Frequently Asked Questions about local weather etc

Well in truth few visitors to the site send in questions so here area few that may be of general interest judging by search engine queries...

Last update 10 June 2012

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How far back do accurate weather records go ?
Weather data has only been reliably and continuously recorded in the UK since 1659 as part of the Central England Temperature (CET) series, although this does not cover Kent of course. Note also that rainfall data has been reliably recorded in England and Wales only since 1766. Reliable historical weather data in detail for Tonbridge and Kent in the public domain is difficult to track down.
What are the highest and lowest temperatures ever recorded in Tonbridge?
The "HIGHEST" temperature recorded was the famous former UK record 100.5 ºF / 38.1 ºC measurement in a Glaisher screen on the 22nd July, 1868 by Dr Hunsley Fielding. Dr Fielding was a fellow of the Royal Society and a meticulous recorder of the weather who obtained certificates of accuracy for his instruments. However, the thermometer was situated over gravel rather than grass and the base of the screen open to heat radiation from the ground, not meeting modern Met Office requirements. It is now accepted that this value, when compared with the standard Stevenson screen required for official meteorological purposes, was between 35 - 36ºC.

It is almost certain that the highest temperature on the 10th August 2003 equalled or exceeded this reading but there are no longer any officially accepted recording instruments in the town. The nearest official station at East Malling Research reached 37.4 ºC / 99.3 ºF.

I have not been able to find any records of the COLDEST ever temperature in Tonbridge but a book dated 1934 in the reference library notes a minimum of 5 ºF / -15 ºC on 6th February 1895, although it is likely that the all-time minimum is rather lower. The lowest temperatures are not confined to the notoriously cold winters of the past - on 19th January 1966, East Malling dropped to -16 °C, and at the end of January 1972, -17 °C [Weather of Britain - Roger Stirling]. Tunbridge Wells has an unofficial minimum of -23ºC in 1819-20 "although no details of exposure are evident", a former official weather station there recorded a minimum of -15.6 °C in one 30 year period.
What are the highest and lowest temperatures ever recorded in Kent?
On the 10th August 2003, certainly the hottest day in reliable weather history in the UK, Faversham (Brogdale) recorded 38.5 ºC / 101.3 ºF and Gravesend touched 38.1 ºC / 100.6 ºF. However, it should be noted that these measurements are questioned by some due to the sites being more sheltered than recommended in the strictest Met Office guidelines. The UK record accepted without question is 38.1 ºC at Kew (Royal Botanic Gardens). Greenwich Observatory (still regarded as in Kent by many and definitively so pre 1974 boundaries!) recorded 37.5 ºC.

In the legendary winter of 1947, Kent's coldest ever official temperature was recorded at Elmstone (east of Canterbury): -6 ºF /-21.3 ºC on 29/30th January.
What about the driest and wettest years in Tonbridge?
The driest year appears to be 1921 when just 15.73 ins / 400mm was recorded. 2005 was another unusually dry year with unofficial readings of between 425 and 450mm (16.5 to 17.5 ins).

I have no definite record of the wettest year, but more than 40 ins / 1016mm in a year is rather uncommon. In 1927 the town received 41.66 ins / 1058.2mm. 1951 (total not known), and 2000 - when I recorded 1032.7 mm / 40.66 ins were also exceptionally wet years.
What is the average annual rainfall for Kent?
The average amount of annual rainfall in the county varies quite considerably according to location and altitude. For the most part the average is between 640 - 740mm per year, with higher ground of the Weald and Downs (e.g. Tunbridge Wells) receiving 800mm or more. The yearly average for the nearest official rainfall recording station at East Malling is 640mm. The driest part of the county is the narrow strip alongside the reach of the Thames Estuary with between 520 and 650mm.
What is the heaviest rainfall in a single day in Kent?
On 20th September 1973, the wettest day in Kent was recorded at West Stourmouth where 7.51 inches of rain fell (187.75mm), or nearly four times the monthly average in one day. (source: Bob Ogley)
What is the average annual sunshine for Kent?
The average annual sunshine duration varies from between 1500 hours in West Kent up to 1650 hours in eastern parts of the county. The yearly average for the nearest official sunshine recording station at East Malling is 1568 hours. More than 2000 hours sunshine in a year is rather uncommon but many places exceeded this in both 1990 and 2003. The status of sunniest place in the county is claimed by Ashford, Canterbury and Herne Bay amongst others!

December is the dullest month with little more than 1.5 hours average sun each day, and any month from May to August can be the sunniest with up to 7.5 hours average per day.

What is the "Dewpoint" and why is it important?
Dewpoint (or Dew Point Temperature) is a measure of atmospheric moisture. A higher dew point indicates more water vapour moisture present in the air. It is the temperature to which the air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant), that is, when no more moisture can be absorbed and excess water vapour must be released as, for just one example, fog.

In the UK a higher dewpoint relative to air temperature usually indicates an air mass of warm maritime / tropical origin and usually the temperature does not fall very much overnight. In hot summer weather a high water vapour content gives the uncomfortable "sticky" or "oppressive" feeling. A low dewpoint relative to air temperature suggests air imported from a cooler or cold, dry Polar / Continental source which feels "fresh" and can allow a deep fall of overnight temperature in calm conditions.

Dewpoint is a more helpful measurement of air moisture than Relative Humidity % in many ways and useful for forecasting fog, frost or thunderstorms; as well as the likely minimum overnight temperature in stable conditions.
What exactly is the "Solar Radiation" measurement?
The Solar Radiation or irradiance measurement is not a measure of sunshine duration but energy flow or flux from the sun's electromagnetic radiation measured in Watts per square meter . This energy flow (insolation) is what causes heating of the earth's surface and what makes solar power generation possible. Solar radiation is variable according to the time of year, the elevation of the sun above the horizon, and the obscuration from cloud or haze. This is measured for meteorological purposes with a horizontally level sensor surface. The higher the reading, the more energy is available for heat conversion at the earth's surface or the generation of solar power.

In the southern UK, the maximum daily possible amount of solar radiation in W/m² varies from under 400 in midwinter to nearly 1200 in midsummer (the solar irradiance above the atmosphere at the mean earth-sun distance is approximately 1370 W/m² - the solar constant).

During daylight some solar radiation will penetrate even total overcast or thick fog. It can also be reflected, for example, from the base of clouds; this is diffuse radiation as opposed to direct radiation, and is also present during a short period of twilight before sunrise or after sunset. All of this makes accurate conversion into actual sunshine duration difficult. By setting a variable "sunshine threshold" and using complex algorithms it is possible to calculate (fairly crudely and approximately) the duration of bright sunshine in hours in a day, however surrounding buildings mean that my own sensor has limited exposure to direct sunlight.

For solar power generation in the UK latitudes solar collectors need to be tilted to an incline of 35 - 38º for maximum efficiency (the higher further north). The average property in the UK would require approximately 3 000 kWh per year of solar radiation for domestic water heating, and in the southernmost counties it is actually quite feasible to supply 40 - 50% of the energy need with solar power with a few square meters of collectors on a south facing roof - all subject to the inevitable vagaries of British weather!
What do the various "apparent temperature" readings mean?
WIND CHILL takes account of how wind speed affects our perception of coldness of the air temperature. Most noticeable in shade air temperature of 10ºC or below, wind takes heat away from any insulating layer of clothing and we feel colder than in calm conditions.

HEAT INDEX uses the shade air temperature and humidity readings to determine how hot we really feel. A higher humidity (water vapour content) in the air makes it more difficult for our bodies to cool by evaporating moisture through perspiration and that makes us feel less comfortable. On the other hand low humidity allows perspiration to evaporate more quickly and we feel cooler. The Heat Index reading is irrelevant below 14ºC.

THW (TEMPERATURE, HUMIDITY, WIND) INDEX. This is a slightly more sophisticated calculation which incorporates the heating and/or cooling effects of the shade air temperature, humidity and wind to provide a "feels like" temperature.

THSW (TEMPERATURE, HUMIDITY, SUN, WIND) INDEX. This is another more sophisticated calculation as above which also includes the heating effect of sunshine to provide an indication of what the temperature "feels like" in the sun. Unfortunately, although the weather station calculates the THSW and logs the data on the computer database, there is no means to automatically display this reading on the data updates on the website.

What is an "Automatic Weather Station"?
An Automatic Weather Station is an array of weather sensors at which data observations are made at predetermined intervals. The data is transmitted automatically either using a wireless or cable link to a separate recording unit or console. The console can be linked to a computer so that all the data can be stored in a database and retrieved later (this usually requires special software).
How much does an Automatic Weather Station cost and where can I buy one?
Firstly you should think about what data you want to collect and how you want to store and access it, as well as how much you wish to spend of course. A wireless sensor to observe only temperature and humidity readings and allow you to transfer recorded data from an indoor console onto a PC can now cost as little as £40. More sophisticated stations which also record air pressure and rainfall start at about £120 but you could spend up to £1000 or even more for the best quality equipment. You also can expect to have to spend more on mounting the sensors for the best possible exposure and specialist software to store and access the data and produce reports on a PC. You may want to buy an internet domain and website hosting for showing your own data, and for frequent updates a broadband connection is a great benefit.

There are a few specialist suppliers in the UK and I would recommend that you telephone and/or arrange to visit one or more to discuss your requirements and find equipment within your budget to meet your needs. I am happy to recommend the following suppliers:

Meteorologica (Morecambe, Lancs)
Weathershop (Eastbourne, East Sussex)
Prodata (Ely, Cambs)

It is also worth joining a weather forum such as UK Weatherworld, The Weather Outlook or Net-Weather to get advice and opinions from people who already own equipment.