You may have learned in science class that lightning is caused by ice in the clouds. However, lightning is hot, and ice is cold, which makes this fact a bit confusing. What gives?
The theory is that grains of ice bump against each other in large updrafts inside clouds, knocking off electrons and becoming positively charged. The positives move to the top of the cloud, and the negatives to the bottom, setting up a static charge.
This theory is still debated, though. In fact, some scientists from the National Space Science and Technology center in Huntsville, Alabama, decided to test this idea. They used a satellite called the Tropical Rainfall Measurement Mission, or TRMM, to gather data on a million clouds, literally.
They used radar to tell how much ice was in a cloud, and used an optical sensor to count how many flashes of lightning occurred. If lightning really is caused by ice, the two should go together.
In fact, they did. The researchers found flashes were correlated with ice in over 99% of the cases. This strongly supports the theory that ice pellets jostling together inside clouds cause a static build-up which is released as super-hot lightning.