Compare this year's temperature to a movie of an average year of daily mean surface temperature. This is the climatology. This climatology covers the period from Jan. 1 1946 until Dec. 31, 2002.
It is difficult to distinguish this year's temperature from
an average year's temperature. So we look at the anomaly (the
difference between them).
is the surface temperature
anomaly this year and an average year in the period from 1948 to 2002.