Ch6 1 Stability a Stability of an equilibrium state refers to the tendency of the system to return to equilibrium if Stable the system will return to equilibrium disturbed i ii Unstable the system will accelerate away from equilibrium iii Neutral the system will neither return to nor accelerate away from equilibrium b Atmospheric stability is concerned with what will happen to a parcel of air at equilibrium if it is displaced i 2 Adiabatic Motion Specifically how where will it move in response c As we see most interesting weather occurs when the atmosphere is unstable a Adiabatic process a process in which an air parcel does not exchange heat with its surroundings b All vertical motion occurs adiabatically c Consider a parcel at the surface Recall that air pressure always decreases with height i ii The parcel will push out on the ambient air until pressure is equilibrated volume increased iii Its temperature must drop ideal gas law d As an air parcel rises it expands and cools e As an air parcel sinks it compresses and warms 3 Dry Adiabatic Lapse Rate DALR a The rate at which any unsaturated air parcel will cool with height b Recall unsaturated air means relative humidity is greater than 100 c 10K km d 5 5 F 1000ft 4 Moist Adiabatic Lapse Rate MALR a As a parcel rises it cools b Sometimes it cools to its dewpoint and becomes saturated relative humidity equals 100 c Moist saturated adiabatic lapse rate the rate at which a saturated air parcel cools with height i Varies with temperature ii Average value 6K km or 3 3 F 1000ft 5 Adiabatic Motion a Once a parcel becomes saturated it cools at the MALR b Also once a saturated parcel is lifted condensation occurs and a cloud forms c Recall that condensation releases latent heat i e it s a warming process d The release of latent heat offsets some of the cooling that occurs as the parcel rises e Thus MALR DALR f MALR DALR even if the parcel is sinking g A saturated parcel will warm at the MALR as it sinks h As it is sinking evaporation is occurring i Recall evaporation removes latent heat from the atmosphere i e it s a cooling process j The removal of latent heat offsets some of the warming that occurs as the parcel sinks 6 Determining Stability a Compare the temperature of the rising parcel to the temperature of its surroundings i ii If the parcel is colder than the environment it will sink back to its original level stable If the parcel is warmer than the environment it will continue to rise unstable b This is because of the parcel s buoyancy 7 Environmental Lapse Rate a This is the lapse rate change in temperature with height for the environmental air i e the b The environmental lapse rate does not have to be equal to the DALR or the MALR It can be surroundings anything 8 Absolutely Stable a We say that the atmosphere is absolutely stable when the environmental lapse rate is less than a The potential temperature is the temperature a parcel would have if it were lowered b adiabatically to 1000 mb It represents the potential warmest temperature a parcel can achieve through an adiabatic process the moist saturated adiabatic lapse rate i Unsaturated parcel 1 Colder than surroundings if lifted descent ii Saturated parcel 1 Colder than surroundings if lifted descent 9 Creating Stability a Low lapse rates or temperature inversions i Nighttime radiational cooling ii Cold air advection at the surface b The atmosphere is the most stable in the mornings 10 Potential Temperature 11 Neutral Stability a For unsaturated air b For saturated air i i Environmental lapse rate DALR 1 Unstable with respect to saturated air Environmental lapse rate MALR 1 Stable with respect to unsaturated air 12 Absolutely Unstable dry adiabatic lapse rate super adiabatic i Unsaturated parcel 1 Warmer than surroundings if lifted ascent ii Saturated parcel 1 Warmer than surroundings if lifted ascent a We say that the air is absolutely unstable when the environmental lapse rate is greater than the b Deep layers in the atmosphere are rarely absolutely unstable 13 Conditionally Unstable a We say that the atmosphere is conditionally unstable when the environmental lapse rate is between the dry adiabatic and moist saturated adiabatic lapse rates i Unsaturated parcel 1 Colder than surroundings if lifted descent ii Saturated parcel 1 Warmer than surroundings if lifted ascent b The average environmental lapse rate is 6 5K km c This value is between the DALR and MALR d Thus the atmosphere is commonly in a state of conditional instability 14 Destabilizing the Atmosphere i a High lapse rates Cool air aloft 1 Cool air advection 2 Radiational cooling from clouds or air ii Warm air at the surface 1 Daytime solar heating 2 Warm air advection iii Mixing iv Large scale lifting 15 Cloud Development a Can be attributed to four major causes Surface heating free convection i ii Orographic uplift topography iii Convergence iv Uplift along frontal systems 16 Cloud Development Assumptions a No mixing takes place between the air parcel and its surroundings i e adiabatic process b Only a single thermal produces a single cumulus cloud c The cloud forms when RH 100 d The rising air in the cloud remains saturated 17 Convection a A thermal pocket of warm air breaks away from the surface and rises i By adiabatic cooling the air may reach the dew point and saturate a cloud forms b Convection must be part of a circulation for continuity there can t be a vacuum in the atmosphere i Air must sink around the thermals blue sky 18 Atmospheric Levels a b Level of Free Convection LFC the height at which a parcel becomes buoyant i Lifting Condensation Level LCL the height at which a parcel becomes saturated The parcel will rise on its own above this level The parcel will condense water vapor i ii This is the cloud base height of cumulus clouds c Both of these assume adiabatic motion 19 Cumulus Growth cumulus clouds 20 Orographic Uplift a Stability of the atmosphere above the condensation level is critical to the development of b Absolutely stable areas act as a cap to the cloud preventing further upward growth a Orographic uplift is forced lifting along a topographic barrier i Air that hits a mountain is forced up and over it b This causes cooling and subsequent cloud development i Because condensation removes moisture from the air on the windward side of the mountain the leeward side is usually dry rain shadow 21 Other Clouds a Because clouds emit radiation from the top and absorb from the bottom of the deck the
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