How it is worked out: each hour from 6am to midnight is split by its forecast. The chance of rain in that hour goes to the rain types (heavy if 4 mm an hour or more; light drizzle gets its own slice when that part of the day looks wet), and the rest of the hour goes to its dry-weather type, picked in this order: windy, hot, fog, frost, then by cloud cover. So the slices are the share of the day you can expect each kind of weather.