As you will undoubtedly find out soon, detailed climate data is vital to any energy efficient and responsive passive building design, but it can often be the hardest thing to actually get your hands on - even for some highly populated areas. Anywhere there is a major airport you can be reasonably sure that detailed hourly data exists somewhere. If not, then you are really at the mercy of your local meteorological station, some of whom take only three-hourly readings, sometimes with no wind or solar radiation data. Even if the readings are hourly, and complete, you may only be able to buy monthly summary data. If you can get hourly data, you may not be able to get a digital copy, just mountains of folded printer paper.

These problems are all par for the course. The experience from a range of international projects suggests that data for locations in the US, Canada and Australia tends to be relatively easy to obtain, generally at low cost and in a commonly used format. European data is usually a little more difficult, more expensive and often in a slightly more idiosyncratic format. Major centres in Asia keep good records, but anywhere outside a major city is almost impossible to obtain. This is also true of much of Africa and the Middle-East. Having never done a project in South America it would be remiss to comment, but if anyone else would like to share experiences there, we would be most interested.

Important Hourly Data

There are a wide range of climatic factors that can be recorded. However, many are interrelated and can often be derived from a combination of other measures. In terms of hourly data, the following is the minimum required for both pre-design site analysis and post-design thermal performance simulation:

1. Dry-Bulb Temperature (°C or °F)
2. Relative Humidity (%)
3. Solar Radiation (W/m²)
4. Wind Speed (km/h, mph or m/s)
5. Wind Direction (Deg, 1/4, 1/8 or 1/16ths)
6. Cloud Cover (%, tenths or octa)
7. Rainfall (mm or in)

Not surprisingly, these coincide with the basic requirements of The WeatherTool software, a weather visualisation and analysis tool available as a free download from this website. It is actually shareware, however you can use all of its visualisation and analysis functions to learn about climate data without having to registering it.

Obtaining the right data for The Weather Tool means obtaining digital hourly data sets from some source or other. If you overcome that hurdle, then the software can import almost any ASCII file format, either fixed or column separated, as well as some binary files. As there are so many different 'standards' for weather data, you will most likely also need to know the format of the data within the file(s) in case you need to build up an import/export profile, a relatively simple process within the software itself.

Weather File Formats

There are a plethora of weather file formats available. It seems that almost every new analysis tool takes pride in using its own 'standard' file format - the Weather Tool included unfortunately - though, like every other developer before me, I reckon mine is the only logical way of doing it and cannot possibly understand why anyone else would want to store it any other way. That said, there are generally two very distinct types of weather data available in digital form, ASCII and binary files.

ASCII Weather Files

ASCII stands for American Standard Code for Information Interchange. An ASCII file has all its data stored as text in the file, or as a series of human readable alphanumeric characters. I say human readable here because they consist of recognisable characters, however there are very few ASCII weather file formats that are actually human comprehensible.

Such files almost invariably record a single hour's worth of climate data on a single line within the file, with a full year requiring at least 8760 lines (365x24). The order in which values for date, time, temperature, humidity or cloud cover actually occur within that line is determined by the file's format (not its 'type' which is ASCII). In addition to the values themselves, many formats include data uncertainty flags to indicate whether the values are actual measurements, interpolated from measures made at other times or mathematically generated because recorded data was not available.

Within ASCII weather files there are two further distinct groups:

• Fixed format files
  In this type of file, values occupy fixed columns within each line. For example, the first two characters may represent the day number, the next two the month number, the next two the hour of the day, the next five the dry bulb temperature in °C, etc. Such files are characteristic of computer programs written in the FORTRAN programming language as it is particularly adept at handling such a record format. This means that each line is exactly the same length and each value is therefore right-padded to the required length with zeros or spaces.
• Delimited files
  These files use a special character, usually a coma or tab, to separate or delimit each value in the line. This means that each values can be as long or as short as needed, provided the delimiting character is present between each one. See the related links section below for detailed information on some of the commonly used examples of ASCII weather file formats.

ASCII weather files tend to be quite large (usually more than 800K each) and, as they require a lot of parsing by the computer program as they are read in or written out, tend to be quite slow to work with.

Binary Weather File

Binary files tend to be stored as on disk in almost the same way as they are stored in memory and accessed by the computer program that uses them. As such, they are usually relatively small (< 200K) and very quick to read and write. However, they are completely unreadable by humans and, unless the detailed binary format is made available by the developer, unreadable by other programs as well.

The Weather Tool uses its own proprietary binary file format called a .WEA file - simply because it is quick to read in and relatively small in size. However, even though its format is readily available, it is doubtful that any programs other than the SQUARE ONE suite of analysis tools will ever use it. What it does is provide an efficient means of storing and transferring large amounts of weather data for use by software that can convert it to any other ASCII format required.

The point is, however, that unless you know for sure that the software you have can read a particular binary file format, it will likely end up of absolutely no use to you. You will not be able to simply work out its format by loading it into a spreadsheet program and looking at it.

Related Links

User's Manual for TMY2s

http://rredc.nrel.gov/solar/pubs/tmy2/data.html

TMY Data Handbook

http://www.rt66.com/rbahm/tmyman.htm

Energy-10 Weather Files

http://www.sbicouncil.org/enTen/weather.html

Summary of WYEC2 Record Format

http://www.cmc.ec.gc.ca/climate/wyec2.htm