Precipitation Processes:

Why does it fall on us?

• 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

Evaporation and Condensation

• 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 bythe amount of pressure exerted on the atmosphere by water vaporwater vapor

• saturation vapor pressure (SVP) – the max. vapor pressure

Vapor Pressure

• 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)

Relative Relative HumidityHumidity

(RH)(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)

• 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 (DP)

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

• 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

Cooling Air to the Dew Point

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

• 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

The Environmental Lapse Rate

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

• 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

Radiation 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

Advection Fog

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.