Part 2. Water in the
Atmosphere
Chapter 5.
Atmospheric Moisture
The Hydrologic Cycle
The
Hydrologic
Cycle
shows how
H2O cycles
from the
surface and
subsurface
to the
atmosphere
and back to
the surface.
Water covers 70% of the Earth’s 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
Solid surface
prevents
evaporation
Evaporation exceeds Evaporation equals
condensation -condensation
Saturation vapor
pressure
Indices of Water Vapor Content
Humidity
• 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
condensation
than evaporation
(>100%
humidity;
supersaturation)
More evaporation
than condensation
(<100% humidity;
undersaturation)
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 Temperature
Actual 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)
Relative Humidity:
80%
100%
100%
When the air temperature drops to the dewpoint, the relative humidity is 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 atmosphere
Atmospheric water does not freeze at 0oC
(32oF)
Leads to supercooled water
At or below -40oC (-40oF) = spontaneous
nucleation
High Humidities and Human Discomfort
Heat index
• Combines heat and humidity factors
High humidity reduces evaporation
• Reduction in the cooling power of
perspiration
Heat Index Tables
Processes for Heating and Cooling Air
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
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
T=8.5°C
T=9.0°C
T=9.5°C
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.
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
Dew
Liquid 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 Fog
Warm, moist air moving over a cooler surface
• Diabatic process
Upslope Fog
Adiabatic process from upslope advection
Precipitation Fog
Evaporating rain
Steam Fog
Water 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