These graphs show the departure from the 100-year average of the total monthly precipitation 
amounts from 1950-1998 for BWI airport.

The shaded blue regions represent drier than normal conditions and the shaded red areas are wetter 
than normal conditions.

The dashed lines separate the seasonal boundaries.  For a given year, winter is defined as December of 
the previous year through February of the current year.

These diagrams clearly illustrate that it is normal for the weather to deviate from the mean from day to 
day. In addition, the year to year differences can also be quite dramatic for a given month or even season.