What do you think of when you hear the word “sky"? Fluffy clouds floating on a light breeze? Maybe you think of popular phrases, such as “lighter than air" or “disappear into thin air"? Whatever your particular thoughts might be, most of us think of the sky as ethereal, unsubstantial and almost not there, as is if it's essentially nothing.

But is that an accurate view of the sky? Is there really nothing up there? Is it weightless? Not quite! In fact, it's actually much heavier than you could probably imagine.

Air is all around you. It starts at the ground and goes up, up, up all the way to the top of the sky. All that air is not empty. It's made up of all sorts of gas molecules. And guess what? Those molecules all have weight.

What does all that air add up to? Would you believe the entire atmosphere weighs around 5 million billion tons? Yikes! How heavy is that? Some scientists estimate it's about the same weight as 570,000,000,000,000 adult Indian elephants! You're probably WONDERing how we keep from being crushed under all that weight.

Luckily for us, all that weight is distributed evenly over Earth's surface, which you know is really big. Still, the average weight of air pressing on you — called air pressure — is almost 15 pounds on each square inch of your body (14.7 pounds per square inch to be exact!).

Since air moves about easily, it presses on all sides of you. If all that weight was just on top of your head, it might knock you to the ground. Instead, it presses evenly all around you…even from the inside. That's right! You breathe air into your lungs and it presses outward from your insides. The even distribution of all that air pressure keeps us from collapsing under the pressure.

Air pressure can be measured with a tool called a barometer. The air pressure reading on a barometer is called the barometric pressure. Meteorologists use barometers to detect changes in air pressure, which often signal changes in the weather.

A drop in barometric pressure means a storm is probably on the way. On the other hand, a rise in barometric pressure usually foretells sunny skies.

If you live in a mountainous area, you can test the weight of the atmosphere for yourself. Just grab a barometer and take a measurement at the bottom of a mountain. Then travel to the top of the mountain and measure it again. The pressure at the top of the mountain will be less, because you're higher in the atmosphere and that means there's less air above you, so it weighs less!

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Tomorrow’s Wonder of the Day features lions and tigers…oh my!