Lornshill Academy Geography Department Higher Revision Physical Environments - Atmosphere
Physical Environments Atmosphere Global heat budget The earth s energy comes from solar radiation, this incoming heat energy is balanced by the amount of heat escaping back into space. This balance is called the Earth s Heat Budget. Incoming solar heat, insolation, from the sun is absorbed and reflected meaning not all the heat reaches the earth s surface. 26% of the energy is reflected back into space by the atmosphere and 18% of the heat is absorbed by the atmosphere due to dust particles retaining the heat. This leaves 56% which travels to the earth s surface. Not all of this is absorbed. 6% is reflected from the earth s surface and this makes up the earth s albedo. This means only 50% reaches the earth s surface and is balanced out by the long wave radiation escaping back into the atmosphere. Latitudal Variation As well as variations in the amount of energy received from the Sun across the Atmosphere. There is also variations in the amount of energy received from the Sun across the latitudes of the Planet. Between approx. 35 N and 35 S there is a surplus of energy because insolation exceeds outgoing radiation. The curvature of the Earth means the same amount of insolation is spread over a much great surface area the further north or south from the equator. The Sun s heat is more concentrated nearer to the Equator. The angle the insolation has less atmosphere to travel through near to the equator and so less energy is lost by reflection and absorption. Due to the different land covering more reflection (albedo) takes place near the Poles (ice / snow covered) than near the Equator (tropical forests). This means there is a surplus of energy at the equator and a deficit at the poles. Energy is therefore transferred from areas of surplus to areas of deficit in two ways: By atmospheric circulation By oceanic currents. 2 P a g e
Atmospheric Circulation We have established that some parts of the globe receive more heat from the sun than others. It is this difference in temperature that leads to the formation of winds. Global winds account for 80% of heat transfer Rising air = low air pressure Sinking air = high air pressure The Hadley Cell 1. At the Hadley Cell intense heating at the equator causes warm air to rise into the upper atmosphere creating low pressure areas called the Doldrums. 2. The rising air is forced north and south due to the coriolis force where it cools and sinks at 30 o North and 30 o South creating a high pressure area called the Horse Latitudes. 3. On reaching the earth s surface the Hadley cell is completed as some of the air is returned back to the equator via the north-east and south-east trade winds where the cycle is repeated. Thermally direct cell, powered by warm air rising at equator. The Polar Cell 4. At the Polar Cell, cold, dense air sinks creating a high pressure area. Some air moves north and south to lower latitudes via the Polar Easterlies where it expands as it moves into more space and is warmed by the earth s surface. The air eventually meets warmer air brought via the North and South Westerlies at 60 o North and 60 o South where upon it rises and creating a low pressure area. Also a Thermally direct cell powered by cold air sinking at the Poles. 3 P a g e
The Ferrel Cell 5. The final cell involved is the Ferrel Cell. It obtains its energy from the Hadley and Polar Cells. This cell feeds warm air to high latitudes and transfers cold air back to the sub-tropics for warming. This cell is Thermally indirect and is powered by the other two cells. Coriolis Force Air is deflected by the spinning of the earth (CORIOLIS FORCE), which means that in the Northern Hemisphere winds are deflected to the right, and in the Southern Hemisphere to the left. Rossby Waves These are large belts of fast moving winds traversing the globe at high altitudes of between 10,000 and 12,000 metres. The pattern is wavelike due to the influence of temperature and pressure differences between land and sea areas. Jet Streams There are streams of very fast moving air known as jet streams within these waves. These occur due to differences in temperature between the air masses. These waves and jet streams contribute greatly to the movement of energy throughout the world. Ocean Currents You should be able to describe and explain the general pattern of ocean currents on a world map. The currents form a pattern of large loops. Each loop is called a GYRE! 4 P a g e
Land Masses If there were no landmasses, the ocean circulation would be largely controlled by surface wind system, with 3 closed loops in each hemisphere. The distribution of the major land masses break down this pattern and only in the Pacific and Atlantic Oceans, where there is sufficient room, do we see the development of loops or gyres which are controlled by pressure cells. When currents meet land masses their waters tend to diverge and flow away in opposite directions. Rotation of the Earth The coriolis force deflects winds and currents to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The currents therefore move clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere. Winds Prevailing winds tend to move waters in the same direction. In the Tropics, the Trade Winds push them westward towards the equator and in the higher latitudes the Westerlies drive them in the opposite direction. Temperature difference and convection The water at the equator is warmer and less dense compared to polar waters where it is colder and dense. Due to this convection currents are set up resulting in the poleward flow of warm light water. The warm water will eventually cool, become dense and sink. The cold water will eventually warm, become less dense and return to the equator. This cycle is repeated over and over. This process is essential for maintaining the energy balance and bringing up vital nutrients from the depths in Polar Regions which are then redistributed to lower latitudes. Case study Atlantic Ocean In the Northern Atlantic a series of currents combine to circulate water in a clockwise direction called a Gyre. At the equator water warmed by the overhead sun starts to move in a north westerly direction towards the Caribbean by the North equatorial current, transferring warm seas to higher latitudes. The current then hits the North American continent and is deflected in a north easterly direction towards Europe forming the warm Gulf Stream current. Again higher latitudes receive warmer water. Cooler water from the Arctic & Northern Canada flow south and mix with the gulf stream cooling it, which is then sent southwards to Africa as the gulf stream is deflected by Europe. This cooler current flows towards Africa as the Canaries current, which is then deflected by Africa towards the equator. This spreads cooler water to the equatorial region, where it is then heated up by the sun. 5 P a g e
The prevailing North Easterly Trade & Westerly winds, from falling air at the sub tropics, help to circulate the currents. The Coriolis Effect, whereby the earth spins from the west to east also aids the clockwise circulation of the currents. The combined effect of these two factors help to circulate the warm and cold water around the globe. Density differences due to differential heating result in chilled water sinking to the ocean floor and returning equatorwards. Cold dense polar water sinks, then spreads towards the equator where it pushes up the less dense warmer water which moves off towards the polar areas Intertropical Convergence Zone Air Masses Air masses are large volumes of air with relatively uniform characteristics. Air masses originate where the surface geography is constant such as; over oceans, deserts, large plains and ice covered areas. As air slowly moves over these areas it acquires uniform temperature and humidity characteristics. The ITCZ is where the North Easterly trade winds meet the South Easterly trade winds. The air at the equator is being heated by the overhead sun and therefore rises, the ITCZ can therefore also be described as a band of low pressure. Due to the earth spinning on a tilted axis the position of the overhead sun migrates northwards, when we have our summer solstice (longest day) and then migrates southwards, our winter solstice (shortest day). This means the ITCZ is also going to travel north and south throughout the year. This means the heavy rain showers on the West coast of Africa are very seasonal. Origin, Nature and Characteristics of Air Masses Tropical Maritime Origin - Atlantic Ocean in Tropical Latitudes. Nature (what it is like before it moves) - Warm, moist and unstable (water picked up through evaporation over Atlantic). Weather Characteristics - Brings warm, humid and rainy weather. 6 P a g e
Tropical Continental Origin- Sahara Desert (large land mass in tropical latitudes). Nature (what it is like before it moves) - Warm, dry and stable (no water picked up through evaporation due to being over land). Weather Characteristics - Brings dry and warm weather. The ITCZ moves to follow the Thermal Equator. This moves due to the tilt of the Earth (23½ ), moving north towards June / July and then south. This means it passes over some places (near the actual equator) twice. When the ITCZ moves North the Tropical maritime air mass follows influencing the weather. When the ITCZ moves South the Tropical continental air mass has more influence. The Inter-tropical Convergence Zone (ITCZ) is where the North-East Trade Winds (Tropical Continental ct air mass) and the South-West Trade Winds (Tropical Maritime mt air mass) meet. At this point an area of uplift is created. This causes instability and periods of heavy rainfall. Variations The Rainfall patterns vary for a number or reasons: The further North the less the peak rainfall due to the lessening impact of the ITCZ. The less annual rainfall due to the increase influence of the Tropical continental air mass. The earliest peak rainfall is in Lagos as the ITCZ passes this first. Lagos has two peaks as the ITCZ passes overhead twice. Further South the larger annual rainfall due to the influence of the Tropical maritime air mass. July In July, the sun moves to the Tropic of Cancer and drags the ITCZ with it. The result is that moist mt air penetrates quite far north. 7 P a g e
January In January, the sun moves to the Tropic of Capricorn and drags the ITCZ with it. The result is that dry ct air from the Sahara almost extends to the coast. Farmers in North Africa rely on the ITCZ (especially this) and the mt to reach them in July. Over the last few decades it has been noted that the ITCZ is not travelling as far north as it used to, causing obvious disastrous consequences. ITCZ's effect on the region's climate The ITCZ has varying effects on climates. Gao Gao, with around 200 mm of rainfall per year, is a hot desert climate, with only a limited amount of precipitation in summer as the ITCZ migrates north. Gao s climate is influenced by the hot, dry ct air for most of the year and as can be seen in the graph below, it therefore has fewer days of rain and very low total annual precipitation. This is because it is to the north of the ITCZ for most of the year. 8 P a g e
Abidjan Abidjan, with around 1700 mm of rainfall per year, is a tropical rainforest climate. As the graph below displays, it has a twin-peak regime with a major peak in June and a smaller peak in October/November. It is on the Gulf of Guinea coast and is therefore influenced by hot, humid mt air for most of the year. This results in a higher total annual precipitation and a greater number of rain days. The twin precipitation peaks happen because the ITCZ moves north in the early part of the year, bringing rainfall and then south later in the year, again bringing rainfall. Bobo-Dioulasso Bobo-Dioulasso has a total annual precipitation of around 1000 mm and has a clear wet season/dry season regime. As the graph above shows, it receives more rain days and heavy summer precipitation from June until August when the ITCZ is furthest north. This brings rainfall to the area as the mt air mass is dominant. 9 P a g e