- CLOUD SEEDING
Not every cloud produces rain. The reason for this is that the water drops making up a cloud are very much smaller than raindrops. In fact about a million cloud droplets are required to produce just one drop large enough and heavy enough to fall as rain. The size is important, because the speed at which a spherical object falls through air is proportional to its radius. In other words small cloud droplets can be held stationary or carried aloft by rising air currents which are not strong enough to prevent large raindrops from falling.
|Nuclei: the central and most important part of an object, movement, or group, forming the basis for its activity and growth.
Pyrotechnic: relating to fireworks.
The key to the formation of rain is the mechanism by which millions of cloud droplets are fused together, to become raindrops. It is sometimes possible for man to assist with this process, thus making clouds give more rain than they could on their own. It must be emphasised that man can only do this if suitable clouds are already present as there is
no way of making rain-clouds appear in a cloudless sky. Under drought conditions when the air is so dry that clouds either fail to form or are too small cloud seeding is useless. There is no known way of increasing the water content of the atmosphere. However, it is often possible to cause rain to drop from the sky onto the ground.
Most rain is formed by the action of minute particles which have the special property of being able to induce super cooled water droplets to freeze on contact with them. As soon as this happens to a cloud droplet, it grows rapidly. This is because the equilibrium of water vapor pressure over ice is
less than over super cooled water. The frozen cloud drops will thus take water vapor from the air in the cloud and will be replaced by the air taking water from the cloud droplets still in liquid form. The frozen ones will continue growing until they are large enough to fall from the cloud. Unless some of the cloud droplets can somehow be caused to freeze, they would all remain tiny, super cooled water drops, too small to fall out of the cloud.
SILVER IODIDE SEEDING TECHNIQUES
Man can often initiate the rain process by providing artificial ice-forming nuclei. The most common way of doing this is by injecting microscopic silver iodide crystals into the cloud. This is done by burning a specially prepared chemical composition. Sometimes, this is simply silver iodide dissolved in acetone which is burned at the base of a convective cloud, making use of the up-draughts to carry the chemical to the cloud-top, where it will do its work. However, it is often easier to use pyrotechnic cartridges containing silver iodide which can be fired into the cloud-top from a Very pistol. The cartridge then burns as it falls downwards through the cloud leaving a trail of silver iodide smoke in its wake.
Silver iodide does not seem to have much effect unless the cloud-top extends beyond about 6 000m above sea level, where the temperature is colder than -10°C. In other words, the cloud is worth seeding only if its depth is at least twice the height of the base above ground. From a high-flying aircraft it is easy to assess whether or not a cloud has reached the necessary -10°C level. However, this would be virtually impossible for a farmer armed with rain-rockets on the ground. This is normally only of benefit to one’s neighbor because the rain produced by seeding is deposited after the cloud has been carried by the wind for at least half an hour.
Some clouds are so tall that they will give plenty of rain anyway and seeding would be a waste of time and effort. These clouds have tops extending to levels where the temperature is colder than – 25°C, where almost any type of dust particles not soluble in water can act as an ice-forming nucleus. Some clouds even extend enough for water to freeze without the action of ice-nuclei. Seeding is most successful on clouds reaching to about 7 000m above sea level where the temperature is -15°C and only particles with particular surface properties can act as freezing nuclei. Such particles are rare in Nature and Man can make good the deficit. This means that the effect of seeding is to replace light showers by moderate showers: i.e. to increase 2mm – 15mm, rather than to turn moderate falls into heavy torrential ones.
Many people will ask how it is possible to establish that the seeding really did produce extra rain and that the cloud would not have given the same rain had it been left alone. This can only be answered by a long series of randomised experimental trials, in which only half the clouds are seeded and the others deliberately left alone so as to provide a basis for comparison 1. Even then sensitive statistical
|Coalescence: come together to form one mass or whole.
tests are required in order to demonstrate that there is an overall difference between the rainfalls of the seeded and non- seeded clouds, as there is a considerable overlap between the two categories. Some natural clouds produce substantial rain and some seeded clouds fail to rain, but on the average, the seeded clouds give much more rain than those which are left alone.
The experimental findings also indicate that seeding prolongs
the lifetime of a rain-cloud. In other words, if left alone it will disperse and evaporate sooner. In fact, there are signs that the rainfall is increased in regions up to 100 miles or more downwind of the area of seeding, although the mechanism responsible for this is not properly understood. However, it seems unlikely that cloud seeding is merely a matter of ‘robbing Peter to pay Paul’.
In sonic countries, cloud seeding is used to reduce the damage inflicted by hail. A heavier dose of silver iodide is fired into the cloud and greatly increases the number of nuclei used as centres for the growth of hailstones. More hailstones are produced but of smaller size so subsequently the overall damage is less.
SALT OR UREA SEEDING
In tropical regions it is known that rain can fall from clouds whose tops are warmer than 0°C. This means that freezing is not the only method by which cloud droplets grow into raindrops. Another method is simply the coalescence of the drops. When there is a large range in the size of different cloud droplets, the convective up-currents will lift the larger ones more slowly than those smaller and eventually some drops become large enough to fall downwards as rain. Man can stimulate this process as well by dispersing salt or urea into a cloud. These are both hygroscopic substances and each little salt or urea particle in the cloud readily becomes a centre for the growth of an extra-large cloud drop.
Cloud seeding should be regarded as a source of available water and not as a means of terminating or preventing drought.
In this respect, it is similar to the building of a dam or the sinking of a borehole. On average, the successful seeding of a cloud produces an additional 100 000 tons of water on the ground and if this is farmland then the value of this water is certainly greater than the cost of the seeding operation.
Figure 1: Shows a firing mechanism, shooting silver iodide into clouds for seeding purposes.
Figure 2: Shows different methods one can use to seed clouds.
- 2. WEATHER RECORDS
When planning a cropping program for a farm it is wise to study the local weather records, both for the farm and district. A good farmer should keep a record of the daily rainfall and the daily maximum and minimum temperatures, so that he knows when frost is most likely to occur in winter. Rainfall is measured by means of a gauge and a large farm usually has two rain-gauges situated far apart. A rain gauge is shown below:
Figures 3 and 4: Show different types of rain gauges, manual and automatic.
Source: sullivan.sbcisd Source: cedadocs.badc.rl
The rainwater is collected in the rain gauge which is already marked in millimetres. However if one does not have a marked rain gauge a bottle can be used and each day it is poured into a measuring cylinder which is graduated in millimetres so that the total rainfall for that day can be read. It is important to check the rain gauge every day and keep a daily record of rainfall. From this you can calculate monthly and annual rainfall figures.
- 3. PENTADES
It is common to divide the year and particularly the summer rainy season into 5-day periods called Pentades. These are then further divided into Dry and Rainy Pentades and a Rainy Pentade is one which fulfills the following:
- Look at the 3 days in the middle of the Pentade and ignore the first and last days;
- The total rainfall of the 3 middle days must be more than 40mm and;
- Only 1 day of the 3 must have less than 8.5mm of rain, and each of the other 2 days must have more than 8.5mm.
If these conditions are fulfilled, it is a Rainy Pentade and if they are not although rain may have fallen it is not regarded as enough to wet the soil properly, help seeds to germinate or a crop to grow; therefore this is a Dry Pentade.
The chart on page 9 shows how the Pentades are set out for the season with Pentade No.1 being at the beginning of January. By keeping a similar chart for your farm and colouring or shading in the Rainy Pentades, you can study the rainfall pattern for the season, and also see when the first planting rain occurs; the first Rainy Pentade being the first planting rain. By entering rainfall on a daily basis you can calculate the total monthly rainfall and the average, or mean, for the month. The total of the twelve months will give you your annual rainfall.
Information needed for accurate rainfall records. Gauge: ………………….
|Mean to date
Centre Pentade is high rainfall.
- Rainfall total for three days is more than 40mm, and
- Not more than one day has less than 8.5mm.