If you're throwing a party in the garden some day and conversation begins to flag, look
up at the sky and pop the question, 'what's a cloud made of?' People will look at you
as if you're mad. Most have probably never thought about it and if some have they're
likely to answer, 'water vapour'. But it isn't you know. Water vapour is a gas and you
can't see it, it's mixed in the air with the oxygen and nitrogen, all invisible. No, a cloud
is made of tiny water droplets, too small and light to fall under gravity. In the late 1940s
an American meteorologist theorised that if a means could be found to convert the small
droplets of a cloud into large drops they might become heavy enough to fall to the earth
as rain. Thus a means might be available to alleviate periods of dry weather or even
more serious long-term droughts. The problem was how to convert the small droplets
into large drops. It was suggested that spraying dry ice (super-cooled particles of solid
carbon dioxide) into the cloud at very high levels might do the trick and experiments
were undertaken to test the theory. The main trouble is that even after spraying you can
never be absolutely certain that the rain, if any, was due to the spraying and had not
occurred naturally. Lengthy trials lasting many years were undertaken in America and
elsewhere and it was eventually decided that, in general, artificial rain stimulation was
unpredictable in both location and amount and was also uneconomic in relation to the
high costs of the seeding and little has been heard of the proposal since.
In the early 50s I was a meteorologist in East Africa and in one of those years a
serious drought occurred. Crops failed, game died and the Masai tribe and others that
lived mainly on their cattle were in serious difficulties. Drawing on exaggerated
newspaper reports of the American experiments, our political masters insisted that the
Meteorological Department do something about it, make some rain and solve the
country's problems. Naturally enough, no extra money was provided to support the work
and we were therefore left with only our own very limited resources, ensuring that any
manufactured rain really would be produced as the cheapest of the cheap.
We believed that rainfall in the tropics occurs under different conditions and for
different reasons from that in high latitudes and the dry ice technique was unlikely to be
successful. We had therefore to dream up our own operating method. Following a good
deal of thought and lengthy after-hours arguments we hit on a possible method that
seemed, at least theoretically, to offer a degree of hope. Everybody used to know that
salt is hygroscopic even if they didn't know what 'hygroscopic' meant. I use the past
tense because modern salt has additives to prevent it happening so the effect is not so
often seen nowadays. Hygroscopic means 'to absorb water'. Salt in saltcellars used to
absorb water and become damp. That's why saltcellars used to clog up, to the annoyance
of diners struggling to season their eggs. We thought that if we could introduce dry salt
into a cloud large water drops might accumulate around the salt grains and, if we were
lucky, become heavy enough to fall as rain. We crossed our fingers.
Getting the salt into the cloud was a problem debated at length and then somebody
said that if salt in the shops was impregnated with stuff to stop it absorbing water there
didn't seem much point in the whole idea and we could all go home. Since going home wasn't on we decided to make our own salt by boiling some seawater and precipitating
out the salt. Good idea except that as soon as the salt appeared it absorbed all the water it
could and became useless for going up into the sky. We decided we'd have to dry it
immediately before scattering it in the cloud, if we ever could scatter it in the cloud.
How were we going to get the dried salt into the cloud so that it could be scattered?
We couldn't throw it out of an aeroplane because we didn't have one, nor could we
afford one and even if we could have done, it would have had to be a little one and little
aeroplanes don't like going into big clouds, and for us the bigger the cloud the better.
So was there an alternative? Now in those days all meteorological departments had small
balloons (and hydrogen to fill them) to send up into the sky to find out which way the
wind aloft was blowing because aircraft were always wanting to know. Debate produced
the answer. Fill the balloon with hydrogen and attach a film canister to it with a length of
string. Film came in little aluminium cylinders about two inches long. We punched holes
in the bottoms of several cylinders and, to act as wicks, lengths of string that had
previously been soaked in a gunpowder solution were stuck through the holes, the
lengths depending on various anticipated heights of clouds. Next in the cylinders was
about half an inch of gunpowder and above that to the top of the cylinder came the sea
salt, held in with a thin rubber cap. The salt was dried and ground to a fine powder
before being put into the cylinder and the rubber cap served to keep it dry before release.
When the burning wick reached the gunpowder the cylinder would explode, the salt
would be blown out and distribute itself into the cloud. Rain would eventually fall, the
drought would be over, and we would have earned our keep. That was our theory.
All fine and dandy. The next step was to try it out.
The Masai plains provided a perfect location for the experiment. In the early part of
the year beautiful cauliflower-topped cumulus clouds drift over the area looking as
though they'd love to provide showers of rain if only the process could somehow be
kick-started. We hoped our aluminium cylinders would do just that. On the savannas
new green shoots soon follow any small fall of rain and it didn't matter where the rain
fell because the cattle could easily be driven to the new foliage. Of course one had first
to find a suitable cloud and get into a position, not underneath it, but from a point where
the wind would drift the balloon and cylinder into it. This involved perilous and bumpy
journeys across the savannah, circumnavigating anthills, prides of lions, dry streambeds,
occasional grumpy rhinos, patches of thorn trees and other impedimenta designed by
nature to frustrate budding rain stimulators. Often by the time all this had been done the
cloud would have died out or drifted away so we'd have to start again. However, an
objective having eventually been attained, it remained only to guess, some would say
'calculate', the necessary length of fuse (string), apply a match, then sit down, have
a drink and hope to hear an explosion in the cloud and see some rain 10 to 30 minutes
afterwards. Or, of course, look for another cloud, preferably in the direction from which
one had just come, and repeat the performance.
Don't get the idea that the savannas were ever inundated with rain. They weren't, but at
least some rain fell after occasions of seeding, whether due to us or not, and the Masai
certainly came to believe in the great white rain-giving gods. So much so that when some
real seasonal rain eventually arrived and alleviated the drought a party was held to which
folk came from miles around. Local eminences were invited, the politicians amongst them
claimed all the credit, and the Meteorological Department was a guest of honour. Sitting
under thorn trees, we dined in style on goat meat and cattle blood with fermented milk to
taste, sang Masai songs, watched the young bloods dance and hoped it would soon be over.
That party did eventually end, but not the rain-making. Too many people heard about it
and East Africa being a dry country, we soon had demands and invitations to alleviate
droughts from southern Tanganyika to the Northern Frontier Province of Kenya and from
the coast to western Uganda. Perforce the methods and techniques changed, became far
more menacing and hilarious and perhaps improved. But that is another story.