Definitions
Rain is liquid precipitation, drops of fresh water that fall from clouds, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet.
A cloud is a visible mass of water droplets or frozen ice crystals suspended in the Earth's atmosphere above the surface of the Earth from which rain or other kinds of precipitation fall.
Elementary Level
A cloud is water in the atmosphere (air) that we can see. It is where rain and snow comes from.
Water on the earth evaporates (turns into gas) and goes up into the sky. There, when it is colder, the water condenses: it changes from a gas to drops of water or crystals of ice. We see these drops of water as clouds. The drops fall back down to earth as rain, and then the water evaporates again. This is called the "water cycle".
Clouds on other planets are sometimes just collections of gases.
How Clouds Form
The atmosphere always has a little bit of water vapor that humans can not see. Clouds form when the atmosphere can no longer hold all the invisible air vapor. Any more water vapor condenses into very small water drops.
Warm air holds more water vapor than cool air. So if warm air with lots of water inside cools, it can form a cloud. These are ways air can cool enough to form clouds:
- when it is cooled by the ground at night, making a fog
- along warm and cold fronts (where the weather is very hot or cold),
- where air goes up the side of a mountain and cools as it goes higher
- when warm air goes over a colder thing (such as cool water in a lake).
Clouds are heavy. The water in a cloud can have a mass of several million tons. Every cubic metre of the cloud has only about 5 grams of water in it. Cloud droplets are also about 1000 times heavier than evaporated water, so they are much heavier than air. They do not fall, but stay in the air, because there is warm air all round the heavier water droplets. When water changes from gas to droplets, this makes heat. Because the droplets are very small, they "stick" to the warm air.
Sometimes, clouds appear to be brilliant colors at sunrise or sunset. This has shown to be due to pollution in the air.
Types of Clouds
There are different sorts of cloud, because of how fast the air goes up and if the air stands still or is moving. Also, some clouds make more rain, or make thunder and lightning. These differences come from how big the water droplets are, and how they join together.
There are three basic types of clouds:
- Stratus clouds are low, flat gray clouds. Most stratus clouds are near the sea. They are sometimes called "high fogs". Light rain often falls from stratus clouds. When precipitation falls from stratus clouds they are usually called nimbostratus.
- Cumulus clouds are white puffy clouds. They are usually seen on bright days. They have flat bottoms, but the shape of the top is often different. When a cumulus cloud grows into a thunderstorm, it is called a cumulonimbus cloud.
- Cirrus clouds are high, thin clouds. They are made up of ice crystals intead of water drops. Thin cirrus clouds are sometimes called mares' tails because they look like the tails of a horse.
What is Rain
Rain are droplets of water falling from clouds in the sky that are bigger than 0.5 mm. Droplets of water that are about 0.2mm to 0.5mm big are called drizzle. Rain is a kind of precipitation. Precipitation is any kind of water that falls from clouds in the sky, like rain, hail, sleet and snow. Rain is part of the water cycle.
Convectional rain happens in places of the world that are hot and wet. Sometimes, it also takes place in tropical deserts and inland areas during summer, when temperatures are hot. During the day, the sun makes the ground very cold. Air near the ground surface is heated by conduction. The heated air expands, becoming less dense and rises in a strong upwards air current. When the temperature of the rising air falls to the dew point, water vapor shrinks into thick clouds and forms convection rain. Depending on the temperature it may fall as sleet or snow.
Relief rain usually happens along coastal areas where a line of hills runs along to the coast. When wet onshore wind from the sea meets a mountain, hill and barrier, it is forced to rise along the slope and cools. When the air temperature falls to its dew point, water vapor condenses to form clouds. When the clouds can no longer hold the water droplets, relief rain begins to fall on the windward slope of the mountain. On the leeward slope, air sinks, it is warmed and further dried by compression. Therefore, the leeward slope is known as rain shadow. Moist winds blow in from the sea and are forced to rise over the land. The air cools and the water vapour condenses, forming rain drops. Relief rain is also a very dense and cold mixture of precipitation.
Frontal rain is when a cold front meets a warm air front. The less dense warm air rises and condenses forming clouds. These clouds get heavier and it eventually rains. The cold air front tends to come from the north west and the warm air front comes from the south west.
A Rainstorm is a sudden heavy fall of rain. It may cause floods on land which is not much above the level of the sea for example, flat coastal land and river basins. Too much water can make rivers overflow and cause floods. Also, landslides may happen. This is bad news as people may drown. For example, over 6 million people died just last year from rainstorms.
Advanced Level
Rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface. On Earth, it is the condensation of atmospheric water vapor into drops of water heavy enough to fall, often making it to the surface. Two processes, possibly acting together, can lead to air becoming saturated leading to rainfall: cooling the air or adding water vapor to the air. Virga is precipitation that begins falling to the earth but evaporates before reaching the surface; it is one of the ways air can become saturated. Precipitation forms via collision with other rain drops or ice crystals within a cloud. Rain drops range in size from oblate, pancake-like shapes for larger drops, to small spheres for smaller drops.
Moisture moving along three-dimensional zones of temperature and moisture contrasts known as weather fronts is the major method of rain production. If enough moisture and upward motion is present, precipitation falls from convective clouds (those with strong upward vertical motion) such as cumulonimbus (thunderstorms) which can organize into narrow rainbands. In mountainous areas, heavy precipitation is possible where upslope flow is maximized within windward sides of the terrain at elevation which forces moist air to condense and fall out as rainfall along the sides of mountains. On the leeward side of mountains, desert climates can exist due to the dry air caused by downslope flow which causes heating and drying of the air mass. The movement of the monsoon trough, or intertropical convergence zone, brings rainy seasons to savannah climes. Rain is the primary source of freshwater for most areas of the world, providing suitable conditions for diverse ecosystems, as well as water for hydroelectric power plants and crop irrigation. Rainfall is measured through the use of rain gauges. Rainfall amounts are estimated actively by weather radar and passively by weather satellites.
Rain Formation
Air contains water vapor and the amount of water in a given mass of dry air, known as the Mixing Ratio, is measured in grams of water per kilogram of dry air (g/kg). The amount of moisture in air is also commonly reported as relative humidity; which is the percentage of the total water vapor air can hold at a particular air temperature. How much water vapor a parcel of air can contain before it becomes saturated (100% relative humidity) and forms into a cloud (a group of visible and tiny water and ice particles suspended above the Earth's surface) depends on its temperature. Warmer air can contain more water vapor than cooler air before becoming saturated. Therefore, one way to saturate a parcel of air is to cool it. The dew point is the temperature to which a parcel must be cooled in order to become saturated.
There are four main mechanisms for cooling the air to its dew point: adiabatic cooling, conductive cooling, radiational cooling, and evaporative cooling. Adiabatic cooling occurs when air rises and expands. The air can rise due to convection, large-scale atmospheric motions, or a physical barrier such as a mountain (orographic lift). Conductive cooling occurs when the air comes into contact with a colder surface, usually by being blown from one surface to another, for example from a liquid water surface to colder land. Radiational cooling occurs due to the emission of infrared radiation, either by the air or by the surface underneath. Evaporative cooling occurs when moisture is added to the air through evaporation, which forces the air temperature to cool to its wet-bulb temperature, or until it reaches saturation.
The main ways water vapor is added to the air are: wind convergence into areas of upward motion, precipitation or virga falling from above, daytime heating evaporating water from the surface of oceans, water bodies or wet land, transpiration from plants, cool or dry air moving over warmer water, and lifting air over mountains. Water vapor normally begins to condense on condensation nuclei such as dust, ice, and salt in order to form clouds. Elevated portions of weather fronts (which are three-dimensional in nature) force broad areas of upward motion within the Earth's atmosphere which form clouds decks such as altostratus or cirrostratus. Stratus is a stable cloud deck which tends to form when a cool, stable air mass is trapped underneath a warm air mass. It can also form due to the lifting of advection fog during breezy conditions.
Coalescence occurs when water droplets fuse to create larger water droplets, or when water droplets freeze onto an ice crystal, which is known as the Bergeron process. Air resistance typically causes the water droplets in a cloud to remain stationary. When air turbulence occurs, water droplets collide, producing larger droplets. As these larger water droplets descend, coalescence continues, so that drops become heavy enough to overcome air resistance and fall as rain. Coalescence generally happens most often in clouds above freezing, and is also known as the warm rain process. In clouds below freezing, when ice crystals gain enough mass they begin to fall. This generally requires more mass than coalescence when occurring between the crystal and neighboring water droplets. This process is temperature dependent, as supercooled water droplets only exist in a cloud that is below freezing. In addition, because of the great temperature difference between cloud and ground level, these ice crystals may melt as they fall and become rain.
More about Clouds
A cloud is a visible mass of water droplets or frozen ice crystals suspended in the Earth's atmosphere above the surface of the Earth or other planetary body. Clouds in the Earth's atmosphere are studied in the nephology or cloud physics branch of meteorology. Two processes, possibly acting together, can lead to air becoming saturated: cooling the air or adding water vapor to the air. Generally, precipitation will fall to the surface; an exception is virga which evaporates before reaching the surface. Clouds can show convective development like cumulus, be in the form of layered sheets such as stratus, or appear in thin fibrous wisps as with cirrus. Prefixes are used in connection with clouds: strato for low cumulus-category clouds that show some stratiform characteristics, nimbo for low to middle stratiform clouds that can produce moderate to heavy precipitation, alto for middle clouds, and cirro for high clouds. Whether or not a cloud is low, middle, or high level depends on how far above the ground its base forms. Some cloud types can form in the low or middle ranges depending on the moisture content of the air. Clouds have Latin names due to the popular adaptation of Luke Howard's cloud categorization system, which began to spread in popularity during December 1802. Synoptic surface weather observations use code numbers for the types of tropospheric cloud visible at each scheduled observation time based on the height and physical appearance of the clouds. While a majority of clouds form in the Earth's troposphere, there are occasions where clouds in the stratosphere and mesosphere are observed. Clouds have been observed on other planets and moons within the Solar System, but due to their different temperature characteristics, they are composed of other substances such as methane, ammonia, or sulfuric acid.
Topics of Interest
A rainbow is an optical and meteorological phenomenon that causes a spectrum of light to appear in the sky when the Sun shines on to droplets of moisture in the Earth's atmosphere. It takes the form of a multicoloured arc. Rainbows caused by sunlight always appear in the section of sky directly opposite the sun.
Acid rain is a rain or any other form of precipitation that is unusually acidic, i.e. elevated levels of hydrogen ions (low pH). It can have harmful effects on plants, aquatic animals, and infrastructure through the process of wet deposition. Acid rain is caused by emissions of sulfur dioxide and nitrogen oxides which react with the water molecules in the atmosphere to produce acids. Governments have made efforts since the 1970s to reduce the release of sulfur dioxide into the atmosphere with positive results. Nitrogen oxides can also be produced naturally by lightning strikes and sulfur dioxide is produced by volcanic eruptions.
Cloud seeding, a form of weather modification, is the attempt to change the amount or type of precipitation that falls from clouds, by dispersing substances into the air that serve as cloud condensation or ice nuclei, which alter the microphysical processes within the cloud. The usual intent is to increase precipitation (rain or snow), but hail and fog suppression are also widely practiced in airports.
For more information:
https://en.wikipedia.org/wiki/Rain
https://en.wikipedia.org/wiki/Cloud
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)
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