Free When Science Goes Wrong by Simon Levay

of hundred miles behind it, a cold front marking the leading edge of the pursuing arctic air. (An animation of the advancing weather system is available online – see the list of sources for this chapter.)
    At about 2am on Thursday, the Met Office’s Cyber 205 began chewing on the data that described the weather situation at midnight, using the fine-mesh model. After an hour or so, it spat out the result: yes, the depression would be deep enough to cause stormy conditions after all, but the centre would track up the English Channel, perhaps grazing the very tip of Kent in southeast England, on its way to the North Sea and the Low Countries. This seemed like good news for Britain, because it meant that the right-hand side of the depression, where the strong winds were likely to occur, would affect the English Channel and northern France, sparing the British mainland.
    This tendency for the strongest winds to occur to the right of the centre of an extra-tropical cyclone is a phenomenon that is also seen with hurricanes. It is caused by the fact that, on the right side, the speed of the entire advancing system adds to the speed of the cyclonic circulation, whereas on the left side it subtracts from that speed. This effect is not very pronounced with hurricanes, whose eyes typically progress quite slowly – perhaps 10 mph. But extra-tropical cyclones move much faster – the centre of the cyclone that caused the Great October Storm raced across Britain at more than 50 mph. This caused a very large difference between wind speeds on the left and right sides of the storm track: to the left, gentle breezes wafted from the northeast; to the right, hurricane-force gales blew from the southwest.
    Having finished its work on the fine-mesh model, the Met Office’s computer turned its attention to the global model. Because it was now about 3am, some more recent data could be fed into the model that had not been available for the running of the fine-mesh model. Whether on account of the new data, or for some other reason, the output of the global model was quite different from that of the fine-mesh model. It predicted that the centre of the depression would veer left, make landfall in southwest England about midnight, and cross the country well north of London during the small hours of Friday morning. What was more, the model predicted that the pressure at the centre of the depression would be 965 millibars. This was 48 millibars below mean atmospheric pressure and, in combination with the predicted steep pressure gradient to the south of the advancing centre, it would be enough to generate very strong winds in the region of England to the southeast of the storm track.
    Thus, when the Bracknell forecasting team for Thursday, October 15 came on duty, they were faced with a quandary: their two computer models predicted utterly different conditions for the following night. Either southeast England would experience a major windstorm, or it wouldn’t.
    This was a situation where the task of weather forecasting was suddenly thrown back into the laps of human beings, and specifically into the lap of the senior forecaster on duty that day. (The position is now called chief forecaster.) I haven’t been able to find out that person’s name. Ewen McCallum, a senior forecaster who was present in the operations room on that day (but not on duty), wouldn’t tell me; others I spoke with professed not to know, and the reports issued after the storm didn’t identify him. Evidently, there has been some desire to protect the identity of the person who (in terms of his official status at least) was most responsible for the erroneous forecast. As McCallum commented, ‘it was a there-but-for-the-grace-of-God-go-I kind of thing.’
    McCallum did give me one insight. The then head of the Central Forecasting Office (and thus the boss of the senior forecasters) was a man by the name of Martin Morris. During the previous week, McCallum told me, Morris had