So I think all good tornado talks need to start with an awesome tornado shot（发射）. And this is not that awesome tornado shot. All right. That was the first thing I ever saw. It was really cool and it was really scary. And the reason I was showing it to you guys was because that's why I got into the field in the first place.

  So even though it's a bad photograph, it was really cool to be out there the first time. But now I'm taking a real tornado footage（电影片段）. And fast forward a few years here. This is a few years ago during a field project called Vortex（旋涡） 2 where myself and a bunch（束） of other scientists were out there surrounding（围绕） tornadoes with different types of instrumentation and trying to figure out how tornadoes form. So it's a big question that we're trying to answer. It sounds like a very basic one, but it's something that we're still trying to figure（ 认为） out.

  We're also still trying to figure out what the winds are like near the surface. We know what the winds are like above building level, but we really don't know what the winds are like at the surface and how that relates to what we're seeing above building level. So most tornadoes form from what we call supercell thunderstorms. And these supercell thunderstorms are what you commonly（普通地） think of as tornado-producing storms. They're big, rotating（旋转） thunderstorms that happen a lot of times in the midsection of the United States. But the problem is that even because they're rotating up above doesn't mean they're rotating at the surface.

  And when we look at these storms, when you look at these pictures, when we look at the data（资料） that we have, they all kind of look the same. And that's really problematic（ 成问题的） if we're trying to make tornado forecasts（预测） or tornado warnings because we only want to warn on the storms or forecasts（预测） about the storms that are going to actually make a tornado. One of the big, critical（决定性的） distinguishing（区别） features that we think between these storms is something about the rear（后方的） flank downdraft. So these big, rotating thunderstorms have this downdraft that wraps（裹） around the rear（后方的） edge of it, hence（因此） the rear（后方的） flank（侧面） downdraft. But we think how warm that is, how buoyant（有浮力的） that air is, and then also how strong the updraft（上升气流） that's wrapping（裹） into makes a big difference on whether or not there's going to make a tornado or not.