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climate: air and wind

Global air circulation & pressure systems:

  • as a result of the rotation of earth on its axis, the resultant Coriolis effect, ocean gyres & temperature variations with latitude, global air circulation tends to form a pattern of pressure systems which move from the west to the east:
    • equatorial trough:
      • a long & narrow region of low pressure called the intertropical convergence zone (ITCZ) or equatorial trough
      • it is centred at 15°N in July and 5°S in January with some local variations depending on surface & season
      • the band is widest over areas of land rather than over seas.
      • results in:
        • equatorial light winds - the doldrums
        • equatorial westerlies (north of 20°S in summer)
          • due to low temperature of sea surface off coast of Somaliland which stabilises atmosphere there & increases surface pressure over western part of Indian Ocean
        • trade winds:
          • these are reliable, moisture-laden east winds occurring mainly at 15° latitude that emerge from oceanic high pressure cells
          • these flow towards the equator and are particularly evident on the east sides of the oceans & stronger in the southern hemisphere
          • the Hadley cell:
            • the trade winds become increasingly unstable as they flow over low latitude water resulting in considerable convective ascent of air which then returns polewards in anti-trade winds above the trade winds, with the upper flow deflected to the east by the Coriolis effect, finally descending as it cools due to giving off its heat as radiation, creating a subsidence inversion about 500m deep with its base at 1-2km altitude. This is the Trade Wind Inversion which limits the vertical growth of fair-weather cumuli in the Trades.
          • the Walker circulation is a feature of atmospheric circulation over equatorial Pacific related to the El Nino
        • tropical cyclones:
        • monsoons:
          • secondary circulations occurring over a wide area, characterised by  a seasonal reversal of the steady, prevailing wind direction, and by only rare alternations of high & low pressure systems
          • extend as far north as Japan, China & India but only as far south as northern Australia (17°S) & Madagascar
          • in Asia, are due to fixed high over Mongolia in January but moist airflows from Arabian Sea & southern Indian Ocean in July as a result of northward shift of the ITCZ. The switch from dry to wet air brings floods starting at end of May in Sri Lanka and in early July in north-west India
          • in Australia, the “wet” lasts from September to April
    • subtropical anticyclonic bands:
      • this is a band which consists mainly of:
        • a series of high pressure systems:
          • these are centred between 25°S-30°S in the southern hemisphere - more southerly in January
          • these are centred at ~ 35°N in the northern hemisphere
          • occasionally, a high may become stationary for 1-2weeks - a “blocking high” - deflecting oncoming lows & creating extremes of weather nearby:
            • these are most likely to occur at western ends of oceans where the warmer equatorial waters flow polarwards adjacent to relatively cold continents
            • in Australia, ~10 per year (more common in late winter) form over the warm Tasman Sea, where if they form in summer can create a heat wave in SE Aust by causing persistently hot, dry northerly winds to come from inland Australia and may cause prolonged rains in eastern coast
        • embedded low pressure systems:
          • mid-latitude cyclones:
            • these are created by bulges in the boundary between different airstreams, notably between easterly flowing polar air & westerly mid-latitude air, which break off and in the southern hemisphere move north eastwards
          • stationary lows:
            • can also occur in the lee of mountains and over relatively hot land (eg. heat low over WA for much of summer)
        • frequent frontal activity
        • dominated by westerly winds
    • subpolar troughs:
      • occur at 50-60°S & 60°N
      • the Subantarctic trough is located at the latitude of the oceanic convergence and the limit of pack ice
    • polar highs:
      • are due partly to the higher density of colder air
      • these deflect wind to form polar easterlies
    • upper troposphere winds:
      • at elevations above 500mb or 5600m, there are winds that are counterparts of the surface winds, balancing equatorial surface flows by poleward winds & vice versa
      • strong westerly flows near the tropopause at 30°-45° latitude which lurch side to side in huge waves called Rossby waves
        • within these there are regions where the wind blows faster than 60knots called jet streams

Surface winds:

  • surface winds are the result of the net effect of various factors operating within a particular air mass such as:
    • pressure gradients - air tends to flow from places of high pressure and towards those of low pressure
    • temperature gradients - air tends to flow from places of cold temperature and towards those of warmer temperature
    • rotations of Earth & the atmosphere - the Coriolis effect
    • inertia of the wind & its vorticity
    • local variations in contour of earth's surface
  • within an air mass, there may be several types of winds operating concurrently at different regions or heights with irregularities constituting turbulence:
    • a squall is a wind that rises suddenly to at least 11m/s, lasts for at least a minute, & then dies suddenly
    • a gust is shorter-lived:
      • in designing buildings in Australia, allow for 3sec storm gusts that occur ~ once in 50yrs at 10m elevation 50m/s in Sydney & 40m/s in most other capital cities
    • gustiness may be expressed as the difference between minimum & maximum speeds, or as a percentage of the mean
  • the boundary between adjacent air masses (eg. between a high and a low pressure system) is called a frontal zone:
    • it is characterised by a relatively abrupt horizontal variation of dewpoint, wind direction & strength
    • the frontal zone is tilted with the colder, heavier air beneath & rarely extends higher than 5km
    • the band where the zone touches the ground is called a front
      • it is a cold front the the cold air is advancing under the warm air
        • the advance of a cold front pushes the warmer air upwards creating cloud & potential for thunderstorms in advance of the front
        • fronts usually take 2 days to get from Perth to Melbourne, moving faster in winter when there are stronger upper westerlies
        • avg. no. of fronts/month: Sydney 5-9; Perth & Melbourne: 5 in Feb; 10 in August;
      • it is a warm front if the colder airmass is retreating
        • these are unusual over Australia as they form well to the west of the continent & swing well south by the time they reach Australia's longitudes
    • fronts do not occur at either latitudinal extremes where air conditions are relatively uniform, but occur over a wide range of latitudes south of 25°S
    • travelling fronts are those that break away from the semi-permanent fronts such as those that lie at latitudes 40-50°S which relate to the oceanic fronts (these are analogous but are boundaries of different water masses in the ocean)
    • in addition, there are more localised moving fronts:
      • commonly called cool changes
      • occurs esp. on summer afternoons in assoc. with sea breeze

types of winds:

  • geostrophic winds:
    • is a result of the balance between the Coriolis effect & the pressure-gradient force of differences in air pressure systems, winds flow almost parallel to adjacent isobars as shown in weather maps, and in the southern hemisphere:
      • winds tend to to spin anticlockwise out of a high pressure system (anticyclone) creating divergence and central subsidence of air and thus clear skies
      • winds tend to to spin clockwise into a low pressure system creating convergence resulting in central ascent of air and cloudiness
    • the velocity of the winds depends on:
      • local pressure gradient - the closer the isobars, the greater the gradient & the stronger the winds
      • latitude (which governs the Coriolis effect)
  • thermal wind:
    • is a result of the balance between the Coriolis effect & the temperature-gradient force
    • in the southern hemisphere:
      • thermal wind is directed clockwise around a patch of low temperature
    • examples:
      • westerlies of the upper troposphere:
        • as isobaric surfaces mostly slant towards the pole in each hemisphere due to contraction of cold air causing thermal wind to become stronger with height
      • polar-front jet stream
  • local winds:
    • winds that move from cold areas to warm areas:
      • seabreeze:
        • a regular onshore daytime wind occurs in coastal regions depending on:
          • difference between land and sea temperatures - thus mainly in summer in southern Australia
          • strength & direction of prevailing geostrophic wind
          • distance from coastline
          • obstruction by mountains
        • as the sea breeze flows over hotter land, the cold air moves under the warmer land air, forcing it upwards and potentially producing cloud at the convergence zone (the boundary of the seabreeze and the warmer land air)
        • the convergence zone moves inland as the day progresses, usually starting about 9am and initially moving inland at rate of ~3km/hr, but as the inland heats up and the temperature differential increases, the breeze becomes stronger and the convergence zone travels inland more rapidly, perhaps up to 10km/hr
        • the convergence zone may reach inland areas quite distant from the coast:
          • eg. “Fremantle Doctor” has even reached Kalgoorlie (360km inland) where they arrive at 9-10pm
        • the warm air that is forced upwards circulates back over the seabreeze and subsides again over the sea, creating a local circulation
        • a similar phenomenon can occur with large lakes resulting in a lake breeze
        • sea breeze index:
          • a sea breeze will occur if the sea breeze index falls below a critical value for the specified region
          • sea breeze index = (geostrophic wind speed in m/sec)2/(land surface temp - sea surface temp)
        • sea breezes may collide due to the topography:
          • sea breezes from Port Phillip Bay interacts with the Dandenong Ranges to form a wind pattern the “Melbourne eddy”.
      • land breeze:
        • as the land cools on clear nights, an offshore breeze may arise, but these are generally weaker than sea breezes
      • country breeze:
        • as rural areas tend to cool faster than urban areas overnight, a breeze from the country into urban areas may occur
    • effect of mountains:
      • may force wind through gullies
      • create undulating waves in the air flow often with wavelengths of 12km which result in lenticular wave clouds in the upper parts of the wave cycle which run parallel to a mountain range (eg. over Sydney when a westerly is blowing)
      • high mountains force air masses to rise and create cloud before it passes over the mountain which results in precipitation on the windward side and dry winds on the lee side
        • eg. Great Dividing Range - wet on coastal side but dry on inland side
      • Foehn winds:
        • winds which flow down the lee side of high mountains are heated by adiabatic compression & may reach a given level at a temperature higher & drier than that of the air at the same level on the windward side due to the foehn effect:
          • precipitation on the mountain yields latent heat of condensation which creates extra warmth
        • the effect is more obvious in winter and if their onset is sudden, may lead to a rapid rise in temperature:
          • eg. “snoweaters” in Canada
          • lee of the Blue Mountains when westerlies blow in winter
      • descending winds caused by downhill gravitational flow:
        • katabatic winds:
          • sloping land allows air cooled by land to flow down into a valley by gravity resulting in a “fall wind
          • these are often quite shallow being only 100-200m deep and create inversions in valleys which lifts warmer, moist air and if that air is near its dewpoint will result in fog in the valleys
          • an extreme example is that which occurs at the steep high edge of the snow-covered Antarctic Plateau, where the aerial cascade affecting Mawson has an average speed of 20m/s, equivalent to a permanent gale & some days may reach 45m/s.
      • ascending winds caused by convection:
        • anabatic winds:
          • these are daytime winds created by heating of hillsides and result in valley winds up a mountain

alteration of wind by obstacles:

  • wind hitting a cliff:
    • the wind flow will be directed above the cliff at greater speed while a “bubble” region adjacent to the cliff face extending away from the cliff face by a distance of approx. half the cliff height will have turbulence and gusty winds which on average may be more into the wind direction rather than in the same direction - but don't rely on this reversal to stop your boats hitting rocks at the cliff face!
  • wind flowing over land which ends as a cliff:
    • a separation bubble of increased turbulence with flow reversal will form for a distance beyond the cliff approx. 4 x the cliff height
  • wind flowing over a small island (less than 100m high and 1km diameter):
    • a flow separation and reversal area with increased turbulence and higher wind gusts but lower average speed may form on the leeward side if the island is steep enough on the lee side for the given aerodynamic roughness of the island.
    • thus if the islet is covered by trees or bushes, separation will occur if the leeside slope is greater than about 20degrees, whereas it must be greater than about 30 degrees if it is covered by grass.
    • the potential wind gusts in the leeward region can be estimated approximately by the equation:
      • max. potential wind gust = (undisturbed wind speed at height 1/5th of islet height) x (1 + (7 x islet height/islet diameter))
    • the downwind extent of this reversal is usually no more than 3-4 x height of the islet but may extend to 10x height if the islet has a steep rocky crest. The average speed of the wind in this wake will have regained 90% of its original speed by some 10x islet diameter while the turbulence persists for a little further.
    • the wake disturbance also spreads out horizontally on either side of the islet.

wind force:

  • wind force (eg. on a sail)  = area of sail x air density x force coefficient x (wind speed)2 
  • air density is approx. 1.2 kg/m3 
  • thus every time wind speed doubles, the sail area needed must be reduced to a quarter, but in reality, what is done is reduce the force coefficient by changing the angle of the sail to the wind and adjusting the set of the sail. Also this depends on the aspect ratio of the sail - a tall one is more efficient than a short fat one, especially in strong winds - as wind speed increases with height - but a tall sail results in a greater “heeling moment” or tendency to pull the boat over sideways.

wind speed:

  • estimating ground wind speed from distance between isobars on a weather map:
    • assuming 2mb isobars at mid-latitudes such as Melbourne:
      • 2000km between isobars (ie. width of Australia) then < 2 knots
      • 1000km then 2-3 knots
      • 500km then 4-6 knots
      • 250km then 8-11 knots
      • 125km then 17-20 knots
    • wind speeds are generally faster near the equator for same pressure gradient
    • detailed calculation:
      • geostrophic wind speed  in m/sec = 77.5 x horizontal pressure gradient in mbar per metre / Coriolis parameter
      • Coriolis parameter = 2 x angular velocity of earth's rotation in radians per second x Sin(latitude)
      • angular velocity of earth's rotation = 7.29 x 10-5 radians per second
    • you need to take into account local topographic effects, local fronts, storms, etc. as well.
Beaufort scale and description Wind speed Effect on land Effect on water (wave height on deep water and with adequate fetch & wind duration)
0 calm < 1 knot or < 1kph smoke rises vertically surface looks like a mirror
1 light wind 1-3 knots or 1-5kph smoke follows wind; wind vanes don't work ripples can be seen - waves 1cm high
2 light breeze 4-6knots or 6-11kph leaves rustle, people feel wind on faces small wavelets with glassy appearance - 6cm waves
3 gentle breeze 7-10knots or 12-19kph leaves,twigs in constant motion; light flag will extend large wavelets & crests begin to break; 20cm waves
4 moderate breeze 11-16knots; 20-28kph dust, loose paper raised; small branches move small waves & some whitecaps; 60cm waves
5 fresh breeze 17-21knots;29-38kph small leafy trees sway; inland waters form crested wavelets moderate sized waves to 1.3m & many whitecaps
6 strong breeze 22-27knots;39-49kph large branches move; power lines whistle large wves to 2.5m, extensive whitecaps, some spray
7 near gale 28-33knots;50-61kph whole trees move; people have some difficulty walking into wind sea heaps up; waves to 4.5m; white foam from breaking waves is blown in streaks
8 gale 34-40knots;62-74kph twigs break off trees; people have difficulty walking into wind moderately high waves break & form spindrift; well-defined streaks; waves to 7m possible;
9 strong gale 41-47knots;75-88kph structural damage can occur high waves; dense streaks of foam along wind lines; crests of waves begin to roll over; spray may affect visibility; waves may reach 11m
10 storm 48-55knots;89-102kph trees uprooted; severe structural damage may occur very large waves with long over-hanging crests; sea is chaotic & tumbling & takes on a dense white apearance; waves may reach 16m
11 violent storm 56-63knots;103-117kph widespread damage but rare on land small-medium sized ships may be temporarily blocked from view by waves; foam everywhere; waves may reach 22m
12 hurricane >64knots; >118kph rare on land air filled with foam & spray severely limiting visibility; waves may reach higher than 22m.
climate/climate3.txt · Last modified: 2013/02/02 00:28 by gary1