The force exerted on the surface by the column of air above it is called atmospheric pressure or air pressure. It is measured by means of an instrument called barometer.

In urban and industrial areas smoke provides plenty of nuclei which helps in the formation of fog and mist. Smog is a condition when fog is mixed with smoke.

Warm ocean currents raise the temperature in cold areas while the cold currents decrease the temperature in warm ocean areas.The Labrador current which is a cold current, lowers the temperature near the north-east coast of North America.

Humidity is the amount of water vapour present in air. Absolute Humidity is the weight of actual amount of water vapour present in unit volume of air.Relative Humidity is the Ratio of water vapour in air at a particular temp. to total amount of water vapour required to saturate the same air at same temperature. It is expressed in %Specific Humidity is the Weight of water vapour present in per unit weight of air. It is not affected by change in pressure or temperature.

Wind is an air movement having both direction and speed. Depending on the cause of the winds, they can blow in many directions.The Easterly winds blow from east to west whereas the westerly winds blow from west to east.

Salinity is directly positively related to seawater density.On an average, seawater density increases with increasing salinity and vice versa.E.g. Density of pure water is 1.00 g/cm3 and density of water with 35% salinity is 1.028 g/cm3.

Coral lives a symbiotic life. Inside the sac of each coral polyp lives one-celled algae called zooxanthellae.Algae gives off oxygen and other nutrients that the coral polyp needs to live and in return the polyp gives the algae carbon dioxide and other substances the algae needs.

A tribe can be defined as an aggregate of people linked together by means of social ties, a common language and the recognition of a common cultural heritage. Amerinds – The term is applied to the people of mixed European and American Indian blood in North America.

Period of Stable Population (1891 to 1921) –  Between 1891 and 1921, rate of growth of population in India, was low. In these 30 years, population increased by 1.26 crore.It was so because in these years, calamities and epidemics, like famines, plague, malaria etc took a heavy toll of human lives. The epidemic in 1918, took a toll of 140 lakh human lives.

Demographic Dividend – The opportunities provided by a country’s demographics in which the working population is much more than the dependent population is called demographic divided. For taking advantage of this dividend, skill development is must as it allows the youth to get adequate and meaningful jobs.