Rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain forms when a layer of the atmosphere has temperatures that cause the condensation of water vapor into drops of water heavy enough to fall to earth. Two processes, possibly acting together, can lead to air becoming saturated enough to form rainfall: cooling the air or adding water vapor to the air.
Raindrops have sizes ranging from 0.1 millimetres (0.0039 in) to 9 millimetres (0.35 in). Drops larger than this usually tend to break up. Smaller drops are called cloud droplets, and their shape is spherical. As a raindrop increases in size, its shape becomes more oblate, with its largest cross-section facing the oncoming airflow. Large rain drops become increasingly flattened on the bottom, like hamburger buns or parachutes. Contrary to popular belief, their shape does not resemble a teardrop. The biggest raindrops on Earth were recorded over Brazil and the Marshall Islands in 2004 — some of them were as large as 10 millimetres (0.39 in). Rain drops hit the Earth at a velocity of between 4.5 and 20 miles per hour.
Air contains water vapor and the amount of water in a given mass of dry air determines whether rain can form. The amount of moisture in air is referred to as its relative humidity, which is the percentage of water vapor the air can hold at a particular temperature. The temperature of the air mass determines how much water vapor it can hold before it becomes saturated (100% relative humidity). The dew point is the temperature to which an air mass must be cooled in order to become saturated.
Evidence of an air mass reaching full saturation is when water vapor forms into a cloud. Clouds are groups of tiny visible water and ice particles suspended above the Earth's surface. Warmer air can contain more water vapor than cooler air. Therefore, one way to saturate an air mass is to cool it.
Clouds stay suspended above the earth because air resistance causes the water droplets to remain stationary. When wind causes air turbulence that moves the cloud, water droplets within the cloud collide, producing larger droplets. As these larger water droplets descend, they continue to collide and combine (coalescence) allowing the drops to become heavy enough to overcome air resistance and fall as rain. Sometimes, however, there is not enough moisture to allow the droplets to grow enough to continue their descent to earth. Virga is precipitation that begins falling to the Earth but evaporates before reaching the surface.
There are four main mechanisms for cooling the air to its dew point: (1) adiabatic cooling, (2) conductive cooling, (3) radiational cooling, and (4) evaporative cooling.
(1) Adiabatic cooling occurs when air rises to cooler layers of the atmosphere and expands. The air can rise due to convection, which are large-scale atmospheric motions causing moist air to rise, condense into convective clouds. When enough moisture accumulates, rain drops form and fall as precipitation. This same process occurs in mountainous areas. Heavy precipitation is possible when moist air is forced up the slope to an elevation with a temperature cool enough to cause the moist air to condense and fall out as rainfall along the sides of mountains.
{2} Conductive cooling occurs when an air mass comes into contact with a colder surface, such as when air blows over a warm lake or ocean onto the surface of colder land (lake effect).
(3) Radiational cooling occurs when heat is radiated into space from the surface of the Earth.
(4) Evaporative cooling occurs when moisture is added to the air through evaporation, which forces the air temperature to cool and reach saturation.
Let's face it rain is something that not a lot of us desire. However rain plays a key role in the cycle of returning water back to our earth. So it is obvious that we need rain in order for the world to continue functioning.
Top Facts about Rain
1. The umbrella was originally invented to protect people from the hot sun.
2. Rain starts off as ice or snow crystals at cloud level.
3. Light rain is classified as being no more then 0.10 inches of rain an hour.
4. Heavy rain is classified as being more then 0.30 inches of rain an hour.
5. Louisiana is the wettest state in the U.S, which received an annual rainfall of 56 inches.
6. Rain drops do not fall in a tear drop shape, they originally fall in the shape of a flat oval.
7. Rain that freezes before it hits the ground is known as frozen rain.
8. Rain is recycled water that evaporated from our worlds lakes, rivers, oceans, seas etc.
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