chapter 4 moisture and atmospheric stability
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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 - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 4
Moisture and Atmospheric Stability
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Steam Fog over a Lake
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The Hydrologic Cycle
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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
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States of water
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States of Water
• Ice– Frozen
• Liquid Water– Liquid at room temperature
• Water Vapor– Gas
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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
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Water’s Changes of State
Water’s Changes of State
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Ice
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Water
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Condensation of Water VaporGenerates Fog
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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)
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Humidity Is the
Content of Water Vaporin the Air
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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
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SaturationVapor PressureVaries withTemperature
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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
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Relative Humidity
Changes with Added Moisture
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Saturation Mixing-RatioFor every 10°C increase in temperatures, the saturation vapor pressure doubles
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Changes with Temperature
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Higher TemperatureLower Relative Humiditywith MORE Moisture!
Lower TemperatureHigher Relative Humiditywith LESS moisture!
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Daily Changes in
Relative Humiditywith
Temperature
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Dewpoint temperature
• Dewpoint– The temperature to which a parcel of air needs to be cooled to in order to reach saturation
Sling Psychrometer
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ColdDrinking-glassesChill
SurroundingAir
to the Dew-Point
Condensation on Cold Drinking-glasses
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Dew Point Temperatures
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Dew Point Temperatures
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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
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Adiabatic Temperature Changes
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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)
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Dry & Wet Adiabatic Rates
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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
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Lifting Processes
Orographic
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Heavy Precipitation in Mountains
Snow Pack in the Rocky Mountains
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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”
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Rain Shadow Desert
Death Valley
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WetterWindward Locations
and LeewardRain Shadows
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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
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Frontal Wedging
• Masses of warm and cold air collide, producing a front
• Warmer, less dense, air rises over colder, denser, air
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Frontal Wedging
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Convergence
• When air flows in from more than one direction
• Air ascends, cools, and forms clouds
• Florida provides an excellent example of convergence
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Convergence
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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
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Convection
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As long asairin a
balloonis
hotter than the
surrounding air,it will rise
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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
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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
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AdiabaticCooling
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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
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Atmospheric Stability
Absolute Stability
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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
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Absolute Instability
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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
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Conditional Stability
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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,
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An Unstable Atmosphere
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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
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Chapter 4 Humidity and Moisture
Next – Condensation and Precipitation