Humidity

Characteristics of waterHumidity

Condensation

Forms of water

• Water – liquid form – between 0 and 100 degrees C

• Ice – solid form– Below 0 degrees C

• Water vapor – gas form – Above 100 degrees C

• Sun’s energy converts water from liquid or solid form into gas form

Water in the air

• Through Evaporation– Liquid changing to a

gas

• Examples: – Lakes and rivers

(water to water vapor)– Steam is not vapor

• Vapor to Liquid: Condensation

• Through sublimation– Solid changing directly

to a gas

• Examples– ice caps (ice to water

vapor)– Mothballs

• Vapor to Solid: Deposition

Humidity

• Warm air can hold more water than cold air

• Why?

• Warm air has higher energy and prevents condensation

Specific humidity

• Also called absolute humidity

• The actual amount of humidity in the air expressed in grams

• What is the absolute humidity of our previous example

• Air can hold 12g of vapor and is holding 9g

• SH = 9g

Specific Humidity

• The amount of water that can be held in the air is limited

• When the specific humidity is equal to the maximum amount of water it can hold (RH = 100%)

– At this point the air is Saturated

Saturated

• When the air is saturated no more water vapor can enter it

• Water begins to condense – Remember condensation happens

• Again warm air can hold more water

Relative Humidity

• The amount of H2O in the air compared to the amount air can hold at a given temperature (expressed as a percent)

• Example:– At a certain temperature, air can hold 12g of

vapor; however, it only has 9g– RH = 9/12 x 100 or 75%

Finding Relative Humidity• Sling Psychrometer

– Wet bulb and dry bulb – As you sling the Sling Psychrometer water

evaporates– Use a chart to discover the humidity by the

difference between the wet and dry bulb temperature and the dry bulb temperature

Dry Bulb

Wet Bulb

Dry Bulb Temperature

• Air temperature at a given time • Does not change

• That is it. . . Just the temperature

Wet bulb temperature

• When the wet bulb stops dropping you have found the temperature at which the air is saturated

• Called the Dew point

• Relative Humidity – 100%

• Condensation begins

Relative humidity values