Thursday, 5 April 2012

Trends in cloudiness and temperature

One of the fundamentals of the consensus approach to climate change is that increasing temperature should lead to increasing water vapour and cloudiness. One type of data measured but not readily available is ‘hours of bright sun’. Initially it was measured by the Campbell-Stokes solar recorder, developed in middle of the 19th century. It uses a glass sphere to direct sunlight on to specially prepared card which shows a ‘burn’ mark when the sun is shining. Recently, radiation is measured by more modern methods. The hours of bright sun varies inversely with cloudiness.

Data on hours of bright sun, among other parameters, are posted on the website of Hungarian Met Office for four climate stations for the period 1910 to 2000.  ( The following chart shows data for 4 sites for the whole of the period when they all have hours of sun data. There appears to be a break in the data measurement method around 1970; before that period there is a greater variability and less consistency between the stations, after that the data seem to be more consistent. (Note that we have presented the full data set even though part of it is not consistent with later data and could legitimately be excluded.)  From 1970 to 2000 the data show a rising trend of 0.018 hours per year. This is equivalent to an increase of 0.54 hours over the 30 year period relative to average of 5.3 hours of sun per day.

Another similar data set is available from the Australian Bureau of Meteorology website at Here the data are expressed as daily solar exposure in MJ m-2. In this case they are measuring solar radiation which also has an inverse relationship with cloud cover. We have chosen the 5 stations because they have data and are in different parts of Australia. The trend line shows a steady climb over the 21 years with data again indicating a reduction in cloudiness. (To create the trend line we have replaced the 2005 data which has missing data for all stations by the average of preceding and following year). The trend is equivalent to an increase of 0.45 MJm-2 over the 21 years relative to average of 17.9 MJm-2.

The final data set we consider is from the UK and was downloaded from the Met Office web site at From the stations on the web site we extracted monthly hours of sun data for 10 stations which were more-or-less complete from 1931 to 2011. The next chart shows the data for all 10 stations and the average. There is some sign of trend in the data with a minimum around 1970/80.

As this graph is a bit confusing we have plotted the average of the 10 stations on a separate graph and also a 20-year moving trend line. A moving trend line calculates the slope of the 20-year trend every year from the 20th year with data up to the end of the data set.  This shows that in the 20 years up to 1966, the trend was -0.037 hours per year. In the 20 years preceding 2000 the trend was +0.041 hours a year.  Given that hours of sun varies inversely with cloudiness, this demonstrates that not only does cloudiness decrease with increasing temperature but increases with decreasing temperature.

 These charts using disparate data sets but they all show one thing; temperature and cloudiness are closely related.


Anonymous said...

You might try to take a look at USCRN data, too. Their oldest stations only start in 2000 but they provide very reliable data as they have digital solar irradiation meters.

Administrator said...

Thanks for the lead but I'm really interested in long-term data.