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STABILITY & CLOUD DEVELOPMENT Chapter 6 Importance of Clouds  Release heat to atmosphere  Help regulate energy balance  Indicate physical processes Atmospheric Stability  Clouds from as air rises and cools  Adiabatic processes: change in temperature without giving or

removing  Dry rate = 10°C/1000 m  Moist rate = 6°C/1000 m  Why the difference?

 Stability is a state of equilibrium in terms atmospheric movement; no vertical movement occurs

Determining Stability  Warm air rises or is unstable  Cool air sinks or is stable  Compare air parcel lapse rate to environmental lapse rate

Determining Stability  Stable environment  Environmental lapse rate less than moist lapse rate  If an air parcel is forced it will spread horizontally and form stratus clouds

 Usually occurs over a cool surface (radiation, advection)  Inversion: warm over cool.

Determining Stability  Subsidence Inversions  Occurs when air slowly sinks over a large area. ○ The layer compresses and shrinks vertically as it sinks, so the upper part sinks more than the lower part, and thus warms more than the bottom.

 Strong subsidence exacerbates air pollution due to the lack of vertical motion.

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 Strong subsidence exacerbates air pollution due to the lack of vertical motion.

 Inversions act as “lids” on vertical air motion, can exacerbate pollution.

Determining Stability  An Unstable Atmosphere  Environmental lapse rate greater than the dry adiabatic lapse rate

 As air parcel rises it forms a vertical cloud  Convection, thunderstorms, severe weather

Determining Stability  A Conditionally Unstable Atmosphere  Moist adiabatic lapse rate is less than the environmental lapse rate which is less than the dry adiabatic lapse rate

 Stable below cloud base, unstable above cloud base  The atmosphere is usually in this state.

Determining Stability  Causes of Instability: Lifting or Mixing  Warming of surface (insolation, advection, warm surface) ○ Cool air aloft (advection, radiation cooling in clouds)

 Mixing occurs due to thermals or wind

Cloud Development  Clouds develop as air parcels rise and cool below the dew

point.  Four cloud formation processes: 1. Surface Heating/Convection 2. Topographic Uplift 3. Convergence of Surface Air 4. Uplift Along Fronts

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Cloud Development  Convection  Differential land surface heating creates areas of high surface temperature.

 Air above warm land surface heats, forming a ‘bubble’ of warm air that rises or convection.

 Cloud base forms above the level of free convection at the condensation level.

Cloud Development  Topography  Orographic uplift  Orographic clouds  Windward, leeward  Rain shadow  Lenticular clouds

Cloud Development  Topic: Adiabatic charts  Adiabatic charts show how various atmospheric variables change with height: pressure, temperature, humidity.

Cloud Development  Changing cloud forms  Stratus clouds can change to cumulus clouds if the top of the cloud cools and the bottom of the cloud warms.

 Alto cumulus castellanus: towers on alto stratus  If moist stable air without clouds is mixed or stirred it can form stratocumulus clouds.

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Homework for Chapter 6  Chapter 6 Questions for Review, p. 161  #2, 5, 6, 11, 12, 15, 19

 Chapter 6 Problems and Exercises, p. 162  #5 (a, b)

Project for Chapter 6  None

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