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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