Is it warmer upstairs or downstairs? If you’re thinking warm air rises, then it’s got to be upstairs, right? If you’ve ever stood on a ladder inside your house and compared it to the temperature under the table, you’ve probably felt a difference.

So why is it cold on the mountain and warm in the valley? Leave it to a science teacher to throw in a wrench just when you think you’ve got it figured out. Let’s take a look at whether hot air or cold air takes up more space. Here’s what you do:

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  • water
  • plastic bottle
  • balloon
  • stove top and saucepan or the setup in the video

Download Student Worksheet & Exercises

1. Pour a couple of inches of water into an empty soda bottle and cap with a 7-9″ balloon. You can secure the balloon to the bottle mouth with a strip of tape if you want, but it usually seals tight with just the balloon itself.

2. Fill a saucepan with an inch or two of water, and add your bottle. Heat the saucepan over the stove with adult help, keeping a close eye on it. Turn off the heat when your balloon starts to inflate. Since water has a high heat capacity, the water will heat before the bottle melts. (Don’t believe me? Try the Fire-Water Balloon Experiment first to see how water conducts heat away from the bottle!)

3. When you’re finished, stick the whole thing in the freezer for an hour. What happened to the balloon?

What’s going on? So now what do you think? Does the same chunk of air take up more or less space when it’s hot? Cold? When you heat air, it expands because the air molecules move around a lot more when you add energy. The molecules zip around and wiggle like crazy, bouncing off each other more often than when they are cooler.

Getting back to our original question, the upstairs in a house is warmer because the pockets of warm air rise because they are less dense than cool air. The more the molecules move around, the more room they need, and the further they get spaced out. Think of a swimming pool and a piece of aluminum foil. If you place a sheet of foil in the pool, it floats. If you take the foil and crumple it up, it sinks. The more compactly you squish the molecules together, the more dense it becomes. You can read more about density in Unit 3.

As for why mountains and valleys are opposite, it has to do with the Earth being a big massive ball of warm rock which heats up the lower atmosphere in addition to winds blowing on mountains and changes in pressure as you gain altitude… in a nutshell, it’s complicated! What’s important to remember is that the Earth system is a lot bigger than our bottle-saucepan experiment, and can’t be represented in this way.


  1. Draw a group of molecules at a very cold temperature in the space below. Use circles to represent each molecule.
  2. True or False: A molecule that heats up will move faster.
    1. True
    2. False
  3. True or False: A material will be less dense at lower temperatures.
    1. True
    2. False


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