Hampstead’s Storm Of The Century
By Paul Homewood
This year marks the 40th anniversary of one of the most remarkable weather events in the UK during the 20thC.
The Hampstead Storm of 14th August 1975 dropped an massive 170.8mm of rain in considerably less than three hours. The Met Office give a figure of 169mm for 155-minutes, potentially a record for the UK. (At Walshaw Dean in West Yorkshire, it is claimed that 193mm fell in 120 minutes in 1989, but the Met Office have reservations about this.)
Philip Eden describes the event:
During Tyssen-Gee’s tenure befell Hampstead’s most notorious meteorological event — the Hampstead Storm of 14 August 1975 — and the repercussions of this storm were felt more widely than the area where rain fell. Four of London’s main-line railway stations were flooded and closed, a large part of the London Underground was brought to a standstill as tunnels were inundated and the electricity supply failed, and a Promenade concert in the Albert Hall, far distant in west London, was delayed by over an hour due to the late arrival of several orchestral players and the conductor (not to mention a substantial proportion of the audience).
The rain started at exactly 1715 BST, and ended at 1950. The storm was centred over Hampstead Heath and scarcely moved during the entire duration of its most intense phase. The Society’s weather-recording station must have been close to the point of maximum rainfall, for the rain gauge there recorded 170.8mm of rain — something like three months’ worth of rain in considerably less than three hours. A 20-minute walk away across the Heath, Golders Hill Park measured 131.3mm while three other gauges in the vicinity, Lanchester Road in Highgate, Waterlow Park, and Parliament Hill, also logged more than 100mm. And yet just 6km or so away from the storm centre in districts such as Kingsbury, Wembley, Acton, Fulham, Finsbury, the City of London, Hackney, and Walthamstow, there was almost no rain at all.
Although the point of maximum rainfall was probably within a few hundred metres of the rain-gauge at Hampstead observatory, it is highly unlikely that the site of the gauge and the wettest spot coincided. Furthermore, it is probable that the Hampstead rain-gauge under-recorded the actual quantity of precipitation because many of the large hailstones that fell at the peak of the storm could have bounced out of the gauge’s funnel. Tyssen-Gee conducted a simple experiment to test the hypothesis. He threw a number of ice-cubes almost vertically into the rain-gauge funnel, and found that approximately 30 per cent of them jumped out again, and several others broke into fragments which scattered both in and out of the funnel. Thus it is by no means impossible that nearby some 175 to 200mm of rain could have fallen.
Could it happen again? Statistical calculations indicate that such a large quantity of rain in under three hours probably occurs about once every 20,000 years at Hampstead, although the margin of error either side of that figure would be very large indeed. For the record, there has been only one occasion anywhere in the British Isles when more rain has fallen in as short a time (193mm in two hours at Walshaw Dean Reservoir, West Yorkshire, on 19 May 1989) and there is still some doubt as to the authenticity of that figure.
A report carried out in 2011 by a firm of engineering consultants, Haycock, which reviewed the storm, confirmed that the total could have been as much as 200mm, when hail was properly factored in. This report compared the storm with 1:10,000 year rainfall, and concluded:
Within the HiDEP project (2010) we have determined that the 1:10,000 year rainfall for 4.4 hours is 135mm.
We often hear claims of “1 in umpteen thousand year” events, but this case study rather puts such claims into perspective. While the storm may have been a 1:10,000, or 20,000 year event AT THIS PARTICULAR LOCATION, there are are thousands upon thousands of such sites in the UK alone. Statistically, events such as this one are perfectly common somewhere or other.