Geography 1 (41100). Prof. Dean . Wint., Jan 12, 2012 . Lect and Chp 6
1
Chapter 6
Atmospheric Moisture
I. The Impact of Atmospheric Moisture on the Landscape
a. Atmospheric moisture affects the Earth in the short and long term
i. Short Term= puddles, flooding, snow, ice, etc.,
ii. Long Term= Weathering and erosion and if present growth of
vegetation.
b. The ground loses water through run-off and evaporation, and it gains
water by rain.
II. The Nature of Water: Commonplace but Unique
a. Water occupies 70% of the surface of the planet, is the most
distinctive substance on Earth, and is the most essential ingredient for
life.
b. The Water Molecule
i. Built of atoms built of protons, neutrons and electrons, which
are held in valence shells.
ii. Two or more elements can be held together to form a molecule.
iii. Atoms can be held together by an ionic, covalent (where
elements share an electron) or a hydrogen bond. Depending on
the type of bond polarity can develop in the molecule.
c. Properties of Water
i. Is generally found as a liquid.
ii. While most liquids continue to contract as they freeze, water
contracts to a specific point and then expands. This unique
property is important in weathering, where water penetrates
the cracks of objects.
iii. Water also becomes less dense as it approaches its freezing
point so water in lakes or oceans rises as it freezes, so that
lakes freeze from the top down.
iv. Because of its hydrogen bonding, water creates a high surface
tension that allows water to bead together.
v. Surface tension and adhesion allow water to rise vertically up
through narrow openings in an action called capillarity.
vi. It is considered the universal solvent.
Geography 1 (41100). Prof. Dean . Wint., Jan 12, 2012 . Lect and Chp 6
2
vii. It has a high heat capacity, which is its ability to hold onto heat
energy.
III. Phase Changes of Water
a. Water is found in three stages: liquid, solid, and gas.
b. Water vapor can easily change to form ice or water.
c. Through a process of phase changes, where latent heat is either stored
or released, water can move between its three stages.
d. Latent Heat- is the amount of energy that must be released or
absorbed/stored to create a phase change between different states. If
you want to move from 1gram of ice to water you need 80 cal and
once you have changed that to water you have applied 80 cal of
energy to the ice and made water that will have latent heat within it.
To turn it back to ice you have to remove those 80 cal of energy out.
IV. Water Vapor and Evaporation
a. Evaporation: is the conversion of liquid water to a gaseous state
through an increase in heat and latent energy, which breaks many of
the hydrogen bonds that keep water together as water becomes more
agitated by heat.
i. The rate of evaporation depends on the temperature of the
water, the air, the amount of water vapor in the air and
whether the air is moving or not.
b. Makes up as much as 4% of total atmospheric volume and exists in the
Troposphere.
c. Vapor pressure is the amount of pressure exerted by water vapor.
d. Warmer temperatures evaporate more water.
e. Windiness increases evaporation.
f. Evapotranspiration (relates to plants)
i. Is water that comes from (1) soil and other inanimate surfaces
or from plants.
V. Measures of Humidity- amount of water vapor in the air
a. Actual Water Vapor Content
i. Absolute Humidity-the mass of water vapor in a given volume
of air.
ii. Specific Humidity- the mass of water vapor in a given mass of
air.
Geography 1 (41100). Prof. Dean . Wint., Jan 12, 2012 . Lect and Chp 6
3
iii. Vapor Pressure- contribution of water vapor to total
atmospheric pressure
iv. Relative Humidity- how close the air is to saturation with
water vapor.
v. Saturation represents the maximum amount of water vapor
the air can hold.
vi. Saturation depends on temperature.
vii. Saturation vapor pressure- is the maximum possible vapor
pressure at a given temperature.
viii. Temperature and relative humidity are inversely related. As
one increases the other decreases. Generally this relationship
will also mean that as relative humidity increases so does
atmospheric pressure.
ix. Dew point temperature: is the critical air temp at which
saturation is reached.
x. Sensible temperature: is a concept dealing with how
temperature is sensed by the human body. The implication of
this theory is that on a dry humid day, your body will feel like
the air is not as hot as the thermometer says it is, therefore the
sensed temperature is low.
VI. Condensation
a. Conversion of water vapor to liquid water
b. In order for condensation to take place, the air must be saturated,
which usually, but not always, happens when the air is cooled to a
temperature below the dew point.
c. Surface tension makes it nearly impossible to grow pure water
droplets.
d. Supersaturated air is made if there is no surface available on which
condensation can occur.
e. Need particle to grow droplet around, a cloud condensation nuclei.
f. Liquid water can persist at temperatures colder than 0C without a
nuclei.
VII. Adiabatic Processes
a. Adiabatic process= cooling of air by air expansion in higher altitudes.
Geography 1 (41100). Prof. Dean . Wint., Jan 12, 2012 . Lect and Chp 6
4
b. You cannot have rain without rising air and as it cools through the
adiabatic process it cools enough to begin to condensate.
c. The dry adiabatic lapse rate = 10C per 1000 meters (does not mean
the air is dry but rather is unsaturated)
d. Lifting condensation level (LCL)
e. Saturated adiabatic lapse rate.
f. Parcel lapse rates versus environmental lapse rate.
VIII. Clouds
a. 50% of Earth is at any time covered by clouds, which provide all the
precipitation on Earth.
b. Influence on radiant energy
c. Classification on the basis of form and altitude (3 primary forms) and
they often are transitioning between one and another. There are 7
additional specific types that are combinations of the primary 3.
i. Cirrus clouds- thin and wispy
ii. Stratus clouds- cover the entire sky
iii. Cumulus clouds- massive, rounded and sometimes billowing
upward.
d. Cloud Types
i. High clouds- thin and white, composed of ice crystals.
ii. Middle clouds- puffy indicate settled weather, lengthy clouds
are indicator of changing weather.
iii. Low clouds- widespread and associated with somber skies.
iv. Clouds of vertical development.
1. Grow upward from low bases to heights of over 15km
occasionally
e. Fog
i. Simply clouds on the ground, but develop differently.
ii. Radiation: develops from heat loss form the ground.
iii. Advection: warm air moving horizontally over a cold surface.
iv. Upslope: humid air climbing up a topographic slope.
v. Evaporation: water vapor is added to cold air.
IX. Dew and Frost
a. Dew
i. Usually originates from terrestrial radiation.
Geography 1 (41100). Prof. Dean . Wint., Jan 12, 2012 . Lect and Chp 6
5
ii. Moisture condensation on surfaces that have been cooled to
saturation.
iii. Will appear as water droplets
b. Frost
i. Occurs when air temperature lowers to saturation point, when
the saturation point is below OC
ii. Will appear as large numbers of small white crystals.
X. The Buoyancy of Air
a. The buoyancy of air is how freely and extensively it rises.
b. Stable air- parcel of air is nonbuoyant, will not rise without an
external force. Is generally when cold air is beneath warm air.
i. Is not associated with cloud formation and precipitation.
c. Unstable air- parcel is positively buoyant, will rise without an external
force.
d. Conditional instability.
e. Determining Air Stability
i. Is related to the lapse rate of the surrounding air (the
environmental lapse rate), the dry and saturated adiabatic
rates and the temperature.
ii. A rough indication can be directly observed by viewing the
state of the sky.
XI. Precipitation
a. Condensation alone is insufficient to produce raindrops.
b. Collision/Coalescence= warm clouds, where condensation droplets
must coalesce into drops large enough o fall as precipitation.
c. Ice-Crystal Formation= cool clouds, ice crystals grow at the expense of
the water droplets until they are large enough to fall.
d. Forms of Precipitation: Rain, Snow, Sleet, Glaze (freezing rain), Hail.
XII. Atmospheric Lifting and Precipitation
a. Convective Lifting- warming from unequal heating.
b. Orographic Lifting- lifting by air being forced over a topographic
barrier.
c. Frontal Lifting- When unlike air masses meet but do not mix.
d. Convergent Lifting- when air converges and due to crowding lifts.
XIII. Global Distribution of Precipitation.
Geography 1 (41100). Prof. Dean . Wint., Jan 12, 2012 . Lect and Chp 6
6
a. High precipitation regions = tropics
b. Low precipitation regions = deserts and poles
c. Isohyet= a line joining points of equal quantities of precipitation
d. Factors that determine precipitation
i. Determined by the nature of the air mass involved and the
degree to which the air is uplifted.
ii. Dry lands are most prominent on the western sides of
continents in subtropical latitudes.
iii. Dry regions in the midlatitudes are most extensive in central
and southwestern Asia.
iv. In the high latitudes, there is not much precipitation anywhere.
e. Seasonal Precipitation Patterns.
i. Most pronounced over continental interiors.
ii. Coastal regions are often more balanced.
iii. The seasonal shifting of major pressure and wind systems,
follows the sun.
iv. Summer is the time of maximum precipitation over most of the
world.
v. The most conspicuous variation in seasonal precipitation is
found, predictably, in monsoon regions.
f. Precipitation Variability
i. Is the expected departure from average precipitation in a given
year.
XIV. Acid Rain
a. Refers to the deposition of either wet or dry acidic materials from the
atmosphere on Earth’s surface.
b. Principal acids are sulfuric and nitric
c. Number of hydrogen ions defines PH
d. Dust-free air has a pH of 5.6, therefore any precipitation below 5.6 is
considered acidic.
e. Acid deposition has caused more erosion on the marble Parthenon in
Athens in the last 30 years than in the previous 30 centuries.
f. A major problem of acid rain is that much of the pollution is deposited
at great distances from its source.
Geography 1 (41100). Prof. Dean . Wint., Jan 12, 2012 . Lect and Chp 6
7