Humidity is not humorous… But what really *is* humidity? Let’s discuss! Do I get a point for discussing DEW POINT? This heat wave has me thinking of when I was a kid, and got confuddled by the meaning of the term HUMID! And how about that phrase HEAT INDEX, which to me seemed  always to perplex… And what about DEW POINT, what’s its point?Do I get a point for discussing DEW POINT? This heat wave has me thinking of when I was a kid and confuddled by the meaning of the term HUMID and that phrase HEAT INDEX which to me seemed to perplex…

Humidity has to do with how much water’s in the air. When I was a little kid and heard meteorologists talk about 100% humidity, I’d get really confused because wouldn’t that mean we’d basically be in a swimming pool? There are at least 2 problems with that confused me’s thinking:

  1. “water” isn’t always a liquid &
  2. that % humidity is RELATIVE HUMIDITY (RH) – it tells you, for the current temperature, how much water’s currently in the air vs the max amount of water that could be in the air. It’s related to, but different from DEW POINT, which tells you, for how much water’s currently in the air, how how low would you have to drop the temp before dew starts forming (water vapor (gas) condenses into liquid water)  

The closer to the dew point, the higher the humidity and the HEAT INDEX takes into account the RH & the dew point to tell you what temperature it “feels like”

When we say “water” we usually think of *liquid* water – if you ask for a glass of water & your waiter brings you a cup of ice or a sealed vial of water vapor you’d probably be upset (or at least confused). But those are water too, just with more (gas) or less (ice) energy. Water’s just the non-science-sounding word for dihydrogen monoxide – 1 (mono) oxygen bonded to 2 (di) hydrogens).

These bonds are covalent bonds, formed by sharing electrons (negatively-charged subatomic particles). Water’s really “sticky” because the oxygen doesn’t share fairly – it hogs electrons, making it partly positive & the H’s partly negative – and opposites attract so they like to hang with oxygens from other water molecules. But if the molecules get enough energy to overcome these attractions they can jiggle free from these intermolecular (between-molecule) forces (IMFs) & escape into the gas phase. More on such transitions:

In liquid water, the water molecules have enough energy to slide past one another, but they can’t fully “escape” into the air because they’re close together & on the way out they bump into other molecules that they bind to -> so molecules in a LIQUID are moving around but forming & breaking bonds as they go (kinda like a swing dance) – and a really fast one! bonds are constantly breaking and reforming. When a molecule breaks free it normally forms a new bond (with that same water molecule or a different one) within 0.1ps (picoseconds (1 trillionth of a second!)

But add more energy and the molecules are moving so fast they don’t have time to get “caught” before they leave -> they escape into the air as a GAS. Once they’re in the air and out of the close vicinity they can move far away from other water molecules so they don’t have to worry about getting caught any more

Temperature is a measure of the average amount of energy the molecules have. The warmer it is, the more energy the water molecules in liquid water get, and thus the more likely they are to escape. And the more energy the water molecules in the gaseous, water vapor, phase get, the more likely they are to be able to stay gassy – they can run far away from the liquid surface and zip around so quickly they can’t get “caught” by other water molecules in the air.

But at the same time, those water molecules, while tiny, still have mass & as more and more water escapes the liquid, it weighs down on the liquid water, making it harder for liquid molecules to escape – they have more external pressure to overcome. And there are more water vapor molecules banging back into the water’s surface, where they can get trapped.

Eventually a dynamic equilibrium is reached where the rate water molecules are leaving the liquid water & entering the air (evaporation) equals the rate water molecules are leaving the air & entering the liquid (condensation). So even though water molecules are still moving and interchanging, there’s no net effect. 

In high humidity, you’re closer to equilibrium being reached meaning you’re close to the point where you can’t evaporate more unless you condense an equal amount.

So, due to this and rain replenishment, the whole ocean doesn’t evaporate to nothingness (for which the fishies are grateful). But this pressure also means that it’s harder for your sweat to evaporate. And this makes you feel hotter. This is because when the water evaporates, it uses a tiny bit of all the heat to break free. And tiny bits add up to big bits so you can actually feel a cooling effect. 

How much water is in the air vs. the liquid when equilibrium is reached depends on the temperature. You might hear it said that “Warm air can hold more moisture” – but this is a misleading  statement because the “air” itself has nothing to do with it. In a mixture of gases, the molecules are all moving so fast & are all so far apart that whatever other air molecules (O2, N2 etc.) are in there don’t matter to the water molecules. So there’s plenty of *room* for water in the air – the problem’s getting there (evaporation) and staying there. 

When you have higher temps, more liquid water molecules have the energy needed to escape to the gas form. And if they collide with other molecules while there, they have a better chance of being able to re-escape if caught. The more water molecules in the air the higher the humidity.

But it will still “max out” which is called 100% relative humidity (RH). It’s called *relative* humidity because it’s the percent of water in the air vs the amount that theoretically could be there at that temperature. 

If you had 100% absolute humidity, on the other hand, you’d be saying the “air” is all water. Absolute humidity is a measure of how much water is actually in the air. It’s just a measurement, not a calculated value.

Specifically, absolute humidity is the mass of water divided by the mass of dry air in a certain volume, usually reported in (g/m3)(grams of moisture per cubic meter of air). Relative humidity is the current absolute humidity divided by the maximum possible absolute humidity. 

Another way to talk about how much water’s in the air is the partial pressure which is the portion of the total atmospheric pressure that’s contributed by the water. So we can also define RH as the ratio of the current water vapor pressure (Pw) to the saturation water vapor pressure (Pws) at that temperature: RH = 100% * Pw/Pws

The absolute humidity is related to something called the DEW POINT which, instead of a concentration is a temperature. The DEW POINT says – say you take a sample of the current air with however much water vapor’s in there. Start taking away the water’s energy by decreasing the temperature. how low would you have to go in order for the water vapor that’s currently in the air to start going back to liquid form (condensing into “dew”)? 

If there’s a lot of water in there, you won’t have to lower the temp much before the crowded water molecules start colliding with one another, now with insufficient energy to break free again. So the higher the dew point, the more moisture is in the air.

But if there’s not much water in there, even if you slow the water molecules down a bit they’re still a lot less likely to collide with other water molecules so you have to lower the temp a lot more to get dew to form. So the lower the dew point, the less moisture is in the air. 

Note that with a fixed amount of moisture like this, when you change the temperature, you don’t change the dew point, but you DO change the relative humidity. As you get closer to the dew point you increase the relative humidity because the air’s getting more and more crowded. In fact, what you’re trying to do is find the temperature at which you have 100% RH. 

dew point = temperature at which, for a certain amount of moisture, you have 100% relative humidity 

The dew point can be more useful than relative humidity for telling you how humid it feels because it tells you about how much moisture is actually there, and that’s what you really care about/feel when you’re in a heat wave – you care about how drenched you are not how drenched you could be – it’s not one of those “be grateful because it could be a lot worse” things. 

The closer the actual temperature is to the dew point, the muggier it feels. The HEAT INDEX combines temperature & dew point to tell you the temperature it “feels like.” There are a couple different formulas used & NOAA puts out a heat index graph you can consult – but when using keep in mind that the chart refers to temperatures in the shade and if you’re in the direct sun you’ll feel even hotter. But if it’s windy, it can help. 

This post is part of my weekly “broadcasts from the bench” for The International Union of Biochemistry and Molecular Biology. Be sure to follow the IUBMB if you’re interested in biochemistry (@the_iubmb)! They’re a really great international organization for biochemistry.

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