What is Cooling Tower? What is the white cloudy stuff rising from it?

 The main function of cooling tower is to cool down and condense the hot water vapour coming out of the power plant and get liquid water back to use it again.

Inside the cooling tower there is only mesh structure made of pipe lines, in which hot water vapour is supplied and cooled down into liquid vapour but natural convection, as shown below.

But nothing is 100% efficient. During this cooling process, about 2% of water vapour escapes from condensation and get out of the tower from the top and that we see as white cloudy stuff. In a huge size of cooling tower of power plant running on full load, water escapes 6000 gallon per hour in the form of vapour.

Shape of cooling tower helps to increase the cooling rate by creating the flow of air inside of it. Warm, moist air naturally rises due to the density differential compared to the dry, cooler outside air. Warm and moist air is less dense than drier air at the same pressure. This moist air buoyancy produces an upwards current of air through the tower. This creates the low pressure region at the bottom of Cooling Tower allowing to flow the air from the outside to inside, from bottom to top. This effect is called Stack Effect.

Image credit: Google