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Hampstead’s Storm Of The Century

March 17, 2015

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.

  1. March 17, 2015 10:38 pm

    Reblogged this on the WeatherAction News Blog.

  2. March 18, 2015 12:05 am

    Thanks Paul,

    One I had glossed over before – and in the middle of a notable hot spell. The MetO report for the month makes interesting reading.

    Click to access Aug1975.pdf

    Hopefully someone reading can add more.

    I used to live near the equator and once witnessed a record storm system that dropped over 500mm in 24 hours back in ’78. The rain was like sheets and the streets disappeared below pale brown rivers. I was used to storms but remember this one going on and on. Had I not lived enough up a hill my experience may not have been so favourable (hence why hillside property comes at a premium!).

    Years after when people used to comment to me on a thunderstorm in the UK I used to laugh ‘it’s nothing’. The nearest deluge I’ve seen here was 3rd August 2004 when I got drenched through running 20 meters from the bus to shelter (Boscastle was later that month). Compared to Hampstead it was just ~40mm fell but it was impressive. Thankfully I missed the hail, but the lightening was unsettling.

    A local West London report which covers the sewage washed into the Thames

    This is from a Geography teaching site which mentions the ‘freak storm’. Aside the climate change bumf it does sensibly discuss the flooding problems that await London should one of these freak storms turn up again…and it will.

    We suffer similar problems here where our sewage pipes are not built for the population pressure or the changing British climate (see Lamb quote below). Every time we get a really heavy downpour it floods the bottom of one street, although no houses are directly flooded everything starts backing up. I have to keep the outside pipes clean or everything starts backing up and the manhole covers start oozing filth. The past few years have been challenging. New houses are not welcome here at least not without the infrastructure development – and there’s the nub. Lamb understood this.

    “I have always thought it a misfortune that the general introduction of plumbing into British homes coincided with the quite unusual run of mild winters between 1896 and 1936. [my house was built during this ‘unusual run’] And possibly some of the modern glass architecture and the hill-top sites with an open south-west aspect which became so desirable a few years ago seem less to be recommended in the 1950s.”

    HH Lamb – The Changing Climate (Routledge Revivals): Selected Papers Note – the paper was ‘The changing climate, past and present; which appeared in Weather, October 1958, Vol 145, pp. 299-318

  3. tom0mason permalink
    March 18, 2015 2:35 pm

    And global CO2 level were how high when this happened?

  4. RichardCannes permalink
    October 11, 2015 11:15 am

    Living near Cannes in France, I’ve experienced a similar storm here one week ago, interesting to see the parallel (we had 107mm in 1 hour ; 175mm in 2 hours), however there have been 20 deaths here (most, but not all, in underground car parks and underpasses). It’s quite incredible to see how much destruction (of appartments, cars) can result from two hours of rain. I’ve added one link to a very complete newspaper article:

  5. John R Jones permalink
    October 30, 2016 12:24 pm

    I clearly remember Michael Fish doing a review of 1975’s weather at the end of the year on BBC 1 which I recorded on my cassette player and replayed it many times. I was still in school and only 15 years of age.

    He said:
    “It did rain on occasions though, and on August 14th a violent storm broke over Hampstead in North London. This produced getting on for seven inches of rain in the space of only a few hours, the likes of which had never been recorded before in London. In fact the chance of such a violent storm occurring is probably the order of about once in every twenty thousand years”.

    I have been fascinated by this storm and other intense acts of weather in the U.K. as well as having experienced some extremes but not on this scale.

    I came across a friend who I asked where he was from and when he said he had been brought up in Hampstead I mentioned this storm to him. He remembered it and how parts of Hampstead was flooded being on the Heath which wouldn’t have been expected but didn’t remember the year or date.

    It was interesting to speak to someone who experienced the storm to some extent and especially the after effects. I don’t think he realised the headlines and publicity this storm got.

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