Overview
Pressure is not only a matter of altitude but also the temperature. If the volume of a space doesn't change, then as the temperature increases, so does the pressure. The molecules and atoms that comprise the air 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 due to localized decreasing temperature.
| TOTAL TIME | 10 minutes |
|---|---|
| SUPPLIES | Two empty 2-liter bottles, hot tap water |
| PRINTED/AV MATERIAL | None |
| TEACHER PREPARATION | None |
| SAFETY FOCUS | Tornado safety |
Procedure
- Place two cups of hot tap water into each two 2-liter bottle.
- Place your thumb over each bottle opening and shake the bottles. This ensures the air inside the bottle is warmed.
- Pour the water out of each bottle and quickly screw a bottle cap on only one of the two bottles.
- Stand both bottles side-by-side and observe over the next five minutes.
Discussion
As air cools inside the uncapped bottle, the molecules loose their energy, leading to a lowering of the pressure inside of the bottle. This lowered pressure pulls outside air through the bottle opening, immediately equalizing the pressure.
As with the uncapped bottle, air inside the capped bottle also cools as molecules loose their energy. However, with no replacement air able to enter, the cooling temperature results in lower air pressure inside compared to outside the bottle. The higher outside pressure begins to slowly crush the capped bottle, changing the volume until the inside and outside pressures are equal.
Building a Weather-Ready Nation
There was a time when the National Weather Service advised you to open a window in your house to equalized the pressure when a tornado was approaching. The belief was that a house could blow up from higher pressure trapped inside.
However, research has shown that the pressure difference is only about 10%, and houses can handle this pressure difference through vents in bathrooms and kitchens. What actually destroys a house is high speed wind and debris slamming into the structure.
Therefore, if a tornado approaches your house, LEAVE THE WINDOWS ALONE! The windows will likely break as debris hits the glass. Instead, seek shelter in the interior of your dwelling, on the lowest floor and away from windows in order to place as much protection between you and flying debris as possible.