National Weather Service United States Department of Commerce

Learning Lesson: Crunch Time


Pressure is not only a matter of altitude but also is dependent upon the temperature. As the temperature increases so does the pressure. The molecules and atoms that comprise the air we breathe gain energy as they absorb heat. That increase in energy results in faster moving atoms which we observe as an increase in pressure.

The opposite occurs when the temperature decreases. As the molecules lose energy, their motion is decreased and we observe a decrease in pressure. The students will see a plastic 2-liter bottle crushed by the normal atmospheric pressure in the room by this decrease in pressure.

TOTAL TIME 10 minutes
SUPPLIES Two empty 2-liter bottles, hot tap water
SAFETY FOCUS Tornado safety


  1. Place two cups of hot tap water into each two 2-liter bottle.
  2. Place your thumb over each bottle opening and shake. This ensures the air inside the bottle is warmed.
  3. Pour the water out of each bottle and screw a bottle cap on only one of the two bottles.
  4. Stand both bottles side-by-side and observe over the next five minutes.


Initially, the pressure inside the capped bottle will be higher than the surrounding environment as the temperature inside of the bottle will be higher than outside. As the air inside of the bottle cools the bottle will begin to collapse.

This cooling is a result of the energy of the molecules inside the bottle decreasing leading to a decrease in pressure. As their energy decreases so does their velocity and therefore the pressure inside the bottle decreases. The outside pressure will begin to exceed the inside pressure and slowly crush the bottle.

However, the uncapped bottle remains unchanged. As the air cools inside, the drier outside air flows in to take up the space thereby keeping the pressure the same both inside and outside of the bottle.

Building a Weather-Ready Nation

There was a time the National Weather Service advised you to open a window in your house to equalized the pressure both inside and out when a tornado was approaching. The idea was to prevent your house from blowing up caused by high pressure trapped inside.

Research has shown the pressure difference is only about 10% and houses can handle this pressure difference by vents in bathrooms and kitchens. What destroys a house is debris slammed into the structure carried along in the tornadic wind.

Therefore, if a tornado approaches your house, LEAVE THE WINDOWS ALONE! The tornado will open them for you as debris hits the glass. Instead seek shelter in the interior of your dwelling, on the lowest floor, away from windows, in order to place as much protection between you and flying debris possible.