Odds from Ch'ch Gardens rainfall records

[RWood]

I loaded down the rainfall monthly records for Christchurch Gardens, the longest-running NZ site (1863+). There are no records for 1885-1893, and quite a few other missing months early on, but from mid-1898 the record is almost complete.

One mildly interesting point is that over the entire record (1863-2006), the 3 wettest months on average are July, May and June, with a gap of 8mm or more back to the remaining ones. However, with many drier Mays and wetter Augusts in the last 30-40 years, the more recent 30-year averages (1971-2000) show the 3 wettest months as July, August and June, with a 10mm gap back to the rest. Whether this apparent "shift" will remain is of course unknowable.

Longest run of wet(ter) years: 1974-1980 (7)
Longest run of drier years: 1926-1933 (8)

With the monthly values, the skew in distribution in all 12 cases means there are many more "dry" months than "wet" ones. No month has had more than 4-5 years in a row above average.

Noteworthy "dry" runs of months: April, 1926-1935
October, 1870-1882
October, 1957-1969
December, 1964-1975
December, 1994-2003

In 1911-1935, March was near or below average for rainfall 22 times out of 25.

Consecutive drier months: November 1987 - April 1989 (18)