Unit 4-1:Humidity and Dew Point

REMOVE HEAT

Evaporation

• As molecules in liquid form absorb heat, they evaporate.– Evaporation is the phase change from liquid

to gas.

• Molecules of water enter the atmosphere as water vapor.– This causes humidity, cloud formation, and

precipitation.

Specific Humidity

• Specific Humidity – How much water vapor is actually present in the air. – Expressed as the number of grams of water

vapor in one kilogram of air.

• Saturated – When the air is holding the maximum amount of water.

• Amount of water vapor that can be held doubles for every 11°C the temperature rises.

Relative Humidity

• Relative Humidity – comparing the amount of vapor in the air to how much the air can hold at its current temperature. – Amount * 100 = Relative Humidity (%)

Capacity

• Relative humidity is also determined by a sling psychrometer.

Reads Current Air Temp. Dry Bulb

Reads Air Temp after water evaporates, pulling heat away from the thermometer. Wet Bulb

You can determine the relative humidity from this tool by using a relative humidity table that compares the wet bulb and dry bulb temperatures.

Dew Point

• As water evaporates (goes from liquid to gas), it takes heat away from the environment.

• As water condenses (goes from gas to liquid), it gives heat to the environment.

• As the temperature of the air decreases, it also decreases the amount of water vapor the air can hold.

Dew Point

• If the amount of water vapor in the air is greater than the amount the air can currently hold (due to the air’s temp dropping), then the water condenses.

• Dew Point – temperature at which saturation occurs.

Dew Point

• Air can loose heat in 4 ways:– Contacting a cold surface– Radiating heat– Mixing with cooler air– Expanding as it rises

• In order for condensation to occur, there needs to be cooling below the dew point and a condensation nuclei.

Dew Point

• Condensation Nuclei: tiny particles needed for water vapor to condense upon.– Examples: Salt, dust, sulfur trioxide (from

fire), etc.

• Can also condense on a surface.– Like when you have condensation forming on

the outside of a cool drink or on the window after a hot shower.

Dew and Frost

• Dew and frost occur when the air temperature falls below the dew point and water condenses on the surface of the earth.

• Dew occurs when the air temperature is above freezing

• Frost occurs when the air temperature falls below freezing.