Geography & EVS

Distribution of Temperature:The sun is continuously radiating heat energy into space. This energy is known as solar radiation. Of the total radiant energy received on the outer surface of the earth’s atmosphere only less than half reaches the earth’s surface. The heat is lost in its way either by scattering and reflection or by absorption by water vapor and ozone etc. The incoming solar radiation received on the earth’s surface is known as the insolation. The amount of insolation varies from one place to the other under the influence of the factors such as the latitude of the place, the nature of the surface and the duration of the sunlight. Vertical rays of the sun in lower latitudes are more effective in heating the surface of the earth than the oblique rays in higher latitudes. Also in general the insolation over the oceans and lakes etc. is more effective than over the land masses and over the snow surfaces.The total annual insolation is maximum near the tropics and at the equator and goes on decreasing towards the poles. The radiant energy which passes through the atmosphere does not appreciably heat the atmosphere on its onward journey. This is so because the incoming radiation from the sun is in form of short wave radiation and the atmosphere cannot absorb the short wave radiation. Instead the earth absorbs the insolation and radiates this energy to the atmosphere in the form of long wave radiation which is absorbed by the water vapor and the carbon dioxide in the atmosphere. Thus the atmosphere receives heat energy second hand via the earth surface. The radiation emitted by the earth is known as terrestrial radiation.So the atmospheric temperature is highest not at 12 noon but around 2 p.m. The other processes by which the atmosphere gets heated are convection and advection. The transfer of heat from warmer parts to the cooler parts by the movement of heated particles moving upwards in the atmosphere is called convection. The process of horizontal transfer of heat by winds is known as advection. These factors affect both the horizontal and the vertical distribution of temperature. The horizontal distribution of temperature is governed by latitude, relative distribution of land and sea, winds, ocean currents etc. The horizontal distribution of temperature is shown on a map by means of isotherms. They are the lines joining places with an equal temperature. The line of the highest mean annual temperature is called the mean thermal equator and it is situated slightly north of the geographical equator.The atmosphere is heated mainly from below. Therefore the lowest layer of the atmosphere in contact with the earth’s surface is the warmest and the temperature decreases as the height increases. Occasionally the temperature in the lower layers of the atmosphere is I found to be increasing with height in stead of decreasing. Such a condition is called the inversion of temperature. The inversion may occur when the surface of the earth cools rapidly during long winter night and the air coming in contact with it becomes colder while the air in the upper zone of the atmosphere is still warmer and has not lost all the heat received during the day. The inversion may also occur when an air mass blowing over a very cold surface such as a snow filed blows over a comparatively warmer area and the warmer local air is uplifted by the invading colder air. This happens due to advection from colder to the warmer regions. This phenomenon is observed in mountain valleys also where the air on the upper slopes gets cooled very rapidly at night and the cold air descends down the valley side slopes at night pushing the warmer valley air upwards. The temperature inversion in the mountains prevents occurrence of frost on the middle and upper slopes of valleys while the cold air filling the valley bottoms causes frost in the lower areas. Many of the tourist resorts and hotels etc. in the mountain regions are also constructed at the upper slopes of the valleys so that they do not experience very cold conditions in winter.The isotherms on a map of the world trend east to west or parallel to the latitudes. However, due to the variations of temperature on account of land and sea distribution, they are not perfectly parallel to the isobars and they do not remain straight lines also. In the month of July the isobars bend towards north as one moves from the oceans to the continents. On the other hand in the month of January, the isobars bend towards south as they move from oceans to the continents. This is a result of the differential heating of the land and the sea.On the basis of the temperature the world is divided into three main temperature belts, the torrid zone extending between the Tropic of Cancer and the Tropic of Capricorn; the temperate zone extending between the tropics and the polar circles; and the frigid zones extending poleward of the polar circles.

SPHERICAL COORDINATE SYSTEM: We are all familiar about Cartesian coordinate system. It is the most common system. It is either two-dimension or three-dimension. In two-dimension, it has x and y axes. While in three-dimension, it has x, y, and z axes. The two figures shown are the 2-d (figure 1) and 3-d system (figure 2), respectively.We can also convert a given Cartesian coordinate system, either 2-d or 3-d, to another system like spherical. If we have x, y, and z for Cartesian, we have also r, ?, and f for Spherical system. Figure 3 shows a spherical coordinate system.To convert (r, ?, f) to (x, y, z):x = r sin? cosfy= r sin? sinfz= r cos?To convert (x, y, z) to (r, ?, f):r = v(x2 + y2 + z2)? = arccos (z/r)f = arctan(y/x)Lunar & Solar Eclipse :A “lunar eclipse” and a “solar eclipse” refer to events involving three celestial bodies: the Sun (“solar”), the moon (“lunar”), and the Earth. A lunar eclipse occurs when the Earth passes between the Moon and the Sun, and the Earth’s shadow obscures the moon or a portion of it. A solar eclipse occurs when the Moon passes between the Earth and the Sun, blocking all or a portion of the Sun. An eclipse can be total, partial, or annular. A total solar eclipse is when the moon blocks out the Sun entirely, a partial eclipse is when it blocks out a portion of the Sun, and an annular eclipse is when the moon is at its furthest point in orbit. It will not cover the Sun completely that’s when you can see a thin ring of light emerging from the outside rim of the moon. How are a lunar eclipse and solar eclipse different? A lunar eclipse occurs at night and a solar eclipse occurs during the day. There are only certain times when either of them can occur. A lunar eclipse can only occur when the moon is directly opposite the Sun in the sky — a full moon. Even though there is a full moon each month, obviously a lunar eclipse does not occur on a monthly basis because the Sun isn’t exactly in line with the Earth and the moon. The moon’s orbit is actually tilted 5 degrees more than that of the Earth; otherwise, we would see a lunar eclipse each month. We can see lunar eclipses more readily than solar eclipses, and it has to do with proximity. The Moon is much closer to the Earth (well over 300 times closer than the Sun!), so the Earth has a much greater chance of blocking sunlight to the Moon, compared to the Moon blocking light from the Sun. Also, a lunar eclipse can be seen from a greater portion of the Earth. Solar eclipses, on the other hand, are more rare and when they do happen can only be seen by a very narrow segment of people on Earth, for a short period of time. It is quite safe to watch a lunar eclipse with the naked eye, while watching a solar eclipse without eyewear protection can seriously damage your eyesight. You can use a telescope to get a clearer view of the moon during an eclipse and really see what is happening. A solar eclipse has always had a more profound effect on humans than a lunar eclipse. This is probably because of the importance of the Sun to all life on Earth. In ancient China, a solar eclipse was thought to be the dragon coming to eat the Sun. The effect that an eclipse has on all life on Earth is of particular interest to scientists. They eagerly await a solar eclipse because it helps them to gather more knowledge about the Sun and its position with respect to Earth.     iibm

Earth’s Rotation and Revolution: Being a planet of the sun, the earth revolves around the sun. Besides this it also rotates on its own axis. Thus the earth has two motions, the rotation and the revolution.RotationThe earth rotates on its axis from west to east. The axis is an imaginary line passing through the northern and the southern poles of the earth. The rotation of the earth is completed in about 24 hours and it is called the daily motion of the earth. This motion is responsible for the occurrence of day and night. One rotation of the earth is completed when a given heavenly body crosses the observer’s meridian two times in succession. The period of rotation is calculated with reference to a star and with reference to the sun. When it is calculated with reference to a star, it is called a sidereal day and when it is calculated with reference to the sun, it is called a solar day.The solar day is a time period of 24 hours and the duration of a sidereal day is 23 hours 56 minutes. This difference of four minutes between a solar day and a sidereal day is due to the fact that the position of the earth keeps changing with reference to the sun due to the earth’s revolution around it while with reference to a star at infinity it will remain unchanged. Thus a sidereal day is the actual time taken by the earth for a rotation of exactly 360 degrees on its axis.As a result of the apparent motion of the sun, the appearance of the night sky as seen from the earth changes from day to day. The stars rise every day 4 minutes earlier than the preceding day and every night one degree of celestial sphere is added to the eastern sky while one degree is lost from the western sky.RevolutionThe movement of the earth around the sun in its orbit is called revolution. This movement of the earth is also from west to east. The period of revolution is one year (365 1/4 days). The orbit of the earth around the sun is elliptical and not circular. Due to this the distance between the earth and the sun keeps changing. When this distance is minimum the earth is said to be in perihelion (around January 3) and when the distance is the maximum, the earth is said to be in aphelion (around July 4). The average of the maximum and minimum distances is called the mean distance and this distance of the sun from the earth is 150 million kilometers.The apparent annual track of the sun through the fixed stars in the celestial sphere is called the ecliptic and an imaginary plane passing through this plane and extending outward through all points is called the plane of ecliptic. This plane is imagined to be horizontal. The axis of the earth (rotational or polar axis) makes an angle of 66 1/2 to the plane of ecliptic. The earth’s axis points constantly to the same point (the polar star) in the celestial sphere. As a consequence the latitude on the surface of the earth at which the sun’s rays fall vertically keeps changing as the earth moves it its orbit around the sun. Due to this the earth attains four critical positions with reference to the sun. On 21st March the earth is so positioned with reference to the sun that the sun’s rays are vertical at the equator and the entire world experiences equal day and night. A similar situation occurs on September 23. These positions of the earth are called the equinoxes the vernal equinox and the autumnal equinox, respectively). On 21st of June the sun’s rays are vertical over the Tropic of Cancer as the north pole of the earth is inclined at its maximum towards the sun. This position is called the summer solstice and at this time the north pole experiences a long continuous day and the south pole a long continuous night. The northern hemisphere has the summer season at this time and the southern hemisphere experiences winter now. Also the days are longer than the nights in the northern hemisphere at this time. On December 22, the position of the earth with respect to the sun is such that the south pole is inclined at its maximum towards the sun and the Tropic of Cancer receives the vertical rays of the sun. This position is called the winter solstice when the sun shines continuously in the south polar region and it is a long continuous night at the north pole. This is the winter season in the northern hemisphere and the summer in the southern hemisphere. During the winter solstice the days are longer than the nights in the southern hemisphere. Thus, the variation in the duration of day and night and the change of seasons are due to the earth’s revolution and the inclination of the axis of the earth. Also the seasons are reversed from the northern to the southern hemisphere.At the time of the summer solstice there is continuous day at the north pole as the circle of illumination passes through the Arctic Circle beyond the pole. As a result at the places situated beyond the Arctic Circle the sun will be visible continuously even when it is night at the same longitude in the lower latitudes. This phenomenon is called the midnight sun. A comparable situation occurs in the Antarctic Circle during the winter solstice when the circle of illumination passes through this circle beyond the south pole.

24. Dead zones • Dead zones are large areas in the ocean that have low oxygen concentration. • The marine life in these areas mostly suffocates and dies or if they are mobile like the fish then, they leave the area. • Though at many times, dead zones occur naturally, scientists are also of the opinion that they are created due to increased human activity. • The main cause of the zones created by humans is nutrient pollution. Excess nutrients (nitrogen and phosphorus) can result in the overgrowth of algae, which later decomposes in the water consuming excess oxygen, depleting the supply available for the marine life. Dead zones can be found in virtually every oceanic body, the largest encompassing almost the whole bottom of the Baltic Sea. Another large dead zone is located in the Gulf of Mexico. Other dead zones occur off the western coastsof North and South America and off the coast of Namibia and western coast of India. Global warming triggered by climate change is predicted to lead to an expansion of these dead zones. However, it is not certain whether the climate change would lead to the removal of the last traces of oxygen from the bay as well.