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