A single river can carry three times as much water as the entire Mississippi, they provide almost half the annual rain fall for parts of the American Pacific coast, but when they remain in one area for a day or more, they can produce violent flooding.

Heavy winds, lots of water vapor, a strong * not moving fast, adds up to a big flooding.

The rivers were only recently discovered because they form over the world's oceans, we still don't have enough data to forecast their floods. 

An example of the challenges we face in weather prediction is that while this model can actually produce an atmospheric river like we see right here, a week ago today we looked at the forecast for today, and it said there should be a strong atmospheric river creating heavy rain in northern California, while instead it is in Canada, it's a huge difference.

To fill the gap in data, Ralph explores the rivers on research lights, the rivers form in front of banks of cold air, they creat channels of wind that carry huge quantities of water vapor from the ocean to the coast at up to 100 miles an hour.

These are like conveyor belts for water vapor, they are very narrow, and yet they do 90% of the water vapor transport for the global atmosphere at those altitudes.

When the air flow hits a mountain, it rises up, shedding its water as rain and snow, Ralph has tracked how the rives move with the changing seasons.