Table 5.1 compares typical number concentrations in different size ranges of gases aerosol particles and hydrometeor particles under lower tropospheris conditions. It indicates that number and mass concentrations of gas molecules are much larger than are those of particles. The number concentration of aerosol particles decrease with increasing particle size. The number concentration of hydrometeor particles is typically less than is that of aerosol particles but the mass concentration of hydrometeor particles always exceeds that of aerosol particles. Aerosol particle size distributions can be divided into modes which are regions of the size spectrum space in which district peaks in concentration occur . Usually each mode is described analytically with a log normal function which is a bell curve distribution on a log-log scale as shown in figure 5.1a figure 5.1b shows the same distribution on a log-linear scale.