We have underground rivers, like the one in Puerto Princesa which was dubbed the 7th wonder of the world, but I did not know there are also aboveground rivers. Yes, there is water flowing like a river up there in the sky. They are called atmospheric rivers. The recent heavy rains and flooding in the state of California in the United States is caused by this weather phenomenon.
According to the National Oceanic and Atmospheric Administration (NOAA) of the US, atmospheric rivers are relatively long, narrow regions in the atmosphere 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 in the U.S. When the atmospheric rivers make landfall, they often release this water vapor in the form of rain or snow.
Another agency, the United States Geological Survey (USGS), said that the formation of an atmospheric river starts near the equator. The sun heats the earth most directly at the equator, and these warm temperatures cause water to evaporate and rise into the atmosphere. Some of that water vapor is pulled away from the equator by atmospheric circulation, forming a narrow band that transports the water vapor to other regions like a conveyer belt.
Did you know that the Philippines, being near the equator, is one of the sources of atmospheric rivers? I read an article written in March 2016 by former U.S. Vice President and climate activist Al Gore where he said that the rapidly warming ocean water surrounding the Philippines is the origin of the “atmospheric river” which fueled the record downpours that flooded the American South that year. Al Gore mentioned two atmospheric rivers that affected the U.S.- the Pineapple Express which hit California and the Mayan Express which struck the South-Central U.S.
Atmospheric rivers flow in the lowest part of the atmosphere, only about half a mile (800 meters) to a mile (1600 meters) above the ground. When they reach the coasts and flow inland over mountains, the atmospheric river is pushed upwards, causing much of that water vapor to condense and fall to the ground as rain or snow, creating an atmospheric river-driven storm.
On average, Atmospheric rivers are 400-600 kilometers wide. They move with the weather and are present somewhere on the Earth at any given time. They can be hundreds to thousands of kilometers long, and though they are narrow in the context of weather systems, "narrow” can mean up to 483 kilometers across.
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. Most of them however 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.
Since Atmospheric rivers are fueled by heat, climate change will most likely make them worst in the future.