The Great Storm of 1987, mapped

If you went to bed in London on Thursday 15 October 1987, it was a wet, ordinary autumn night: drizzle, 9°C, a breeze you would barely notice. By the morning, 15 million trees were down across the south of England, roads were blocked, and millions of homes had no power.

I'm a weather forecaster, and the Great Storm is the night every British forecaster knows by heart. I have rebuilt it from the data, hour by hour, to show what happened while the country slept. Some of it surprised even me.

23:00: the warm air arrives

Here is the moment that gives me goosebumps. At 22:00 the temperature in London was 9.3°C. An hour later it was 14.1°C. By 01:00 it was 16.6°C, warmer in the middle of the night than it had been all afternoon.

That leap is the storm's warm sector arriving: a tongue of mild, damp air from far to the south, wrapped round the low as it raced in from the Bay of Biscay. Forecasters watch for it because it means the low is deepening and the wind is about to come. It's also the name of this shop. The wind followed it within the hour.

05:00 to 07:00: the pressure leaps

Look at the top line. All night the pressure over London slid down, to its lowest at about 05:00. Then, as the low swept away to the north-east, it shot back up: by about 8 hPa in a single hour. On an ordinary day the pressure might move that much in a whole day. That steep climb is the cold air flooding in behind the storm, and it came with the worst of the wind. The temperature tumbled back to 9°C.

A barometer on a hall wall that night would have been the most dramatic thing in the house: down, down, down, and then up so fast you could watch the needle move.

The night in four maps

The storm itself, from the same data. The colour is the strongest gust, purple the fiercest. The white lines show the wind's flow, the black lines are isobars, and the red L marks the centre of the low.

Why so many trees came down

Three things lined up. It had rained hard for days, so the ground was soft and roots let go. It was early October, and the trees were still in full leaf, like sails. And winds like these almost never reach the south-east, so its trees had never had to stand up to them. Kew Gardens lost hundreds of trees in a night, and Sevenoaks famously lost six of the seven oaks that gave the town its name.

The timing was cruel and kind at once. The strongest winds came in the small hours, so few people were out. Had the same storm come through at rush hour, the death toll would have been far higher.

The forecast, and the line that would not go away

At lunchtime that Thursday, Michael Fish told BBC viewers that a woman had rung to ask if a hurricane was on the way, and that there wasn't. He has since said he was talking about a hurricane off Florida. Strictly, he was right about the word: a hurricane is a tropical storm, and this was not one, even if its gusts reached hurricane force. But the line stuck to him for the rest of his career.

The real problem was bigger than one remark. The forecast models of 1987 saw the world in boxes roughly 150 km across, and there were few observations over the Bay of Biscay, where the storm was growing. The models struggled to show how deep the low would get or where its worst wind would land. The inquiry that followed changed how the Met Office observes, forecasts and warns, and it led to the National Severe Weather Warning Service.

What we know now

Years later, researchers looking back at the storm found something nobody knew about in 1987: a narrow, fast stream of air descending from the cloud head on the storm's southern side, bringing the wind's full force down to the ground. They called it a sting jet. It helps to explain how a gust of 115 mph reached Shoreham, and forecasters now watch for sting jets in every deep Atlantic low.

Bert, Warm Sector