The atmospheric pressure is measured with a mercury barometer and the unit of measurement is millibar (mb).

(i) Isotherms are imaginary lines drawn on maps, connecting points that have equal temperatures.
(ii) Temperature varies from place to place, season to season and continent to continent.

The different atmospheric layers are Troposphere, Stratosphere, Mesosphere, Thermosphere and Exosphere

(i) The blanket of air that surrounds the Earth is called the atmosphere.
(ii) It is held close to the earth by gravitational attraction.
(iii) Atmosphere plays on important role in predicting weather phenomenon.
(iv) It is a mixture of gases, water vapour and dust particles in different proportion.

The term cyclone is a Greek word meaning "coil of a snake". Cyclones are centres of low pressure where, winds from the surrounding high pressure area converge towards the centre in a spiral form. Due to the rotation of the earth, the cyclonic winds in the northern hemisphere move in anti-clockwise direction, whereas they move in clockwise direction in the southern hemisphere.
Cyclones can be classified into Tropical cyclones, Temperate cyclones and Extratropical cyclones.
Tropical cyclones: Tropical cyclones develop in the Intertropical convergence zone [ITCZ]. They are formed due to the differential heating of land and sea. Tropical cyclones are known as 'cyclones' in Indian ocean, 'typhoons' in the western pacific ocean, 'hurricanes' in the Atlantic and eastern Pacific ocean, 'baguios' in Philippines and 'willy willy' in Australia. Tropical cyclones often cause heavy loss of life and property on the coasts and become weak after reaching the landmasses.
Temperate cyclones: Temperate cyclones are formed along a front where hot and cold air masses meet in mid-latitudes between 35° and 65°N and S. Temperate cyclones do not become weak like the tropical cyclones on reaching the land. Temperate cyclone commonly occurs over the North Atlantic Ocean, North West Europe, Mediterranean basin. Mediterranean basin's temperate cyclones extend up to Russia and India in winter. In India it is as called western disturbances.
Extratropical cyclones: Extratropical cyclones occur in the latitudes between 30° and 60° in both the hemispheres. They are also called as mid-latitude cyclones. They collect energy from temperature differences which are found in higher latitudes. Extratropical cyclones produce mild showers to heavy gales, thunderstorms, blizzards, and tornadoes.

Atmosphere:
The blanket of air that surrounds the Earth is called the atmosphere. The five atmospheric layers are Troposphere, Stratosphere, Mesosphere, Thermosphere and Exosphere.
Troposphere: The lowest layer of the atmosphere is the troposphere. The Greek word 'tropos' means 'turn' or change. The layer extends up to 8 kms at the poles and up to 18 kms at the Equator. The temperature decreases with increasing height. Almost all weather phenomenon take place in this layer. Hence it is called weather making layer. The upper limit of the troposphere is called as tropopause.
Stratosphere: Stratosphere lies above the troposphere. It extends to a height of about 50 km above earth surface. Since this layer is a concentration of ozone molecules, it is also referred as ozonosphere. The temperature increases with increase in height in this layer, Large jet planes normally fly here. The upper limit of the stratosphere is called as stratopause.
Mesosphere: Mesosphere extends between 50km and 80km. The temperature increases with increasing height. Radio waves transmitted from earth are reflected back to earth from this layer: Most of the meteors nearing the earth, get burned here. The uppermost limit of the mesosphere is the mesopause.
Thermosphere: Thermosphere exists above the mesosphere. It extends to about 600 km. The composition of gases in the lower thermosphere is more or less uniform, hence it is called "Homosphere". The upper portion of the thermosphere has uneven composition of gases and hence it is referred as "Heterosphere". Here the temperature increases with increasing height. Ionosphere is a layer of the thermosphere that contains Ions. and free electrons.
Exosphere: The uppermost layer of the atmosphere is called exosphere. This layer is extremely rarefied with gases and gradually merges with the outer space. This zone is characterized by aurora Australis and aurora borealis