Raindrops form when millions of tiny water droplets collide together in clouds to form larger ones. This process is called coalescence. Eventually, they become too heavy and fall out of the clouds as rain.
Rain can fall from a number of different types of clouds. Nimbostratus clouds can produce rain as well as other types of precipitation. The cumulonimbus clouds of thunderstorms produce rain as well. Very small raindrops are called drizzle. Stratus clouds or stratocumulus clouds sometimes produce a light mist or drizzle. The island of Kauai in Hawaii gets 40 inches (1 meter) of rain in a year - the United States' highest average yearly rainfall.
Rain falls at different intensities that are defined by the National Weather Service. These intensities are light, moderate, and heavy. A rainfall rate for a heavy intensity is greater than 0.30 inches per hour (>0.76 cm/hr).
Although they look like teardrops in cartoons, a raindrop is not really that shape. A raindrop's shape is dependent on its size. Typical raindrops are shaped like hamburger buns because of the large air pressure on the bottom of the drop that flattens it. Air pressure on the sides of the drop is much less than the bottom, allowing the drop to expand in size.
Have you ever noticed an earthy smell in the air that often comes out before it rains? The odor may be from bacteria in soil that produces an aromatic gas. These gases are pushed into the air as rain falls on the soil. Winds are the reason the smell may hit you before the rain shower.
Acid rain falls on industrialized areas with large amounts of pollution. It also falls in places that are downwind from air pollution sources. This type of rain includes acidic air pollutants from power plants, factories, houses, and cars; acid rain can have harmful effects on our ecosystems.