There are two major
aspects of global climate, both related to latitude: variation in the distribution of solar radiation with respect to time (seasonal) and space. There are two major
aspects of global climate, both related to latitude: variation in the distribution of solar radiation with respect to time (seasonal) and space.
First: Variation in TIME – the seasons. Since the earth's axis of spin always points in +/- the same direction relative to the cosmos at large (the N. pole always points towards the Pole Star – which is why that star never seems to move in the sky, either through the night or across the seasons) and the same spin axis is not perpendicular to the plane of earth's orbit around the sun (it's ~23.5° from the vertical), then the effective latitude of any given point on earth cyclically increases and decreases as the earth moves around the sun. This means that daylength and radiation levels increase and decrease throughout the year, these effects being most marked at higher latitudes (see the diagram a little lower, to the right) - that is, we experience seasons. Longer days and stronger radiation we call summer, and so on.
The four main landmarks on earth's orbit (shown in the figure above right) are:
two solstices (northern summer, ~June 21, to the left; northern winter, ~December 21, to the right), when the axis tilt is normal with respect to the orbit – one pole is tilted either 23.5° towards or 23.5° away from the sun – and daylength is a maximum or minimum, depending on which hemisphere you're in;
two equinoxes (~March 20; ~September 22), when the axis tilt is tangential to the orbit – tilted neither towards nor away from the sun – and daylength is exactly 12h. everywhere, from pole to pole, because the sun rises due east and sets due west everywhere on those two days. The two solstices, mid-winter and mid-summer, are separated by two equinoxes, either the spring (vernal) or fall (autumnal) equinox.
As well as thus dictating seasonality, the axis tilt of ~23.5° from the vertical also defines:
latitudes above which the sun is permanently above the horizon at midsummer solstice and permanently below it at midwinter solstice: these are the Arctic and Antarctic Circles – at (90°- 23.5°) = 66.5°. The Arctic Circle is visible in the diagram above;
latitudes below which the sun is directly overhead on at least one day a year: the Tropics of Cancer and Capricorn – at 23.5° either side of the equator. Within the tropic latitudes, the sun is directly overhead twice a year – exactly every six months on the equator. This means that, in the tropic latitudes, the mid-day sun is part of the year in the southern sky and the rest of the year in the northern sky.
Make sure you can imagine all this. Excellent images and accompanying explanatory text on this theme may be found here. A terrific animation showing how earth's spinning and orbiting motions combine to give the seasonal changes in daylength, together with the connection between the earth's axis-tilt and the location of the tropics and the Arctic & Antarctic Circles is available here. Click the "Show Earth Profile" button at bottom left and then use the slider to show how things change as you move through the year. Take care as you approch the equinoxes and solstices! You can also hit the "Play" button, but it moves pretty quickly! |
Second: Variation in SPACE – latitudinal patterns.
Major global patterns
in climate are driven by latitudinal variation in the amount of solar
energy that impinges on the planet's surface and the
effects of this variation in solar heating on the behaviour of the
atmosphere. This variation in heating has three primary causes:
1. Lower latitudes receive more radiation per unit area (see diagram at left), because any given-sized "packet" of radiation hits earth at angles close to 90° (i.e. it arrives ~perpendicular to the earth's surface), but that same energy impinges much more obliquely at higher latitudes - it gets spread over a larger area.
2. At low latitudes the arriving energy passes directly through a minimum depth of absorbing
and scattering atmosphere, while at
higher latitudes, the energy's oblique path through the atmosphere is
longer, meaning that more energy is scattered away into the cosmos.
3. At lower latitudes, incoming radiation suffers minimal reflection from the atmosphere and from the earth's surface, whereas the low angle of incidence at higher latitudes means that more energy is reflected away.
The sum of these sources of variation in irradiation
according to latitude (see diagram to the right) is a corresponding variation
in the heating of the earth's surface, as short-wave solar radiation is
absorbed by the earth. This energy heats the earth, and then gets
re-radiated as long-wave radiation, and this form of the energy heats the atmosphere.
The relatively great heating of the earth's surface around the equatorial
belt sets up a massive upward convection current in the atmosphere in the
equatorial region (see diagram immediately below, left), and this broad belt of
strongly rising air, which girdles the equator, takes lots of water vapour
up to high altitudes (about 10,000 m), where it cools, and the vapour condenses,
falling back to earth as rain. Thus, much of the equatorial zone
is both hot and humid, and typically with low atmospheric pressure.
The mass of air, now much colder and drier, cannot fall straight back down to earth, but must move out of the way to make way for the next day's strong updraft. So it must move away from the equator, either north or south (the diagram at left only shows the northern hemisphere). Meanwhile, at ground level, the upward convection current, centred on the equator, produces a lowered air-pressure (at L) which draws surface air towards the equator from higher latitudes, either north or south. At about 20-30° north and south of the equator, this equator-wards movement of surface air begins to pull down the air mass from higher altitudes, which, if you recall, has just traveled up from the equator...... and this air mass, now cooled and dried out, descends towards the earth, making a high-pressure zone (H, above). As it descends, it warms, both through compression (ever wondered why your bicycle pump gets so hot? - it isn't just friction), and from radiant heat from the earth's surface. This descending air mass was dry to start with (most of its moisture was lost as tropical rain), but as it heats up on the way down, it develops a serious water debt, and hits the ground very dry.
This is because of the Coriolis Effect,
which is an apparent force due to the earth's rotation. The earth
spins from west to east (anticlockwise as viewed from the north pole),
and this has the effect of making anything that moves (such as an air
mass)
veer, with respect to the earth's surface, towards the right in the northern hemisphere and towards the left
in the s. hemisphere. Thus the subtropical winds (the Trade Winds)
are from the northeast and southeast, while the prevailing winds in temperate
latitudes are primarily westerlies, bringing oceanic air onto continents
from their western margins. The dominance of these westerly winds
is what lies behind the fact that, in temperate latitudes, westerly continental
locations are milder and moister than easterly ones at the same latitude,
since they get the moisture of the mild oceanic air masses. Coastal
B.C. is much milder and moister than Newfoundland. Korea is much
colder than Spain. London, England is much milder than Kamchatka.
At the temperate latitude (40-50°) low pressure zones, there is a continuous
"conflict" between the air masses converging on them from the south and
the north, and this generates the great climatic instabilities that are
typical of the region. This instability contrasts with the high pressure
zones at ~30° and ~60°. The main atmospheric circulation
picture is completed by a third cell, rotating equatorwards at ground level,
bringing very cold dry air down to earth in the polar regions. That's
why polar regions are not only very cold but also very dry. 
Local climate is affected by anything else that moves the air mass up and down or affects its water-load, such as mountain ranges or the proximity of oceans and large lakes. As a relatively moist, moving, air mass hits high mountains, it is forced to rise, gets cooler, and the moisture condenses, dropping rain on the lower and mid slopes of the windward side of the mountain-range. As the air descends on the other side, it gets warmer, and its moisture debt increases. Thus, in the lee of mountains, it is often especially dry e.g. the Mojave desert, in the lee of the very high Sierra Nevada in s. California (see below).
The deep interiors of large continents (e.g. east-central Asia, Mongolia
- see world map above), are so far downwind from sources of moisture that
most of that moisture has already been lost from the air-mass - thus the
air is dried, and there can be little rain, and often desertic conditions
prevail outside of the main subtropical desert belt, in this case giving
the Gobi Desert of northern China and adjacent parts of Mongolia, and the
other central Asian deserts around the Caspian and Aral Seas. 
In North America , both the high mountains in the far west and the huge size of the continental mass itself have dramatic effects on the simple latitudinal climatic systems discussed earlier, such that, when we focus on the continent alone, and ignore the planet as a whole, the major latitudinal belts are only somewhat in evidence. Rather, the major precipitation regimes seem largely to be disposed longitudinally (see map of North American precipitation regimes left; or a more vivid one of the USA here). You will appreciate this steadily drying climate as we drive west from London towards the Rockies.
A primary message in this map is that local conditions can impose important distortions on the overall global latitudinal pattern; the other side of this coin is that, if we simply look at such local patterns, we can gain a completely erroneous impression of the nature of the global picture, and the factors which govern it. Note in the map particularly the high rainfall generated along the mountainous west coast, the arid lands beyond those mountains, the semi-arid and subhumid prairies in the continent's interior, and the generally humid temperate eastern regions. On other continents, different individual local patterns are generated by the major features of their local topographies.
Global heating and earth's rotation set up comparable global movements in the enormous body of the earth's oceans. Again, the moving masses are influenced by the Coriolis Effect, to give major surface currents (largely driven by surface winds) that are usually clockwise in the northern hemisphere (e.g. the Gulf Stream-Canaries current system in the N. Atlantic, or the Kuroshio-California system in the N. Pacific) and anticlockwise in the s. hemisphere e.g. the Benguela-Brazil current in the S. Atlantic).
Until recently, we only knew about these surface ocean currents, but now we recognize that there is a very important 3-D oceanic system called the Thermohaline Circulation (driven primarily by density differences through variation in temperature and salinity), which provides for massive heat redistribution about the globe. This complex oceanic circulation, taking place at all depths, with upwellings and downwellings, profoundly affects the temperature of the ocean surface, in turn affecting conditions on nearby land-masses e.g. subtropical coastal upwellings of very cold water off Baja California, Peru and Namibia generate peculiarly high aridity onshore, making some of the world's most arid deserts, such as the Vizcaino of Baja California, the Atacama of Peru and the Namib of s.western Africa.
A convenient way to summarize much information about the climate of
a locality is a Climate Diagram. These diagrams were invented by
Walter & Lieth , and have been widely adopted by most ecologists, geographers
and others as an efficient tool. With a bit of practice you can make
a pretty good guess at the vegetation of a locality just from inspection
of the climate diagram. A key to the interpretation of climate diagrams
is to be found below, to be used in conjunction with the sample diagrams
opposite, and those shown on the rainfall map of the Sonoran desert.
Ankara and Odessa are grasslands (steppe); Douala is tropical rainforest; Hohenheim is deciduous forest.
X-axis: months (N. Hemisphere Jan.-Dec.; S. Hemisphere July-June).
Y-axis: one div. = 10°C or 20mm rain. a = locality; b = altitude above sea-level; c = length of record (if two, first = temp.; second = rainfall); d = mean annual temp. °C; e = mean annual ppn. in mm; f = mean daily min. temp. of coldest month; g = lowest temp. recorded; h = mean daily max. of warmest month; i = highest temp. recorded; j = mean daily temp. variation; k = curve of mean monthly temp.; l = curve of mean monthly ppn.; m = period of relative drought; n = period of water excess; o = mean monthly ppn. >100mm. on 1/10 scale; q = months with daily min. temp. <0°C; r = late or early frosts; s = mean duration of frost-free period in days.
Not all the information is shown in all diagrams.
Whittaker (who popularized the Five Kingdom classification system) noted that the major vegetation types of the world are substantially predicted by two primary climatic variables: mean annual temperature and total annual precipitation. A simplified version of a diagram with these two axes and the various major biomes mapped onto it is shown below (all these biomes are represented in North America and Mexico):
The number of biomes differs according to the authority, but the seven shown in the diagram are commonly recognised. Often, the tropical forest group is split into a) the superhumid "rainforest" biome and b) the seasonal (drought-deciduous) forest biome, while the relatively warm and highly humid temperate conifer (and sometimes deciduous) forests are recognised as Temperate Rainforest, such as those of the northwestern American coast (conifer), the southeastern coast of Australia and the southwestern coasts of New Zealand and Chile (mixed conifer and broadleaf). Finally, a maritime, summer-drought/winter-rain assemblage called chaparral is often acknowledged, which is typified by woody shrubs with leathery evergreen foliage. This is found in coastal central Chile, southern California, southwestern Australia, the Cape region of S. Africa, and around the margins of the Mediterranean Sea.
When we plot the geographic occurence of biomes (variously defined) about the globe we see patterns that strongly recall that of climate. This is no surprise, given the close integration between climate and vegetation. Similarly, soils and primary production shows very similar distributions, as we have seen. So now we have described, from several directions, the phenomena lying behind the fact that, for example, the low-latitude (equatorial) zone is humid, very highly productive and occupied by a very large number of species of plants, largely trees. A closer look at those trees will show that, when adult, many of them have the same overall morphology, through evolutionary adaptive convergence: they are very tall, have a smooth, almost limbless trunk, and a dense canopy at the top; they usually have large, simply-shaped, somewhat leathery leaves, which persist for several years. In similar fashion, boreal forests are characterised by trees which are conical in shape, often with branches to the ground, and evergreen, drought-adapted, needle-leaves. Again, arctic regions are extremely cold and dry and occupied by low-statured, often evergreen plants. Finally, deserts show vegetation which is often spiny, succulent, and with great capacities for sustaining high levels of temperature and drought.
Though most of us start out with the image of deserts as endless, pitilessly-hot, wastes of sand and rock, and though there are such deserts, this picture is nothing like true of many. Much desert land is covered by low-diversity scrubby vegetation (albeit sparse) of short bushes and small trees, but some deserts have a quite luxuriant and very diverse vegetation, including good-sized trees e.g. the Sonoran desert of s.w. U.S. and Mexico.
Many trees are capable of accessing deep ground-water, and are thereby independent of local rainfall. Such plants are called phreatophytes: a mesquite tree (Prosopis) will have as much or more woody growth below ground as above. Examples are: Cercidium, Acacia, Prosopis, Mimosa, Olneya.
Some plants can absorb dew, and a diversity of plants and lichens that do this are epiphytic, sitting in the branches of trees and shrubs e.g. Tillandsia.
Upper slopes occupied by a diverse vegetation, including trees, bushes and larger cacti (when present); middle slopes dominated by bushes (including Ambrosia, Krameria), including bush-type cacti; lower slopes and flats typically occupied by smaller bushes (including Ambrosia, Larrea); water courses support larger bushes and trees, even out on the flats. Saline flats and basins are occupied by halophytes, which are capable of tolerating higher concentrations of salt in the tissues (e.g. Atriplex); some are capable of excreting salt through special glands (e.g. Tamarix).
