May is normally the sunniest month of the year in Ireland

This could be a thread that could open and close with its title, there isn't a lot more to say you might think ... but I will anyway, details details.

Sunshine hours are measured at a number of locations in Ireland and normal values have been calculated (on the met-ie website under climate of Ireland, 30 year averages).

I don't know what technology is used now or in Ireland, I had some experience as a part-time weather observer for the Toronto City station which is near the University of Toronto and measured sunshine from 1882 onwards, my contribution was on June 1st, 1969 because Pele was in town for a soccer game and you could see the playing field from the rooftop of the weather office so I volunteered to measure everything that day. And I discovered (a) Pele was a damn good soccer player and (b) you measure sunshine by placing a cardboard strip in a globe that then burns the strip as the Sun moves along its path in the sky. You can then calculate how many hours of sunshine occurred, but one might note that a location like that would be in the shadow of nearby tall buildings near sunrise and sunset, so a better location at an airport would be unobstructed all the time.

Sunshine can only happen when the Sun is above the horizon, so there are two different measures of the amount of sunshine, the absolute totals, and the relative percentages of possible sunshine.

May in Ireland has an average of 6 hours of sunshine at the variety of stations available and it varies from 5.8 hours in the midlands to 6.9 hours in the sunny southeast, and 6.5 hours at the northern tip which has the advantage of a longer daylight period, so really that is not an indication of clearer skies. The table below shows the average daily sunshine at various locations for the recent 30-year period 1981-2010, except where noted (two locations closed down their sunshine recording in 2007 and 2008 as noted so the thirty years are made up with years from the late 1970s). The table shows the next highest sunshine daily average and what month(s) that occurred, and also the number of days with zero sunshine in May. That is often the lowest value for the year, but not at all these locations, for example, Casement has an average of 1.1 days with zero sunshine in August and 1.6 days in July.both lower than May's 1.8 days.

Location _____ Avg daily sun ___next highest ___ days with zero sun hrs (May)

Rosslare (78-07)___ 6.9 _______ 6.3 July _____________1.7

Malin Head _______ 6.5 _______ 5.5 June ____________ 2.3

Cork _____________6.2 _______ 5.8 June ____________ 1.8
Dublin ___________ 6.2 _______ 5.8 June ____________ 2.0
Belmullet _________ 6.1 _______ 5.2 April, June _______ 2.0
Casement _________6.0 _______ 5.3 June ____________ 1.8
Valentia __________ 5.9 _______ 5.3 June ____________ 2.8
Shannon _________ 5.8 ________5.2 June ____________ 1.9
Mullingar (79-08)___ 5.8 _______ 5.0 June ____________ 1.9

Looking at the 1961-1990 averages (these do of course overlap the above by the ten years 1981-90) the amount of sunshine has increased slightly at most locations, the 1960s must be the culprit there, and in fact Casement then had almost equal sunshine amounts in May and June (both 5.7-something). A greater precision was used in those tables (2 dec instead of one).

A sunshine peak in May is unusual, but two other climate zones have a similar spring maximum (April-May), namely the central arctic islands of Canada and southeast Asia (where the summer monsoon puts an end to steadily rising sunshine hours). I imagine there may be locations elsewhere that are similar, for example near the Mexican border with Texas. The more standard sunshine peak is either June or July, for example at Toronto the peak is 8 hours a day on average in July, May has a similar amount to what you get in Ireland.

You could compare months by percentage of possible sunshine too. The sun is above the horizon for about 15.5 hours on average in May (in Ireland) and probably a bit over 16 hours at Malin Head, while in April it's about 14 hours. So May should have a higher daily average by a factor of 15.5/14 which is 111 per cent, however, the one location that has April tied for second (Belmullet) has about 17% more sunshine in May than April. So May appears to win this race either way you want to measure it. June and July would have the opportunity for 10 and 4 per cent more sunshine respectively, compared to May, while August would have 6 per cent less opportunity.

If we say that the average across all of Ireland is 6 hours a day and that from a potential of 15 (allowing for some horizon blockout) then May has a sunshine percentage of 40% of possible hours, and this month has been above 50% (about 54 at present). This is a little below the normal values of sunshine across most of North America, for example, but not a lot below (55 to 70 per cent would be the spread for most inland locations, the cloudier coasts are similar to Ireland).

This May has been particularly sunny. In our contest, we use the six locations Belmullet, Casement, Dublin, Shannon, Valentia and Cork. Up to yesterday the running average sunshine per day was about 8 hours and the surplus about 30 per cent. On 2nd of June we can report in this thread how the national sunshine data ended up relative to both normal and long term extremes. I'm guessing that some May in the past managed to achieve 9 or even 10 hour averages, the spread in monthly sunshine is often from 50 to 150 per cent of normal values.

The least sunny months, as would surprise nobody, are December, January, November in that order. They have average amounts of 1 to 2 hours. That comes from opportunities lasting only 8 or 9 hours so the percentage comparison is a bit better, perhaps 20% compared to May's 40%.

Why is "high summer" cloudier than late spring? (at least in the overall statistics, some years like 2013 may have turned out differently). The main reason I can see is that blocking high pressure is more likely to occur in May than in June or July, and also, the ocean warms up near shore which can lead to longer periods of morning fog and low cloud at the west coast locations (notice that Belmullet falls off from May to June faster than the east coast stations). Another factor is that tropical moisture begins to join the circulation more in June which is normally the start of the tropical storm season. Moist southwest flows are poor for sunshine especially on the west coast.

Rosslare being closer to Britain and the continent starts to show the bimodal peak in May and July which often displays due to a tendency for June stagnant low pressure reducing the hours. That stagnant low is often located around central Britain or the North Sea so that it has less influence on the sunshine in other parts of Ireland, where June tends to be sunnier than July. A sunny June of course can lead the way in any year, but the tendency is for June to fall behind both May and July in some climate zones. This is the case around where I live too, July and August are often a lot sunnier than June.