1 Dec 2012

Potten End Newsletter Article 7

This article originally appeared in the Potten End Newsletter, December 2012.

Potten End Weather

“A snowball in the face is surely the perfect beginning to a lasting friendship.”
― Markus Zusak, The Book Thief

We’re finally in to the winter season with this issue of the Newsletter, and there can be no more obvious subject than snow.

Snow is formed when moist air in the atmosphere is at a temperature of 2°C or less. The moisture in the air then freezes to form tiny ice crystals. These ice crystals collide, join together and form snowflakes. If enough of them join together, they become heavy enough to fall to the ground.

Surprisingly, the heaviest snow falls in the UK tend to occur when the temperature is between zero and 2°C. Above that temperature, the snowflakes melt and fall as sleet or rain.

Snowflakes that fall through dry, cool air will be small and powdery and they won’t stick together. This is the kind of snow that skiers prefer.

At slightly higher temperatures, snowflakes will melt around the edge and join together to form larger wetter flakes. This is the kind of snow that is perfect for making snowmen and snowballs because it sticks together easily.

In Potten End we get snow on average between 10 and 20 days each year, though much of this snow does not settle. Snow does settle and remain on the ground for around 5-10 days each year on average (statistics based on Met Office data from 1971 to 2000).

Fresh-fallen snow settles at a depth of roughly 12 to 1 of equivalent rain. This means, for example, that 30cm (or 1 foot) of snow depth is approximately equal to 2.5cm (or 1 inch) of rain.

Forecasts

It is very difficult to accurately forecast snow for the UK because the UK is positioned between the Atlantic Ocean and mainland Europe and because of the volatile weather patterns we experience.
This means it’s a very fine line between forecasting rain or snow.

White Christmas?

Most people think of a White Christmas as a beautiful white covering of snow falling on Christmas day. The official definition is significantly less romantic in that it involves the observation of a single flake of snow falling at a particular location at any time on 25th December. It’s even less romantic to think that that single flake of Christmas snow may be a part of a thoroughly unpleasant mixed shower of rain and snow. Pity the poor Met Office employee who has to stand outside in the freezing snow watching and waiting for that snowflake! To be honest, we’re pretty sure it doesn’t happen exactly like that!

The chances of this Christmas being White are currently being estimated at around 20% for the country south of Yorkshire/Lancashire. But there is a huge amount of uncertainty around this figure.

Whatever the weather, I wish you all a happy and peaceful Christmas!

1 Nov 2012

Potten End Newsletter Article 6

This article originally appeared in the Potten End Newsletter, November 2012.

Potten End Weather

“For man, autumn is a time of harvest, of gathering together.
For nature, it is a time of sowing, of scattering abroad.”
— Edwin Way Teale

An Indian Summer

At the time of writing, the notoriously unreliable long range forecasts make mention of an Indian Summer. It’s a phrase often heard, but what does it mean and where did it come from?

An Indian Summer is a warm, calm spell of weather occurring in autumn, especially in October and November. The earliest recorded use of the phrase was in America in 1778 and it is thought to have been in wide use there before that time. It wasn’t in popular use in the UK until the 1950s.

The Indian part of the phrase refers to Native Americans living on the east coast who used to rely on the extended warm season to complete the harvest and increase their food stores for the winter.

A record high temperature of 29.4°C was recorded in the UK on 1st October 1985 in the town of March in Cambridgeshire during one Indian Summer giving rise to the favourite quiz question: “When was the October temperature record set in March?”.

Fog and Mist

Fog and mist is formed when a moist cool, stable air mass is trapped underneath a warm air mass. Water vapour condenses in to tiny water droplets which remain suspended in the air. The source of the water vapour is usually ground-based like a river, lake or just wet ground.

The difference between fog and mist is simply visibility. If you can see less than 1km then it is fog, otherwise it’s mist. There are more strict definitions and classifications for aviation purposes.

In 1952, a thick fog, which became smog due to airborne pollution, turned in to the worst pollution event in the history of the UK. It was a particularly severe phenomenon caused by a period of cold weather with a slow-moving high pressure zone and windless conditions that collected and contained air pollution from coal fires. It lasted from Friday 5th to Tuesday 9th December, caused huge disruption and is estimated to have killed more than 12,000 people in the aftermath. As a result of the tragic loss of life, the Clean Air Act of 1956 was passed.

Potten End Skies

With clear skies it will be a great year to see the Leonids meteorite shower this year. A young crescent moon will set shortly after sunset which means the skies will be nice and dark – perfect for meteor watching. The shower peaks between midnight and dawn on Saturday November 17th. You need to look to the East in the constellation of Leo and you can typically expect to see an average of 10-15 pale yellow and blue meteors an hour and perhaps as many as 40 an hour if you’re lucky.

November Expectations

Crunching the numbers in our weather database, we find an average Potten End November to have around 80mm of rain, with an average daily high temperature of around 10°C and an average daily low temperature of just above 4°C. The autumn rain has steadily diminished in recent years which is partly why we are short of water in the summer. Whatever happens, make sure you enjoy your autumn!
“It was one of those perfect English autumnal days
which occur more frequently in memory than in life.”
— P. D. James

1 Oct 2012

Potten End Newsletter Article 5

This article originally appeared in the Potten End Newsletter, October 2012.

Potten End Weather

“Autumn is a second spring when every leaf is a flower.”
― Albert Camus
Autumn is upon us, it’s getting colder and the nights are drawing in. Long range ‘White Christmas’ forecasts are being published but at the moment these are more for entertainment value than serious contingency planning. If you’re interested, the chances of a white Christmas are currently 20% south of Yorkshire and Lancashire and 25% north of there.

The meteorological seasons are different to the commonly seen astronomical seasons which are based on the equinoxes and solstices. In 1780 the Societas Meteorologica Palatina, an early international organisation for meteorology, defined seasons as groupings of three whole months. As a result in the northern hemisphere autumn starts on 1st September, winter starts on 1st December, spring starts on 1st March and summer starts on 1st June.

Wind Chill

With the increasingly cold weather, we’ll start to hear about wind chill in the forecasts. More recently this has been referred to as the ‘Feels Like‘ temperature. It’s the apparent temperature felt by people (as well as other warm blooded creatures) during cold and windy conditions. It’s not just a rough figure guessed at by forecasters. There is a sophisticated formula used to calculate the ‘Feels Like’ temperature that takes into account  things like modern heat transfer theory, skin tissue resistance and, of course, wind speed at around 5 feet which is the average height of a human face. We use the formula to provide the ‘Feels Like’ temperature on our web page (www.pottenend.org). It’s only calculated for temperatures below 10°C.

Jet Stream

This past summer, the Jet Stream has played a huge part in making our weather foul or fabulous. Existence of the Jet Stream was hinted at after the 1883 eruption of the Krakatoa volcano. The plume of ash from the volcano was observed over several years and became known as the “equatorial smoke stream”. In the 1920s, a Japanese meteorologist observed the jet stream using weather balloons launched near Mount Fuji. It wasn’t until 1939 that the term Jet Stream was coined by German meteorologist Heinrich Seilkopf. The original German term was Strahlströmung (or Strahlstrom in modern German). The real understanding of the nature of Jet Streams came during World War II when transatlantic flyers noticed tail winds in excess of 100mph.

There are two jet streams per hemisphere. The subtropical and the polar. Occasionally they merge in to one. It is the polar jet stream that broadly defines our weather and depending on whether it moves further north or further south, the weather is warmer or colder. In general, if it moves north, then the UK has better, warmer weather. This year, and in 2007, the polar jet stream stayed mostly south for the summer which resulted in severe flooding.

Outlook

We’re expecting nothing unusual for October’s weather. A low pressure area will dominate, bringing rain and temperatures of around 14-17°C which is average or slightly above average. We may enjoy the odd bright spell, but not for long.

In the night sky we can expect to see a great show of Orionid meteors from October 20th to the 24th in the south east. There should be about 20 pale yellow and blue meteors an hour. The Orionid meteors are produced by dust from the tail of Halley’s comet.

1 Sep 2012

Potten End Newsletter Article 4

This article originally appeared in the Potten End Newsletter, September 2012.

Potten End Skies

From Meteorology to Meteors and Beyond!

A slight change of course in this issue takes us from meteorology to meteors and beyond.
We’re lucky to have relatively dark skies in Potten End. Free from the pollution of excessive lights, the skies are dark enough to view lots of interesting celestial objects that town and city dwellers will sadly miss.

Meteors or shooting stars are not as rare as you might imagine. Spend half an hour looking up at a clear night sky and you’ll often see a couple in the corner of your eye. You have to be lucky to see them directly as they pass very rapidly. It’s a little late to see the Perseid meteor shower that peaks every year between the 9th and 14th of August but this year the skies were slightly overcast during the critical time so you haven’t missed much. In perfect conditions the Perseid meteor shower can be quite spectacular as it produces over 100 visible meteors an hour with colours ranging from pale yellow to orange and blue.

Meteors are caused by small particles of debris (ranging from dust to the size of a small pebble) burning up in the atmosphere between 75km and 100km above the Earth leaving a visible trail of hot gases and melted meteoroid particles. The Perseid meteor shower is caused by dust particles from the tail of the Swift-Tuttle comet entering Earth’s atmosphere at around 132,000 miles per second.
Regular meteor showers are named after the area in the sky from which they appear to originate. The Perseid meteors appear to radiate from the constellation in the North East named after the Greek hero Perseus, and have been regularly observed for at least 2,000 years with the first written observations made in 36 AD in the Far East.

The next meteor showers to look out for are:
  • the Orionids from October 20th to the 24th which should produce about 20 pale yellow and blue meteors an hour.
  • the Leonids from November 17th to the 18th which should produce about 40 pale yellow and blue meteors an hour.
  • the Geminids from December 13th to the 14th which should produce about 60 multi-coloured meteors an hour.
September has a few interesting celestial events. The equinox (when night and day time are equal) occurs on 22nd September. The days start getting shorter from that point. The full moon closest to the equinox is called the Harvest Moon and that happens on September 30th. It’s called the Harvest Moon because the light from the full moon allowed farmers to work longer hours in the fields in the autumn.

The International Space Station (ISS) will be visible towards the end of September. You can hardly miss it as it is extremely bright and fast moving – quite different to a high flying jet plane or a satellite – but you’ll have to be up and about very early in the morning to see it.

I’m hoping for better weather and clearer night skies in the weeks ahead, but all long range forecasts continue to indicate unsettled weather for the foreseeable future. However it’s worth taking a long look at the night sky whenever you get the time. There’s always a chance of seeing something truly awe-inspiring.

“Look up at the stars and not down at your feet”
Professor Stephen Hawking,
London Paralympic Opening Ceremony 2012.

1 Jul 2012

Potten End Newsletter Article 3

This article originally appeared in the Potten End Newsletter, July 2012.

Potten End Weather

There are an enormous number of old wives tales and country sayings that relate to the weather. Some of them are complete hogwash, but many of them have a basis in scientific truth.

Perhaps the most common, “Red sky at night, shepherd’s delight. Red sky in the morning, shepherd’s warning” is older than you might think and even appears in the Bible (Matthew 16) from as early as 1395. Undoubtedly the saying originated significantly earlier than that.

There is a scientific explanation behind the saying. When the sun is close to the horizon both at dawn and dusk, the sunlight travels to us through more atmosphere than at other times of the day. The red light is able to travel a more direct course and reflects off clouds giving a red sunrise or sunset. Bluer light is more scattered and less of it is reflected from the clouds.

In the UK and in the northern hemisphere in general, weather predominantly comes from the west. If red light is reflected off the clouds in the morning then those clouds are probably headed our way bringing with them an increased chance of rain. And if we see red clouds at night, then the chances are that the clouds have already passed us and we’re in for a nice day tomorrow.

The Rain in June

“And when it rains on your parade, look up rather than down.
Without the rain, there would be no rainbow.”
G. K. Chesterton.
From the weather archives, the average rainfall for the month of June is 38.4mm. The last measurement for June before publication showed we had 128mm of rain (333% more than local average) and the indications were that more was on the way.

So why are we still in a drought according to our local water company? The reason is that Veolia Central take most of their water from underground. They say that “Following two years of very dry weather before April, our groundwater levels remain very low.”

Because most rainfall in the spring and summer is either absorbed by plants or is lost to evaporation, very little of the rain makes it to the aquifers. However, we’ve had exceptional rainfall recently and the good news currently is that the aquifers are starting to fill up. They are still at a “Notably low” level according to the Environment Agency, but at least that’s one level up from the “Exceptionally low” level that they were at a couple of weeks ago.

At least the garden isn’t suffering too much from the hosepipe ban at the moment.

23 Jun 2012

June Rain Compared to the Average

From our short-term records (only going back to 2006), the average rainfall for June is 38.4mm.

By the 5th of June, we’d just passed the monthly average.

By the 11th of June, we’d had twice the monthly average.

By the 15th of June, we’d had three times the monthly average.

At the time of writing we’ve had 130mm of rain and are expecting heavy overnight rain.

Update 1pm, 24th June: We’ve had another 16mm of rain overnight and this morning bringing the monthly total to 146mm.

Four times the monthly average (153.6mm) seems quite reachable at this time. Especially as Wimbledon is imminent.

18 Jun 2012

Drought Situation – June Update …

A quick update to this post: We’ve now had a total of 116mm rain this June.

A glimmer of hope on the hosepipe ban horizon: One of the three aquifers that our local water company uses has risen from “Exceptionally Low” status to  “Notably Low”. This is an indication that some of the rain is getting through to the underground reservoirs.

See the weekly water situation report here: http://www.environment-agency.gov.uk/static/documents/Research/WE_120612(2).pdf

The Environment Agency are due to publish a new weekly water situation report  in the next day or so. Watch this space.

12 Jun 2012

Record Rainfall for June …




… and it’s not over yet!
  
June Rain Total
2012 105mm
2011 81mm
2010 22mm
2009 57mm
2008 21mm
2007 11mm
From our records, the monthly average rainfall for June is 38.4mm. As I write this it’s the evening of the 12th of June and in these 12 days we’ve had 105mm of rain so far! That’s over 270% of the average.

Forecasts for the foreseeable future (around 5 days) show more rain is on the way. Longer range forecasts, though far less accurate, predict a continuing grim remainder of June with temperatures below average and still more rain.

End to the Hosepipe Ban?

So is there an end to the hosepipe ban in sight? Not according to our local water company (Veolia Central). Veolia Central take most of their water from unerground water sources and they say that “Following two years of very dry weather before April, our groundwater levels remain very low.”
Veolia Central add: “Unfortunately at present, we cannot relax the temporary use ban further, as we need to conserve our supplies to prepare for the possibility of a third dry autumn and winter.”

They go on to explain that most rainfall from April to September is absorbed by trees and other plants or is lost to evaporation, and that in order to move us out of a drought situation, we need “prolonged and substantial rainfall between October and March“.

So, unless Veolia Central’s measurements show that a lot of the rainfall in recent weeks has not been absorbed or evaporated, and that it has made it down to the underground reservoirs, then it looks like the hospipe ban will remain in place.

It’s not like the garden is suffering too much at the moment!

Increasing Trend

There also appears to be a general trend of increasing rain in June over the last few years. Sadly this corresponds to some theories about what will happen to the UK climate as global warming increases, but this is such a small statistical sample that no credible link can really be made.
June Rainfall from 2007 to 12th June 2012

1 Jun 2012

Potten End Newsletter Article 2

A condensed version of this article originally appeared in a special commemorative edition of the Potten End Newsletter, June 2012. Also, there were some minor mathematical errors in the original article – they have been corrected here.

Potten End Weather

I’ve taken a bit of a departure from my planned series of articles to briefly write about the extraordinarily wet April we’ve just had.

Also, as this is a special commemorative issue of the Newsletter, I’ve been digging around in the weather archives of the Met Office and other sources to find out what the weather was like over 60 years ago.

The Wettest April on Record

The rain it raineth on the just
And also on the unjust fella;
But chiefly on the just, because
The unjust hath the just’s umbrella.
– Lord Bowen
The Met Office declared April 2012 the wettest since records began in 1910. And it certainly felt that way. In fact, our region of the UK received 237% more rain than the average April rainfall from 1971-2000.

How do our Potten End records compare to that? Well, we measured a total of 113mm of rain in April. This is the wettest April we have on record by quite some margin. Here are the rainfall totals for April in previous years:
  • 2011: 8mm
  • 2010: 12mm
  • 2009: 20mm
  • 2008: 45mm
And it rained every day from April 17th to the end of the month. The wettest day being Sunday 29th April with a total of 21mm falling that day.

Surprisingly, the sunshine amounts for April have not been far off the average.
Despite all that rain we’re still in drought conditions with a hosepipe ban because we still haven’t had enough rain in the east and south-east this year.

April has been in stark contrast to March, which was the fifth driest on record for the UK.

The Queen’s Weather

As this is the commemorative edition of the Newsletter, some have been wondering what the weather was like sixty years ago.

In February 1952 when Queen Elizabeth II was proclaimed Queen, the month started out mild and dry, but became grey and a little wet towards the end. It was the driest February compared to the surrounding years with only 11mm of rain that month. In February 1951, 121mm of rain was recorded.

In 1953, the month before the Coronation had been dry and warm with temperatures reaching nearly 32C in the week before the actual event. But the weather turned at the end of May and the day of the Coronation was decidedly cold, grey and wet. Though it did dry out in the afternoon, the high temperature was only 12C.

And This Year?

Because of the lead time in producing articles for the Newsletter, it is not possible to provide meaningful forecasts for the Queen’s Diamond Jubilee bank holiday. All long range forecasts tend to be very vague with a lot of wriggle room. Depending on where you look and how you interpret trends, the weather will always be some combination of hot, windy, wet, cold and dry.

As an example of just how variable long range forecasts can be, here are just some of the forecasts at the time of going to press:

There are indications that a high pressure zone will develop to the north of the UK which could mean a westerly or south-westerly flow will bring changeable conditions resulting in a ridge of high pressure which could bring some pleasant weather. On the other hand that could also mean that low pressure systems will be forced to take a more southerly track resulting in prolonged wet weather.
My favourite prediction is that the high pressure zone extends and moves south causing warm air to be pulled up from Africa or southern Europe. This gives rise to the effect known as the “Spanish Plume” heat wave. OlĂ©!

1 May 2012

Potten End Newsletter Article 1

This article originally appeared in the Potten End Newsletter, May 2012.

I don’t recall exactly when my obsession with the weather started, but I know I was very young. Frightened by thunder and lightning, my parents encouraged me to count to measure the gap between flash and crash to determine whether a storm was approaching or leaving. We watched the birds flying high to see if it would be sunny. We looked at dry pine cones or noted whether the cows were standing or laying down to indicate whether we could expect rain.

Weather lore like this nearly always has some basis in fact and I may explore these in future articles.

My weather obsession grew slowly but surely, culminating in purchasing a fully automated, computer connected weather station just before the turn of the century. Then, nearly seven years ago I started recording the weather data so I could build up a historical record.

Connecting the weather station to the internet was the next logical step. The Potten End weather website has been around quietly in some form or other for quite a few years, but recently time and resource has been found to turn it in to something a little more publicly accessible and informative.

Drought and the Hosepipe Ban
With nearly seven years of data to look at, it’s possible to see how our rainfall has diminished in the last couple of years.
  • April 2007 to March 2008: 560mm
  • April 2008 to March 2009: 682mm
  • April 2009 to March 2010: 669mm
  • April 2010 to March 2011: 481mm
  • April 2011 to March 2012: 472mm
Those figures show that rainfall dropped by 25% to 30% over the last two 12-month periods.
So it’s clearly not just the water companies’ leaky pipes that are causing the problems.

Potten End Weather on the Internet
Potten End weather is available in many forms on the internet. On the website at www.pottenend.org you can view all of the recent weather measurements. The graphs can show data going back up to 90 days.

The web site is still under development. It will mature, change and improve over the coming months. So keep an eye on it!

You can also subscribe to the Potten End Weather Twitter feed at @pottenend. Weather information is tweeted several times a day.

Please note that if you send twitter messages to @pottenend about local information or events, they can be re-tweeted to all of the Potten End Weather twitter followers.

1 Jan 2012

Disclaimer

The Weather station is a voluntary, unfunded and amateur service located in Potten End.

Availability and accuracy of information and data on this website are not guaranteed and are subject to verification and amendment.

No liability is accepted for any errors or omissions in information or data contained in these pages, or in information or content accessed from the links within this site.

No responsibility is accepted for any loss or injury resulting from use of this website.

These pages are provided for personal interest only and any use made of it is entirely at your own risk.

Data and Service Limitations

The weather station is fully automated.

Failures, inacuracy and loss of data are likely but we do our best to patch-up hardware and fix software issues as soon as possible.

Precipitation is measured to the nearest 0.2 of a millimetre. Very light rain may not be measured at all. Thawing snow and ice will cause inaccurate readings.

About the Weather Station

The Weather Station is amateur, unfunded and located in Potten End, Hertfordshire, UK.

Data is published via the website, via RSS and occasionally via Twitter.

Data is sent to the Met Office and Weather Underground every five minutes.

This web site contains public sector information licensed under the Open Government Licence.