The situation at colder temperatures is similar to that described above, but there are some important differences that arise because of the peculiar properties of water and because of the difference between liquid and solid particle collisions. Because of the strong interactions of water molecules, some extra energy is needed to initiate the growth of very small water particles from vapor. For the growth of liquid droplets in clouds near the surface, the presence of water-containing aerosol particles greatly reduces the amount of energy needed, requiring only a small excess of vapor pressure over the saturated amount (i.e., the relative humidity must only reach values of about 100.1% to form droplets). However, at higher altitudes there are not only many fewer aerosols available but they do not help initiate the growth of an ice crystal nearly as well as they can help droplet growth, so ice clouds do not begin to form until the vapor pressure exceeds saturation by a much larger amount (relative humidity with respect to ice usually must reach values as much as 101%). In fact, many ice clouds start instead by forming liquid droplets at temperatures well below freezing(down to as low as about -30°C) and then freezing them. The peculiar property of water is that at temperatures below freezing the saturation vapor pressure over liquid droplets is much higher than over ice crystals at the same temperature. Once these cold droplets begin growing, they quickly freeze, exposing them to a much higher vapor pressure. The consequence is that the ice crystals grow much more quickly to larger sizes in the range from 20-100 µm and they keep growing below the cloud, reaching sizes of a few hundred microns, because the relative humidity is still > 100% with respect to ice below the initial cloud base. These large particles also collide as they fall, but ice crystals have a more difficult time sticking together; nevertheless, at temperatures nearer freezing, some liquid droplets are encountered that help stick the crystals together. So when the air motions are stronger, very much larger frozen particles can be produced. In the violent vertical motions of strong thunderstorms, for example, the particles can fall and rise many times, producing large hail stones that have been known to reach sizes > 10 cm (105 µm)., Clouds affect the climate but changes in the climate, in turn, affect the clouds. This relationship creates a complicated system of climate feedbacks, in which clouds modulate Earth's radiation and water balances. Clouds cool Earth's surface by reflecting incoming sunlight., The International Satellite Cloud Climatology Project (ISCCP) provides data on various aspects of cloud variables like cloud amount, optical thickness, and cloud top temperature. The data, which have been collected since 1983, are available at 3-hour and monthly intervals with global coverage..