pvlib.snow.loss_townsend(snow_total, snow_events, surface_tilt, relative_humidity, temp_air, poa_global, slant_height, lower_edge_height, string_factor=1.0, angle_of_repose=40)[source]#

Calculates monthly snow loss based on the Townsend monthly snow loss model.

This model is described in 1.

  • snow_total (array-like) – Snow received each month. Referred to as S in 1. [cm]

  • snow_events (array-like) – Number of snowfall events each month. Snow events are defined as days in the month that have snowfall greater than 1 inch. May be int or float type for the average events in a typical month. Referred to as N in 1.

  • surface_tilt (float) – Tilt angle of the array. [deg]

  • relative_humidity (array-like) – Monthly average relative humidity. [%]

  • temp_air (array-like) – Monthly average ambient temperature. [C]

  • poa_global (array-like) – Monthly plane of array insolation. [Wh/m2]

  • slant_height (float) – Row length in the slanted plane of array dimension. [m]

  • lower_edge_height (float) – Distance from array lower edge to the ground. [m]

  • string_factor (float, default 1.0) – Multiplier applied to monthly loss fraction. Use 1.0 if the DC array has only one string of modules in the slant direction, use 0.75 otherwise. [-]

  • angle_of_repose (float, default 40) – Piled snow angle, assumed to stabilize at 40°, the midpoint of 25°-55° avalanching slope angles. [deg]


loss (array-like) – Monthly average DC capacity loss fraction due to snow coverage.


This model has not been validated for tracking arrays; however, for tracking arrays 1 suggests using the maximum rotation angle in place of surface_tilt. The author of 1 recommends using one-half the table width for slant_height, i.e., the distance from the tracker axis to the module edge.

The parameter string_factor is an enhancement added to the model after publication of 1 per private communication with the model’s author. The definition for snow events documented above is also based on private communication with the model’s author.



Townsend, Tim & Powers, Loren. (2011). Photovoltaics and snow: An update from two winters of measurements in the SIERRA. 37th IEEE Photovoltaic Specialists Conference, Seattle, WA, USA. DOI: 10.1109/PVSC.2011.6186627