Part 2. Water in the Atmosphere

Chapter 5.

Atmospheric Moisture

The Hydrologic Cycle

Water covers 70% of the Earth’s surface

The Hydrologic Cycle shows how H2O cycles from the surface and subsurface to the atmosphere and back to the surface.

The movement of water vapor molecules exerts vapor pressure

Evaporation -- H20 goes from liquid phase to gas phase

Condensation -- H20 goes from gas phase to liquid phase

Evaporation exceeds condensation

Solid surface prevents evaporation

Evaporation equals condensation -- Saturation vapor pressure

Indices of Water Vapor ContentHumidity • The amount of water vapor in air

– Expressed in many ways

Vapor pressure• Pressure exerted on the atmosphere by water

vapor– Dependent on temperature and density

Saturation Vapor Pressure = maximum water vapor pressure possible (100% humidity)

Saturation vapor pressure line: condensation equals evaporation(=100% humidity; saturation)

More evaporation than condensation (<100% humidity;

undersaturation)

More condensation than evaporation (>100% humidity; supersaturation)

Measures of Water Vapor Content in the Air• Absolute Humidity• Specific Humidity• Relative Humidity• Mixing Ratio

Air Temperature

Relative Humidity

The relationship between Relative Humidity and TemperatureActual amount of water vapor in air

Water vapor content for saturation

For same water vapor content:

Air at 14°C has relative humidity

of 60%

Air at 25°C has relative humidity

of 30%

Dew Point = Temperature above freezing at which saturation occurs (i.e., dew forms)

Frost Point = Temperature where saturation occurs below the freezing point (i.e., frost forms)

When the air temperature drops to the dewpoint, the relative humidity is 100%

Relative Humidity:

80%

100%

100%

Conditions that can lead to saturation

Addition of water vapor to the air (by evaporation)

Mixing cold air with warm, moist air

Cooling air to the dew point (by IR radiation)

Larger drops have less curvature than smaller ones

The greater the curvature of a drop, the greater the rate of evaporation from the drop; very small drops can have supersaturated conditions near them

Humidity near droplet surface = 100%

Humidity near droplet surface > 100%

Small droplets require higher Relative Humidities to remain liquid without completely evaporating

Condensation in the atmosphere normally occurs around condensation nuclei. (Water vapor does not condense in pure air.)

Condensation nuclei can be dust, ash, spores, soot, salt, etc., also called (hygroscopic nuclei).

Dissolved hygroscopic nuclei in water droplets reduce the evaporation rate of the droplets

Water droplets in the atmosphere can be supercooled (below 0° C)

Deposition (water vapor directly to ice) in the atmosphere occurs around ice nuclei

Supercooled water in the atmosphereAtmospheric water does not freeze at 0oC (32oF)

Leads to supercooled water

At or below -40oC (-40oF) = spontaneous nucleation

High Humidities and Human DiscomfortHeat index• Combines heat and humidity factors

High humidity reduces evaporation • Reduction in the cooling power of

perspiration

Heat Index Tables

Diabatic process -- A process that changes the temperature of a gas, liquid or solid through the direct addition or removal of heat energy

Adiabatic process -- A process that changes the temperature of a gas, liquid or solid without any addition or removal of heat energy

The Second Law of Thermodynamics -- Energy always transfers from areas of higher temperature to areas of lower temperature

Processes for Heating and Cooling Air

Dry adiabatic cooling

When air rises rapidly without condensation, it cools at the dry adiabatic lapse rate -1oC/100m (-5.5oF/1000ft). When air sinks rapidly without condensation, it warms at the dry adiabatic lapse rate 1oC/100m (5.5oF/1000ft).

Saturated (or moist) adiabatic cooling

When saturated air rises rapidly, it condenses and cools at the saturated adiabatic lapse rate of about -.5oC/100m (about -3.3oF/1000ft). Air cannot stay saturated when it sinks; it always sinks at the dry adiabatic lapse rate.

T=9.5°C

T=9.0°C

T=8.5°C

The Environmental Lapse Rate is the change in air temperature with height as measured by a rising weather balloon

The environmental lapse rate changes with the the time of day and variations in wind direction

A comparison of adiabatic and environmental cooling rates

Air inside rising balloon tries to cool at the adiabatic lapse rate

Environmental lapse rate (air temperature outside balloon)

Forms of Condensation

DewLiquid condensation on surface objects

Frost (white or hoar)Deposition in below-freezing conditions

Frozen Dew Dew formation followed by a temperature drop

Creates a tight surface bond

Radiation Fog• Diabatic chilling of near surface due to radiational cooling

• A slight breeze is required

Advection FogWarm, moist air moving over a cooler surface

• Diabatic process

Upslope Fog

Adiabatic process from upslope advection

Precipitation FogEvaporating rain

Steam FogWater evaporated into cold, dry air

Radiation fog in the Central Valley

Different types of fog found throughout the U.S.

Formation and Dissipation of Cloud Droplets

Clouds formed through adiabatic cooling of rising air

50 m above the LCL (lifting condensation level) all condensation nuclei are used

Additional growth occurs instead of new drop formation