Two weather stations stand about nine kilometres apart, one on the roof of a school in the city and one on a hill above the reservoir. On the last Thursday of September they recorded the same afternoon, and the two records read very differently.
The city station was at thirty-one degrees at two in the afternoon and nineteen degrees at eight in the evening. Six hours, twelve degrees. The bulletin said the temperature had fallen steadily, so the fall belonged to each hour equally: two degrees an hour, hour after hour. At five o'clock, three hours in, the reading was twenty-five degrees, exactly as that rate of change predicts.
The hill station started cooler and ended warmer: twenty-nine degrees at two, twenty-three at eight. The same six hours, but only six degrees — one degree an hour.
Drawn on the same axes, the two records are two straight lines going down, and the city's line is the steeper one. That word is doing real work. It does not say the city was hotter, although at two o'clock it was. It says the city cooled faster.
A forecaster reading those two lines is not really looking at temperatures at all. She is looking at how fast each number is moving, which is the only part of the picture that tells her what tonight will be like.