Condensation nuclei are tiny particles of solid or liquid in the atmosphere that trigger water vapor to begin condensing into water droplets and forming clouds. Any particles that are attractive to water molecules, namely charged or polar particles, are suitable nuclei, but under normal conditions they must be large enough to likely encounter a water molecule. Once this occurs, each additional molecule helps the nuclei attract more molecules.
Condensation nuclei are essential to the formation of clouds that are actually clustered liquid water droplets suspended in air, and this requires that small particles are somehow carried into the atmosphere. The main ways in which this occur are fires and ocean sprays. The wave actions of oceans can cause tiny droplets of seawater, or even sea salt, to be carried aloft by air currents.
Fires deliver large numbers of polar compounds into the atmosphere. Whereas once, natural fires and the odd volcano were the only ways this occurred, in modern times many human technologies deliver charged particles to the atmosphere. It is uncertain what net effect human activities have on cloud activity, however, as humans tend to suppress natural occurrences of fires as well. When mixed with these more potent materials, even dust from the soil can serve as condensation nuclei.