Meteo 3: Chapter 4 Water Vapor and Clouds Read Chapter 4

Who Cares About Water Anyways?  Phase changes of water important for energy transport in atmosphere  Clouds!  Precipitation!

Overview  Cloud: Collection of liquid water drops or ice crystals  Clouds form as –1) Water vapor condenses onto small particles known as condensation nuclei to form liquid water drops, or –2) Water vapor deposits onto small particles known as ice nuclei that allow for ice crystal formation  In a cloud, water can be present in all three phases at the same time

Hydrologic Cycle  Global precipitation = Global evaporation

Terms  Evaporation: liquid water molecules break bonds with other molecules to escape to gaseous phase  Condensation: Water vapor returns to liquid state  Sublimation: Ice changes directly to water vapor  Deposition: Water vapor changes directly to ice  Transpiration: Plants releasing water vapor into air

Temperature and Evaporation  Evaporation occurs when liquid water molecules gain enough kinetic energy to break bonds –The higher the temperature of water, the higher the kinetic energy of its molecules, thus the higher the evaporation rate  Evaporation is a cooling process

Balance of Evaporation & Condensation  Net Condensation: Condensation exceeds evaporation  Net Evaporation: Evaporation exceeds condensation  Vapor Pressure: Water vapor’s contribution to total pressure

Equilibrium Vapor Pressure & Temperature

Birth of Clouds  Relative Humidity: (vapor pressure / equilibrium vapor pressure) * 100 –At saturation (rate of condensation = rate of evaporation), RH = 100% –Clouds form when RH exceeds 100% by a few tenths of a percent –Water vapor condenses onto CCN…some hygroscopic, meaning they attract water vapor –In summer, when RH exceeds 80%, net condensation occurs on some particles (pollution), leading to haze…associated with poor air quality

Mechanisms to induce cloud formation  For clouds to form, there must be net condensation  We can get this by cooling the air –As temperature lowers, molecular speeds decrease, and water vapor gathers near CCN –The amount of cooling needed is related to ratio of vapor pressure / equilibrium vapor pressure (RH)

Fog Formation by Cooling Air

Cooling via lifting  Air pressure (density) decreases with height  Rising air parcels expand, cooling as they do work on environment  If vapor pressure > equilibrium vapor pressure => condensation

Clouds due to Lifting

Orographic Lifting: Lifting by Terrain  Windward side of mountain, facing prevailing wind, is extremely wet  Leeward side, sheltered from wind, very dry…known as rain shadow

Clouds due to Terrain

Orographic Lifting: California

Mixing Warm & Cold Air Masses

Assessing Air’s Moisture Content  Problems with RH because denominator depends on temperature –Cold, dry air masses can have a high RH, even if they hold little water vapor –Relative humidity varies with time of day –

Dew Point: Absolute measure of water vapor  Dew Point: Temperature air must be cooled (at constant pressure) to reach saturation –Less than or equal to temperature –Higher the dew point, more water vapor in air –Frost point if air temperature below 32ºF –Measured with a hygrometer or sling psychrometer –Changes by evaporating water into air, mixing drier air from above, wind blowing in moist or dry air from another region (air dries behind cold front, moistens before cold front)

Applying Dew Point to Weather Forecasting  1) Cloud Base Height –Temperature of rising air decreases faster than dew point…has a decent chance of eventually reaching dew point  2) First-Guess Low Temperature  3) Severe Weather –High dew points indicate enhanced risk