london has regular, light rain throughout the year. july is the warmest month, with an average temperature at greenwich of 13.6 °c to 22.8 °c. the coldest month is january, with an average of 2.4 °c to 7.9 °c. the average annual precipitation is fairly low at 583.6 mm, and february is normally the driest month. drought is sometimes possible, especially during longer heatwaves in summer. snow is uncommon but usually falls at least once each winter and heavy snow is rarer and does not happen every winter. while snow is uncommon in central london itself, there is more snow in the outer areas; this is because the "urban heat island" the big city generates makes the city about 5 °c warmer than surrounding areas in winter.