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Some hardware provides derived quantities, such as dewpoint, windchill, and heatindex, while other hardware does not. This service can be used to fill in any missing quantities, or to substitute a software-calculated value for hardware that has unreliable or obsolete calculations.

By default, the service can calculate the following values, although the list can be extended:

  • altimeter
  • appTemp
  • barometer
  • cloudbase
  • dewpoint
  • ET
  • heatindex
  • humidex
  • inDewpoint
  • maxSolarRad
  • pressure
  • rainRate
  • windchill
  • windrun

The configuration section [StdWXCalculate] consists of two different parts:

  1. The subsection [[Calculations]], which specifies which derived types are to be calculated and under what circumstances.
  2. Zero or more subsections, which specify what parameters are to be used for the calculation. These are described below.

The service StdWXCalculate can be extended by the user to add new, derived types by using the XTypes system. See the wiki article Extensible types (XTypes) for how to do this.


The data source to be used for historical data when calculating derived quantities. It should match a binding given in section [DataBindings]. Optional. Default is wx_binding.


The data binding used by the StdWXCalculate service should normally match the data binding used by the StdArchive service. Users who use custom or additional data bindings should take care to ensure the correct data bindings are used by both services.


This section specifies which strategy is to be used to provide values for derived variables. It consists of zero or more options with the syntax:

obs_type = directive[, optional_bindings]...
where directive is one of prefer_hardware, hardware, or software:

directive Definition
prefer_hardware Calculate the value in software only if it is not provided by hardware.
hardware Hardware values only are accepted: never calculate the value in software.
software Always calculate the value in software.

The option optional_binding is optional, and consists of either loop, or archive. If loop, then the calculation will be done only for LOOP packets. If archive, then the calculation will be done only for archive records. If no binding is specified, then it will be done for both LOOP packets and archive records.

Example 1: if your weather station calculates windchill using the pre-2001 algorithm, and you prefer to have WeeWX calculate it using a modern algorithm, specify the following:

      windchill = software

This will force WeeWX to always calculate a value for windchill, regardless of whether the hardware provides one.

Example 2: suppose you want ET to be calculated, but only for archive records. The option would look like:

      ET = software, archive


In the absence of a [[Calculations]] section, no values will be calculated!

However, the version of weewx.conf that comes with the WeeWX distribution includes a [[Calculations]] section that looks like this:

        pressure = prefer_hardware
        altimeter = prefer_hardware
        appTemp = prefer_hardware
        barometer = prefer_hardware
        cloudbase = prefer_hardware
        dewpoint = prefer_hardware
        ET = prefer_hardware
        heatindex = prefer_hardware
        humidex = prefer_hardware
        inDewpoint = prefer_hardware
        maxSolarRad = prefer_hardware
        rainRate = prefer_hardware
        windchill = prefer_hardware
        windrun = prefer_hardware


The StdWXXTypes class is responsible for calculating the following simple, derived types:

  • appTemp
  • cloudbase
  • dewpoint
  • ET
  • heatindex
  • humidex
  • inDewpoint
  • maxSolarRad
  • windchill
  • windDir
  • windRun

A few of these types have an option or two that can be set. These are described below.


This subsection contains several options used when calculating ET (evapotranspiration). See the document Step by Step Calculation of the Penman-Monteith Evapotranspiration for the definitions of cn and cd.


The length of time in seconds over which evapotranspiration is calculated. Default is 3600 (one hour).


The height in meters of your anemometer. Default is 2.0.


The albedo to be used in the calculations. Default is 0.23.


The numerator constant for the reference crop type and time step. Default is 37.


The denominator constant for the reference crop type and time step. Default is 0.34.



Controls which algorithm will be used to calculate heat-index. Choices are new (see, or old. The newer algorithm will give results down to 40°F, which are sometimes less than the sensible temperature. For this reason, some people prefer the older algorithm, which applies only to temperatures above 80°F. Default is new.


This section is used for specifying options when calculating maxSolarRad, the theoretical maximum solar radiation.


Controls which algorithm will be used to calculate maxSolarRad. Choices are bras ("Bras"), or rs (Ryan-Stolzenbach). Default is rs.


The coefficient atc is the "atmospheric transmission coefficient" used by the 'Ryan-Stolzenbach' algorithm for calculating maximum solar radiation. Value must be between 0.7 and 0.91. Default is 0.8.


The coefficient nfac is "atmospheric turbidity" used by the 'Bras' algorithm for calculating maximum solar radiation. Values must be between 2 (clear) and 5 (smoggy). Default is 2.



Indicates whether the wind direction should be undefined when the wind speed is zero. The default value is true: when the wind speed is zero, the wind direction will be set to undefined (Python None).

To report the wind vane direction even when there is no wind speed, change this to false:

        force_null = false


This class is responsible for calculating pressure-related values. Given the right set of input types, it can calculate barometer, pressure, and altimeter. See the Wiki article Barometer, pressure, and altimeter for the differences between these three types.


Some of the calculations require the temperature 12 hours ago (to compensate for tidal effects), which requires a database lookup. There may or may not be a temperature exactly 12 hours ago. This option sets how much of a time difference in seconds is allowed. The default is 1800.



Which algorithm to use when calculating altimeter from gauge pressure. Possible choices are ASOS, ASOS2, MADIS, NOAA, WOB, and SMT. The default is ASOS.


This class calculates rainRate from recent rain events.


The algorithm calculates a running average over a period of time in the past. This option controls how far back to go in time in seconds. Default is 1800.