Rain occurs when the water held in warm, damp air condenses and falls. The primary difference between light rain and heavy rain is the amount of moisture that is present in the air mass, which is proportional to the size of the air mass. Typically, heavy rains are caused by pressure fronts, while lighter rains are caused by convective processes.
In addition to rain that is caused by pressure fronts, other atmospheric phenomena cause heavy rain. For example, low pressure zones, such as those surrounding tropical depressions, tropical storms or hurricanes, often cause large amounts of rain to fall.
Heavy rainfall can cause extensive damage to people and property. Torrential downpours can cause widespread flooding and mudslides. Additionally, dangerous lightning and high-speed winds often accompany heavy rains, especially if they were caused by a tropical storm. If the temperature differential between two adjacent fronts is extreme, the rain storm tends to be stronger.
Under some conditions, the precipitation may fall as snow, freezing rain or sleet instead of rain. For example, if the temperature and pressure surrounding a rain cloud are within a narrow range of parameters, the water may become super-cooled and freeze immediately upon contact with a surface.