Some Storm-Related Weather Trends in Weymouth
DRAFT
SUMMARY
A previous report (http://www.geoffkirby.co.uk/CoastalReport.pdf) examined flood risks in the
vicinity of Weymouth Harbour caused by extreme tidal effects, low barometric pressures and
high winds. This report examines primarily the flooding risks over a broader area surrounding
Weymouth caused by high rainfall. This exploits the unique 127-year record from the
Weymouth Weather Station. These data show that rainfall trends are not conforming to those
predicted by climate modelling for the Southern England region. Indeed, rainfall trends in
Weymouth are cyclical with no clear long-term trends. The cycles have periods between 10
and 80 years. In general, the rainfall patterns being experience in this century were also
experienced in the 20th century.
As an example, whilst the annual rainfall has shown no significant change since 1881, the
number of wet days each year is now about fifty days higher than the period 1940 - 1995.
Weymouth is becoming wetter; the same annual rainfall is falling on a much greater number of
days compared with the second half of the 20th century. However, the present number of
annual wet days is much the same as experienced in the 1930s.
An analysis of wind statistics allowed a simple empirical equation to be proposed for predicting
extreme wind frequency.
10 December 2009
Geoff Kirby BSc(London), BSc(Open)
6 College Lane, Weymouth, Dorset, DT4 7LP
 01305-787253
geoff@geoffkirby.freeserve.co.uk
Some Storm-Related Weather Trends in Weymouth - Geoff Kirby
Draft
CONTENTS
Summary
Cover
1. Introduction
Page 2
2. Rain records analysed
Page 7
a.
Climate change forecasts
Page 7
b.
Long-term rainfall
Page 9
c.
Decadal rainfall
Page 13
d.
Seasonal rainfall
Page 17
3. Rain and barometric pressure
Page 25
4. Wind
Page 32
5. Conclusions
Page 36
6. Acknowledgements
Page 37
7. References and notes
Page 38
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1.
Introduction
In Part 1 of this series of reports
1
a mathematical model was generated to enable sea levels
and wave heights to be forecast for Weymouth Harbour. This model enabled predictions to
be made for the frequency of a sea defence wall being overtopped either by a consolidated
flow of water when the sea level is higher than the wall or by ‘slopping’ when the wave crests
exceed the wall height but the underlying sea level is below wall height.
The report concentrated on flooding of the Town Centre by water overtopping the existing or
a future sea wall. However, in the 20th century the great majority of floods in the Weymouth
peninsula occurred in the Park District and were not primarily due to sea levels being
excessive.
All of the floods shown in figure 2 (overleaf) were due primarily to excessive rainwater causing
the River Wey to burst its banks. In some cases the river flooded the town because high tides
in Weymouth Harbour prevented the sluice gates opening in Westham Bridge. However, this
was not always the case.
Figure 1 - The Weymouth peninsular
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Figure 2 - A collage of 20th century floods in Weymouth
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This second report examines trends in eighty-one years of Weymouth’s daily weather records
and studies the relationships between rainfall and other meteorological parameters such as
barometric pressure and wind strength.
This report also examines the ‘return frequency’ of extreme weather events which potentially
can cause flooding. After a flood there are always calls for greater protection to be installed and rightly so. However, the cost of installing protection has to be balanced against the
frequency with which such events occur and how long the useful life of such a protection
scheme might be.
By way of illustration consider an extreme case of the hurricane of 1824.
This raised waves on 23 November 1824 estimated at tens of metres in height which swept at
high speed over Chesil Bank and inundated fields well inland from the northern shoreline of
The Fleet. Fleet Church was largely destroyed and many local people were left homeless.
Melcombe Regis
2
Esplanade was totally destroyed and the town was extensively inundated.
Many inhabitants died as the sea poured over the beach and ran through to the Backwater.
About 50 people were killed at Chesil on Portland and 80 cottages were destroyed 3.
Now imagine a similar event occurring in 2010. Weymouth’s Esplanade and Harbour are still
vulnerable to severe storm damage and there would be very extensive flooding of the Town
Centre and the Park District.
One reaction in 2010 would be to blame global warming; a modern media-driven but baseless
diagnosis.
There would be an outcry against coastal planners for not having provided protection against
this freak hurricane and there would be pressure to build expensive protection in anticipation
of the next such event.
And yet, a storm of the severity of 1824 has occurred only once in recorded history of the
area and so providing protection against a repeat of this event would seem hard to justify.
The aftermath of the extensive damage caused by Hurricane Katrina to New Orleans in 2005
shows that there is also a political dimension to decisions about the provision of expensive
protection of communities as well as engineering and scientific considerations.
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In this report Weymouth’s historical weather database is examined for trends in rainfall, wind
speed and barometric pressure; all of which can either directly cause flooding or can
exacerbate conditions under which flooding could occur.
It is hoped that this report will contribute to the scientific decision-making process.
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2.
Rain records analysed
a.
Climate change forecasts
Climate modelling experts predict changing rainfall patterns in southern UK as a result of
climate change. The technique used is to run different models under a range of input
assumptions and then pool the results to determine a mean and spread of possible changes.
The estimates of precipitation change for the South West England are summarise below 4.
These have been derived using the ‘Weather Generator’ program 5.
Change in mean winter precipitation
2020
2050
2080
Low
Medium
High
Low
Medium
High
Low
Medium
High
+6%
+7%
+6%
+12%
+17%
+18%
+19%
+23%
+31%
Change in mean summer precipitation
2020
2050
2080
Low
Medium
High
Low
Medium
High
Low
Medium
High
-7%
-8%
-5%
-14%
-20%
-20%
-12%
-24%
-30%
Table 1 - Predicted mean change in precipitation
The changes in precipitation are relative to the mean for 1961 - 1990.
The table covers three assumed greenhouse gas emissions achievements, ‘Low’, ‘Medium’
and ‘High’. These are defined in terms of many factors including GDP growth, population
growth, land use change, future aircraft use, achievement of energy efficiency targets, etc 6.
The above figures are the medians lying within large bands of uncertainty. For example, the
figure of +7% highlighted in red is bounded by lower and upper decile figures of -2% and
+20% respectively.
Evidence to support these predictions is weak. For example, we read
7 8
“There has been an increase in average winter precipitation in all regions of the
UK between 1961 and 2006. However this trend is only statistically significant
above background natural variation in Northern England and Scotland where
increases of 30 to 65% have been experienced.”
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“There has been a slight decrease in average summer precipitation in most
regions of the UK between 1961 and 2006. However this trend is not statistically
significant above background natural variation.”
“There are no statistically significant trends in the average number of rain days
or mean sea level air pressure for any region of the UK between 1961 and
2006.”
We see that the lack of supporting evidence is due, in part, to a lack of statistically meaningful
data trends. The problem may well lie with the relatively short set of daily weather records
collected at Ringway Airport between 1961 and 1990 used to calibrate the ‘Weather
Generator’ program
9.
This is where the Weymouth daily weather database has the
advantage having been started in 1927
10.
This not only has a better long-term statistical
significance but applies locally. It is well known that Weymouth weather is sometimes very
different from Dorchester which is only 12 km to the north behind the Ridgeway Hills which
significantly change the weather pattern between the two towns.
In general, the predictions for the south west of England are for wetter winters and drier
summers with more days having intense rainfall in the winter than now.
What can Weymouth’s weather records tell us?
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b.
Long-term rainfall
Annual Rainfall
1600
Average Rainfall (mm/yr)
1400
From daily records
From annual records
1200
1000
800
600
400
200
0
1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
Year
Figure 3 - Annual rainfall records
In figure 3 the annual mean rainfall has been plotted using annual summaries before 1927
and the average of daily measurements after that date.
The figure tends to show no overall change in annual rainfall since 1881 although there are
short-term variations from year to year which may be no more than random variations not
associated with any underlying physical mechanism.
Table 2 shows the probability of observing rainfall on any day in the period 1926 - 2008
exceeding the amount shown stated.
No Rain
>0 mm
>2 mm
>4 mm
>6 mm
>8 mm
>10 mm
>12 mm
>14 mm
>16 mm
0.612
0.388
0.228
0.160
0.115
0.082
0.056
0.042
0.0300
0.0225
>18 mm
>20 mm
>30 mm
>40 mm
>50 mm
>60 mm
>70 mm
>80 mm
>90 mm
>100 mm
0.0124
0.0066
0.00270
0.00076
0.00027
0.0000995
0.0000995
0.0000663
0.0000332
0.0000332
Table 2 - Probability of observing rainfall exceeding the stated amount on any day,
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Figure 4 shows data in table 2 plotted and it can be seen that the cumulative probability
distribution is close to an exponential curve until a rainfall exceeding about 50 mm after which
the probability of observing large amounts of rainfall in a day remains much the same.
The extreme right-hand data point is for 18 July 1955 when 182 mm of rain fell in Weymouth
causing extensive flooding in the Town Centre and the Park District
Distribution of Daily Rainfall (1926-2008)
1
Cumulative Probability
11.
0.1
0.01
0.001
0.0001
0.00001
0
20
40
60
80
100
120
140
160
180
200
Daily Rainfall (mm)
Figure 4 - Probability of observing rainfall on any day exceeding stated value
It may be surprising to see that rain has fallen on nearly 40% of days over the past eighty-two
years. The impression that days are drier than this may well arise because we are mostly
aware of rain in the daytime rather than rain that falls only overnight. This analysis does not
distinguish between the distribution of rainfall during 24-hourly periods.
Figure 5 shows the probability of observing rainfall on any day over the same eighty-two year
period.
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Probability Distribution of Daily Rainfall (1926 - 2008)
Probability of Observing Rainfall
1
0.1
0.01
0.001
0.0001
0
2
4
6
8
10 12 14 16 20
Daily Rainfall (mm)
24
30
40
50
60
Figure 5 - Probability of observing rainfall on any day within stated values
Date
18/07/1955
15/07/1937
11/07/1977
22/10/1966
05/06/1983
18/09/1999
18/10/1955
05/08/1997
02/07/1957
28/09/1991
21/09/1949
02/07/1950
25/07/1954
16/11/1935
04/09/1974
13/11/1940
10/08/1960
27/12/1979
14/09/1927
27/12/1928
mm
182.0
83.0
78.0
57.0
56.0
53.7
51.0
50.6
50.0
50.0
49.0
46.0
46.0
44.1
44.0
43.0
42.0
42.0
41.5
41.5
Date
01/12/2005
11/07/1953
04/11/1966
13/10/1939
22/10/2003
08/10/1988
24/02/1933
25/05/1941
15/10/1966
13/11/1974
21/09/1976
31/07/1978
13/06/1980
25/06/2006
07/11/1926
18/06/1957
06/11/1969
09/09/2002
01/10/1927
22/06/1983
mm
41.1
41.0
41.0
40.0
39.8
39.0
38.9
38.0
38.0
38.0
38.0
38.0
38.0
37.9
37.0
37.0
37.0
36.1
36.0
36.0
Date
27/11/1929
26/01/1940
12/01/1948
21/01/1962
25/09/1967
30/05/1979
23/10/2005
06/03/1941
08/10/1960
18/03/1964
06/08/1966
03/08/1974
29/11/1985
31/08/1988
09/08/1989
13/12/1989
30/12/1993
06/09/1927
29/10/1937
29/12/1955
mm
35.3
35.0
35.0
35.0
35.0
35.0
34.4
34.0
34.0
34.0
34.0
34.0
34.0
34.0
34.0
34.0
34.0
33.3
33.0
33.0
Table 3 - The dates and rainfall for the sixty wettest days in Weymouth (1926 - 2008)
It is curious to note that six out of the top thirteen wettest days fell in July.
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Figure 6 - Average number of days each year delivering more than a specified rainfall.
Figure 6 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 generally falling.
However, the number of days each year when rain falls has increased by about 50 days since
1990 but the number of days on which it rains hard has not changed. This means that there
are more days with light rain.
However, this increase in the number of wet days until 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.
The long drought of 1976 shows up quite clearly in figure 6.
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c.
Decadal rainfall
Days per Decade with Rainfall
Number of Days per Decade
2200
Scaled from 9 years
2000
1800
1600
1400
1200
1000
800
600
400
200
0
1930s
1940s
1950s
1960s
1970s
1980s
1990s
2000s
Figure 7 - Number of wet days each decade
Figure 7 shows the number of days each decade on which measurable rainfall was observed.
The figure for the 2000s covers only nine years and so the data have been scaled up.
It can be seen that there is a modest trend for the number of wet days per decade to increase
from the 1940s after a drop from the wet decade of the 1930s. There is a hint of a cyclical
behaviour with a period of 30 - 40 years superimposed upon an underlying increase. This is
highly speculative however.
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Days per Decade with Rainfall Exceeding 10 mm
Number of Days per Decade
300
Scaled from 9 years
250
200
150
100
50
0
1930s
1940s
1950s
1960s
1970s
1980s
1990s
2000s
Figure 8 - Number of days each decade with rainfall exceeding 10 mm.
Figure 8 shows the number of days each decade on which at least 10 mm of rain fell. There
is no clear trend in these data.
Number of Days per Decade
80
Days per Decade with Rainfall Exceeding 20 mm
Scaled from 9 years
60
40
20
0
1930s
1940s
1950s
1960s
1970s
1980s
1990s
2000s
Figure 9 - Number of days each decade with rainfall exceeding 20 mm.
Figure 9 shows the number of days recording more than 20 mm of rain each decade. There
appears to be a distinct trend for a regular and possibly cyclical variation having a period of
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about six decades. If this is a true interpretation of the data then the 2010s could deliver a
50% increase in the number of days producing at least 20 mm of rain. This will have a
significant effect on flooding of the River Wey catchment and potential flooding area.
Number of Days per Decade
20
Days per Decade with Rainfall Exceeding 30 mm
Scaled from 9 years
15
10
5
0
1930s
1940s
1950s
1960s
1970s
1980s
1990s
2000s
Figure 10 - Number of days each decade with rainfall exceeding 30 mm.
Figure 10 shows the number of days recording more than 30 mm of rain each decade. There
appears again to be a distinct trend for a regular and possibly cyclical variation having a
period of about six or seven decades. However, the number of days in the sample is too small
to be statistically significant.
If the apparent trend is valid this predicts that the number of very wet days will increase
throughout the 2010s and 2020s to reach double the figures observed in the 1990s and
2000s.
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Number of Days per Decade
7
Days per Decade with Rainfall Exceeding 40 mm
6
5
Scaled from 9 years
4
3
2
1
0
1930s
1940s
1950s
1960s
1970s
1980s
1990s
2000s
Figure 11 - Number of days each decade with rainfall exceeding 40 mm.
Figure 11 shows the number of days recording more than 40 mm of rain each decade. It is
obvious that the 1950s will be remembered for torrential downfalls and, indeed, the Park
District experienced some of its worst floods in that decade.
Overall the number of days in the sample is too small to allow trends to be derived.
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d.
Seasonal rainfall
Figure 12 shows the mean quarterly rainfall from 1927 to 2008 inclusive.
More rain falls in the period October - December inclusive than in other quarters.
Also shown on this chart are the Standard Deviation (SD) figures for each quarter.
Average Rainfall by Quarter
300
Mean
250
Rainfall (mm)
SD
200
150
100
50
0
1
2
Quarter
3
4
Figure 12 - Average quarterly rainfall
It is interesting to predict the rainfall for the rest of the 21st century combining the above chart
with the forecasts on table 1.
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1000
Predicted Rainfall Trends - Southern England
Low Emissions Scenario
Rainfall (mm)
800
Winter
Summer
Annual
600
400
200
0
1980
2000
2020
2040
2060
2080
2100
2120
Year
Figure 13 - Predicted rainfall trends for Southern England
1000
Predicted Rainfall Trends - Southern England
Medium Emissions Scenario
Rainfall (mm)
800
Winter
Summer
Annual
600
400
200
0
1980
2000
2020
2040
2060
2080
2100
Year
Figure 14 - Predicted rainfall trends for Southern England
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1000
Predicted Rainfall Trends - Southern England
High Emissions Scenario
Rainfall (mm)
800
Winter
Summer
Annual
600
400
200
0
1980
2000
2020
2040
2060
2080
2100
2120
Year
Figure 15 - Predicted rainfall trends for Southern England
We see from the above three charts that the rainfall is predicted to reduce in the summer
months and increase in the winter; there being a modest rise in overall rainfall (up to 6%) for
all three emissions scenarios by the end of the 21st century.
Total Rainfall in 1st Quarter
400
Total Rainfall (mm)
350
300
250
200
150
100
50
0
1920
1940
1960 Year 1980
2000
Figure 16 - Rainfall in the first quarter of each year.
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Figure 16 shows the annual rainfall (mm) in the first quarter of each year and it can be seen
from the five-year running average (the blue curve) that there has been no significant change
in rainfall patterns over the eighty-one year period.
The ‘Weather Generator’ predictions for winter rainfall in Southern England are for increases
per century of 22%, 27% and 41% respectively for ‘Low’, ‘Medium’ and ‘High’ greenhouse gas
emissions over the 21st century.
If anything, the trend in figure 16 shows a general decrease in rainfall in the first quarter of the
year with no sign of the forecast increase.
It is clear from figure 16 that there is a cyclic variation in mean rainfall with a period of about
10 - 11 years which is close to the mean period of sunspot cycles which is 10.7 years
12.
There is a long history stretching back to Norman Lockyer in 1868 purporting to show that
there are links between sunspot cycles and weather
13.
The cases for and against an influence of sunspots and their cycles on weather will no doubt
rage on for decades 14 15 16 17 18.
0.8
Global Land+Ocean Surface Temperature Anomaly (C) (Base: 1951-1980)
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
1860
1880
1900
1920
1940
1960
1980
Figure 17 - Global Mean Temperature
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Figure 18 - Global Mean Carbon Dioxide concentration
19
However, the lack of a strong correlation between Global Mean Temperature and CO 2
concentrations - see figures 17 and 18 above - shows that the latter is not the only
mechanism actively driving climate change. Many mechanisms have been proposed
including:
20,
-
Volcanic sulphurous particles ejected into the atmosphere
-
The post-WW2 increased use of leaded fuel resulting in increased aerosol
concentrations 21,
-
Decadal variations in the North Atlantic Oscillation ocean flows
-
Sunspot variations 23 .
22,
In order to investigate the possible influence of sunspot cycles on the First Quarter rainfall
records, the dates when each recent sunspot cycle started 24 have been superposed onto
figure 18 as shown below.
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Total Rainfall in 1st Quarter
400
Total Rainfall (mm)
350
300
250
200
150
100
50
0
1920
1940
1960 Year 1980
2000
2020
Figure 19 - Sunspot cycles superimposed onto figure 18
When the actual sunspot cycles are superimposed onto the data we see an initially good
correlation over the period 1927 - 1985 but this falls apart after 1985.
It should be clear that sunspot cycles are not, after all, a factor in Weymouth’s rainfall.
Total Rainfall in 2nd Quarter
300
Total Rainfall (mm)
250
200
150
100
50
0
1920
1940
1960 Year 1980
2000
Figure 20 - Rainfall in the second quarter of each year.
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Figure 20 shows the annual rainfall (mm) in the second quarter of each year and it can be
seen from the five-year running average (the blue curve) that there has been no significant
increase since 1927 although there is a possible cyclical pattern with a period of about 17
years.
Total Rainfall in 3rd Quarter
350
Total Rainfall (mm)
300
250
200
150
100
50
0
1920
1940
1960 Year 1980
2000
2020
Figure 21 - Rainfall in the third quarter of each year.
Figure 21 shows the annual rainfall (mm) in the third quarter of each year and it can be seen
from the five-year running average (the blue curve) that there has been a generally reducing
rainfall since about 1960.
However, the third quarter mean rainfall in the 1990s and 2000s was about the same as in
the late 1930s suggesting a long-term cycle in the weather.
It is interesting to see that the spectacular daily rainfall of 182 mm in July 1955 does not give
a dominant contribution to that quarter's result as shown by the green circle on figure 21. In
fact, the UK record-breaking rainfall in July 1955 appears to have been the culmination of
fifteen years of increasing third-quarter rainfall after which the rainfall eased off.
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Total Rainfall in 4th Quarter
500
450
Total Rainfall (mm)
400
350
300
250
200
150
100
50
0
1920
1940
1960 Year 1980
2000
2020
Figure 22 - Rainfall in the fourth quarter of each year.
Figure 22 shows the annual rainfall (mm) in the fourth quarter of each year and it can be seen
from the five-year running average (the blue curve) that there has been a small but scarcely
significant decrease from 1927 to 1980 with a possible small rise since then.
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3.
Rain and Barometric Pressure
It is traditional weather lore that low barometric pressure increases the likelihood of rain. This
is the assumption upon which barometer dials are designed as shown below.
Figure 23 - Traditional barometer dial
Every day since 1927 measurements and visual observations have been made of twenty-six
weather-related parameters at the Weymouth Weather Station.
In this section the database is used to examine the relationship between rain and barometric
pressure.
The importance of any correlation between rainfall and barometric pressure lies in the fact
that they both have a significant effect on the probability of flooding in Weymouth.
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0.8
Effect of Barometric Pressure
0.6
D H(P) (m)
0.4
0.2
0
-0.2
-0.4
-0.6
970
980
990
1000
1010
1020
P (mB)
1030
1040
Figure 24 - The relationship between barometric pressure and sea level.
Figure 24 shows the sea level height excess above the predicted astronomical tidal height
plotted against barometric pressure as derived in Part 1 of this series of reports
25.
It can be
seen that a reducing barometric height causes the sea level to rise. The regression equation
predicts an increase of 12.2 mm/mB which is close to the value corresponding to hydrostatic
rise in sea water.
Thus, if a pressure of (say) 990 mB occurs the sea will, on average, be about 300 mm higher
than predicted. This is a significant amount in Weymouth Harbour
26
where the tidal variations
are relatively small; mean Spring Tides varying between 0.2 m at low tides and 1.4 m at high
tides 27.
A sea level rise of 300 mm can significantly increase the threat of flooding. If this coincides
with heavy rainfall the River Wey may not be able to discharge through the sluices at
Westham Bridge and the rain water could backup and flood from Radipole Lake into the
town; mainly into the Park District but also into the Town Centre if conditions are unfavourable
in Weymouth Harbour.
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Approximately 30,000 daily weather records have been analysed to determine the
relationship between barometric pressure and rainfall.
That there is no deterministic relationship between rainfall and barometric pressure will first
be illustrated by referring to the period encompassing the spectacular rainfall and flooding on
18 July 1955.
Barometric Data for 1955
Barometric Pressure (mB)
1040
1030
1020
1010
1000
990
29/05/55
08/06/55
18/06/55
28/06/55 08/07/55
Date
18/07/55
28/07/55
07/08/55
Figure 25 - barometric records for Summer 1955 in Weymouth
Figure 25 shows the barometric pressure recorded at the Weymouth Weather Station in the
summer of 1955. These records are not unusual and the pressure is generally above the
mean of 1017 mB. Rain fell on the days indicated by the vertical red arrows and there is a
broad correlation between dips in pressure and rainfall. However, on 18 July 1955 as
indicated by the blue arrow, 182 mm of rain fell on Weymouth and 280 mm fell at Martinstown
28.
And yet, there is no indication from the barometric record that such vast quantities of rain
were expected. In fact, at the time of the disastrous rainfall and consequent flooding the
barometric pressure was slightly above the mean of 1017 mB.
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This rainfall caused exceptional flooding with a great deal of damage especially in
Martinstown and along the River Wey Valley all the way to Weymouth Harbour. Some of the
photographs in figure 2 were taken during this event.
There is, however, a broad statistical relationship between rainfall and barometric pressure.
Incidence of Wet Days
1.0
Frequency of Wet days
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
1035
1030
1025
1020
1015
1010
1005
1000
995
990
985
980
975
970
0.0
Barometric Pressure Range (mB)
Figure 26 - Incidence of wet days as a function of barometric pressure.
Figure 26 shows the probability that a day will experience precipitation as a function of
barometric pressure. Clearly, the lower the barometric pressure the greater probability that a
day will be wet. This should come as no surprise to even the most casual of weather
observers.
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Quantity of Rain Fall
8
7
Rain fall (mm)
6
5
4
3
2
1
1035
1030
1025
1020
1015
1010
1005
1000
995
990
985
980
975
970
0
Barometric Pressure (mB)
Figure 27 - Average rainfall on days that are wet as a function of barometric pressure.
Figure 27 shows the average rainfall on the wet days as a function of barometric pressure.
This chart shows that when barometric pressure is low the wet days deliver more rain than
wet days having high barometric pressure. Again this accords with everyday observation.
However, the combination of low barometric pressure, frequent wet days and each wet day
delivering more rain combine to increase flooding risks in Weymouth.
Mean Rainfall Per Day (mm)
7
Sea Level Rise and Rainfall
6
5
4
3
Decreasing Barometric
Pressure
2
1
0
-0.2
0
0.2
0.4
Sea Level Rise in Weymouth Harbour (m)
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Some Storm-Related Weather Trends in Weymouth - Geoff Kirby
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Figure 28 - Mean effect of barometric pressure on rainfall and sea level rise in Weymouth
Harbour.
Figure 28 shows how the mean rainfall on any day is linked to mean sea level rise due to
barometric pressure changes. It can be seen that very low levels of barometric pressure can
lift the sea level by over half a metre whilst threatening as much as seven millimetres of rain
in any day.
These are of course mean values and there will be much variation about the values plotted.
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4.
Wind
There is a problem with Weymouth’s Weather Station records for wind.
The wind speed data appears to have been affected by the changing location of the recording
equipment and it may be unsafe to draw long-term conclusions.
16
Average Wind Speed (mph)
14
12
10
8
6
4
2
0
1950
1960
1970
1980 Year 1990
2000
2010
Figure 29 - Winds speeds in Weymouth
Figure 29 shows the wind speed record from 1949 averaged over a one-year moving window.
Before 1949 wind speeds were recorded as Beaufort Numbers which cannot accurately be
related to actual wind speeds.
It can be seen that there is an overall downwards trend in wind speed with a dramatic drop
between 1969 and 1992 as indicated by the red arrows. This change is almost certainly due
to the change in location of the recording equipment in these years. In 1969 the recorders
were moved from the Corporation Yard on Westwey Road to Westhaven Hospital. The latter
is near the crest of a hill on the outskirts of the town whereas the Corporation Yard was in the
centre of the town close to the Inner harbour and surrounded by buildings. One might expect
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the winds to be stronger on the later, more exposed site. However, the above chart shows the
winds recorded as being about 4 mph lower.
In 1982 and 1983 the recorder was moved within the Westhaven Hospital grounds and this
may account for the sudden rise in mean wind speed seen in the chart at around that time.
In 1992 the equipment was moved because of vandalism which had been a big problem
since the 1950s. The current location is a secret but is significantly different from the
Westhaven site. This move almost certainly accounts for the jump in mean annual wind
speed recorded in 1992.
Various sources predict that the climate predictions also indicate that Britain will be windier.
One study suggests 30 percent more gales in Wales and Southern England in winter,
increasing the risk of another storm like that in 1987, which left £2 billion of damage in its
wake.
So, is it getting more windy in Weymouth as climate modellers are predicting?
Despite the discontinuities in the data caused by location changes, there do seem to be two
trends in operation in figure 29.
Firstly, there is an overall trend downwards of about 2 - 3 mph per decade which is seen in
periods where the recorder was in the same location, i.e., the 1970 - 1979 and the 1995 2005 decades.
Secondly, there seem to be peaks in the data approximately every 10 - 11 years as indicated
by the blue arrows.
In the rest of this analysis only wind data for 1993 - 2008 inclusive will be used when the
weather station was at its present site.
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Smoothed Wind Speed
Wind Speed (kts)
15
10
5
0
1990
1995
2000
Year
2005
2010
Figure 30 - Wind speed smoothed over running half-year window.
Figure 30 shows that, for a self-consistent run of measurements, there is a downwards trend
in the mean wind speed from 1993 to 2007 after which the trend recovers to the 1996 value.
The annual winter high wind effect is superimposed upon this trend.
There is no evidence that the predicted increase in wind speeds due to climate change is yet
occurring in Weymouth.
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Probability of Wind Exceeding Stated Value
1.0000
Prob = 3.0*exp(-0.2W)
Probability
0.1000
0.0100
0.0010
0.0001
0
10
20
30
40
50
Wind Speed (kt)
Figure 31 - Probability of observing greater than stated wind speed
Figure 31 shows the probability of observing a wind speed greater than shown. The error bars
indicate that the number of data values is small towards the right-hand of the chart with only
three samples in the last point plotted.
Conditional upon there being no significant trend in wind speed distribution over longer
periods of time than used in the analysis (5,844 daily values) then the empirical curve shown
on figure 31 may be used to extrapolate to higher speeds observed over a longer period of
time.
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Wind Speed and Barometric Pressure
Average Wind Speed (kt)
7
6
5
4
3
2
1
0
975
980
985
990 995 1000 1005 1010 1015 1020 1025
Barometric Pressure (mB)
Figure 32 - The relationship between barometric pressure and average wind speed
Figure 32 shows the relationship between barometric pressure and average wind speed. This
shows that the strongest wind speeds are observed, on average, when the barometric
pressure is around 990 mB with generally lower wind speeds at lower and higher pressures.
This may be because the strongest winds in a cyclonic or anti-cyclonic circulation are not at
the centre of the ‘eye’ of the storm where the pressure is lowest but somewhere between the
centre and the outer edge.
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5.
Conclusions
A previous report (http://www.geoffkirby.co.uk/CoastalReport.pdf) examined flood risks in the
vicinity of Weymouth Harbour caused by extreme tidal effects, low barometric pressures and
high winds.
This report examines primarily the flooding risks over a broader area surrounding Weymouth
caused by high rainfall. This exploits the unique 127-year record from the Weymouth
Weather Station.
These data show that rainfall trends are not conforming to those predicted by climate
modelling for the Southern England region. Indeed, rainfall trends in Weymouth are cyclical
with no clear long-term trends. The cycles have periods between 10 and 80 years. In general,
the rainfall patterns being experience in this century were also experienced in the 20 th
century.
As an example, whilst the annual rainfall has shown no significant change since 1881, the
number of wet days each year is now about fifty days higher than the period 1940 - 1995.
Weymouth is becoming wetter; the same annual rainfall is falling on a much greater number
of days compared with the second half of the 20th century. However, the present number of
annual wet days is much the same as experienced in the 1930s.
An analysis of wind statistics allowed a simple empirical equation to be proposed for
predicting extreme wind frequency.
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6.
Acknowledgements
Ariel photographs are copyright Dorset County Council 2000 and are reproduced here with
permission.
Figure 2 is compiled from photographs in the Dorset County Library Archive and are
reproduced with permission.
The Weymouth weather data has been compiled as a result of the dedicated and untiring
work of a succession of volunteers. Bob Poots is the current weather observer. He very
generously made the data available to me in a suitable machine readable format.
Last and certainly not least I acknowledge the never failing patience of Sandra - seen on the
front cover of this report - who lives with a very old and eccentrically obsessive scientist. Her
untiring and uncritical support makes life worth living.
Biographical Notes. Geoff Kirby graduated in 1960 from London University with a First Class
Honours degree in Physics. He spent most of his working life at Portland with the Ministry of
Defence. In 1992 he took early retirement having been Head of the Oceanographic and
Sonar Performance Department for nine years. He then worked as a sole trading consultant
to a variety of companies as well as the MoD until finally retiring in 2004. In 2000 he
embarked on an Open University BSc degree course in Environmental Sciences with an
additional year studying the History of Mathematics. He graduated just before his 67 th
birthday.
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7.
References and notes
1
Sea Level And Flood Risk Forecasts For Weymouth Harbour Geoff Kirby
(25 November 2009) available for download at
www.geoffkirby.co.uk/CoastalReport.doc or www.geoffkirby.co.uk/CoastalReport.pdf
2
What is now commonly named ‘Weymouth’ is actually Melcombe Regis. Historically
Weymouth was the area to the south of the harbour. Both areas will be referred to as
‘Weymouth’ here.
3
http://www.soton.ac.uk/~imw/chestorm.htm (accessed 02 December 2009)
4
http://ukclimateprojections.defra.gov.uk/content/view/982/527 (accessed 02 December
2009)
5
http://ukclimateprojections.defra.gov.uk/images/stories/UKCP09_WGenerator.pdf
(accessed 03 December 2009)
6
http://www.grida.no/publications/other/ipcc_sr/?src=/Climate/ipcc/emission/091.htm
(accessed 02 December 2009)
7
Environment Agency (2002) How your region might be affected in the 2050s .
http://www.metoffice.gov.uk/climatechange/guide/ukcp/map/ (accessed 03 December
2009)
8
http://ukclimateprojections.defra.gov.uk/content/view/512/9/ (accessed 02 December
2009)
9
http://ukclimateprojections.defra.gov.uk/images/stories/UKCP09_WGenerator.pdf
(accessed 02 December 2009)
10
In fact, weather measurements were made from 1881 but these are only available as
annual summaries.
11
About 280 mm fell in the Martinstown area about 10 kms north west of Weymouth.
This was a UK rainfall record for 54 years until November 2009 when slightly more rain
fell in Cumbria.
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12
http://en.wikipedia.org/wiki/Solar_cycle (accessed 02 December 2009)
13
http://projects.exeter.ac.uk/nlo/about/nlockyer.htm (accessed 07 December 2009)
14
You can’t control the climate, Phillip Stott, New Scientist 20 September 2003 p 25
15
The Chilling Stars H. Svensmark and N. Calder, Icon Books (2008) ISBN 9781840468-66-3
16
Sunspots are up, here comes the rain New Scientist 8 November 2008 p10
17
Saved by the sun New Scientist, 16 September 2006, p32
18
A fake fight A. Thorpe New Scientist 17 March 2007 p 24
19
http://en.wikipedia.org/wiki/File:Mauna_Loa_Carbon_Dioxide-en.svg (accessed
02 December 2009)
20
http://en.wikipedia.org/wiki/Global_dimming#Probable_causes (accessed 24
November 2009)
21
http://www.newscientist.com/article/dn16976-did-lead-cause-global-cooling.html
(accessed 24 November 2009)
22
http://www.newscientist.com/article/mg20126955.400-north-atlantic-is-worlds-climatesuperpower.html (accessed 24 November 2009)
23
Saved by the Sun New Scientist 16 September 2006 p 32
24
http://en.wikipedia.org/wiki/Solar_cycle (accessed 02 December 2009)
25
Sea Level And Flood Risk Forecasts For Weymouth Harbour Geoff Kirby
(25 November 2009) available for download at
www.geoffkirby.co.uk/CoastalReport.doc or www.geoffkirby.co.uk/CoastalReport.pdf
26
This rise will also be observed throughout Weymouth Bay.
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27
http://www.pol.ac.uk/ntslf/hilo.php?port=weymouth (accessed 02 December 2009)
28
The rainfall at Martinstown was a UK record until November 2009 when it was
exceeded by rain falling in Cumbria.
GJK01305
Keywords:- Weymouth Dorset climate change tides tidal wind rainfall sunspots floods
flooding park district barometric pressure Dorset coast forum
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