Color key for the plots:
1: At least one observation in a 2-degree latitude-longitude region at any time in the 25-year period.
2: On average, at least one observation in half of the months (=150 observations).
3: On average, at least one observation in all of the months (=300 observations).
4: On average, at least one observation for each day (=109500 observations).
The blue colors do not mean anything.