Class #4: Stability, cloud development, and precipitation. Chapters 6 and 7. Stability & Cloud development. Chapter 6. Fig. 6-CO, p. 140. Fig. 6-1, p. 142. Importance of Clouds. Release heat to atmosphere Help regulate energy balance Indicate physical processes. Atmospheric Stability.
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Class #4: Stability, cloud development, and precipitation
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Importance of Clouds • Release heat to atmosphere • Help regulate energy balance • Indicate physical processes Class #4 Monday, July 12, 2010
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 Class #4 Monday, July 12, 2010
Determining Stability • Warm air rises or is unstable • Cool air sinks or is stable • Compare air parcel lapse rate to environmental lapse rate Class #4 Monday, July 12, 2010
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. Class #4 Monday, July 12, 2010
Determining Stability • Special Topic: Subsidence Inversions • Strong subsidence exacerbates air pollution due to the lack of vertical motion. • Pollution is not diluted. Class #4 Monday, July 12, 2010
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 Class #4 Monday, July 12, 2010
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 Class #4 Monday, July 12, 2010
Determining Stability • Causes of Instability • Cool air aloft (advection, radiation cooling in clouds) • Warming of surface (insolation, advection, warm surface) Class #4 Monday, July 12, 2010
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. Class #4 Monday, July 12, 2010
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. Class #4 Monday, July 12, 2010
Cloud Development • Topic: Adiabatic charts • Adiabatic charts show how various atmospheric variables change with height: pressure, temperature, humidity. Class #4 Monday, July 12, 2010
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. Class #4 Monday, July 12, 2010
