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Last edited 06 Oct 2022
Design summer year (DSY)
The UK Meteorological Office (MO) collects weather data from stations across the UK. These stations measure different characteristics of the weather at hourly intervals. The characteristics include; air temperature, wind speed, wind direction and air pressure as well as others. The Chartered Institute of Building Service Engineers (CIBSE) have synthesised this data into two different types of files; Design Summer Year (DSY) and Test Reference Year (TRY). These files are interpolations of actual weather data that are often used in the dynamic thermal modelling of buildings to assess energy use or overheating and to evidence meeting the requirement of the Building Regulations Part L and (the soon to be released) Part O.
 Design summer year (DSY)
Design Summer Year (DSY) data represents an averaged year of weather. The DSY file has the advantage of simplicity as it actually shows a year of weather as if it were single and continuous, rather than composite and made up from average months. Initially a baseline was created by taking the mean temperatures from April to September, this was then adapted to create probabilistic DSYs with a set of years that better describe overheating events, their relative frequency and severity. Although this improved the data sets, it is less able to account for extreme temperature values within individual months or incident solar radiation, both of which are significant aspects of building performance and overheating analysis.
A Test Reference Year (TRY) is a similar weather data reference file but one that models or represents a typical year in a certain location. As such it has a greater number of reference points within it, so might be considered more complex but also more accurate (depending on the selected reference years used). TRY files have been updated at various points in time (initially based on 1984 to 2004 then 1984 to 2013) and are available for 14 locations across the UK. More recently with the onset of climate change, differing weather patterns and their extremes, institutes in more and more countries have updated their TRYs and started to work on Future TRYs and DSYs.
 Future TRYs and DSYs
Future weather files based on existing Design Summer Years (DSYs) and Test Reference Years (TRYs) that incorporate different climate change scenarios (UK climate projections) are available for three different time periods;
- 2020s (2011-2040)
- 2050s (2041-2070)
- 2080s (2071-2100)
In the case of standard TRYs and DSYs the data is based on relatively recently recorded events, in the case of future scenarios the likely hood of a particular weather being likely has to be considered and transparently described.
UK climate projections are based on different emission scenarios (High - Low) relating to ongoing climate mitigation efforts. These directly relate to the representative concentration pathways (RCPs) which are used internationally to show different scenarios and the likely resulting global mean temperatures (RCP2.6, 4.5, 6.0 and 8.5).
Percentiles are a way to describe different probabilistic assumptions within these scenarios, in terms of future climate change. For example a 90th percentile in a high emissions scenario means there is a 10% chance that temperatures will fall above the given threshold for a particular time period, so above the 90th percentile in a high emissions scenario might be considered a worst case, whilst the 10th percentile in a low emissions scenario the best case. That is to say that for 90% of the time the temperatures will be lower than the temperature specified for the former example described or 10% for the latter.
The following emissions scenarios are available for use in dynamic building simulations from the Chartered Institute of Building Service Engineers at https://www.cibse.org/knowledge/cibse-weather-data-sets:
- 2020s – High emissions scenario – 10th, 50th, 90th percentile,
- 2050s – Medium – 10th, 50th, 90th,
- 2050s – High – 10th, 50th, 90th,
- 2080s – Low, 10th, 50th, 90th,
- 2080s – Medium – 10th, 50th, 90th,
- 2080s – High – 10th, 50th, 90th.
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