Introduction : The scientists began to delineate since a long time how the atmosphere is fueled by solar heating which is first absorbed at the surface of the earth. The heat from the Sun falls over the oceans and other water masses. The water thus heated rises into the sky and gets transferred to the atmosphere in the form of latent heat in gaseous water vapour which gets condensed due to adiabatic lapse rate at the dew point level into water droplets that form the clouds. Thus clouds are the converters where by the gaseous water vapour is condensed over dust particles to form into water droplets, thus turning latent heat into sensible heat which can be sensed by thermometer and utilized to drive atmospheric circulations.

The tropical atmospheric circulations operate by means of thermally direct (Hadley ) cells with their ascent concentrated in a narrow equatorial “trough zone” and with mean sinking motion outside this zone. At the higher levels in the atmosphere, the tropical cells transport energy and moment towards the poles to make up the radiation deficits in those zones and drive the wind circulations in the temperate and the polar regions.For cloud cover for various countries in different months,refer to the following web site for evaluation of their potential for giving rain-fall.

These cumulus clouds produce more than 3/4ths of the rain that falls on our planet, dominating in the thirsty tropical and subtropical zones. Since the atmosphere runs an inefficient engine about 2% of the energy drives the planetary wind system while most of the energy is lost to space by radiation. In the temperate latitudes the cyclones draw the energy stored in air mass contrasts and become the driving mechanism in the planet’s Westerlies which at high levels snake around the globe in a wave pattern and with their instabilities exert influence over the fluctuating weather. Over the mid latitude oceans cumulus clouds influence the occasional explosive deepening of winter storms that trigger a major readjustment in the wind pattern over the whole hemisphere.

If a number of these cumulus clouds can be modified to some extent a powerful tool would be in the hands of man kind for managing their water resources taming the cyclones and modifying the regional climates. How to explore the different possibilities of modifying the clouds determines the ways in which man can modify cloud systems to tap the abundant sky water to improve the living standards of mankind and conserve nature for sustainable development. In order to provide additional annual precipitation for improving drinking water, agriculture, hydro power generation, fog dispersal in airports and metropolitan cities, forest growth, pollution control, summer heat reduction, flood and cyclone damage reduction and environmental improvement, it is necessary to understand what is already known and what is yet to be known about clouds in general and the large cumulus clouds in particular and how cloud seeding operations are to be conducted and evaluated.

Cloud formation :

A cloud is nothing but a visible conglomeration of very small particles of water or ice or of both in the atmosphere. The moisture that goes into the atmosphere is due to heating of the water from the surface of lakes, streams and oceans by the Sun and it will be in the form of invisible vapour. Like other gases in the atmosphere exert pressure on the earth’s surface, the water vapour also exerts pressure on the earth. At any given temperature and pressure, the atmosphere cannot hold unlimited water in the form of water vapour and hence evaporation from any water body comes to an end when the atmospheric air reaches a state of saturation with the water vapour and the partial vapour pressure exerted is called the saturation vapour pressure. More moisture is held by the atmosphere when it is warmer, it means that a warm unsaturated moist air when cooled becomes saturated or even super saturated, that is, the relative humidity exceeds 100% or the dew point equals the atmospheric temperature. The excess water vapour changes into the liquid or solid phase in the form of dew, fog, mist, frost and cloud. Such changes of phase occurs due to condensation , freezing or even sublimation depending upon the temperature. But this process needs the assistance of some surface and this is easily available as dust and other particles from the ground. But in the free atmosphere where suitable surfaces are not readily available super saturation occurs.

[absolute humidity: water content in air];

 A variety of measures are usedto describe the content of water vapor in air; one is the content by volume, which is between 0.3% (hot desert) and 4% (rain forest).It is  estimated at about at 5 gm per Cubic meter in the atmosphere over  green hilly areas near coastal  zones

 In the free atmosphere the temperature of an air parcel as it rises into the sky decreases at the rate of 6oC to 10oC per km due to adiabatic lapse rate. If a parcel of air at the ground level with a temperature of 30oC and relative humidity of 75% contains 20 grams of moisture per cubic meter rises into the sky to a height of 1km. Its temperature becomes 24oC and it can retain about 19 grams of moisture per cubic meter of air and consequently the remaining moisture in the air parcel gets condensed as liquid water over dust particles known as cloud condensation nuclei and the liquid droplets form the clouds. These cloud droplets form in the sizes of 1 to 50 microns in diameter with an average of 10 to 20 microns ( a micron is one thousandths of a millimeter and the human hair is 75 microns in thickness)

Cloud droplets of 250 microns provide drizzle while cloud drops between 500 to 5000 microns provide heavy rains. The following figures shows how the formation of clouds occur and the different sizes of cloud droplets and rain drops. (Fig-1)

The following table gives details about the various sizes of particulates in the atmosphere and the cloud droplets and the rain drops. Characteristics of particles, cloud drops and rain drops (Table-1)

Fog, dew or mist do not contribute much to the rainfall because the water content is small and the evaporation losses are high. (Fig-2)  

Condensation of moisture occurs when the moist air ascends into the sky and cools adiabatically and the mechanism of ascent may be convection , Orography or frontal uplift. The condensation mechanism in the atmosphere to cool the moisture laden air is achieved in the following ways.

1. Radiative Cooling: Radiative exchange of heat from the surface of the earth and the tops of clouds

2. Advective Cooling: Passage of warm air over a cold land or sea surface.

3. Convective Cooling: Vertical rise into the sky of air resulting in its expansion due to the decreased atmospheric pressure known as adiabatic cooling. This vertical uplift is caused by mechanism like rising air caused by a mountain barrier known as Orographic lift. The gliding of a mass of warm air over a cold air mass is known as Frontal lift.

4. Mixing of two different air masse with different temperatures often resulting in cooling.

If the air with the moisture is free from impurities even if it expands and cools it remains as vapour even at temperatures below dew point. Fortunately the atmosphere contains many impurities and pollutants as millions of minute particles including hygroscopic aerosols.

The water vapour firstly condenses on hygroscopic and other particles and changes into droplets of liquid water and these particles are known as condensation nuclei (CCN). Once the condensation starts the process continues indefinitely and the clouds are formed. The condensation nuclei are of different sizes and originate from the dust, sea spray and the combustion products. Based on the size (diameter) they are known as Aitkeen nuclei (less than 0.1 microns) large nuclei (0.1 to 1.0 microns) and giant nuclei (more than 1 micron). One micron size is one millionth of a meter or 1000th part of a millimeter) Most of these particles are water soluble. In the presence of suitable condensation nuclei which are also known as cloud condensation nuclei (CCN) the condensation of vapour starts when humidity is about 75%. All water vapour does not condense at 0oC and continues to exist in the air as super cooled water without becoming ice crystals upto a temperature of –40oC. But if such super cooled droplets come into contact with ice particles they freeze as ice crystals. (Table-2)

Heat is consumed by the water for its evaporation and conversely when water vapour condenses the heat is liberated. For instance the evaporation of one gram of water by boiling requires the consumption of 540 calories of heat and when one gram of water vapour is condensed it releases again the same quantity of heat. This liberated latent heat of condensation is absorbed by the air parcel that becomes warmer and lighter as compared to the surrounding environment and begins to rise. If continued moisture supply is available more and more condensation will occur, resulting in the formation of fog or mist. When this fog is lifted above the ground a sheet like cloud known as stratus cloud forms. If this condensation occurs above the ground level after some moist air is forced vertically upwards as updrafts, then small heaps of clouds form and these clouds may not have a growing tendency and hence may dissipate without giving rain.

However when large areas in the sky are covered with streets of puffy cumulus (heap) clouds, the larger ones among them with intense updrafts or vertical upward motion of air at a rate of 5 to 10 meters per second, the clouds may grow by as much as 6km in thickness, known as towering cumulus clouds. These clouds contain water droplets only in super cooled state.  Additional growth of such clouds may result in the freezing of water droplets. If the tops of these clouds grow upto about 16km and may assume anvil shapes indicating outflow of air from the top of the clouds, such clouds are known as cumulo-nimbus (violent rain) clouds. The shape of the cloud and its appearance are determined by the nature, number and size of nuclei and droplets and the weather characteristics. The clouds are always in a continuous evolution process and hence display many varieties of forms.

Depending upon the height of the cloud-base and thickness the clouds may be classified as follows: 




1. High Clouds

5 to 14 kms


2. Medium Clouds

2 to 7 kms



3. Low Clouds

0 to 2 kms




4. Clouds with Vertical growth

0.5 to 9 kms

0.5 to 18 kms

Large cumulus



Alto-status clouds give light rain while Nimbo-stratus ones give continuous rain and sometimes with heavy down pour accompanied by hail.  Other cloud forms generally do not give rain. (Fig-3) 

The cumulonimbus clouds are heavy dense masses with large vertical growth and look like tower and mountains.  The upper portion of the cloud develops fibrous structure that looks like an anvil or plume.  It has liquid droplets at bottom and ice crystals in the portion above and often produces lightending thunder, showers and hail.  The Nimbo-stratus clouds are dark grey, sometimes reaching low and precipitate rainfall.  About one percent of the earth’s water vapour is present in the atmosphere and helps the cloud growth.  But how to modify the clouds to get the required water at a given place remained a perplexing issue.(Fig-4)


Slow and prolonged ascent of air in a low pressure area or irregular stirring motion produces a layer type clouds known as “stratiform clouds”. Convective currents leading to violent ascent of air masses due to insulation caused by heat from the sun create heaps of clouds known as “cumuliform clouds”. All the clouds can be divided into four classes based on the thickness and height of the cloud base as shown below (Table-3)


Clouds play an important role in weather forecasting. Feather like Cirrus (curl of hair) clouds in the middle and low latitudes scattered in the sky during summer indicate clearing of  the weather.

Rain-clouds :

If cumulus clouds are gradually developing into cumulo-nimbus clouds on hot and humid days, they indicate an approaching thunder storm and hence cloud seeding has to be planned in advance to prevent the development of hail storms that cause severe damage to crops and properties. If stratocumulus clouds cover the morning sky and the wind is gusty, clouds will develop during the day time. Cloudiness and rainfall are interrelated in the tropical equatorial belt where there will be substantial cloud cover with maximum amount of rainfall.

The clouds are localised and have great vertical development. Due to the descending air currents in the subtropical belt, rainfall and cloud cover will be less and deserts are located in  such regions.  In the equatorial belt, cloudiness does not vary much from month to month. But the cloud maximum occurs between 10o and 20oNorth and South latitudes during summer months with substantial rainfall. Daily variations in cloudiness is based on the kind of clouds in the sky.

The maximum cumulus or cumuliform clouds occur during the early and middle afternoon periods while stratus and stratiform clouds appear to the maximum extent during the morning periods. While altostratus clouds give light rain, towering cumulus clouds provide short spells of showers. While Nimbostratus clouds give moderate continuous rain, cumulonimbus clouds give heavy rain, often accompanied by hail stones. Several other kinds of clouds do not provide rain. Stratus clouds are in the lowest level in the sky.  When there is a depression within 200 to 300 kms from the sea coast stratus clouds in the shape of cotton pieces moving fast, touching the tall trees are found along the seacoast. This cloud is always thin and has a stratiform or horizontal top. If the depression close by is intensifying further, hundreds of these stratus clouds are seen moving like an invading army.

Cumulus clouds are 5000 ft thick and are generated by the summer heat when some moisture is present in the atmosphere. They disappear by night when the convection currents subside. They have rugged or cauliflower shaped tops and are known as fair weather cumulus clouds. During summer when there is moist air and good convection cumulus clouds grow vertically and reach the freezing levels around 6km. Super cooled water often triggers sharp rainfall that lasts upto half an hour.

If there is stable upper air layer, the top of the cumulus cloud gets flattened and the cloud becomes a strato-cumulus cloud. Small strato-cumulus clouds do not shed any rain. Alto-cumulus clouds when closely packed look like a flock of sheep and they do not give any rain. If the sky is covered with alto cumulus clouds for 2 to 3 days it indicates that a low pressure system within 300 km is developing into a depression or a cyclone. Cyclonic storm winds associated with a depression cause a large scale convergence for about 300 kms around the depression at the lower levels of the atmosphere. Altostratus clouds then from with a base round 8000 ft and a thickness of 5000 ft to 10,000ft and its spread cloud be around 300 kms around the depression. By its gradual growth and thickness, it provides corresponding increase in rainfall reaching upto heavy downpour which continues until the depression disappears. The rain may last for 2 to 3 days.

The high clouds that form above 7 km in the sky are known as cirrus clouds that are very thin and contain ice-crystals. These clouds do not give any rain. The thunder cloud, the giant king of the sky, is known as cumulo-nimbus cloud which is classified as low cloud with its base around 1.5 km and its top reaching 12 km in the sky. They provide rain for about half an hour in dry summer and they dominate the sky during monsoon periods for 2 to 3 hours and produce heavy rains. The Nimbo-stratus cloud, a very dark Stratiform cloud gives moderate rainfall for a short time.

Among the clouds the most important ones that provide rainfall are the tall cumulus the large Strato cumulus, the thick Alto-stratus and the dark Cumulo-nimbus. Since the human eye can observe the horizon upto 40 kms, it is necessary to cultivate sky reading to predict the rain which is influenced both by the vertical extent as well as the horizontal dimensions of the clouds.