The Global Climate
The weather at any place changes daily, sometimes hourly. Climate is most simply expressed as the average weather over a period of several years. As for weather, it is convenient to measure climate quantitatively, by using average temperature, average pressure, average rainfall and so on. Averages may be calculated monthly, yearly or over a number of years. Sometimes it is also helpful to know climatic extremes. Two places in different parts of the world may have the same average yearly temperature, but different ranges throughout the year. Manchester (UK) and Warsaw (Poland) have similar annual averages, but the yearly average temperature range for Warsaw is twice as large as that for Manchester, with much colder winters and warmer summers.
Climatic differences throughout the world are caused in the first instance by the differing amounts of solar radiation received at different parts of the Earth and at different times of the year. More solar radiation is received nearer the equator than near the poles where the angle of incidence of radiance is greater.
Solar radiation at the Earth's surface
During the course of the Earth's orbit around the Sun (a year), the angle of maximum incidence of the Sun at the Earth's surface changes. This is due to the tilt of the Earth's orbit, 23.5º from the perpendicular. Warmest temperatures at a particular location on the Earth occur when that location is tilted towards the Sun - during the summer. Winter occurs when that part of the Earth's surface is tilted away from the Sun. Consequently, summer and winter occur at opposite times of the year in the northern and southern hemispheres. Near the equator, the angle of incidence of solar radiation remains high throughout the year and seasonal patterns of temperature are not evident.
General circulation of the atmosphere
If solar radiation were the only factor affecting climate, all places at the same latitude would have the same average temperature. However, the world-wide systems of winds which transport warm and cold air very great distances away from the source regions, influence significantly the climates of the world. The average world-wide wind system is called the general circulation of the atmosphere.
The general global movement of air occurs according to the same physical principles discussed in an earlier section. Air is heated and rises at the equator where the highest levels of solar radiation occur. Surface air from sub-tropical regions blows equatorward to replace the rising air, much as for a sea breeze in coastal areas. The rising air spreads outwards and descends at higher latitudes, completing the circulation of air movement. This circulation is called a Hadley Cell.
Although the physical reality of Hadley Cells has been questioned, they provide an excellent means for describing the way in which heat is transported across the Earth by the movement of air. Other circulation cells exist in the mid-latitudes and polar regions. The general circulation serves to transport heat energy from warm equatorial regions to colder temperate and polar regions. Without such latitudinal redistribution of heat, the equator would continue to heat up whilst the poles would continue to cool down.
The effect of the Earth's rotation is to cause winds to swing to their right in the northern hemisphere, and to their left in the southern hemisphere. Thus the equatorward movement of air swings to form the northeast and southeast trade wind of tropical regions; poleward moving air forms the westerlies associated with the belt of low pressure systems at about 50 to 60° north and south. Where air is found to descend, high pressure develops, for example at sub-tropical latitudes and near the poles. Where air is rising, atmospheric pressure is low, as at the equator and in the mid-latitudes where frontal systems develop.
Idealised global airflow and Hadley cells
The effect of land and sea
Of course, the simple picture of global air movement sketched above is complicated by the position of continents and oceans. Land surfaces react quickly to radiation gain and loss, becoming warm in summer, cold in winter. The oceans react far more slowly and during the summer they are cooler than the adjoining land, whilst in summer they are warmer.
This effect of the Earth's surface is to produce relatively high pressure over cold areas and low pressure over warmer ones, producing large modifications to the wind belts. During winter, for example, a large anticyclone develops over Asia, centred on Siberia where temperatures can fall to -50°C and below. A weaker winter anticyclone develops over north America. In the northern hemisphere average pressure is low at middle and high latitudes, where the Icelandic and Aleutian Lows in the North Atlantic and North Pacific oceans develop respectively.
Global isobaric patterns for January (top) and July (bottom)
In summer, the landmasses warm up, and the winter high pressure over Asia is replaced by a large low pressure system, centred over northern India. The Icelandic and Aleutian Lows weaken and the subtropical highs become stronger. During all seasons there is a belt of relatively low pressure near the equator where the air is rising, but this tends to swing northward and southward with the seasonal changes. The subtropical oceanic high pressure cells tend to swing likewise. Similar but reverse pressure differences occur in the southern hemisphere, although less pronounced than in the northern hemisphere because of the absence of really large land masses. In particular, a continuous belt of low pressure circumnavigates the globe at high latitudes.
The build up of high and low pressure systems as described affects the simple pattern of winds shown above. Most noticeably, a southwesterly monsoon develops in the Indian Ocean, blowing towards the low pressure over Asia, during the northern hemisphere summer. In winter, this is replaced by a north to north-easterly airstream.
Global wind patterns for January (top) and July (bottom)
Global distribution of temperature and rainfall
Average temperature is approximately halfway between the averages of day maximum and night minimum temperatures. In January, lowest temperatures occur over the northern continents, Siberia and northern Canada, where the excess of radiation loss to radiation receipt is greatest. The North Pacific and North Atlantic are warm and the prevailing westerlies carry warmth to the adjacent land, particularly into Europe. Eastern coastal areas, on the other hand, have prevailing winds from the cold continental interiors and are much colder than at corresponding latitudes on western coasts. The warmest areas are the land masses of the southern hemisphere, particularly South Africa and Australia.
In July, the northern continents are strongly heated. The hottest temperatures are the desert areas of the Sahara, Arabia, northwest India and California with average temperatures well in excess of 30°C. Whilst equatorial regions receive the most solar radiation, they are somewhat cooler than the deserts of the sub-tropical, since considerable energy is consumed in evaporating the abundant moisture that precipitates there. Global yearly average temperatures are shown in the figure below.
Global average temperatures for January (top) and July (bottom)
The highest rainfall totals occur near the equator; this is to be expected because the air here is rising, and being warm, is capable of storing considerable amounts of water vapour. Most of the rainfall in the tropical belt is thus convective, with prolonged heavy showers and frequent thunderstorms. At very high latitudes, precipitation is low because air is too cold to contain much water vapour. The subtropical high pressure belts are regions of very low rainfall, due to stable atmospheric conditions associated with descending air. The northern temperate mid latitudes have moderate rainfall, much of it frontal in nature, which diminishes into the interiors of North America and Asia. Rainfall, too, shifts with the north-south movement of the Sun and the seasons, particularly the equatorial rainbelt. Global yearly average is shown in the last figure.
Global yearly average precipitation