Properties of the Atmosphere

1. Air Density

Ears "pop" due to air pressure differences. Photo courtesy of CK-12. 

If your ears have ever "popped", you have experienced a change in air pressure. Ears "pop" because the air pressure is different on the inside and the outside of your ears

The air desnity (the number of molecules in a given volume) decreases with increasing altitude. This is why people who climb tall mountains, such as Mt Everest, have to set up camp at different elevations to let their bodies get used to the decreased air density. 

Why does air density decrease with altitude? Gravity pulls the gas molecules towards the Earth's center. The pull of gravity is stronger closer to the center, at sea level. Air is denser at sea level, where the gravitational pull is greater. 

2. Air Pressure

Gases at sea level are also compressed by the weight of the atmosphere above them. The force of the air weighing down over a unit of area is known as its atmospheric pressure, or air pressure. Why are we not crushed? The molecules inside our bodies are pushing outward to compensate. Air pressure is felt from all directions, not just from above.

At higher altitudes the atmospheric pressure is lower and the air is less dense than at lower altitudes. That's what makes your ears pop when you change altitude. Gas molecules are found inside and outside your ears. When you change altitude quickly, like when an airplane is descending, your inner ear keeps the density of molecules at the original altitude. Eventually the air molecules inside your ear suddenly move through a small tube in your ear to equalize the pressure. This sudden rush of air is felt as a popping sensation.

Source: Pressure and Density of the Atmosphere. Retrieved from http://www.ck12.org/book/CK-12-Earth-Science-Concepts-For-High-School/r16/section/9.3/ on August 26, 2013. 

3. Temperature vs. Heat

Temperature is a measure of how fast the atoms in a material are vibrating. High temperature particles vibrate faster than low temperature particles. Rapidly vibrating atoms smash together, which generates heat. As a material cools down, the atoms vibrate more slowly and collide less frequently. As a result, they emit less heat. What is the difference between heat and temperature?

• Temperature measures how fast a material’s atoms are vibrating.
• Heat measures the material’s total energy.

Heat energy is transferred between physical entities. Heat is taken in or released when an object changes state, or changes from a gas to a liquid, or a liquid to a solid. This heat is called latent heat. When a substance changes state, latent heat is released or absorbed. A substance that is changing its state of matter does not change temperature. All of the energy that is released or absorbed goes toward changing the material’s state.

For example, imagine a pot of boiling water on a stove burner: that water is at 100^oC (212^oF). If you increase the temperature of the burner, more heat enters the water. The water remains at its boiling temperature, but the additional energy goes into changing the water from liquid to gas. With more heat the water evaporates more rapidly. When water changes from a liquid to a gas it takes in heat. Since evaporation takes in heat, this is called evaporative cooling. Evaporative cooling is an inexpensive way to cool homes in hot, dry areas.

Substances also differ in their specific heat, the amount of energy needed to raise the temperature of one gram of the material by 1.0^oC (1.8^oF). Water has a very high specific heat, which means it takes a lot of energy to change the temperature of water. Let's compare a puddle and asphalt, for example. If you are walking barefoot on a sunny day, which would you rather walk across, the shallow puddle or an asphalt parking lot? Because of its high specific heat, the water stays cooler than the asphalt, even though it receives the same amount of solar radiation.

Source: Temperature and Heat in the Atmosphere. Retrieved from http://www.ck12.org/book/CK-12-Earth-Science-Concepts-For-High-School/r16/section/9.10/ on August 26, 2013. 

4. Air Temperature

Earth's atmosphere. Photo courtesy of NASA Goddard Video and Photo/Flickr. 

As you've learned, temperature varies throughout the layers of Earth's atmosphere. Warm air rises, but why? Gas molecules are able to move more freely, and if they are uncontained, as they are in the atmosphere, they can take up more or less space. 

• When gas molecules are cool, they are sluggish and do not take up as much space. With the same number of molecules in less space, both air density and air pressure are higher.
• When gas molecules are warm, they move vigorously and take up more space. Air density and air pressure are lower.

Warmer, lighter air is more buoyant than the cooler air above it, so it rises. The cooler air then sinks down, because it is denser than the air beneath it. This is convection.  The property that changes most strikingly with altitude is air temperature. Unlike the change in pressure and density, which decrease with altitude, changes in air temperature are not regular. A change in temperature with distance is called a temperature gradient.

Source: Temperature of the Atmosphere. Retrieved from http://www.ck12.org/book/CK-12-Earth-Science-Concepts-For-High-School/r16/section/9.4/ on August 26, 2013. 

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