El Niño, the great mass of warm water currently spreading across the Pacific, is probably the most closely studied contemporary weather phenomenon because it poses danger for millions of people. It pushes up the average temperature around the world, changes rainfall patterns and makes extremes of weather worse. In theory, in Europe, it will make northern countries colder and frostier in the coming winter and the south wetter, perhaps a welcome change if the rain is spread evenly across parched lands. In practice, with the world already warming so fast and the Arctic ice reducing all the time, no one knows whether this prediction will hold true or where the demarcation line between cold and dry or warmer and wetter will be. Britain, which is roughly halfway, could experience either or both. So far we do not even know how large the El Niño will be, because it has to be measured in both the warmth of the Pacific and the length of time it lasts. All we know is that it is happening and still growing. So, despite all our forecasting knowledge and computer power, we can only watch and wait to see what occurs, with the unhappy foreboding that at least some effects will be extreme and very bad news.