Meteorology

Temperature and humidity

Water Vapor

• Source of all condensation and precipitation

• Most important gas in the atmosphere for understanding atmospheric processes

• Zero to 4% by volume

Water Changes State

• Always requires heat

• Heat is either absorbed or released

• Water must pass through the atmosphere in the form of water vapor

• If all the water vapor was in a global layer it would only be 2 mm deep

Solid to Liquid

• Melting– Heat is transferred to ice water– Temperature of the water remains a constant 0

degrees C until all ice has melted– Heat breaks apart crystal structure of ice forming

liquid water

Liquid to Gas

• Evaporation– Changing a liquid into a gas– Requires 2500 joules of energy to convert 1

gram of liquid water to water vapor

Liquid to Gas

• “Evaporation as a cooling process”– Energy is needed to evaporate water– Cooling effect after getting out of the pool– Energy needed comes right from your skin

• Condensation– Water vapor changes to liquid– Generates clouds and fog

Solid to Gas

• Sublimation– Conversion of solid directly to a gas– No liquid state– Dry ice (frozen carbon dioxide)– Generates “smoke”

• Deposition– Conversion of vapor directly to a solid– Frost on cold objects, grass, windows

Humidity

• Humidity – the amount of water vapor in the air• Saturation

– Balance between liquid water and water vapor– Closed jar: half water, half air– Water begins to evaporate increases pressure in

the air– Pressure is caused by more water vapor molecules

moving into the air– Pressure in air continues to increase– Forces more water molecules to return to liquid– When a balance occurs, air is said to be saturated

Saturated Air

• Depends on temperature

• Warm air contains more water vapor than cold air

Relative Humidity

• Ratio of air’s actual water vapor content compared to the amount of water vapor the air can hold at the time

• How much water vapor can the air hold?– Depends on temperature and pressure

• Indicates how near the air is to saturation

Relative Humidity

• How can it be changed?

– 1 – adding or removing water vapor• Occurs naturally• Oceans and smaller bodies of water

– 2 – varies with temperature• Lowering air temperatures causes INCREASE• Raising air temperatures causes DECREASE

Dew Point

• The temperature at which one parcel of air would need to be cooled in order to reach saturation

• If the air was cooled further it would condense

• This would cause dew,

fog, clouds

How do you measure humidity?

• Hygrometer – used to measure relative humidity

• Psychrometer– Two thermometers side by side– One is dry bulb, the other wet bulb– Larger the difference on the reading the lower

the relative humidity– If air is saturated the thermometers will have

the same reading