Atmosphere Ch2

8/13/2019 Atmosphere Ch2

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ATMOSPHERE

Objective: -To understand the structure of the atmosphere

Desired Learning Outcomes:-

1. Study the limits of atmosphere2. Analyze the various layers of atmosphere3. Understand the various important constituents of atmosphere

1. Introduction. The Atmosphere is the gaseous envelope surrounding theearth. In meteorology we study whole atmosphere because any event taking placeany where in the atmosphere, however small, will have repercussions elsewhere,therefore forecasters gather information from an area much wider than that of theirimmediate area of interest.

2. Constituents of Atmosphere. Air is a mixture of various gases and theirproportion in the atmosphere is as follows:-

Nitrogen 78.1%Oxygen 20.9%Argon 0.9%Carbon dioxide, Hydrogen, Ozone Traces

3. Apart from these gases, there are two more important constituents of theatmosphere namely, water and solid particles. Their importance will be discussed inlater chapters.

4. Limits of Atmosphere. There is no clear cut boundary between theatmosphere and space. Atmosphere slowly fades out into space at heightssomewhere in between 500-1000 km. However at 300 km there are enough airmolecules present to ignite an incoming meteor. It is important to note that almosthalf the mass of atmosphere lies below 18000 feet due to gravity of the earth.

Layers of Atmosphere

5. It has been noticed that as one moves away from earth to greater heights theair exhibits different characteristics of temperature. This happens due to certainnatural phenomenon occurring at different heights such as ozone formation,ionization etc. So due to these differences at different heights the whole atmosphere

has been divided into different layers (Fig 2.1). These layers are discussed brieflyhere.


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Fig 2.1

6. Troposphere. This is the layer adjacent to earth and contains the maximummass of atmosphere. This is the only layer which contains water vapors, so all thecloud formation takes place in this layer. Temperature decreases with height at anaverage rate of 1.98 degree centigrade per 1000 feet, till it reaches average value of-56.5OC. Depth of troposphere is maximum at equator (because at highertemperatures atmosphere expands) and minimum at poles (due to very lesstemperatures, atmosphere contracts).

7. Tropopause. At the upper limit of troposphere temperature reaches an

average value of 56.5O

C and then stays constant till certain height before it startsto increase again. This isothermal layer (where temp stays constant) is known astropopause. It is a boundary between troposphere and stratosphere.

8. Stratosphere. The atmospheric layer just above the tropopause is thestratosphere. The average altitude of the top of this layer is 22 miles. Characteristicsof this layer are a slight increase in temperature with increase in height and the nearabsence of water vapors and clouds. Occasionally a strong thunderstorm will breakthrough the tropopause and in very rare instances an aggregate of ice crystals will


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form the "mother of pearl" cloud. In this layer temperature rises because of Ozoneformation.

(a) Ozone layer. Ozone saves all living beings on earth from thedangerous short wave ultra violet radiations by absorbing them and not lettingthem reach the earths surface. Ozone is produced in the following manner

O2+U / V = O+OO2+O = O3 thenO3+U/V=O2+O

Fig 2.2

9. It is a continuous process (as shown in Fig 2.2) and the SW radiations areabsorbed in the process of making and breaking of ozone. In this manner theconcentration of Ozone remains constant and an increase in temperature of airtakes place. In troposphere there is no ozone formation because of lesser intense

radiations and stronger cohesion of oxygen molecules. Above stratosphere inmesosphere oxygen becomes too diffused and an oxygen molecule after break updoes not find another O2in close vicinity to form O3.

10. Except for a substantial increase in the amount of ozone, the composition ofthe stratosphere is the same as that of the troposphere. Ozone is important becauseit absorbs most of the deadly ultra violet rays from the sun. Aircrew flying throughareas of higher ozone concentration may experience irritation to eyes, nose andmouth or coughing symptoms associated with ozone sickness.

11. Mesosphere. Heat absorption stops at the end of stratosphere due toseizure of ozone formation. The temperature starts to drop again with the increase in

height and this marks the beginning of mesosphere. The temperature drops tolowest value in the atmosphere i.e. 86.5OC. Strartopause is the boundary betweenmesosphere and stratosphere.

12. Ionosphere. Due to very low pressures in the upper mesosphere and lowerthermosphere, ionization of gases starts. This part of these two layers is known asionosphere. These ionized atoms can reflect the long wave electromagneticradiations back towards Earth. That is the reason that long wave radio frequenciescan also be heard from the other side of the globe.


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13. Thermosphere. Here mostly the gases are found in atomic form which is sothin that they rarely collide with each other because of which they can accelerate tovery high velocities. That is why the temperatures here shoot up to as high as1500OC (Kinetic temperatures).

14. Exosphere. This is the last layer, in which atoms soar up into space forsome time and then again settle back.

Temperature profile

15. Fig (2.3) shows the average temperature profile in the atmosphere from sealevel to the thermosphere. The temperature at the stratopause is much the same asat Mean Sea Level, while the coldest zone is found at the mesosphere.

Important Constituents of Atmosphere

14. Following are four major atmospheric constituents which play an importantrole in world weather.

15. Water. Water is the only substance which changes states (vapour, liquid,solid) at normal atmospheric conditions and is present in atmosphere in all three

forms. It has following important functions:-

(a) It is responsible for all weather, e.g. rain, clouds, snow, fog, hail etc(Ch-9/10).

(b) It keeps the atmosphere warm by absorbing the out going terrestrialradiations (Ch-3).

Fig 2.3


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(c) It transports heat in atmosphere by absorbing heat during evaporationat one place and releasing it during condensation at a different place (Ch-5).

16. Solid Particles. Microscopic particles of sand, sea salt, smoke, dust,carbon etc suspended in the air play a vital role in condensation and sublimationprocesses (ch-5) to form a rain drop or a snow flake respectively. With out them noweather activity is possible but their abundance causes problem of poor visibility.

17. Carbon Dioxide. Just like water CO2 also absorbs out going long waveterrestrial radiations (Ch-3) thus keeping the atmosphere warm. Without CO2 &water vapors in the atmosphere the temperature of air during night will drop so lowthat existence of life will become questionable. Nowadays the amount of CO2in theatmosphere is increasing continuously due to burning of fossil fuels; as a result moreheat is being absorbed by the atmosphere. This is one of the major reasons thatoverall temperature of the world is on the rise. This phenomenon is also known asGreen House Effect.

18. Oxygen. In ionosphere O2breaks down into atomic form after absorbinghigh temperature short wave solar radiations. In stratosphere also, O2forms ozonethus absorbing heat. All these processes combine to produce comparatively warmlayers in the high atmosphere and also help life to sustain over the earth by saving itfrom deadly U/V radiation.

Questions for Study & Discussion

DLO 1

Till what limits is our atmosphere extended into space?

DLO 2

Explain the structure of the atmosphere in terms of various layers.

DLO 3

What are the important constituents of atmosphere and their role in worldweather?

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Atmosphere Ch2