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The Global Climate
9.1. Average weather
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 (see Figure 9.1).
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 (see Figure 9.2). 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.
Figure 9.2. The seasons
9.3. 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 lesson 3. 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 (see Figure 9.3). 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 (see Figure 9.3). 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.
Figure 9.3. Idealised global airflow and Hadley cells
9.4. The effect of land and sea
Of course, the simple picture of global air movement sketched in Figure 9.3 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 -40°C. 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 (see Figure 9.4).
Figure 9.4 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 in Figure 9.3. Most noticeably, a southwesterly monsoon develops in the Indian Ocean, blowing towards the low pressure over Asia, during the northern hemisphere summer (see Figure 9.5). In winter, this is replaced by a north to north-easterly airstream.
Figure 9.5. Global wind patterns for January (top) and July (bottom)
9.5. 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 Figure 9.6.
Figure 9.6. 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 Figure 9.7.
Figure 9.7. Global yearly average precipitation
a) How does climate differ from weather?
b) List the main factors that influence the climate of a particular region.
c) Describe how and why Hadley Cells form and explain the idealised global wind system in Figure 9.3 of the Information Sheet.
d) Explain how the distribution of continents and oceans affects climate.
e) Explain the distribution of rainfall around the world in Figure 9.7 of the Information Sheet.
Notes for Teachers
After lesson 9, students should understand:
Answers to Questions
a) Whilst weather describes the conditions temperature, pressure, sunshine, cloudiness, precipitation and wind, climate is the average weather over a period of several years.
b) The most important factor influencing the climate of a particular location is how much solar radiation it receives, particularly how much it receives per unit area. This is determined by the latitude. Areas in higher latitudes receive less solar radiation per unit area, and are therefore likely to be colder, particularly in winter, since seasonal differences are greater at this time. The climate of a particular region will also be influenced by the effect of the global wind system and the distribution of land and sea in that area.
c) Hadley Cells form in the same way as sea breezes (lesson 3). Most solar radiation is received at the equator. Air here is heated by the surface and rises before dispersing north and south. This air releases considerable energy through condensation and precipitation, and descends again at about latitudes 30° north and south. Some of this air returns along the surface towards the equator, generating the trade winds, whilst the remainder moves poleward to meet the air blowing equatorward from the high latitudes. The two air masses meet in the mid latitudes to generate the frontal systems, forcing the warm tropical air to rise. Because the Earth is rotating, the movement of air is deflected, to the right in the northern hemisphere, and to the left in the southern hemisphere. The resulting idealised atmospheric circulation is sketched in Figure 9.3 of the Information Sheet.
d) Because the continents heat up and cool down much faster than the oceans, this has considerable influence on global climatic patterns. The centre of large continents such as Asia experience large ranges of temperature between summer and winter. Nearer coastal regions the affect of the ocean modulates the seasonal changes in temperature. In the UK, average summer temperatures are only about 10°C warmer than those of winter.
e) Highest levels of precipitation are to be found near the equator. Here, the immense masses of rising air are cooled, generating large thunderstorms and frequent heavy downpours. Nearly all of the rain in these regions is convective. Some areas may receive over 2500mm (100 inches) in a year. In sub-tropical latitudes, particularly in Africa and Australia, the descending air prevents the occurrence of much precipitation. Sometimes, droughts can last for several years and annual average rainfall is barely 250mm. Higher levels of precipitation occur in the mid-latitudes where frontal systems are generated. Although summertime rain in these areas can be convective, the majority of precipitation throughout the year is frontal in nature. Precipitation amounts drop off further into the interior of the large continents, far away from the large oceanic water sources. Low levels of precipitation occur again in the polar regions where absolute humid levels are too low to permit the generation of significant precipitation amounts.