Whenever one thinks about clouds, invariably, they will also think of rain. Rain is the precipitation of liquid drops of water. There are also other forms of precipitation such as snow, sleet and hail. Raindrops generally have a diameter greater than 0.5 mm (0.02 in). They range in size up to about 3 mm (about 0.13 in) in diameter, and their rate of fall increases, up to 7.6 m (25 ft) per sec with their size. Larger drops tend to be flattened and broken into smaller drops by rapid fall through the air. The precipitation of smaller drops, called drizzle, often severely restricts visibility but usually does not produce significant accumulations of water. Lines drawn through places with the same amount of rainfall are called isohyets.
Amount or volume of rainfall is expressed as the depth of water that collects on a flat surface, and is measured in a rain gauge to the nearest 0.25 mm (0.01 in). Rainfall distribution of a place is described according to the amount of rain received: very heavy (>2000mm); heavy (1500-2000mm); moderate (750-1500mm); light (250-750mm) and very light (0-250mm).
The rain gauge is used to measure rainfall. It consists of 2 copper cylinders: an outer 13cm diameter cylinder and an inner cylinder with a jug. Rain is collected in the bottle by means of a funnel which has a tapered end so that the rain water will not evaporate easily. Rainfall collected in the glass bottle of the rain gauge will be poured into a measuring cylinder that is graduated in millimeters. It has an tapered end to measure small amount of rain accurately.
There are generally three types of rainfall: orographic rain (also known as relief rain), convectional rain and frontal/cyclonic rain.
Artificial precipitation will also be discussed below:
Orographic rain results when near-saturated warm maritime air is forced to rise when confronted by a coastal mountain barrier. As air rises, it expands dynamically due to a decrease of air pressure. Due to expansion, cooling of the air then takes place, followed by condensation into water droplets to form clouds as the relative humidity rises until the dew point reaches the level of condensation, causing air to be saturated. The height at which condensation occurs is called the level of condensation. When the clouds get too heavy to be suspended, heavy rain falls. As the air descends on the leeward side of the mountain range, it becomes compressed and warmed. Condensation ceases creating a rain shadow effect where little rain falls. One example is the Murray basin in S.E.Australia which is sheltered by the Eastern Highlands from SE onshore winds. If the mountain is very high, after crossing the mountain, the descending air is compressed and becomes warm adiabatically, forming a warm dry wind known as Fohn wind or Chinook. One example is the Calgary in the Rockies in Canada.
Convection rain commonly occurs in warmed or heated areas such as equatorial/tropical regions, where there is almost daily occurrence and even distribution of rain, and temperate areas in summer. It is also common in the intertropical convergence zone (ITCZ). It happens when the ground surface is locally overheated and the adjacent air, heated by conduction, expands and rises. During its ascent, the air mass remains warmer than the surrounding environmental air. Further cooling of the air causes the water vapour in the air to condense into water droplets and it is likely to become unstable forming towering cumulonimbus clouds. The rain comes in thunderstorm and fall heavily in the afternoon. The rain is intense but short-lived which last for minutes and is localized. It is usually accompanied by lightning and common in the tropics and temperate continental summers. Convectional rain is usually associated with tropical depression (typhoons) and thunderstorms.
Frontal / Cyclonic Rain
Frontal/Cyclonic rain is caused by cyclonic activity and occurs along fronts. It is formed when two air masses of different temperature, humidity and density meet such as the meeting of a tropical maritime air mass and a polar air mass. A zone called a front separates them. At the warm front, the lighter warm air rises gently over the heavier cold air which remains close to the ground. As the warm air rises, it expands and cools and condenses to form clouds (altostratus). The rain falls steadily for a few hours to several days. At the cold front, the cold air forces warm air up aggressively, causing it to rise quickly and condense, forming cumulonimbus clouds. The heavy rain that falls is of short duration. Such rain is usually associated with temperate depressions (temperate cyclones).
Artificial precipitation is used when despite the presence of moisture and lifting, clouds sometimes fail to precipitate rain. This circumstance has stimulated intensive study of precipitation processes, specifically of how single raindrops are produced out of a million or so minute droplets inside clouds. Two precipitation processes are recognized: (1) evaporation of water drops at subfreezing temperatures onto ice crystals that later fall into warmer layers and melt, and (2) the collection of smaller droplets upon larger drops that fall at a higher speed. Efforts to effect or stimulate these processes artificially have led to extensive weather modification operations within the last 20 years. These efforts have had only limited success, since most areas with deficient rainfall are dominated by air masses that have either inadequate moisture content or inadequate elevation, or both. Nevertheless, some promising results have been realized and much research is now being conducted in order to develop more effective methods of artificial precipitation.
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