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Chapter 5: Cloud Development and Precipitation Atmospheric Stability Atmospheric Stability Determining stability Cloud development and stability Precipitation processes Precipitation types Measuring precipitation Atmospheric Stability Determining 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. A Stable Atmosphere environmental lapse rate absolute stability stabilizing processes subsidence inversions An Unstable Atmosphere absolute instability warming of surface air destabilizing processes superadiabatic lapse rates • Unstable air tends to be well-mixed. • Stable air provides excellent conditions for high pollution levels. 1 Conditionally Unstable Air Cloud Development and Stability conditional instability dry and moist adiabatic lapse rates 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 Precipitation Processes • 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. 2 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 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? Stepped Art Fig. 5-22, p. 124 3 Precipitation in Clouds Precipitation Types accretion ice crystal process Rain rain drizzle virga shower Snow • Virga is much more commonly observed in the western US, because the humid climate of the eastern US reduces the visibility. • Snowflake shape depends on both temperature and relative humidity. Snow Grains and Snow Pellets Sleet and Freezing Rain sleet freezing rain rime snow fallstreaks dendrite blizzard snow grains snow pellets graupel • Sleet makes a ‘tap tap’ sound when falling on glass. 4 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 5
