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Chapter 6

1 Concordia University Geog/Sci-381 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




 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.


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7 Concordia University Geog/Sci-381 Chapter 6 8 Concordia University Geog/Sci-381 Chapter 6



 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.

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


Fig. 6-6, p. 145 11 Concordia University Geog/Sci-381 Chapter 6 Fig. 6-6, p. 145 12 Concordia University Geog/Sci-381 Chapter 6

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 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



 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.



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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


Mixing tends to steepen the lapse rate, making the atmosphere more: a.  stable b.  unstable


Pyrocumulus Clouds

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



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.


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Cloud Development

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


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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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