Clouds form when water vapor in moist packets of air condenses onto atmospheric aerosols, such as particles of dust. The transition from dry particle to liquid water droplet is known as activation. The threshold for activation is a function of an aerosol's size and chemical composition, as well as of the relative humidity of the local air. As a given particle's size and composition are fixed, activation occurs principally when a packet of air crosses a water saturation level called critical supersaturation.