Weather options
The Weather tab defines the environmental conditions for a SunSolve Yield simulation. A weather file provides time-series meteorological data — irradiance, temperature, wind and, optionally, atmospheric parameters — at each time step. From this data, SunSolve derives the sun’s position, solar spectrum, and atmospheric conditions that feed into the main solving algorithm. The Options panel controls how these quantities are computed, including the choice of solar-position algorithm, spectral model, and any overrides to atmospheric parameters.
The Weather tab is organised into three collapsible panels: Selection, Options, and Graph.
Selection
Section titled “Selection”Weather file
Section titled “Weather file”Choose a weather file to provide time-series meteorological data for the simulation.
Location — Filters the list of available weather files by geographic location. If only one weather file exists for a location, it is selected automatically.
Weather file — Selects a specific weather file from those available at the chosen location. When a weather file is selected, SunSolve automatically:
- Sets the simulation date range to the full extent of the weather file
- Detects the observation period definition (start, middle, or end of period)
- Detects whether the file contains solar position data (zenith and azimuth)
- Updates the site latitude, longitude, and altitude
Date range
Section titled “Date range”A date range picker constrains the simulation to a subset of the weather file’s time span. The picker displays dates in local legal time.
Both the start and end dates must fall within the bounds of the selected weather file.
Options
Section titled “Options”The Options panel is divided into two groups of settings: solar position and determination of incident spectra (including atmospheric overrides).
Solar position
Section titled “Solar position”Determines how the sun’s position in the sky is computed at each time step.
Calculation method — Choose between:
- Calculate (default) — Computes the solar position from the site coordinates and time using the selected algorithms. This reveals the additional settings described below.
- Load from weather — Reads solar zenith and azimuth directly from the weather file. No further algorithm choices are shown.
When set to Calculate, the following sub-options appear:
Vector — The algorithm used to determine the solar position without atmospheric refraction. For details on each algorithm see solar vector models.
Refraction — Adjusts the computed solar position to account for atmospheric refraction (bending of light through the atmosphere). See atmospheric refraction models.
Solar time — The algorithm used to convert UTC and legal time into local solar time.
For a full description of the solar position algorithms see solar position models.
Solar position relative to time stamp
Section titled “Solar position relative to time stamp”These settings control how the observation time stamp in the weather file is interpreted when computing the solar position. For a detailed explanation of how observation periods and time offsets affect the solar position calculation, see time steps.
Time stamp definition — Override the observation period defined by the weather file. Enable the checkbox to select one of:
- Start of period — The time stamp marks the beginning of the measurement interval.
- Middle of period — The time stamp marks the midpoint of the measurement interval.
- End of period — The time stamp marks the end of the measurement interval.
- Instantaneous — The measurement applies at exactly the time stamp (period length is zero).
When left unchecked, the observation period from the weather file metadata is used.
Time offset — Only available when the time stamp definition is set to Instantaneous. Specifies an offset in hours between the irradiance measurement and the reported time stamp. Range: −12 to +12 hours.
Determination of incident spectra
Section titled “Determination of incident spectra”Controls how the solar spectrum is determined for each time step.
Calculate, Default. Physical model using SPECTRL2 (clear sky) and Ernst 2016 (cloudy sky) based on atmospheric conditions.
Always AM1.5g, Standard global reference spectrum (ASTM G173-03) scaled to the irradiance at each time step.
Always AM1.5d, Standard direct reference spectrum (ASTM G173-03) scaled to the irradiance at each time step.
When Calculate is selected, the atmospheric overrides section described below becomes available.
Atmospheric parameter overrides
Section titled “Atmospheric parameter overrides”When the spectral calculation model is set to Calculate, SunSolve uses atmospheric parameters to determine the solar spectrum at each time step. By default these values come from the weather file, but each parameter can be overridden with a constant value that applies to every time step.
Each override follows the same pattern: enable the checkbox to activate the override, then enter a value. When the checkbox is unchecked, the weather file value is used (or a default if the weather file does not contain that parameter).
The override precedence for each parameter is:
- User override — the value entered in the UI (if the checkbox is enabled)
- Weather file — the value from the weather file for each time step (if present)
- Default — a built-in fallback value
Precipitable water vapour, cm (or mm), 1.4164 cm, Total column water vapour in the atmosphere. If not in the weather file and no override is set, it is calculated from relative humidity and temperature when available.
Aerosol OD at 500 nm, (dimensionless), 0.084, Aerosol optical depth at 500 nm, a measure of atmospheric turbidity.
These atmospheric parameters feed into the SPECTRL2 and Ernst 2016 spectral models to calculate clear-sky and cloudy-sky spectra respectively. For a detailed description of how these parameters are used, see solar spectra.
For more details on the specific impact of each atmospheric component, see impact of atmosphere on spectrum.
Calculate mode vs. Always AM1.5g
Section titled “Calculate mode vs. Always AM1.5g”Calculate with all atmospheric parameters left at their default values will not produce the same spectrum as Always AM1.5g. There are two key differences:
- Air mass varies with solar position. In Calculate mode, the spectral shape changes throughout the day and year as the air mass changes with solar zenith angle. Always AM1.5g uses a fixed spectral shape, scaled to the irradiance at each time step, regardless of where the sun is in the sky.
- Albedo differs. The AM1.5g reference spectrum (ASTM G173-03) was generated using the SMARTS model with a wavelength-dependent ground albedo (LightSoil surface type). In Calculate mode, SunSolve uses SPECTRL2, which applies a single wavelength-independent albedo value defaulting to 20%. The two approaches differ both in magnitude and in their wavelength dependence, which affects the spectral shape of the diffuse light.
The Graph panel displays a time-series chart of the selected weather file data over the chosen date range. The chart updates when the weather file selection or date range changes.