Cooling and heating degree days by UK region
Updated 23 October 2026 · SEO Dons Editorial
What is a degree day, and which base temperatures are used?
A degree day measures how far the outdoor temperature sits from a reference, or base, temperature over a period. CIBSE's TM41 describes degree-days as "a summation of the differences between the outdoor temperature and some reference (or base) temperature over a specified time period". Two conventions appear in Met Office material. For heating, its UKCP09 dataset sums the degrees by which the air temperature is below 15.5°C, day by day. For cooling, its Climate Data Portal counts the degrees by which a day's average temperature is above 22°C: an average of 22.5°C adds 0.5 cooling degree days and 27°C adds 5.
As arithmetic, a day averaging 5.5°C adds 10 heating degree days at a base of 15.5°C. TM41 stresses that the base temperature is the key issue, because in buildings it relates to the energy balance of the building and system, so 15.5°C and 22°C are conventions rather than rules for every building.
Where are official UK degree-day figures published?
| Publisher and dataset | What it gives | Coverage and form |
|---|---|---|
| Department for Energy Security and Net Zero: DUKES weather and Energy Trends | Heating degree days; mean series from 2002 in DUKES, quarterly and monthly data in Energy Trends | National series, as Excel spreadsheets |
| Met Office: averages maps in the climate averages tool | Heating and cooling degree days as map layers, for 1991 to 2020 and 1961 to 1990 | UK maps based on 1 km HadUK-Grid |
| Met Office Climate Data Portal | Annual cooling degree days: projections for warming levels, with 1981 to 2000 and 2001 to 2020 baselines | 12 km grid, online dataset |
| Met Office UKCP09 regional values | Annual cooling degree days on a 1961 to 1990 baseline | 14 administrative regions, text files (older set) |
| CIBSE TM41 (2006) | The method: theory and application | Technical memorandum |
Among the official HTML pages checked, none gives a regional table of both measures that can be read as text. The Met Office shows its averages as maps, and the DESNZ series is national. An engineer who needs a defined base temperature and daily data for one site should state the source and base used.
Why can't a monthly average give cooling degree days?
Because cooling degree days accrue only on days when the daily average passes 22°C, and monthly averages sit well below that. In August 2026, the Met Office's county breakdown put Greater London's mean temperature at 20.7°C, the highest it quotes, still under the base. In the table below the July mean is 15.4°C to 19.0°C, which is 3.0°C to 6.6°C under the cooling base. Any cooling degree days come from individual hot afternoons, which are rarer in Edinburgh than at Heathrow, and only daily data can count them.
How do the regions compare? An illustrative table
The table uses one Met Office station per region and its 1991 to 2020 averages. The mean temperature is the average of the monthly mean daily maximum and minimum. The heating estimate multiplies each month's days by the gap between 15.5°C and that mean (zero if the mean is above 15.5°C), sums the year and rounds to the nearest 100. It is illustrative arithmetic: using monthly means understates real degree days, which count each cold day separately, so use it to compare rows and not for energy reporting.
| Region | Met Office station | January mean (°C) | July mean (°C) | July gap to 22°C base (°C) | Illustrative heating degree days, base 15.5°C |
|---|---|---|---|---|---|
| Scotland | Edinburgh (Royal Botanic Garden) | 4.5 | 15.4 | 6.6 | 2,200 |
| North East | Durham | 4.1 | 15.8 | 6.2 | 2,200 |
| North West | Rochdale | 4.2 | 16.0 | 6.0 | 2,200 |
| Yorkshire | Sheffield | 4.6 | 17.1 | 4.9 | 2,000 |
| East Midlands | Nottingham (Watnall) | 4.3 | 16.9 | 5.1 | 2,000 |
| West Midlands | Pershore | 4.8 | 17.4 | 4.6 | 1,900 |
| East of England | Cambridge (NIAB) | 4.8 | 17.8 | 4.2 | 1,900 |
| London | Heathrow | 5.5 | 19.0 | 3.0 | 1,600 |
| South East | Reading (University) | 5.1 | 17.8 | 4.2 | 1,800 |
| South West | Yeovilton | 5.3 | 17.1 | 4.9 | 1,900 |
| Wales | Cardiff (Bute Park) | 5.5 | 17.5 | 4.5 | 1,700 |
| Northern Ireland | Aldergrove | 4.7 | 15.4 | 6.6 | 2,100 |
The pattern is clear even with this crude method: London's estimate is about 1,600 against about 2,200 for the North East, North West and Edinburgh, while London's July mean sits 3.0°C under the cooling base and Edinburgh's 6.6°C. Heating dominates in the north; cooling exposure is concentrated in the south and east. The guide to the hottest places in the UK ranks stations and records.
What are degree days for, and what are they not for?
TM41 says they are a tool for assessing and analysing weather-related energy use in buildings, with two types of user in mind: designers checking the likely energy consumption of a design, and energy managers assessing existing buildings. In practice that means normalising consumption, comparing sites in different regions and checking whether a bill is high because of weather. For example, a building that used 24,000 kWh of gas in a month with 400 heating degree days used 60 kWh per degree day; if the next month has 500, about 30,000 kWh would be expected, and a bill far above that points to a problem. The numbers are invented to show the method.
They are not a way to size plant. Sizing depends on peak conditions and heat gains, set out in a load calculation; see how to size commercial air conditioning. CIBSE splits its weather files by purpose, with Test Reference Years for annual energy assessments and Design Summer Years for overheating risk, and the Met Office calls its cooling degree day results rough estimates of overall demand changes on a large scale.
What do the regional contrasts change in practice?
In the north and Scotland, a long heating season makes envelope, heat recovery and heating plant the main energy levers, and cold outdoor air favours the approach in the free cooling guide. In London and the South East, July sits closest to the cooling base, so cooling efficiency and controls deserve more attention. Climate change moves every row; the guide to whether the UK will need air conditioning covers the projections. Each region page on the locations directory has local detail, and a reader who wants a contractor's view can send an enquiry through the site's form.
Sources
- TM41 Degree days: theory and application, CIBSE, 2006; weather data, CIBSE; checked 22 October 2026.
- Met Office: UKCP09 heating degree days; Annual Cooling Degree Days projections; UKCP09 regional cooling degree days; August 2026 county breakdown; checked 22 October 2026.
- Department for Energy Security and Net Zero: DUKES weather and Energy Trends: UK weather; checked 22 October 2026.
- Met Office, location-specific long-term averages and station tables for 1991 to 2020: Edinburgh (Royal Botanic Garden), Durham, Rochdale, Sheffield, Nottingham (Watnall), Pershore, Cambridge (NIAB), Heathrow, Reading (University), Yeovilton, Cardiff (Bute Park), Aldergrove; checked 22 October 2026.
Common questions
Which base temperature should I use?
TM41 says the base temperature relates to the building's energy balance and system, so it should reflect the building. The Met Office conventions are 15.5°C for heating and 22°C for cooling. Whatever is chosen, state it alongside the figures.
Can I use degree days to size a chiller or heat pump?
No. Degree days summarise energy-related weather over a period, while plant is sized for peak load, which needs a load calculation. CIBSE provides separate weather files for energy assessments and for overheating checks.
Why do these figures differ from other published degree days?
The heating estimates here are illustrative: they use monthly mean temperatures from one station per region, which understates real daily degree days, and a 15.5°C base. Published series use daily data and may use a different base.
Where can I get degree days for my own site?
For national heating degree days, DESNZ publishes annual and monthly series. For maps of heating and cooling degree days, the Met Office climate averages tool has them for 1991 to 2020. Site-level work needs daily data and a stated base temperature.
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