1. Fans don’t cool the room temperature, so turn them off when you leave.

Box fans; ceiling fans; drum fans; tower fans — they are great at keeping you cool, as long as you are in the room. If you leave them running when you’re not home, the only thing they do is waste energy.

Sweating often feels gross, but it’s the body’s natural air conditioning. When water evaporates, it literally absorbs the energy (heat) off your body. It is a cooling process, which is why when there’s a breeze, whether it’s summer or winter, we feel cooler.

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Fans make us feel cooler because they create a breeze, which evaporates the moisture on our skin more efficiently than stagnant air.

2. Don’t rely on relative humidity — check out the dew point.

This is counter-intuitive, we know, but relative humidity is pretty useless for actually determining how humid it is. Many humid days may only reach 40 or 50 percent humidity, which sounds pretty dry.

Instead, we use the dew point to figure out how much moisture is in the air. It’s the temperature the air must cool to in order for dew to form, or the temperature at which the air is “saturated” with moisture — when it can’t get any more humid. Any dew point below 60 is pretty comfortable, but once you start getting near the 70-degree mark, things can get oppressive quickly.

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3. Cotton is not the only fabric that can keep you cool and dry on a hot day.

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Cotton is a lovely, natural fabric. It’s breathable and absorbs water, which prevents sweat from building up on your skin. It also has the advantage of being comfortable and tends to be hypoallergenic. But there are a few other fabrics that you might consider on a hot and humid day.

If you can deal with wrinkles, linen is a great option. It’s made from the natural fibers of the flax plant. It also doesn’t feel as heavy as cotton does when it absorbs sweat.

Rayon is another possibility, especially in a dry heat (or if you’re not a heavy sweater). It’s a synthetic fiber that does not absorb moisture, but it’s extremely light-weight. The downside to rayon is that it also wrinkles easily, and it tends to be a pain to launder.

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A polyester blend is ideal if you’re going to be active in the heat. Although the synthetic material doesn’t allow your skin to “breathe” as well as cotton or linen, it dries incredibly fast, which means you won’t be running or hiking in a heavy shirt bogged down with moisture. These materials tend to be the go-to hot weather choice for athletes.

4. There’s no such thing as “heat lightning.”

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When lightning flashes, there’s always a thunderstorm somewhere to blame. It might be clear overhead, but what you’re seeing is the lightning from a far-off storm that’s beyond the horizon. Often these storms are so far away that you won’t be able to hear the thunder. Things like wind direction, terrain and precipitation can all influence whether you hear thunder from a distant storm.

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5. Despite how it feels, humid air is actually less dense than dry air.

When the humidity gets high, the air seems dense. It feels like you can cut it with a knife. But in reality, humid air is actually less dense than dry air. It sounds crazy — how can air become less dense if we add more water vapor to it?

The density of air is measured using its mass and its volume. And gas (air, in this case) is a special thing because a specific volume of gas will always have same number of molecules in a fixed volume, no matter what kind of molecules we add.

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As it turns out, water molecules are lighter than nitrogen and oxygen. So if you pump more water molecules into the air, which pushes out the heavier molecules, the air becomes less dense.

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This phenomenon also makes physical activity even harder on hot, humid days — there’s less oxygen to breathe.

6. Washington is not miserably humid because it was built on a swamp.

We hear this one all the time. For whatever reason, people seem to think that Washington was built on a swamp. It wasn’t, but even if it had been, it would not affect our weather hundreds of years later.

“Within the original city’s boundaries (the area south of Florida Avenue), only about 2 percent of the total area fits the definition of a swamp,” writes Don Hawkins, a Washington historian. “It was almost entirely laid out over well-drained terraces and hills. In fact, for a riverside site, it was amazingly free of swampiness.”

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There is one area that flooded periodically back in the day, and thus was moderately swampy — the region at the base of Capitol Hill. Early Washingtonians tried to solve this flooding problem by building a canal, but it was poorly constructed, Hawkins says. They fixed the problem not by repairing the canal but by asking private developers to drain the land.

What’s there now? The Reflecting Pool.

7. This is normal — D.C.’s hottest weather typically comes in July.

In the grand scheme of things, this is pretty normal. For many locations on the East Coast, July tends to be the hottest month. In Washington, the average high temperature is 88.4 degrees in July. In August, that falls to 86.5 degrees.

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