If the forecast high temperature in your city is 36 degree Celsius (96.8 degree F), but the heat index makes you feel like it’s 42 degrees (107.6 degree F) – the humidity is to blame. But why does humidity make it feel hotter?

Short Answer: The body releases sweat to cool itself off. Each time the sweat evaporates off the skin, it takes away a bit of heat from the body. But during high humidity days, the sweat cannot evaporate off the skin as the air is already saturated with water. Hence you get hotter.

In case you want a longer explanation, Hank Green from SciShow has got you covered.

Our body’s built-in thermostat likes to keep our internal temperature at a steady 37 degrees Celsius so that all of our enzymes can function properly in order to keep us alive. When we need to cool off, our blood vessels open up, letting blood flow to our arms and legs so heat can escape through skin. Our sweat glands also start to produce moisture, which takes a bunch of heat with it when it evaporates. But, when it’s humid out, the effectiveness of sweat evaporation over the skin is reduced. So we end up holding onto the extra heat, and we just get hotter.

Well, it’s amazing how our skin helps us regulate our body temperature quite effectively by evaporation, even when sweating is not involved.

Humidity is the amount of water vapour present in the atmosphere. Although it causes you to feel hotter, it plays a crucial role in our daily weather. Most of the observable weather phenomena such as rain, storms, thunder, or lighting all happen because of humidity. Also, did you know without precise knowledge of humidity, it’s impossible to make a weather forecast?

We usually hear about the term relative humidity in the forecast. It tells us how close the air is to saturation, where there’s as much water vapor in the air as possible. But as Hank explains, the relative humidity forecast isn’t that useful for figuring out how hot it’ll feel, because the saturation point increases with the temperature. The higher temperature increases gets, the more energy water molecules have. Which means more of them can zoom around in the gas phase.

Let’s say you have two days where the relative humidity is 50%. If the first day forecast is 30 degrees Celsius and 35 degrees the second day, there’s more moisture in the air on the second day, because it’s 50% of a much higher saturation point. The humidity on the second day is going to feel even worse.

When it comes to forecast, the dew point is a lot more useful than the relative humidity. Dew point is the temperature at which the amount of water vapour in the air becomes saturated and condenses into a dew or fog. It tells us the amount moisture present in the air in absolute terms. So the higher the dew point, the grosser you’ll feel, because more moisture is present in the air, or atmosphere.

On the first day where the forecast was 30 degrees out with 50% relative humidity, the dew point would be 18 degrees. On the second day, though, the dew point would be 23 degrees. It would be steaming hot, even though the relative humidity would still be 50%.

According to meteorologists, as long as the dew point temperature doesn’t go beyond 18 degrees, you’ll be comfortable. If it does, it will start to feel muggy and quite uncomfortable. After all, “it’s not the heat, it’s the humidity!”

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