Motivating Myth: Warm air holds more water vapor than cold air. Or alternatively, cold air can hold less water vapor than warm air.

Have you ever been taught that "warm air holds more water vapor than cold air," or perhaps heard it when reading or watching a story about weather? If you search around on the Web, you can find plenty of sites that explain processes like cloud formation with the idea that cold air can't hold as much water vapor as warm air. The explanations usually go something like this: "air cools to the point where it can't hold any more water vapor, and liquid water drops form." But, don't believe everything you read on the Internet! This idea is scientific garbage, and it poorly describes what's really happening when net condensation causes liquid water droplets to form.

For starters, let's examine what accepting this myth really implies. By accepting this myth, we're basically treating air like a sponge, and once all the pores in the sponge get filled with water, it can't absorb any more water, so water starts dripping from the saturated sponge. But, air isn't like a sponge. Air is also not like a hotel, which posts a "No Vacancy" sign when all of its rooms are filled with water vapor. If these ideas sound a little silly, it's because they are!

What we call "air" is really mostly empty space with tiny molecules flying around independently of each other. If we had a box filled with air, the "air" molecules (oxygen, nitrogen, carbon dioxide, etc.) would occupy a really tiny fraction of the space in the box, regardless of the temperature. In other words, no matter what the temperature is, there's always enough room for more water vapor molecules. So, the idea that colder air doesn't have enough room to hold more water vapor molecules is nonsense!

So, why is the myth that "warm air holds more water vapor than cold air" so common? Well, it's an "easy" explanation, and sometimes folks (even those who should know better) take unfortunate shortcuts.