ENVS120 Atmospheric Moisture and Precipitation Notes Weeks 4

ENVS120 (ENVIRONMENTAL SYSTEMS PART 2: CLIMATOLOGY) Dr Themba Dube University of KwaZulu-Natal School: Agricultural, Earth and Environmental Sciences Discipline: Geography 6. ATMOSPHERIC MOISTURE AND PRECIPITATION ...

ENVS120 (ENVIRONMENTAL SYSTEMS PART 2: CLIMATOLOGY) Dr Themba Dube University of KwaZulu-Natal School: Agricultural, Earth and Environmental Sciences Discipline: Geography 6. ATMOSPHERIC MOISTURE AND PRECIPITATION  Content 1. Humidity measurement 11. Thunderstorms, entrainment, 2. Humidity terms seeding, squall wind, microburst 3. Adiabatic process 12. Rainshadow 4. Lapse rates 13. Tornado 5. Stable and unstable air 14. Enhanced Fujita Scale (EFS) 6. Cloud, nuclei, supercooled water 15. Air pollution (types, reaction, smog) 7. Cloud forms 16. Pollution settling 8. Precipitation measurement 17. Urban effects 9. Precipitation forms 18. Heat island, dome, plume 10. Precipitation production 19. Climate and env. Effects of urban (spontaneous/convective; forced air poll. rise)  Humidity Specific humidity = ratio of mass of water vapour to mass of dry air and moist air. Humidity = amount of water vapour present in air. q = mv/(mv + ma) Saturated air = air holding max. quantity of water Units = g/kg vapour possible at a specified temperature. Range is 0.2 g/kg in cold Antarctic/Arctic air to 19 g/kg Quantity increases with T increase. in warm equatorial air. Vapour pressure (e): units = Force/Area Absolute humidity = Mass of water vapour to volume of dry air pV = mv/V, where mv is mass of water vapour, and V is volume of dry air. Mixing ratio = Mass of water vapour to mass of dry air. r = mv/ma Fig.: Global specific humidity.  Saturation vapour pressure (es) = represents max. amount of water vapour that can be held by air of a given T and p. At constant pressure, es increases with T, nearly doubling for each 10OC increase. Clausius Clapeyron Fig.: Saturation specific humidity and temperature. statement.  Relative humidity = % ratio of water vapour present to max. quantity at saturation. RH = (e/es)x100 Increase in RH by: (a) evaporation from free water surface. (b) decrease in air T. Daily cycle of RH: decrease during the day as air T increases; increase at night as air T decreases.  Dew point: Critical air T at which air is saturated. Precipitation requires lift of large mass of air to higher altitudes. T decreases below dew point near the surface  condensation as dew or frost. Adiabatic process: Change of T within a gas because of compression or expansion, without gain Hygrometer: measures RH. or loss of heat from outside. Sling psychrometer: pair of thermometers (dry and Compression causes heating; expansion causes wetbulb) moving rapidly through air. cooling. Degree of cooling of wetbulb by evaporation is Rising air expands, undergoes adiabatic cooling. greater for air of lower humidity. Dry adiabatic lapse rate (DALR): rate at which air is Sling psychrometer: pair of thermometers (dry and cooled as it rises, when no condensation is wetbulb) moving rapidly through air. occurring. Degree of cooling of wetbulb by evaporation is Value: 10OC/1000m. greater for air of lower humidity. Dew-point temperature also decreases with Condensation and the adiabatic process increase in altitude at 2OC/1000m. Precipitation: particles of water, liquid or solid, tat Condensation sets in at altitude where adiabatic fall from atmosphere and reach ground cooling brings air to dew-point temperature. Fig.: Adiabatic cooling in a rising parcel of air. Lapse rates A Constant lapse rate. B Lapse rate growing smaller with altitude increase. Fig. XXX C Lapse rate increasing with altitude increase. D Constant lapse rate below and above inversion. E LR beginning with production of inversion (common in cool nights). DALR applies to T change in dry air Dry air = air with RH < 100%.  Stable air Air forced over mountain/topographic Fig.: XX irregularity  on windward side, it becomes colder than surrounding air  descend on leeward side. Here Environmental lapse rate (ELR) of surrounding air < adiabatic lapse rate (DALR) of Fig.: XX rising air (ELR < DALR).  Unstable air Forced uplift, e.g. mountain  rising air becomes progressively Fig.: XX warmer than surroundings  acceleration of uplift on windward side. Here ELR > DALR Fig.: XX  Wet adiabatic lapse rate (WALR): reduced lapse rate when condensation is taking place in rising air. Liberation of latent heat partially offsets adiabatic cooling. Value: 3 to 6OC/1000m. Varies with condensation rate. Cloud particles Cloud: Dense mass of suspended water or ice particles in diameter range 20 to 50 microns. Nucleus: core of solid matter upon which water condenses to form cloud particles, e.g. sea salts. They are hygroscopic/hydrophillic (property of susbstance to absorb water from surroundings). Fig.: Breaking or spilling waves from an open ocean are an important source of condensation nuclei.  Supercooled water: Water in liquid state in droplets well below freezing temperature of 0OC. May occur in cloud droplets of T -12OC. Mixture of water and ice at -12OC to -30OC. Ice crystals below -30OC. Cloud forms Stratiform clouds: layered. Cumuliform: globular. Clouds are classified on basis of (a) altitude, and (b) form  4 families, namely High, Middle, Low, and Clouds of vertical development. Fig.: Cloud families.  Family A: High clouds; 6-12 km; formed from ice Family D: Clouds of vertical development; extend particles. through great altitudes. Cirrus: delicate, wispy, fibrous. Cumulus: white woolpack masses; associated with fair weather; dense, congested forms yield Cirrostratus: veil-like layer, make halo around sun showers. or moon. Cumulonimbus: large, high, dense, thunderstorm Cirrocumulus: globular, in rows or layers. cloud; yield rain, hail thunder, lightning. Family B: Middle clouds; 2-6 km. Fog: Cloud layer in contact with land or water surface; or very close to that surface. Altostratus: blanket-like. Radiation fog: produced by radiational cooling of Altocumulus: globular; in layers. basal air layer. Family C: Low clouds’ ground level to 2 km. Associated with low-level temperature inversion. Stratus: dense, low layer. Usually nocturnal. Nimbostratus: stratus yielding rain. Advection fog: Condensation within moist basal air layer moving over cold land or water surface. Stratocumulus: Low layer of globular or roll-like cloud masses. Air losses heat to surface beneath. Fig.: Advection fog as moist maritime air moves onshore after crossing a current of cold water.  Precipitation forms Rain: falling water droplets, 1000 to 2000 microns in diameter. Formed by condensation of cloud droplets in warm clouds. Snow: falling ice crystals, clotted together into snowflakes. Sleet: ice grains or pellets produced by freezing of falling rain. Generated in cold air layer below warm layer aloft. Hail: large pellets or spheres of ice formed in updrafts of cumulonimbus cloud. Glaze: ice coating on ground, trees, wires, formed as rain freezes upon landing. Ice storm: occurrence of heavy glaze.  Precipitation measurement Units: depth of fall per unit time, e.g. cm/hr; cm/day. Instrument: raingauge. Fig.: Weather radar display.  Precipitation production 2 mechanisms of rise of large air masses (a) spontaneous rise or convection, (b) forced rise.  types of precipitation: Convective, orographic, and frontal (a) Convective precipitation Moist air is warmed at the surface expands, becomes less dense than surrounding, cooler air, and is buoyed upward. At the lifting condensation level, clouds begin to Fig.: Convectional precipitation in warm clouds typical of the form. equatorial and tropical zones.  Convection: Spontaneous rise of moist air in convection cell. Convection cell: updraft system in which bubble-like mass of air rises rapidly because of its lower density. Heating of lower air during the day can generate small convective system. Liberation of latent heat during condensation is energy source for all strong convection cells rising Fig.: The Bergeron process. In cold clouds, precipitation forms high in atmosphere. as water vapour evaporates from supercooled liquid cloud drops. The water vapour is then deposited on ice crystals, forming snowflakes.  Unstable air: Air significantly moist and warm to yield heat through condensation, giving rise to convectional activity. Typical of equatorial, tropical zones and midlatitudes in summer. Has steep ELR (12OC/km); steeper than DALR (ELR > DALR). Steep ELR develops by heating of lower air from underlying warm ocean or land surface. Often associated with large water vapour content (high specific humidity). Stable air: ELR < DALR  air resists being lifted. Fig.: Atmospheric stability depends on how ait temperature changes with altitude at a particular location at a particular time.  Thunderstorms Fig.: Lightning Thunderstorm: Intense, local convectional storm associated with cumulonimbus cloud and yielding heavy precipitation. Thunder, lightning, and hail commonly present. Entrainment: drawing in of surrounding air by air rising rapidly in a thunderstorm cell. Flattened anvil top of cumulonimbus cloud is formed by downwind motion of ice cloud where prevailing wind is strong. Fig.: Anatomy of Cloud seeding: fall of ice crystals from anvil top of a severe cumulonimbus cloud, serving as nuclei of thunderstorm condensation at lower levels. Strong downdraft strikes ground as destructive squall wind.  Fig.: Microburst is an intense downdraft or downburst that accompanies the gust front. Passing through a microburst, an air plane first experiences a strong headwind, then a strong tailwind that can cause the aircraft to lose lift and crash.  Artificial cloud seeding using hygroscopic chemicals e.g. silver iodide can increase precipitation under favourable conditions. (b) Orographic precipitation Precipitation induced by forced rise of moist air over a mountain barrier. Cooling by adiabatic process causes condensation, clouds and precipitation. Fig.: Orographic precipitation.  Rainshadow: Zone of aridity on lee of mountain barrier. Result of adiabatic warming of descending air. Chinook winds: Strong updrafts on lee of mountain. Isohyet: lines joining places of equal precipitation. Tornado A tornado is a small but intense spiralling vortex of rising air associated with a strong updraft of an intense thunderstorm. It descends from the base of a severe thunderstorm. Fig.: Tornado.  Formation of mesocyclones and tornadoes. The combination of vertical wind shear with very strong convection produce mesocycles that extend through the troposphere. Rapidly rotating circulations extending from the bottom of the mesocycles to the surface can then develop into Fig. Formation of mesocyclones and tornadoes. tornadoes.  Tornado destruction The enhanced Fujita scale (EF0- EF5) rates the severity of tornadoes, based on the nature and amount of damage done to structures. Fig.: Enhanced Fujita intensity scale of tornado damage.  Air pollution Pollutants: foreign matter injected by man into atmosphere as particulate matter and as chemical pollutants. Particulate matter: solid and liquid particles capable of being suspended for long time in the air. Chemical pollutants: gasses other than normal atmospheric gases from populated, industrialised regions., e.g. carbon monoxide (CO), sulphur dioxide (SO2), oxides of nitrogen (NO, NO2, NO3 ), hydrogen compounds, ozone (O3). SO2 + O2  SO3 + O SO3 + H2O  H2SO4  acid rain Smog: mixture of particulate matter and chemical pollutants over urban areas. Fig.: Smog in Beijung, China on a day in March 2008.  Minor concentration of pollutants normal in stagnant air layers. Pollutants introduced in urban areas, largely by fuel combustion, and in rural areas by smelting, mining, quarrying, and farming. Natural pollutants: volcanic dust, sea salts, pollen, grass smoke and forest fires, blowing dust, bacteria, viruses. Major sources of pollution Emissions and sources make different contributions to air pollution. Vehicular exhausts: CO, hydrocarbons, nitrogen oxides, Pb. Electricity-generating plants: SO2, particulates, fly ash. Fig.: Smog and haze. Space heating, refuse burning are minor sources.  Pollution settling mechanisms Persistent inversions can trap pollutants, resulting in low air quality and smog. Fallout: gravity fall of particulates, reaching the ground. Low-level inversion results from radiational cooling at night; creates stable conditions. Washout: downsweeping of particulate by Pollutants trapped beneath inversion lid. precipitation. Low-level inversion: top surface of inversion Photochemical reactions: Action of sunlights layer, resisting mixture of air above and below upon pollutants gases to synthesise new toxic the lid. compounds or gases, e.g. sulphuric acid (H2SO4) derived from SO2; Ozone (O3) Upper-level inversion: caused by subsidence produced by sunlight action upon nitrogen of air in anticyclone over basal cool layer, oxides and hydrocarbons. creating inversion lid. Low-level inversions A layer of warm, dry air from a persistent fair- weather system rides over a cool, moist Steepened ELR over urban area allows warm marine air layer at the surface to create a air and pollutants to rise until level of stability persistent temperature inversion. is reached.  Modification of urban climate Effects of urbanisation on energy balance Reduced transpiration. Pavements and masonry hold more heat than soil. Thermal effect is to convert city to desert environment. Fig.: Rural and urban surfaces. Fig.: Typical variation of air temperature between urban and rural areas. Heat island: persistent region of higher air temperatures centred over city. Pollution dome: broad, low, dome-shaped layer of polluted air formed over urban areas at times when winds are weak or calm. Fig.: Pollution dome and pollution plume. Pollution plume: trail of polluted air carried downwind from a pollution source.  Other climatic effects of urban air pollution Acid deposition and its effects Reduced visibility. Acid rain: rainwater having abnormally high content of acid ions, principally sulphate ion of Reduced UV penetration. H2SO4, and ions of nitric acid (HNO3). Increased incidence of fog. Rainwater pH of acid rain lies between 4 and 5. Values below 4 commonly observed over heavily Increased convectional precipitation. industrialised areas in midlatitudes. Environmental effects of atmospheric pollution Acid rain results from input of pollutants of fossil fuel combustion. Human: harmful e.g. H2SO4 from SO2, ethylene from hydrocarbon compounds, O3, CO from Acid rain leads to excessive leaching of plant vehicular exhausts. nutrients from soil and to serious disturbances of aquatic ecosystems of lakes and streams. Plants: harmful e.g. O3, SO2. Crops and trees Numerous lakes in affected areas now have no damaged. fish. Corrosive action of H2SO4, affect masonry, metals. O3 damages exposed rubber. Lead fallout from polluted urban air threat to life. Pb additives to gasoline have been phased out. 7. AIR MASSES AND CYCLONIC STORMS