Download Relative Humidity

Chapter 4: Moisture and Atmospheric Stability
The Atmosphere:
An Introduction to
Meteorology, 12th
Lutgens • Tarbuck
Lectures by:
Heather Gallacher,
Cleveland State University
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Movement of Water
Through The Atmosphere

Hydrologic cycle:
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Water: A Unique Substance

Water:
 Water is the only liquid on the surface of the Earth in large
quantities.
 It exists in all forms on Earth.
 Ice (solid state) is less dense than liquid.
 Water has a high heat capacity.
 It has a unique ability to form hydrogen bonds (H2O).
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Water’s Changes of State

Ice, liquid water, and water vapor:
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Latent Heat

Latent heat:
 Adding heat to melt ice does not result in a temperature
change.
 Melting 1 gram of ice requires 80 calories.
 Latent heat of melting
 Freezing 1 gram of water releases 80 calories.
 Latent heat of fusion
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Latent Heat

Latent heat is also during evaporation, the process
of converting a liquid to a gas.
 The latent heat of vaporization is the energy absorbed by
water during evaporation.
 ~ 600 calories/gram for water
 Evaporation is a cooling process.

Condensation is the reverse process, converting a
gas to a liquid.
 The process when water vapor changes to the liquid state
is the latent heat of condensation.
 Energy is released, which warms the surrounding air.
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Latent Heat

Sublimation:
 Sublimation is the process that turns a solid to a gas.
 Disappearing ice cubes in freezer are a result of sublimation.

Deposition:
 Deposition is the reverse process of changing a vapor to a
solid.
 Frost accumulating in a freezer is a result of deposition.
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Humidity: Water Vapor in the Air

Humidity is amount of water vapor in the air.
 Absolute humidity is the mass of water vapor in a given
volume of air.
 The mixing ratio is the mass of water vapor in a unit of air
compared to the remaining mass of dry air.
 Vapor pressure is defined as the part of the total
atmospheric pressure attributable to its water-vapor
content.
 Relative humidity indicates how close air is to saturation
rather than the actual quantity of water vapor in the air.
 Dew point is the temperature to which air needs to be
cooled to reach saturation.
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Vapor Pressure and Saturation

Vapor pressure is that
part of the total air
pressure attributable
to water vapor
content.
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Vapor Pressure and Saturation

Saturation:
 Saturation is the equilibrium point between evaporation
and condensation.
 It increases temperature.
 Humid air equals a high vapor pressure.
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Relative Humidity

Relative humidity:
 Relative humidity is the ratio of the air’s actual water
vapor content and amount of water vapor required for
saturation at a certain temperature.
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Relative Humidity

How relative humidity changes:




100% relative humidity equals saturation.
If water vapor is added, relative humidity goes up.
If water vapor is removed, relative humidity goes down.
A decrease in temperature equals an increase in relative
humidity.
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Relative Humidity
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Relative Humidity

Natural changes:
 Daily temperature changes affect relative humidity.
 Temperature changes are caused by advection, the primarily
horizontal component of convective flow (wind).
 Temperature changes are also caused through convection,
where some of the air in the lowest layer of the atmosphere,
heated by radiation and conduction, is transported by
convection to higher layers of the atmosphere.
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Dew-Point Temperature

Dew point:
 The dew point is the temperature air is cooled to reach
saturation.
 Dew-point temperature is a measure of actual moisture
content.
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How is Humidity Measured?

Hygrometer:
 A hygrometer measures moisture the content of air.
 A hair hygrometer operates on the principle that hair changes
length in proportion to changes in relative humidity.
 A psychrometer consists of two identical thermometers;
one (dry thermometer) measures air temperature, the other
called the “wet bulb,” measures the dryness of air.
 The larger the difference in temperatures between the wet
and dry, the lower the relative humidity (with a table).
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Adiabatic Temperature Changes

An adiabatic process is one in which no heat is
transferred.
 When air is compressed, it warms.
 When air expands, it cools.
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Adiabatic Temperature Changes

Adiabatic cooling and condensation:
 Cooling occurs when air moves up and it expands and
cools.
 Unsaturated air cools at a rate of 10° C/1000m; this is called
the dry adiabatic rate.
 Condensation is triggered when air rises high enough to
reach its saturation point and clouds form, called its lifting
condensation level.
 When air ascends above the lifting condensation level, the
rate at which it cools is reduced. The slower rate of cooling is
called the wet adiabatic rate (because the air is saturated),
which varies from 5° C/1000m.
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Adiabatic Temperature Changes
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Processes That Lift Air

Orographic lifting occurs when elevated terrains,
such as mountains, act as barriers to the flow of air.
 When air reaches the leeward side, much of its moisture
has been lost.
 Adiabatic cooling can generate clouds and copious
precipitation. Many of the wettest places in the world are
located on windward mountain slopes.
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Processes That Lift Air
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Processes That Lift Air

Frontal wedging:
 Masses of warm and cold air collide, producing fronts.
Cooler, denser air acts as a barrier over which the warmer,
less dense air rises. This process is called frontal wedging.
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Processes That Lift Air

Convergence:
 Converging horizontal
air flow results in
upward movement.
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Processes That Lift Air

Localized convective lifting:
 Unequal surface heating causes localized pockets of air to
rise.
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The Critical Weathermaker: Atmospheric
Stability



Stable air resists vertical movement.
Unstable air rises due to buoyancy.
Environmental lapse rate is the actual temperature
of the atmosphere.
 Air temperature is measured at various heights in the
atmosphere.
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The Critical Weathermaker: Atmospheric
Stability
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The Critical Weathermaker: Atmospheric
Stability
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The Critical Weathermaker: Atmospheric
Stability

Conditional instability is the most common type of
atmospheric instability.
 This situation prevails when moist air has an environmental
lapse rate between the dry and wet adiabatic rates.
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Stability and Daily Weather

How stability changes:
 Instability is enhanced by the following:
 Intense warming of the lowest layer of the atmosphere
 Heating of an air mass from below
 General upward movement of air caused by orographic
lifting, frontal wedging, and convergence
 Radiation cooling from cloud tops
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Stability and Daily Weather

How stability changes:
 Stability is enhanced by the following:
 Radiation cooling of Earth’s surface after sunset
 Cooling of an air mass from below as it traverses cold surface
 General subsidence within an air column
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Stability and Daily Weather

Temperature changes and stability:
 When air is cooled from below, it becomes more stable,
often producing widespread fog.
 In winter, air is rendered sufficiently unstable when cold,
dry air passes over a warm, wet surface, which can often
produce lake effect snow over the Great Lakes.
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Vertical Air Movement and Stability

Subsidence is a general, downward air flow.
 Usually, surface air is not involved.
 This results with stable air and clear, blue, cloudless skies.
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End Chapter 4
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