* see also:

• work, energy & power
• Climate - Introduction 1 - atmosphere and temperature
• Climate - Introduction 3 - air & wind
• Climate - Introduction 4 - Climates 

• 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)
• man's use of groundwater for irrigation between 1993–2010 has moved 2150 GTon of water from underground to the oceans causing a 6mm rise in sea levels and increasing the earth's tilt by drift of Earth's rotational pole of 78.48 cm toward 64.16°E ¹⁾

• 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

• 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

• 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

• 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

• 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;

• hail generally forms when there are these four factors present:
  • deep moist convection
  • adequate updraft to keep the hailstone aloft for an appropriate amount of time
    • high convective available potential energy (CAPE), especially > 1000J/kg as these lead to high upward velocities within a thunderstorm
    • lower precipitable water values have the potential to produce large hailstones when significant CAPE is present as there is less gravity water weight loading to oppose upward convection
    • high wind shear such as in a supercell allows CAPE to be maximised by separating the updraft and downdraft
  • sufficient supercooled water near the hailstone to enable growth as it travels through an updraft
    • high altitude regions will be closer to cold layers of upper atmosphere and storms there are more likely to produce hail
    • low freezing level (the height of atmosphere above which it is freezing)
      • at low elevation, if the freezing level is closer to the surface than 650 millibars, strong thunderstorms have a good probability of producing hail that will reach the surface
      • freezing level can be found by examining the morning or afternoon Skew-T Log-P plot or forecast sounding
      • dry mid-level air entrained into the storm is evaporatively cooled and can lower the freezing level as well as create strong surface wind gusts
  • nucleation: a piece of ice, snow or dust for it to grow upon
• hail falls when the thunderstorm's updraft can no longer support the weight of the ice
• nearly all severe thunderstorms probably produce hail aloft, though it may melt before reaching the ground
• multi-cell thunderstorms produce many hailstones, but they are not usually very large as the mature phase is short and not enough time for large hail to form
• supercell thunderstorms have sustained updrafts that support large hail formation by repeatedly lifting the hailstones into the very cold air at the top of the thunderstorm cloud
  • hail 2 inches (5 cm) or larger in diameter is generally associated with supercell thunderstorms