57 years of qualitative ship-board weather reports are used to assess
apparent trends in precipitation occurrence over the global oceans.
Positive trends of up to ~15% per decade, relative to
the long term mean precipitation frequency at a location, are found over
most tropical and temperate ocean areas, with negative trends of up to
~5% per decade being found principally at higher
latitudes. While it cannot be ruled out that the observed trends are an
artifact of gradual changes in shipboard weather reporting habits or
procedures over time, no specific candidate for such a change has been
identified that could explain the existence of robust positive and
negative trends and their apparent geographic coherence.