At the end of each decade, the National Climatic Data Center updates the so-called "normals" or "averages" of temperature, the numbers forecasters use for comparison in daily and monthly reports. These averages are actually computed using 30 years of data. The averages that we've used for the last ten years were computed from the period 1961 to 1990.

But now that another decade is over, it's time to update the numbers, with the new averages computed using data from 1971 to 2000. Essentially, data from the 1990s replaces data from the 1960s in these new averages. Now, consider that around here, the 1960s was the coldest decade of the last century, while the 1990s was the warmest. As a result, the new averages are higher than the old, with nearly all of the increase coming in overnight low temperatures, by as much as three degrees in some months. Average daytime high temperatures changed very little.

Though the new averages were released a few weeks ago, the Weather Service won't officially use them until January 1, when I'll have plenty more to say about the implications of the change.