三伏天是一年中最难熬的日子，不仅是因为三伏天气温高，还因为三伏天湿度大。空气中的湿度大为什么会让我们感觉更热呢？

Humidity refers to the amount of water contained in the air. As the air's temperature changes, so does the amount of water the air can hold.

湿度指的是空气中所含的水分。随着气温变化，空气中的水分含量也会变化。

One of the ways your body cools itself is by sweating. The sweat then evaporates from your skin, and it carries heat away from the body as it leaves.

身体自我冷却的方式之一就是出汗。汗水从你的皮肤表面蒸发时，会带走身体的热量。

However, humidity throws a wrench in that system of evaporative cooling. As relative humidity increases, the evaporation of sweat from your skin slows down. Instead, the sweat just drips, which leaves you with a damp T-shirt and none of the cooling effect. When the humidity spikes, we effectively lose a key tool that would normally cool us down.

然而，湿度却会让蒸发冷却机制失灵。随着空气中的相对湿度增大，汗水从皮肤表面蒸发的速度则会减慢。停留在皮肤表面的汗水会把你的T恤浸湿，而且一点也不会让你感到凉快。当湿度很高时，我们实际上失去了一个自我降温的法宝。

When we think of wind on a hot day, we think of a nice, cooling breeze. That's the normal state of affairs, but when the weather is really, really hot—think high-90s hot—a dry wind actually heats us up. When it's that hot out, wind actually draws sweat away from our bodies before it can evaporate to help cool us down. Thanks to this effect, what might have been a cool breeze acts more like a convection oven.

当我们想到热天里的风，我们想到的是清凉宜人的微风。正常情况下是如此的，但是当天气非常非常热的时候（比如高于32摄氏度时），干燥的风其实会让我们更热。在天气很热时，风会在汗水尚未蒸发时就将其吹干，从而让我们无法借助汗水蒸发来降温。也正因如此，大热天里的风不仅不能带来凉意，反而更像是烤箱的对流加热。

However, being in the shade can make quite a big difference. Direct sunlight can add up to 15 degrees to the calculated heat index.

不过，在阴凉处则会有很大不同，因为阳光直射会让炎热指数上升15度之多。