Why Does the Shower Curtain Blow In?

There you are, minding your own business, taking a hot, relaxing shower. Then, as the minutes pass and the room gets steamy, you find yourself fighting the advances of a renegade shower curtain. No matter what you do, the curtain will not stop, not until the water is shut off. Why is that? What would possess the curtain to come after you like that? The answer can be found in your basic meteorology or geosciences class, or even while watching the local weather.

If you have been to the beach, you may notice that during the day the breeze rolls in from the water. You may also notice that the sand can get unbearably hot in a short period of time in direct sunlight, while the water temperature remains relatively the same throughout the day. This is due to the higher specific heat of the water, meaning it takes a lot more sunlight and heat to raise the temperature of the water than it does the sand. So as the day goes on, the sand gets hotter than the water, creating a pressure differential between the air above the sand and the air above the sea. Hot air is less dense, and therefore lighter than cold air, so this hotter, lighter air over the sand rises. As this air rises, it creates a void (the air pressure falls), so the cooler air from the sea is pulled in to fill this void, creating the breeze you feel during the day. The cycle continues like this until nightfall, when the sand cools much faster than the water after the sun sets, and the process is reversed.

The same thing happens in your shower as at the beach, except in the shower, there is a curtain between the hot and cold air masses. You create a tiny pressure system in your bathroom, where hot air from the inside of the shower rises up out of the stall and over the top of the curtain, while cold, dense air is pulled in to replace it. Since there is a shower curtain in the way of that entering cold air mass, the curtain is pulled in with it, enveloping you in a vinyl cape.

Now you know! Amaze your friends with your wealth of useless knowledge!