Upload masks
The area average of a field can be restricted to a user-defined mask, as well as the usual land and sea masks. These masks have to be uploaded first. They should be in the format
# optional comments
longitude1 latitude1 [optional information, is ignored]
longitude2 latitude2 [optional information, is ignored]
longitude3 latitude3 [optional information, is ignored]
...
The path is closed by drawing a segment from the last line to the first. A grid point is included if the line from the South Pole to the point crosses an odd number of segments, it is given weight 0.5 if it is on a segment, and is excluded if it crosses an even number of segments. The definition can be reversed by checking the box "include South Pole".

There are a few standard masks:

Please contact me if you have another dataset that you would like to have in this list.