Chapter 4

Moisture and Atmospheric Stability

Steam Fog over a Lake

The Hydrologic Cycle

Moisture-related Terms

• Transpiration– The release of water vapor into the atmosphere by plants

• Latent Heat– Energy absorbed or released to change the state of water

– Measured in Calories•A calorie is the amount of heat needed to raise the temperature of 1g of water by 1°C

States of water

States of Water

• Ice– Frozen

• Liquid Water– Liquid at room temperature

• Water Vapor– Gas

More Moisture Terms• Evaporation

– The phase change from liquid to gas – Endothermic / absorbs latent heat

• Condensation– The phase change from gas to liquid– Exothermic / latent heat is released

• Sublimation– Phase change of solid directly to gas

• Deposition– Phase change of gas directly to solid

•Frost is an example of deposition

Water’s Changes of State

Water’s Changes of State

Ice

Water

Condensation of Water VaporGenerates Fog

Humidity – Water Vapor in Air

• Humidity– The general term for the amount of water vapor in the air

• Absolute Humidity– Mass of water vapor in a given volume of air•Mass of water vapor (g) / volume of air (m3)

• Mixing Ratio– Mass of water vapor in a unit of air compared to the remaining mass of dry air•Mass of water vapor (g) / mass of dry air (kg)

Humidity Is the

Content of Water Vaporin the Air

Vapor Pressure and Saturation

• Vapor Pressure– The part of total atmospheric pressure attributable to its water vapor content

• Saturation– Balance between evaporation and condensation

• Saturation Vapor Pressure– Pressure of water vapor in a saturated environment

SaturationVapor PressureVaries withTemperature

Relative Humidity

• Relative Humidity– Ratio of the air’s actual water vapor content compared with the amount of water vapor needed for saturation at that temperature and pressure

Relative Humidity

Changes with Added Moisture

Saturation Mixing-RatioFor every 10°C increase in temperatures, the saturation vapor pressure doubles

Changes with Temperature

Higher TemperatureLower Relative Humiditywith MORE Moisture!

Lower TemperatureHigher Relative Humiditywith LESS moisture!

Daily Changes in

Relative Humiditywith

Temperature

Dewpoint temperature

• Dewpoint– The temperature to which a parcel of air needs to be cooled to in order to reach saturation

Sling Psychrometer

ColdDrinking-glassesChill

SurroundingAir

to the Dew-Point

Condensation on Cold Drinking-glasses

Dew Point Temperatures

Dew Point Temperatures

Adiabatic Temperature Changes

• The Basis of Cloud Formation• Adiabatic Temperature Changes mean no heat added or subtracted– When air expands, it cools– When air is compressed it warms– Example: Pumping up a tire

•Air compressed in tire causes it to warm

•Escaping air is cool

Adiabatic Temperature Changes

Adiabatic Cooling• Parcel

– A volume of air• Dry Adiabatic Rate

– Unsaturated air / 10°C per 1000 m (1km)

• Lifting Condensation Level– Parcel reaches saturation / condensation begins

• Wet Adiabatic Rate– Latent heat absorbed from the evaporation processed is released at the LCL due to condensation– this reduces the adiabatic rate / BTW 5°C and 9°C per 1000 m (1km)

Dry & Wet Adiabatic Rates

Processes that lift air

• Orographic Lifting– Air is forced to rise over mountains

• Frontal Wedging– Warmer, less dense air is forced over cooler, denser air

• Convergence– A “pile-up” of horizontal air flow results in upward movement

• Localized Convective Lifting– Unequal surface heating causes small parcels to rise due to buoyancy

Lifting Processes

Orographic

Heavy Precipitation in Mountains

Snow Pack in the Rocky Mountains

Rain Shadow Desert

• When air climbs over a mountain, most of the moisture is lost as precipitation on the windward side

• The air descends on the leeward side

• As it descends, it compresses, warms, and becomes very dry

• Very little precipitation occurs in the “Rain Shadow”

Rain Shadow Desert

Death Valley

WetterWindward Locations

and LeewardRain Shadows

Rain Shadow Deserts• On the windward side of the Sierra Nevada in California, giant sequoias and Douglas firs are found

• On the leeward side, you can find Death Valley

• In addition, fast moving windward systems can produce downslope warm dry winds

• These winds are often called “Chinook” and can warm the adjacent areas by 10°C (18°F) during winter

Frontal Wedging

• Masses of warm and cold air collide, producing a front

• Warmer, less dense, air rises over colder, denser, air

Frontal Wedging

Convergence

• When air flows in from more than one direction

• Air ascends, cools, and forms clouds

• Florida provides an excellent example of convergence

Convergence

Localized Convective Lifting

• Unequal heating causes some places to be warmed more than other places, i.e., parking lot versus wooded area

• Parcel of air that is heated will rise – these parcels are often called thermals

• Birds and hang gliders ride on thermals

• This is called localized convective lifting

Convection

As long asairin a

balloonis

hotter than the

surrounding air,it will rise

Atmospheric Stability• If an air parcel is cooler than the surrounding environment, it tends to sink, and does not rise – called stable air

• If an air parcel is warmer than the surrounding environment, it tends to rise – called unstable air / like a hot air balloon

• How high does the parcel rise? Until its temperature is the same as the surrounding environment

Environmental Lapse Rate

• The actual temperature of the atmosphere at any height in the atmosphere, based on observations

• Adiabatic changes are based on a parcel of air moving vertically in the atmosphere – does not include horizontal movement or mixing

• Air that rises, that is cooler than the surrounding environment, will sink if allowed to

AdiabaticCooling

Absolute Stability

• The environmental lapse rate is less than the wet adiabatic lapse rate

• If temperature increases with altitude, an inversion exists and conditions are relatively stable

Atmospheric Stability

Absolute Stability

Absolute Instability

• The environmental lapse rate is greater than the dry adiabatic lapse rate

• The ascending parcel of air is always warmer than its environment

• Often occurs in summer and warmer months

• Generally confined to the first few km of the troposphere

Absolute Instability

Conditional Instability

• Moist air has an ELR between the wet and dry adiabatic rates

• The atmosphere is stable in respect to an unsaturated parcel of air, but unstable in respect to a saturated parcel of air

Conditional Stability

Stability and Weather• Days with low clouds and light precipitation probably involve stable air forced aloft

• Days with puffy clouds, such as in summer, likely result from unstable conditions

• Unstable conditions– Intense solar heating, air masses heated from below, orographic lifting, fronts, and convergence

• Stable conditions– Radiative cooling of earth’s surface after dark,

An Unstable Atmosphere

Subsidence

• Sinking air is called subsidence• Associated with High Pressure systems

• Usually associated with blue cloudless skies

• More subsidence-warming occurs aloft than at the surface

Chapter 4 Humidity and Moisture

Next – Condensation and Precipitation