Why Do Balloons Get Smaller in the Cold?

Whether you fill balloons with helium or air, you are actually filling them with a gas. A large number of molecules are needed to fill the balloon's volume. These molecules are temperature sensitive. When the temperature drops, helium becomes denser. Its molecules lose energy, slow down and move closer together to conserve heat. This reduces the volume inside the balloon. Because the helium molecules are moving closer together rather than outward toward the balloon's outer shell, the balloon becomes smaller. The helium molecules are no longer less dense than air.

At first, the air molecules are much larger than the tiny helium molecules. Both are affected by the cold, but helium is affected much more than air.

All balloons are porous - which means they have tiny holes in their surface through which the gas can escape. You can't see them, but they're there. That's why all balloons deflate over time and don't float forever! Helium molecules are much smaller than air molecules and therefore escape through the small holes more easily than air molecules. This is why inflated designs last much longer than helium-filled balloon designs.

When gas molecules come into contact with the cold and shrink in size, they are more likely to escape through the small holes in the balloon. The longer they are in the cold, the more molecules can escape. If the balloon is left in the cold for too long before being returned to the warm environment, the number of molecules in the balloon will decrease. Even if the molecules will expand again, their number will be reduced, so the balloon will not fully re-inflate to the same size as it was before being placed in the cold.

This may affect both the inflation design and the helium design, but helium is already much smaller, so they are much less likely to "inflate" if left in a cold environment for too long and then returned to a warm environment.

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