[RWood]

{Note: Because of the difficulty in selecting stations with long-term records and no significant changes of site, I've limited the remarks to that zone - I suspect something similar has happened in some southern areas, though to a lesser extent. Some NZ sites show a drop for a time from the 1950s, but no marked increase afterwards.}

March sunshine records at Blenheim, Nelson and Wellington show an appreciable drop over several decades of the last century, followed by a larger rise for the following 2 decades: The values shown are the averages in hours for the given period.

Blenheim 1930-1953 225.7
1954-1988 205.2
1989-2009 233.0

Nelson 1922-1953 226.4
1954-1988 200.6
1989-2009 238.2

Wellington 1928-1953 195.0
1954-1988 183.7
1989-2009 210.2

It looks as if March 2010 is going the same way - but of course there's no way of knowing if/when the Marches will revert to a cloudier phase. On shorter timescales, odd things have happened - most recent Septembers have been rather unsettled, but from the late 1940s to the late 1950s September was probably the most settled month of the year, on average. Even much-maligned October had a pretty good run in the 1960s. Annual numbers too can show lengthy runs of "deviant" values - at Kelburn and its prior sites, the average rainfall for 1906-1935 was at least 180mm less than for the entire period of record (1863+).

This post has been edited by RWood: 09 March 2010 - 11:10 AM

[TokWW]

In each case, the third period is greater than the first, hence a noticeable, repeatable trend. Assuming no man made influence could have made this change, then are we considering the ocean temperatures and similar effects - as the warmth of the ocean will encourage the evaporation rate and uptake by the wind, to provide for cloudier times... but then it also depends too, on the prevailing wind and over which ocean it travels... hmmm

Graeme

This post has been edited by TokWW: 09 March 2010 - 11:50 AM

[RWood]

TokWW, on 09 March 2010 - 12:49 PM, said:

In each case, the third period is greater than the first, hence a noticeable, repeatable trend. Assuming no man made influence could have made this change, then are we considering the ocean temperatures and similar effects - as the warmth of the ocean will encourage the evaporation rate and uptake by the wind, to provide for cloudier times... but then it also depends too, on the prevailing wind and over which ocean it travels... hmmm

Graeme

Yes, good thoughts - the kind of thing that makes weather/climate so difficult and fascinating. I should add that in the 3rd period, rainfall in the 3 locations is down slightly (more so in the case of Blenheim) on the 2nd period, but not as low as in the 1st period.

This post has been edited by RWood: 09 March 2010 - 12:37 PM