Summary

Climate modelling experts predict changing rainfall patterns in southern UK as a result of running climate change models, e.g.,

"Climate change simulations indicate a trend towards larger rainfall totals during winter and reductions in summer (especially in southern UK), and an increase in the intensity of precipitation (especially during winter)" [1].

"The weather is changing too. Experts expect our summers to become drier, but when it does rain it will probably rain harder. At the same time UK winter rainfall may increase by between 5 and 20 per cent, leading to worse and more frequent flooding." [2]

"Winters will become wetter and summers may become drier throughout the UK. Snowfall amounts will decrease throughout the UK. Heavy winter precipitation will become more frequent." [3]

We see that wetter winters and drier summers are expected with more days having intense rainfall in the winter than now.

What does the Weymouth weather record predict based on past trends?

The graphs below tend to show no overall change to annual rainfall. However, the number of days each year when rain falls has increased by 50 days since1990 but the number of days on which it rains hard has not changed. This means that there are more days with light rain. This trend for more wet days affects every season with April, May and June showing slightly greater increase than other quarters.

However, this increase in the number of wet days since 1990 follows a prolonged period of decreasing number of wet days since about 1940. The figures for wet days are now about the same as in the late 1920s and 1930 strongly suggesting that global warming is not the cause of these changes but, rather that they are due to a long-term natural cycle.

Figure 4 shows a clear short-term cycle at work with a period of 15 - 20 years; evidence that global warming is not the cause of the observed changes in rainfall.

Actual average rainfall shows a decrease in the second half of each year. This broadly agrees with the predictions by climate modellers but this is coincidental. This is because there was an increase in summer and autumn rainfall before about 1958 - see figure 9. As a result, summer and autumn rainfall is much the same as in the 1930s. 

Thus, the long-term changes in rainfall are almost certainly due to long-term natural weather cycles and not due to global warming.

ANNUAL RAINFALL

Daily rainfall has been recorded since 1927 at a variety of locations around Weymouth. The chart below shows the data averaged using an annual moving window to smooth out seasonal changes.

Figure 1

The red curve is based on daily records whilst the blue curve is based on records of the annual rainfall provided by private observers. 

This chart shows that there has not been any significant change in annual rainfall over the past eight decades in Weymouth.

Figure 2

Figure 2 shows the number of days each year from 1927 to 2004 upon which rain fell in Weymouth. 

It can be seen that the number of wet days was about 50 days higher than average in the 1920s and 1930s and again around 1995 - 2004 although the current trend is for Weymouth's wet days to be about 20 more than the long-term average.

There is no evidence that Weymouth's long-term trend is for more wet days each year. Indeed, since 2000 the number of wet days has been falling.

The long drought of 1976 shows up in figure 2 quite clearly.

If we count the number of days upon which rain exceeding a given amount fell then there are no long-term trends up or down. Thus, the number of days on which heavy rain falls has stayed pretty well constant since 1927.

SEASONAL RAINFALL

Figure 3

Figure 3 shows the number of wet days in the first quarter of each year; this being counted as the months of January, February and March. The red curve is a five-year running average.

It can be seen that there has been an increase in wet days of about 25% in Weymouth since about 1990.

Figure 4

Figure 4 shows the number of wet days in the second quarter of each year; this being counted as the months of April, May and June. The red curve is a five-year running average. 

It can be seen that there has been an increase in wet days of about 50% in Weymouth since about 1990 and that there is a clear 15 - 20 year cyclic variation in number of wet days.

Figure 5

Figure 5 shows the number of wet days in the third quarter of each year; this being counted as the months of July, August and September. The red curve is a five-year running average. 

It can be seen that there has been an increase in wet days of about 40% in Weymouth since about 1990.

Figure 6

Figure 6 shows the number of wet days in the fourth quarter of each year; this being counted as the months of October, November and December. The red curve is a five-year running average. 

It can be seen that there has been an increase in wet days of over 50% in Weymouth since about 1990.

Although there seems to be a consistent increase in the number of wet days in Weymouth in every quarter since 1990, the number of wet days now is much the same as in the 1930s so this cannot be taken as evidence of 'global warming'.

Figure 7

Figure 7 shows the annual rainfall (mm) in the first quarter of each year and it can be seen from the five-year running average (the red curve) that there has been an increase of about 25% since 1927. Thus, winters may be getting wetter.

Figure 8

Figure 8 shows the annual rainfall (mm) in the second quarter of each year and it can be seen from the five-year running average (the red curve) that there has been no significant increase since 1927. 

Figure 9

Figure 9 shows the annual rainfall (mm) in the third quarter of each year and it can be seen from the five-year running average (the red curve) that there was a peak in about 1957 after which this quarter has been experiencing reducing rainfall. This now stands at about 50% below the 1957 peak. However, the mean rainfall in the third quarter in 1990 was about the same as in 1935 suggesting a long-term cycle in the weather and that global warming is not responsible.

Figure 10

Figure 10 shows the annual rainfall (mm) in the fourth quarter of each year and it can be seen from the five-year running average (the red curve) that there has been a small but scarcely  significant decrease since 1927. 

REFERENCES

1.   Jones PD and Reid PA (2001) 'Assessing future changes in extreme precipitation over Britain using regional climate model integrations.' International Journal of Climatology 21, 1337-1356.

2.    Environment Agency (2002) How UK Weather Will Change. http://tinyurl.com/9ck87

3     UK Climate Impacts Programme  

  http://www.ukcip.org.uk/climate_change/how_uk_change.asp