Sources: PowerPoint, Web, text (p. 9-12)

1. A place where potential evapotranspiration is significantly higher than precipitation.

2. Index of Aridity. The index is 5 in Tucson, AZ.

3. Radiation (greater sunlight intensity = more), Temperature (higher = more), Relative Humidity (lower = more), Wind Speed (higher = more), and Plant Activity (higher = more).

4. Elevation, Latitude, Greenhouse gas concentration in atmosphere

5. Hot air rises, cold air descends.

6. Rising air cools, descending air heats (that is why it is hotter in Tucson than on Mt. Lemmon).

7. Warm air holds more water potentially.

8. Absolute humidity is the actual amount of water in the air.
Relative humidity (RH) is the actual amount of water in the air as a percentage of the maximum amount of water the air could hold at that temperature.

9. When air cools, RH increases. When air heats, RH decreases.

10. When air rises, it cools, so RH increases. When air descends, it heats, so RH decreases.

11. When air rises, it is more likely to rain. When air descends, it is less likely to rain.

12. The temperature at which the RH of the air becomes 100%

13. More likely.

14. From the Pacific in winter, and from the Gulf of California and the Gulf of Mexico in summer.

15. Convection (when air is heated and rises), Orographic (when air rises as it is blown up the side of a mountain), Frontal (when air rises as it is pushed up the leading edge of a cold front). As the air rises (by all the above means) it cools and its capacity to hold water decreases so the relative humidity increases and the chance of rain increases.

16. The 23.5 degree tilt of the Earth and the Earth's revolution around the sun. As the Earth revolves around the sun, the angle each hemisphere is pointing toward or away from the sun changes. When the northern hemisphere is pointed most perpendicularly to the sun (June 21st), it is summer in the northern hemisphere but this is when it is winter in the southern hemisphere because June 21 is when the southern hemisphere is pointed most away from the sun.

17. It is cooler in the southwest in the winter because the northern hemisphere is tilted away from the sun and so the sun's rays must pass through more atmosphere (which reflects and absorbs the sun's energy) and because the sun's energy is spread over a larger area due to the sun's rays coming in at a low angle. It is warmer in the southwest in the summer because this is when the northern hemisphere is tilted toward the sun and the sun's rays are most perpendicular so are most concentrated and travel through the least amount of atmosphere (thus are reflected and absorbed less so that more reaches the ground).

18. Descending air in the Hadley Cells at 30 degrees N and S latitudes: as the air descends, it heats, causing the relative humidity to decrease which causes the potential evapotranspiration rate to increase and the chance of precipitation to decrease.
Descending air on the downwind side of mountain ranges (the Rainshadow Effect): as the air descends, it heats, causing the relative humidity to decrease which causes the potential evapotranspiration rate to increase and the chance of precipitation to decrease.
Warming of air as it moves from being over a cold ocean to over relatively warmer land: as the air heats over the warmer land the relative humidity decreases which causes the potential evapotranspiration rate to increase and the chance of precipitation to decrease.
From being far away from a source of water.

19. Below is an image of Hadley Cells, Ferrel Cells, and Polar Cells. Note that air rises at 0 and 60 degree latitudes N and S (rains alot) and the air descends at 30 and 90 degrees N and S latitude (deserts). Notice prevailing wind direction is determined by looking at the air flow in the cells against the ground and then deflecting the air clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere. To name the direction of the winds, imagine you are standing with your face into the wind and record the direction in which you are facing (e.g., the winds are called Westerlies at our latitude, even though they are coming from the southwest). With the ocean currents, remember that in general the ocean currents are flowing clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere; therefore, along the west side of a continent, the water is flowing from the pole and so is cold (along the east coast, the water is flowing from the equator and so is warm).