Moisture and Atmospheric Stability

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Moisture and Atmospheric StabilityChapter 4

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4.1 Movement of Water Through the Atmosphere

+Movement of Water Through The Atmosphere

97% of all water is salt water

Remaining 3% is stored in ice sheets in Antarctica and Greenland

Only 0.0001% is found in the Atmosphere in the form of water vapor

+Movement of Water Through The Atmosphere

Hydrologic Cycle:The continuous

exchange of water among oceans, atmosphere and continents

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4.2 Water: A Unique Substance

+Water: A Unique Substance

Water:1. Water is the only liquid on the surface of the

Earth in large quantities.

2. It exists in all forms on Earth.

3. Ice (solid state) is less dense than liquid.

4. Water has a high heat capacity.

+Water: A Unique Substance

It has a unique ability to form hydrogen bonds (H2O)

When ice forms on a body of water it insulates the underlying layer of water and slows the rate of freezingThis is how fish can survive in a lake during the

winter :)

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4.3 Water’s Changes of State

+Water’s Changes of StateIce, liquid water, and water vapor:

+Water Changes States

IceTight orderly network of molecules vibrating in

a fixed state

WaterMolecules can slide past one another

GasVery random erratic motion

+Latent Heat

Whenever water changes state heat is lost to its surroundings.

Latent heat is hidden heatAdding heat to melt ice does not result in a

temperature change.Melting 1 gram of ice requires 80 calories.

Latent heat of meltingFreezing 1 gram of water releases 80 calories.

Latent heat of fusion

+Latent Heat

EvaporationThe 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.

+Latent Heat

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.Forms clouds and fog

+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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4.4 Humidity: Water Vapor in the Air

+Humidity: Water Vapor in the Air

Humidity is amount of water vapor in the air.1. Absolute humidity is the mass of water vapor

in a given volume of air.

2. The mixing ratio is the mass of water vapor in a unit of air compared to the remaining mass of dry air.

Meteorologists like to use this one because it is not affected by changes in pressure or temperature

+Humidity: Water Vapor in the Air

3. Vapor pressure is defined as the part of the total atmospheric pressure attributable to its water-vapor content.

4. Relative humidity indicates how close air is to saturation rather than the actual quantity of water vapor in the air.

5. Dew point is the temperature to which air needs to be cooled to reach saturation.

**We will talk more about these 3

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4.5 Vapor Pressure and Saturation

+Vapor Pressure and Saturation

Vapor pressure is that part of the total air pressure attributable to water vapor content.More water vapor equals

more vapor pressure

+Vapor Pressure and Saturation

Saturation: Saturation is the equilibrium point between

evaporation and condensation.Temperature dependentFor every 10°C (18 °C) increases in temperature, the

amount of water for vapor needed for saturation doublesHumid air equals a high vapor pressure.

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4.6 Relative Humidity

+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 and pressure.

Relative humidity indicates how near the air is to saturation rather than the actual quantity of water vapor in the air.

+Relative Humidity

At 25°C, air is saturated at 20 g/kg

So if we had 10g/kg at 25°C then the relative humidity is 10/20 or 50%

+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.

+Relative Humidity

A decrease in temperature equals an increase in relative humidity.

When air aloft is cooled below its saturation level, some of the water vapor condenses to form clouds. Clouds are made of liquid droplets, so the moisture is no longer apart of the water-vapor content of the air.

+Relative Humidity

+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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4.7 Dew-Point Temperature

+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.

+Dew-Point Temperature

In nature, cooling below the dew point cause water vapor to condense, typically as dew, fog or clouds

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4.8 How is Humidity Measured?

+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.

These are harder to use and need to be calibrated often

+How is Humidity Measured?

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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4.9 Adiabatic Temperature Changes

+Adiabatic Temperature Changes

Remember condensation occurs when sufficient water vapor is added to the air or more commonly when the air is cooled to its dew point temperature.

An adiabatic process is one in which no heat is transferred.When air is compressed, it warms.When air expands, it cools.

+Adiabatic Temperature Changes

Adiabatic cooling: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.

+Adiabatic Temperature Changes

Adiabatic condensation: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.

+Adiabatic Temperature Changes

+Adiabatic Temperature Changes

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4.10 Processes That Lift Air

+Processes That Lift Air

4 mechanisms that cause to air to rise1. Orographic lifting in which air is forced to rise

over a mountainous barrier

2. Frontal wedging in which warmer less dense air is forced over cooler dense air

3. Convergence is a pileup of horizontal air flow that results in upward motion

4. Localized convective lifting is unequal surface heating causes localized pockets of air to rise because of their buoyancy.

+Processes That Lift Air

Orographic lifting occurs when elevated terrains, such as mountains, act as barriers to the flow of air.Adiabatic cooling can generate clouds and

copious precipitation. Many of the wettest places in the world are located on windward mountain slopes.

When air reaches the leeward side, much of its moisture has been lost.

+Processes That Lift Air

Rain shadow desert- happens on leeward mountain side, adiabatic warming making ran less likely

+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.

+Processes That Lift Air

Convergence:Converging

horizontal air flow results in upward movement.

Can also happen when an obstacle slows or restricts horizontal air flow

+Processes That Lift Air

Localized convective lifting:Unequal surface heating causes localized pockets of

air to rise.Can rise high enough, condense and form clouds

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4.11 The Critical Weather maker: Atmospheric Stability

+The Critical Weathermaker: Atmospheric Stability

Stable air resists vertical movement.

Unstable air rises due to buoyancy till it reaches a temperature that is equal to its surroundings.

Environmental lapse rate is the actual temperature of the atmosphere.Air temperature is measured at various heights in

the atmosphere.

+The Critical Weathermaker: Atmospheric Stability

Absolute StabilityThe environmental lapse rate is less than the

adiabatic rateThe rising air continues to cool

Temperature inversionType of environmental lapse rate when the temp

in a layer of air increases with altitude rather than decreasing

+The Critical Weathermaker: Atmospheric Stability

+The Critical Weathermaker: Atmospheric Stability

Absolute InstabilityThe environmental lapse rate is greater than the

adiabatic rateThe ascending parcel of air is always warmer

than its environment and will continue to rise because of its buoyancy

Occurs most often during the warmest months on clear days when solar heating is intense

+The Critical Weathermaker: Atmospheric Stability

+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.

+The Critical Weathermaker: Atmospheric Stability

+In Summary: Atmospheric Stability

The stability of air is determined by measuring temperature of the atmosphere at various temperatures (environmental lapse rate)

A column of air is unstable when the air near the bottom of this layer is significantly warmer than the air aloft

+In Summary: Atmospheric Stability

Air is considered stable when the temperature decreases gradually with an increase in altitude

Most stable condition occurs during a temperature inversion when the temperature actually increases with height.Under these conditions there is little vertical air

movement

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4.12 Stability and Daily Weather

+Stability and Daily Weather

On a dreary overcast day with light drizzle, stable air is forced aloft

On a day with towering clouds forming the atmosphere is unstable

+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 belowGeneral upward movement of air caused by

orographic lifting, frontal wedging, and convergence

Radiation cooling from cloud tops

+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

+Stability and Daily Weather

Temperature changes and stability:As air moves horizontally over the surface with

different temperatures changes in stability occurEx: When warm air from the Gulf of Mexico

moves north over the snow covered Midwest, the air is cooled from below, it becomes more stable, often producing widespread fog.

+Stability and Daily Weather

Ex: In winter, polar air move south over over the Great Lakes. Polar air is then rendered sufficiently unstable when the cold, dry air passes over a warm, wet surface, which can often produce lake effect snow over the Great Lakes.

+Vertical Air Movement and Stability

Subsidence is a general, downward air flow.

Usually, surface air is not involved.

When there is a general downward airflow, the upper portion of the subsiding layer is heated by compression more than the lower. So, the net effect is to stabilize the air because the air aloft is warmed more than the surface air.The warming effect of a few 100m of subsidence is enough

to evaporate the clouds. Resulting in clear blue skies :)

+Final Summary

The air’s stability or, lack of it, determines to a large degree whether clouds develop and produce precipitation and whether that precipitation will come as a gentle shower or a violent down pour.

+Final Summary

In general, when stable air is forced up, the associated clouds have very little vertical thickness and precipitation, if any, is light.

In contrast, clouds associated with unstable air are towering and are frequently accompanied by heavy precipitation.