First Alert Weather In-Depth: A river in the sky
ROCHESTER, N.Y. — Over the past few years, a new weather term has caught the eye of many, and the term is “atmospheric river.” Fun fact, it isn’t a “new” term, as it has been around for a long time. What is an atmospheric river? Is it only unique to a certain part of the world?
An atmospheric river is exactly what you think it is, a river in the atmosphere. Just like a flowing river on the surface of the Earth, an atmospheric river holds a mass amount of water in the form of water vapor within a small stream. Just as the Mississippi or the Amazon, these atmospheric rivers can stretch for thousands of miles.
For example, atmospheric rivers are more known in the Western U.S. and typically stretch from Hawaii to the West Coast, but they can occur anywhere around the world. If you have ever hear of the “Pineapple Express” that is the local term for the moisture stream that stretches from Hawaii to the West Coast. For an atmospheric river to form, you’ll need a strong area of low pressure which pulls an immense amount of moisture and terrain to lift the air and squeeze out as much rain/snow as possible. They are more well known in the Western U.S. because of the large terrain changes within a short distance.
As the stream of water vapor interacts with the mountains of the Sierra-Nevada and Cascades the moisture is forced to lift, cool, and condense. This process will form clouds and eventually rain or snow. Just like lake effect snow that we see locally, the greater the elevation the greater the lift. The terrain influence on the West Coast helps to squeeze as much moisture out of the atmosphere as possible when the atmospheric river targets the west coast.
Think of it as wringing out a wet rag. When the rag is dry and no moisture is available, nothing will be squeezed out. However when the rag was just used and you wring it out a bucket of moisture can fall out. Depending on the duration of these events, over 10 inches of liquid precipitation can be squeezed out even over 100 inches of snow can fall within just one atmospheric river. To end, atmospheric rivers aren’t just unique to the Western U.S. as we see them in the Eastern U.S. too.
During anytime of the year with a strong enough storm we can have moisture stretch from the Gulf of Mexico to Rochester, New York. As mentioned above though, the terrain influence isn’t as strong. So, we don’t see the full brunt of what the moisture holds, but a good amount of rain or snow can fall locally.