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Atmospheric Moisture Chapter 6
The Impact of Atmospheric Moisture on the Landscape
• Forms haze, fog, clouds, rain, sleet, hail and snow • Streams and rivers flood • Causes weathering and erosion • Plant and animal life
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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
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
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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
• 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.
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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
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
• 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) – 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
• Cloud layer on the ground • Visibility less than 3300 feet • Difference between cloud and fog = how it forms • Impacts human life
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• 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
Precipitation• All precipitation originates in clouds • Most clouds do not yield precipitation • How does precipitation form?
– Collision and coalescence – Ice crystal formation
• Types – 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