Weather data

General description

Weather or climate files are textual files that contain data about weather conditions data for specific location for some time period. They contain data such as sky and sun irradiances, temperatures, wind speed etc.

Weather files can contain real data recorded for some time period on a location. Example of such files are Satel-Light weather files. Although these data are recorded and real data, they don't represent typical climate conditions for some location, and aren't a good source for daylighting calculations.

Another format contains weather data which are statistically analyzed and organized as test reference years (TRY) for some location. They represent typical weather conditions, and as such are suitable for daylighting calculations. Typical time step in these files is one hour, and files contain data for one year period. Example of such format is Energy Plus Weather Format (EPW).

Where to find EPW weather data ?

Files for many places on Earth can be found on:

• Energy Plus: https://www.energyplus.net/weather
• OneBuilding: http://climate.onebuilding.org
• Photovoltaic Geographical Information System (PVGIS) : https://re.jrc.ec.europa.eu/pvg_tools/en/#MR

And companies in many countries can provide specific climate files.

SketchUp model location

For daylight calculations, it is important to locate the model correctly. This feature is in the SketchUp menu:
Window-> Model info-> Geo-location

Weather data inside DL-Light extension

• Building location

  Exact building location coordinates are used in combination with weather data to generate correct sky luminance/radiance distribution for simulations. The 3 possible choices for buiding location are:

  • location of SketchUp model,
  • location of the weather data if used in the current extension,
  • define a custom location.

  Custom location is defined with latitude and longitude:

  
  • Latitude North [-90°, 90°] (North latitude is treated as positive, and South as negative.)
  • Longitude East [-180°, 180°] (East longitude is treated as positive and West as negative.)
  
  
• Time zone and daylight saving time (DST)

  Dates and times are relative to a time zone, a priori that of the location of the building.

  The legal time zone (used by EPW files) differs from the geographic meridian for many countries.

  The default time zone proposed by SketchUp, based on the meridian, can lead to calculation errors.

  The only extension using daylight saving time is sDA/ASE. Others extensions never use daylight saving time (DST). This practice is common to most norms and standards, LM-83 (sDA/ASE) being an exception. The SunExposure plugin in "single moment" mode is a special case, it allows to use a local solar time (the sun is south at noon).

  Consequently:

  • The times of the calculation periods are interpreted according to the legal time of the chosen time zone, without taking into account the change to summer time. (The concept of legal time without daylight saving time is of course paradoxical.)
  • The time zone to use is that of the EPW climate file. To ensure the consistency of the calculations, plugins that do not use climatic data can propose to select an EPW file to use the reference of its time zone.

  Climate data files from Energy Plus never use daylight saving time. In practice no EPW uses daylight saving time. (The sDA/ASE plugin automatically makes the necessary time corrections.)

  Extract from documentation of EPW on bigladdersoftware.com :
  « Note: EnergyPlus processed weather files available on the EnergyPlus web site: http://www.eere.energy.gov/buildings/energyplus/cfm/weather_data.cfm have neither special days specified nor daylight saving period. »

• Clear sky calculations

  SunExposure extension provides additional possibility to select climate with all clear skies. Goal is to determine maximal potential for sun exposure of some building location, regardless of actual climate for the location. If this option is selected, calculation can be run for any period and time step can be set on values lower than 1h.

Other climatic elements

Direct sun visibility

DL-Light uses the sun visibility criterion of the World Metheorological organization, direct sun is visible on the sky if direct normal irradiance of the sky is above 120 W/m2.

Standard time versus solar time

Solar time is based on Apparent solar time, the standard time is based on Mean solar time. The gap is about a few seconds. For more details, see the Wikipédia page on Solar time.

All extensions use the standard time except for extension Sun Exposure which allows a calculation in solar time in "single moment" mode. The time is then interpreted as a local solar time independent of longitude and time zone: the sun is south at 12:00.