Class VI

PBL Weather and Climate

Reading Material

Atmosphere; Composition and Structure

INTRODUCTION

The Earth is the only known planet on which life exists. The present condition and properties of the Earth’s atmosphere support life. But, what is atmosphere? Atmosphere is a thin gaseous layer surrounding the Earth. It is very thin when compared to the size of the Earth and contains a mixture of gases, yet, this thin layer has its own influences on various processes that take place on Earth.

The atmosphere is special because it contains life-sustaining oxygen in large quantities (about 21% by volume). In fact, it took millions of years and a wide range of different processes in order to reach its present condition .

The atmosphere constantly exchanges energy and matter with other components of the Earth – the lithosphere, hydrosphere and biosphere. Our atmosphere extends from a few meters below the Earth’s surface to a height of about 60,000 km. However, about 90% of the atmosphere is within the first few kilometresfrom the ground. This means that most of the mass of the atmosphere is near planetary surface as the gravity pulls the atmosphere towards the Earth’s centre.

THE IMPORTANCE OF ATMOSPHERE

Our atmosphere is very important for sustaining life on Earth, containing life-supporting oxygen in large quantities. A constant concentration of oxygen is maintained through the oxygen cycle. The oxygen cycle does not take place in isolation but it takes place along with other bio-geo chemical cycles. These cycles connect the atmosphere with the hydrosphere (water on Earth), lithosphere (land on Earth) and biosphere (life on Earth). The atmosphere does not function by itself, but it functions in conformity with other spheres on Earth.

Our atmosphere is the transparent layer through which, life-sustaining solar radiation passes and reaches the Earth’s surface. Solar radiation is the only source of energy for photosynthesis to take place on Earth supporting plant life and therefore all other forms of life.

COMPOSITION OF THE ATMOSPHERE

The Earth’s lower atmosphere is a mixture of many gases called air. The two main gases in air are nitrogen and oxygen. Together, they form about 99 per cent of dry air by volume. The remaining 1 per cent is mostly argon and carbon dioxide. The atmosphere also contains tiny amounts of helium, hydrogen, neon, ozone, krypton, and other gases.

  • Nitrogen- 78%
  • Oxygen- 21%
  • Argon- almost 1%
  • Carbon Dioxide- .03%
  • All Other Gases- .01%

Gas molecules at the bottom of the atmosphere are squeezed together by the gases above them. As a result, 99 per cent of the atmosphere’s weight is found within about 32 kilometres of Earth’s surface. Half the atmosphere’s weight is within 5.5 kilometres.

Air always contains some water vapour. Water vapour enters the air by evaporation from the oceans and from water or plants on land. The amount of water vapour varies with location, season, and time of day. Most of the water vapour is near the surface, and the percentage decreases with height.

Dust, another part of air, includes tiny grains of rock, dirt, pollen, salt crystals from sea spray, soot from fires, chemicals from factories, and bacteria. Dust helps form fog and rain. Water vapour condenses around some dust grains, forming tiny water droplets.

Incoming Solar Radiation and Ozone Layer

When solar radiation passes through the atmosphere, harmful ultra violet radiation gets “absorbed” by the ozone layer, which is also a part of our atmosphere. The ozone layer prevents about 95% of harmful ultra-violet radiation from reaching the Earth’s surface. When solar radiation passes through the troposphere (the lowest layer of the a atmosphere) part of the radiation is reflected back by clouds, scattered by atmospheric constituents (thereby reducing visibility) absorbed by atmospheric constituents and the remainder reach the Earth’s surface (land or water).

Outgoing Backward Radiation and the Greenhouse Effect

The heated Earth emits energy in the form of infrared radiation at night and this radiation is absorbed by carbon dioxide, water and a few other gases. This process results in the “greenhouse effect”. Thus, the atmosphere is kept warm during night; otherwise it would become extremely cold and intolerable for living organisms.

Weather Patterns

The Earth is not heated by solar radiation uniformly due to its inclination. As a result, different weather patterns exist across the Earth at any one time. In order to compensate for these differences, air circulates which results in winds. Winds can be of a global scale as well as on a local scale. They can be responsible for disastrous storms like cyclones, dust storms, and tornadoes etc. These wind currents also influence water currents in the oceans which in turn also affect wind currents. Consequently, understanding the atmosphere and its functions and behaviour is quite complex. Because of all these processes, the atmosphere is very dynamic.

STRUCTURE OF THE ATMOSPHERE

Scientists divide the atmosphere into four layers based on temperature changes.

The Troposphere

The layer closest to the Earth is called the troposphere. The troposphere starts at the Earth’s surface. Its thickness depends on the latitude. At the equator the troposphere is about 18 kilometres thick; at the poles it is only about 8 kilometres thick. The gases of the troposphere are essential to life on Earth. The Earth’s weather occurs in the troposphere. Temperatures gradually decrease with altitude in the troposphere. The top of the troposphere is called the tropopause. There, the decrease in temperature stops. At the poles, the tropopause temperature is about -55 degrees Celsius.

Stratosphere

The second layer is the stratosphere. It reaches from the tropopause to a height of about 50 kilometres from the Earth. The stratosphere is clear and dry. It has strong, steady winds and only few weather changes. Because of its steady weather conditions, jet aircraft fly in the stratosphere. The lower part of the stratosphere is as cold as the tropopause, then it warms up steadily to its top, at thestratopause. The absorption of sunlight by ozone is what makes the stratosphere’s temperatures increase with height. The ozonosphere is therefore located in the stratosphere.

Mesosphere and Thermosphere

The third and fourth layers are the mesosphere, in which temperatures drop again, and the thermosphere, in which temperatures rise again. The top of the thermosphere is around 500 kilometres from the Earth. In the thermosphere, nitrogen and oxygen atoms absorb solar energy, causing the temperature to rise.

Ionosphere

At heights between about 65 and 500 kilometres above the Earth, the air is highly ionised. The ions are formed when ultraviolet rays from the sun knock electrons off oxygen atoms. This part of the atmosphere is called the ionosphere. It stretches from the lower mesosphere to the top of the thermosphere. The ions and electrons are concentrated in layers at four different levels. Each layer reflects radio waves of different wavelengths. The ionosphere therefore reflects radio waves back to Earth which greatly increases the area in which they can be received. The ionosphere does not reflect the waves used to transmit television. These waves, however, can be picked up and re-broadcast by special satellites orbiting high above the Earth. The ionosphere is affected by solar events. Eruptions on the sun send out large amounts of short-wave radiation which disrupts radio communication.

The Auroras

Solar eruptions also send out ionised particles. Since they are electrically charged, these particles are deflected by the Earth’s magnetic field to the North and South Poles. At the poles, the ionised particles interact with air molecules to form auroras, coloured displays of light in the night time sky.

The Project

Introduction:

The reading material talks about the major components of the atmosphere and their classification in layers based upon thermal changes. The basic information about Auroras and incoming and outgoing solar radiation has also been given and in the end the importance of atmosphere for land and people has been explained in detail.

The Task:

  • The teacher will make groups and the students will be provided with a graph paper. They will plot the temperature of different layers accordingly after reading the material carefully. They will label the layers as well. Show temperatures in degree centigrade on the X axis and altitude in km above sea level on Y axis.
  • They will scan the graph paper and add that to the power point presentation or they can straight away plot the graph on excel and paste it on power point slide.
  • They can make an OHT in case they opt to use OHP for presentation.
  • Separate the boundaries between each layer.
  • Mark the general location of the ozone layer.
  • Mark the ionosphere to know which layer has the ionization.
  • Find out more than what is written in the reading material about the formation of Auroras.
  • Find photographs of Auroras.
  • Find an appropriate video of either of the auroras, not more than 6 minutes duration.
  • Prove that the Earth’s atmosphere is important for the land and people by comparing it with Venus and Mars.

Number of Periods:

Four (4) periods are required to carry out this project.

Success Criteria:

  • The duration of power point presentation or OHT will not be more than 10 minutes.
  • The presentation will include slides beginning with the details of members of group to references in this order

Main topic

Group details

Introduction (what is the task)

Materials used

Video link

Conclusion

References (for materials used)

  • Every group member has to present a part in front of the audience
  • Slides should be relevant to the topic
  • Slides should not be overwhelmingly colourful or contain too much information

Suggestions:

  • Information can be gathered from multiple sources such as different websites of SUPARCO, NASA, HAARP, encyclopedia etc.
  • Students can use different sources which may include facts and figures, tables, charts, diagrams
  • Library period can be integrated with this project.