Introduction

In this chapter modes of cloud droplet nucleation and ice-crystal nucleation are examined as well as parameterizations of number concentrations of cloud condensation nuclei and ice nuclei. The nuclei in general are small aerosols of various sizes called Aitken aerosols O(10−2 μm), large aerosols O(10−1 μm), giant aerosols O(100 μm), and ultra-giant aerosols O(101 to 102 μm). Nucleation by cloud condensation nuclei and ice nuclei is called heterogeneous nucleation as it involves a foreign substance on which cloud water and ice water can form, compared to homogeneous nucleation, for which no foreign substance is needed for nucleation. Supersaturations have to exceed values not found on Earth (e.g. 400%) for homogeneous nucleation of liquid droplets, which is discussed at length in Pruppacher and Klett (1997). An examination of the Kelvin curve described in the next section shows why this is so. As homogeneous nucleation does not occur on Earth for liquid particles, it is not parameterized in models. In general, cloud condensation nuclei made of some salt compound such as sodium chloride (table salt) are the most effective for heterogeneous nucleation of liquid droplets. Heterogeneous nucleation of liquids can be a function of several variables, such as temperature, vapor pressure or supersaturation, pressure, and factors or activation coefficients related to the composition of aerosols involved. As a result the means of expressing heterogeneous nucleation have become more complex over the years as a result of new observations and new techniques to represent nuclei numbers.