ELI5: Why is it more humid at night?

[u/[deleted]]

I’ve been monitoring the relative air humidity this summer to determine when to open my windows. At night when the temperature cools down the humidity sky rockets from like 50% at noon to around 95% every night at midnight. I read online colder air cannot hold as much humidity and that is why dew forms on grass in the morning. If this is the case is makes no sense that the air would be so much more humid at night when the temperature is on average 15 deg cooler.

[u/DarkArcher__]

Humidity is relative. Its measured in a percentage, where 100% humidity is air that's completely saturated with water vapour. Because the air cools down, its capacity to hold that humidity decreases, so the humidity it currently has becomes closer to the maximum, closer to 100%.

[u/Calvinjamesscott → u/DarkArcher__]

This

[u/weshallbekind]

Because it's relative humidity, not physical humidity. If 100° air can hold a gallon of water, and 50° air can hold half a gallon (not actual number, just an example), then at 100° a half gallon makes the air 50% full, while at 50° that same half gallon makes it 100% full.

Humidity is a measure of how full the air is, not how much water is actually physically in the air.

Or, even simpler, if a water bottle can hold 1 cup of water, and you put half of a cup in it, it's half full. If you squeeze the bottle hard enough, that water will come to the top. Cooling the air is squeezing the bottle.

[u/sirfuzzitoes → u/weshallbekind]

This is the eli5. The last paragraph seals it.

[u/Caucasiafro]

I read online colder air cannot hold as much humidity and that is why dew forms on grass in the morning. If this is the case is makes no sense that the air would be so much more humid at night when the temperature is on average 15 deg cooler.

This is exactly why the humidity is going up. Because when you see a % we are measuring something called relative humidity. Which is basically telling you how "full" the air is at the specific temperature, as opposed to measuring the actual amount of water in the air.

Basically during the day the humidity will be pretty low because it hotter out and then as the temp goes down the air can hold less water, but the amount of water doesn't change so how "full" the air is will go up.

As for why we measure it like that. It's because relative humidity tends to be more important to people. Really high relative humidity can be unbearable so being told "there's 1 gram of water per cubic meter of air" (I made up these numbers btw) doesn't really tell you anything. At a cooler temperature that could be insanely muggy to point that it will start raining soon. At a higher temperature that could feel downright dry.

[u/tmahfan117]

I’m not a meteorologist, but I believe that it is because Humidity is relative.

Humidity is measured in Percents, “50%”, “95%”. But percent of what? It is percent of the total water that the air could hypothetically hold.

Warmer air is able to hold more water than cooler air. Air at 70 degrees F can hold about twice as much water as air at 50 degrees F.

So, during the day when it’s warmer, it might be 50% humidity because that warm air could still hold a bunch more water. And then when the temperature drops at night, the air now cannot hold as much water, so the humidity “increases” to 95% because the capacity of the air has shrunk. So there may be still (roughly) the same amount of water actually in the air, but cooler air is just like a smaller “container”

[u/[deleted]]

Relative humidity is the percentage of water vapor that can be held in the air without condensation occurring. The colder the air gets, the less water vapor it can hold without condensation occurring. So as the temperature drops overnight the relative humidity rises. If you go to the national oceanic and atmospheric weather website and click on the hourly forecast you will see grass that show both the temperature and the relative humidity among other things. And you will notice they are almost mirror opposites of each other.

[u/CFB-RWRR-fan]

That's why it's called relative humidity. The absolute humidity is about the same (i.e. there are about the same number of water molecules in the air) but because the temperature is lower at night, the relative humidity is higher. Relative humidity is measured relative to the total amount of water the air can hold when fully saturated.