You don’t have to hop on a plane to visit the tropics when you live in the southeastern United States—it can often feel like you’re already there. It gets humid in this corner of the country. Not just regular humid, mind you, but so disgustingly moist that you can almost feel the air slosh across your skin as you walk out the front door. But what is it that makes the Southeast so humid compared to the rest of the country?

Mugginess during the summer is a problem just about everywhere you go in the United States. The corn fields of Iowa can see a higher dew point than a rainforest. But even there the steaminess doesn’t last as long as it does in the Southeast. Much of it has to do with the region’s proximity to the Gulf of Mexico, which usually feels like bathwater on a good day. The water in some parts of the Gulf of Mexico can heat up to 90°F during the peak of the summer, and the water isn’t quick to cool down once cold fronts start sweeping through in the fall and winter. The warmth of the Gulf and the Caribbean Sea to its south keep moisture in plentiful supply.

It’s not just the water itself that contributes to the mugginess. The water vapor over the ocean doesn’t migrate inland on its own—weather patterns drag it inland and keep it locked in place. Strong winds blowing counterclockwise around low-pressure systems often help bring this tropical moisture inland, especially during the cooler months when you expect to shiver rather than sweat. During the summer, though, persistent ridges of high pressure keep the southeast feeling gross with a moist southerly wind. These “heat domes” deflect most weather systems approaching from the west, basically locking the Southeast into a state of sultriness for weeks and even months at a time.

The constant moisture isn’t merely uncomfortable—it can be downright dangerous. The human body is able to cool itself when sweat evaporates from the surface of exposed skin. But sweat has a harder time evaporating when there’s too much moisture in the air, which could cause a person to overheat. This phenomenon is measured with the heat index, and it’s the cause of thousands of heat-related illnesses and deaths every year.

All of that moisture makes both day and night downright miserable. If you’ve ever been to the desert during the warmer months, you know firsthand that even on a day when the high temperature exceeds 100°F, the mercury can plummet once the Sun goes down and get chilly enough to require a light jacket. The wild temperature fluctuations in desert regions are due to the dryness of the air there. Moist air has a higher heat capacity, so it takes a lot longer to warm up and cool off.

Meanwhile, the gross humidity levels in the Southeast on most days keep it from getting excessively hot, but it also keeps the nights from cooling off very much. The lack of nighttime relief compounds the danger posed by heat and humidity.