Q:  How are anomalies calculated?

A:  An anomaly is the difference between an actual value and some long-term average value.

For example, if

   X = actual value of average temperature for January, 1982 and
   Xbar = long-term average temperature for January (an average over many years)
   anom = anomaly value for January, 1982

then

   anom = X - Xbar
 

--Michael Bell.