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Unformatted text preview: Stability & Cloud Development Stability This chapter discusses: 1. Definitions and causes of stable and unstable Definitions atmospheric air atmospheric 1. Processes that cause instability and cloud Processes development development Stability & Movement Stability Figure 7.1 A rock, like a parcel of air, that is in stable equilibrium will return rock, to its original position when pushed. to If the rock instead departs in the direction of the push, it was in If unstable equilibrium. unstable Movement & Temperature Movement
Rising air Rising expands, using energy to push out, which slows and adiabatically cools the air. the A parcel of air parcel may be forced to rise or sink, and change temperature relative to environmental air, which is sampled using radiosonde balloons. balloons. Figure 7.2 Dry & Moist Adiabatic Rates Dry A parcel of rising dry air cools at approximately 10° C/km, while parcel latent heat released into the parcel by condensing vapor keeps the moist adiabatic lapse rate nearer to 6° C/km. moist Because the environmental air cools more slowly, the parcel of air Because held by the helicopter is always cooler and sinks back to its original position. original Figure 7.3A Figure 7.3B Stable Atmospheric Conditions Stable Figure 7.4 Cold air below warm air is considered stable because cold air has a Cold higher density, and when displaced will gravitate back below the warm, lighter, air. warm, A morning atmosphere has this stable vertical profile, keeping fog morning unmixed. unmixed. Subsidence Inversion Subsidence Figure 7.5 Cooler surface air is called a temperature inversion, caused by Cooler radiational cooling, cold advection, or subsiding air that shrinks in total thickness. total As it sinks and shrinks, its top lowers a greater distance and warms As more than the air resting at earth's surface. more Absolutely Unstable Atmosphere Absolutely Absolutely unstable Absolutely conditions indicate that a lifted parcel of air, whether dry or moist, will be warmer than the surrounding environmental air, and hence continue to rise. hence Figure 7.6A Conditionally Unstable Conditionally Conditions are initially stable Conditions when the environmental lapse rate (LR) is between the dry and moist adiabatic LR. and Air that is dry is always cooler Air and sinks, but when the parcel's dew point is reached it cools more slowly, and may become warmer than the surrounding air, creating instability. instability.
Figure 7.7A Stabililty Summary Stabililty
Environmental Environmental temperatures determine stability for rising parcels of air. of The atmosphere is The absolutely unstable when the environmental lapse rate (ELR) is steeper than the dry LR, absolutely stable when the ELR is less steep than the moist LR, and conditionally unstable otherwise. otherwise.
Figure 7.8 Heat as Instability Trigger Heat
As the environmental lapse rate As (ELR) steepens, it becomes more unstable. more Heating air below, through Heating contact with a hot surface or fire, will cause the ELR to steepen. steepen. Cooling air aloft by clouds Cooling radiating energy or cold advection will also increase instability. instability.
Figure 7.9 Mixing as Instability Trigger Mixing
Winds that mix Winds upper and lower atmospheric air will steepen the environmental lapse rate. lapse Wind forced Wind convection will warm descending and lower air while cooling rising and upper air. air. Figure 7.10 Layer of Air Instability Layer Figure 7.11 Lifting a stable layer of air causes it to stretch out vertically, with Lifting the top section experiencing a greater elevation change and more cooling than the lower section, even when both cool at the dry adiabatic rate. adiabatic Convective Instability & Storms Convective
Lifting a stable layer of Lifting air that is saturated below and unsaturated aloft will cause the moist air to cool more slowly, and create a very steep and unstable environmental lapse rate. rate. Severe thunderstorms Severe are associated with such processes. processes.
Figure 7.12 Cloud Development Triggers Cloud Figure 7.13A Convection & Clouds Convection
Pockets of warm Pockets air rise as thermals with invisible water vapor, and at the dew point temperature condensation creates the cloud base. base. Rising air from Rising below is replaced by sinking air from above, creating areas of blue sky. blue Figure 7.14 Cumulus Clouds & Clear Sky Cumulus Figure 7.15 Development of Cumulus Cloud Development Figure 7.16 The panels reveal vertical trends for temperature and moisture The during cloud development. during The level of free convection is where rising air becomes warmer The than environmental air. than Stability & Cloud Height Stability Environmental Environmental lapse rates (ELR) determine the stability, and as the time of day changes the ELR, the cloud profile can change. can Figure 7.17A Cumulus to Cumulonimbus Cumulus Figure 7.18 Topography & Clouds Topography Figure 7.19 Winds blowing moist air toward a mountain will experience Winds orographic uplift to an elevation where dew point is reached and clouds are formed. clouds When the condensed moisture falls as rainfall, the leeward side of When the mountain is kept in a rain shadow. the Mountain Wave Clouds Mountain Figure 7.20 Figure 7.21 As moist air condenses into clouds on the windward side of the As mountain, sinking and swirling winds on the leeward side shape a lenticular mountain wave cloud. lenticular Changing Cloud Forms Changing Uniform winds may Uniform align a large region of cirrocumulus and stratocumulus clouds into rows of cloud streets. streets. Figure 7.22 Billow & Castellanus Clouds Billow Figure 7.23 Figure 7.24 Rapid changes in wind speed with elevation can create Rapid wavelike billows at the cloud top, while conditionally unstable air above a cloud may cause lifting and condensation in the shape of castles. shape Mixing & Stratocumulus Mixing Figure 7.25 When air is stable and close to saturation, then forced wind mixing When of the atmosphere: of a) steepens the environmental lapse rate b) moves moist low-level air aloft The cooler and moister air aloft then condenses into a deck of clouds. ...
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This note was uploaded on 03/15/2010 for the course ATOC ATOC 250 taught by Professor Gyakum during the Spring '09 term at McGill.
- Spring '09