In this method, common plants with known limits to their range are used. Sunset publishes a series that breaks up climate zones more finely than the USDA zones, identifying 45 distinct zones in the US, incorporating ranges of temperatures in all seasons, precipitation, wind patterns, elevation, and length and structure of the growing season. In addition, the Köppen climate classification system can be used as a more general guide to growing conditions when considering large areas of the Earth's surface or attempting to make comparisons between different continents. The Trewartha climate classification is often a good "real world" concept of climates and their relation to plants and their average growing conditions. Australian hardiness zones The Australian National Botanic Gardens have devised another system keeping with Australian conditions. The zones are defined by steps of 5 degrees Celsius, from −15–−10 °C for zone 1 to 15–20 °C for zone 7. They are numerically about 6 lower than the USDA system. For example, Australian zone 3 is roughly equivalent to USDA zone 9. The higher Australian zone numbers had no US equivalents prior to the 2012 addition by USDA of zones 12 and 13. The spread of weather stations may be insufficient and too many places with different climates are lumped together. Only 738 Australian stations have records of more than ten years (one station per ), though more populated areas have relatively fewer hectares per station. Mount Isa has three climatic stations with more than a ten-year record. One is in zone 4a, one in zone 4b, and the other is in zone 5a. Sydney residents are split between zones 3a and 4b. Different locations in the same city are suitable for different plants. Canadian hardiness zones Climate variables that reflect the capacity and detriments to plant growth are used to develop an index that is mapped to Canada's Plant Hardiness Zones. This index comes from a formula originally developed by Ouellet and Sherk in the mid-1960s. The formula used is: where: Y = estimated index of suitability X1 = monthly mean of the daily minimum temperatures (°C) of the coldest month X2 = mean frost free period above 0 °C in days X3 = amount of rainfall (R) from June to November, inclusive, in terms of R/(R+a) where a=25.4 if R is in millimeters and a=1 if R is in inches X4 = monthly mean of the daily maximum temperatures