THE year 2015 was probably the hottest since meteorological records began. It certainly ended with a flourish. On North America’s east coast, dreams of a white Christmas were banished by springlike temperatures. In New York, for instance, the mercury hit 22°C (72°F) on Christmas Eve. Europe, too, enjoyed unseasonal warmth. But this was no festive gift, for the warm, moist air that caused it also brought humungous storms. In South America flooding has forced 130,000 Paraguayans from their homes. In the United States tornadoes before and after Christmas have killed at least 29 people. Thirteen more have drowned in floods caused by a storm that this week tracked across the Atlantic (see map), where it may add to the misery of people in large parts of northern England, who have already been inundated several times this year, the Christmas period included (see article).

As The Economist went to press, forecasters were warning that this storm, dubbed Frank by British meteorologists, may develop into what is known as a bomb cyclone, undergoing a sudden, drastic drop in air pressure at its centre in a way that will suck warm air from the tropics and funnel it northward. If these predictions prove correct, the temperature at the North Pole is likely to rise a little above freezing. Though that is still chilly by most people’s standards, it is an extraordinary 30°C above the average for this sunless time of year.

One explanation for the weird weather, at least in the Americas, is El Niño—a phenomenon in which a slackening of trade winds over the Pacific allows warm water to slosh back eastward, increasing the amount of heat and moisture in the atmosphere in a way that has various predictable effects across the tropics. The floods in South America are part of a typical Niño pattern, and the tornadoes in the United States tend to fit, too.

Another factor is that the polar vortex, which traps cold air in the Arctic, has taken a form which permits balmier than normal weather in much of the northern high latitudes. This week’s bomb cyclone may change that, though. Bits of Europe could be in for a cold new year.

Climate change is a contributor too. The greenhouse effect warms the oceans as well as the atmosphere, and they have stored up quite a lot of heat in recent years. The oceans are therefore unusually warm—not just the eastern Pacific, but the Indian and Atlantic, too.

Kevin Trenberth of the National Centre for Atmospheric Research, in Boulder, Colorado, points out that a lack of hurricanes—which most people would welcome—may explain some of the effects around the Atlantic at the moment as heat normally released by summer hurricanes stayed in the ocean. As ever, connecting weather patterns across the seasons and across the globe is difficult. But learning how to do so is becoming ever more important.