Understanding Pressure and What It Has To Do With the

Atmosphere

Activity: Hot air balloon:Activity: Molecules in a box Come inside and review observations.

Molecules of a gas behave in a very simple manner. They move in a straight line and at a high speed, until they bump into something.

More molecules means more pressure.

What causes an increase in air pressure?

More Pressure

Less Pressure

More molecules means more pressure.

Higher temperature (meaning faster molecules!) means more pressure.

What causes an increase in air pressure?

HOT

HOT

HOT

More molecules means more pressure.

Higher temperature (meaning faster molecules!) means more pressure.

A smaller container means more pressure.

What causes an increase in air pressure?

Directly over this 1 inch x 1 inch square there are 14.7 pounds of air. That’s the weight of Nitrogen, Oxygen, Argon, Neon, etc.

Note: In Denver, Colorado it’s 12.2 pounds of air since we are at an elevation of 5280 feet.

Demonstrations: 1)Blowing a stream of air over the top of notebook paper2)Drinking glass with water and index card

Pressure• Pressure is defined as a

force applied per unit area

• The weight of air is a force, equal to the mass m times the acceleration due to gravity g

• Molecules bumping into an object also create a force on that object, or on one another

• Air pressure results from the weight of the entire overlying column of air!

Slide from: Scott Denning, 2007 Atmospheric Science Class

Sea Level Value Units of Pressure: (average) 1 atmosphere 760 mm. of mercury 29.92 in. of mercury 33.9 ft. of water 1013.25 millibars

How do we measure pressure?

Why does pressure decrease with altitude?

Remember:

Pressure = mass*gravity/unit area

As you go higher, you have less mass above you.

Slide from: Scott Denning, 2007 Atmospheric Science Class

Back to the Hot Air Balloon…

What happens when the balloon goes up?

2. Some air molecules escape through opening.

3. Air pressure in balloon goes down, so balloon goes up.

4. Outside pressure is greater, sinks below balloon.

Can you explain what’s happening with a Weather Balloon, filled with helium

with no opening at the bottom?

Atmospheric Atmospheric MeasurementsMeasurements

Helium-filled weather balloons are released from over 1000 locations around the world every 12 hours. Balloons carry Radiosondes high into the atmosphere.

These instruments document temperature, pressure, humidity, and winds aloft as they ascend.

Balloons are only partially filled with helium so as they ascend to about 25 miles above the earth they expand as they rise and get 24 - 32 feet across before they burst.

Slide modified from: Scott Denning, 2007 Atmospheric Science Class

Present Atmospheric Composition

Slide from: Scott Denning, 2007 Atmospheric Science Class

Vertical Thermal Structure

• Heated from below by water vapor and dry air.

• Heated in stratosphere by ozone absorption