Humidity Characteristics of water Humidity 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 H 2 O 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