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climate:climate1

climate - atmosphere and temperature

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.)

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
      • dewpoint = dry bulb temp - (100-RH)/5) and thus RH = 100 - 5(dry bulb temp - dewpoint)
      • 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:
    • 15kph for 33,000 population and 43kph 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
    • Fohn winds are warmer and drier after passing over mountains

mathematical models of temperature

mean monthly average temperatures for a region

  • many regions can be modeled with the following equation:
    • mean temp for month = (annual mean temperature range x cos (pi(month of year - offset factor)/6)) + minimum monthly mean temp

temperature determinants

  • thermal advection (horizontal movement of air)
    • the temperature of this air mass will be moderated by:
      • soil temperature
      • snow cover
      • cloud cover
    • wind speed / direction
    • pressure pattern
  • local heating / cooling:
    • insolation
      • day length
      • cloud
      • latitude
      • snow cover
      • suspended particles
    • radiation
      • cloud cover
    • latent heat from dew formation
    • dewpoint (sets a quasi-limit for overnight cooling in a barotropic environment)
      • higher dewpoints cause lower maximums and higher minimums (eg. Wilsons Prom esp. if also windy and cloudy)
  • adiabatic
    • ascent with wind going up sloping terrain
    • descent with wind coming down sloping terrain
    • downdraft from a storm bringing air from high in the atmosphere to the surface
  • wind speed
    • determines how much local temperature will build up during the day and how much the surface air cools at night - in general, the greater the speed the lower the maximum temps and the higher the minimums (eg. Wilsons Prom)
    • in light winds:
      • during the day heat can build right at the surface without being significantly mixed with cooler air aloft. This can form what is known as the superadiabatic lapse rate.
      • at night it does not allow radiationally cooled air at surface to mix with warmer air aloft resulting in colder air at the surface than if winds had mixed it
  • mesoscale effects
    • topography
    • urban heat island effects
    • water bodies
  • effect of rain
    • rain will cause evaporational cooling at the surface
      • The low level dewpoint depression determines how much surface temperatures will cool.
      • A high dewpoint depression will result in a greater evaporational cooling.

air temperature when air rises or falls (adiabatic lapse rates)

  • The dew point is the temperature to which air must be cooled to become saturated with water vapor. When cooled further, the airborne water vapor will condense to form liquid water (dew). When the temperature is below the freezing point of water, the dew point is called the frost point, as frost is formed via deposition rather than condensation to form dew. In the air, the condensed water is called either fog or a cloud, depending on its altitude when it forms.
  • At dew point, the rate of condensation of water exactly equals the rate of evaporation of water
  • relative humidity of 100% indicates the dew point is equal to the current temperature and that the air is maximally saturated with water.
  • When the moisture content remains constant and temperature increases, relative humidity decreases, but the dew point remains constant.
  • Increasing the barometric pressure increases the dew point.
  • At sea level pressures, max. water content of air varies with temperature: ~3% at 30degc; ~2% at 22degC; ~1% at 10degC;
  • approximate relationship for relative humidity (RH) > 50%:
    • dewpoint = dry bulb temp - (100-RH)/5) and thus RH = 100 - 5(dry bulb temp - dewpoint)
  • a comfortable dewpoint is 10-16degC higher is getting a bit too humid and lower, the skin starts to dry out

descending air

  • when air falls the temperature rises at 10degC per 1km fall in elevation while the dewpoint rises at 2degC per 1km irrespective of the degree of water saturation as the air will become less saturated as it falls and not hit 100%
  • Once air begins the sink the relative humidity will decrease below 100% since the temperature increases at a rate more than the dewpoint increases when air sinks.

ascending air

  • while unsaturated temperature will fall at 10degC per 1km rise and dewpoint will fall by 2degC per 1km rise
    • calculating cumuliform convective cloud base altitude formed when air is lifted mechanically from the ground eg. convergence of airmasses
      • approx altitude in meters from the ground = (ground dry bulb temp - ground dewpoint in degC) x 120
  • if saturated the adiabatic lapse rate for both temperature and dewpoint will be the same and will vary depending upon degree of saturation:
    • rate is closer to 4 C per kilometer if the dewpoint of the air is very high and the rate is closer to 10 C per kilometer if the dewpoint is very low.
    • When the dewpoint is very low, the air is almost dry even if it is saturated.
    • When air has a high dewpoint and the air is saturated there will be an abundance of condensation when the air rises. Since condensation warms the air it partially cancels out the 10 C per kilometer cooling that unsaturated air has.

air temperature movement on the horizontal plane (thermal advection)

  • low level (LL) advection is from ground to 550mbars
  • upper level (UL) advection is at elevations between 550mbars and ~150mbars (tropopause)
    • The upper levels are much more uniform in temperature and the flow is more zonal and geostrophic thus thermal advection is usually weak.
  • warm air advection (WAA) is the movement of warmer air toward a fixed point on a horizontal plane
    • LL WAA is common behind warm fronts and ahead of cold fronts.
    • LL WAA contributes to rising air because warm air is less dense than cold air.
    • warmer air expands to a larger volume than cold air, this expansion in the low levels pushes the air above the low levels up also but this upward motion is slow and on the synoptic scale and much less than the updraft of a thunderstorm, but at a rate of synoptic upward vertical motion which varies generally between 1 and 30 centimeters per second.
  • cold air advection (CAA) is the movement of colder air toward a fixed point on a horizontal plane
    • NB. evaporative cooling and air cooling at a fixed point due to radiational cooling are not CAA
    • LL CAA is common behind cold fronts.
    • LL CAA contributes to sinking air as cold air is denser than warm air. This contraction in the low levels forces a sinking motion aloft to compensate for the reduction of volume in the LL and thus skies will be generally clear behind cold fronts
  • NB. Evaporative cooling, condensation warming, solar heating, complex topography, and radiational cooling contaminate thermal advection
climate/climate1.txt · Last modified: 2021/07/07 23:51 by gary1