Sunday, November 9, 2014

Shock horror. England's rain comes in mini-monsoons, not metered daily amounts.

Here's a headline from todays' Sunday Mail we are seeing with increasing and monotonous frequency these days:

10 days rain in just 6 hours! Wow!  Is that a record?  Nope. (Mail on Sunday, 9 November 2014)

Is 10 days rain in 6 hours really so unusual? I know we in Britain are famed for our frequent spells of rain (that help to keep the gardens and countryside green most of the year). But do we really have our annual rainfall spread so evenly over the 365 days of the year that 10 days worth in a single day (or even 6 hours)  warrants a newspaper headline?

I've been meaning to look more closely into this question for a while, but have been putting it off (not being a regular visitor to meteorological data). But a few taps on laptop keys have confirmed my hunch that our media are making mountains out of molehills, or should that be monsoons out of moisture.

First I had to choose a location. I went for London, Heathrow airport, close to where this blogger was born, my father and two uncles, all brothers, having worked at one time or another for Brenards News Agency, the brothers later as PR chiefs for BEA and the London Airport Authority. Boy, did those first generation jets make a noise when they took off at all hours (before nighttime take-offs were banned).

So Heathrow it is:
Average annual rainfall, London Heathrow.
Certainly our rain in England (well, the drier south-east) is spread fairly evenly across the year, typically with between 40 and 60mm per month, with October a notable exception (70mm).

If one counts up the monthly figures, they come to a total of 630mm per year, which is an average of 1.73mm per day. So 10 days worth, the mini-monsoon of our headline writers, is 17.3mm. How many days per year at Heathrow experience 17mm or more of rain in a single day (let's not bother with whether it comes in 6 or 24 hours - either way  you'll need your umbrella up or windscreen wipers going).

Now let's look at a chart showing actual daily rainfall for this current year to date,  again for Heathrow.

Daily precipitation at Heathrow Nov2013- Nov2014. Click to ENLARGE.

NB. Posting that chart has just reminded me that what I've been referring to as rainfall is described by the meteorologists as "precipitation" and will include all forms of liquid AND solid H2O that fall on us (rain, hail, sleet and snow).

OK, let's draw a line across that chart at the 17mm figure, corresponding to 10 days precipitation.

Now observe there are EIGHT occasions in the last 12 months on which daily precipitation has exceed the 17mm 10 day average. That's 8 times per year, or once every 6 or 7 weeks on average.And they consider a each of those occasions to warrant a headline, as if to imply  that our rainfall generally comes as an even drizzle for a few hours at a time;

I rest my case. Once again, our headline writers have omitted to do their homework, and/or not bothered to make a phone call to the appropriate folk who deal with this kind of information on a daily basis.  One could say it's a kind of dumbing down, inasmuch as a false picture is being promoted re the nature of "weather", a topic that is close to most of our hearts in England, although  rarely if ever permitted to go closer to our heads, thanks to the tabloid mentality that still exists in "Fleet Street".

Many years ago, back in the 70s as I recall, there was a similar media obsession over statistics. It concerned road accidents on bank holiday weekends. More than one newspaper would warn of the coming weekend "carnage on our roads" and frighten us all to death by speculating on how the number of deaths and injuries  would compare with previous years.  Whose turn is it for the Grim Reaper this time? This shroud-waving went on with monotonous regularity, with much denunciation of Government inaction, poor road maintenance, poor vehicle maintenance, speeding, alcohol etc etc but all of a sudden it stopped.Yup,  just like that. Why?

Might it have been that small concise letter to the Editor of  Times (nope, not mine) pointing out that the average number of road accidents on a bank holiday weekend were scarcely any different from any other 3 day period!

There's another myth re English weather that needs disposing off, namely that we have a wet climate. That may be true of the western seaboard where moisture-laden Atlantic winds meet the high ground over Cornwall, Wales, the Lake District, the Scottish highlands etc But it's certainly not true for the eastern side that lies in the rain shadow. There's an amazing difference, over 3 times as much,  between Princetown on Devon's Dartmoor (1998mm per year) and London (592mm)  roughly the same latitude, just 225 miles or so miles apart.

I once read somewhere that Southend-on-Sea  ("Sarfend") on the Thames estuary, a short distance from London, gets no more rain each year than ... wait for it - Jerusalem. I must try and get confirmation of that figure.

Southend-on-Sea has the lowest average annual rainfall in the UK.  517mm.

The comparable figure for Jerusalem: 550mm!

BTW: it wasn't until this science bod was called on to teach an Earth Science Unit for GCSE Combined Science that he learned what caused weather. He still finds the single sentence encapsulates the majestic simplicity of science at its very best, (He even made a fill-in-the-missing-letters quiz of that sentence in this blog's side bar).

Here it is:

Weather is due to unequal heating of the Earth's surface.

It's because it's a sphere, see, so its impossible for all parts that face the Sun to receive the same amount of solar radiation per unit area at the same time, even on a cloudless day. The further from the approximate Equator, the closer to the poles, the more oblique the presentation of Earth to Sun, the smaller the radiant flux density.

No comments: