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 breath gain energy as they absorb heat. That increase in energy results in faster moving atoms which we observe as an increase in energy.

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.


The bottle that was capped will eventually begin to collapse. This is a result of the cooling air inside that bottle. The air cools because the molecules and atoms inside the bottle lose energy as they collide with the bottle side that is exposed to the cooler surrounding air.

As their energy decrease so does their velocity and therefore the pressure decreases. Since the pressure inside the bottle decreases, the force of the air outside the bottle begins to 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.

Live Weatherwise

There was a time the National Weather Service advised, when a tornado was approaching, to open a window in the house to equalized the pressure inside and out. The thought was high pressure trapped inside a house caused to blowup due a rapid fall of pressure outside resulting from the tornadic winds.

Research has shown the pressure difference is only about 10% and houses can handle this as there are vents in bathrooms and kitchens to relieve the pressure. What destroys a house is debris slammed into the structure by the force of the 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.

Fast Facts

We can only swim down to a relatively shallow depth before the increased pressure from the water crushes our bodies. At a certain point, outward pressure in the lungs exceeds the structural integrity of the rib cage, and the rib cage collapses would kill a human being.

As such, the world's record SCUBA dive is 1090 feet 4½ inches (332.35 meters) set in 2014.

However, whales can withstand this pressure because their bodies are more flexible. Their ribs can collapse safely under pressure, which keeps them from rupturing. This allows sperm whales, which dive to depths of 7,000 feet (2,100 meters) or more to hunt for giant squid.