What are Atmospheric Rivers?

About Atmospheric Rivers:

• Atmospheric rivers are long, concentrated regions in the atmosphere that transport moist air from the tropics to higher latitudes.
• They are responsible for 90 percent of the movement of moisture from the tropics toward the poles. 
• They are the largest transport mechanisms of freshwater on Earth. 
• They form when large-scale weather patterns align to create narrow channels, or filaments, of intense moisture transport.
• Ten or more atmosphericrivers can be happening at once across the globe.
• A well-known and strong one is the Pineapple Express, with moisture transported from the tropical Pacific around Hawaii to the US and Canadian West Coasts.
• Features:

o Typically,250 to 375 miles wide, atmospheric rivers can stretch more than a thousand miles long.

o They are more frequent on the East Coast than they are on the West Coast.

o They primarily occur during the winter of the respective hemisphere, when extratropical cyclones are most prevalent. 

o The average atmospheric river carries an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River (second-longest river in North America).

• When atmospheric rivers run up against mountains or run into local atmospheric dynamics and are forced to ascend, the moisture they carry cools and condenses, so they can produce intense rainfall or snowfall.
• While they are an incredibly important source of rainfall, they can also bring flashflooding, mudslides, and landslides, sometimes killing people and destroying property.

· When atmospheric rivers pass over land, they can cause conditions similar to those of hurricanes with intense and rapid rainfall, cyclone-force winds, and significantly increased wave heights. 

Q1: What are Tropics?

The Tropics are a region of the Earth, roughly defined as the area between the tropic of Cancer and the tropic of Capricorn. Although topography and other factors contribute to climatic variation, tropical locations are typically warm and experience little seasonal change in day-to-day temperature. An important feature of the Tropics is the prevalence of rain in the moist inner regions near the equator, and that the seasonality of rainfall increases with the distance from the equator.

Source: Another 'atmospheric river' storm to hit California with potentially deadly floods