CS 755: CLIMATE, AGRICULTURE AND ENVIRONMENT BY REV. PROF. MENSAH BONSU
COURSE CONTENT Introduction to Climatology • Differentiating between weather and climate • Partitioning the Atmosphere • Climatic elements and their usefulness • Solar energy and air temperature • The earth inclination and temperature variation • The lapse rate and temperature inversion
COURSE CONTENT Introduction to Climatology • Air pressure and winds • pressure gradient force • The convection system • Land and sea breezes • Mountain and valley breezes • Cariolis effect • Frictional effect • Global air circulation pattern
COURSE CONTENT Introduction to Climatology • Ocean currents and their effects on precipitation • Moisture in the atmosphere • Air masses and storms • Climate regions of the world
INTRODUCTION TO CLIMATOLOGY • Weather: State of the atmosphere at a given time and place e.g. temperature, wind and precipitation. • Climate: Long-term average weather conditions in a place or region or trends in weather data that have been accumulated over an extended period of time e.g. tropical climate, sub-tropical climate etc.
PARTITIONING THE ATMOSPHERE • Troposphere is the lowest layer of the earth’s atmosphere; it extends about 10 km above ground. • Stratosphere is the next layer of the earth’s atmosphere after troposphere; it extends approximately 10 to 24 km above the ground. • The imaginary boundary separating the troposphere and the stratosphere is called tropopause.
CLIMATIC ELEMENTS AND THEIR INFLUENCE ON HUMAN EXISTENCE (USEFULNESS) • The troposphere contains all the air, clouds and precipitation of the earth. • The earth climatic differences make us understand the way people use the land. • Climate is key to understanding the distribution of world population.
DESCRIPTION OF ELEMENTS CONSTITUTING WEATHER CONDITIONS Solar energy and air temperature The intensity and duration of solar radiation at any given place vary and are controlled by: • The angle at which the sun’s rays strike the earth • The number of daylight hours
DESCRIPTION OF ELEMENTS CONSTITUTING WEATHER CONDITIONS The temperature variation of the earth and the earth’s inclination • The axis of the earth connecting the north and the south poles is tilted about 23.5° from the perpendicular. • If the earth were not tilted in this way, the solar radiation received at a given latitude would not vary during the course of the year. • When the Northern Hemisphere is tilted directly toward the sun, the sun’s vertical rays are felt 23.5 °N latitude (Tropic of Cancer). This position occurs in June 21, and we have summer for the Northern Hemisphere, and winter for the Southern Hemisphere.
DESCRIPTION OF ELEMENTS CONSTITUTING WEATHER CONDITIONS The temperature variation of the earth and the earth’s inclination • About December 21, the vertical rays of the sun strike near 23.5 °S latitude (Tropic of Capricorn); it is the beginning of summer in the Southern Hemisphere and onset of winter in the Northern Hemisphere. • The tilt of the earth makes the length of days and nights vary during the year. One half of the earth is always illuminated by the sun at any particular time. It is only at the equator that there is light for 12 hours each day of the year.
DESCRIPTION OF ELEMENTS CONSTITUTING WEATHER CONDITIONS The Lapse rate and temperature inversion • Within the troposphere, temperatures are usually warmest at the earth’s surface and decrease as elevation increase. This rate of change of temperature with altitude in the troposphere is called lapse rate, and the average is about 6.4 °C per 1000 meters. • Sometimes the earth radiation is so rapid that it causes temperatures to be higher above the earth surface than at the surface itself. This particular condition is called temperature inversion.
DESCRIPTION OF ELEMENTS CONSTITUTING WEATHER CONDITIONS Importance of temperature inversion Warm air at the surface may be blocked by relatively warmer air above the surface due to temperature inversion. If the trapped surface air, which is relatively cooler, is filled with automobile exhaust emissions or smoke, a serious smog condition may develop close to the surface. Smog = smoke + fog
AIR PRESSURE AND WINDS • There is a drop in atmospheric pressure when air heats up and a rise in pressure when air cools down. • The lighter air moves to the top while the heavier air moves to the bottom, causing the heavier air to spread horizontally. • Therefore air moves from heavy (cold) air locations (high air pressure) to light (warm) air locations (low air pressure). • The greater the differences in air pressure between places, the stronger the wind.
PRESSURE GRADIENT FORCE • The differences in the nature of the earth’s surface, e.g. water and green forest, may cause zones of high and low pressures to develop. • Pressure differences between areas create pressure gradient force, which causes air to blow from an area of high pressure toward an area of low pressure. • Heavy air stays close to the earth surface and forces the upward movement of warm (light) air, producing winds. The velocity or speed of the wind is directly proportional to the pressure differences.
PRESSURE GRADIENT FORCE • If distances between high and low pressure zones are short, pressure gradients are steep and strong wind velocities develop. • When zones of different pressures are far apart, the pressure gradients are not great, and gentle air movements occur. THE CONVECTION SYSTEM Warm air rises as cool air descends. The circulating motion of ascending warm air and descending cool air is known as Convection.
LAND AND SEA BREEZES • One example of a convectional system is Land and Sea Breezes. • Evaporative cooling (latent heat of vaporization), causes the water surface to be cooler than the Land surface during the day • The warmer air over the land surface rises vertically (low pressure), and the cooler air from the water surface (high pressure) flows to take the place of the ascending warm air and a cooling breeze results on Land – Sea Breeze • During the night, the land cools faster than the water surface and the opposite occurs, that is, Land Breeze toward the sea.
MOUNTAIN AND VALLEY BREEZES (i.e. TOPOGRAPHIC WIND EFFECT – ANABATIC WIND) • During the day, the air above the slope of the valley will be heated to a higher temperature than that of the center of the valley. The warm air above the slope rises while the cooler air of the valley moves to the upslope to give rise to valley breeze. During the night, mountain breezes occur. The air of the mountain slopes cools and descends to the valley. Thus bringing cool breeze to the valley – mountain breeze.
CARIOLIS EFFECT As winds move from high pressure zone to low pressure zone, they tend to be deflected toward the right in the Northern Hemisphere and toward the left in the Southern Hemisphere. This deflection is called cariolis effect. The cariolis effect and the pressure gradient force produce spirals rather than straight patterns of wind. Spiral of wind characterize the earth’s air circulation system of many storms’.
FRICTIONAL EFFECT • The movement of wind is slowed down by the frictional drag of the earth’s surface. • The effect is strongest at the surface and declines with elevation until it becomes ineffective at about 1,500 meters above the surface. • The frictional effect decreases the magnitude of wind speed and changes the direction of wind flow.
THE GLOBAL AIR CIRCULATION PATTERN Sub-tropical high pressure zone • Because of solar heating, the air in the equatorial zone is warm (lighter) and tends to move away from the equatorial low pressure in both the northerly and southerly directions. • As the equatorial air rises, it cools and eventually becomes dense. The lighter air near the surface cannot support the cool, heavy air. • The heavy air falls, forming surface zones of high pressure called sub-tropical high pressure, which are located 30 °N and 30 °S of the equator.
NORTH-EAST TRADES IN THE TROPICS When the cooled air reaches the earth surface, the part that moves in the northerly direction undergoes cariolis effect in the Northern Hemisphere to give belts of wind called North-east trades in the tropics. SOUTH WESTERLIES IN THE MID-LATITUDES The part of the cooled air that moves in the southerly direction also undergoes cariolis effect to give rise to south-westerlies in the mid-latitudes.
SUB POLAR LOW PRESSURE ZONE A series of ascending air cells also exists over the oceans to the north of the westerlies called sub polar low pressure zone. These areas tend to be cool and rainy. THE POLAR HIGH • The polar easterlies connect the sub-polar low areas to the polar high areas. • the general global air circulation pattern is modified by local wind conditions.
OCEAN CURRENTS AND THEIR EFFECT ON PRECIPITATION • The winds of the world set ocean currents in motion. • Differences in density of water cause water to move from a zone of high density to a zone of low density. • Thus wind direction and differences in density cause water to move in various paths from one part of the ocean to another
OCEAN CURRENTS AND THEIR EFFECT ON PRECIPITATION • Cold ocean currents near land cause the air just above the water to be cold while the air above this cold zone is warm. This condition prevents convection effects, thus denying moisture to nearby land. That is why coastal deserts of the world border cold ocean currents. • Warm ocean currents bring moisture to the adjacent land area, especially when prevailing winds are landward.
MOISTURE IN THE ATMOSPHERE Cloud Formation • Descending air in the high pressure zones yields cloudless skies. • As warm, moist air rises, clouds form. This kind of cloud formation that often accompanies heavy rain is the CUMULONIMBUS.
EL-NINO • El-nino condition prevails as the result of the interaction of the atmospheric pressure and ocean temperature. • Under normal circumstances, in the south pacific ocean, trade winds blow warm surface water west-ward and allow cold water to come to the surface along the South American Coast. This condition maintains the contrast in water temperature.
EL-NINO • But when a condition called the Southern Oscillation occurs, there is warming in the eastern pacific, enhancing the usual temperature contrasts between the equator and the poles. Atmospheric pressure rises near Australia, the wind falters and El-nino is created off the coast of South America. • The greater the temperature disparity combined with moisture availability from the pacific ocean, the more severe the weather.
AIR MASSES AND STORMS • Air masses are large bodies of air with similar temperature and humidity characteristics throughout. They form from a source region. • When two different air masses come into contact, a front develops and the possibility of storms developing is created. • If the contrasts in temperature and humidity are sufficiently great, or if the touching air masses are moving in opposite direction, waves might develop in the front.
AIR MASSES AND STORMS • As the waves enlarge, cooler air may move along the surface, while warm air moves up and over the cold air, the rising warm air creates a low pressure centre and precipitation accompanied by winds develops into a storm or cyclone. • Tropical cyclone or hurricane begins in a low pressure zone over warm waters, usually in the Northern Hemisphere. In the developing hurricane, the warm, moist air at the surface rises, which helps to suck up air resulting in the formation of thick cumulonimbus clouds.
CLIMATE TYPES AND THEIR LOCATIONS • TROPICAL Associated with earth areas lying between the Tropic of cancer in the North of the equator and the Tropic of Capricorn in the South of the Equator. • DRYLAND Associated with areas in the interior of continents where mountains block west winds, or inlands far from the reaches of moist tropical air.
CLIMATE TYPES AND THEIR LOCATIONS • HUMID MID-LATITUDE Mountain ranges, warm or cold ocean currents, particularly land-water configuration bring about variations in the middle latitudes. • SUBARCTIC AND ARCTIC Located toward northern areas and into the interior parts of the North America and Eurasian Landmasses.
CLIMATE REGIONS OF THE WORLD The two most important elements that differentiate weather conditions are temperature and precipitation.
TYPES OF RADIATION • Radiation refers to the emission of energy in the form of electro-magnetic waves from all bodies whose temperature is above 0°K. • Solar Radiation • Shortwave radiation whose wavelength ranges from 0.3 – 3 micron meters (3000 – 30,000 A°) (Angstron). • Half consists of visible light (0.4 – 0.7 micron meters). • Corresponds to the emission of a black body whose temperature is 6000°K (solar constant). • It reaches the outer surface of the atmosphere at a nearly constant flux of 2 Langley/minute (or 2 calories/min cm2).
Solar Radiation • It changes its flux and spectral composition while passing through the atmosphere as a result of reflection, absorption and scattering. • Reflection: About ⅓ is reflected back to space as a result of the atmospheric composition e.g. water vapour/clouds. It can be as high as 80% when the sky is completely overcast with clouds. • Absorption and scattering of solar radiation cause only about half of the original flux density to finally reach the ground.
Solar Radiation • Direct solar radiation is the part that reaches the ground without being reflected or scattered. • Sky radiation: Part of the reflected and scattered solar radiation that reaches the earth. • Global Radiation = Sky Radiation + Direct Radiation
B. Terrestrial Radiation • Part of the solar radiation that reaches the earth surface is radiated (or emitted back) to space as terrestrial or longwave radiation (infra red). • The temperature of the earth surface is about 300°K. Therefore, the terrestrial radiation is of much lower intensity and greater wavelength than solar radiation. • The wavelength of the terrestrial radiation is therefore long in the range of 3 – 50 micron meters (longwave radiation).
C. Blackbody Emittance and Spectral Distribution • A blackbody is one which absorbs all radiation reaching it without reflection and emits all radiation at maximal efficiency. The sun is an example of a blackbody.
RADIATION LAWS • Plank’s Law: There are two principles: 1st Principle: ℮ = hv………………. (1) Where: ℮ = energy per photon h = Plank’s constant v = frequency of radiation v = c/λ……………………….(2) Where: c = speed of light λ = wavelength Combining (1) and (2) gives ℮ = hc/λ………………………(3)
RADIATION LAWS 2nd Principle: The intensity distribution of energy emitted by a blackbody as a function of wavelength and temperature: Eλ = 2πhc2/ λ5 [exp (hc/KT)-1] Where: K = Boltzmann’s constant T = Absolute Temperature (°K) Eλ = Spectral emittance of blackbody
RADIATION LAWS II. Stefan – Boltzmann’s Law: The total energy emitted by a body integrated over all wavelengths is proportional to the fourth power of the absolute temperature: i.e. Jt = εᵹT4 Where: ε = emissivity coefficient ᵹ = Stefan-Boltzmann’s constant ε = 1 for a blackbody
RADIATION LAWS III. Wien’s Law: The maximum energy per unit wavelength emitted λm is given by: λm = 2897/T…………………(1) Where T = absolute temperature, K
D. Greenhouse Gases • There are absorptive gases that occur in the atmosphere and are responsible for the partial trapping of emitted longwave from the earth that causes global warming. The principal greenhouse gases are: water vapour (H2O), carbon dioxide (CO2), Ozone (O3), Methane (CH4) and nitrous oxide (N2O).
SOIL MANAGEMENT PRACTICES AND GREENHOUSE GAS EMISSIONS Agriculture and soil management practices that contribute to greenhouse gas emissions are: • Animal production • Agricultural Residue Burning • Application of nitrogen mineral fertilizers • Application of crop residues to soil • The use of nitrogen fixing crops in soil management • Production of paddy rice • Tillage and direct emission from soil • Land use change
1. Animal Production • Enteric Fermentation: Methane production from herbivores is a by-product of enteric fermentation, a digestive process by which carbohydrates are broken down by micro-organisms into simple molecules for absorption into the blood stream. Both ruminants (e.g. cattle, sheep) and non-ruminants (e.g. horses and pigs) produce CH4 although ruminants are the largest source.