Ever heard the phrase ‘It’s not the heat, it’s the humidity’? People say this because humidity, which is the amount of water vapor in the air, can make hot temperatures even more unbearable than they already are.We hear a lot about relative humidity in weather reports. This is the amount of water vapor in the air relative to what the air can hold. Let’s say that a certain parcel of air can hold 30 grams of water vapor per cubic meter of air, but it only has 15 grams of water vapor per cubic meter of air. We simply divide the amount of water present by the amount of water possible, so 15 divided by 30, and then multiply it by 100 to get a percent. So in this case, 15 / 30 = 0.5, multiplied by 100 gives us 50%. So the relative humidity is 50%, which means the air contains about half of the total water vapor it could possibly hold.Humidity plays an important role in both weather and climate on Earth, and we should be thankful that it does because it makes our planet hospitable for us! Water vapour is a key greenhouse gas, which helps block harmful UV rays from the sun as well as traps heat on Earth that makes life possible.Water vapour in the air comes from water on land and in the oceans. It evaporates into the air, which is an important part of the water cycle on Earth. Without this step, we would have no rain or snow, which help cycle water back to the ground and feed our rivers, lakes and streams that we depend on for drinking water.However, because it is such a powerful greenhouse gas, water vapour can be quite the double-edged sword. Like a nice blanket on a cold day, we depend on water vapour in the atmosphere to give us just the right amount of protection and warmth. But if someone comes along and adds more blankets on you when you don’t need them, this will likely make it too warm for you to be comfortable. The same thing happens to Earth when too much water vapour is in the air, but Earth can’t remove the extra blankets and simply continues to warm all over. This feeds a vicious cycle on Earth because as the planet warms, the humidity increases. But, with more humidity comes more warming, which then leads to more humidity and so on.