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/1000m Moist rate = 6°C/1000m  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

Stepped Art Fig. 6-2, p. 143

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

Determining Stability  Special Topic: Subsidence Inversions Strong subsidence exacerbates air pollution due to the lack of vertical motion. Pollution is not diluted.

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 unstable above cloud base Atmosphere usually in this state

Determining Stability  Causes of Instability Cool air aloft (advection, radiation cooling in clouds) Warming of surface (insolation, advection, warm surface)

Cloud Development  Clouds develop as an air parcel rises and cools below the dew point.  Usually a trigger or process is need to initiate the rise of an air parcel.

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 at level of free convection.

Stepped Art Fig. 6-16, p. 152

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.