Patterns of climate
Many of the most important physical factors of the land environment
have very distinct patterns of variation in different parts of the
world. The climate of an area is the whole range of weather
conditions, temperature, rainfall, evaporation, water, sunlight,
and wind that it experiences through all the seaons of the year.
Many factors are involved in the determination of the climate of an
area, particularly latitude, altitude, and position in relation to
seas and land-masses. The climate in turn largely determines the
species of plants and animals that can live in an area.
Climate varies with latitude for two reasons. The
first reason is that the spherical form of the earth results in an
uneven distribution of solar energy with respect to latitude, as
shown in Figure na. As the angle of incidence of the sun's rays
approaches 90°, the area over which the energy is spread is
reduced, so that there is an increased heating effect. In the high
latitudes, energy is spread over a wide area; thus polar climates
are cold. The precise latitude that receives sunlight at 90°
at noon varies during the year; it is at the Equator during March
and September, at the Tropic of Cancer (23-45°N during June,
and at the Tropic of Capricorn (23'45°S) during December. The
effect of this seasonal fluctuation is more profound in some
regions than in others.
The second reason is that variations also result
from the pattern of movement of air masses. In an idealized picture
that assumes a uniform surface to the Earth. Under these conditions
air is heated over the equator, and therefore rises (causing a low
pressure area) and moves northwards. As it moves northwards it
gradually cools and increases in density until it descends, where
it forms a sub-tropical region of high density. Air from this high
pressure area either moves towards the equator, forming the Trade
Winds, or else moves polewards. This latter air eventually meets
cold air currents moving south from the polar region, over which
air is cooled and descending (causing a high pressure area). Where
these two air masses meet, a region of unstable low pressure
results, in which the weather is changeable.
This idealized picture is complicated by the
Coriolis effect (named in honour of the French mathematician
Gaspard Coriolis, 1792-1843, who analysed it), which results
from the east-west rotation of the Earth. This force tends to
deflect a moving object to the right of its course in the Northern
Hemisphere and to the left in the Southern Hemisphere.
The distribution of oceans and land-masses
modifies this simple picture yet further. Because heat is gained or
released more slowly by water than by land- masses, heat exchange
is slower in maritime regions, while at the same time humidities
are higher. In summer, therefore, continental areas tend to develop
low- pressure systems as a result of the heating of land-masses and
the conduction of this heat to the overlying dry masses.
Conversely, in winter the reverse situation occurs, continental
areas becoming cold faster than the oceans, and high pressure
systems developing over them. Because most of the Earth's land
areas occur in the Northern Hemisphere, the ideal situation shown
in the diagram is disrupted to a far greater extent in the Northern
than in the Southern Hemisphere.
In addition to the heating and cooling effects of
land-masses, climate is also affected by altitude. On average the
air temperature falls by up to 6° C for every 200-metre rise
in height, but this varies considerably according to prevailing
conditions, especially the aspect and steepness of slope and the
wind exposure. Because of this tendency for temperature to fall
with increasing altitude, the organisms inhabiting high tropical
mountains—such as Mount Kenya in East Africa—may be
more like the flora and fauna of colder regions than that of the
surrounding lowlands. However, although in general temperature
falls as one ascends such mountains, other environmental conditions
do not mirror precisely those found at higher latitudes. For
example, the seasonal variations in day-length typical of high
latitude tundra areas are not found in the "alpine" regions of
tropical mountains. Also the high degree of insolation resulting
from the high angle of the sun produces considerable diurnal
fluctuations in temperature which are not found in tundra regions.
It is not surprising therefore that the altitudinal zonation of
plants and animals should not reflect precisely the global,
latitudinal zonation.
Zonation of life forms
Plants show strong reactions to the physical factors of their
environment but, because of their inability, for the most part, to
move in response to them, they react by changes in their pattern of
germination or growth. Seeds normally require rather precise
conditions of temperature, soil moisture and light for successful
germination. Adult plants respond to the environment in the size
and shape to which they grow, and changes in their environmental
conditions produce faster growth or periods of dormancy.
Interaction of factors is also important in plants. Green plants
require sunlight for their photosynthesis and, up to certain
levels, photosynthesis increases with increase in light intensity.
But light also has a heating effect on the tissues of the plant
and, since many of the chemical reactions within the tissues
operate well only at fairly low temperatures, very high light
intensities tend to inhibit photosynthesis or even to damage the
plant.
The climate of an area, then, is the result of
the many varying factors that affect the region, and the Earth's
surface accordingly experiences a great variety of climates
distributed over it in an intricate pattern; they can be classified
into five divisions, though each contains scores of regional
variants.
Each of these climatic types and their major
sub-divisions has a number of characteristic plant and animal
communities that have evolved so that they are well- adapted
to the range of environmental factors in them; such characteristic
communities are called Homes. The distinctions between
biomes are not necessarily related to the taxonomic classification
of the organisms they contain, but rather to the life-form
(the form, structure, habits, and type of life-history of the
organism in response to its environment) of their plants and
animals. This concept of the life-form was first put forward by the
Danish botanist Christen Raunkaier in 1903. He observed that the
most common or dominant types of plants in a climatic region had a
form well suited to survive in prevailing conditions. Thus in
Arctic conditions, the most common plants are dwarf shrubs and
other low-growing plants; these have no extensive above-ground
growth that would be broken by heavy winter snowfalls, and their
buds are carried at or just below the surface of the soil where
they obtain the maximum protection from cold and wind in the long
winter. In warmer climates, the characteristic types of vegetation
are trees or tall shrubs that carry their buds and reproductive
structures well above the ground because they are rarely exposed to
severe weather conditions. Deserts usually contain small plants,
mostly quick- growing annuals, with little above-ground growth, and
buds and survival structures below the soil surface, because of the
risk of drought. Animals also show distinct life- forms adapted to
different climates, with cold-resistant, seasonal, or hibernating
forms in cold regions and forms with drought-resistant skins or
cuticles in deserts. Nevertheless, animal life-forms are usually
far less easy to recognize than are those of plants and,
consequently, most biomes are distinguished by the plants they
contain and are named after their dominant life-form.
There is no real agreement among biogeographers
about the number of biomes in the world. This is because it is
often difficult to tell whether a particular type of vegetation is
really a distinct form or is merely an early stage of development
of another, and also because many types of vegetation have been
much modified by the activities of man. The following list would,
however, be generally accepted as representing the main climatic
divisions.
Tundra
Northern
coniferous forest
Temperate
forest
Tropical rain
forests
Temperate
grassland
Tropical
grassland or savannah
Chaparral
Deserts