What is weather data, and how is it collected?

Weather is a combination of meteorological metrics encountered at a given time. While an on-site personal weather station (PWS) can capture real-time weather data, public weather datasets are widely available. Actual hourly values for air temperature, solar radiation, wind speed or direction, and many other metrics are collected daily at thousands of sites worldwide. Multi-year data from a 10+ year collection period are synthesized into a single representative year to understand climate trends over time. These synthesized files are required inputs for most environmental simulation engines.

TRY, TMY, and TMYx Datasets

One of the earliest weather datasets, the Test Reference Year (TRY), was created with historical weather data from 1948 to 1975. TRY was limited because it did not contain actual solar data, and the data selection process excluded all weather extremes, resulting in a mild representative year. In light of these shortcomings, a new dataset, the Typical Meteorological Year (TMY), emerged in 1981, including actual solar radiation values. TMY is the preferred dataset used by many simulation engines today. The methodology used to create a TMY dataset is similar to the TRY but with one key difference. TMY uses the most typical months instead of isolating the most typical year. For example, data for January might be sourced from 1996, while data for May comes from 2017.

TMY3 is the latest dataset (1991- 2005) available from EnergyPlus, and the newer TMYx database (2004-2018) is available from Climate.OneBuilding. Each TMY zip file comes with several file formats that serve different purposes.

A TMY3 zip file for a location contains the following:

• Design Day (DDY) files are based on ASHRAE Design Conditions and include a collection of climate design data that engineers primarily use to design systems.

• EnergyPlus Weather Format (EPW) files contain 8,760 values for each meteorological parameter included in a TMY dataset, which corresponds with the number of hours in a year.

• Statistical Report (STAT) files contain expanded EnergyPlus weather statistics.

In addition to the TMY3 files, TMYx contains the following additional files:

• CLM is a file type derived from an EPW file for use in a UK-based software for energy modeling and systems design, ESP-r.

• RAIN files contain hourly precipitation in meters per hour at locations available.

• WEA files are a more flexible weather file format that can deviate from the annual, one value per hour format. They can be for a portion of the year or contain fewer meteorological metrics for the year.