Climate - Introduction 2
Water:
- 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:
- 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
- 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
- Clouds:
- 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:
- 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
-
- Rain:
- 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
-
- droughts:
- 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;
-
