Tasmania is the coolest region in Australia and differs from the mainland in several ways, mainly in that it has more defined seasons. Though often pleasant, the weather can be unpredictable: winter days are often clear and sunny, but it's not unheardof to see snow on Mt Wellington in December, and some days seem to have all four seasons thrown in! The best solution? Wear a few layers of clothes that you can add to or strip off as necessary.
Hobart's Summer evenings seem to last forever, though there's still a bit of a chill in the air once the sun eventually goes down. From November to February you can expect maximum temperatures in the mid twenties most days. There's usually a handful of days in Summer where the temperature tops 30 degrees centigrade, but don't expect the persistent, sweltering heat and humidity that you'll find on the mainland in Summer.
Autumn sees the changing of leaf colours in Hobart's parks and gardens. It can be a very beautiful season; temperatures are usually mild and consistent up until around April, after that the weather becomes more variable.
Winter brings on snowfall on the mountains like Mount Wellington, behind Hobart, but snow down to sea level is rare. Much of winter is overcast, with rain showers, but with quite a few crisp, clear days as well. Daily maximum temperatures in winter would average 10 - 15 degrees celcius.
Spring can be glorious but is often unsettled. September and October can be very windy and blustery, but by late October and early November the days are long and the weather is warming up properly. Daylight savings starts on the first Sunday of October, giving an extra hour of daylight in the evenings.