1. Do Now: Air rises ___________ and ____________
Air sinks ___________ and ____________
Draw this picture and show what happens to air and show where clouds would form:
Mountain
water
land

2. Atmospheric Moisture
pages

3. Moisture Exists in solid, liquid, and gas in atmosphere
Water enters the atmosphere by:
Evaporation – liquid changes into gas
Transpiration – plants release water vapor
Evapotranspiration – evaporation + transpiration together
Remember it’s the Sun that provides the energy for these to occur

4. Factors affecting Evaporation:
amount of energy available (more = faster evaporation)
surface area of the water (spread out = faster evaporation)
degree of saturation (more saturated = slower evaporation)
wind speed greater wind speed = greater evaporation)

5. Humidity
Absolute Humidity - water vapor in each unit volume of air (or moisture capacity)
- hotter air can hold more water vapor than cold air
Relative humidity – ratio of the amount of water vapor in the air to maximum amount it can hold
The one you see in forecasts

6. Which is heavier? H2O O2 N2 N2 N2 O2 O2 H2O
Which is heavier dry air or wet air?
H2O O2 N2 N2
vs. N2 O2 O2
H2O
Would a baseball travel further on a night where the air is saturated or dry?

7. Dew Point
Dew point – temperature at which air is saturated (filled with water)
If air temp = dew point relative humidity = 100%
If air temp drops below dew point: water vapor condenses to liquid water

8. Measuring Moisture
Sling Psychrometer – instrument used to measure relative humidity and dew point
Dry bulb temp – ordinary thermometer
Wet bulb temp – thermometer with wet wick around it

9. To convert relative humidity and dew point
You must use page 12 of ref. table
Subtract: dry bulb temp – wet bulb
Locate dry bulb on left side
Locate answer to step 1 on top of chart
See where those two numbers meet ****Make sure you are using the correct chart: dew point or relative humidity

10. Try a couple examples:
1. What is the relative humidity where the dry bulb temperature is equal to 18oC and a wet bulb temperature of 15oC?
2. What is the dew point temperature when dry bulb temperature is equal to -4oC and a wet bulb temperature of -5oC?

11. Cloud Formation Air rises and cools Air reaches dew point
Water begins to condense (gas -> liquid)
Water droplets form on condensation nuclei (dust particles / aerosols)

12. Air Masses

13. Air masses
Air Masses – large body of air in the troposphere with similar characteristics (pressure, moisture, temp.)
Form when air remains stationary over an area
Where an air mass originates determines its characteristics

15. Different Characteristics
Continental = dry
Maritime = wet
Polar or arctic = cold
Tropical = wet

16. Symbols on reference table
cP = continental polar = dry and cold
cT = continental tropical = dry and warm
mP = maritime polar = wet and cold
mT = maritime tropical = wet and warm