Transcript Chapter 5 Cloud Formation - Cal State LA

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.