Download chapter5

Chapter 5: Cloud
Development and
Precipitation






Atmospheric Stability
Determining stability
Cloud development and stability
Precipitation processes
Precipitation types
Measuring precipitation
Atmospheric Stability
Atmospheric Stability




stable and unstable equilibria
air parcels
adiabatic process
adiabatic lapse rates
• Stability does not control whether air will rise or sink.
Rather, it controls whether rising air will continue to rise
or whether sinking air will continue to sink.
Determining Stability
A Stable Atmosphere




environmental lapse rate
absolute stability
stabilizing processes
subsidence inversions
• Stable air provides excellent conditions for high
pollution levels.
An Unstable Atmosphere




absolute instability
warming of surface air
destabilizing processes
superadiabatic lapse rates
• Unstable air tends to be
well-mixed.
Conditionally Unstable Air


conditional instability
dry and moist adiabatic lapse rates
Cloud Development
and Stability
Cloud Development and
Stability




surface heating and free convection
uplift along topography
widespread ascent
lifting along weather fronts
Convection and Clouds


thermals
fair weather cumulus
• Fair weather cumulus
provide a visual marker of
thermals.
• Bases of fair-weather
cumulus clouds marks the
lifting condensation level,
the level at which rising air
first becomes saturated.
Topography and Clouds


orographic uplift
rain shadow
• The rain shadow works for snow too. Due to frequent
westerly winds, the western slope of the Rocky Mountains
receives much more precipitation than the eastern slope.
Precipitation
Processes
Collision and Coalescence
Process



terminal velocity
coalescence
warm clouds
• A typical cloud droplet
falls at a rate of 1
centimeter per second.
At this rate it would take
46 hours to fall one mile.
Stepped Art
Fig. 5-9, p. 116
Ice Crystal Process




cold clouds
supercooled water droplets
saturation vapor pressures over liquid
water and ice
accretion
• The upper portions of
summer thunderstorms are
cold clouds!
Fig. 5-22, p. 124
Stepped Art
Fig. 5-22, p. 124
Cloud Seeding and
Precipitation


cloud seeding
silver iodide
• It is very difficult to determine whether a cloud seeding
attempt is successful. How would you know whether
the cloud would have resulted in precipitation if it hadn’t
been seeded?
Precipitation in Clouds


accretion
ice crystal process
Precipitation Types
Rain




rain
drizzle
virga
shower
• Virga is much more commonly observed in the western
US, because the humid climate of the eastern US
reduces the visibility.
Snow




snow
fallstreaks
dendrite
blizzard
• Snowflake shape depends on both temperature and
relative humidity.
Sleet and Freezing Rain



sleet
freezing rain
rime
• Sleet makes a ‘tap tap’ sound when falling on glass.
Snow Grains and Snow
Pellets



snow grains
snow pellets
graupel
Hail


updraft cycles
accretion
• A hailstone can be sliced open to reveal accretion
rings, one for each updraft cycle.
Stepped Art
Fig. 5-35, p. 134
Measuring
Precipitation
Instruments


standard rain gauge
tipping bucket rain gauge
• It is difficult to capture rain in a bucket when the
wind is blowing strongly.
Doppler Radar and
Precipitation


radar
Doppler radar
Stepped Art
Fig. 5-39, p. 135