The water cycle is often taught as a simple circular cycle of evaporation, condensation, and precipitation.
Although this can be a useful model, the reality is much more complicated. The paths and influences of water through Earth’s ecosystems are extremely complex and not completely understood.
Liquid water evaporates into water vapor, condenses to form clouds, and precipitates back to earth in the form of rain and snow.
Water in different phases moves through the atmosphere (transportation).
Liquid water flows across land (runoff), into the ground (infiltration and percolation), and through the ground (groundwater).
Groundwater moves into plants (plant uptake) and evaporates from plants into the atmosphere (transpiration).
Solid ice and snow can turn directly into gas (sublimation).
The opposite can also take place when water vapor becomes solid (deposition).
Atmospheric rivers are relatively long, narrow regions in the atmosphere – like rivers in the sky – that transport most of the water vapor outside of the tropics.
These columns of vapor move with the weather, carrying an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River. When the atmospheric rivers make landfall, they often release this water vapor in the form of rain or snow.
Although atmospheric rivers come in many shapes and sizes, those that contain the largest amounts of water vapor and the strongest winds can create extreme rainfall and floods, often by stalling over watersheds vulnerable to flooding.
These events can disrupt travel, induce mudslides and cause catastrophic damage to life and property.
A well-known example is the “Pineapple Express,” a strong atmospheric river that is capable of bringing moisture from the tropics near Hawaii over to the U.S. West Coast.
Not all atmospheric rivers cause damage; most are weak systems that often provide beneficial rain or snow that is crucial to the water supply. Atmospheric rivers are a key feature in the global water cycle and are closely tied to both water supply and flood risks — particularly in the western United States.