If you’ve ever left a helium-filled balloon in a cold room or vehicle, you probably came back to a shriveled-up piece of latex. The balloon didn’t actually deflate because the same amount of helium is still inside it. Temperature affects the density of gases such as helium, which is why helium-filled balloons appear to deflate in cold temperatures.

You may have heard people say helium is lighter than air, but that’s not strictly true. It’s more accurate to say that helium is less dense than air. This is because air molecules are packed together more tightly than helium molecules are. The density of any solid, liquid or gas is the mass per unit volume and can be measured in several ways, but the most accurate way to calculate it is to divide its mass in kilograms by its volume in cubic meters. The density of helium is about 0.18 kg/m3, while the density of air at sea level is about 1.3 kg/m3. Air consists of 78 percent nitrogen, 21 percent oxygen, and 1 percent other gases such as argon and water vapor.

At room temperature, helium molecules move about freely and are spread far apart, which is why helium balloons float in the air at room temperature. Other gases that are less dense than air are hydrogen, neon, nitrogen, ammonia, methane and carbon monoxide.

When the temperature drops, helium becomes denser. Its molecules lose energy, slow down and move closer together to conserve heat. This decreases the volume inside the balloon. Because the helium molecules are moving closer together, rather than outward toward the shell of the balloon, the balloon shrivels and shrinks. The helium molecules are no longer less dense than air.

Don't think that once your helium-filled balloon shrinks and is lying on the floor instead of floating in the air, it's worthless. The same amount of helium is still inside the shell of the balloon. Simply move the balloon to a warmer place. The helium molecules get an energy boost, loosen up, move away from one another and expand. The balloon fills out and floats again.