Climate - Introduction 1
The atmosphere:
- there is negligible atmosphere beyond 32,000km because the centrifugal
forces of earth's spinning exceeds its gravitational forces at this distance
- air is squashed by the layers of air above it resulting in higher
pressures closest to earth
- air pressures: sea level 870-1080mb (mean = 1013mb); 3km 700mb; 10km
300mb; (1mb = 100 pascals)
- half of the mass lies in 1st 5.6km
-
- 1st 80km - the homosphere:
- 1st 15km:
- this is the troposphere
- this region is primarily heated by convection and is normally
characterised by:
- air, being transparent, is not significantly heated directly
by solar radiation, but mainly is heated from the ground by
vertical convection, hence a positive
lapse rate > 2degC/km (usually 5-7) in the temperature gradient
- increasing wind with height
- clouds
- vertical churning of the air by wind & convection
- the top of this layer is bounded by the "tropopause"
:
- it is defined as the region where lapse rates fall less than
2degC/km and is usually 8km at the poles and 15km over equator
- NB. at 8.7km, the peak of Mt Everest is >halfway to the
tropopause
- it has a sub-layer:
- 1st 1km is called the planetary boundary layer
- the wind's direction & speed are affected by the
roughness of the ground
- 15-60km:
- this is the stratosphere
- it has a negative lapse rate as a result of warming primarily due
to dissociation of oxygen
- the electrosphere:
- at ~50km, a positively charged layer half a million volts more
positive than the ground
- readily conducts electricity and is formed by connecting the
+ve charge contributions from the cloud tops of thousands of
thunderstorms around the world that occur each day
- the top of this layer is called the stratopause
- 60-80km:
- this is the mesosphere
- whilst it also is primarily heated by dissociation of oxygen, it
has a positive lapse rate with temperatures falling to minus 90degC
by 80km as a result of loss of heat due to radiation out to space
- occasionally, clouds can be seen that shine at night (noctilucent
clouds) and usually occur in the cold, summer polar mesopause at
approx. 85km altitude - see http://lasp.colorado.edu/noctilucent_clouds/
- the top of this layer is called the mesopause
- 80-32000km - the heterosphere:
- it is also called the thermosphere (strictly > 100km) as it
is characterised by a negative lapse rate as a result of heating
primarily by UV light
- temperatures can reach > 30deg C as the sparse gas molecules
are quickly warmed by absorption of even a small amount of daytime
solar radiation
- 80-700km:
- this region is the ionosphere
- there is just sufficient molecules here to be ionised and the
elevation is high enough so that ionising radiation can reach it
without getting blocked out
- this layer has electrical conductivity and reflects radio waves
back to earth & plays a leading role in the optical effects of
the aurora as well as playing a role in the cycle of atmospheric
electricity
Temperature:
- global temperatures and climate change:
- over the past 2.5 million years there have been 50 glacial &
inter-glacial periods.
- over the past 400,000 years, earth has been colder for 90% of the
time, with brief warmer periods of about 10,000 yrs.
- the peak of the last glaciation was 20,000 yrs ago when it was the
coldest it has ever been.
- we are now at the end of a warmer period, and many believe earth will
get colder - perhaps over the next 10-1000yrs.
- earth has become warmer from 1860 to 2000, although it had been also
warmer in Roman and Medieval times with a cool period between 1550 and
the 19th C when the Thames used to freeze over.
- a cooling trend took place between 1940-70, when temperatures again
began to rise reaching a peak in 1998 which coincided with the biggest
El Nino event in the 20th C.
- temperature measurement:
- thermometer ambient temperature: as measured by a dry bulb thermometer
- wet bulb globe temperature: measured in the shade:
- = (0.7 x natural wet bulb temp.) + (0.2 x black globe temperature)
+ (0.1 x dry bulb thermometer temperature in shade)
- apparent temperature (AT): takes into account humidity & wind speed
- apparent temperature in the shade = dry bulb temperature +
(0.33 x water vapour pressure in hPa) - (0.70 x wind speed) - 4.00
- wind speed in km/hr & assumes elevation of 10m
- water vapour pressure in hPa = (relative humidity in % / 100)
x 6.105 x exp(17.27 x dry bulb temp/(237.7+dry bulb temp))
- for AT in the sun in Australia, the sun adds 8 deg C to this
when at its maximum height in the sky in summer:
- AT = dry bulb temp + (0.348 x water vapour pressure in hPa) - (0.70 x wind speed)
+ (0.70xQ/(wind speed + 10)) - 4.25
- Q = Net radiation absorbed per unit area of body surface
(w/m2)
- ref: www.bom.gov.au/products/IDV65079.shtml
(NB. USA measures this differently.)
- see thermal
stress on BOM website
- atmospheric temperature vs elevation:
- positive lapse rates (ie.
air temperature gets colder with elevation):
- normally, atmospheric air gets colder the higher it is away from
the earth's surface as a result of lower greenhouse effects, this
results in a "lapse rate" of ~5degC reduction in
temperature for every 1000m elevation until the tropopause is
reached when temperatures begin to rise or stay much the same
depending on latitude (surprisingly the coldest atmosphere is above
the tropics where the strong convection forces air higher so that it
the tropopause is higher (~15,000m) and temperatures thus fall to a
minimum of minus 80degC whereas most other latitudes the tropopause
is at ~10,000m & temperatures fall to a minimum of minus 40degC)
- adjacent to mountains, the lapse rate is altered by heat
radiated from the mountain, especially in light winds, so that
the lapse rate may reach 6.5degC/1000m or more.
- examples:
- Canberra at elevation of 570m, tends to be 4deg cooler
than Sydney at 40m
- in most regions, air temperature outside aircraft flying
at 10,000-15,000m is usually minus 40 to minus 60deg C
- negative lapse rates (ie. air temperature gets warmer with
elevation) occurs in the following circumstances:
- in conditions where "inversions" occur
- polar winters where air temperatures at ground may be minus
30degC, and rise in the 1st 1000-2000m to minus 10deg, and then for
elevations higher than this have the usual lapse rates as for other
regions
- stratosphere:
- most latitudes at heights > 10,000m (but temperatures do
not get higher than minus 25degC by 30,000m) except for:
- tropical regions where negative lapse rates commence at
elevations > 15,000m
- polar winters
- solar radiation:
- amount of radiation received by the earth from the sun (insolation)
depends on:
- distance apart (closest on Jan 4 (perihelion), most distant
on July 4 (aphelion))
- this would suggest that southern hemisphere summers would be
hotter than northern hemisphere, but in actuality, this tendency
is often masked by the effects of different amounts of ocean in
the 2 hemispheres)
- earth's 23°27' tilt (marked on the globe as the Tropic of
Capricorn and Tropic of Cancer):
- this is responsible for the seasons so that:
- in southern hemisphere, the longest day is on Dec22
(solstice) as the south pole is tilted most towards the sun
at that time, and vice versa on Jun 21/22 (winter solstice)
- the midday sun is directly overhead at the equator on the
equinoxes (Mar 21 and Sep 21) when the earth's axis is
exactly perpendicular to its orbital plane.
- thus this determines the length of day light in a region for a
particular time of the year as well as determining the angle of
the sun in the sky for a particular region, with the amount of
heat reaching the earth is reduced at lower angles due to:
- the area heated by the sun is larger and thus less intense
- the sun's rays must pass through more atmosphere resulting
in attenuation of heat
- sunspot activity:
- sunspots are associated with an increase in emissions of UV
radiation, electrons & protons (solar wind)
- there tends to be a maximum every 10-12yrs (eg. 1947,1957,
1969) with particularly marked maximum every 85yrs
- atmospheric density:
- volcanic activity results in decreases in solar
radiation by up to 10-15% (eg. mid-1880s, 1902, 1912) due to
increased debris
- upper atmosphere blocks wavelengths < 0.29um (ie. 5%
of solar radiation):
- wavelengths < 0.1um are absorbed in thermosphere
- wavelengths < 0.24um are absorbed by dissociation of
oxygen in stratosphere
- UV wavelengths = 0.25um are absorbed by ozone in
stratosphere
- lower atmosphere:
- clear skies:
- block at least a further 20% of solar radiation due
to:
- Rayleigh scattering of light above 2000m
elevation:
- mainly scatters blue light hence blue sky
above & redder near horizons
- turbidity due to aerosols:
- the amount blocked is higher in cities (8%
less radiation passes in Melbourne vs rural)
- carbon dioxide and water mainly block wavelengths
above 0.7um
- 18% of the radiation reaches earth as diffuse
radiation rather than direct sunlight
- 6.5-7.5% of global shortwave radiation reaching
Melbourne is UV light
- cloud:
- overcast days allow only 25% of solar radiation to
reach surface compared with 75% in clear skies
- heat radiation from earth:
- heat is radiated from all objects:
- at a rate proportional to the 4th power of its temperature in
degrees Kelvin
- sun at 6000K radiates 160,000 x the same surface area that
earth (300K) does
- at a dominant wavelength which is inversely proportional to its
temperature in degrees Kelvin:
- sun = 0.5um; earth = 10um;
- see also: heat radiation
- this radiated heat will be maximal in dry, clear skies and minimal on
cloudy skies:
- in many areas, frost is likely to occur overnight if that day's
maximum temperature + the dewpoint temperature is less than 25degC:
- the dewpoint temperature is an indicator of humidity &
hence difficulty in heat escaping
- frost usually occurs if the ground temperature on short turf
falls below minus 1 degC
- on a clear day with minimal winds, there is a diurnal variation in
radiated heat, with peak being in late afternoon coinciding with
maximum temperature and lowest point at dawn, coinciding with minimum
temperature:
- during the morning and early to mid afternoon, heat gained from
insolation is greater than heat lost from earth's radiation,
resulting in gradual increase in surface temperatures, reaching a
peak late afternoon, but then the temperature falls as rate of heat
gain from insolation falls below rate of heat lost from earth's
radiation
- green-house effect:
- the greater the density of the atmosphere, the more the lower
energy heat frequencies are trapped and thus heat the earth
- water vapour contributes 95% to the natural greenhouse effect
while carbon dioxide contributes 3.6% and human activity contributes
0.12%.
- without the natural greenhouse effect, the average earth
temperature would be minus 18 deg C instead of plus 15 deg C as it
is now.
- even though carbon dioxide levels have been shown to be rising in
the famous 1960-91 graph, this was preceded by a rise in global
temperature as had occurred on previous occasions over the past
100,000 yrs.
- urban heating:
- urban areas tend to be warmer than rural areas due to:
- man-made heat from industry, transport, living animals
- eg. Sydney - is about 25% of insolation in summer and almost
50% of insolation in winter!
- high thermal mass construction materials absorb insolation and
radiate it at night
- drainage of water from city prevents evaporative cooling
- reduced albedo (reflectiveness) - eg. bitumen instead of plants
- smog may reduce radiated heat more than insolation
- central urban areas are often 3 degrees hotter than outskirts,
especially at dawn
- this effect is removed by winds exceeding:
- 8knots for 33,000 population and 23 knots for 8 million population
- wind:
- winds move air from one region (which may be a different temperature)
to another:
- sea breezes moderate coastal temperature extremes
- the pressure systems dictate wind strength, direction & where
they come from:
- winds from polar regions or from snow-capped mountain ranges
or frosty inland regions tend to be very cold
- winds from hot arid regions tend to be dry & hot
- air masses may be warm or cold following a front depending on the
type of front, which may suddenly change the air temperature as the
front passes over
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