Small airborne particles on to which water vapour condenses to form cloud droplets; in the absence of CCN it is possible for the relative humidity to exceed 100%. The most effective CCN are 0.2–2.0 μ‎m in diameter and are hygroscopic. Hygroscopic nuclei include a variety of substances, including dust, smoke, sulphate, and salt (NaCl). There are typically 5–6 million CCN per litre of air over land and about 1 million per litre of air over the oceans far from land.