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climate - water and rain

Water on earth:

  • water vapour in atmosphere = 20 million, million tonnes
  • water in lakes, river & underground = 200 million, million tonnes
  • water in icecaps = 20,000 million, million tonnes
  • water in oceans = 1.4 million, million, million tonnes (ie. ~100x that in the ice caps)


  • evaporation is not the same as boiling, it occurs at the liquid surface at any temperature
  • Dalton equation:
    • evaporation rate = Ku(liquid's vapour pressure - ambient atmospheric water vapour pressure)
      • liquid's vapour pressure is dependent on its temperature
      • ambient atmospheric water vapour pressure is dependent on atmospheric temperature and humidity
      • K = factor dependent on stirring of surface by wind & the surface roughness
      • u = wind speed, but as K is almost inversely proportional to u, Ku is almost constant at 0.5, but will almost double for the extra roughness of ocean surface when wind increases above 6.5m/s.
  • mean evaporation rates from a pan rises approx. exponentially with mean temperature:
    • 10deg C ⇒ ~1mm/day
    • 15degC ⇒ ~2.5mm/day
    • 20degC ⇒ ~4.5mm/day
    • 25degC ⇒ ~7mm/day
  • in Australia, average annual evaporation rates range from:
    • Tasmanian wilderness = 500mm/yr
    • NZ = 650-950mm/yr
    • SE Australian coast incl. Melb, Sydney = 1000mm/yr
    • Qld coast, Adelaide, SW WA coast = 1250mm/yr
    • Kalgoorlie = 3300mm/yr
  • southern hemisphere avg evaporation rates vs latitude:
    • from oceans:
      • latitudes 0-30deg ⇒ 1100-1200mm/yr
      • latitudes 30-40deg ⇒ 890mm/yr
      • latitudes 40-50deg ⇒ 580mm/yr
      • latitudes 50-60deg ⇒ 230mm/yr
    • from continents:
      • latitudes 0-10deg ⇒ 1200mm/yr
      • latitudes 10-20deg ⇒ 900mm/yr
      • latitudes 20-50deg ⇒ 400-500mm/yr
      • latitudes 50-60deg ⇒ 200mm/yr

Water content in atmosphere:

  • the amount can be specified in several ways:
    • vapour pressure (mb) = 1.3 x absolute humidity (g/m3)
      • saturated vapour pressure = 12mb at 10degC & doubles with every 11degC increase in temperature
    • dewpoint temperature:
      • the temperature to which air must be cooled for the moisture present to provide saturation
      • air cooled below its dewpoint results in dew forming
      • dewpoint = dry bulb temp - (100-RH)/5) and thus RH = 100 - 5(dry bulb temp - dewpoint)
      • if wind is lifted 1km by a hill, the pressure of each of the air's components is reduced by 13%, a fall of this magnitude in the saturated vapour pressure corresponds to a reduction of dewpoint temperature by ~2degC
      • see also: combating dew
    • relative humidity:
      • this is the ratio of the air's vapour pressure to the saturated vapour pressure at the air's temperature
    • precipitable amount:
      • amount of water in a column of air from the ground including vapour & cloud droplets (usually only 2.7% of total amount)
      • there is a close correlation between surface dewpoint and total atmospheric moisture
      • mean annual precipitable water varies with latitude, being maximal at equator and decreasing steadily away from it although there are some regional variations


cloud types:

  • 3 main types of clouds:
    • convective:
      • up to 10km thick with updraft velocities of 3-20m/sec and horizontal size up to 10km
      • cumulus clouds have 1g/cu.m water (1.5 for cumulonimbus) with bases often at 1km elevation
    • orographic:
      • often up to 200km wide and up to 1km thick with updraft velocities 1-10m/s
      • stratocumulus clouds often have bases1.5km and extend to 3-4km elevation
      • altocumulus exist within 3.5 to 6.5 km elevations
    • layer (stratus):
      • can be 1000km wide, but often only 100m thick and updraft velocities of only 0.1-0.2m/sec
      • stratus clouds are usually in elevations below 2km & have 0.25g/cu.m water (0.5 for nimbostratus)
      • alto-stratus clouds are usually in elevations 2-8km & have only 0.1g/cu.m water
      • cirro-stratus clouds are usually in elevations 8-20km at tropics and 3-8km at poles

cloud formation:

  • if saturated air is cooled to its dewpoint, water condenses out forming a cloud or dew
  • cloud droplets are 0.2-20um size & are no more than a millionth the size of rain droplets
  • clouds are usually formed by moist air being either
    • uplifted:
      • orographic:
        • moist wind is uplifted due to presence of hills or mountains in its path
      • frontal:
        • warm moist air masses are uplifted in front of an incoming cold front or local front such as that produced by a sea breeze
      • convergence:
        • associated with low pressure systems and tropical cyclones
        • land breezes at night converging around most sides of a large lake or bay
        • confluence of air from northern & southern hemispheres near the equator
      • convective:
        • convection upwards of moist air over a warm surface:
          • hot land - eg. fair-weather cumulonimbus thunderstorms
          • warm oceans - eg. off NSW coast where cold, moist air from south passes over warm East Australian Current
          • tropical nocturnal thunderstorms:
            • within 10deg. of equator, just before dawn, if sea temp. < 28degC, the upper air cools overnight, allowing the warm moist air over the ocean to rise
    • or just cooled or mixed with colder air:
      • radiation inversion:
        • ground inversion:
          • as the ground cools overnight, the air nearest the ground cools which may result in fog
          • inhibited by clouds, urban heating, or by stirring caused by winds
        • cloud tops:
          • as the tops of layer clouds have high albedo, they absorb little sunshine to warm them, and so cool by losing long-wave radiation to outer space resulting in the smooth top of a layer cloud, an example of an inversion layer. Inversion layers represent extreme stability and cause:
            • limitation of any ruffling of the surface layer by vertical air movement to an extent dependent on the depth of the inversion & its intensity (the difference between the top and bottom temperatures), although can be penetrated by a plume of hot gases from a bushfire or by a vigorous convection cloud.
            • less aircraft lift above it as the air is warmer, thus planes experience a jolt when flying through it
            • more aircraft lift flying below it, thus may cause pilots to have difficulty landing
            • sound is reflected beneath an inversion since waves travel faster through the upper warmer air & so are bent back towards ground, thus to a ground observer, aircraft noise is much greater when they descend below the inversion layer
            • air pollutants and clouds tend to be trapped below the inversion layer
            • there is decoupling of the winds with those below it being still whilst those above it are now no longer slowed from ground friction and become faster
      • advection inversions:
        •  cold air flowing from higher ground to pass under lower warmer air:
          • eg. valleys
        • warm air flowing over a cold ocean which cools the lower layers of the wind
          • eg. Los Angeles onshore west winds; hot northerly winds passing over southern ocean causing a sea fog;
        • cool sea-breeze passing under warm air, typically forms an inversion layer at several hundred metres elevation
      • subsidence inversions:
        • compression of any layer of descending air due to higher pressures at lower elevations & the spreading of air sideways, typically causing an inversion layer 300-600m deep at an elevation of 1-2km
      • boundary of a cold front forms an inversion layer
      • trade wind inversions:
        • at low latitudes where trade winds blow over the oceans resulting in subsidence inversion is several hundred metres thick & about 500m high, increasing to 2km elevation downwind



  • requires both:
    • cloud formation
    • raindrop nucleation:
      • a cloud droplet is so small that its terminal velocity in still air is only a few millimeters/sec which is much less than the updraught velocity in a cloud, thus the resultant motion is upwards
      • for rain to reach the ground, droplets or crystals must reach at least 0.1mm diameter which requires the aggregation of thousands or millions of droplets
      • aggregation of cloud droplets requires special active nuclei:
        • “warm clouds”:
          • clouds at low altitudes or latitudes that have temperatures above freezing
          • particles of hygroscopic materials such as sea salt with diameters > 5um
            • clouds 1km from coast may have 10 nuclei/L, whilst at 100km inland, only 1 nucleus/L
          • seeding can be done with ammonium nitrate ground to 3um
        • “cold clouds”:
          • if the droplet is cooled to minus 40degC, the tendency towards freezing is so great, that it overcomes the need for a nucleating foreign body, and forms ice spontaneously ⇒ “homogeneous nucleation”
          • between 0degC and minus 40degC, the readiness of a cloud to form rain depends on the presence of ice nuclei, consisting mainly of volcanic dust, clay, and soil particles with a crystalline structure similar to ice with sizes typically > 0.1um, however, if there are too many such nuclei (eg. in drought conditions when there is much dust in the air ⇒ drought begets drought), the drops do not get big enough to fall as rain.
          • seeding can be done by either dry ice or silver iodide

rainfall variability:

  • mean annual rainfall on coastal regions often correlate with adjacent sea mean annual temperatures
  • rainfall for a region tends to have persistence:
    • a wet year tends to be followed by another wet year
    • a wet day tends to be followed by a wet day (Melbourne: 67% chance in Sept, falling to 47% in Jan)
    • a dry day tends to be followed by a dry day (Melbourne: 81% chance in Jan, falling to 54% in July)
  • seasonal variability for a region is dealt with elsewhere, but in general is determined by probability of:
    • tropical cyclones (mainly summer in tropics)
    • onshore moist winds:
      • trade winds (eg. NE coast Australia in spring & summer) - less if El Nino year
      • anticyclonic latitude in relation to region's latitude ⇒ Qld dry in winter
    • frontal systems - esp. for SE Australia, more common in winter
    • convection thunderstorms - esp. in summer



  • are difficult to define, but generally is regarded as occurring if the annual rainfall for a region is in the range of the driest 10% of years for that region
  • major droughts in Australia:
    • 1864-8; 1880-6; 1888; 1895-1903; 1911-6; 1918-20; 1940-1; 1944-5; 1946-7; 1957-8; 1965-6; 1967-8;
climate/climate2.txt · Last modified: 2021/07/07 23:51 by gary1

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