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Title Photo Page

““Rain! whose soft architectural hands have Rain! whose soft architectural hands have power to cut stones, and chisel to shapes of power to cut stones, and chisel to shapes of

grandeur the very mountain.grandeur the very mountain.””——Henry Ward Beecher (Brainyquote.com)Henry Ward Beecher (Brainyquote.com)

6Atmospheric Moisture

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Vocabularyabsolute humidity (p. 149)acid rain (acid deposition) (p. 173)capillarity (p. 143)condensation (p. 144)condensation nuclei (p. 153)convective lifting (p. 167)convergent lifting (p. 169)dew (p. 159)dew point (dew point temperature) (p. 151)dry adiabatic rate (p. 153)evaporation (p. 144)evapotranspiration (p. 147)fog (p. 157)frontal lifting (p. 169)hail (p. 166)humidity (p. 149)

hydrologic cycle (p. 146)isohyet (p. 169)lifting condensation level (p. 153)orographic lifting (p. 168)precipitation (p. 162)rain (p. 165)rain shadow (p. 168)relative humidity (p. 149)saturated adiabatic rate (p. 153)snow (p. 165)specific humidity (p. 149)stable (air) (p. 159)sublimation (p. 144)supercooled water (p. 153)unstable (air) (p. 161)

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Impact of Moisture on the Landscape• Precipitation

– Floods– Snow and ice– Affects wind– Erosion

• Weathering• Terrestrial Vegetation• Natural Resource

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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.

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The Nature of Water: Common place but Unique• Occurs in three forms in the atmosphere

– Ice– Liquid– Water vapor

• Fig. 6.1

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– Fig. 6-4

• Properties of Water– Changes State

• Liquid• Solid• Vapor

– Expands Upon Freezing• Important in weathering of rock• Basis of shelf ice and icebergs

– Adhesion (“Sticky”)• Surface tension• Capillary action

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• Fig. 6-5

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.

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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.

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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.

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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.

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• Fig. 6-6

– Latent Heat

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• Fig. 6-7

Water Vapor and the Hydrologic Cycle• Hydrologic Cycle

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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.

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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.

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Hydrologic Cycle

• Hydrologic cycle intricately related to many atmospheric phenomena.– Important determinant of climate:– Rainfall distribution– Temperature modification

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– Fig. 6-8

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

3. whether the air is still moving.

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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.

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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

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• 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 changes.

• Temperature-Relative Humidity Relationship

• Fig. 6-11

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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

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• 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

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Condensation• Phase change of gas

as to liquid– Water vapor to water

droplets• Requirements

– Decrease in temperature (usually)

– Condensation nuclei• tiny atmospheric particles

of dust, smoke, and salt that serve as collection centers for water molecules.

• Fig. 6-12

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– 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

Adiabatic Processes

Fig. 6-14

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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.

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• 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

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• Comparisons of Lapse Rates

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– Fig. 6-16: Temperature changes in air as it crosses over a mountain

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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.

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Clouds

• Clouds are classified on the basis of two factors

• Form • Altitude

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Clouds

• Table 6-1

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.

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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.

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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.

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Cloud FamiliesFour 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).

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– Subtypes of Cloud Forms• High clouds• Middle clouds• Low clouds• Clouds of vertical

development

• Fig. 6-18

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Cloud Types and Identification

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Cirrus

Figure 7.22

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Cirrostratus

Figure 7.22

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Altocumulus

Figure 7.22

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Altostratus

Figure 7.22

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Nimbostratus

Figure 7.22

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Stratus

Figure 7.22

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Cumulus

Figure 7.22

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Cumulonimbus

Figure 7.22

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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.

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• Distribution– United States and southern Canada

Fig. 6-21

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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

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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 atwhich the surrounding air is of equal density (equilibrium level).

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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

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– Conditionally Unstable Air

• Fig. 6-24

Conditional instability—intermediate condition between absolute stability and absolute instability. Occurs whenan 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 heat provides buoyancy).

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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.

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• Determining Atmospheric Stability (continued)– Visual Determination

Fig. 6-26

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Precipitation• Most clouds do not yield

precipitation.• Condensation alone is

insufficient to produce raindrops.

– Fig. 6-27

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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

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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.

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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.

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– Ice Crystal Formation• Cold clouds

– Fig. 6-28

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• 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.

Forms of Precipitation

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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 lifting2. Orographic lifting3. Frontal lifting4. Convergent lifting

• More often than not, the various types operate in conjunction.

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• Forms of Precipitation– Convective– Orographic

• Fig. 6-32

– Frontal– Convergent

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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

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Global Distribution of PrecipitationAverage Annual Precipitation

Very High Levels• Tropical regions

– ITCZ– Trade winds– Monsoon areas

• Upper Middle Latitudes– West coasts– Orographic lifting

Very Low levels• Subtropical latitudes

– Subtropical High Pressure dominates

• Middle Latitudes– Rain shadow areas

• High Latitudes– Low evaporation rates– Cold, dry air

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• Fig. 6-34

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– Shifting of ITC Zone– Worldwide Summer Maximum– Monsoon Areas

• Seasonal Precipitation Patterns

- Fig. 6-35 top

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– U.S. Average January and July precipitation.• Fig. 3-16 top and bottom, dissolve overlay, toggle

• Precipitation Variability

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– Percent Departure from Average in a Given Year• Precipitation Variability (continued)

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Acid Rain• Sulfuric and Nitric Acids in Rain

– Acidity • Fig. 6-38

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– Acid Rain in the United States