Weather Logistics UK

Heating degree days

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Heating Degree Days (HDD)

Free Download - Energy Demand Calculator (High gas or heating oil bills?)

Our climate calculator reports the temperature climatology for today (and next week) in 19 towns / cities of the United Kingdom. It can be found on our climate change page. To see how the long range weather can affect the heating requirements for your home or business, please download our climate calculator. This will provide you information about the temperature bounds at HDD anomalies of 80, 90, 110 and 120%. The calculator is full customisable, enabling you to easily estimate your fuel requirements for oil, gas or electricity for heating on a day-to-day or weekly basis.

Energy Forecasts.png

Please download the climate and energy demand calculator by clicking this link (<100 KB .xls)

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What is a HDD?

Predictions of Heating degree days (HDDs) are a derivative product of the outdoor temperature. A HDD reflects the demand for energy needed to heat a home or business. The heating requirements for a given building at a specific location are proportional to HDD value at the same location. HDD values are reported over various different time periods. On a daily basis, the HDD is obtained by calculating the difference between diurnal (day and night) average outdoor temperature and a baseline value. An accumulated HDD is obtained by multiplying the daily HDD by the number of days in the reported period. Seasonal HDD values are reported by Weather Logistics UK, which must be divided by a factor of three to obtain the monthly HDD.

For extreme winters, the HDD can be up to 50% higher than during a milder winter. As a result the energy demand for heating is much greater, placing pressure on the gas and oil supplies for home and industry. HDD are generally higher for large households in rural locations, where average daily temperatures are often lower than within the towns and cities. HDD values are extremely useful for predicting winter energy bills and organising deliveries of heating oil.

What is a CDD?

A similar derivative of temperature analysis or predictions is a Cooling Degree Day (CDD). This value reflects the amount of energy used to cool a home or business and is also directly proportional to the outdoor temperature. CDDs are most useful in the summer months in the extra-tropics, but are also applicable to other seasons in other parts of the world. When the day and night temperature exceeds a certain threshold businesses or a home can become too warm for comfort, for server housings or on transport systems. Air-conditioning systems can be switched on to cool the work-place down to the desirable temperature, but require large amounts of energy to function. CDD units and a HDD units are not equivalent, corresponding to different kWh (a measure of power consumption). In the UK, energy bills are usually much lower during the summer than the winter months, however extreme weather conditions can vastly increase summer energy demand. An example of an unprecedented summer energy demand was during the European heat wave of 2006. During 2006 an increase in the use of air conditioning in the city of London (UK) outstripped the winter energy demand for the first time on record. The intensity of heat was exacerbated by the urban heat island effect, pronounced due to an exceptionally long period of hot and sunny conditions.

How do I calculate a HDD or CDD?

Weather Logistics UK calculates the HDD and CDD values for you, or provides anomaly values that can simplify your household bill calculations. Their respective calculations are currently performed using the following equations:

HDD = Tbase - (Tmax + Tmin) ÷ 2.0
CDD = (Tmax + Tmin) ÷ 2.0 - Tbase

Tbase is the base temperature i.e. the temperature where no heating or cooling is required.

For calculations on this website Tbase = 15.5°C (59.9°F).

Tmax and Tmin are the maximum and minimum daytime temperatures for the local region where you wish to perform your calculation

  • Note that for a period where both HDD and CDD apply, you may have to calculate the absolute values for each and add them together to form an overall degree day (DD) value. For example:
DD = |Tbase - (Tmax + Tmin) ÷ 2.0| ≡ |(Tmax + Tmin) ÷ 2.0 - Tbase|

Updated HDD / CDD data (May 1st 2011 onward)

From 1st May 2011, Weather Logistics UK is upgrading its system for generating degree day data to comply with the UK Climate Projections of DEFRA. This is the method used in the UKCP09 Weather Generator and its Threshold Detector. The UK temperature threshold for calculations (base temperature) will remain at 15.5°C, yet some minor changes will be implemented. These are detailed as follows:

Weighted Summation to Calculate HDD (UKCP09)
Daily situation HDD daily increment
Tmin ≥ Tbase 0
Tmax < Tbase Tbase – Tmean
Tmean < Tbase and

Tmax *>* Tbase

0.5 × (Tbase – Tmin) –

0.25 × (Tmax – Tbase)

Tmean > Tbase and

Tmin < Tbase

0.25 × (Tbase – Tmin)
≤ changed to > : Error appears in logic on UKCP09 pages


Tmin = minimum daily temperature

Tmax = maximum daily temperature

Tmean = mean daily temperature = 0.5 × (Tmean + Tmax)

Tbase = threshold (base) temperature (15.5°C)

Base temperature

The base temperature is a critical temperature, where no net heating or cooling is required to maintain a building at a desirable temperature. Since different buildings have different energy ratings, the base temperature varies considerably. Modern homes are often better insulated than older constructions. They often contain loft and cavity insulation, double glazed windows and are largely draft free. Larger homes that are less well insulated generally require much more heating to reach the desired room temperature. At Weather Logistics UK we use a standard temperature baseline of 15.5°C (59.9°F) - suitable for a large proportion of buildings in the UK.


To convert °F HDD to °C HDD: °C HDD = (5 ÷ 9) x (°F HDD)

To convert °C HDD to °F HDD: °F HDD = (9 ÷ 5) x (°C HDD)

Interpreting HDD anomaly graphs at Weather Logistics UK

Annual maximum and minimum temperature graph for Aberdeen, based on 1961 - 1990 climatology data from the UK Meteorological Office
  • To plot your own HDD graphs for your UK town / city, please download our climate calculator using the link at the top of this page.

Worked example

The figure above shows the average daily maximum (red) and minimum (blue) temperatures with solid lines for Aberdeen in Scotland (UK). The data is based on a [1961 - 1990 climatology of monthly temperatures] from UK Meteorological Office data. Monthly data has been smoothed to indicate the gradual transition of annual temperatures for several UK towns and cities, reporting an error of ±0.5°C (0.9°F). The figure also indicates upper and lower bounds of HDD (CDD) anomalies by blue and red dashed lines. For example the average day and night temperature for Aberdeen on 1st January is +1.6°C (34.9°F). When the minimum and maximum temperatures are equal to the lower dashed blue and red lines respectively energy consumption for heating would be 20% greater than the climate average. In contrast, during a warmer interlude with a daily mean outdoor temperature of +4.4C, 20% less household heating would be require to reach a standard room temperature (HDD down -20%). For this day the daily maximum temperature would be on the upper red and blue dashed lines.

The difference between these two contrasting days (upper and lower dashed lines) correspond to a 50% difference in household heating bills. Although different households require a different amount of energy to warm them to room temperature, the percentage or perturbation to your normal heating bill can be estimated by using HDD forecasts for your location. Weather Logistics UK provides HDD anomaly forecasts in its autumn and winter weather forecasting packages, whilst providing CDD anomalies during the summer season.

HDD / CDD Anomalies - Applications to local energy bills

Weather Logistics UK produces seasonal forecast for four seasons, including: winter (Dec, Jan, and Feb), spring (Mar, Apr, and May), summer (Jun, Jul, and Aug), and autumn (Sep, Oct, and Nov). It reports HDD and CDD anomalies, as well as absolute values for 9 regions within England, Wales, Scotland and Ireland. Anomalies can provide a better insight into the amount of household heating or cooling. We recognise that your personal HDD may differ wildly from a long term regional average. For example your town may be 100m above sea level in the Peak District and as a direct result you may find that your local HDD is much higher than the nearest regional value reported for the Midlands. To refine estimates of your HDD you will need a bit of extra information regarding your local weather that is rather computationally expensive on our behalf.

Worked examples to calculate your gas or heating oil bills

Below are two examples, one for the summer and another for the winter period. There are two pieces of information that are required here. The first bit, is that you require a seasonal forecast (from us) that provide predictions of the HDD or CDD. In our example, winter 2012 is predicted to be cold with a HDD +20% (higher than climate average). It is then necessary to conduct your own research, using some of the hyperlinks below to discover your local HDD climatology data for the winter. Similarly local CDD can be obtained for summer weather predictions, as indicated below. For the summer example we assume that the 2011 CDD is -15% (below the long term average). The calculations are then conducted as follows:

  • 2. Calculate the long term (seasonal) HDD or CDD value for your location using one of the following methods. For example, if you may want to know how winter 2012 energy bills compare to the long term average (or summer 2011 cooling bills for your aircon unit). To calculate a seasonal average, you must add the monthly or daily degree day data for the respective season.
  • Add monthly data averages for the respective season (recommended) OR using daily data:

Summer example:

June CDD + July CDD + August CDD = Summer CDD
June 1st CDD + June 2nd CDD + ... August 31st CDD = Summer CDD

Winter example:

Dec HDD + Jan HDD + Feb HDD = Winter HDD
Dec 1st HDD + Dec 2nd HDD + ... Feb 29th HDD = Winter HDD (leap year!)

Note that for a true climatology, you will need to sum all the winter or summer seasons during the 30 year reference period 1961 - 1990, then divide by a factor of 30.

  • 3. Supposing you know your local HDD is 1000 units for an average winter period. You also discover that your local CDD is 100 units for the summer period. From a seasonal forecast, you obtain data that indicates a winter 2012 HDD of +20% and a summer 2011 CDD of -15%. To combine these two pieces of data you need to multiply the DD by the appropriate factors:

Winter 2012 example:

HDD (1961 to 1990) × ([HDD anomaly + 100%] ÷ 100%) = HDD (2012 seasonal prediction) = 
1000 × ([100 + 20] ÷ 100%) = 1200 units

Summer 2011 example:

HDD (1961 to 1990) × ([CDD anomaly + 100%] ÷ 100%) = CDD (2011 seasonal prediction) = 
200 × ([100 - 15] ÷ 100%) = 170 units

Note that CDD values can be represented by negative HDD values. You should calculate the HDD or CDD value separately, as combining the two to form an overall energy bill requires a substitution HDD = -CDD, or by the method described by UKCP09 (see above).

Green policy

Weather Logistics UK undertakes all its calculations on a dual processor desktop machine. It vastly conserves its own energy by reporting regional data rather than producing high resolution data. There are no in-house supercomputers eating away at the national grid here. In fact our temperature calculator provides daily average temperatures for many town and cities in the UK. It uses a very simple non-linear calculation and consumes only a tiny bit of your own computer's processing power.

To conserve energy supplies and to help battle climate change, Weather Logistics UK recommends using energy to cool or heat homes only where necessary and in moderation. Air conditioning units vastly increase the demand for energy, particularly during the summer months, placing additional pressures on the national grid supply. Climate change is expected to increase the energy demand in the United Kingdom, as indicated by the UK government on its UK Climate Projections UKCP09 website.

External links

Red box.png * Weather Logistics UK: Jet stream - Blocking interaction model (5 to 9km) jet stream - blocking interaction to simulate large scale and long term air flow for the British Isles. Seasonal forecast product includes HDD and CDD anomaly predictions.

Sources of free HDD data:

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