Mostly arid to semi-arid, few areas in the province receive more than 400 mm (16 in) of rainfall per annum and the average annual rainfall over the province is 202 mm (8.0 in). Rainfall generally increases from west to east from a minimum average of 20 mm (0.79 in) to a maximum of 540 mm (21 in) per year. The west experiences most rainfall in winter, while the east receives most of its moisture from late summer thunderstorms. Many areas experience extreme heat, with the hottest temperatures in South Africa measured along the Namibian border. Summers maximums are generally 30 °C (86 °F) or higher, sometimes higher than 40 °C (104 °F). Winters are usually frosty and clear, with southern areas sometimes becoming bitterly cold, such as Sutherland, which often receives snow and temperatures occasionally drop below the −10 °C (14 °F) mark.