A weather file is a text file containing daily observations of temperature, humidity, wind, solar radiation, and precipitation at various elevations that profiles a particular climate zone's annual weather stream. The recorded weather stream is greatly oversimplified, however, it is sufficient for practical simulations. 

Weather files are used by energy modelers and building consultants in simulations where a building's performance is reviewed in its anticipated weather stream. By doing these energy simulations one can learn a building's annual energy use and costs, annual peak electrical demand, heating load, and cooling load. If the simulations are done early enough in the design process, the information learned can then be used to find optimal design solutions can elevate a building's performance.

Weather files are formatted into ".epw" files. EPW's were developed by the United States Department of Energy (DoE) to be a standard weather data format, to which several other data formats could be converted. These have been derived from a wide range of sources which includes TMY2, TMY3, IWEC, and RMY. In general, it is recommended to use ‘TMY3’ (Typical Meteorological Year 3) as it is more accurate unless it is too far from the subject site, in which case it is better to use ‘TMY2’. The world's weather files can be easily accessed and downloaded at EnergyPlus. 

To see which locations have available weather files check the National Climatic Data Center (NCDC) website here, or see here for an epw map by Mostapha S. Roudsari.

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