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SUMMARY

There appears to be no trend in annual average sunshine for Weymouth between the 1890s and 2005.

However, since 1927, the average sunshine has increased steadily in the quarter October, November and December and fallen slowly in the half-year from January to June.

In the third quarter, July to September, there has been no significant trend in the data.

Summer days were slightly more overcast in the 1940s and 1970s but there is not a strong overall trend.

The sunshine recording instrument has been moved several times, vandalised and replaced in the period 1927 - 2005 so the analysis may be fraught with uncertainty.

ANALYSIS

Climate change modellers do not have as much to say about possible future trends in sunshine levels as they do about temperature, rainfall, winds and sea level.

It is inferred that the number of sunshine hours recorded per year may increase as summers become hotter and less wet and cloudy. This will be offset to some extent by wetter winters. But, the potential hours of sunshine in winter are far less than in summer so one might predict more hours of annual sunshine as global warming gets underway.

The sunshine records for Weymouth may not be reliable. One reason is that the recording equipment has been vandalised several times and in 1991, the recording ball was stolen and not replaced for many months. This explains the gap in the blue curve on figure 1 and the 'drop out' around 1991 in the other graphs.

Another reason was that publication of sunshine figures in the national press was considered to be a good source of advertising for Weymouth as a holiday resort. Great efforts were made to get the highest possible legitimate number of hours of sunshine and certainly to try to get these figures to exceed the records being supplied to the national press by other south coast resorts such as Eastbourne and Brighton.

As the Weymouth weather website states [1]:

"To tempt holidaymakers to seaside resorts every last minute of sunshine was essential and to that end the sunshine recorder was installed at different sites around the town over the years. It has been reported that some resorts entertained underhand methods such as using hot needles to burn the sun cards and thus claim greater amounts of sunshine than actually occurred."

Thus, we have to be cautious about interpreting the sunshine records analysed below.

Figure 1

Figure 1 shows the daily sunshine hours averaged using a one-year moving window (blue curve) and with a five-year moving window (red curve). The green curve is for annual averages recorded by individuals before the 'official' data collection started in 1927.

There is no significant trend in the data.

Figure 2

Figure 2 shows the daily sunshine hours averaged using a one-year moving window (blue curve) and with a five-year moving window (red curve) for the first quarter of each year namely, January, February and March.

There appears to be a significant trend in the data. The sunshine averages drop until about 1990 and then appear to rise rapidly.

However, this could be an artifact created by the theft of the recorder ball in 1991 and the installation of a new ball later that year combined with the move to a new site in 1992 which has a lower horizon than the previous site.

The current levels of sunshine are similar to those existing in the 1940s and earlier which would seem to rule out global warming as a reason for the trends.

Figure 3

Figure 3 shows the daily sunshine hours averaged using a one-year moving window (blue curve) and with a five-year moving window (red curve) for the second quarter of each year namely, April, May and June.

There appears to be little significant trend in the data. 

Figure 4

Figure 4 shows the daily sunshine hours averaged using a one-year moving window (blue curve) and with a five-year moving window (red curve) for the third quarter of each year namely, July, August and September.

There appears to be little significant trend in the data. 

Figure 5

Figure 5 shows the daily sunshine hours averaged using a one-year moving window (blue curve) and with a five-year moving window (red curve) for the fourth quarter of each year namely, October, November and December.

There appears to be a significant upwards trend in the data. 

Figure 6

Figure 6 shows the number of days in Summer upon which no sunshine was recorded. The blue curve is a five-year running average.

It can be seen that Summer days were slightly more overcast in the 1940s and 1970s but there is not a strong overall trend.

REFERENCES

1.  Weymouth Weather Website

http://www.weymouth.gov.uk/main.asp?svid=137&svaid=268&svapid=2449

 

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