Download Atmospheric Moisture

Atmospheric Moisture
Chapter 6
The Impact of Atmospheric Moisture on the
Landscape
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Forms haze, fog, clouds, rain, sleet, hail and snow
Streams and rivers flood
Causes weathering and erosion
Plant and animal life
Water’s Unique Properties
• Exists in 3 states
• Pure water is
colorless, odorless
and tasteless
• Changes states as
energy is absorbed
or released
• Drives daily weather
patterns
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The Nature of Water
• 70% of the surface of the Earth
• Solid below 32o F and expands when freezing
• Density decreases with freezing
• Liquid from 32o F to 212o F
• Boils at 212o F and becomes water vapor
Latent Heat
• To change states, water must expend or absorb
energy
• Expending energy creates heat
• Absorbing energy removes heat
• Sublimation
Evaporation
• The escape of water molecules
from a liquid into the air as water
vapor
– Warm water evaporates more than
cold
– Warm air promotes evaporation
• Water cannot keep vaporizing
and entering the air without limit
– At any given temperature there is a
maximum amount of vapor
– The higher the temperature, the
higher the maximum amount of
water vapor
Humidity
• Refers to water vapor in the air
• The capacity of air to hold water is dependent on
temperature
• Warmer air can hold more water vapor
• Cooler air can hold less water vapor
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• Relative Humidity - a ratio that compares the
actual amount of water vapor in the air to the
water vapor capacity of the air
• Expressed as a percentage
• 0 to 100%
• Weather forecasts
• Saturation is the point where air reaches 100%
relative humidity
• Any additional water vapor or decrease in temperature
results in condensation
Condensation
• Opposite of evaporation
• In order for condensation
to occur the air must be
saturated
• Surface for condensation
to occur is required
• Condensation molecule
bumps into each other and
combine into larger droplet
• Dew Point
– When air is cooled, water vapor
capacity decreases and relative
humidity increases.
– Cooling can bring unsaturated air
to saturation point
– The temperature at which
saturation is reached is called the
dew point.
Atmospheric Stability
• Parcel is used to describe a body of air that has
specific temperature and humidity characteristic
• Two opposing forces
• Buoyancy
• Gravity
• Warm air is less dense than cold air
• Warm air rises and expands
• Cool air descends and compresses
• Stability refers to the tendency of an air parcel to
either remain in place, rise or descend.
• Stable = resist upward movement
• Unstable = continues to rise
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Adiabatic Processes
• Only way for large air masses to be cooled to the
dew point is for it to gain altitude
• Only way to develop clouds and produce rain is
by adiabatic cooling
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As air rises it cools
As air descends it warms
• Dry Adiabatic Rate(DAR) - the rate at which
unsaturated air cools
– As unsaturated air rises, it cools at 5.5o F per 1000 feet
– As air cools it’s capacity to hold water vapor decreases
– Descending air warms at the DAR
• Lifting Condensation Level (LCL)
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Altitude at which air cools to the dew point
Air reaches 100% saturation
Clouds form
Normally visible
• Saturated Adiabatic Rate (SAR) - the rate at
which saturated air cools
– Release of latent heat slows the cooling of air
– As saturated air rises, it cools at 3.3o F per 1000 feet
• Dry Adiabatic Rate - Rate at which unsaturated air cools
• Lifting Condensation Level - point that air becomes saturated
Clouds
•Tiny droplets of water or ice
crystals
•Covers 50% of earth
•Creates all precipitation
•Influences solar energy
Fog
• Minor form of condensation
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Cloud layer on the ground
Visibility less than 3300 feet
Difference between cloud and fog = how it forms
Impacts human life
• Advection Fog
• Surface air migrating to another place
• Warm moist air over cooler ocean, lake or snow
• Moist air flowing to higher elevation along a hill or
mountain
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• Radiation fog
• Cooling of a surface chills the air directly above it to
the dew point, creating saturation
• Occurs over moist ground
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Precipitation
• All precipitation originates in clouds
• Most clouds do not yield precipitation
• How does precipitation form?
– Collision and coalescence
– Ice crystal formation
• Types
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Rain
Sleet
Snow
Hail
Convective lifting
• Air heated and becomes unstable, rising above the Lifting
condensation level
• Covers a small area, but multiple cells can form close by
• Warm parts of the world and warm seasons
• Common in the mid-west United States
Orographic Lifting
• Topographic barrier that blocks air movement
• Precipitation on windward side
• Rain shadow on leeward side of barrier
Frontal Lifting
• Point where two air masses of different pressure
meet
• Warm air is forced to rise, cool and possible
cloud formation and precipitation
• Cool polar air meeting warm tropical air
• Midlatitudes
Convergent Lifting
• Least common type of lifting
• Uplift because of crowding of air masses
• Associated with cyclonic storm systems
(hurricanes, tropical storms, etc)
• Low latitudes