their cells, allowing water to freeze between cell walls and the cells to survive. Plants normally considered hardy may not survive freezing if they are not readily acclimated, which renders them unable to use these mechanisms. Hardiness ratings Various hardiness ratings are published. In the United States (US), the most widely used is the U.S. Department of Agriculture (USDA) system of hardiness zones based on average minimum yearly temperatures. This system was developed specifically for the extremely diverse range of conditions in the US, from baking desert to frozen tundra. Another commonly used system is the Sunset Climate Zone system. This system is much more specific to climates (i.e. precipitation, temperature, and humidity based) and less dependent on the yearly minimum. In contrast the United Kingdom (UK) and Western Europe have an oceanic climate, and experience a narrower range of temperatures that is tempered by the presence of the Gulf Stream. This results in areas like western Scotland experiencing relatively mild winter conditions and cool summers that enable the growing of some subtropical plants, despite being well to the north of subtropical climate areas. The Royal Horticultural Society has published a set of hardiness ratings applicable to the UK. The ratings range from H1a to H7. H1a, higher than , applies to tropical plants permanently under glass in heat; while H7, below , applies to very cold-tolerant plants such as heathers. Most outdoor plants in the UK fall within the range H4, (hardy in the average winter) to H5, (hardy in a cold winter). Also, the average minimum temperature in the UK is much warmer than the average minimums in much of the northern US, while the average maximums in the northern US are often much warmer in summer than the UK. In addition to cold tolerance, plant hardiness has been observed to be linked to how much stress specific plants are undergoing into the winter, or even how fast the onset of cold weather is in a specific year. This means that often stressed plants will exhibit less cold tolerance than plants that have been well maintained. Plants may also die if the winter changes from balmy to exceptionally cold in a short period of time. Other hardiness Tolerance to other climatic factors are often omitted from the "hardiness" discussion. This section provides a very brief overview of the topics. Heat tolerance may be displayed alongside cold-hardiness, such as