Feeling the Heat

There is no such thing as normal weather. The average daytime high temperature for New York City this week should be 14C, but on any given day the mercury will almost certainly fall short of that mark or overshoot it, perhaps by a lot. Manhattan thermometers can reach 18 in January every so often and plunge to 10 in July. And seasons are rarely normal. Winter snowfall and summer heat waves beat the average some years and fail to reach it in others. It's tough to pick out overall changes in climate in the face of these natural fluctuations. An unusually...