyn, Bob. “A Surge from an Atmospheric River Drove California’s Latest Climate Extremes.” Inside Climate News, 2 Feb. 2021, insideclimatenews.org/news/02022021/a-surge-from-an-atmospheric-river-drove-californias-latest-climate-extremes. Accessed 16 Apr. 2026.

“California Storm: After 5 Days, the Rain Has Stopped. In Its Wake, 9 Dead, a Trail of Destruction.” The Los Angeles Times, 8 Feb. 2024, www.latimes.com/california/story/2024-02-05/california-storm-weather-los-angeles-2024. Accessed 16 Apr. 2026.

Dacre, H. F., et al. “How Do Atmospheric Rivers Form?” American Meteorological Society, 1 Aug. 2015, journals.ametsoc.org/view/journals/bams/96/8/bams-d-14-00031.1.xml. Accessed 16 Apr. 2026.

Davis, William B., et al. “Track the Latest Atmospheric River to Hit the West Coast.” The New York Times, 16 Apr. 2026, www.nytimes.com/interactive/2024/us/atmospheric-river-california-west-tracker.html. Accessed 16 Apr. 2026.

Erdman, Jonathan, and Rob Shackelford. “What is an Atmospheric River? Explaining the Blessing and Curse.” The Weather Channel, 1 Nov. 2025, weather.com/science/weather-explainers/news/atmospheric-river-explained. Accessed 16 Apr. 2026.

“General Information About Atmospheric Rivers.” Center for Western Weather and Water Extremes, cw3e.ucsd.edu/wp-content/uploads/AR_FAQ’s.pdf. Accessed 16 Apr. 2026.

Hill, Alice C., and Tess Turner. “How Big a Climate Threat Are Atmospheric Rivers?” Council on Foreign Relations, 22 Mar. 2024, www.cfr.org/article/how-big-climate-threat-are-atmospheric-rivers. Accessed 16 Apr. 2026.

Mendez, Emely. “A Climate Expert Explains Why Atmospheric Rivers Are Causing Historic Rainfall in California.” Columbia Climate School, 5 Feb. 2024, news.climate.columbia.edu/2024/02/05/a-climate-expert-explains-why-atmospheric-rivers-are-causing-historic-rainfall-in-california. Accessed 16 Apr. 2026.

Steenburgh, Jim. “Atmospheric Rivers.” University of Utah, 31 Mar, 2017, www.inscc.utah.edu/~steenburgh/classes/5210/lecture_notes/AtmosphericRivers.pdf. Accessed 16 Apr. 2026.

“What Are Atmospheric Rivers?” National Oceanic and Atmospheric Administration, 21 Feb. 2025, www.noaa.gov/stories/what-are-atmospheric-rivers. Accessed 16 Apr. 2026.

Wulfeck, Andrew. “Mapped: What a Barrage of 56 West Coast Atmospheric River Events Looks Like.” Fox Weather, 23 Apr. 2025, www.foxweather.com/weather-news/atmospheric-river-recap-2024-2025-california-northwest. Accessed 17 Apr. 2026.

Zhou, Yang, et al. “Atmospheric River Frequency‐Category Characteristics Shape US West Coast Runoff.” Journal of Geophysical Research: Atmospheres, vol. 130, no. 2, 16 Jan. 2025, doi:10.1029/2024jd041805. Accessed 16 Apr. 2026.

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