2.4. THE SKY COMPONENT


If you are setting up a point-in-time illuminance, visualization, or glare run, you must create a sky environment using the Sky component. For other simulation types (annual daylight, radiation maps, and radiation images) skies are generated internally by the simulation components themselves.

In Radiance, skies are mathematical luminance distributions that describe the brightness of the sky and sun. There are several different models to choose from, including CIE standard skies based on different degrees of atmospheric clarity, and Perez skies based on measured weather data.


INPUTS

  • [Type] Type

    string

    The Radiance sky model type. Which one should you choose? Well, that depends on the purpose of your study. The default selection is the CIE ClearSkyWithSun model, because it represents a worst-case scenario for glare. It is helpful to think of this option as one extreme, and the Overcast option as the other. When studying the daylight conditions at a particular place and time, these skies represent two ends of a range of possible conditions based on weather.

    • ClearSkyWithSun
      CIE standard clear sky with sun.

    • ClearSkyWithoutSun
      CIE standard clear sky, sky component only.

    • IntermediateSkyWithSun
      CIE standard intermediate sky with (somewhat subdued) sun.

    • IntermediateSkyWithoutSun
      CIE standard intermediate sky, sky component only.

    • Overcast
      CIE standard overcast sky. No direct sun, zenith is three times brighter than horizon.

    • Uniform
      A completely uniform sky distribution.

    • PerezFromWeatherFile
      Perez sky distribution using the direct normal and diffuse horizontal irradiance listed in the weather file for the hour in question. Note that weather files are a collection of actual historical months of meteorological data. In aggregate they typify the conditions observed over a long collection period, but in isolation the conditions for a single hour are not particularly meaningful.

    • PerezFromGlobalHorizontalIrradiance
      Perez sky based on a custom global horizontal irradiance value. This is useful if you have specific measured data you have collected yourself (using an exterior meter) or acquired from a source other than the weather file. This selection should be used in conjunction with the Global Horizontal Irradiance (GHI) input on this component (the input is hidden by default).

    • PerezFromDirectAndDiffuse
      Perez sky distribution based on custom direct normal and diffuse horizontal irradiance values. This selection should be used in conjunction with the Direct Normal (Dir) and Diffuse Horizontal (Dif) Irradiance inputs on this component (they are hidden by default).

    • Uniform
      Utah colored sky model. This is Mark Stock's implementation of the Preetham, Shirley, Smits model for sky color in Radiance, including stars, moon, and planets (Venus, Jupiter, and Mars). Note that lunar brightness is accurate, though phase geometry isn't shown. This is the only option that permits below-horizon sun positions. You can set the turbity of Utah skies using the Turbidity (T) input, and the year using the Year (Y) input (both hidden by default).

  • [Loc] Location

    string

    The location is provided by an EPW weather file. You can select an option using the dropdown menu or by connecting an external source (see dropdown parameters). The dropdown items are pulled from EPW files found in the C:/DIVA/WeatherData directory. An EPW file input as a full path is also permissible. The sky component will pull the latitude, longitude, and time zone from the weather file, but will not use its meteorological data unless the PerezFromWeatherFile sky type is selected.

  • [M] Month

    integer [1, 12]

    Month of year.

  • [D] Day

    integer [1, 28-31]

    Day of month.

  • [H] Hour

    number [0, 24)

    Hour of day.

  • [GHI] Global Horizontal Irradiance

    number [Watts/m2]

    Measured global horizontal irradiance. Use this input in conjunction with the PerezFromGlobalHorizontalIrradiance sky type.

  • [Dir] Direct Normal

    number [Watts/m2]

    Measured direct normal irradiance. Use this input in conjunction with the PerezFromDirectAndDiffuse sky type.

  • [Dif] Diffuse Horizontal

    number [Watts/m2]

    Measured diffuse horizontal irradiance. Use this input in conjunction with the PerezFromDirectAndDiffuse sky type.

  • [T] Turbidity

    number

    Atmoshpheric turbidiy, for Utah skies only. A value of 2 is very clear, while 10 is very hazy. The default value is 2.45.

  • [Y] Year

    integer

    The Common Era year, for Utah skies only. The default value is the current year.


OUTPUTS

  • [S] Sky

    Sky (custom data type)

    Sky environment based on the above inputs. The output can be connected to the Sky input of an Illuminance, Visualization, or Glare component: