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  through conduction loses energy • energy is transferred from areas of high temperature toward those of lower temperature • 2 nd 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 •1 st Law of Thermodynamics  • e.g. inflating a bicycle tire expanding air cools, compressed warms

• rising air expands  cools  rises through a less dense atmosphere • expand and cool at the

dry adiabatic lapse rate =

1 o C/100 m • eventually cools to DP  which saturation occurs) aka lifting condensation point (height at • if parcel continues to rise it cools at saturated adiabatic lapse rate (SALR) = 0.5

o C/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 0 o C • deposits white ice crystals  • e.g. car windshield known as hoar frost • phase change from vapor directly to solid (deposition) • diabatic process

Frozen Dew

• results when saturation occurs slightly above 0 o C  formed, when Temp drops liquid dew freezes liquid dew • 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.