Our Earth is surrounded by a blanket of air from all sides. This enveloping blanket is called atmosphere. The atmosphere consists of a mixture of more than ten gases. In terms of volume, 78.05% of the atmosphere is nitrogen, 20.94% is oxygen and nearly 1% carbon dioxide. The remaining gases include argon, helium, neon, carbon monoxide, ammonia etc. The atmosphere also contains water vapour and dust particles, hydrocarbon, hydrogen oxide and sulphur compounds in variable amounts.
Since all these gases are different forms of matter, they exert a downward force on the surface of the Earth and are regulated by the force of gravity. Man is rarely conscious of this weight since it presses on him from all directions, and his body is fully adjusted to it. The weight of the atmosphere can be determined by taking into account the masses of these gases and the force of gravity (weight = mass x gravity). We know that one mole of nitrogen weighs 28 gm. and that of oxygen 32 gm. Mole is the basic unit for measuring the amount of any substance in terms of atoms and molecules.
The total weight of the atmosphere is about 5.5 million billion metric tons. More than half of this is upto 5.5 km from the Earth’s surface. In fact, 97% of its total weight is contained within the first 30 km.
Now the question arises why the lower atmospheric layers have more weight?
As we rise from the Earth’s surface, the density of air gradually decreases and hence the atmospheric pressure also decreases. At a height of 5.5 km above ground, the pressure is reduced to half of that at the sea level. The concentration of the gases near the Earth’s surface is more due to the force of gravity of the Earth. The farther we move away from the Earth’s surface, the less is the force of gravity and hence the density of the gases also decreases. That is why the lower layers of the atmosphere contain more weight as compared to upper layers.