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Unformatted text preview: Chapter 6 Chapter Stability & Cloud Stability Development Development I. Clouds I. Add beauty to the sky Add beauty Release latent heat Release latent into the atmosphere into Maintain energy Maintain balance balance Provide precipitation Provide precipitation Visually indicate Visually indicate atmospheric stability atmospheric II. Atmospheric Stability
What makes air rise and sink? What Why do clouds form on some days and not on Why others? What determines the shape and size of What clouds? clouds? Knowing the air’s stability will help us answer Knowing these questions. these The tendency of the air to rise or sink dictates The the formation of clouds. Stability is how we measure this tendency. measure II. Atmospheric Stability II. Stability refers to a state of equilibrium An Air Parcel An An Air Parcel is body An Air of air encased in an imaginary elastic wrap imaginary It can freely expand It and contract and There is no heat There exchange between the parcel and the environment environment A process with no process heat exchange is an Adiabatic Process Adiabatic The Lapse Rate The The Lapse Rate is the rate of temp decrease with The Lapse altitude in the atmosphere altitude Given as positive values (example: 10°C/1000 m) Given even though it describes temp decrease. even A negative lapse rate negative indicates a temp inversion (temp increasing with height). increasing Adiabatic Lapse Rate Adiabatic Since pressure decreases Since with altitude: with rising parcel expands sinking parcel contracts Adiabatic = no heat exchange Air molecules push parcel Air wall using their internal energy energy Rising parcel cools due to Rising expansion (and sinking parcel
warms due to compression) warms The cooling or heating rate The is the adiabatic lapse rate adiabatic Dry and Moist Adiabatic Lapse Rates Rates As rising a parcel cools, its RH increases. As When it cools down to dew point, RH=100%, and When condensation begins. condensation Further cooling will release latent heat and thus Further cooling slows down. cooling The cooling rate after condensation is called moist The adiabatic lapse rate. Before condensation, it is called adiabatic Before dry adiabatic lapse rate. dry Adiabatic lapse rates: 10oC/km for dry parcels (dry adiabatic lapse rate) C/km for 4-10oC/km for moist parcels, depending on moist 4-10 C/km depending rate varies with moisture amount (moist adiabatic lapse rate) lapse Vertical Sounding Vertical Twice a day, an instrument attached to a balloon is Twice sent up from many sites across the country and the world. The temperature, among other things, is measured The vertically throughout the atmosphere as the balloon rises. This is called a sounding. rises. Weather Balloons: The BalloonBorne Sounding System (BBSS) Weather Balloon and Radiosonde Radiosonde (Credit - Vaisala) http://avc.comm.nsdlib.org/cgi-bin/wiki_grade_interface.pl?Weather_Balloons III. Determining Stability III. Stability is determined by comparing the lapse rate of Stability a rising parcel with its surroundings (environmental lapse rate). lapse If parcel lapse rate is smaller than its surroundings: rising parcel cools slower than surrounding air, and it is rising warmer than the environment. warmer Warmer parcel will keep rising – unstable atmosphere. Conversely, if parcel lapse rate is larger than its Conversely, surroundings: surroundings: parcel is cooler than surrounding air, and is cooler than parcel the environment the Cooler parcel sinks back to its original level – stable Cooler atmosphere atmosphere Absolutely Stable Absolutely Absolutely Stable Absolutely The atmosphere is always absolutely stable The absolutely when the environmental lapse rate is less than the moist adiabatic lapse rate. Air lifted from any level will sink back to its Air starting position (parcel is always colder). starting Since absolutely stable air strongly resists Since upward motion, an air parcel will, if forced to rise, tend to spread out horizontally. rise, Clouds such as stratus usually form in stable Clouds air. air. Conditions for a Stable Atmosphere Atmosphere Warm the air aloft Warm air advection Warm Cool the surface air Radiational cooling Cold air advection Air moving over a cold Air surface surface A layer of air sinks A Sinking Layer of Air Sinking Why are Inversions Important? Why Morning fog development Trap pollutants Trap Determining when an inversion dissipates can Determining be important for thunderstorm development be Types of Inversions Types Subsidence Inversions typically form in regions of large scale typically sinking motion (under the subtropical highs, under the left entrance / right exit region of jets) or on the periphery of convective cells. jets) Radiation Inversions where would you expect a radiation where inversion to develop? and when? (NEXT SLIDE!) SLIDE!) A Subsidence Inversion Subsidence A Radiation Inversion Radiation
Ground cooling at night conducts heat to ground via conduction and causes an inversion! Neutral Stability Neutral No preference for vertical motion The environmental lapse rate is equal to The the dry and/or moist adiabatic lapse rate the The parcel has the same temperature The and density of the environment at all levels levels Absolutely Unstable Absolutely Absolutely Unstable Absolutely Absolute instability results when the environmental lapse rate is greater than the dry adiabatic lapse rate. Air parcels lifted from any level will Air continue to rise because they are warmer and less dense than the air around them. around Deep clouds such as cumulonimbus are associated with a deep unstable layer. associated Conditions for an Unstable Atmosphere Atmosphere Cool the air aloft Cold air advection Clouds emitting IR Clouds radiation radiation Warm the surface air Daytime solar heating Warm air advection Air moving over a Air warm surface warm
Any process that steepens the environmental lapse rate Conditionally Unstable Conditionally Conditionally Unstable Conditionally Conditional instability occurs when the environmental lapse rate is between the moist and dry adiabatic lapse rates. and This type of stability depends on whether or This depends on not the rising air is saturated. air When the rising parcel is unsaturated (dry) the atmosphere When is stable is When the parcel is saturated (moist) the atmosphere is When the unstable. unstable. Conditional instability means that, if unsaturated air Conditional could be lifted to a level where it becomes saturated, instability would result. instability Conditions for an Unstable Atmosphere Atmosphere Mixing Mixing Convection Turbulent Eddies Lift a layer of air Stability and Lapse Rates Stability IV. Cloud Development IV. Clouds form as Clouds air rises, expands, cools, and water vapor condenses onto condensation nuclei nuclei How do we get How air to rise? air Lifting Mechanisms Lifting Lapse Rate and Cloud Thickness Lapse Vertical temperature profile Vertical and cloud thickness and Stable layer inhibits Stable cloud development cloud Thermals and Clouds Thermals
When thermals When are lifted to condensation level, they level they condense and form clouds form Convective Cloud Bases Convective Formation of a Cumulus Cloud Formation Clouds due to Orographic Lift Clouds Clouds due to Orographic Lift Clouds Cloud Streets and Billows Cloud Summary Summary Vertical temperature profile or lapse rate determines atmospheric stability. If the lapse rate > 10°C/1km, atmosphere is absolutely unstable < 4°C/1km, atmosphere is absolutely stable = 4-10°C/1km, the atmosphere is conditionally unstable Stability is a major factor in cloud development. Deep unstable layer is potential for tall and convective clouds. Shallow unstable or slightly stable layer favors layer clouds Atmosphere is typically conditionally unstable, so moist air is more unstable than drier air. Heating atmospheric column from below or cooling it at top destabilizes the air. Cooling it from below or warming it above stabilizes the air. ...
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- Spring '08