The Black Cloud

by Fred Hoyle

  Kingsley, Alexandrov, and Weichart spent the afternoon arguing. In the evening they collected Marlowe and Yvette Hedelfort and went to Parkinson’s room.

  “Look, Parkinson,” began Kingsley, after drinks had been poured, “I think it’s up to you to decide what London, Washington, and all the other cities of sin are to be told. Things aren’t quite as simple as they seemed this morning. I’m afraid the hydrogen isn’t really as important as you thought, Yvette.”

  “I didn’t say it was important, Chris. I simply asked a question.”

  “And you were quite right to do so, Miss Hedelfort,” broke in Weichart. “We’ve been giving far too much attention to the temperature problem and overlooking the effect of the Cloud on the Earth’s atmosphere.”

  “Not clear until Dr Marlowe finished work that Earth would be in Cloud,” grunted Alexandrov.

  “That’s true enough,” agreed Weichart. “But now the decks are cleared we can get into action. The first point is one of energy. Each gram of hydrogen that enters the atmosphere can liberate energy in two ways, first by its impact with the atmosphere and second by combination with oxygen. Of these the first yields more energy and is therefore more important.”

  “My God, this only makes it worse,” exclaimed Marlowe.

  “Not necessarily. Think what’ll happen when the gas of the Cloud hits the atmosphere. The very outside of the atmosphere’ll become extremely hot, because it’s on the outside where the impact will take place. We’ve calculated that the temperature of the outer parts of the atmosphere’ll go racing up to hundreds of thousands of degrees, perhaps even to millions of degrees. The next point is that the Earth and the atmosphere are spinning round and that the Cloud will be hitting the atmosphere from one side only.”

  “From what side?’ asked Parkinson.

  “The Earth’s position in its orbit will be such that the Cloud will come at us from the approximate direction of the Sun,” explained Yvette Hedelfort.

  “Although the Sun itself won’t be visible,” added Marlowe.

  “So the Cloud will be hitting the atmosphere during what would normally be the daytime?”

  “That’s right. And it will not be hitting the atmosphere during the night.”

  “And that’s the crux of the matter,” continued Weichart.

  “Because of the very high temperature I was talking about, the outer parts of the atmosphere will tend to blow outwards. This won’t happen during the “daytime” because the impact of the Cloud will hold it in, but at “night” the upper atmosphere will stream outwards into space.”

  “Oh, I see what you’re meaning,” said Yvette Hedelfort. “Hydrogen will come into the atmosphere during the “daytime” but it will blow out again during the “night”. So there will not be any cumulative addition of hydrogen from day to day.”

  “That’s exactly right.”

  “But can we be sure that all the hydrogen will be evaporated off in this way, Dave?’ asked Marlowe. “If even a small proportion of it were retained, say one per cent or a tenth per cent, the effect would be disastrous. We’ve got to keep in mind how very small a disturbance — small from the astronomical point of view — could still wipe us out of existence.”

  “I’d feel confident in predicting that effectively all the hydrogen will be evaporated away. The danger is rather the other way, that too much of the other gases of the atmosphere will also get evaporated into space.”

  “How can that be? You said only the outer parts of the atmosphere would be heated.”

  Kingsley took up the argument.

  “The situation is this. To begin with, the top of the atmosphere will be hot, extremely hot. The bottom of the atmosphere, the part where we live, will be cool to start with. But there’ll be a gradual downward transfer of energy, tending to heat up the lower parts.”

  Kingsley put down his glass of whisky.

  “The whole point is to decide how fast the downward transference of energy will be. As you say, Geoff, only a very slight effect would be utterly disastrous. The lower atmosphere might be heated sufficiently to cook us, quite literally to cook us, all done to a turn quite slowly, politicians included, Parkinson!”

  “You’re forgetting that we shall survive longest, because our skins are thickest.”

  “Excellent, a point to you! Of course the downward transfer of energy might be fast enough to cause the whole of the atmosphere to be blown off into space.”

  “Can this be decided?”

  “Well, there are three ways of transferring energy, they’re just our old friends, conduction, convection, and radiation. We can be pretty sure already that conduction isn’t going to be important.”

  “Nor convection either,” broke in Weichart. “There’ll be a stable atmosphere with a rising temperature as you go outwards. So there can be no convection.”

  “So that leaves radiation,” concluded Marlowe.

  “And what will the effect of radiation be?”

  “We don’t know,” said Weichart. “It’ll have to be calculated.”

  “You can do that?’ queried the persistent Parkinson.

  Kingsley nodded.

  “Can calculate,” affirmed Alexandrov. “Will be bloody great calculation.”

  Three weeks later Kingsley asked Parkinson to see him.

  “We’ve got the results from the electronic computer,” he said. “Good thing I insisted on having that computer. It looks as though we’re all right so far as radiation is concerned. We’ve got a factor of about ten in hand and that should be safe enough. There’s going to be an awful lot of lethal stuff coming downward from the top of the atmosphere though — X-rays and ultra-violet light. But it seems as if it won’t get through to the bottom of the atmosphere. We shall be pretty well shielded down at sea-level. But the situation won’t be so good in the high mountains. I think people will have to be brought down. Places like Tibet will be impossible.”

  “But, by and large, you think we’ll be all right?”

  “I just don’t know. Frankly, Parkinson, I’m worried. It’s not this radiation business. I think we’re all right there. But I don’t agree with Dave Weichart about convection, and I don’t think he’s as confident as he was. You remember his point about there being no convection because of the temperature increasing outwards. That’s all very well under ordinary conditions. Temperature inversions, as they’re called, are well known, particularly in Southern California, where Weichart comes from. And it’s quite true that there’s no vertical movement of the air in a temperature inversion.”

  “Well then, what are you worried about?”

  “The top of the atmosphere, the part that the Cloud is hitting. There must be convection at the top, because of the impact from outside. This convection certainly won’t penetrate through to the bottom of the atmosphere. Weichart’s right there. But it must penetrate downwards a little way. And in the region in which it does there’ll be a big transference of heat.”

  “But so long as the heat doesn’t get to the bottom will that matter?”

  “It may do. Consider things as they’ll occur day by day. The first day there’ll be a little penetration of the currents. Then at night we shall lose not only the hydrogen that has come in during the day but also the part of the atmosphere down to which the currents have penetrated. So in the first day and night we shall lose an outer skin of our atmosphere, quite in addition to the hydrogen. Then the next day and night we shall lose another skin. And so on. Day by day the atmosphere will be stripped off in a series of skins.”

  “Will it last for a month?”

  “That’s exactly the problem. And I can’t tell you the answer. Maybe it won’t last ten days. Maybe it’ll last a whole month quite easily. I just don’t know.”

  “Can’t you find out?”

  “I can try, but it’s horribly difficult to make sure that every important factor is included in the calculations. It’s much worse than the radiation problem. Undoubtedly we can get some sort of answe
r but I don’t know that I’d give it much weight. I can tell you right now that it’s going to be a touch-and-go business. Frankly again, I don’t believe we shall be much wiser six months from now. This is probably one of those things that are too complicated for direct calculation. We shall have to wait and see, I’m afraid.”

  “What am I to tell London?”

  “That’s up to you. You certainly ought to tell ’em about evacuating high mountain districts even though there aren’t any high enough to matter in Britain. But I leave it to your judgement how much of the rest you tell ’em.”

  “Not very nice, is it?”

  “No. If you find it getting you down I’d recommend a talk with one of the gardeners, Stoddard by name. He’s so slow that nothing would worry him, not even the atmosphere being sprayed off.”

  By the third week in January the fate of Man was to be read in the skies. The star Rigel of Orion was obscured. The sword and belt of Orion and the bright star Sirius followed in subsequent weeks. The Cloud might have blotted out almost any other constellation, except perhaps the Plough, without its effect being so widely noted.

  The Press revived its interest in the Cloud. “Progress reports’ were published daily. Bus companies were finding their Night-time Mystery Tours increasingly popular. “Listener research’ showed a threefold increase in the audience for a series of B.B.C. talks on astronomy.

  At the end of January perhaps one person in four had actually observed the Cloud. This was not a suffficient proportion to control public opinion, but it was sufficient to persuade the majority that it was high time that they took a look for themselves. Since it was scarcely possible for a majority of town dwellers to move at night into the country, suggestions were made for the shutting off of town lighting systems. These were at first resisted by municipal authorities, but resistance only served to change polite suggestions into strident demands. Wolverhampton was the first town in Britain to impose a nightly black-out. Others quickly followed, and by the end of the second week in February the London authorities capitulated. Now at last the population at large was starkly aware of the Black Cloud, as it clutched like a grasping hand at Orion, the Hunter of the Heavens.

  A closely similar pattern of events was repeated in the U.S., and indeed in every industrialized country. The U.S. had the additional problem of evacuating much of the western states, since a considerable area of populated territory there lies above 5,000 feet, the safe limit set in the Nortonstowe report. The U.S. Government had of course referred the matter to its own experts, but their conclusions turned out not to differ significantly from those of Nortonstowe. The U.S. also undertook to organize the evacuation of the Andean republics of South America.

  The agrarian countries of Asia were strangely unmoved by the information supplied to them through the United Nations. Theirs was a ‘wait and see’ policy, which might really be said to have been the wisest course of all. For thousands of years the Asian peasant had been accustomed to natural disasters — ‘acts of God’ as the lawyers of the West called them. To the oriental mind drought and flood, marauding tribes, plagues of locusts, disease, were to be suffered passively, and so was the new thing in the sky. In any case life offered them little and consequently was not set at an unduly high price.

  The evacuation of Tibet, Sinkiang, and Outer Mongolia was left to the Chinese. With cynical indifference nothing at all was done by them. The Russians, on the other hand, were punctilious and prompt in their evacuation of the Pamirs and of their other highland areas. Indeed genuine efforts were made to shift the Afghans, but Russian emissaries were driven out of Afghanistan at pistol point. India and Pakistan also spared no effort to ensure the evacuation of the part of the Himalaya south of the main watershed.

  With the coming of spring in the northern hemisphere the Cloud passed more and more from the night sky to the day sky. So, although it was spreading rapidly outside the constellation of Orion, which was now completely obscured, its presence was far less obvious to the casual observer. The British still played cricket, and dug their gardens, as indeed did the Americans.

  The widespread interest in gardening was favoured by an exceptionally early summer which started in mid-May. Apprehension was widespread certainly, but it was lulled to a vague outline by week after week of wonderfully clear sunny weather. Vegetable crops were ready for eating in late May.

  The Government was not nearly so pleased by the excellent weather. The reason underlying it was ominous. Since its first detection, the Cloud had by now completed about ninety per cent of the journey to the Sun. It had of course been realized that more and more radiation would be reflected by the Cloud as the Sun was approached, and that a consequent rise of temperature would take place on the Earth. Marlowe’s observations suggested that there would be little or no increase in the amount of visible light, a prediction that turned out to be correct. Throughout the whole of the brilliant spring and early summer there was no noticeable increase in the brightness of the sky. What was happening was that light from the Sun was impinging on the Cloud and being re-radiated as invisible heat. Fortunately, not all the light impinging on the Cloud was re-radiated in this fashion, otherwise the Earth would have become entirely uninhabitable. And fortunately quite a large fraction of the heat never penetrated inwards through our atmosphere. It was reflected and bounced back into space.

  By June it became clear that the temperature of the Earth was likely to be raised everywhere by some thirty degrees Fahrenheit. It is not commonly realized how near the death temperature a large fraction of the human species lives. Under very dry atmospheric conditions a man can survive up to air temperatures of about 140° Fahrenheit. Such temperatures are in fact attained in a normal summer in low-lying regions of the Western American desert and in North Africa. But under highly humid conditions, the death temperature is only about 115° Fahrenheit. Temperatures at high humidity up to 105° Fahrenheit are attained in a normal summer down the eastern seaboard of the U.S. and sometimes in the Middle West. Curiously, temperatures at the equator do not usually run above 95° Fahrenheit, although conditions are highly humid. This oddity arises from a denser cloud cover at the equator, reflecting more of the Sun’s rays back into space.

  It will accordingly be appreciated that the margin of safety over much of the Earth amounts to no more than 20°, and in some places to very much less than this. An additional rise of 30° could be viewed therefore only with the greatest apprehension.

  It may be added that death results from the inability of the body to get rid of the heat that it is constantly generating. This is necessary in order to maintain the body at its normal working temperature of about 98° Fahrenheit. An increase of body temperature to 102° produces illness, 104° produces delirium, and 106° or thereabouts produces death. It may be wondered how the body can manage to rid itself of heat when it happens to be immersed in a hotter atmosphere, say in an atmosphere at 110°. The answer is by evaporation of sweat from the skin. This happens best when the humidity is low, which explains why a man can survive at higher temperatures in low humidity, and indeed why hot weather is always pleasanter when the humidity is low.

  Evidently much would depend in the days to come on the behaviour of the humidity. Here there were grounds for hope. The heat rays from the Cloud would raise the temperature of the surface of the land more rapidly than the sea, and the air temperature would rise with the land while the moisture content of the air would rise more slowly with the sea. Hence the humidity would fall as the temperature rose, at any rate to begin with. It was just this initial fall of humidity that produced the unprecedented clarity of the spring and early summer in Britain.

  First estimates of the heat rays from the Cloud underrated their importance. Otherwise the American Government would never have placed their scientific advisory establishment in the western desert. They were now obliged to evacuate men and equipment. This made them more dependent for information on Nortonstowe, which therefore increased in importance. But Nortonstow
e had its own difficulties.

  Alexandrov summed up the general opinion at a meeting of the Cloud investigation group.

  “Result impossible,” he said. “Experiment wrong.”

  But John Marlborough averred that he was not wrong. To avoid an impasse it was agreed that the work should be repeated by Harry Leicester, who otherwise was concerning himself with communication problems. The work was repeated and ten days later Leicester reported back to a crowded meeting.

  “To go back to the early phases. When the Cloud was first discovered it was found to be moving in towards the Sun at a speed of slightly less than seventy kilometres per second. It was estimated that the speed would gradually increase as the Sun was approached, and that the average speed eventually attained would be around eighty kilometres per second. The upshot of observations reported a fortnight ago by Marlborough is that the Cloud is not behaving as we expected. Instead of speeding up as it approaches the Sun it is actually slowing down. As you know, it was decided to repeat Marlborough’s observation. The best thing will be to show a few slides.”

  Only one person was pleased with the slides — Marlborough. His work was confirmed.

  “But damn it all,” said Weichart, “the Cloud must speed up as it falls through the Sun’s gravitational field.”

  “Unless it gets rid of momentum in some way,” countered Leicester. “Let’s look at that last slide again. You see these tiny pips right away over here. They’re so small that they might be a mistake, I’ll grant you. But if they’re real they represent motions of about five hundred kilometres per second.”

  “That’s very interesting,” grunted Kingsley. “You mean the Cloud is firing off small blobs of material at very high speed, and that’s what is making it slow down?”

  “You could interpret the results in that way,” answered Leicester.

  “At least it’s an interpretation that conforms with the laws of mechanics, and which preserves sanity in some degree.”