This image shows raindrops falling into a puddle.
Click on image for full size
Courtesy of University Corporation for Atmospheric Research
Rain is precipitation that falls to the Earth in drops of 5mm or greater in diameter according to the US National Weather Service. Virga is rain that evaporates before reaching the ground.
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 Statesí 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 raindropís 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.
Shop Windows to the Universe Science Store!
The Fall 2009 issue of The Earth Scientist
, which includes articles on student research into building design for earthquakes and a classroom lab on the composition of the Earthís ancient atmosphere, is available in our online store
You might also be interested in:
One process which transfers water from the ground back to the atmosphere is evaporation. Evaporation is when water passes from a liquid phase to a gas phase. Rates of evaporation of water depend on factors...more
Nimbostratus (weather symbol - Ns) clouds are composed of water droplets and belong to the Low Cloud (surface to 2000m up) group. They are dark gray with a ragged base. Nimbostratus clouds are associated...more
Cumulonimbus (weather symbol - Cb) clouds belong to the Clouds with Vertical Growth group. They are generally known as thunderstorm clouds. A cumulonimbus cloud can grow to such heights that it actually...more
Thunderstorms are one of the most thrilling and dangerous types of weather phenomena. Over 40,000 thunderstorms occur throughout the world each day. Thunderstorms form when very warm, moist air rises into...more
Drizzle is light precipitation that is made up of liquid water drops that are smaller than rain drops. Drizzle can be so light that only a millimeter of accumulation is measured at the Earth's surface....more
Stratus (weather symbol - St) clouds consist of water droplets and belong to the Low Cloud (surface-2000m up) group. They are uniform gray in color and can cover most or all of the sky. Stratus clouds...more
Stratocumulus (weather symbol - Sc) clouds consist of water droplets and belong to the Low Cloud (surface-2000m) group. These clouds are low, lumpy, and gray. These clouds can look like cells under a microscope...more