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THIS PAGE WAS REWRITTEN AND UPDATED IN JANUARY 2010

SUMMARY

The average annual minimum temperature in Weymouth has been rising since about 1985 after being steady since records began in 1927. It is now higher than at any time since records started in 1927.

The average annual maximum temperatures have not changed significantly between 1927 and 2008 apart from a peak around the 1940s when maximum temperatures averaged about one degree Centigrade above the long-term mean.

The overall annual mean temperature in Weymouth in the first decade of the 21st century was about one degree Centigrade above the pre-1990s mean.

Changes in average temperatures over seasons of the year show no dramatic changes. Any changes are within a 2 degree Centigrade window and are suggestive of long-term cyclical changes rather than a general warming trend.

The most dramatic changes are seen in the number of days each year on which the temperature exceeds a stated value - see figure 6. This shows that 'heat waves' occurred in the 1940s and 1970s with relatively few really hot days in the 1930s, 1960s and in recent decades. There is no evidence that Weymouth summers are getting hotter with more extreme temperatures. Indeed, the number of very hot days in recent years has been about one-half of the number of very hot days experienced in the 1940s and 1970s.

There was a slight increase in the number of frosty days from 1927 to about 1990 after which the number dropped dramatically. There has been a rise since about 2000.

ANALYSIS

Daily maximum and minimum temperature records have been collected in Weymouth since 1927. This analysis uses all the data up to the end of 2008.

Figure 1

Figure 1 shows the annual average minimum and maximum temperatures as well as the annual daily average. The green curves are five-year running means.

The average annual minimum temperature in Weymouth has been rising since about 1985 after being steady since records began in 1927. It is now higher than at any time since records started in 1927.

The average annual maximum temperatures have not changed significantly between 1927 and 2008 apart from a peak around the 1940s when maximum temperatures averaged about one degree Centigrade above the long-term mean.

The overall annual mean temperature in Weymouth in the first decade of the 21st century was about one degree Centigrade above the pre-1990s mean.

Figure 2

Figure 2 shows the average maximum and minimum temperatures in the first quarter of each year with a five-year average plotted in green. It can be seen that maximum temperatures have been fairly constant throughout the eighty-year period. Minimum temperatures were largely unchanged although there is some evidence of a small rise in minimum temperatures since about 1990.

Even so, mean temperatures are much the same as values recorded in the 1940s.

Note that the exceptionally cold winters of 1947 and 1962 stand out in the above chart.

Figure 3

Figure 3 shows the average maximum and minimum temperatures in the second quarter of each year with a five-year average plotted in green. It can be seen that maximum temperatures rose from 1927 until 1960 after which they fell. Since 2000 the maximum mean temperature has started to rise again but values are still lower than in the 1930s and 1940s.

Minimum temperatures were largely unchanged until 1960 when they fell by about 1.5 degrees. Since 1980 minimum temperatures have been rising and are now slightly higher than at any time since 1927.

Figure 4

Figure 4 shows the average maximum and minimum temperatures in the third quarter of each year with a five-year average plotted in green. It can be seen that temperatures have remained largely steady since records started in 1927.

Figure 5

Figure 5 shows the average maximum and minimum temperature in the fourth quarter of each year with a five-year average plotted in green. It can be seen that temperatures have remain fairly steady since 1927.

Figure 6

Figure 6 shows the number of days each decade on which the daily maximum temperature exceeded the stated value.

There has been much media concern about the effects of extreme high temperatures with claims that recent summers have produced very hot temperatures which have resulted in many deaths. In fact, the excess number of deaths due to hot summers is greatly offset by any reduction in winter deaths due to milder winters. About seven times as many deaths occur in Europe each year due to excessive cold than due to excessive warmth in summer - see the book 'Cool It" by Bjorn Lomborg (2007) p 17 (ISBN-10: 030738652X)

Figure 6 shows that the hottest periods were around the 1940s and 1950s and again around the 1970s and 1980s after which the number of very hot days has reduced confounding claims that Weymouth's summers are getting hotter.

For five consecutive days starting 15 August 1947 the temperature exceeded 30 degrees centigrade.

In recent years such temperatures have been rare with 4 August 1991 recording 31.0 degrees centigrade and 10 August 2003 recording 31.9 degrees centigrade. Temperatures exceeding 30 degrees centigrade have not been observed on any other days from 1991 to the end of 2009.

The maximum temperature recorded in 2009 was a mere 24.5 degrees Centigrade.

There is no evidence that Weymouth's summers are getting hotter or that extreme temperatures are becoming more frequent. In fact, if anything, in both cases the opposite is true.

Finally, Figure 7 shows the number of days each year upon which a frost was recorded with the white curve showing a running five-year mean.

Figure 7

It can be seen that there was a slight increase in the number of frosty days from 1927 to about 1990 after which the number dropped dramatically. There has been a rise since about 2000.

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