A ‘dryline’ is triggering severe weather in the Plains. Here’s what that means.

Imagine the most oppressively-humid Florida-like weather you can. It might feel like you’re swimming through it. It’s “air-you-can-wear” — walking outside feels like a sauna.

Now imagine the type of bone-dry air you’d feel in the dead of winter, where your hands crack and your eyes sting.

Now imagine these air masses are only about 10 miles apart. It sounds wacky, but weather is weird sometimes. The zone between the exceptionally humid and the uber-dry is called a dryline.

If you live on the Plains, you probably hear the word dryline a couple times a week during the spring. It’s an integral part of the forecast, and plays a big role in Tornado Alley’s characteristic severe weather.

The dryline is the boundary between arid air from the desert Southwest and humid air streaming northward from the Gulf of Mexico. Relative humidity can easily drop 80 percent or more over just a few miles as the dryline sloshes back and forth. It’s a semi-permanent feature during the spring months, becoming more sharply defined at some times than others. But it’s the violent clash between the two air masses that can spawn some of the most vicious storms on earth.

A dryline acts a bit like a cold front. The cold front causes thunderstorms because cool, dense encroaching air nudges the warm air ahead of high up in the atmosphere. Drylines behave similarly. Dry air is much denser than moist air. Water molecules have a lower density than the stuff dry air is made of (mainly nitrogen), so the more water molecules in the air, the less dense it becomes.

So the dense, dry air kicks up the warm, moist air when it surges in. The dryline is commonly known for spinning up supercell thunderstorms across Tornado Alley — exactly as it will do this week.

This week, dew points are surging into the upper 60s in Texas, Oklahoma, Kansas, and parts of Nebraska. As strong winds from the south draw in tropical air, it will start to feel like a sauna. Just west, however, a very dry blob of air is building in with dew points in the teens and single digits. Over the Texas Panhandle, each cubic meter of air contains the equivalent of just over two grams of liquid water. To the east, that number is eight times higher.

Unlike with cold front storms, it doesn’t get cold once the dryline storms pass, because the triggering air mass is hot.

Most of the time the dryline erodes as summer takes hold. Drylines tend to be very springlike features of the Great Plains.