More than 70 tornadoes were reported over the weekend in the Midwest and Great Plains of the United States, killing at least three people, with at least ten more still missing and significant damage in Arkansas and Texas.

More tornadoes are expected in the coming days. The fierce storms can happen at any time of year but are most common in May and June in North America, when atmospheric conditions tend to be most right for their formation. Tornadoes remain deadly and relatively unpredictable, despite recent advancements in weather science.

Here’s what scientists do know:

A tornado, or twister, is a violently rotating column of air that extends between the Earth's surface and a cloud, usually a cumulonimbus cloud. Most tornadoes last