Precipitation Processes: Why does it fall on us?

1. Precipitation Processes: Why does it fall on us?

2. Evaporation and Condensation • evaporation  liberation of water molecules, requires energy • water vapor increases in air as surface water evaporates • Upon saturation, condensation will begin • saturation: equilibrium between evaporation and condensation • Sublimation: ice  vapor Deposition: vapor  ice

3. Vapor Pressure • Dalton’s Law: total pressure consists of the individual partial pressures • of the gases in atm • vapor pressure - the amount of pressure exerted on the atmosphere by • water vapor • saturation vapor pressure (SVP) – the max. vapor pressure

4. Relative Humidity (RH) • indicates the amount of water vapor in the air relative to the possible • maximum • RH = content/capacity • saturation (100% RH): content = capacity • RH is dependent on air temperatureand total water vaporpresent • the saturation vapor pressure for warm air is much higher than cold • air (exponential relationship)  temperature dependency

5. Relative Humidity(RH) • RH = content/ capacity • saturation: content = capacity (100%) • dew point temperature = temp at which a given mass of air becomes saturated • daily patterns (high RH in morning; low in afternoon) • expressing RH: vapor pressure (mb) and specific humidity (g/kg)

6. Dew Point (DP) • temp at which a given mass of air becomes saturated • increase vapor content • chill air • good indicator of moisture content in air • high DP – abundant vapor present in atm. • if DP is much lower than air temperature RH is low • If DP is equal to air temperature RH is high

7. Dew point/temperature relationships in a) unsaturated air b) and c) saturated air

8. Cooling Air to the Dew Point • Condensation occurs when: • moisture is added to air • cold air is mixed with warm, moist air • air temperature is lowered to the Dew Point (DP) • Condensation  clouds  precipitation? • Change temperature by: • diabatic processes – adding/removing heat • adiabatic processes - no addition/removal of heat

9. Diabatic Processes • involves the addition/removal of heat energy • e.g. movement of air mass over a cool surface  loses energy • through conduction • energy is transferred from areas of high temperature toward those of • lower temperature • 2nd Law of Thermodynamics • associated with fog development

10. Adiabatic Process • when temperature changes w/o addition/removal of heat • Cloud formation: primarily due to temperature changes • with no heat exchange with surrounding environment • 1st Law of Thermodynamics  expanding air cools, compressed warms • e.g. inflating a bicycle tire

11. rising air expands  cools  rises through a less dense atmosphere • expand and cool at the dry adiabatic lapse rate = 1oC/100 m • eventually cools to DP  aka lifting condensation point (height at • which saturation occurs) • if parcel continues to rise it cools at saturated adiabatic lapse rate • (SALR) = 0.5oC/100m • sinking air is compressed and warms at DALR Dry adiabatic cooling

12. The Environmental Lapse Rate • environmental (ambient) lapse rate (ELR) refers to an overall decrease in air temperature with height • The ELR is related to the distance btw a parcel of air and the surface (heat source) • ELR changes diurnally from place to place

13. Forms of Condensation: Things That Make it Wet • saturation  droplets or ice crystals • condensation/deposition  clouds, fog, dew, frost • Dew • liquid condensation on surface • occurs early morning on windless, cloudless days • air immediately above ground cools, reaches Dew point • diabatic process

14. Frost • ~ to dew BUT saturation occurs below 0oC • deposits white ice crystals  known as hoar frost • e.g. car windshield • phase change from vapor directly to solid (deposition) • diabatic process

15. Frozen Dew • results when saturation occurs slightly above 0oC  liquid dew • formed, when Temp drops liquid dew freezes • forms thin sheet of ice, tightly bound to surface • dangerous – black ice

16. Fog • can be considered a cloud with base at ground level • air has either been: • cooled to dew point • had moisture added (breath) • mixed with warm moist air (steam fog) • 3 different types associated with dew point • radiation • advection • upslope

17. Radiation Fog • occurs when near surface air chills diabatically through loss of • longwave rad’n  reaches dew point • requires cloudless nights and light wind to create mixed layer • excess wind speed will enable warmer air to mix with near surface • air  evaporate the fog • ‘burns’ off with sunrise – evaporates from below due to surface • heating • e.g. Central Valley, CA (Tule Fog) • Coast ranges, Sierra Nevada with • light winds, cold conditions in winter • abundant moisture in atm

18. Advection Fog • occurs when warm moist air moves across a cooler surface • air is chilled diabatically to saturation • common on the U.S. west coast  warm, moist air from Pacific advects over the cold California current • Frequently develop near boundaries of opposing ocean temperatures • e.g.: northeast coast of the U.S., Gulf Stream and Labrador current

19. Upslope Fog • develops due to adiabatic cooling • occurs when air is lifted over topographic barriers, mountains • air expands and cools as it rises • common in region between Great Plains • and Rocky Mountain foothills

20. Different types of fog found throughout the U.S.