Introduction to Atmosphere

Introduction to Atmosphere

Atmosphere is the gaseous envelope surrounding our earth. It extends up to 800 km over earth surface. Atmosphere is attached to the earth by the gravitational force of earth. You need to understand that our atmosphere also moves in same speed as well as direction of earth.

Composition of Atmosphere

Atmosphere is a combination of many gasses. The main constituents of the atmosphere are nitrogen and oxygen. Nitrogen constitutes 78 % and oxygen constitutes 21%. In addition, there is one percent of Carbon di Oxide and a few trace gases.

Nitrogen to Oxygen Ratio

The ratio of Nitrogen to Oxygen is 4:1 by volume and 3:1 by weight. The percentage of oxygen in the atmosphere reduces with increasing height. Therefore, supplementary oxygen is required above 10,000 feet.

Homosphere vs Hetrosphere

Up to 80 km from the earth’s surface the constituents of the atmosphere are constant. This is called as Homosphere. Above 80 km the constituents are not constant. This is called as Hetrosphere.

Water Vapour

Water vapour forms very small portion of the atmosphere. In fact, it forms a very small component which cannot be called as trace. However, in Meteorology, water vapour plays a crucial role in weather and climate.

Density of Air

The density of air is maximum near the surface of the earth and reduces as we go higher and higher. In other words, the air density reduces with height or atmosphere becomes rarer with height. In our atmosphere 50% of the air is present between the surface and 6 km. Similarly, 75% of the air column is found below 10 km and 99 % is found below 35 km. As you can see the amount of air above 35 km is just 1%.

Trace Gasses in Atmosphere

There are a lot of trace gasses in the atmosphere. The major trace gasses are Argon (.93%), Carbon di oxide (.035%), Neon, Methane, Helium, Ozone, Hydrogen, Krypton, Xenon, Nitrous di Oxide, Iodine, Carbon Monoxide, Carbon Dioxide, Sulphur Dioxide, Water vapour and some solid particles

Classification of Trace Gasses

The percentage of some of the gases vary from one place to another. These are called variable gasses. Water vapour, Carbon Monoxide, Sulphur Dioxide, Nitrous Dioxide and Methane are examples of variable gasses. Greenhouse gasses are those gasses which partially absorb long wave radiation from the sun. Excess generation of greenhouse gasses is one of the main reasons for global warming. Carbon Dioxide is produced by burning fuel. Thankfully, plants absorb Carbon Dioxide thus keeping it within the limits. Ozone is a gas found in our upper atmosphere which helps in absorbing Ultra Violet Radiation. Ozone is found between the heights of 10 to 50 km from the earth’s surface. The maximum concentration of ozone is found between 20 to 25 km above the surface of earth. The solid particles found in the atmosphere are the primary reason for smog. Examples of solid particles are sea salt or dust particles.

Thermal Structure of Atmosphere

Solar radiation from the sun reaches earth directly without any resistance from the atmosphere. Our atmosphere is heated by the earth’s surface. That is the reason for lower temperature in hill stations. Heat is transferred from one part of the atmosphere to another by various means. Sensible heat or the heat we can measure using a thermometer is transferred by means of conduction, convection, and radiation. Latent heat is the heat which cannot be measured easily by thermometer. The latent heat is transferred by means of evaporation, condensation and sublimation.

Saturation of Air

Saturation is the condition in which the air cannot hold any more of water vapour. Fully saturated air means the air which is already holding the maximum possible amount of water vapour. At higher temperatures, a lot more of water vapour is required for saturation. Therefore, in the tropics, at least 4% of water vapour s required for saturation due to higher temperatures. In the poles, since the temperatures are low, negligible amount of water vapour is sufficient for saturation. When we say ‘saturated air’ we mean that the air parcel consists of at least 4% of water vapour. In other words, the relative humidity is 100 percent (RH = 100 %). Similarly, ‘Un saturated air’ contains less than 4% of water vapour. In such a case the relative humidity would be less than 100 percent. (RH < 100 %)