Humidity

Water Most abundant 71% of surface Hydrologic cycle

Phase changes -transfers of latent energy- Gas (water vapor) Liquid Solid (ice)

Phase changes -transfers of latent energy- Gas (water vapor) evaporation Liquid Solid (ice)

Phase changes -transfers of latent energy- Gas (water vapor) LATENT HEAT OF VAPORIZATION + 597 cal/gm evaporation Liquid Solid (ice)

Phase changes -transfers of latent energy- Gas (water vapor) LATENT HEAT OF VAPORIZATION + 597 cal/gm evaporation Liquid Solid (ice) melting

Phase changes -transfers of latent energy- Gas (water vapor) LATENT HEAT OF VAPORIZATION + 597 cal/gm evaporation Liquid Solid (ice) melting + 80 cal/gm LATENT HEAT OF FUSION

Phase changes -transfers of latent heat energy- Gas (water vapor) LATENT HEAT OF VAPORIZATION + 597 cal/gm sublimation evaporation Liquid Solid (ice) melting + 80 cal/gm LATENT HEAT OF FUSION

Phase changes -transfers of latent heat energy- Gas (water vapor) LATENT HEAT OF VAPORIZATION 597 80 100 +777 cal/gm + 597 cal/gm sublimation evaporation Sublimation in winter: low temp, intense sunlight, winds; 100 to raise it to boiling point; Heat required to change phase without temp change Liquid Solid (ice) melting + 80 cal/gm LATENT HEAT OF FUSION

Phase changes -transfers of latent heat energy- Gas (water vapor) +777 cal/gm + 597 cal/gm sublimation evaporation deposition (-777) condensation (- 597) freezing (-80) Liquid Solid (ice) melting + 80 cal/gm

Expressions of atmospheric humidity: 1. vapor pressure 2. relative humidity 3. dew point 4. mixing ratio

1. Vapor Pressure The total pressure of a mixture of gases equals the sum of the pressures exerted by the constituent gases. Dalton’s Law Water vapor is one of the gases in the atmosphere that contributes to air pressure. Vapor pressure is the portion of the air pressure due to water vapor.

Saturation concept Constant two-way interchange at surface; EVAPORATION: more molecules enter gas phase. CONDENSATION: more enter liquid phase. EQUILIBRIUM: equal amount become liquid and gas. At equilibrium, vapor pressure is SATURATION VAPOR PRESSURE vapor liquid

Saturation vapor pressure At higher temperatures, the amount of water vapor in the atmosphere (and therefore the saturation vapor pressure) is higher. Raising temperature increases energy of molecules and more readily escape surface as gas. Initially evaporation prevails, but eventually a new equilibrium is established. Higher temp, higher water vapor conc, higher equilibrium/saturation V.P.

Saturation vapor pressure At higher temperatures, the amount of water vapor in the atmosphere (and therefore the saturation vapor pressure) is higher. Notice the difference between vapor pressure and saturation vapor pressure Vapor pressure (on graph axis) is partial pressure exerted by the actual amount of water vapor in the atmosphere What if…. temperature is 30ºC and vapor pressure is 20 mb? Net evaporation or net condensation?

Saturation vapor pressure At higher temperatures, the amount of water vapor in the atmosphere (and therefore the saturation vapor pressure) is higher. if temperature drops to 20º ?

Saturation vapor pressure At higher temperatures, the amount of water vapor in the atmosphere (and therefore the vapor pressure) is higher. There is less energy available for evaporation ; condensation will occur, then a new equilibrium is reached.

“Warm air can hold more water vapor than cold air” “When temperature is higher, there’s more thermal energy for evaporation , so there is more water vapor in the air” “Warm air expands and can hold more water vapor” “There’s always plenty of room for water vapor molecules”. (Average intermolecular distance is 145 cm.)

Now look at inset graph:

Now look at inset graph: Below freezing, sat. vapor pressure over ice is different than saturation vapor pressure over (supercooled) water.

Now look at inset graph: Below freezing, sat. vapor pressure over ice is different than saturation vapor pressure over (supercooled) water. Which is greater? Does it take more water molecules to saturate air over water or over ice? Over water

Supercooled water is super cool! Water can exist as liquid at temps below freezing if it doesn’t have a means to undergo nucleation. Water droplets need a freezing nucleus (ice crystals) Small droplets can supercool. The vapor pressure for a curved surface (like a droplet) is higher than for a flat surface. If the radius is small enough (like a very small water droplet), the amount of supersaturation needed for condensation to occur is so large, that it does not happen naturally.

2. Relative humidity Amount of moisture in air relative to saturation amount, expressed as %. actual amount water vapor X 100 saturation amount

If we have a parcel of air at 10ºC and the vapor pressure is 6 mb, what is the relative humidity? 6 X 100 = 50% 12

If temperature increases (and actual amount of water vapor stays same), what happens to relative humidity?

3. Dew Point Temperature Temperature at which cooling air becomes saturated. Dew Clouds Frost

Vapor Pressure= 10 mb Morning: 10°C ? Temperature Relative humidity Dew point 10°C ?

Vapor Pressure= 10 mb Morning: 10°C 83% ? Temperature Relative humidity Dew point 10°C 83% ?

Vapor Pressure= 10 mb Morning: 10°C 83% 5ºC Temperature Relative humidity Dew point 10°C 83% 5ºC

Vapor Pressure= 10 mb Morning: 10ºC 83% 5ºC Afternoon: 30ºC ? Temperature Relative humidity Dew point 10ºC 83% 5ºC Afternoon: 30ºC ?

Vapor Pressure= 10 mb Morning: 10ºC 83% 5ºC Afternoon: 30ºC 24% ? Temperature Relative humidity Dew point 10ºC 83% 5ºC Afternoon: 30ºC 24% ?

Vapor Pressure= 10 mb Morning: 10ºC 83% 5ºC Afternoon: 30ºC 24% Temperature Relative humidity Dew point 10ºC 83% 5ºC Afternoon: 30ºC 24%