Transcript Slide 1
Precipitation Processes: Why does it fall on us? 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 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 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 temperature and total water vapor present • the saturation vapor pressure for warm air is much higher than cold air (exponential relationship) temperature dependency 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) 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 Dew point/temperature relationships in a) unsaturated air b) and c) saturated air 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 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 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 • 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 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 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 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 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 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 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 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 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 Different types of fog found throughout the U.S.