JetStream - An Online Weather School Learning Lesson: Crunch Time

The Atmosphere Learning Lessons

1.	AM in the PM
2.	Heavy Air
3.	A Pressing Engagement
4.	Going with the Flow
5.	Crunch Time
6.	The "Wet" Barometer
7.	The "Dry" Barometer
8.	Melts in your Bag, not in your Hand
9.	Canned Heat
10.	It's a Gas, Man
11.	Sweatin' to the Coldies
12.	The Rain Man
13.	Water, Water Everywhere

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Learning Lesson: Crunch Time

OBJECTIVE	Demonstrate the effect of heat on pressure.
OVERVIEW	A plastic 2-liter bottle will be crushed by the normal atmospheric pressure in the room.
TOTAL TIME	10 minutes.
SUPPLIES	Two empty 2-liter bottles, hot tap water.
PRINTED/AV MATERIAL	None
TEACHER PREPARATION	None
SAFETY FOCUS

Background

Pressure is not only a matter of altitude but also is dependent upon the temperature. As the temperature increases so does the pressure. The 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 atoms loose energy, their motion is decreased and we observe a decrease in pressure.
Procedure

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

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 atoms inside the bottle loose 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 weight of the air outside the bottle begins to crush the bottle. However the uncapped bottle remains unchanged. As the air cools inside, outside air moves in to take up the space thereby keeping the pressure the same both inside and outside of the bottle.
Live Weatherwise

Severe Thunderstorm Safety The weight of molecules also affects the weather. One measure of the severity of a thunderstorm is the wind speed. In addition to the size of hail, the National Weather Service defines a severe thunderstorm as one containing wind speed of 58 mph (50 kts) or greater. The weight of all of the molecules in wind of 58 mph (50 kts) can create hazardous weather conditions such a blowing down phone and power lines, trees, and make driving hazardous. When the National Weather Service issues a Severe Thunderstorm Warning it means a thunderstorms with wind gusts to 58 mph (50 kts) or greater and/or hail size of 3/4" or greater is occur or about to occur near you. Discuss severe thunderstorms with your family. Everyone should know what to do in case all family members are not together. Discussing disaster ahead of time helps reduce fear and lets everyone know how to respond during a severe thunderstorm. Take an American Red Cross first aid and CPR course to learn how to treat burns and how to give rescue breathing and administer CPR. Everyone should know how to respond, because severe thunderstorms can strike almost anywhere at anytime. Postpone outdoor activities if thunderstorms are likely. Many people take shelter from the rain, but most people struck by lightning are not in the rain! Postponing activities is your best way to avoid being caught in a dangerous situation.

Fast Facts

We can only swim down to a relatively shallow depth, about 250 feet, 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. Obviously, this would kill a human being.

However, whales can withstand this pressure because their bodies are more flexible. Their ribs are bound by loose, bendable cartilage, which allows the rib cage to collapse at pressures that would easily snap our bones.

A whale's lungs can also collapse safely under pressure, which keeps them from rupturing. This allows sperm whales, which dive to depths of 7,000 feet (2,100 m) or more to hunt for giant squid that live at these great depths.

Back: Air Pressure

National Weather Service Southern Region Headquarters 819 Taylor Street Fort Worth, Texas 76102	www.srh.weather.gov Updated: December 30, 2004

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