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Unit 11: Atmospheric Moisture and the
Water Balance
Properties of Water
Water Vapor & its
The Hydrological Cycle
Condensation, Clouds
Precipitation Processes
Surface Water Balance
The hydrological cycle in action.
Discuss the various forms of water and understand the
important heat transfers that accompany changes of these
physical states
• Explain the various measures of atmospheric humidity, how
they are related, and the processes responsible for
• Outline the hydrologic cycle and the relative amounts of
water that flow within this cycle.
• Understand the process of evaporation.
• Examine the conditions necessary for the formation of clouds.
• Introduce the concept of precipitation
• Describe the Earth’s surface water balance and its variations
Phase Changes of Water, Latent Heat
Heat is consumed in evaporation, melting; heat is
released in condensation, freezing (sublimation).
Schematic view of the molecular structure of water in its three physical states and heat-energy exchange among those
states. The latent heat-exchange numbers between the arrows are explained in the text (values are for 0°C).
Measurements of water vapor
Vapor pressure is the pressure exerted by water vapor molecules
Saturated vapor pressure is maximum pressure of water vapor at that
Dew point is the temperature the air must be cooled to reach
Relative humidity is ratio of water vapor in air
to maximum the air can hold at that
RH is usually highest when daily temperature is lowest.
Specific humidity is the mass of water
Vapor in the air per unit mass of air.
Mixing ratio is the mass of water vapor in
The air to the mass of dry air containing the
Water vapor.
Variation of saturation vapor pressure (mb) with temperature (C). The curve is nearly a pure exponential. At
temperatures below 0C saturation values over supercooled water are greater than over ice.
Relative Humidity
Figure 5.7
Humidity Patterns
Figure 5.10
Figure 5.12
Evaporation, Evapotranspiration
• The rate of evaporation depends on: temperature, humidity, wind speed
and water quality (salinity)
• Plants lose water to the atmosphere through transpiration
• Evapotranspiration is the loss of water from the soils and plants to the air
• Potential evapotranspiration (PE) is the maximum evapotranspiration lost
when abundant water is available
• Actual evapotranspiration (AE) is the amount of water lost in any actual
amount of soil moisture
• AE is usually less than PE, except when there is abundant water available,
like a swamp or water surface. In deserts, PE might be very high, while AE
is very low.
Fig 12.2
Hydrologic cycle.
The numbers attached to the stages express each value as the volume
of water divided by Earth surface area. Thus the values shown represent the depth of water
(centimeters per year) associated with each mass transfer. All can be directly compared to the
global average precipitation rate, which is about 100 cm/year.
Water in the Hydrosphere
• Most of the water is salt water in the ocean
• Most of the fresh water is locked up in ice
sheets and glaciers
• Most of the liquid fresh water is in the
Fig 12.3
Distribution of water in the hydrosphere. The middle and lower bars show the percentage distribution of the 2.8
percent of total hydrospheric water that is fresh. Of that freshwater component, only about one-tenth is easily
available to humans.
Water Usage in the United States, 2005
Which states use the most water?
Total water withdrawals (millions gallons per day) for the United States in 2005.
Condensation and Clouds
• For water vapor to condense into liquid, there must be
• -condensation nuclei for water molecules to condense upon
• -sufficient water vapor to reach saturation by either cooling
or through evaporation of more water vapor
• Condensation at the ground is dew or frost (if temperature is
below freezing)
• Condensation near the ground can form fog, if the cloud is in
contact with the surface
• Condensation above the ground can form clouds
Condensation near the ground forms fog, a
cloud in contact with the ground.
As the ground and surface air cools to the dewpoint, water vapor condenses into a
radiation fog (cooling by longwave radiation overnight) here in East Africa.
Advection Fog
Figure 5.20
Evaporation Fog
Figure 5.21
Valley Fog
Figure 5.25
Figure 5.22
Figure 5.23
Cloud Types
Schematic diagram of the different cloud types – stratus,
cumulus, and cirrus, arranged by their typical altitude.
Precipitation Processes
The Ice-Crystal Process-requires the coexistence of ice and super-cooled
water droplets in the cloud. Ice grows at the expense of water droplets that
evaporate water molecules which adhere to the ice until they are large
enough to fall as snow. If the air is warm enough, the snow melts and rain
The Coalescence Process-requires different sizes
of water droplets within warm clouds. Larger
droplets grow by falling faster and sweeping up
smaller droplets by coalescing until they are
large enough to fall as rain.
Source: http://collider.com/singin-in-the-rain-60th-anniversary-blu-ray-review/191992/
Types of Precipitation
• Besides rain and snow, there is:
• Sleet-melting ice that refreezes before reaching
• Freezing rain-melting ice that freezes on contact
with a frozen surface
• Hail-ice particles that grow within
clouds that have strong updrafts
• Graupel-soft, partially melted hail
Four major forms of precipitation. (A) A rainstorm douses the Ponderosa Pine Forest near Flagstaff, Arizona. (B)
Falling snow accumulates in south-central Alaska. (C) Freezing rain forms an icy coating on pine needles on a
golf course in Wawona, near Yosemite National Park, California. (D) Golf-ball-sized hailstones litter the
countryside following a storm in northern Texas.
Four Forms of Precipitation
C-freezing rain
The Water Balance
• Based on methods devised by climatologist C. Warren
Thornthwaite, calculates inputs and outputs of water at the
Earth’s surface based on simple formulae using monthly
temperatures and precipitation of a station.
• When PE is greater than P (precipitation), there is water loss
• When P is greater than PE, there is a water gain
• By calculating monthly PE values and comparing with P, one
can calculate amounts of surplus (runoff) and deficit (not
enough soil moisture) in surface water.
• Different climates exhibit different water balances
Water Balance for Different Climates
Range of water balance conditions found at the surface of Earth. (A) Baghdad, Iraq,
experiences a constant deficit because potential evapotranspiration normally exceeds
precipitation. (B) At Tokyo, Japan, the situation is reversed, and a constant water surplus
is recorded. (C) At Faro, Portugal, the intermediate situation occurs, with a combination
of surplus and deficit at different times of the year.
Water Balance Averages by Latitude
Average annual latitudinal distribution of precipitation, evapotranspiration, and runoff in cm per year. The
arrows show the direction of the water vapor flux by the atmospheric circulation.
Global Distribution of Annual Evaporation, Evapotranspiration
Fig 12.10?
Global distribution of annual evaporation and evapotranspiration in centimeters, with land elevations
adjusted to sea level. Red isolines show the pattern over land; blue isolines over the oceans.
Global Distribution of Annual Precipitation
Source: http://www-das.uwyo.edu/~geerts/cwx/notes/chap10/global_precip.html
Global distribution of annual precipitation in millimeters/day.

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