Climates on a Rotating Earth

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• Climates on a Rotating Earth
• We can divide the study of climate into a number
  of sub-areas.
• The global pattern of penetration and absorption
  of solar energy. That energy is the driving force
  for most of what follows.
• The Input Solar Radiant Fluxes over the Globe
• The solar energy flux which reaches the outer
  limit of the atmosphere is 2 calories/cm2/minute.
  That's called the solar constant. Due to the
  elliptical orbit of the earth, the solar
  constant varies by about 15 from season to
  season.

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Only about 1/2 the energy striking the outer
surface of the earth's atmosphere actually
reaches ground level the remainder is reflected,
re-radiated, or absorbed within the atmosphere.
Heres what happens to the other half
21 is reflected by clouds back into
space 5 is reflected by dusts and
aerosols 6 is reflected by the earth's
surface 3 is absorbed by clouds 15 is
absorbed by dust, water vapour and CO2
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Why are the tropics, i.e. the low latitudes,
warmer than the poles? It isnt duration of
sunlight. The total number of daylight hours in a
year is constant for all points on earth over
the year every site averages 12 hours of daylight
and 12 hours of night per day. It is input
intensity. The input of solar energy, measured in
calories, is not evenly distributed over
latitude the rate of input, and total input, are
both higher in the tropics. It is also
atmospheric thickness. At high latitudes, where
the surface is 'tilted away' from the sun, the
effective thickness of the atmosphere is greater.

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The intensity of sunlight, measured as
calories/unit area, differs with latitude because
a sunbeam covering an area of 1 cm2 at the upper
surface of the atmosphere is spread over a
differing surface area on earth at differing
latitudes. Its simple trig At the equator, the
1 cm2 sunbeam is absorbed by an area which also
measures 1 cm2. Anywhere else the surface of the
earth is at a tilt with respect to the beam. Its
angle of incidence is (90 - L). Representation
really requires spherical geometry. Simplifying,
at 45, energy input is spread over an area 1 cm
wide, but 1.414 cm long. For every calorie/cm2 at
the equator, only 0.707 calories/cm2 are
available at 45o latitude.
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Because the earth's axis is at a 23o tilt with
respect to the plane of the earth's orbit around
the sun, the solar equator moves seasonally.
Solar intensity varies with solar
latitude. North of the Arctic Circle and south
of the Antarctic Circle (each at approximately
67o latitude) there are 'days' and 'nights' 24
hours long. The circles mark the map latitude
where the 'solar latitude' reaches 90o on at
least 1 day of the year. A fair part of seasonal
temperature variation is explained by considering
seasonal patterns in solar latitudes.
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• Consider Windsor ( 42o N latitude)
• On June 21 the solar equator is at 23oN. Our
• effective solar latitude is not what the map
  says
• (42o), but 42 - 23, or 20oN.
• Our effective solar latitude on December 21 is
  42
• 23, or about 65oN, and our days are shorter.
  Both
• the energy input per unit area per unit time
  and the
• duration during which we receive that input
  are
• reduced.

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Atmospheric heating results from 2 energy inputs
1. absorption of incoming radiation, which
accounts for 18 of incoming energy, and is
unlikely to change much over geological time
scales on earth However, changing the albedo
(reflectance) of the earth's surface (e.g.
asphalt parking lots) increases both
absorption and re-radiation as infra-red. 2.
re-radiation of infra-red energy from the
earth's surface, and its absorption by CO2 in
the atmosphere. Absorption by increased
concentration of CO2 is the enhanced greenhouse
effect', and adds significantly to the heat
load of the atmosphere.
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Global Patterns of Air Circulation Begin at the
equator, forgetting that the earths surface is
covered by irregular land masses as well as
water, and that the earth rotates on its
axis. Begin by considering the pattern at an
equinox, when the solar equator and the map
equator coincide. Consider the flow as if it were
two-dimensional, rising and falling on a plane in
the atmosphere. At the equator the intensity of
solar energy input is at its maximum, and the
atmosphere is warmed most. Hot air rises. As the
hot air rises it expands in the more rarefied
atmosphere of higher altitude. To expand, the air
does work, spends energy.
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That energy has to come from the parcel's own
energy supply. Spending it means the parcel
cools. Cooling occurs at a characteristic rate,
called the adiabatic lapse rate. Rising creates
a low pressure area at the equator it's
occurring continuously, thus forcing a flow in
the upper atmosphere away from the
equator. Cooling caused by rising in the
atmosphere and additional cooling caused by
displacement from the equator causes a gradual
increase in the density of the air mass we're
following. If hot air rises, then cold air sinks.

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By the time the air mass has reached about 30o
solar latitude (N or S), its density is higher
than that of the atmosphere beneath, and the air
mass sinks back to the surface. That produces a
band of consistently high pressure at what are
termed the 'horse latitudes'. The reverse of what
happened when the air rose happens when it falls.
The parcel of air is compressed by parcels
surrounding it, which are at higher pressure at
lower elevation work is done upon the falling
air that energy input warms the falling air at
the adiabatic lapse rate.
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As the descending parcels approach the earths
surface, a portion of the descending air is
deflected toward the equator (by earths
rotation), and completes a circulation cell. That
air produces what we call 'trade-winds. The
remainder of the descending air mass is deflected
poleward. In the general neighbourhood of 45-50o
latitude, the warmer air from equatorial
circulation meets air masses from a cold, polar
circulation cell. The polar cell results from the
descent of very cold, dense air masses near the
poles, and their spread to lower latitudes. The
poles are thus another zone of fairly steady high
pressure.
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Where polar and deflected equatorial air meet,
there is a zone of unstable pressure. Unstable
pressure leads this region to be characterized by
storms. We live about there.
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Global Rainfall Patterns As hot air rises, it
cools at the adiabatic lapse rate. We are
assuming no further input of energy (i.e.
absorption of solar energy) as the air mass
rises. The adiabatic lapse rate for dry air is
10oC/km. We must specify dry air because water
vapour has a higher thermal capacity than the
gases of dry air. As a rising air mass cools, it
may become saturated. If it cools further, water
vapour will condense on particulate matter in the
atmosphere. Should condensation occur, the heat
of vaporization of the condensing water vapour
(approximately 585 cal/gm) is released into the
air mass, slowing the rate of cooling to 6oC/km.
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As our air mass rises at the equator, it reaches
100 relative humidity, and further rise and
cooling causes water to condense out on
particulates in the air (dust), forming clouds.
As the water droplets get bigger, they fall as
rain. We can now explain the large scale global
latitudinal 'bands' of high rainfall and deserts.
In the tropics, at very low solar latitudes, say
5o on either side of the solar equator, there is
a low pressure zone where solar heating causes a
rising flow of air. As this air rises and cools
adiabatically, water vapour condenses. The result
is almost daily rainfall, usually in the evening.
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The next latitudinal zone where weather pattern
is determined by the global pattern of air
circulation is the zones surrounding 30oN and S
latitude. There cold air masses descend toward
the surface, warming adiabatically as they
descend. As air warms, its relative humidity
decreases. The air mass will only rarely reach
100 relative humidity, only rarely will
condensation produce clouds, and rain is
unlikely. Instead, the warm air at the land
surface will 'absorb' evaporation from the warm
surface into the unsaturated atmosphere. The
result is the world's great deserts.
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In the southern hemisphere these are the Atacama
desert in Chile, the Kalihari desert of southern
Africa, and the Central Desert of Australia. In
the northern hemisphere they are the Gobi desert
of Manchuria, the Sonoran desert of the
southwestern U.S. and Mexico, and the Sahara of
Africa.
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Gobi
Sahara
Sonoran
Kalahari
Atacama
Central desert
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At both the northern and southern latitudinal
boundaries of the desert zones are fairly narrow
zones in which precipitation shows consistent
patterns of seasonal variation. On the
equatorial side is a zone which receives most of
its precipitation in the summer and little
precipitation during its winter. At the high
latitude margins of deserts, the pattern is
exactly the opposite. Rain (or precipitation,
whatever its form) falls principally during the
winter season during the summer their solar
latitudes produce a moderate, desert-like
climate.
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Where warm equatorial and cold polar air masses
meet, the meeting of the air masses causes a
general rising flow. Adiabatic cooling during the
rise leads to rainfall, but this is a diffuse
belt, and rainfall is not predictable at any
specific location or time. Finally, at extreme
latitudes we find Arctic and Antarctic polar
deserts. These areas receive extremely low
amounts of precipitation annually they are zones
of stable high pressure where cold air descends
back toward the surface, and where rainfall (or
snowfall) is therefore unlikely. These areas
cover latitudinal zones from around the polar
circles (65o or so) to the poles.
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Surface Topography and Precipitation Patterns As
surface winds pass over terrestrial topography,
the air masses comprising them necessarily must
rise and fall. Those upward and downward
movements subject air masses to the same
adiabatic changes in temperature, and therefore
are also of great importance in determining
precipitation patterns. On the leeward side of
every mountain chain there is a 'rain shadow', a
region of low rainfall and on the windward side,
particularly along mountain slopes, there is
typically a fairly 'wet' climate
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Follow an air mass which begins at the western
edge of the Rockies at a comfortable 20oC, and a
moderately high relative humidity. As the air
rises up the western slope, it cools
adiabatically, initially at the 'dry' adiabatic
lapse rate of 10oC per km. Assume that the air
reaches saturation (100 relative humidity) at
10oC. When the air has risen halfway (1 km) up
the mountain, it is saturated. As it continues to
rise and cool from 1 to 2 km elevation, clouds
will form and rain will fall. Condensation
releases the heat of vaporization, so that
cooling occurs at the lower, saturated adiabatic
rate of 6oC/km.
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Air which had cooled to 10o at 1 km cools to 4o
at the peak of the mountain. As the air descends
on the leeward side, it warms adiabatically. As
it warms, the relative humidity drops. Since the
air is now unsaturated, the rate of warming is
the 10oC unsaturated rate. When this air has
descended to 1 km, its temperature is 14oC, and
at the eastern base of the mountains it is 24oC.
The leeward side is warmer, and since the air is
unsaturated, rainfall is an uncommon
occurrence. The rain shadow phenomenon and
adiabatic temperature changes which affect
likelihood of rainfall are important in many
areas.
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• Regions in the middle of continents typically
  undergo seasonal extremes in climate hot
  summers and cold winters. There are 2 parts to
  explanation of extreme seasonal fluctuations at
  mid-continent
• Water has a high thermal capacity. The presence
  of
• large bodies of water nearby (e.g. oceans,
  the Great Lakes) tends to moderate temperature
  fluctuations.
• 2.Temperatures can fluctuate more rapidly and to
  wider extremes when air is 'dry' (i.e.
  unsaturated) than when air is at or very near
  saturation.
• Climatologists use a measure called
  'continentality
• to indicate the combination of variation in
• temperature and humidity suggested in explanation.

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Climate Resulting From the Earth's Rotation The
last factor to add is the daily rotation of the
earth on its axis, and effects on air and water
circulation. Rotation produces what is called the
Coriolis effect. The Coriolis force represents
conservation of momentum for objects moving over
the surface of a rotating earth. Air masses
moving latitudinally deflect from the simple N-S
patterns indicated previously in global air
circulation.
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Think of yourself as standing at the equator. At
the equator the earth is approximately 40,000 km
in circumference. Standing still for 24 hours at
the equator, the rotation of the earth will have
caused you to move 40,000 km. The air which
surrounds you moves at the same 40,000 km per
day, assuming you feel no wind. Now consider air
descending at 30o latitude. The earths
circumference is about 34,600 km at 30o.
Equatorial air descending at 30o is moving faster
than that, even including friction, which would
decrease its actual velocity from 40,000 km/day.
The descending air spreads northward and
southward.
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Rotation of the earth is from west to east
(that's why the sun rises in the east and sets in
the west, in case you've lost track). Therefore,
that's the direction of deflection of winds in
the air mass moving away from the equator
(towards higher latitude) at 30o - from west to
east, or westerly winds. Frictional drag as air
spreads from the solar equator is important.
Otherwise the difference in velocity (167.7
km/hr) would produce continuous hurricane force
winds.
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A westerly deflection (west?east) occurs in
descending air moving toward more extreme
latitudes. In the southern hemisphere, this
deflection produces the 'roaring 40's'. The
same forces produce cyclonic storms (hurricanes,
tornados, monsoon winds).
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What about the air that deflects back toward the
solar equator? Frictional drag near the surface
(in the 1 km nearest the surface) has slowed this
air mass to within a few km/hour of the
rotational velocity at 30o, i.e. wind speed is
only a few km/hr. As this air moves toward the
equator, it now has a horizontal velocity lower
than the rotational velocity of the earth's
surface over which it is passing. Frictional drag
tends to accelerate this air, but it is
nevertheless deflected from east to west. These
northeasterly winds (north ? south as a result of
Hadley cell circulation, east ? west from
Coriolis deflection) are called the trade winds
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Near the equator frictional drag has caused
surface winds to catch up with surface velocity
there is little N-S velocity air movements are
dominated by the Hadley cell vertical
circulation. As a result surface winds are
usually weak near the equator, and result in a
zone known as the doldrums. Ocean currents are
also directed by Coriolis forces. Generally the
direction of ocean currents is determined by the
effects of surface winds moving surface waters,
effects of land masses (blocking/re-directing),
and Coriolis forces deflecting water movement.
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Where Coriolis forces draw surface waters away
from continental margins, that water must be
replaced. The replacement water is cold,
nutrient-rich upwelling. These waters are the
world's great fishing grounds. Off Newfoundland
the Gulf Stream is deflected towards Europe by
its northward movement. The upwelling produces
the Grand Banks. Off California and northern
Mexico the Japan Current is deflected westward
across the Pacific by southward movement. Off
Peru, where a southern Pacific cell has a return
flow deflection, is the third major Western
hemisphere fishing grounds.
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These upwellings and coastal flows also have a
major impact on the rainfall patterns over nearby
continental areas. Patterns of rainfall along the
west coast of North America in winter and summer
result from westerly winds and the relative
temperature of water and land. Water has a high
thermal capacity, and water temperature results
from the Coriolis force driven Japan current.
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The same relative temperature explains
lake-effect snowbelts in the Great Lakes region
and many other local climate features around the
world.
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References and Readings Hughes, L. 2000.
Biological consequences of global warming is the
signal already apparent? TREE 1556-61. MacArthu
r, R.H. 1972. Geographical Ecology patterns in
the Distribution of Species. Harper Row, New
York. Chapter 1 - Climates on a Rotating Earth.
Post, W.M., T.-H. Peng, W.R. Emanuel, A.W.
King, V.H. Dale and D.L. DeAngeles.1990. The
global carbon cycle. American Scientist
78310-26. Smith, R. 1990. Ecology and Field
Biology 4th ed. Harper Row, NY. Ch. 4.
Climate. Thomas, C.D. et al. 2004. Extinction
risk from climate change. Nature 427 145-148.
good basic chapters on climate - the forces
and results of solar input, atmospheric
circulation, and topography.