Atmospheric Moisture

How does the moisture get in the atmosphere??

• EVAPORATION

• TRANSPIRATION – water evaporated from trees

Factors that affect evaporation rates(adding water vapor to the atmosphere)

1. Temperature – warmer the temperature, evaporation increases

2. Surface Area – As surface area increase, the rate of evaporation increases

3. Relative Humidity – as relative humidity increases, the rate of evaporation decreases

Which will evaporate faster?

4. Surface area – more surface area of water available,the more evaporation

ABSOLUTE HUMIDITY

• Amount of Water Vapor in the atmosphere• Air that is full of water vapor is said to be

saturated• The higher the temperature, the greater

the capacity of the air can “hold” water• SO, • As the air temperature increases, the air can

hold (less or more) water

Relative Humidity (RH)

Percentage of air that is saturated at a given temperature

50% RH = air has ½ the water vapor needed to saturate the air

RH = 100% means Air is saturated, it cannot add any more water vaporor condensation will occur

Dew Point

Temperature at which condensation occurs

It’s a direct measure of the moisture content in the air.

Therefore, as the dew point temp increases the amount of moisture in the air increases

Air Temp = Dew Point, then RH= 100%, which equals and occurs

• As air temperature and dew point get closer together, the probability of clouds and precipitation increases

Relationships

A. If the moisture content remains the same:

As temperature increases, RH

As temperature decreases, RH

decreases

Indirect relationship

increases

Indirect relationship

Measuring Dew Point

Instrument:

Psychrometer has 2 thermometers,

- dry bulb –

- Wet Bulb – a piece of moist cloth wrapped around the bulb

Sling Psychrometer

Air temperature

Psychrometer

• The psychrometer is swung around, the wet bulb temperature drops because of the cooling effect when being

• Wet bulb depression - difference between the dry and wet bulbs

evaporated

ESRT Page 12

• To Find Dew Point (see ESRT p. 12)• Find the wet bulb depression along the top of the chart• Find the air temperature along the left side of the chart• Where the two meet is the dew point (You may have to

estimate)

• Ex:• Dry bulb = 20°C Wet bulb = 17°C •Difference between Wet bulb and Dry Bulb =•• Dew point =

3°C

Dew Point = 15 oC

• Dry Bulb = 11 oC

• Wet Bulb = 4 oC

• Difference between wet bulb and dry bulb = ______

• Dew Point = __________

7 oC

-7 oC

To Find the Relative Humidity

• ESRT Page 12

• Same procedure as finding the dew point

• Dry Bulb = 20 oC Wet Bulb = 17 oC

• Difference between wet and dry bulbs

• = ________ oC3

RH = 74%

• Dry Bulb = 11 oC Wet Bulb = 4 oC

• Difference between wet and dry bulbs

• = ________ oC

• RH = _______

7

26%