In order to get the process of condensation started, it is essential that there should be an abundant supply of microscopic dust particles in the air.
Earlier the weather scientists held the view that the tiny particles of dust, of whatever type they may be, were sufficient for the process of condensation, but of late, it was discovered that condensation does not occur on ordinary dust particles.
For this purpose only such particles are necessary as are capable of attracting or absorbing water. These condensation nuclei are called hygroscopic nuclei. Various types of hygroscopic nuclei ordinarily present in the atmosphere are the following:
(1) Sea salts which include sodium chloride and magnesium chloride, etc.
(2) Sulphur dioxide (SO2), which oxidizes to sulphur trioxide (SO3) which becomes sulphuric acid when mixed with water.
(3) Oxides of nitrogen, especially nitric oxides.
Active volcanoes, ocean spray, forest fires, and meteors which burn out in the atmosphere are the sources of a large number of such hygroscopic nuclei. Salt particles carried over the continents from the oceans by winds are supposed to be the most powerful condensation nuclei.
Besides, such products of combustion as have sulfurous and nitrous acids are also the most effective and active condensation nuclei. According to Pettersen, the salt nuclei may vary in size from 0.1 to 1 micron. A few ‘giant nuclei’ may be as large as 5 microns.
The density of these nuclei varies from It) to 1000 per cm3. The nuclei that are the product of combustion are invariably smaller in size. According to Bruni’s estimate, the number of these particles varies from 2,000 to 50,000 per cubic centimeter.
These nuclei are not uniformly distributed in the air. Their density varies from place to place. In the vicinity of industrial towns the number exceeds 1, 00,000 per cubic centimeter. That is why the atmosphere in large industrial centers is always hazy or foggy (other conditions being favourable).