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Record Sunshine In April

May 7, 2015

By Paul Homewood




The Met Office have just announced that last month was the sunniest April on record in the UK, beating the previous record set in 1942. This continues the trend towards much sunnier springs since the 1980’s. (See here.)

Whatever the cause, and there is evidence to suggest that reduced air pollution has something to do with it, there is inevitably an effect on temperature trends. Unsurprisingly, we find that sunnier months tend to be warmer.




We can also see how April temperatures began climbing at around the same time that sunshine hours were increasing.




A Met Office report, Climate Memorandum 21, published in 2006, found strong correlation co-efficients between sunshine and temperatures in spring time. This report also found that this increase in sunshine hours “could be the result of the Clean Air Acts of 1956 onwards”.


All of this raises the question – just how much of the step up in UK temperatures since the 1980’s been caused by more sunshine, rather than CO2?

  1. May 7, 2015 10:38 am

    Guess that it is time that the sunshine hours were homogenised: that will change the trend!

  2. May 7, 2015 10:58 am

    It’s been noticeable chilly if not in direct sunshine this spring in the East Midlands at least.
    I would think that April temperatures would not be a record even though sunshine hours are. If the sunshine hours were not so high I would suspect April temperature would have been substantially lower.

  3. May 7, 2015 11:49 am

    I suspect that the wind direction (mostly Easterly) has got something to do with it, relatively dry and cloudless continental air.

  4. Steve C permalink
    May 7, 2015 1:13 pm

    I can confirm Lord Beaverbrook’s comment on the E. Midlands – it’s been OK in the sun, but distinctly chilly out of it. Days of lower wind speeds felt very pleasant, though.

    OT, but today The Register is reporting a critical XSS vulnerability in (often default) WordPress plugins, specially mentioning the Twenty Fifteen one. Be aware – this site is much too valuable a resource to lose!

  5. A C Osborn permalink
    May 7, 2015 1:17 pm

    Paul, I doubt very much if the Sunshine is anything to do with less air polution.
    I lived through the 50/60s Smogs and there was not that much less Sunshine than today.
    Also there were a couple of years in the 70s when there was very little sunshine, with high cloud day after day, but not rain.
    I think Cloud Cover is the key, which is controlled by Wind direction as mentioned by climanrecon, plus of course the possibility of Cosmic Ray interactions on cloud formation.

  6. May 7, 2015 1:41 pm
    This study analyses the interannual variability of sunshine duration (SDU) at the urban area of Athens from 1897 to 2011. Observations of total cloud cover (TCC) are also used for a better interpretation of SDU variations. The annual SDU in Athens has increased by +8% (+19 h/decade) over the past century, mainly due to increase in the summer and spring SDU, however, distinct sub periods with decreasing and increasing trends are also discerned. SDU in Athens has undergone an abrupt increase during 1940s with early 1950s being the brightest period of the record. For long periods the course of SDU mirrors TCC, indicating a strong negative correlation between the two variables, nevertheless during the last three decades, both variables reveal trends of the same sign (more evident in spring). Under all-sky conditions, annual SDU decreased by approximately 7% from 1950s to 1980s and increased by 3% thereafter. Under clear sky conditions, the increase of SDU after 1980s is larger, amounting to 9%. Singular spectrum analysis and Continuous Wavelet Transform indicated significant non-linear trends of SDU and an intermittent oscillation, centered at 2.9–3.0 yrs.
    The annual sunshine duration mean time series shows a decrease from the early 1960s to the late 1970s [in Iran], in line with the widespread dimming of surface solar radiation observed during this period. By the early 1980s, there is an increase in sunshine through the end of the 20th century, aligning with a well-known and well-documented brightening period.
    Total global solar shortwave (G) irradiation and sunshine duration were recorded at nine Spanish stations located in the Iberian Peninsula. Averaged series (using the nine locations) showed a statistically significant decrease in annual G [global dimming] from 1950 to the mid 1980s (−1.7%dc−1) [-8.5 W/m-2] together with a significant increase [global brightening] from the mid 1980s to 2011 (1.6%dc−1) [+8 W/m-2].
    Trends in downwelling global solar irradiance were evaluated at high elevation sites on the island of Maui, Hawaii. Departures from monthly means were assessed for the 6-month Hawaiian wet and dry seasons over the period 1988 to 2012. Linear regression analysis was used to characterize trends in each season. For the dry season (May-October), statistically significant (p ≤0.05) positive trends of 9–18 W m-2 (3–6%) per decade were found at all four high elevation stations tested. Wet season trends were not significant, except at the highest elevation station, which had a significant negative trend. No consistent trends in aerosol concentrations have been observed at high elevations in Hawaii, therefore, the observed dry-season brightening is most likely the result of decreasing cloud cover.

    Click to access acp-12-8635-2012.pdf

    Solar radiation incident at the Earth’s surface, also known as surface solar radiation (SSR), is the fundamental source of energy in the climate system, and consequently the source of life on our planet, due to its central role in the surface energy balance and for processes such as evaporation and the water cycle or plant photosynthesis (e.g. Wild, 2009, 2012). Here we report a significant increase of +2.68 W m−2 per decade during the 1981–2010 period, slightly higher than the reported trends in Switzerland by Ruckstuhl and Norris (2009) (+1.8 W m−2 per decade). Norris and Wild (2007) estimated a widespread increase in clear-sky SSR over whole Europe around +2.0–2.3 W m−2 per decade, which is in good agreement with the present study.
    Wild et al. (2009) investigated the global solar radiation from 133 stations from GEBA/World Radiation Data Centre belonging to different regions in Europe. All series showed an increase over the entire period, with a pronounced upward tendency since 2000. For the Benelux region, the linear change between 1985 and 2005 is equal to +0.42Wm-2 per year, compared to the pan-European average trend of +0.33Wm-2 per year (or +0.24Wm-2 if the anomaly of the 2003 heat wave is excluded) (Wild et al. 2009). Our trend at Uccle of +0.5 ( 0.2)Wm-2 per year (or +4% per decade) agrees within the error bars with the results from Wild et al. (2009).
    The Carpathians are the longest mountain range in Europe and a geographic barrier between Central Europe, Eastern Europe, and the Balkans. To investigate the climate of the area, the CARPATCLIM project members collected, quality-checked, homogenized, harmonized, and interpolated daily data for 16 meteorological variables and many derived indicators related to the period 1961–2010….Temperature was found to increase in every season, in particular in the last three decades, confirming the trends occurring in Europe; wind speed decreased in every season; cloud cover and relative humidity decreased in spring, summer, and winter, and increased in autumn, while relative sunshine duration behaved in the opposite way [increased]; precipitation and surface air pressure showed no significant trend, though they increased slightly on an annual basis. We also discuss the correlation between the variables and we highlight that in the Carpathian Region positive and negative sunshine duration anomalies are highly correlated to the corresponding temperature anomalies during the global dimming (1960s and 1970s) and brightening (1990s and 2000s) periods.
    SDR [shortwave downward radiation] annual mean time series [was] averaged over the eight German and the 25 Swiss sites for all-sky (all measured situations), cloud-free and cloudy periods. All-sky solar irradiance (SDR) shows positive trends from 1981 to 2005 at the German and the Swiss sites. The average increase in SDR at all stations is +2.99 [+0.52 to +5.46] W/m-2 per decade. All-sky radiation trends are largely affected by the year 2003, with strongly reduced cloudiness and hence increased shortwave radiation during the extreme summer.
    A pronounced summer warming is observed in Europe since the 1980s that has been accompanied by an increase in the occurrence of heat waves. The authors show that the variance of European summer temperature is partly explained by changes in summer cloudiness. Using observation-based products of climate variables, satellite-derived cloud cover, and radiation products, the authors show that, during the 1984–2007 period, Europe has become less cloudy (except northeastern Europe) and the regions east of Europe have become cloudier in summer daytime. In response, the summer temperatures increased in the areas of total cloud cover decrease and stalled or declined in the areas of cloud cover increase. Trends in the surface shortwave radiation are generally positive (negative) in the regions with summer warming (cooling or stalled warming), whereas the signs of trends in top-of-atmosphere (TOA) reflected shortwave radiation are reversed. The authors’ results suggest that total cloud cover is either the important local factor influencing the summer temperature changes in Europe or a major indicator of these changes.
    Literature estimates for the overall SSR [surface solar radiation] decline during dimming (1950s to 1980s] range from 3 to 9 W m−2, and from 1 to 4 W m−2 for the partial recovery during subsequent brightening [1980s to 2000] (Stanhill and Moreshet 1992; Liepert et al. 1994; Abakumova et al. 1996; Gilgen et al. 1998; Stanhill and Cohen 2001; Alpert et al. 2005; Kvalevag and Myhre 2007; Kim and Ramanathan 2008; Wild 2009).
    Traditionally the Earth’s reflectance has been assumed to be roughly constant, but large decadal variability, not reproduced by current climate models, has been reported lately from a variety of sources. There is a consistent picture among all data sets by which the Earth’s albedo has decreased over the 1985-2000 interval. The amplitude of this decrease ranges from 2-3 W/m2 to 6-7 W/m2 but any value inside these ranges is highly climatologically significant and implies major changes in the Earth’s radiation budget.

    Click to access RAD_2005-3_Wild_et_al.pdf

    A similar reversal to brightening in the 1990s has been found on a global scale in a recent study that estimates surface solar radiation from satellite data. This indicates that the surface measurements may indeed pick up a largescale signal. The changes in both satellite derived and measured surface insolation data are also in line with changes in global cloudiness provided by the International Satellite Cloud Climatology Project (ISCCP), which show an increase until the late 1980s and a decrease thereafter, on the order of 5% from the late 1980s to 2002. A recent reconstruction of planetary albedo based on the earthshine method, which also depends on ISCCP cloud data, reports a similar decrease during the 1990s. Over the period covered so far by BSRN (1992 to 2001), the decrease in earth reflectance corresponds to an increase of 6 W m-2 in absorbed solar radiation by the globe. The overall change observed at the BSRN sites, estimated as an average of the slopes at the sites in Fig. 2A, is 0.66 W m-2 per year (6.6 W m-2 over the entire BSRN period).
    Global [surface solar] radiation has an overall positive, and significant, trend [1983-2010] over the Meteosat disk which is mainly due to a negative trend in the effective cloud albedo, i.e., a decrease in cloudiness. Trends due to changes in the clear sky radiation are small and only induced by trends in the water vapor fields. Trends caused by changes in the direct effects of atmospheric aerosol are not represented because an aerosol climatology is used. … All considered regions show positive trends for the extended CM SAF surface radiation dataset pointing to an increase in solar surface radiation and, thus, a brightening by either a decrease in cloudiness or a decreased atmospheric absorption of solar radiation. However, the extent and also the significance of the trends in the different regions vary substantially. The trend for Europe of 4.35 W m− 2 dec− 1 is in the order of trends derived from surface measurements by Wild (2012) of 2 W m− 2 dec− 1 for the 1980s to 2000 and 3 W m− 2 dec− 1 after 2000.
    [T]here has been a global net decrease [of 3.6%] in 340 nm cloud plus aerosol reflectivity [which has led to] an increase of 2.7 W m−2 of solar energy reaching the Earth’s surface and an increase of 1.4% or 2.3 W m−2 absorbed by the surface [between 1979 and 2011].
    The decadal trend shown in the 5-year running mean indicates a period of rapid increase [solar radiation reaching the surface/brightening] starting in late 1930s and continuing to early 1950s with a change of 10 W m2. The dimming trend from the early 1950s to the late 1980s shows a decrease of 13 W m2. The subsequent increase starting in late 1980s is about 10 W m 2 by 2005. These changes are not confined to a small number of stations in western Europe, but shared by more than 400 other sites where global irradiance has been continuously observed for more than 40 years. … Global solar irradiance showed a significant fluctuation during the last 90 years. It increased from 1920 to 1940s/1950s, thereafter it decreased toward late 1980s. In early 1990s 75% of the globe indicated the increasing trend of solar irradiance, while the remaining area continued to lose solar radiation. The magnitudes of the variation are estimated at +12 W m 2 [1920-1940s/1950s], – 8 W m 2 [1950s-1980s], and +8 Wm2 [early 1990s-2005], for the first brightening, for the dimming, and the recentbrightening periods, respectively.
    Surface incident solar radiation G determines our climate and environment…Data from this summation method suggest that surface incident solar radiation increased at a rate of 6.6 W m−2/decade−1 (3.6%/ decade) from 1992 to 2002 (brightening) at selected sites.

  7. May 7, 2015 3:07 pm

    The dutch study reported here recently showed that the reduction of SO2 has been enormous in terms of the GLC measurements. This is common throughout the western world simce the 1980s when pollution reduction was introduced. The reduced sulphate is probably the most important point in setting the value of the radiative forcing function that drives all the IPCC models. If this falls the forcing function increases greatly and the earth heats even faster ………… demonstrating again that in mathematical models GIGO.

    It may be hard to prove that cloud cover is related to aerosols as AC Osborn notes, but it is a much more likely cause of any real warming than CO2 increases.

  8. Andy DC permalink
    May 7, 2015 9:42 pm

    Another example of extreme weather. Extremely nice weather that is. We are all going to die!

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