Today’s Wonder of the Day was inspired by BOB. BOB Wonders, “If hot air rises why is all the air way above the earth freezing cold?” Thanks for WONDERing with us, BOB!

Do you live in a house that has an upstairs and a downstairs? Perhaps you have a main level and a basement? If not, you've probably been to a friend's house that had an upstairs.

If you've ever slept in an upstairs bedroom, especially during the dog days of summer, you've probably noticed that the upper level is usually warmer than the lower level. Likewise, if you've ever been in a basement, you may have noticed it's cooler than the main level.

What's the deal with that? Does the air get warmer the closer you get to the Sun? Could it be magic? Nope! It's science! Let's take a closer look at how the air around us behaves.

Have you ever heard someone explain that the higher temperatures on an upper floor are the result of hot air rising? They're absolutely correct. Hot air does rise, but it doesn't have anything to do with getting closer to the Sun.

Hot air rises because gases expand as they heat up. When air heats up and expands, its density also decreases. The warmer, less dense air effectively floats on top of the colder, denser air below it. This creates a buoyant force that causes the warmer air to rise.

This is the very science that explains how hot air balloons fly. The balloon catches the air being heated below. The hot air fills the balloon and its buoyant force lifts the hot air balloon high into the sky. As the heat source is dialed back, the air cools, bringing the balloon back to the ground.

While the science of hot air rising seems very straightforward, it might seem a bit counterintuitive to a certain group of people: mountain climbers. If you've ever climbed a mountain or taken a hike to the top of a mountain at a high elevation, you know one thing: it's usually much colder at the top of a mountain than it is at the bottom.

What's going on here? If hot air rises, shouldn't the air at the top of the mountain be really warm? But experience tells us otherwise. The higher in elevation you go, the colder the air gets. Fortunately, science again gives us our answer.

As air heats up and becomes less dense, it rises higher into the air. As it reaches higher elevations, it encounters less air pressure. The lower air pressure at higher elevations allows the air to expand even more. As it continues to expand, it begins to cool. When it cools, it will sink to a lower elevation where it will encounter higher air pressure, begin to contract, and eventually warm up again.

This process goes on and on. In fact, the movement of air up and down throughout the atmosphere accounts for much of the weather we see. Lower air pressures correspond to lower air temperatures, whereas higher air pressures correspond to higher air temperatures.

When you hike to the top of a mountain, the air up there has expanded to the point where it is under much lower air pressure and its temperature is much cooler as a result. As you descend, you encounter air that's more compact (higher air pressure) and warmer as a result.

#### Wonder What's Next?

You might want to dip a toe into tomorrow’s Wonder of the Day before you make the plunge!