What is the primary reason why convection occurs in the atmosphere?

The answer is it’s because there is unequal heating of Earth's surface. Actually the convection occurs in the troposphere but not in stratosphere because lower altitudes of the troposphere are warmer than higher altitudes, unlike in the stratosphere. Troposphere temperature declines with altitude in the troposphere but increases with altitude in the stratosphere.

places of higher salinity. there are parts of the ocean where hardly any rain falls but warm dry winds cause lots of evaporation. this evaporation removes water – when water vapour rises into the atmosphere, it leaves the salt behind, so the salinity of the seawater increases. this causes the seawater to become denser.