Hygroscopic ("water attracting") nuclei are microscopic particles (radii less than a few micrometers) that readily permit condensation on their surfaces, even when the relative humidity is less than 100%. Salt, sulfuric acid, and nitric acid particles are hygroscopic nuclei, also commonly referred to as condensation nuclei. Maritime air masses compared with continental air masses contain an abundance of sea salt particles, making conditions more favorable for cloud or fog development at lower relative humidities.

The graph above shows that clouds within continental and maritime air masses typically have significantly different droplet size distributions. The average cloud droplet sizes measured in marine environments are much larger than those over continents, due to the abundance of large condensation nuclei such as sea salt particles. Droplet size distributions in marine environments are also wider and the droplet concentrations lower.