Iceland weather in September

Climatic data Reykjavik (on any? homepage of a met.Institute):
Reykjavik (not Iceland):
Precipitation: 67 mm - normal NW-European level. Dryer that wettest areas in WEurope
Days with (some) precipitation: 12
temp (day-night) 10°- 5°C 125 hours of sunshine

Note that in and close to the Atlantic Ocean - an individual month can deviate significantly from the statistical average.

Free sources on weather on the web can be rather varied in the quality of their data. One site I came across purported to give you climatic data for a remarkable number of towns on the planet, but after looking at it carefully I discovered it was merely a database of their past weather forecasts, not actually founded in measuring anything.

This source
http://www.weather2travel.com/climate-guides/index.php?destination=reykjavik
gives 86mm rain and 18 days with some precipitation.
This source
http://www.bbc.co.uk/weather/world/city_guides/results.shtml?tt=TT003890
agrees with #1's 67mm, but says 19 days with some precipitation.

Average rainfall can vary with which period you measure the average over. 1960-90 was a commonly used data period for climate statistics not so long ago, and it is generally accepted that things have moved on since then in many places. Free data may be based on an old publication using 1960-90.My perception is that September has been rather wet in recent years, so the 86mm may be based on more recent data. A monthly quantity like 86mm in a month is not untypical of a place like Naples, which obtains a few heavy downpours in a month but the sun shines most of the time. But nor is it untypical of a place like the Scottish Highlands where light rain falls intermittently a lot of the time.

"Days with some precipitation" varies with what you allow as a minimum amount for "some". The second source I gave there states a minimum of 0.25mm; so shows more wet days than a source which uses, say, 1mm, which is also common. My own rain gauge is incapable of measuring less than 0.5mm, so I wouldn't pick up as many days with "some precipitation" as the BBC if I took daily readings from it. You can have quite a prolonged period of misty drizzle and not break the 0.5mm barrier, I know from experience. The thing about Reykjavik is that it is prone to intermittent light rain, so the delivery of even 67mm of rain can occupy a significant period of time.

I would prefer to direct you to the Iceland Met office for data of known source and reliably described: it used to publish some free official climatic stats for Reykjavik and other locations, but since it updated the site they are no longer there. I think it wants to sell climatic data now.

The met. institutes data are normally good. So are BBC (Mine were from Danish Metr. Inst - super quality).

Iviehoff points on a big problem: you an base statistics on a short, resent period and postulate that "this is how weather is now" or on a longer period and have basically more reliable data (Lower St.Dev. of averages).

Another problem is pointed out by iviehoff: What is a day of precipitation? 0.1 mm 1 mm, 5 mm ?
All good sitres will have the definitions on the homepage, The 12 days from dmi.dk are based on 1 mm!!! - so there you have the difference bbc-dmi. I would say that a 0.25mm/day limit gives very pessimistic estimates seen from a tourists point of view.