Humidity

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

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 a 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.

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 This will be important when we study how precipitation happens

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. vapor pressure for curved surface (like a droplet) is higher than for flat surface. If radius is small enough (like a very small water droplet), amount of supersaturation needed for condensation 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

What is FOG? Similar to clouds but forms at surface. To cause surface layer of air to saturate, what has to happen? Surface air has to cool to dew point OR Water Vapor has to be added to the air.

Cool the surface air: RADIATION FOG Surface cools by terrestrial radiative loss of heat during night; if air cools below dew point, condensation occurs. Favored by cloudless skies, light wind to mix lower layer.

Cool the surface: VALLEY FOG Cold, dense air sinks into valley.

Cool the surface: ADVECTION FOG Cold current Air moves horizontally over cold surface; cools to dew point

Advection fog over snow

Cooling the surface: UPSLOPE FOG Air blowing towards mountains, forced to rise and cool.

Adding water vapor to the air: PRECIPITATION FOG Happens as raindrops evaporate while falling.

Add water vapor to air: STEAM FOG Steam fog over Lake Superior, X-mas 2017: warmer moist air over lake mixes with colder air.

a closer look at dew point temperature : Even “clear” air contains microscopic water droplets, BUT evaporation > condensation so drops don’t survive long enough to clump and grow into cloud droplets.

Why do droplets have to grow to become cloud droplets?

Why do we never hear of relative humidities = 100% ? (even if it is raining) Because RH is measured at surface; not at level of clouds. For same reason there can be clouds in sky and a reported RH < 100%

Dry line is a humidity front between dry and wet air

Spread between temp and dew point on station models is indication of humidity