Do not live near bodies of water. Also in the US south where we’ve had excessive heat warnings. I suppose the sun plays into it, but help me understand please. Thank you
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There are two kinds of humidity: relative humidity and absolute humidity.
Absolute humidity is the actual, physical amount of water in a certain amount of air.
What you see in weather forecasts is relative humidity and it’s a percentage of how “full” of water the air is. The maximum amount of water air can hold depends on its temperature. Warmer air can hold more water than colder air.
As temps drop, the actual amount of water in the air stays the same. But the maximum that the air can hold is decreasing. So relative humidity goes up.
Warm(er) air holds more moisture than cool(er) air. The same volume of air can hold more moisture at a higher temperature than at a lower temperature.
As an example, out here in the desert of New Mexico, it’s not unusual to have daytime humidtity in the 8% – 15% range around 95F and then overnight it jumps up to 25% – 35% at 70F with no other changes.
Relative humidity is the percentage of humidity in the air, compared to how much water the air can hold. Cold air cannot hold as much water as hot air. So if the amount of water stays the same, but the air cools off, then the relative humidity will then be higher.
So humidity (that you read on weather) Is typically relative humidity or how much water the air is holding relative to the maximum amount of water the air can hold at that temperature. As air gets hotter it can hold more water, as air gets cooler it can hold less water. So assuming no water condenses if it gets cooler the amount of water in the air stays the same, but the amount that the air can hold goes down causing relative humidity to rise.
The humidity is actually going *down*, but that means all the water in the air has to go somewhere.
If you put a bunch of air in a box and measure what percent of the *stuff* in that box is water, that’s absolute humidity. Relative humidity is how much water the air can hold before no more will evaporate into the air. To be more correct, water will still evaporate into the air, but it’ll condense out of the air at the same rate. When the relative humidity goes down, the moisture already in the air is going to condense out.
During the day, the air is getting hotter and hotter as the Sun blasts energy down to the ground. That warms up the ground, which warms up the air above it. Warm air can hold more moisture than cool air. By evening, the air has gotten as hot as it’s going to get, and absorbed as much moister as it can, and now everything is starting to cool down. As the air cools, the relative humidity starts falling, and the moisture condenses out.
That feels deeply uncomfortable because it means that your sweat can’t evaporate off of you. You’re still sweating, but with the air saturated, the water on your skin just stays there, accumulating more moisture.
What we think of as “humidity” in the US is more accurately called “relative humidity” as it’s a measure of how moisture-saturated the air is. Specifically, it’s calculated from the temperature and the air pressure; so as the day cools and the temperature drops faster than the pressure does, the air loses some of its ability to hold water and the relative humidity rises.
It helps to picture “air” as having “space” in between. The higher the temperature, the more space there is between the air. An imperfect but nice to picture example is water vapor vs liquid water vs ice.
The more space, the more room there is to hold water. Relative humidity is a ratio between how much water is in the air vs how much water the air can hold. If you reduce the temperature, the space between the air shrinks and you can hold less water
Several people have already answered here (hot air can hold more water than cold air), but I thought I’d share an interesting consequence of this. If you reach 100% humidity, it’s hard for the temperature to go down. If the temperature lowers, the air can hold less water, the water condenses, water condensation releases heat, the air heats up and re-absorbs the water, this is a cooling process… and on and on.
So humidity is the measure of how much water can be suspended in air. At 100% the air is full and it rains or condensates on everything.
The amount of water the air can hold is dictated by the temperature. Warm air holds more and cold air holds less.
So if you have 50% humidity at 90 degrees you could have double the water in the air before it rains.
If the temperature drops that doesn’t mean the water has to leave so at 60 degrees you may be only able to hold 70% of what you could at 90.
That would mean the humidity would raise to ~72%>
Because air can hold 7% more water per degree centigrade. So as the temperature drops, the relative humidity goes up.
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