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
half of the mass lies in 1st 5.6km
1st 80km - the homosphere:
1st 15km:
15-60km:
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
-
the top of this layer is called the mesopause
80-32000km - the heterosphere:
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:
apparent temperature (AT): takes into account humidity & wind speed
-
atmospheric temperature vs elevation:
solar radiation:
heat radiation from earth:
heat is radiated from all objects:
this radiated heat will be maximal in dry, clear skies and minimal on cloudy skies:
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
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:
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:
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
temperature determinants
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%:
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
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)
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