Winter solar radiation data layer used in the creation of Land Environments of New Zealand (LENZ) classification. Winter solar radiation reaches a minimum in June, the month when the sun is lowest in the sky and day lengths are at their shortest, hence the layer is the monthly average solar radiation layer calculated for the month of June. Estimates of winter solar radiation across New Zealand were derived from a surface fitted to monthly solar radiation estimates for 98 sites as described for mean annual solar radiation. Data describing monthly humidity was used as a surrogate measure of cloudiness to improve the fit of the surface to the underlying data. This also increases the local accuracy of the surface predictions, as the number of meteorological stations used to fit the humidity surface is more than three times greater than the number of sites used to fit the solar radiation surface. For more details on the creation of these layers see the mean annual solar radiation layer. Additional details such as the climate station locations used in the creation of the layer and error maps are defined in the attached LENZ Technical Guide.
All climate layers used in LENZ were derived either directly or indirectly from mathematical surfaces (thin-plate splines) that use information about the climate, location and elevation of a number of meteorological stations. Locations are described either in terms of their latitude and longitude or their coordinates on a map projection such as the New Zealand Map Grid (NZMG). Each surface is calculated using a process in which data values for each climate station are omitted in turn and its climate is predicted from the surrounding stations. This process is repeated until no further improvement can be made to the fit of the surface to the raw data. Surfaces can be simultaneously fitted to up to 12 variables, typically monthly data for various climate parameters, e.g., monthly estimates of temperature or rainfall. Additional details such as the climate station locations used in the creation of the layer and error maps for this layer are defined in the attached LENZ Technical Guide. Once the surface has been fitted, predictions can be made for any point of known location and elevation. For example, coupling the surface with a digital elevation model, a regular grid of elevation values, allows the generation of digital climate maps as used in the creation of LENZ. Finally, maps that show the standard errors of the predicted values can be derived using results from more sophisticated analyses of the errors associated with the climate surfaces. The majority of the climate station data used in the development of our climate surfaces were derived from summaries of climate observations published by the New Zealand Meteorological Service. Temperature and rainfall data were collected over the period from 1950 to 1980, while data describing humidity and solar radiation consisted of averages of all data collected up until 1980. Some additional short-duration records of rainfall, including information from storage rain gauges, were used to describe geographic variation in high-rainfall mountainous areas of the South Island.