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Chapter 5: Cloud Development and Precipitation Atmospheric stability Determining stability Cloud development and stability Precipitation processes Precipitation types Measuring precipitation Atmospheric Stability We know that air rises, cools and condenses into clouds Atmospheric stability refers to condition of equilibrium We will refer to stable and unstable environments Atmospheric Stability A stable environment is one where the original equilibrium is maintained (things return to where they were An unstable environment is one where things move away from their original position • 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. Atmospheric Stability Remember, when air rises, it moves into an area of lower pressure, expands, and cools. Sinking air is the opposite Adiabatic process – a process in which exchanges no heat with its outside surroundings (how can this happen) Lapse rate – change of temperature with height Dry adiabatic lapse rate - 10°C/1000m or 5.5°F/1000 feet Atmospheric Stability But what happens when air cools (humidity?). If we get condensation, then heat is released and the process is no longer adiabatic. Thus, we have the moist adiabatic lapse rate. Is it less or more than the dry adiabatic lapse rate? Moist adiabatic lapse rate - 6°C/1000 m or 3.3°F/1000 feet. Varies greatly with different moisture content. A Stable Atmosphere So how do we determine stability? We raise a parcel and compare it to its surroundings. If a parcel is colder than its surroundings, it is more dense and will sink (stable) If a parcel is warmer, it is less dense and will rise (unstable) Environmental lapse rate – the rate at which air changes in temperature with height A Stable Atmosphere Absolute stability – when a parcel is colder than the environment at all levels (for both dry and moist adiabats) If forced to rise, clouds will have flat tops and be thin A Stable Atmosphere Atmosphere will be stable with the environmental lapse rate is small So when the air aloft warms and surface cools This can happen during radiational cooling Influx of cold air Air moving over cold surface A Stable Atmosphere Most stable around sunrise Layer of fog or haze can be evidence of a stable atmosphere Because stable atmospheres resist vertical movement, they trap pollutants near the ground and can cause dangerous air quality An Unstable Atmosphere Absolute instability – occurs when air parcels are warmer than their surroundings (parcel will rise) Warming of surface air helps atmos to becoming unstable An Unstable Atmosphere Unstable atmospheres occur when the environmental lapse rate steepens Destabilizing processes: Solar heating of the surfance Warm air brought by wind Air moving over warm surface Superadiabatic lapse rate – when environmental lapse rate exceeds the dry adiabatic lapse rate Conditionally Unstable Air Conditional instability – occurs if we force a cool parcel to a part of the environment where it condenses and becomes warmer Level of free convection – point at which we don’t need to force it up anymore Conditionally Unstable Air Look at the environmental lapse rate and make a hypothesis of what lapse rate you need in order to have conditionally unstable air You need an environmental lapse rate between the moist and dry adiabatic lapse rate Average lapse rate in the troposphere is 6.5°C/1000 m. What is the average stability? Convection and Clouds Primary ways clouds form: Surface heating and convection Topographical uplift Convergence Lifting along a weather front Convection and Clouds Convection and Clouds Topography and Clouds Orographic uplift – forced lifting along a topographic barrier Rain shadow - Due to frequent westerly winds, the western slope of the Rocky Mountains receives much more precipitation than the eastern slope. Convection and Clouds Collision and Coalescence Process How do raindrops become large enough to fall? Condensation alone is just not enough Collision and Coalescence Process One process is collision and coalescence Occurs in warm clouds (tops warmer than 0°C • 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. Collision and Coalescence Process Clouds must have varying droplet sizes Terminal velocity – the point at which gravity equals the air resistance (falls at constant speed Collision and Coalescence Process Larger drops merge with smaller drops in a processing called coalescence Important factor is how long the droplet stays in the cloud Stepped Art Fig. 5-9, p. 116 Ice Crystal Process Ice crystal or Bergeron process Occurs in cold clouds – clouds with temperatures that drop to below freezing Bergeron process states that liquid and ice droplets must co-exist in clouds Ice Crystal Process Supercooled water droplets – droplets that occur as liquid below freezing (middle portion of a thunderstorm cloud) Occurs when there are few ice nuclei Ice Crystal Process Ice crystals form at the expense of water droplets Suppose we have one super cooled droplet and one ice crystal in a saturated environment Since saturated, number molecules evaporating and condense MUST be equal! More vapor molecules over liquid because it is easier to escape liquid to vapor than ice to vapor Ice Crystal Process Since more liquid molecules are going to vapor, it takes more vapor molecules to condense to liquid to keep the equilibrium (saturation) Thus, the saturation vapor pressure above water is greater than that above ice This causes vapor molecules to move towards the ice Removal of water vapor cause water droplet to shrink (not in equilibrium) Ice Crystal Process Ice crystals may also grow larger by accretion – when ice crystals collide with supercooled dropets Fig. 5-22, p. 124 Stepped Art Fig. 5-22, p. 124 Cloud Seeding and Precipitation Cloud seeding – pumping nuclei into clouds to promote precipitation Silver iodide – found to be a good ice nuclei • 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 Rain Rain – falling droplet size greater than or equal to 0.5 mm (0.02 in) Drizzle – falling droplet size less than 0.5 mm (0.02 in) Virga – rain that evaporates before it hits the ground Shower – brief downpours possibility as a result of updrafts Snow Snow – precipitation that falls to the ground as ice crystals. Much precipitation actually starts as snow Fallstreaks – ice crystals and snowflakes that fall from cirrus clouds Dendrite – most common type of fernlike snowflake Blizzard – combination of strong winds, low temperatures, and fine, dry snow • Snowflake shape depends on both temperature and relative humidity. Sleet and Freezing Rain Sleet – precipitation that has thawed and frozen again Freezing rain – supercooled droplets that reach the ground and freeze on contact Rime – accumulation of white ice that occurs when supercooled droplets hit something Snow Grains and Snow Pellets Snow grains Solid equivalent of drizzle Snow pellets Like grains, but bounce Hail Hail develops as droplets are uplifted in the clouds above the freezing level The droplets grows by accretion until it is large enough for gravity to take it to the ground Stepped Art Fig. 5-35, p. 134 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 Can detect precipitation intensity Can detect movement away or to the radar Stepped Art Fig. 5-39, p. 135