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
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