WHAT IS ATMOSPHERE ?

ATMOSPHERE

1.     Introduction to Atmosphere

The surface of the earth is surrounded by a blanket of air and this gaseous sphere surrounding or hanging over the surface of the earth is called the atmosphere. Earth's atmosphere, the blanket of gases surrounding the planet, is the element that, more than any other, enables life to exist. Without its protective insulation, temperature would swing from unbearable cold at night to unbearable heat during the day.

1.2 Composition of the atmosphere

The atmosphere is composed of three elements. They are gases, water vapour and dust particles.

a.       Gas

99% of the atmosphere consists of two gases only- Nitrogen and Oxygen. All other components make less than 1%, while the water vapour content and dust particles differ from place to place, according to temperature conditions.

The gases present in the atmosphere are:

Gases	Percentage
Nitrogen	78.03%
Oxygen	20.99%
Argon	.94%
Carbon Dioxide	.03%
Hydrogen	.01%

b.    Water vapour

Other important part of the atmosphere is water vapour. The sources of the water vapour in the atmosphere are sea, lakes, rivers, soil, plants, etc. we can get water vapour in the atmosphere up to the height of 7500 m only. The percentage of the water vapour decreases as one moves from the equator towards the poles. Water vapour is the only one source of clouds, rainfall, due, hell stone, etc.

c.      Dust Particles

The third important part of the atmosphere is dust particles. Some of these dust particles are visible while others are quite beyond human sight. These dust particles are responsible for such phenomena as sunsets, twilight and haze. There are the nuclei about which condensation takes place. Besides they are effective in absorbing and diffusing solar energy.   

1.3 Extension of the atmosphere

          The earth is surrounded by the gaseous blanket called atmosphere. It is supposed that the atmosphere is extended up to thousands of kilometers from the sea level. 75% of the atmosphere is found up to 11 km from the surface of the earth. 97% of it is found up to the height of 29 km from the surface of the earth. Carbon Dioxide, Nitrogen and Oxygen are the heavier gases which are found in the lower part of the atmosphere.

1.4 Insolation

          The main source of the heat and light to the earth is the sun. The sun is 10 times greater than the earth in diameter and 100000 times larger in the volume. The distance between the earth and the sun is 14,88,00,000 km. It is estimated that the temperature at the surface of the sun is 6,000⁰ Celsius.

Solar heat which is received by the earth is called insolation. Sun is the main source of energy and heat but much of it is lost in its passage through the atmosphere and only a small fraction of it is able to reach the earth.

A.    Factors affecting the Distribution of Insolation

a.     Position of the Sun

b.     Thickness of the Atmosphere

c.      Duration of day and Night

d.     Colour of the Surface

e.      Distance between the earth and the Sun

a. Position of the Sun

          The earth is round and inclined at an angle of to its orbital plane. The insolation is therefore not vertical at all the places. Vertical rays occupy less area than inclined ones. Hence, vertical rays have to heat a smaller area than the inclined ones. The solar radiation is usually vertical at the equator and inclined at the poles. Therefore the equator gets more insolation than the poles.

b. Thickness of the Atmosphere

          Dust particles, water vapours and some gases present in the air absorb, reflect and scatter insolation. If the radius of the obstructing particles is smaller than the wavelength of the insolation, the light is scattered. It is clear that the temperature of lands in higher altitudes is lower that that of the places at lower altitudes. It is why hilly areas even in the equatorial lands are cool.

c. Duration of day and Night

          Except the equatorial areas the length of the days and nights are not equal throughout the year. The days are longer in the summer and shorter in the winter. While the days are shorter the time of the sun shine is short and the days are longer the time of the sun shine is long. The earth gets more sun light while the days are longer.

d. Colour of the Surface

          The colour of the surface of the earth also effects the distribution of the insolation. The shining and the white surface reflect the sun rays but the black colour surface absorbs more sun rays.

e. Distance between the earth and the Sun

          The orbit of the earth is oval. The distance between the sun and the earth is not constant but goes on changing.                                                              The earth is closest to the sun on January 3. This position is called perihelion (164.7 million km away). On July 4, the distance between the sun and the earth is 170.1 million km and the position is called aphelion. More solar radiation is received by the earth when the earth is closer (in perihelion) and less solar radiation when the earth is the farthest (in aphelion).

1.5 Atmospheric Temperature

          The hotness of any place is called the temperature. The hotness of the atmosphere is called the atmospheric temperature. The atmosphere gets the heat mostly by the reflection of the rays from the surface of the earth. The atmosphere gets less heat through the direct radiation of the sun. The methods of getting temperature by the atmosphere are as follows:

a.     Radiation Process

  The surface of the earth gets heat from the sun and it throws the heat towards the atmosphere through radiation process. The heat thrown to the atmosphere is absorbed by dust particles, water vapour, etc. and the air is heated. The cloudy nights are warmer than the clear nights because the heat of the earth's surface cannot go far beyond in the atmosphere by radiation process.

b.    Conduction Process

 Conduction means transmission. Heat is transmitted from heater matter to cooler matter. The lower heated atmosphere heats the upper layer of the atmosphere gradually. This process is called the conduction process.

c.      Convection Process

 While the air is heated it becomes lighter and flows away. The empty place is filled up by cool air. This process is called the convection process. By the convection process whole atmosphere is heated.

d.    Horizontal Transfer of Heat

 The hot air of the equatorial zone is transferred to the poles. In this way whole air is heated. This process is called the horizontal transfer of heat.

1.6 Absorption

 15% to 19% of the solar heat is directly absorbed by the atmosphere because of the presence of water vapour. Water vapour is present in the lower part of the atmosphere. That's why the lower part of the atmosphere is more heated than the upper.

1.7 Measurement of Temperature

           The degree of heat of any body is measured by its temperature. A clinical thermometer is used to measure the temperature of our body. The level of mercury in the tube rises when there is an increase of temperature. When there is a decrease of temperature, the mercury level falls in the thermometer. There are two scales used for graduating a thermometer. These are called Centigrade and Fahrenheit.

a.     Range of Temperature

  The difference of maximum and minimum temperature of a certain place in a certain time is called the range of temperature. For example, if the maximum temperature of Kathmandu on 2^nd July is 30⁰c and the minimum temperature is 20⁰c, the range of temperature of that day is 30⁰ – 20⁰ = 10⁰c. Generally the range of temperature is taken daily, and annually. The range of temperature in the equatorial zone is less and towards the poles is more. The range of temperature is more in the extreme climatic zone and less in the mild climatic zone.

b.    Mean Temperature

  While the maximum and minimum temperature is added and divided the sum by two what we get the result that is called the mean temperature. The mean temperature is also called the average temperature. In this way we can get the mean temperature monthly and annually.

c.      Isotherm

  Isotherm is an imaginary line which joins the places of equal temperature in the maps. The isotherms are not straight because the distribution of the temperature in the world is not equal. It is shown only in the maps. The isotherm does not show the exact temperature because it is located on the basis of altitude from the sea level and mean temperature.

d.    Factors Affecting the Distribution of Temperature

  Temperature is not equal on the surfaces of the earth because the distribution of the temperature is affecting by the different factors. Here are mentioned the factors affecting the distribution of temperature.

                   i.      Latitude

   The 0⁰ latitude which is called the equator gets the vertical (90⁰) sun rays. At 50⁰ N and S latitudes get the angle of 40⁰ sunrays. Like wise at the poles the angle of sun rays is 0⁰. It means increasing the latitudes we can find the decreasing of angle of the sun rays. Vertical sun rays cover the smaller area than the slant sun rays. While the sun rays are more vertical it heats the surface of the earth faster.

                 ii.      Altitude

  The distribution of temperature is affected by many ways. One is the effect of altitude above the sea level. The temperature decreases normally at the rate of 1⁰c per 165 meters of height. The atmosphere gets heated from below upwards by terrestrial radiation. The lower layers of the atmosphere are denser and they contain water vapour, dust and other particles which absorb heat. At higher levels the air is rarified and the absence of water vapour and dust particles permits the escape of radiation back to the space. Therefore the air is cooler in the mountainous regions.

              iii.      Distance from the Sea

  The areas around the oceans experience ocean climate with low daily and annual range of temperature. The landmasses far from the sea experience a continental type of climate. These areas have high daily and annual range of temperature.

       Landmasses get heated faster than water bodies as well as get cooled faster. So the areas around the seas have balance temperature. At day time cooled air blows from the sea to the land and at night warm air blows from the sea to the land. But landmasses far from the seas are more heated at day time and very cooled at night. The temperature is extreme.

              iv.      Prevailing Winds

  Winds modify the temperature of the regions over which they blow, for example, cold winds blowing from the interior of continents during winter reduced the temperature of the places along their paths. Winds blowing from the oceans during summer tend to reduce the temperature of the places along the coast because oceans are cooler than the landmasses.

Winds blowing from the lower latitudes are    warmer than those blowing from higher latitudes.

           v.      Ocean Current

   Ocean currents transport large quantities of water over thousands of kilometers across the oceans. Currents which move from the equatorial to the polar latitudes are warm currents as they transport warm water to cooler regions. Currents which flow from the higher polar latitudes to the equatorial latitude transport cool water to warm regions. These are cold currents along the coastal areas warmer than usual, while the cold currents make the coastal areas cooler than usual.

              vi.      Slope of the Land

     The slope of the land in the hilly and mountain regions differ the temperature because the lands face to the sun rays direction is warmer than the backwards.

     In the Northern hemisphere, the south facing slopes which lies in the shadow.

            vii.      Cloud and Rainfall

During the day, clouds prevent insolation from reaching the earth's surface .Clouds also prevent the escape of terrestrial radiation during the night. The equatorial regions which have cloud On many days experience low range of temperature around 10C.In the tropical deserts clear skies favour high range of temperature around 20c.While the days are very hot, nights are quite chill. 

1.8 Air Pressure

     The earth is surrounded by the air .It is supposed that the air is extended thousands meters above the sea level .The downward force of air of any place is called the air pressure. An   instrument which is used to measure the air pressure is called Barometer.

     We cannot find the equal air pressure on the all surface of the earth .Around1Kg/cm2 air measured in the sea level but increasing the altitude we find the decreasing of the air pressure.

a.  Isobar

          The distribution of the air pressure is not equal in the surfaces of the earth. An imaginary line drawn through the places which have equal atmospheric pressure is called isobar. The lines of isobar are not straight because the atmospheric pressure is varied by different places.

b. Factors Affecting the Air Pressure

          Air has weight. Due to this weight, the air exerts pressure. The pressure exerted by atmosphere at any point is equal in all directions. The atmospheric pressure is measured by Barometer.

                   i.      Temperature

There exists a negative relationship between pressure and temperature. When temperature increases pressure will fall and vice versa. That is why, pressure increase from the equatorial region towards the Polar Regions. Similarly in any place there is low pressure in summer and high pressure in the winter.

                 ii.      Altitude

 

              iii.      Water Vapour

b.                            Pressure Belts of the World

i. Equatorial Low Pressure Belt

           ii.      Sub-tropical High Pressure Belt

        iii.      Sub-polar Low Pressure Belt

        iv.      Polar High Pressure Belt

1.9            Wind

a.     Cause of Wind Origin

b.    Factors Affecting the Direction of the Wind

c.      Kinds of Wind