Ask a Modeler: What is TMYx and How Does it Differ from TMY3 Weather Data?

What is TMYx and how does it differ from TMY3 weather data? Is there anything I should know about where this weather data comes from?
-April Showers

Dear April Showers,

The short answer is that TMYx is a new (albeit confusingly named) update to the TMY family of files (following TMY, TMY2, and TMY3). These files were not officially developed by NREL, as the TMYs were, although they were developed by some of the same people.

For some background, TMY3 is the third iteration of the most popular climate file format (after TMY and TMY2). TMY, standing for ‘typical meteorological year’ can be a file format in itself, though most commonly refers to a methodology being used to create the files, which are most often stored in an EPW (EnergyPlus Weather) comma-separated value (CSV) file format. These three iterations of TMY files have been developed over the years by NREL and have sequentially used more recent data, better data collection (and calculation) methods, and more and more weather stations. These file types are the most commonly used in the United States (as that’s where they were developed). However, several similar formats are used around the globe, as well as reference year files which simply choose a year within a set of measured data to be the most representative of the climate. The legend on this map of weather files will help you understand the various international climate files.

TMYx files are typical meteorological data derived from hourly weather data through 2018 (soon to be through 2020) in the ISD (US NOAA’s Integrated Surface Database) using the TMY/ISO 15927-4:2005 methodologies. Currently, there are more than 13,550 TMYx locations supplied. This is the same dataset and methodology used in the creation of TMY3 files, although some stations may have been taken offline, and new ones added.

One major improvement is the use of the ERA5 satellite derived data set for solar radiation. Previously, the National Solar Radiation Database (NSRDB) was used, which covered most of the Americas but required solar radiation to be calculated for most of the rest of the world. With ERA5, all TMYx files contain data that is satellite derived, which is preferred over calculations that make assumptions based on other climate variables.

There are multiple options within the TMYx filesets. The default file, with a naming convention such as LUX_LU_Luxembourg.AP.065900_TMYx.epw, contains a file derived from all available data up until 2018. (The number 065900 indicates a weather station where the data was measured.) The specific year range is defined in the 6th row of the file’s header and often begins in 1947, especially for locations in the United States. Other files with naming conventions such as LUX_LU_Luxembourg.AP.065900_TMYx.2004-2018.epw specifically describe the years used. These files are the most useful, in this author’s opinion, because they are derived from more recent weather and reflect some of the widespread impacts of climate change. Note that TMY style files intentionally remove extreme events, so the increase in rain, fires, and other storm events are not captured. There is research to create Extreme Meteorological Year files, which attempt to solve this issue, but there is not yet a universally accepted definition of this file format.

In the end, these files still retain the format of EPW files and can be used in all of the same simulation software or opened directly in a text editor or Excel (as a CSV). The EnergyPlus documentation is a good source for those interested in using the raw data.

To download the files and find some more information, I’d recommend visiting http://climate.onebuilding.org/.

Ben Brannon, PE, BEMP
Senior Engineer, Arup

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