GL 107 Earth Systems Science

K. Hannula

Discussion questions for 11/14/07

1.  Describe how evaporation and condensation can change the temperature of surrounding air.

Evaporation cools surrounding air, because heat energy is required to change the liquid into a gas.

Condensation warms surrounding air, because condensing water vapor releases its latent heat of condensation.

2.  Why does sweating cool you off?

Sweat evaporates, cooling your skin.  (If the sweat does not evaporate efficiently, then you do not cool off as well, and end up feeling "hot and sweaty.")

(Really, sweating does allow you to cool off.  You just don't realize you're sweating when the sweat can evaporate.)

3.  Why does 90 degrees in Houston feel more unpleasant than 90 degrees in Durango?

The humidity in Houston is higher, so water does not evaporate as easily.

4.  Define:

• mixing ratio:  mass of water vapor/mass of dry air; a measure of absolute humidity

• relative humidity:  a measure of humidity that compares actual water vapor in the atmosphere to the maximum amount of water vapor possible at a given temperature.  Expressed as a percentage.  At 100% humidity, air is said to be "saturated" with water vapor.

• dew-point temperature :  The temperature at which a parcel of air would be saturated with water vapor.  The higher the dew-point temperature, the more humid the air is.  Dew-point temperature is always less than or equal to the actual temperature.  If the dew-point temperature = the actual temperature, water begins to condense (forming dew, fog, or clouds, or forming ice crystals if the dew-point temperature is below freezing).

5.  If the relative humidity is 70% at 40 degrees F, how should the relative humidity change if the temperature rises?  How should the relative humidity change if the temperature falls?

If the amount of water vapor in the air stays the same, the relative humidity should drop if the temperature rises, and rise if the temperature falls.

6.  What four mechanisms can cause air to rise?

1) Orographic lifting (the rising of air to go over mountains)

2) Frontal wedging (warm air masses rising to go over colder air masses)

3) Convergence (surface winds blowing towards one another, forcing air to rise)

4) Localized convective lifting (air near the ground warming and then rising)

7.  Why is rising air important for formation of clouds?

As air rises, it cools.  If the air cools below its dew-point temperature, clouds will form.

8.  What is a temperature inversion?

A temperature inversion is a situation in which air near the ground is colder than air above it.  Because cold air is denser than warm air, the air near the ground can not rise.  In Durango, temperature inversions happen on cold winter mornings, and can be seen as smoke from the train and wood stoves that does not rise above the College mesa.  In Denver and Los Angeles, temperature inversions are partly responsible for the "brown cloud" of smog that stays near the ground.