It’s usually more humid in the summer because there’s more water vapor available.
I don’t mean that the oceans, lakes and rivers somehow expand in the heat. (Well, maybe a tad.) More precipitation? Perhaps. But it’s not the amount of water itself; the humidity can be quite low over the middle of the ocean. What counts is how much water is being converted to vapor (or gas). It is more humid in the summer because the water supplies, the oceans, lakes, rivers and rains, are warmer, and water has a greater proclivity for making vapor when its temperature is higher.
Notice that I have said nothing at all about the air or its ability to “hold water.” Humidity is the water vapor that comes from water, whether there is any air present or not. When we say, “It’s humid today,” we assume that the “it” in question is the air because, after all, what else is there? But the air plays no role whatsoever in humidity; like Mount Everest, it is simply “there.” It is a bystander.
Think of it this way: We happen to be immersed in a sea of air, just as fish are immersed in a sea of water. If somebody suddenly dumps a load of red ink into the ocean a fish might say, “My, but it’s red today.” But “it” isn’t the water itself; “it” is the ink that has been mixed into the water. Well, humidity is water that happens to be mixed into the air.
Nevertheless, you’ll hear scientists and meteorologists explain humidity and other weather phenomena by talking about the “amount of moisture that the air can hold” and saying that warm air can “hold more moisture” than cold air can. That’s a mistaken and misleading notion. The air isn’t holding on to water vapor; it has no such holding power.
Here’s why. Air and water vapor are both gases, and in gases the spaces between the molecules are so vast that any two gases can mix in any proportions without either one “knowing”, or controlling, how much of the other is there. All the air can do is accept the water vapor, whatever amount the water chooses to give off according to its temperature. It is purely water’s decision as to how eagerly it wants to be in the form of vapor instead of liquid.
Now I suppose you want to know why warm water produces more water vapor than cold water does, right? That’s science for you: Every answer generates more questions.
Water, like all liquids, has a certain tendency for its molecules to leave the surface of the liquid and fly off into the air. That’s because all the molecules are moving with various speeds, and there will always be some of them at the surface of the liquid that have enough energy to go flying right off as vapor. Because molecules move faster on the average at higher temperatures than at lower temperatures, there will be more potential escapees from warm water than from cold. For example, at 86 degrees Fahrenheit (30 degrees Celsius) water produces seven times more vapor molecules in a given space than water at 32 degrees Fahrenheit (zero degrees Celsius).
There is always a sort of tug-of-war going on among the molecules of a liquid. Their speed wants them to fly off as vapor, but their attraction to their fellow molecules wants them to stay in the liquid. At every temperature, water must strike a balance between these two tendencies (Techspeak: The water attempts to come to equilibrium between these two states).
At lower temperatures, the liquid tends to win out; at higher temperatures, the vapor gets the edge because of the higher molecular speeds. (The ultimate limit is when the liquid boils and turns completely to vapor.)
At a given temperature, every liquid has its own preferred balance point between vapor and liquid, because its molecules have their own degree of stickiness toward one another. A liquid whose molecules are stuck tightly together will not form vapor easily, so its balance point will tend to favor the liquid form over the vapor form. Gasoline’s molecules, on the other hand, don’t stick to one another very much at all, so their balance point favors the vapor and gasoline evaporates (vaporizes) much faster than water.
The tendency of liquid molecules to escape and fly off as vapor is called the vapor pressure of the liquid. In Techspeak, we would say that gasoline has a higher vapor pressure than water, and that warm water has a higher vapor pressure than cold water.
Let’s say we’re in a closed box containing some water. The water would soon strike a balance between liquid and vapor, according to the temperature. (The liquid and vapor would be in equilibrium.) If our box suddenly got colder, the water would have to strike a new balance (find a new equilibrium balance between liquid and vapor) based on that new, lower temperature. The new balance would be in the direction of less vapor and more liquid, so some of the vapor would have to condense out and become liquid. There would be rain or dew in our box.
Others may claim that it rained because “there was more water vapor in the cold air than it could hold.” But I never even said there was air in the box, did I? It rained solely because the water shifted its liquid-vapor balance all by itself.
“It” would be just as humid or just as dry in the box if there were no air in it at all, but some other gas with a different reputed “holding power.”