Guest Post by Tony Brown

Section 1 Introduction

A visit to the Botanical gardens in Cambridge was made by the author of this paper on August 7th 2020 between 10.15AM to 12.25pm.  The purpose was to look at the site of the Stevenson screen there,  following the establishment at this location of the highest ever recorded UK instrumental  temperature, confirmed by the Met Office as 38.7 C ( 101.6 Fahrenheit ) taken at the gardens on 25 July 2019, and to determine the possible effects on this record caused by urbanisation.  From the botanic garden web site we note:

 “Analysis of the Garden’s weather records show that over the last 100 years our average temperature has risen by 1.2 Celsius and the hottest day, highest monthly and yearly average have all occurred within the last 20 years. The highest ever temperature recorded at the Garden before this new record was 36.9 C, recorded on 10 August, 2003.”  * See; “Section 5; Temperature trends.”

Cambridge University Botanic Garden records highest ever UK temperature – Cambridge Botanic Garden

Some context is provided by firstly examining the past and present urbanisation of the gardens, the location of the Stevenson screen and there then follows an examination of various temperature recordings locally to determine what affect if any the urbanisation may have had.  

The visit  was  made during one of the hottest spells of the 2020 summer and  in similar conditions to the record, in as much it had been hot in the days running up to the record with  prolonged sunshine and light winds and these were mirrored on the day of the visit. The preceding day, August 6th 2020 was partially cloudy and very warm at 27C, close by at Cambridge Airport.

The day of the visit was sunny from the outset, with little wind. It was very warm on arrival at the parking close to the gardens at 10AM at 27C. (Car temperature reading)*See note 1

Section 2; Urbanisation inside and outside the garden

Figure 1) and 2)  Photos taken within the gardens on 12 August 1957, kindly supplied by the Met Office.

Figure 1

Figure 2

The area immediately around the screen appears to be nicely maintained, although probably a little close to the glasshouses. The screen was subsequently moved and the glasshouse area very considerably augmented over the years most notably with the Tropical Glasshouse in 1989.

The extent of the Botanic Gardens can be seen here;,0.1294666,113m/data=!3m1!1e3

It is suggested this tab is left open whilst this article is being read.  Zooming in and out once or twice will give a good view of the immediate urbanised surroundings and the extent of the buildings within the gardens themselves, and provides essential context for the comments that follow. The weather station was originally established in 1904 and self-evidently the wider setting overall has become more urbanised over the years. The new buildings inside the gardens and immediately outside its curtilage, may further compromise readings.

The current location of the screen is below and to the right of the large named Sainsbury’s Laboratory building, in a close up of the Google photo. It is the small white square set in a green square of grass and with a narrow grass path going from left to right.

The gardens are relatively small at 40 acres and the Stevenson screen on which the 2019 record was recorded is just inside the gardens, approximately 150 yards from the ‘Railway’ Gate in Hills Road that the author accessed the Botanic garden by.

This gate is to the top right of the google photo, between a building marked ‘Apple Research’ and the ‘eye’ shaped ‘Mills and Reeves’ Cambridge. This latter one, called Botanic House is some 7 storeys high and sits beside a very busy main road. It was completed in 2012 and replaced a modest 3 storey building.

Figure 3 ; Botanic House.

The building can be seen in the photo shown above, together with a statue which is marked as “The Hills Road War Memorial”

This provides a chance for further orientation being directly outside the main ‘Railway Gate’ entrance and in a developed area with much recent and current building activity.

There is a gravel path from the Railway Gate to close to the screen, which is in the position previously described. In front of the screen during the 2020 visit was an ‘experimental beds’ area (see figure 4) with up to waist high indigenous plants, mostly dying back. This took up most of the area above the screen and reaching towards the gate, especially the area adjacent to the screen.

Figure 4

To the bottom of the screen area,  was a mostly low cut grass area,  some of which looked newly seeded and was being irrigated by a rotating spray close by, falling just short of the screen which presumably altered the temperature of the screen on the day of the visit. It looked quite different in 2020 to the google photo, where there was little vegetation.

The Sainsbury’s research building (opened 2011) is on the top left of Figure 4, the other substantial buildings  to the right of the screen were for Algal research  opened on 9 May 2016 and included a two storey glasshouse and a hangar type building, the Plant Growth Facility  the end of which was planted. There was air handling units in a compound well to the rear.

Figure 5

Behind the screen (figure 5) is a mature hedge of Leylandii type evergreen trees which looked to have been cut in the last year or so, as much of it was brown, as the height and width reduction had gone into old growth and this type of tree will not make new growth when this happens. The buildings to the left of the Stevenson screen that have seats set in a circular gravel feature (in the Google photo)  is reached by a gravel path shown in Figure 6 below, passing next to the entrance to the Screen area .

Figure 6

The gravel path passing in front of the Algal buildings shown in figure 4 then became a ‘York’ stone type path as it passed immediately in front of the new 2storey Sainsbury’s Laboratory research building. It then widened considerably into a patio type area towards the new cafe area. The heat from the sun even at mid-morning was ‘bouncing off’ the walls and paths, making for an extremely warm canyon of heat that stretched past the café, until the access into the main grounds where it became noticeably cooler.

The Sainsbury building was opened on 27th April 2011 and has hundreds of solar panels on its roof, plus dozens of industrial size air handling units. The building is substantially higher than it appears and what with this, the hedge, the Algal buildings and the other buildings and paths referenced,  all means the screen was fairly well encircled on 3 sides at a variety of distances, and was a noticeable sun and heat trap. In some of these areas it was uncomfortable to even walk through it. The visit was made around 10 days and a year later than the UK record, and in similar weather conditions, so the strength of the sun, its elevation and other factors, such as the heat reflected from surfaces, together with shading and plant growth would be broadly similar. 

 The substantial changes to the gardens over recent years can be seen in this sequence of photos from 2000 to 2008. The 2008 photo shows the beginning of the Sainsbury building constructions during which the Stevenson screen would have been relocated away from the building work.

Some photos of building work in the 2000’s 

Figure 7

Clicking on the link below will enable the images to be enlarged

These however predated the largest changes whereby through 2011/12 the 7 storey “Botanic House” (figure 3) opened just outside the main gate, whilst the Sainsbury building was opened ‘inside’ the curtilage of the gardens with the adjacent Algal building following in 2016. A number of paths have also been created, whilst others are more formalised.

The link below provides a more general look from the 1950’s to 2000’s by each decade

It shows a constant programme of improvement and development over the decades.

2.1: Temperature-urban versus semi-rural

The possible effects of UHI on temperatures are well documented and the differences between a built up area and one less built up can be substantial. Generally, higher temperatures in urban areas are influenced by a number of factors, including the month, orientation, warmth and duration of sun, wind direction and speed, conditions in preceding days, as well as the effects of buildings reflecting warmth, or trees, bushes and paths having an impact, ranging from minimal to substantial.

The point at issue is multi layered, for as well as the UK being a heat island according to the calculations made for Central England Temperature (where an allowance of 0.2C is made) rapidly developing Cambridge is itself a substantial heat island and the considerable urban growth observed immediately around the botanic gardens adds a third level to the UHI effect, whilst the buildings-many large and modern- around the Stevenson screen in the gardens themselves, adds a fourth level.   

There is no reason to believe that the experienced staff at the gardens misread the equipment, nor that the equipment is not functioning correctly, as a follow up visit was made by Met office staff to ensure the screen met their criteria and determine the overall circumstances constituted a record. The criteria was queried directly with the Met office

“….. We have now received the following reply to your question in your email. Our Networks Manager notes that the standards, use application and background across all the elements are a complex subject and has tried to sum this up into the brief statement below.

Stations are operated for a number of purposes and roles, the observed parameters will also vary depending on the requirements for data from the location. Representation of the surrounding environment is sought with metadata collected periodically to capture the environment under which the data is observed. For the extremes product the data is provided from a sub set of stations that are considered representative of the UK climate being experienced’.

Other information was available online confirming the criteria.

“On Monday afternoon (29th July 2019), Phil Johnson, SE England Met Office Regional Network Manager, carried out a climate inspection at the Garden’s weather station to verify the reading. This included checking the weather station instruments against Met Office precision equipment. Based on his report, the Met Office has confirmed the 38.7 degree C recording is now officially the highest temperature ever recorded in the UK. This figure exceeds the previous record of 38.5°C recorded in Faversham, Kent, in August 2003.22C.”

So it has been determined that the equipment is of the correct technical specification and those recording the data do so accurately.

Note: On the morning of the visit by Mr Johnson, the temperature (at the airport 2 miles away,for which data is available) was 22C with a high of 26C later and partly cloudy, so this was substantially cooler than on the day of the record and the sun would not have had the same impact on the reflection of heat from the buildings, paths nor other surfaces, as was observed on 7rth August 2020.

On 25th July 2019 for reference, the maximum temperature was 38C dead at Cambridge Airport approximately two miles away.  Note; within the link below, is a drop down menu showing other months and years.

Weather in July 2019 in Cambridge, England, United Kingdom (

In addition; (on 25th July 2019) “The National Institute of Agricultural Botany (NIAB) published a reading of 38.1°C in the city. But today it was announced that a thermometer in the Cambridge University Botanic Garden weather station had reached 38.7°C.”

Both the airport and NIAB sites are in semi -rural areas but have a number of buildings close by, especially at the airport which also has a tarmac runway.

So the temperature in the general area was around 38C, assuming the airport and the NIAB (an official Met office site) are not affected by UHI and the instruments are calibrated.

Section 3; Temperatures on day of Botanic Garden visit on 7th August 2020

The author was staying in a semi urban area some 2 miles away from the Botanic gardens in a private residence. This showed a shaded temperature reading in the garden, on a reliable max/min thermometer, of 25C at 9.30. This temperature was maintained for most of the 2 mile journey to the gardens, as shown on a car thermometer which read in half degrees and had been shown to be close to the readings of the max/min thermometer.

*Note 1; No claim is made that either device were scientific instruments but they showed close correlation with each other, several digital reading en route to Cambridge the previous day and with digital weather stations at the authors own home.

It was 32C by car thermometer when the author left the gardens at 12.45. and generally, once away from the immediate vicinity, around 28/29C in the 20 minute journey back and 28.7 in the shaded garden area when the author returned.

It remained hot and sunny throughout the rest of the day in all local locations with a little fair weather cloud building in the afternoon. The maximum reached in this general area in the shade of the rear garden of our accommodation was 30.2C as recorded by our max/min thermometer and 30C by car thermometer, reached  at around 5pm. The maximum temperature at Cambridge airport 2 miles away was 36C, at NIAB 35C at a similar distance and at the Botanic gardens it was published at 34.7 C.

On the previous day, 6th August it was partly cloudy and 27C at the Airport and at the Botanic gardens 27C and at our location 26.4C.

Height of location, wind speed and direction, the inclination of the sun and its strength and longevity, will all affect the relative warmth of a location, with the urbanisation factors outlined adding in another dimension. However there still seems a considerable discrepancy between the various sites measured. The quality of official instrumentation and the accuracy of the reading taken by experienced garden staff and the subsequent inspection by Mr Johnson means these factors can be taken as assured. The personal instrumental devices used are of course not calibrated but have been shown to be reasonably accurate (see note 1) and would not account by themselves for a 5 Degree C temperature difference between locations.

Consequently, taking in  to account all these factors, concerns must then be expressed as to whether with all the urbanisation that could be observed, as to whether the reading set on 38.7C on 28th July 2019 should be treated as scientifically accurate enough for a UK record.

Section 4; Criteria for weather stations

The criteria set by the CIMO+ for grading of weather stations is here;

CIMO_Guide_2014_en_I_1-2_Annex_1B.pdf (

The screen site appears to fail the criteria for a Class 1 station due to a variety of factors including the closeness and heat reflectivity of the new buildings within the gardens, which are estimated at approx. 50 metres. The screen area is loosely encircled by buildings on 3 sides at varying distances.

It would seem to fall into class 2 where the distance criteria drop to 30 m.  As the Met office themselves recognise the impact of UHI with their 0.2C current allowance for England as represented by CET,  perhaps a similar more localised uhi impact could be applied, especially  with the noticeable heat effect in the sunshine of the Sainsbury’s building which is topped by numerous air handling units and solar panels.

Whether an important national record should be accredited to a class 2 station with unresolved UHI issues is an open question.

The gardens have over a century of weather recording, during which time the Stevenson screen has been moved a number of times as circumstances changed.  Currently there would seem to be less developed sites within the gardens that might be better suited to the location of the Stevenson screen, if it is to fall within the CIMO rules for a Class 1 site that should presumably be used to establish national records.

“Established by the predecessor of WMO, the International Meteorological Organization, the Commission for Instruments and Methods of Observation (CIMO) ensures the accuracy of weather observation by facilitating the creation of international standards and, thus, the compatibility of measurements.”

Section 5; Temperature trends

Just after the introduction to this piece, this quote was placed just under a link to the highest recorded temp achieved in the UK;

“Analysis of the Garden’s weather records show that over the last 100 years our average temperature has risen by 1.2 Celsius and the hottest day, highest monthly and yearly average have all occurred within the last 20 years. The highest ever temperature recorded at the Garden before this new record was 36.9 C, recorded on 10 August, 2003.”

A look at the development around the screen this century (figure 3) typified by the Sainsbury’s building in 2011, Botanic house just outside the main gate a year later, and the Algal centre in 2016, when viewed against the current placement of the screen, might give pause for thought  as to whether it is development, rather than generally rising temperatures, that is responsible for a proportion of the observed temperature changes of 1.2C, as CET,  plotted from 1900 (figure 4) shows an annual trend rather lower than this value.

Figure 8

However, over the last 20 years temperatures have levelled off substantially, with an annual trend  of some 0.56C over a century, with autumn values actually falling* (figure 9)

Figure 9

CET are daily and monthly temperatures representative of a roughly triangular area of the United Kingdom enclosed by Lancashire, London and Bristol. The monthly series, which begins in 1659, is the longest available instrumental record of temperature in the world. The daily mean-temperature series begins in 1772. Manley (1953, 1974) compiled most of the monthly series, covering 1659 to 1973. Both series are now kept up to date by the Climate Data Monitoring section of the Hadley Centre, Met Office. Since 1974 the data have been adjusted to allow for urban warming: currently a correction of -0.2 °C is applied to mean temperatures.

Parker, D.E., T.P. Legg, and C.K. Folland. 1992. A new daily Central England Temperature Series, 1772-1991. Int. J. Clim., Vol 12, pp 317-342 (PDF)

CET is considered a useful proxy for England and a much wider geographical area. Cambridge is situated in the east of the country, just outside of the specified CET area.  


The day of the authors visit was hot and on subsequently researching the temperature readings from the Airport and the Botanic gardens it appeared that new UK records were set by both locations on the 7th August 2020 .

Cambridge airport reported 36C  on 7th August and NIAB 35C, but the UK record for the 7th August is 34.0 in Bromley in 1975 as can be seen from the chart below (figure 10) . The gardens were slightly lower at 34.7C, so again, a record, if the other locations were discounted for any reasons..  

Records are shown below  

*Gravesend – Swanscombe (Kent) 2003
Kew Botanic Gardens (London) and Wisley (Surrey) 2003
734.093.2Bromley (London) 1975
834.293.6Heathrow Airport (London), Stanstead Abbotts (Herts) 1975
936.798.0Raunds (Northants) 1911; 36.7*/98* Canterbury (Kent)1911; 36.6*/97.9* Epsom (Surrey) and Beddington (London) 1911
1038.1100.6Gravesend – Swanscombe* (Kent) and Kew Botanic Gardens (London) 2003;

Figure 10

TORRO – British Weather Extremes: Daily Minimum Temperatures

 The author subsequently emailed the Met office pointing out the reported temperatures (at the Airport and the Gardens) and the apparent breaking of the record, asked for their confirmation and received this reply  ; 

“The temperature figure provided in my previous email is based off model data – not an actual measurement – so it cannot be taken as a value to be considered where records are concerned. Unfortunately, due to the nature of how we provide historical data as a charged service, I’d be unable to provide the maximum recorded for the day.

Your point on verifying station data  (for a record) is correct – this is usually done within a few days, as you suggested; you can read more about this here.”

Tony Brown February 2021


The Met office for Data and photos

Temperature reading from NIAB

Ed Hoskins for graphics-https://edmhdotme.wordpress.comOnline data gathered from a variety of named web sites

via Watts Up With That?

February 13, 2021 at 12:16AM