Apart from the ice-free Atlantic bringing marine air into Sweden tempering winters, the mildness is further explained by prevailing low-pressure systems postponing winter, with the long nights often staying above freezing in the south of the country due to the abundant cloud cover. By the time winter finally breaks through, daylight hours rise quickly, ensuring that daytime temperatures soar quickly in spring. With the greater number of clear nights, frosts remain commonplace quite far south as late as April. The cold winters occur when low-pressure systems are weaker. An example is that the coldest ever month (January 1987) in Stockholm was also the sunniest January month on record.