How do planetary wind belts affect the climate of a landmass in the mid-latitudes

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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, 23.5� 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

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

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)

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

Questions

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.

After lesson 9, students should understand:

• that climate is "average weather";
• the effect of seasons on climates in different parts of the world;
• the general circulation of the atmosphere and its effect on climate;
• the climatic influences of land and sea;
• the global isobaric, wind, temperature and precipitation patterns.

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.

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Deserts are areas where the rainfall is too low to sustain any vegetation at all, or only very scanty scrub. The rainfall in desert areas is less than 25 mm or 10 inches per year, and some years may experience no rainfall at all. The hot deserts are situated in the subtropical high pressure belts where there is unbroken sunshine for the whole year. Such areas include the Sahara, Saudi Arabia, large parts of Iran and Iraq, northwest India, California, South Africa and much of Australia. Here, maximum temperatures of 40 to 45�C are common, although during colder periods of the year, night-time temperatures can drop to freezing or below due to the exceptional radiation loss under the skies.

The Gobi desert in Mongolia is an example of a cool desert. Though hot in summer, it shares the very cold winters of central Asia. The Arctic and Antarctic regions, too, receive very little precipitation during the year, owing to the exceptionally cold air, but are more usually classified as types of polar climate. Semi-desert areas include the Steppes of southern Russia and central Asia, and the Parries of Canada.

Much of the equatorial belt experiences hot and humid weather. There is abundant rainfall due to the active convection of air that takes place there, and during certain periods, thunderstorms can occur every day. Nevertheless, this belt still receives considerable sunshine, and with the excessive precipitation, provides ideal growing conditions for luxuriant vegetation. The principal regions with an equatorial climate are the Amazon Basin in Brazil, the Congo Basin in West Africa and Indonesia.

Because a substantial part of the Sun's heat is used up in evaporation, temperatures in the tropics rarely exceed 35�C; a daytime maximum of 32�C is more common. At night the abundant cloud cover restricts radiation loss, and minimum temperatures fall no lower than about 22�C. This high level of temperature is maintained with little variation throughout the year. The seasons, so far as they do exist, are distinguished not as warm and cold periods but by variation of rainfall and cloudiness. Greatest rainfall occurs when the Sun at midday is overhead. On the equator this occurs twice a year in March and September, and consequently there are two wet and two dry seasons. Further away from the equator, the two rainy seasons merge into one, and the climate becomes more monsoonal, with one wet season and one dry season. In the northern hemisphere, the wet season occurs from May to July, in the southern hemisphere from November to February.

Between the wet equatorial belt and the subtropical desert regions are areas known as Savannahs. They have a single short rainy season when the Sun is nearly overhead, whilst the rest of the year is dry. Vegetation consists mostly of scrub and grassland, which blossoms during the rainy period, and dies off during the prolonged dry season. Such climates and their associated land types are common in the Sahel in Northern Africa (south of the Sahara), large parts of India and parts of northern Australia.

Temperate climates are those without extremes of temperature and precipitation. The changes between summer and winter are invigorating without being frustratingly extreme. There are two types of temperate climate: maritime and continental. The maritime climate is strongly influenced by the oceans, which maintain fairly steady temperatures across the seasons. Since the prevailing winds are westerly in the temperate zones, the western edge of continents in these areas experience most commonly the maritime climate. Such regions include western Europe, in particular the UK, and western North America at latitudes between 40 and 60� north.

Continentality increases inland, with warmer summers and colder winters as the effect of land on radiation receipt and loss increases. This is particularly true in North America, where the north-south aligned Rocky Mountains act as a climate barrier to the mild maritime air blowing from the west. Maritime climate, on the other hand, penetrates further into Europe where the major mountain range - the Alps - is orientated east-west.

The polar regions are perpetually covered by snow and ice throughout the year. In these high latitude parts of the world, the Sun is never high enough in the sky to cause appreciable melting and the temperature rarely rises above freezing. During the long polar nights, which last six months at the poles, temperature can fall to extremely low values. The lowest ever temperature occurred in Antarctica, where a value of -88�C was recorded.

The north polar region includes the ice-covered Arctic ocean, the Greenland continent and much of northern Canada and northern Siberia. In the southern hemisphere, the vast mountainous continent of Antarctic is covered by snow and compacted ice several kilometres thick.

The Mediterranean climate is a special type of climate that describes a regime of hot summer drought and mid winter rain in the mid latitudes, north of the subtropical highs. This climate occurs most noticeably in the regions around the Mediterranean, from where the climate gets its name, but also in coastal areas of California, South Africa and southern parts of Australia.

In summer, the high pressure belts drift northwards in the northern hemisphere, southwards in the southern hemisphere, They are coincident with substantially higher temperatures and little rainfall. During the winter, the high pressure belts drift equatorward, and substantial rainfall occurs. Whist usually mild, such areas can experience cold snaps when exposed to the icy winds of the large continental interiors.

Describe the type of climate expected at each of the following countries (Refer to Figure 9.4 to 9.7 of Information sheet 9.)

Notes for Teachers

After lesson 10, students should understand:

• The different types of climate that exist around the world (desert, savannah, tropical, temperate, Mediterranean, polar).

For each country, students should identify the type of climate it experiences and determine the winter and summer temperature, and the annual precipitation from Figures 9.6 and 9.7 of lesson 9.

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The British climate is temperate, with no extremes of temperature and rainfall. Winters are usually fairly mild under the influence of the Gulf Stream, whilst summers are neither oppressively hot nor frustratingly cool. The average annual temperature is about 10�C, with a summer and winter average of approximately 15�C and 5�C respectively. The British climate is dominated by the tracks of frontal depressions which form in the Mid-Atlantic and pass across into Europe, bringing rain and frequent bad spells of weather.

A number of principal air masses that affect Britain have been identified, bringing with them characteristic patterns of weather. The more common air masses are shown in Figure 11.1. These air masses are defined according to both their region of origin and their course travelled. Air from Arctic regions, for example, is classified as maritime arctic (mA). This air mass originates in the Arctic and travels across the relatively warm stretch of the North Sea. Maritime tropical air (mT), on the other hand, originates near the Gulf of Mexico and travels across the warm Atlantic before arriving in Britain. Other air masses include maritime polar (mP), continental polar (cP) and continental tropical (cT) (see Figure 11.1).

On an annual basis, the most frequent airflow is maritime, including polar and tropical air masses. When both cyclonic (low pressure) and anticyclonic subtypes are considered, maritime airflow accounts for a quarter of all climate patterns experienced in Britain, reaching a maximum of 35% during December and January. Maritime tropical air is mild in winter. In summer it is cloudy and rather cool but humid. When associated with frontal depressions rain is usually abundant; with anticyclones or high-pressure ridges, settled weather with warm sunny spells.

Maritime polar air streams produce cool, showery weather at all seasons, especially on windward coasts. The air has tracked over a relatively warm sea and becomes unstable as its lower layers are heated. Frequently during the passage of frontal depressions over Britain, a flow of maritime tropical air is followed by maritime polar air as the cold front passes. After a brief interlude of sunshine following the downpour of the cold front, frequent showers, associated with the moist unstable air may develop. These may be of snow during the winter months.

Continental polar air in winter is very cold and temperatures associated with this air stream are usually well below average. The air mass is basically very dry and stable but a track over the central part of the North Sea supplies sufficient heat and moisture to cause showers, often in the form of snow, over eastern England and Scotland. During summer, the airflow is usually warm, since even northern parts of Europe experience high temperatures during this time of year.

Continental tropical air reaches Britain from the Sahara; it is dry and in summer gives rise to heat waves, particularly in the southeast. The lower layers are stable, often capped by a temperature inversion, under which haze may build up. Sometimes, instability develops above the temperature inversion, giving rise to thunderstorms. In winter it gives pleasant mild weather.

The preceding sections described the general airflow that influences Britain, and the general patterns of climate that are experienced. Nevertheless, there are differences in the degree of oceanicity or continentality of the climate in particular areas. Figure 11.2 shows the January and July surface temperature isotherms (lines of equal temperature) for Britain. Coldest temperatures during the winter occur in eastern Scotland and England. The temperature isotherms are orientated north-south, and reveal the warming influence of the maritime tropical airstream to the western half of Britain. The eastern half of Britain experiences greater continentality. In summer, the isotherms are orientated east-west and temperature variations due to latitudinal differences of solar radiation receipt are more evident, with highest temperatures in the south.

The British climate can be divided into four quarters, as shown in Figure 11.3. The northwest quarter is characterised by mild winters and cool summers, the northeast by cold winters and cool summers. The southwest experiences mild winters and warm summers, the southeast cold winters and warm summers. During winter, the western half of Britain experiences a more maritime climate, whilst the east receives influence from the cold air streams from the continent. In summer, climatic differences are more dominated by latitude.

Figure 11.4 shows the general pattern of precipitation in Britain. The western half, and in particular the higher ground, receives considerable rainfall, most of it frontal, but augmented by orographic uplift. Parts of highland Scotland can receive over 250 cm or 100 inches of precipitation per year. Precipitation amounts fall further to the east, particularly in the southeast of England, where certain areas may receive only 50 cm or 20 inches per year. The east of England, and to a lesser extent Scotland, lie in the rain shadow of the Welsh mountains, the Lake District and the Scottish Highlands.

b) What is the principal influence of climate in Britain during the summer?

After lesson 11, students should understand:

• that Britain experiences a temperate climate;
• the different air masses that influence the climate of Britain;
• the differences between maritime temperate and continental temperate climates;
• the special variations in the British climate (temperature and precipitation).

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The types of radiation transfer and nature of the greenhouse effect are explained in detail in lesson 1. Here, an overview is included in the context of climate change. The Sun emits energy in the form of visible light and ultra-violet (UV) radiation, which travels towards the Earth. Some of this energy is absorbed by the atmosphere and some is reflected by the clouds back into space. The rest heats the Earth's surface. The Earth is much cooler than the Sun and re-radiates this heat as infra-red (IR) radiation. Some of this IR radiation is trapped by greenhouse gases in the atmosphere, making the Earth warmer than it would be without an atmosphere. This is called the greenhouse effect, and is shown in Figure 12.1.

Increasing the concentrations of the greenhouse gases will make the natural greenhouse effect more powerful. Some of mankind's activities have resulted in an increase in the amount of greenhouse gases in the atmosphere. These extra greenhouse gases are trapping more of the heat trying to escape from the Earth and this is causing the atmosphere, and consequently the Earth's surface, to heat up. So what is commonly called the greenhouse effect is really the increase of the natural greenhouse effect due to human pollution, and should be called the "enhanced" greenhouse effect.

Man-made emissions of greenhouse gases come from a variety of sources including the burning of coal, oil and gas (the fossil fuels), destruction of the world's forests, vehicle exhausts fumes and aerosols. Carbon dioxide (CO2) is released when fossil fuels are burnt, and also through the destruction of the world's forests. Although CO2 is not the most powerful greenhouse gas it has the greatest concentration in the atmosphere after water vapour. Since 1765, its concentration has increased by 30%. Methane (CH4) emissions come mainly from modern farming methods, but also from coal mining, natural gas production and waste landfill sites. Methane concentrations have increased over 100% since 1765. Nitrous oxide (N2O) emissions come from the productions of fertilisers, nylon, chemicals and the burning of fossil fuels. Chlorofluorocarbons (CFCs) are used in air conditioning units, plastic foams and aerosol cans. CFCs are very powerful greenhouse gases compared to CO2. Even though the concentrations of CFCs in the Earth's atmosphere are very small, their total effect is significant, because they trap much more radiation than CO2. CFCs also destroy the ozone layer. Water vapour (H2O) is the most abundant greenhouse gas. It plays a role in maintaining a stable temperature on Earth through the production of clouds.

Temperature records show the Earth has warmed by 0.5�C during the 20th century. This is shown in Figure 12.2.

Temperature varies naturally from decade to decade and even from century to century. How can one be certain that the rise in temperature that is now occurring is due to mankind's enhancing of the Earth's natural greenhouse effect and not due to natural changes in the climate?

Scientists use computer models to simulate man-made climate change. The results show that the warming over the past 100 years is most likely to be due to mankind, and that if we continue to emit greenhouse gases at the present rate the Earth will continue to warm by 1.5�C every 50 years. This does not sound like much, but it is useful to remember that the Earth warmed by only 5�C at the end of the last ice age; this warming took several thousands of years.

Most greenhouse gases stay in the atmosphere for many tens of years and continue to affect the climate long after being released. If emissions of greenhouse gases stopped today, those already in the atmosphere would continue to affect the climate.

The effects of climate change could have serious impacts on the world. Sea levels may rise, affecting many low-lying coastal regions. Changes in the amount of rainfall will affect agriculture. The most at risk will be those least able to adapt to changes. To minimise these impacts, we must reduce emissions of the greenhouse gases.

There is much uncertainty about how climate change will affect us, but scientists and governments believe that we must start reducing greenhouse emissions now. The Framework Convention on Climate Change (FCCC) represents a first step to achieve this goal. Signed in June 1992 at the Rio Earth Summit, by 162 Governments, the FCCC aimed to reduce greenhouse gas emissions to 1990 levels by the year 2000. In 1997, Governments drew up the Kyoto Protocol which now commits nations to reduce greenhouse gas emissions by 5% from 1990 levels by the year 2012.

i. a doubling of carbon dioxide concentrations in the atmosphere.

ii. a trebling of carbon dioxide concentrations in the atmosphere.

Which are natural?

Which are man-made?

After lesson 12, students should understand:

• the nature of the Earth's greenhouse effect;
• the cause of the enhanced greenhouse effect;
• the key greenhouse gases and their main sources;
• the temperature trends of the last 140 years;
• international commitments to reduce the threat of global warming.

a) From the graph below it can be seen that a doubling of CO2 will lead to a 2.5�C global warming whilst a trebling of CO2 will result in a 4�C global warming.

b) The atmospheric concentration of CO2 must rise to about 610 ppm for a 2�C global warming. If the annual rate of increase is 1.5 ppm, this increase (from 350 ppm) will take about 173 years. In reality, the rate of increase of CO2 in the atmosphere is likely to increase unless measures are taken to halt the rate of emissions. In addition, other greenhouse gases contribute about 40% to the global warming, so a 2�C temperature rise is likely to occur a lot sooner.

c) The other greenhouse gases include methane, nitrous oxide and chlorofluorocarbons or CFCs. Of these, only the CFCs are wholly man-made. They first entered the atmosphere in the 1930's when their potential as refrigerants and coolants was recognised. Molecule for molecule, they are several thousand times stronger at trapping infra-red radiation than CO2 so despite their very low concentrations, they are now accounting for about 20% of the global warming.

d) This question can be treated as a class discussion. The central theme should be the reduction of CO2 by increasing energy efficiency and energy saving. Some of the ideas that can be included are:

• draught proofing in homes;
• turning down the thermostat;
• loft insulation;
• using energy efficient lighting;
• using public transport instead of the car.

e) The time series graphs for annual temperature and precipitation in Manchester, UK are shown below. Students should use the data from the table to draw these graphs. Encourage your students to draw in trend lines demonstrating any real changes in climate. Since the early 1960s there does appear to have been some warming, although the warmest year in the series is 1959. Similarly, a slight reduction in annual precipitation is noticeable.