Transcript Document
Atmospheric Stability Adiabatic Processes The concept of a parcel Parcel and environmental lapse rates Atmospheric dry stability Determining stability Air parcels A parcel is a “blob” of air Small enough to have only one value of T, p, ρ, etc. Large enough to contain a significant number of molecules. (Are there enough particles to talk about temperature as average kinetic energy, for example?) Lapse Rates Parcel lapse rate – the rate at which temperature changes as the parcel is lifted to a higher altitude Environmental lapse rate – the rate at which the air surrounding the parcel changes as altitude increases The Adiabatic Lapse Rate An adiabatic process is one during which no heat is exchanged between the substance in question and its surroundings Many atmospheric motions occur rapidly enough that parcels do not exchange a significant amount of heat with the environment Examples: • • rising air in a thunderstorm Air rising over a topographic barrier (like a mountain) Adiabatic Processes (Chalkboard) The Adiabatic Lapse Rate The adiabatic lapse rate for DRY air on Earth is Γd = g/cp Γd = 9.81 m s-2 / 1004 J kg-1 K-1 Γd = 0.00977 K m-1 Γd = 9.77 K km-1 The Adiabatic Lapse Rate This means that a rising(sinking) air parcel will cool(warm) at a rate of about 10 oC per km of ascent(descent) unless: • • • It exhanges significant mass or heat with the environment It becomes saturated with respect to water vapor It rises(sinks) so slowly that radiation heat transfer is possible The Adiabatic Lapse Rate What is the dry adiabatic lapse rate (Γd = g/cp) in these atmospheres? Atmosphere g (m s-2) cp (J kg-1 K-1) Venus 8.87 844 Mars 3.71 844 Titan 1.352 1039 The Adiabatic Lapse Rate We have thus far only discussed the DRY ADIABATIC LAPSE RATE Water vapor condensation releases 2.5 MJ of energy for each kg of water condensed – this latent heat changes the adiabatic lapse rate for condensing air parcels to the MOIST ADIABATIC LAPSE RATE 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 (Chalkboard) A Stable Atmosphere environmental lapse rate absolute stability stabilizing processes • 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 are different Environmental lapse rate is between the two Atmospheric Moisture Twice now, we’ve mentioned moist adiabatic lapse rates. Maybe we should talk about atmospheric moisture before we go down that road any further… Humidity, Condensation and Clouds Circulation of water in the atmosphere Evaporation, condensation and saturation Humidity Dew and frost Fog Clouds Circulation of Water in the Atmosphere Circulation of Water in the Atmosphere evaporation condensation precipitation hydrologic cycle • The total amount of water vapor stored in the atmosphere amounts to only one week’s supply of precipitation for the planet. Fig. 4-1, p. 80 Stepped Art Fig. 4-1, p. 80 Evaporation, Condensation and Saturation Evaporation, Condensation and Saturation saturation condensation nuclei • In very clean air, about 10,000 condensation nuclei are typically found in one cubic centimeter of air, a volume approximately the size of your fingertip. Humidity Mixing Ratio The ratio of the mass of water vapor in air to the mass of dry air: w = mv / md Usually expressed in g kg-1 Some typical values: • Tropical marine boundary layer air: w ≈ 18 g kg-1 • Polar air: w ≈ 1 g kg-1 • Stratospheric air: w ≈ 0.1 g kg-1 Specific Humidity The ratio of the mass of water vapor in air to the total mass of the air (dry air plus water vapor): SH = mv / (md + mv) w = SH / (1 – SH) SH = w / (1 + w) Vapor Pressure actual vapor pressure saturation vapor pressure • “Saturation” describes a condition of equilibrium: liquid water is evaporating at exactly the same rate that water vapor is condensing. Vapor Pressure Actual vs. Saturation (or equilibrium) vapor pressure… (Chalkboard) Vapor Pressure Saturation vapor pressure depends only on temperature… Formula: e s : Saturation vapor pressure e s0 : Saturation vapor pressure at 273 K = 6.11 mb L 1 1 Latent heat of vaporization = L : es es0 exp 2.5x10 J kg Rv T0 T R v : Gas constant for water vapor = 461 J kg K T0 : 273 K T : Temperature 6 -1 -1 -1 Vapor Pressure Saturation vapor pressure depends only on temperature… Formula: 1 1 es 6.11exp5423 273 T Vapor Pressure Saturation vapor pressure depends only on temperature… Graph: Relative Humidity definition of relative humidity saturation and supersaturation condensation relative humidity and temperature • When the general public uses the term “humidity”, they mean “relative humidity.” Relative Humidity The ratio of the actual vapor pressure to the saturation vapor pressure. rh = e / es Since es depends on temperature, the relative humidity measures closeness to saturation, not actual water vapor content. Fig. 4-5, p. 83 Fig. 4-7, p. 85 Relative Humidity and Dew Point dew point temperature: the temperature to which air must be lowered to reach 100% relative humidity dew point depression and relative humidity • The dew point temperature is useful for forecasting heat index, precipitation probabilities, and the chance of frost. Measuring Humidity psychrometers hygrometers Dew and Frost Dew and Frost dew frost frost point and deposition • Frost is one of the few examples of deposition in nature. Fog Fog radiation fog advection fog upslope fog evaporation (mixing) fog • Fog is an extreme hazard to aircraft. Clouds Classification of Clouds major cloud types cloud appearance cloud base • It’s easy to identify clouds, but it takes practice. The ability to identify clouds allows you to forecast many aspects of the weather using nothing but your eyes. Table 4-2, p. 98 Cloud Identification high clouds middle clouds low clouds clouds with vertical development High Clouds cirrus cirrocumulus cirrostratus • Cirrostratus clouds can sometimes be quite thick. Middle Clouds altocumulus altostratus • Altocumulus clouds are very pretty, especially just after sunrise or just before sunset. Low Clouds nimbostratus stratocumulus stratus • Marine stratocumulus is the most common cloud type in the world. Clouds with Vertical Development cumulus cumulus congestus cumulonimbus • Not all cumulus clouds grow to be thunderstorms, but all thunderstorms start out as cumulus clouds. Some Unusual Clouds lenticular clouds pileus mammatus clouds contrails • Several alleged ‘flying saucer’ reports have turned out to be lenticular clouds. 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. Conditional Stability • Environmental lapse rate between wet and dry adiabatic lapse rates • Lifting Condensation Level (LCL) • Level of Free Convection (LFC) (Chalkboard) Summary of Atmospheric Stability • Absolute Stability = environmental lapse rate greater than both wet and dry adiabatic lapse rates • Absolute Instability = environmental lapse rate less than both wet and dry adiabatic lapse rates • Conditional Stability = environmental lapse rate less than dry but greater than we adiabatic lapse rate. The environment is stable to moist but unstable to dry disturbances. 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