2. The Impact of Atmospheric
Moisture on the Landscape
• Atmospheric moisture
influences landscape
both in short term and
long term.
– Short term, with
puddles, flooding,
snow and ice;
– Long term, with
precipitation integral to
weathering and
erosion, critical to
vegetation.
4
The Nature of Water: Common place
but Unique
• Occurs in three forms in the atmosphere
– Ice
– Liquid
– Water vapor
• Fig. 6.1 5
• Properties of Water
– Changes State
• Liquid
• Solid
• Vapor
– Expands Upon Freezing
• Important in weathering of rock
• Basis of shelf ice and icebergs
– Adhesion (“Sticky”) – Fig. 6-4
• Surface tension
• Capillary action
6
2
3. Evaporation – liquid water
converted to the gaseous form.
Condensation – water vapor
converted to the liquid form.
Sublimation—the process by
which water vapor is converted
directly to ice, or vice versa.
• Fig. 6-5
7
Phase Changes of Water
• In each of the change
processes, there is a gain or
loss of heat, or latent heat.
• To convert one gram of ice
to one gram of liquid water
at 0°C, it requires 80
calories of heat absorbed.
• To raise the temperature of
one gram of liquid water at
0°C to the boiling point, 540
calories of heat must be
absorbed.
• For ice to sublimate to water
vapor, or water vapor to
sublimate to ice, 680
calories must be absorbed,
or released respectively.
8
Phase Changes of Water
• The energy that is absorbed
when water undergoes a
phase change from a solid
to a liquid or a liquid to a gas
is known as the latent heat
of vaporization.
• The energy that is released
when water undergoes a
phase change from a gas to
a liquid or a liquid to a solid
is known as the latent heat
of condensation.
9
3
4. Phase Changes of Water
• Importance of Latent Heat in the
Atmosphere — The absorption and
release of energy during evaporation and
condensation have several effects.
– Water can store energy when it evaporates.
– Water can release heat back to the
atmosphere when it condenses.
10
– Latent Heat
• Fig. 6-6
11
Water Vapor and the Hydrologic Cycle
• Hydrologic Cycle
• Fig. 6-7
12
4
5. Water Vapor and the Hydrological
Cycle
• Water vapor—the gaseous state of water;
atmospheric moisture.
– Changes easily from one state to another with
temperature and pressure changes.
• This ease of changing results in erratic distribution
around the world.
– Can be virtually absent in some parts of world,
constitutes as much as 4% of atmospheric
volume in other parts.
• Essentially restricted to lower troposphere.
13
Hydrological Cycle
The hydrologic cycle is the ceaseless
interchange of moisture in terms of its
geographical location and its physical state:
– Water evaporates, becomes water vapor;
– Goes into atmosphere;
– Vapor condenses, becomes liquid or solid state;
– Returns to Earth.
14
Hydrologic Cycle
• Hydrologic cycle intricately related to many
atmospheric phenomena.
– Important determinant of climate:
– Rainfall distribution
– Temperature modification
15
5
6. Evaporation
• Evaporation—process by which liquid
water is converted to gaseous water
vapor.
– Molecules of water escape the liquid
surface into the surrounding air.
– Water vapor is added to the air when
the rate of evaporation exceeds the
rate of condensation — net
evaporation in this instance.
– Rate of evaporation from a water
surface depends on three factors:
1. The temperature of the water and the
air,
2. the amount of water vapor already in
the air, and
– Fig. 6-8
3. whether the air is still moving.
16
Evapotranspiration
• Evapotranspiration—the process of
water vapor entering the air from land
sources.
– Evapotranspiration occurs through two
ways:
1. Transpiration—the process by which plant
leaves give up their moisture to the
atmosphere;
2. Evaporation from soil and plants.
17
Measures of Humidity
• Humidity
– the amount of water vapor in the air.
• Absolute Humidity
– a direct measure of the water vapor
content of air.
• Specific Humidity
– a direct measure of water-vapor
content expressed as the mass of
water vapor in a given mass of air
(grams of vapor/kilograms of air).
Red line is the maximum
absolute humidity
• Fig. 6-9
18
6
7. • Relative Humidity
• an expression of the
amount of water vapor
in the air in comparison
with the total amount
that could be there if the
air were saturated.
• a ratio expressed as a
percentage.
– Relative humidity
changes if either the
water vapor content or
the water vapor
capacity of the air • Fig. 6-11
changes.
• Temperature-Relative
Humidity Relationship
19
Temperature—Relative Humidity
Relationship
• Also changes if
temperature
changes.
– Relationship
between
temperature and
relative humidity is
one of most
important in all
meteorology.
• Inverse
relationship—as
one increases, the
other decreases.
– Relative humidity
can be determined
through the use of a
psychrometer 20
• Related Humidity Concepts
– Dew Point Temperature
• the critical air temperature at which saturation is reached.
• Cooling is the most common way that air is brought to the
point of saturation and condensation.
– Sensible Temperature
• Temperature as it feels to a person’s body
• Affected by humidity and wind
21
7
8. Condensation
• Phase change of gas
as to liquid
– Water vapor to water
droplets
• Requirements
– Decrease in
temperature (usually)
– Condensation nuclei
• tiny atmospheric particles • Fig. 6-12
of dust, smoke, and salt
that serve as collection
centers for water
molecules. 22
Adiabatic Processes
– Adiabatic
• Large masses of air can be
cooled to the dew point ONLY
by expanding as they rise.
• adiabatic cooling is the only
prominent mechanism for
development of clouds and
production of rain.
– Lapse rate
• the rate at which a parcel of
unsaturated air cools as it rises
Fig. 6-14
23
Lifting Condensation
Level (LCL)
• The altitude at which rising air cools.
sufficiently to reach 100% relative humidity at
the dew point temperature, and condensation
begins.
24
8
9. • Dry Adiabatic Lapse
Rate
– 10ºC (5.5ºF) 1,000 m-1
• Saturated Adiabatic
Lapse Rate
– 6ºC (3.3ºF) 1,000 m-1
• Fig. 6-14 25
• Comparisons of
Lapse Rates
• Fig. 6-15
26
– Fig. 6-16: Temperature changes in air as it crosses over a
mountain
27
9
10. Clouds
• Not all clouds precipitate, but all precipitation
comes from clouds.
• At any given time, about 50% of Earth is
covered by clouds.
• Clouds play an important role in the global
energy budget.
– Receive insolation from above and terrestrial
radiation from below.
– They absorb, reflect, scatter, or reradiate this
energy, and so influence radiant energy.
28
Clouds
• Clouds are classified on the basis of two
factors
• Form
• Altitude
29
Clouds
Three forms of clouds:
1. Cirri form clouds—a cloud that
is thin, wispy, and composed of
ice crystals rather than water
particles; it is found at high
elevations.
2. Stratiform clouds—a cloud
form characterized by clouds
that appear as grayish sheets or
layers that cover most or all of
the sky, rarely being broken into
individual cloud units.
3. Cumuliform clouds—a cloud
that is massive and rounded,
usually with a flat base and
limited horizontal extent, but
often billowing upward to great
heights. • Table 6-1
30
10
11. Cloud Forms
• These 3 cloud forms are subclassified into 10
types based on shape.
– One type may evolve into another.
– Three of these 10 are purely one form, while the
other 7 are combinations of these three.
• Three pure forms:
1. Cirrus cloud—high cirriform clouds of feathery
appearance.
2. Cumulus cloud—puffy white cloud that forms from
rising columns of air.
3. Stratus cloud—low clouds, usually below 6500 feet
(2 km), which sometimes occur as individual clouds
but more often appear as a general overcast.
31
Cloud Forms
• Precipitation comes only from clouds that
have “nimb“ in their name; specifically,
nimbostratus or cumulonimbus.
– Cumulonimbus cloud—cumuliform cloud of
great vertical development often associated
with a thunderstorm.
– Nimbostratus cloud—a low, dark cloud,
often occurring as widespread overcast and
normally producing precipitation.
32
Cloud Families
Four categories based on altitude:
1. High clouds — Altocumulus clouds—found
above 6 kilometers (i.e., cirrus clouds)
2. Middle clouds —between about 2 and 6
kilometers (i.e., altocumulus and alto stratus).
3. Low clouds — below 2 kilometers (i.e.,
stratocumulus and nimbostratus).
4. Clouds with vertical development (i.e.,
cumulus clouds).
33
11
12. – Subtypes of Cloud
Forms
• High clouds
• Middle clouds
• Low clouds
• Clouds of vertical
development
• Fig. 6-18 34
Cloud Types and
Identification
35
Cirrus
36
Figure 7.22
12
15. Cumulonimbus
43
Figure 7.22
Fog
• A cloud whose base is at or very near ground level.
• Types
– Radiation
• forms through loss of ground heat.
– Advection
• forms when warm moist air moves over a cold surface.
– Upslope
• caused by adiabatic cooling when humid air climbs a topographic slope.
– Evaporation
• when water vapor is added to cold air that is already near saturation.
44
• Distribution
– United States and southern Canada
Fig. 6-21
45
15
16. Dew
– Dew droplets
• Dew —the condensation of beads of water on relatively cold
surfaces; if temperature is below freezing, ice crystals (white
frost) forms.
– White frost
Fig. 6-22
46
The Buoyancy of Air
• Atmospheric Stability and Instability
Buoyancy—the tendency of an object to rise in a fluid.
-A parcel of air moves vertically until it reaches a level at
which the surrounding air is of equal density (equilibrium level).
47
Atmospheric Stability
• Stable air—resists vertical movement;
nonbuoyant, so will not move unless
force is applied.
• Unstable air—buoyant, will rise without
external force or will continue to rise after
force is removed.
– Air stability is related to adiabatic
temperature changes
48
16
17. – Conditionally Unstable Air
Conditional instability—
intermediate condition
between absolute
stability and absolute
instability. Occurs when
an air parcel’s adiabatic
lapse rate is somewhere
between the dry and wet
adiabatic rates. Acts like
stable air until an
external force is
applied; when forced to
rise, it may become
unstable if condensation
occurs (release of latent
• Fig. 6-24 heat provides buoyancy).
49
Determining Air Stability
• Accurate determination of stability of any mass
of air depends on temperature measurements,
but one can get a rough indication from looking
at cloud patterns.
– Unstable air is associated with distinct updrafts, which
are likely to produce vertical clouds.
– Cumulous clouds suggest instability.
– Towering cumulonimbus clouds suggest pronounced
instability.
– Horizontally developed clouds, most notably
stratiform, characterize stable air forced to rise.
– Cloudless sky indicative of stable, immobile air.
50
• Determining Atmospheric Stability (continued)
– Visual Determination
Fig. 6-26
51
17
18. Precipitation
• Most clouds do not yield
precipitation.
• Condensation alone is
insufficient to produce
raindrops.
– Fig. 6-27
52
Precipitation
• The Processes
– Still not well understood why most clouds do
not produce precipitation.
– Two mechanisms are believed to be
principally responsible for producing
precipitation:
• Collision and coalescence of water droplets
53
Collision and Coalescence
• Collision/Coalescence—most responsible for
precipitation in the tropics and produces much
precipitation in the middle latitudes.
– Rain is produced by the collision and coalescing
(merging) of water droplets
– No ice crystals because cloud temperatures are too
high.
– Must coalesce enough that the droplets become large
enough to fall.
– Coalescence is assured only if atmospheric electricity
is favorable, so that positively charged droplets collide
with negatively charged ones.
54
18
19. Bergeron Process
• Bergeron process—process by which ice crystal
formation occurs; is believed to account for the majority
of precipitation outside of tropical regions.
– Ice crystals and super cooled water droplets in cloud are in direct
competition for water vapor not yet condensed.
– Ice crystals will attract most of the vapor if liquid droplets are in
state of equilibrium.
– If ice crystals grow at expense of water droplets, the crystals will
grow large enough to fall.
– As they descend, they grow warmer and pick up more moisture,
growing still larger.
– They then either precipitate as snowflakes or melt and
precipitate as raindrops.
55
– Ice Crystal Formation
• Cold clouds
– Fig. 6-28
56
Forms of Precipitation
• Rain—the most common and widespread
form of precipitation, consisting of drops of
liquid water.
– Result of condensation and precipitation in
ascending air that has a temperature above
freezing, but some results from thawing of ice
crystals.
• Snow—solid precipitation in the form of ice
crystals, small pellets, or flakes, which is
formed by the direct conversion of water
vapor to ice.
• Sleet—small raindrops that freeze during
decent, reaching ground as small pellets of
ice.
• Glaze—rain that turns to ice the instant it
collides with a solid object.
• Hail—rounded or irregular pellets or lumps of
ice produced in cumulonimbus clouds as a
result of active turbulence and vertical air
currents. Small ice particles grow by
collecting moisture from super cooled cloud
droplets.
57
19
20. Atmospheric Lifting and
Precipitation
• Significant amounts of precipitation can
originate only by rising air and adiabatic
cooling.
• There are four principal types of atmospheric
lifting:
1. Convective lifting
2. Orographic lifting
3. Frontal lifting
4. Convergent lifting
• More often than not, the various types operate in
conjunction.
58
• Forms of Precipitation
– Convective – Frontal
– Orographic – Convergent
• Fig. 6-32
59
Atmospheric Lifting and
Precipitation
• Convective Lifting
– Showery precipitation with large raindrops falling fast and hard; caused by
convective lifting, which occurs when unequal heating of different air surface
areas warms one parcel of air and not the air around it.
• This is the only spontaneous of the four lifting types; the other three require an external
force.
• Orographic Lifting
– Occurs with orographic lifting, caused when topographic barriers force air to
ascend upslope; only occurs if the ascending air is cooled to the dew point.
– Rain shadow—area of low rainfall on the leeward side of a topographic barrier;
can also apply to the area beyond the leeward side, for as long as the drying
influence continues.
• Frontal Lifting
– Occurs when air is cooled to the dew point after unlike air masses meet, creating
a zone of discontinuity (front) that forces the warmer air to rise over the cooler air
(frontal lifting).
• Convergent Lifting
– Showery precipitation caused by convergent lifting, the least common form of
lifting, which occurs when air parcels converge and the crowding forces uplift,
which enhances instability. This precipitation is particularly characteristic of low
latitudes
60
20
21. Global Distribution of Precipitation
Average Annual Precipitation
Very High Levels Very Low levels
• Tropical regions • Subtropical latitudes
– ITCZ – Subtropical High
– Trade winds Pressure dominates
– Monsoon areas • Middle Latitudes
• Upper Middle Latitudes – Rain shadow areas
– West coasts • High Latitudes
– Orographic lifting – Low evaporation rates
– Cold, dry air
61
• Fig. 6-34
62
• Seasonal Precipitation Patterns
– Shifting of ITC Zone
– Worldwide Summer Maximum
– Monsoon Areas - Fig. 6-35 top
63
21
22. • Precipitation Variability
– U.S. Average January and July precipitation.
• Fig. 3-16 top and bottom, dissolve overlay, toggle
64
• Precipitation Variability (continued)
– Percent Departure from Average in a Given Year
65
Acid Rain
• Sulfuric and Nitric Acids in Rain
– Acidity
• Fig. 6-38
66
22
