Transcript 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