Atmospheric river

An atmospheric river is a long narrow band or filament of condensed water vapor that moves through the lower portion of Earth’s atmosphere. Research indicates that these formations can transport fifteen times the amount of water in the Mississippi River. Atmospheric rivers collect water in tropical 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, picking up moisture vapor and carrying it into the lower portion of the atmosphere known as the troposphere. Once there, this moisture 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 to 603.5 kilometers) and carry varying amounts of water. The rate at which they carry moisture vapor is about the same as that of the Amazon River, or 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. In early 2024, an atmospheric river dropped a month’s worth of rain on San Diego, California, in one day and reportedly caused an estimated $11 billion in damages throughout Southern California. A high amount 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 water vapor transport (IWT). IWV refers to how deep the rain or snow would be if it fell from the river. IWT 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 operation. 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 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 phenomena known as atmospheric rivers are actually the footprint of these cyclones as they move away from the tropical region.

Bibliography

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