Grade 11 Climatology PDF

Welcome to Geography  Plan for “The Atmosphere” What we will cover: The Earth’s Energy Balance Global Air Circulation Africa’s Weather and Climate Droughts and Desertification Mapwork  Energy Transfer in the Atmosphere Earth’s energy is provided by the...

Welcome to Geography  Plan for “The Atmosphere” What we will cover: The Earth’s Energy Balance Global Air Circulation Africa’s Weather and Climate Droughts and Desertification Mapwork  Energy Transfer in the Atmosphere Earth’s energy is provided by the SUN. Energy is important to us because it… Drives winds and ocean currents. Allows plants to grow to produce food. When Earth receives energy from the Sun, what three things may happen to that energy??? 1. Reflected back into space. 2. Absorbed by the atmosphere. 3. Absorbed by the land and water.  Does land or water absorb the Sun’s energy faster??? Think about a sunny day at the beach …  Land…. Why? Because it takes more energy to warm up a body of water than it does to warm up an equal area of land. Think about it…when you walk out onto the sand during the day, it is HOT, so we sometimes RUNNNNNNNN to the cooler water! Why can life exist here on Earth only? Air Movement  Wind The uneven heating of the Earth’s surface by the Sun causes some areas to be warmer than others… …Remember that warmer air expands, becoming less dense than cold air… causing air pressure to be lowered where the air is heated & warm air rises. So… what is wind???  Wind is… the movement of air from an area of higher pressure to an area of lower pressure! The Earth’s Energy Balance  Vocabulary Latitude Longitude Insolation Revolution Why should we learn vocab?  The Unequal Heating of the Atmosphere So, is the earth heated equally? Why do you think so?  Latitudinal Heating Is it hotter at the equator or the poles? Why? Latitudinal Heating cont.  Earth’s Axis So we know that Earth is not heated equally, more insolation strikes the equator than the poles and so the equator is a lot warmer. So what role does the tilt of the Earth play?  Earth’s Axis Earth rotates once around its axis every 24hrs – What is an “axis”? This gives us day and night If the axis was vertical (fig. 2.4a pg75) then we would have exactly 12hrs of sun and 12hrs of night everyday for the whole year. – Would this cause problems? BUT the axis is tilted 23.5⁰ so we get unequal lengths of day and night at different times of the year  RECAP - Seasonal Heating How do you think seasons affect heating?  Watch clip Seasons.mp4 while filing in the table on the next slide Complete the table Transfer of Energy and Energy Balance  Ocean Currents Who doesn’t love the beach?  Ocean Currents Ocean currents play a vital role in pushing hot and cold water around the globe Cold currents move cold water from the poles toward the equator Warm currents move hot water down to the poles  Ocean Currents What would happen if this movement stopped? Winds  Winds What is wind? How does wind affect temperature? What are our major wind belts?  E How World Pressure Belts are Caused C F B D A B D C F E  Homework Complete Activity 4 Page 80 Global Air Circulation  Atmospheric Pressure So we know that the Earth is not heated equally Why?  Energy recap Energy from the sun strikes Earth most directly near the equator. Near the poles, the same amount of energy is spread out over a larger area.  Atmospheric Pressure This different heating gives rise to different atmospheric pressure This difference in pressure causes air movements both vertically and horizontally – and this makes global air circulation Highest recorded wind ever occurred at… Mt. Washington, New Hampshire on April 12, 1934 The speed was 372 km/h  Understanding Air Pressure Changes in air pressure can cause changes in the wind, This affects the amount of humidity in the air, the temperature, and weather forecasting. Normal air pressure is 1013.2 millibars  Air Pressure Defined Air pressure is simply the pressure exerted by the weight of the air above. Average air pressure at sea level is 1 kilogram per square centimeter. Air pressure is not exerted straight down. Air pressure is exerted in all directions. So the pressure pushing up on the object balances air pressure pushing down on an object.  Watch Air Pressure Air Pressure - TedEd  World Pressure Belts Atmospheric pressure is defined as “the weight of the atmosphere pressing onto the surface of the Earth” Temperature plays an important role here – What role?  World Pressure Belts Where air rises we have a LOW PRESSURE forming As air rises, more air swoops in to replace this rising air  CONVERGENCE Low temperature forces air to contract and sink forming HIGH PRESSURES This sinking air hits rock bottom and  Recap Watch clip – Atmospheric Pressure.mp4 Bill Nye  E How World Pressure Belts are Caused C F B D A B D C F E  Tri-Cellular Circulation What is a “cell”? We have three major cells that dominate the weather patterns on Earth Hadley Ferrel Polar  Hadley Cells Located between the equator and 30° North and South Hot conditions at the equator result in intense uplift of warm, moist air, creating areas of extreme low pressure at the surface This low pressure belt is called the Equatorial Low As the air rises and cools condensation occurs and great belts of cloud develop – Thunderstorms and convection rains are common in all equatorial regions  Hadley Cells cont. At about 10 000metres up the air diverges and heads polewards It sinks at roughly 30° North and South Sinking air creates strong High Pressures, called the sub-tropical highs – Sinking air warms up as it sinks thus creating warm, dry ITCZ  E How World Pressure Belts are Caused C F B D A B D C F E  Ferrel Cells Occur between 30 and 60 degrees North and South of the Equator At 30 degrees air from the Hadley meets air from the Ferrel Cells and descends together creating a region of high pressure called the Sub- tropical High Pressure Belt When the air strikes the ground it diverges North and South at 30 degrees Namib Sahara - Algeria  Polar Cells Occur between 60 and 90 degrees North and South Cold regions of the planet At 60 degrees air from the poles and air from the Ferrel cells converge – They meet at the polar fronts – Fronts are where two contrasting air masses meet – Warm air is forced up as the cold air cuts under the warm masses of air Polar Front  Atmospheric Temperature, Pressure and Winds Moving air is called wind. Low temperatures cause air to get heavy and sink High temperatures cause air to heat up and rise Rising air is associated with........... pressure  Atmospheric Temperature, Pressure and Winds Therefore air ALWAYS moves from a HP to an LP But air doesn’t always flow in one straight direction, there are many factors at play There are three factors that combine to control wind: 1 – pressure differences/pressure gradient force 2 – Coriolis effect – affect direction only 3 – friction – affects wind speed and direction  Pressure Differences - Wind is created by differences in pressure…….. - The greater the differences are, the greater the wind speed is. - Isobars are lines on a map that connect places of equal pressure. - Isobars that are closer together indicate a greater pressure gradient than those lines that are farther away  Pressure gradient force Differences in pressure cause a pressure gradient A steep pressure gradient causes greater acceleration of a parcel of air and higher winds A less steep pressure gradient causes a slower acceleration and light winds. The pressure gradient is the driving force of winds. Pressure gradients have both magnitude and direction. The spacing of the isobars represents magnitude. The direction of force is always from areas of high pressure toward areas of low pressure and at right angles to the isobars. Pressure Gradient  Coriolis Force This movement is due to the Earth’s rotation and is named the Coriolis force. The Coriolis effect describes how Earth’s rotation affects moving objects. All free moving objects or fluids, including the air, are deflected to the Left of their path of motion in the Southern Hemisphere. In the Northern Hemisphere, winds are deflected to the Right  Ferrel’s Law William Ferrel developed a simple law that stated: Winds in the Southern Hemisphere are always deflected left and winds in the Northern Hemisphere are always deflected right Coriolis effect  Coriolis Effect As Earth rotates, the Coriolis effect turns winds in the Northern Hemisphere toward the right and winds in the Southern Hemisphere to the left. The shift in wind direction is attributed to the Coriolis effect in four ways: 1 – The deflection is always directed at right angles to the direction of airflow (does NOT mean the wind changes by 90 degrees! But it is pulled in that sharp direction) 2 – The deflection only affects wind direction not wind speed. 3 – The deflection is affected by wind speed - the stronger the wind, the greater the deflection. 4 – The deflection weakens near the equator, becoming nonexistent at the equator (no deflection between 0 and 5 degrees)  Friction Friction affects the wind only within a few kilometers of the Earth’s surface. The higher up you go, the less friction there is Friction acts to slow air movement, which changes wind speed and direction.  Geostrophic Flow These are theoretical winds that result from an exact balance between Pressure Gradient Force (PGF) and Coriolis Force (CF). This condition is called Geostrophic Balance and blows parallel to the isobars But its only theoretical… GEOSTROPHIC FLOW This balance seldom holds exactly in nature. The true wind almost always differs from the geostrophic wind due to other forces such as friction from the ground. Thus, the actual wind would equal the geostrophic wind only if there was no friction and the isobars were perfectly straight (something that can only happen at a very high altitude)  Winds and Air Masses Planetary Winds – these are the major wind belts that blow all year round They are deflected left or right due to...... ? We have three major wind belts namely: Polar Easterlies Westerlies  Effect of Land and Sea The image before assumed the world was uniform all over i.e. Winds blew in these directions regardless of whether they were blowing over land or sea But we know winds are different over land and sea Why?  Effect of Land and Sea Land and sea heat and cool at different rates Does land heat faster than water? What effect will this have on pressure?  Air Mass Characteristics What is an air mass? A large volume of air covering a large area with similar characteristics such as: Temperature Pressure Humidity  Air Mass Characteristics These can be stable or unstable Stable air is subsiding, heavy air that is associated with high air pressure and thus no rain Unstable air is rising, condensing air that could cause rain  Air Mass Characteristics Equatorial Air Masses: Hot, moist air found at the equator. Unstable. Tropical Air Masses: Occurs near the tropics of Cancer/Capricorn. Hot, dry stable air Polar Air Masses: Found 50-60° N/S and is cold, unstable air Arctic/Antarctic Air Masses: Found at the poles, very cold  Regional winds: North East Monsoons The northern hemisphere experiences winter while the Southern hemisphere experiences summer between December and February. The air over Central Asia cools and sinks, causing a high pressure area. This results in winds blowing from Central Asia, towards the ocean where the pressure is far lower, forming cool North Easterlies and North Westerlies The Northeast Monsoon winds will become the Northwest Monsoon winds due to deflection (Coriolis) as it crosses the Equator. India  Regional winds: South West Monsoons The northern hemisphere experiences summer while the Southern hemisphere experiences winter between June and September. The land in Central Asia is heated up and air warms and rises, causing a massive low pressure area. The air over Australia and southern parts of Africa cools and sinks, causing a high pressure area. This results in winds blowing from Australia and Southern Africa, towards Central Asia, forming the Southeast Monsoon winds. The Southeast Monsoon winds will become the Southwest Monsoon winds due to deflection as it crosses the Equator. Indians playing along a flooded street during the Southwest Monsoon between A flooded town in India during the Southwest Monsoon A town destroyed. Flood victims scrambling for aid relief packages.  Class Work Activity 8 page 94  Local Winds: Fohn Winds These are warm, dry winds that descend on the Leeward side of mountains – What is the Leeward side? As air rises up the windward side it cools and condenses to form clouds. The air then loses a lot of moisture and begins its decent down the Leeward side, heating dramatically as it goes  Lapse Rates Remember the Environmental Lapse Rate? Here we use the Dry Adiabatic Lapse Rate (DALR) and/or the Wet Adiabatic Lapse Rate (WALR) Lapse rate is a measurement of how much a pocket of air changes (in ⁰C) with a change in height  Dry Adiabatic Lapse Rate If air is unsaturated and dry (very little moisture) then it will heat/cool far faster than if there was water vapour present – Why? The DALR is measured at a 1⁰C change per 100m 1⁰C/100m  WALR If air is very moist it will heat/cool fairly slowly The WALR is measured at about 0,5⁰C change per 100m 0,5⁰C /100m Why should we care? Droughts Avalanches  Africa’s Weather and Climate What is climate? Does Africa have a large variety of Climates? What different types of climate does SA experience? – Highveld/temperate, semi-arid/desert, Mediterranean, Humid sub-tropical, tropical, Equatorial  Africa’s Weather and Climate Equatorial – Hot all year round, no distinct seasons, lots of rain Tropical – High temperature range, heavy convection summer rains Humid Subtropical – High temp range, mainly summer rains Highveld – High temp range, dry Winter and rainy summer Semi arid/desert – High day temp, low night temp. Very little rain Mediterranean – Hot dry summer, cool wet winters. High temp range  Factors that Influence Temperature and Rainfall There are five main factors that influence Temperature and Rainfall Distance from the equator (Latitude) Distance from the sea Height above sea level (Altitude) Ocean currents and winds Mountains (Aspect)  Distance from the Equator Temperature There is a far more concentrated amount of sunlight on the equator than anywhere else on Earth The further away from the equator you go, the less sunlight you will receive What does this mean for temperature?? Distance from the Equator  Distance from the Equator Rainfall So we know the equator is hotter What does this mean for rainfall in the equator?  Distance from the Equator Rainfall Heated and moisture-laden air constantly rises and cools to form clouds over the equatorial regions This results in large amounts of rain at the equator As we move further away other factors come into play to affect rainfall, but the equator will generally receive the most rain  Distance from the Sea How do you think the ocean effects the temperature and rainfall? Temperature Distance from the Sea Ocean temperatures generally change at a far slower rate than land temperatures So if you are near the ocean, you are far more likely to experience milder winters and cooler summers – Why? Inland conditions are far more extreme, with low winters and hot summers Rainfall Distance from the Sea So clouds are made of water – yes? There is a lot of water at the coast – yes? So will there generally be more rain at coastal areas?  Height above sea level Temperature The higher you go the colder it gets! Why? Air is a lot thinner at high altitudes, meaning the cozy blanket we are so used to down here is very thin up there so all the heat can escape  Height above sea level Rainfall As air rises it cools and condenses So if air travels up a mountain what will happen? Generally mountainous areas receive more rain than flat regions  Ocean Currents and Wind Temperature Ocean currents are generally driven by prevailing winds that blow over them So currents from the equator and tropical places are warmer and bring warm conditions to the land they are next to Polar currents bring cold conditions and lower the temperatures  Ocean Currents and Wind Rainfall Areas along the east coast generally receive more rain as they are next to warm ocean currents West coast regions are generally drier due to the presence of cold ocean currents What are the two currents that pass us?  Mountains Temperature The temperature of mountainous regions largely depends on which way they are facing Rainfall The side the mountain is facing (aspect) determines the amount of rain based on whether or not this is a sunny side or a shady side  Subsidence and Convergence What is Subsidence? What is Convergence? How do clouds form? We have saturated air (saturated with water vapour) rising, cooling and condensing. Why does it condense?  Convergence Convergence Air rushing in and rising Low pressure As the air rises, it cools at 1°C per 100m – DALR As air rises, its capacity to hold water vapour DECREASES Why? – Air consists of millions of particles all vibrating together So we have AIR and WATER VAPOUR mixed together – These particles constantly collide with each other when heated creating kinetic energy (movement energy) – So hot air means there is a lot of energy around. Lots of energy means no time for condensation – This mixing and heating means that the water vapour molecules do not “clump together” as they are too busy being knocked around by other particles – As air rises, it loses this kinetic energy – So now we have the mixing, but not the heating. – This results in condensation SO... air now has less of a hold over the water vapour and its CAPACITY to hold water vapour DECREASES  Subsidence Sinking air High pressures As it sinks, the air heats up adiabatically at 1°C per 100m – DALR As air descends, its capacity to hold water vapour INCREASES Why? – The air is descending and so the pressure is increasing Ever been at the bottom of a “pile-on”? Lots of pressure there, lots of pressure from sinking air – This increase in pressure means that the air in the parcel is now being forced to bump into other particles (including water vapour) – With AIR and WATER VAPOUR mixing under pressure we have an increase in collisions between particles and so the water vapour is not allowed to clump together anymore – Increase in collisions means more heat SO... Air now has a GREATER CAPACITY to hold water vapour and so there is nothing left to form clouds  Effects of Subsidence and Convergence on rainfall in Africa At the equator we have the ITCZ or Inter-Tropical Convergence Zone Massive low pressure belt The Tropical Easterlies converge at the ITCZ resulting in tropical conditions Shifting ITCZ  Subtropical High Pressure Belts Between 15 and 30 degrees North and South we have subsiding air These areas are known as the subtropical high pressure belts As we know, high pressure conditions result in very little condensation and therefore very little cloud cover so very little/no rain occurs here These high pressure belts are one of the main causes of deserts (E.g. Sahara, Kalahari and Namib)  Polar Front Convergence at the Polar Fronts causes air to rise there At the Polar Front mid-latitude cyclones develop These are then pushed North by the Westerly Winds and hit Cape Town, resulting in Cape Town’s cyclonic or frontal rains in Winter  Seasonal Changes The ITCZ does not stay in one place As the Earth sits on a tilted axis and rotates around the sun the various hemispheres receive more sun at certain points in the year. This means that there is more heat in some places than others and with this there is more convergence in these locations Summer Subtropical HP belts Winter Subtropical HP belts Complete Act 8 and 9