Mapping the UK's urban heat islands

In 2008, the Royal Meteorological Society initiated a campaign to map the urban heat islands of UK cities with the help of schools and the general public. The aims of this campaign are to; • create a database of good quality, local measurements that schools could use, as urban heat islands feature in many exam specifications, • involve students and the public in collecting data, thereby raising awareness and understanding of the urban heat island effect, • collect data that could be of use to research groups and city planners, • raise the profile of the Society amongst schools and the general public. The Society will initiate urban heat island measurement campaigns in one UK urban area at a time, in each case working closely with local research groups and schools. To maximise the numbers of participants, it was decided to allow temperature measurements to be made using car thermometers. After consulting the European Automobile Manufacturers' Association, as well conducting a number of experiments, these were found to be reasonably accurate, providing readings were only made after the vehicle had been moving at some speed for a number of minutes. When moving, the thermometers are not affected by engine heat and display current, rather than archived, temperatures. If the numbers of participants are sufficient, random errors in these thermometers can be accounted for. On the morning selected for the investigation, participants were invited to drive to their destination, and then immediately text the outside temperature reading and postcode of their destination to a specified number. It was assumed that the time of the text corresponded to the time of the temperature measurement. In addition, several hundred simple digital thermometers with a precision of 0.1°C and an accuracy of ±1% (manufacturer's specification) were purchased to be distributed to schools in the relevant areas. In November 2008, a trial run of the experiment was carried out in Reading. Although poor weather conditions meant that the results generated were not good enough to be published, many valuable lessons were learnt. The temperature difference between rural and urban areas is usually larger at night than during the day, is most obvious when winds are weak and skies are clear and is greater in the summer than in the winter. However, to enable large scale school and public involvement, we required as late a sunrise as possible and hoped to carry out a measurement campaign in Manchester in January. All Heads of Geography in secondary schools in Greater Manchester received a letter inviting them to participate in the experiment, to which several responded, some requesting thermometers. Altrincham Girls' Grammar School became the lead school for the campaign, and a core group of 10 girls were very active in publicising the campaign to local media and other local schools. The timing of the experiment was restricted by the need to be on a weekday in term time, to allow school participation. It was decided that suitable weather conditions should be identified 48 hours in advance, to allow local media and all those involved to be contacted. This meant that Mondays were also not suitable. Given these constraints, it was actually March before the 48 hour weather forecast suggested that a clear, calm night could reasonably be expected on 6th March. By this time, sunrise was at 0644gmt and most measurements would be made after there would be some solar impact on air temperature. Over 1000 data points were collected in the early morning and, despite weather conditions that turned out to be non-ideal, the results show a detailed picture of the city's urban heat island. Elevated temperatures were experienced in urban areas relative to rural areas. A maximum heat island intensity of ~10ºC was recorded. Lower temperatures were recorded in areas with higher vegetative coverage and low building density (e.g. Farmland, Low Density Residential, Recreational zones) relative to more ‘urbanised' land use types (e.g. Manufacturing, Retail, High Density Residential). Further campaigns will be carried out in the winter of 2009 - 2010. The three main challenges to such a large public involvement urban heat island measurement campaign remain - to involve sufficient people to compensate for large potential random errors, from poorly calibrated or situated thermometers and measurement error. - to reliably identify suitable weather conditions 48 hours in advance - to keep the instructions simple enough for most people to participate correctly. The results are made available at www.metlink.org/urban, where future investigations into the urban heat islands of towns and cities around the UK will be publicised.