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SUMMARY

Despite climate modellers forecasting wider extremes of barometric pressure over southern England, there is no evidence for any such change in Weymouth's eighty-year barometric pressure record. 

There has, however, been a fall in the number of days experiencing pressures below 1,000 mB since 1975 and this is contrary to the claims that global warming is increasing the occurrence of low pressures. 

In fact Weymouth's records suggest an apparent cycle of about fifty years in the record. If this cycle is valid then we should see an increase in the number of low pressure days between 2008 until around 2030.

ANALYSIS

Climate Modellers predict that Britain will get stormier as the global temperature rises, see for example [1]. The evidence presented below suggests the opposite to be true for Weymouth.

Geoff Jenkins, Head of the Hadley Centre, has said that Britain has become twice as stormy in the past 50 years as climate change has forced the deep depressions that used to hit Iceland further south. While low pressure areas which bring high wind and rain are getting deeper, the high pressure areas which bring calm, settled periods are getting stronger. The increased gradients between the two make for more dramatic weather - and for insurance companies, expensive claims for damaged buildings and fences.

"As far as the pressure anomaly is concerned, this is much higher than we predicted in our models, and it basically means that the westerly winds across the British Isles have increased in strength. This is significantly larger than explicable by natural variation, and must be man-made climate change."

The spread of pressure excursion above and below the average is expressed by the Standard Deviation (SD). Geoff Jenkins is saying, in effect, that the SD of the barometric pressure is increasing with time.

Figure 1

The above figure shows the daily barometric pressure for Weymouth over an eighty-year time span. 

It is not evident to the eye that there has been any change in the extremes or spread of the data. 

For a more sensitive test the SD has been computed for a moving window of one year.

 

Figure 2

It is evident that there has been no significant change in the spread of extremes of pressure in Weymouth over the past 80 years.

Figure 3

Figure 3 shows the number of days each year on which the barometric pressure was recorded as below 1,000 mB. The red curve is a ten-year running average.

The fall in the number of days experiencing low pressure since 1975 is contrary to the claims in [1] that global warming is increasing the occurrence of low pressures. 

In fact Weymouth's records suggest an apparent cycle of about fifty years in the record. If this cycle is valid then we should see an increase in the number of low pressure days between 2008 until around 2030.

 

Figure 4

Figure 4 shows the probability of observing a pressure on any day lower than the given value 'P'.

Barometric pressure affects the height of the tide. Low pressures increase tidal heights.

The result on figure 4 will be used in the estimation of future flooding risks in Weymouth over the 21st century, please click here to go to that page.

 

REFERENCES

1.  http://www.guardian.co.uk/uk_news/story/0,3604,1103539,00.html

 

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