Weather data selection

Weather data files represent the local weather conditions at a particular location. The data includes an hourly listing of values for many meteorological and solar radiation properties for, typically, a one-year period. Depending on the purpose of simulation, different types of weather data can be applied. To select the appropriate weather data for your project, be sure you consider:

• The time period included in the weather file
• The location where the weather data was recorded

There are numerous data sets used to generate weather files, and the files are available in a range of file formats. See this page for more technical information on weather data file types.

Weather data - time period

Be sure to select weather data derived over a time period that is appropriate for your analysis. Typical weather data is the most commonly used with applications including annual energy estimation, code compliance, and programs like LEED. Actual weather data is useful for evaluating existing buildings. Future weather data is becoming more commonly used for building designs that seek to account for future climate change scenarios.

Typical year weather data

A typical meteorological year (TMY) is a collation of selected weather data for a specific location, generated from a data bank much longer than a year in duration. It is specially selected so that it presents the range of weather phenomena for the location in question, while still giving annual averages that are consistent with the long-term averages for the location in question.

The most commonly used TMY weather data source is the EnergyPlus Weather Data, which covers hourly TMY weather data files for 1020 US locations and many cities outside the US. The data format provided are DDY, EPW, STAT, and CSV. Some other weather data sources are also available (converted from other sources).

Actual weather data

Actual weather data, also called real-time or historic weather data, are used to forecast near-term building performance and calibrate utility bills in existing buildings for retrofit. This type of weather file is often called an actual meteorological year (AMY) file.

Future weather data

Future weather data are used to estimate potential climate change impacts. Short-term (up to 10 days) weather forecasting provides decision support for building control schemes. Long-term (years or decades) climate change prediction necessitates research for issues related to global warming and its impact to energy consumption.

Design day weather data

Design day is often used for load calculations or sizing HVAC equipment. This data defines the most extreme weather conditions for the geographic location selected for study. Typically, two or more sizing periods are selected to define the heating and cooling extreme weather conditions.

Weather data - location

Be sure to select weather data that was collected from a location that experiences similar weather conditions to your project's site. Many software tools allow you to enter your project's address or zip code and will present you with a list of the closest weather file locations.

Generally available weather files

There are thousands of freely available weather files that are often bundled with BEM software tools. However, this translates to a relatively small number of weather files per country or state so there are some instances where the nearest location to your project site may experience different weather conditions. Also, some of these files are based on data collected at airports which may experience different temperatures and wind patterns than your project's location. Nevertheless, these files are very commonly used and are usually appropriate for energy analysis that seeks to compare design options such as design assistance, code compliance analysis, and programs like LEED.

Local weather files

Some weather data providers offer custom weather data files that are intended to represent the microclimate weather conditions of a particular location more closely than the weather file locations that are generally freely available. These providers offer the ability to divide a region into a grid and select the most suitable location. The files are generated by using a large set of weather station data and interpolation. Because this approach offers a closer match of a particular location, they are often used for energy analysis that seeks to predict annual energy consumption with greater accuracy.

Where to get weather data files

Many BEM software tools include a set of weather data files as part of the installation. If additional weather files are needed, some sources include:

• A large, searchable set of weather files are available on the EnergyPlus website. The site also includes links to many other resources providing weather files (some free, some paid) including future weather and local data.
• IES also maintains a list of weather file resources for downloading or purchasing weather data including international locations, future weather, and local weather data.

Converting weather files to different formats

Weather files come in many different types with different contents and different file extensions. Not all file types are compatible with all software tools. Refer to the page weather file types for more information about tool compatibility along with links to tools that can convert files to different formats so that they can be used by tools that are not natively compatible.