Transcript Climates on a Rotating Earth

Climates on a Rotating Earth
We can divide the study of climate into a number of
sub-areas.
1.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.
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. Here’s 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
Why are the tropics, i.e. the low latitudes, warmer
than the poles?
It isn’t 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.
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. It’s 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.
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.
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.
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.
Global Patterns of Air Circulation
Begin at the equator, forgetting that the earth’s
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.
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.
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.
As the descending parcels approach the earth’s
surface, a portion of the descending air is deflected
toward the equator (by earth’s 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.
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.
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.
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.
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.
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.
Sonoran
Atacama
Sahara
Kalahari
Gobi
Central desert
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.
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.
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
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.
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.
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:
1.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.
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.
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
earth’s 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.
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.
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).
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
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/redirecting), and Coriolis forces deflecting water
movement.
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.
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.
The same “relative temperature” explains ‘lakeeffect’ snowbelts in the Great Lakes region and many
other local climate features around the world.
References and Readings:
Hughes, L. 2000. Biological consequences of global warming: is the signal already
apparent? TREE 15:56-61.
*MacArthur, 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 78:310-26.
*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: 145148.
* = good basic chapters on climate - the forces and results of solar input, atmospheric
circulation, and topography.