Map from Google - Bolzano

In Bolzano (Bozen in German), capital of the province of the same name, in the Trentino-Alto Adige region, the climate is semi-continental, with cold winters and hot summers. The town lies in a valley, 250 metres above sea level. The average temperature of January is 1 °C, that of July is 22.5 °C. The daily temperature range is generally quite high, so in winter, night frosts are the norm, while in summer, even though the days can be hot, nights remain usually cool.
Here are the average temperatures.
Average temperatures - Bolzano
Bolzano J F M A M J J A S O N D
Min (°C) -5 -2 2 5 10 13 16 15 12 6 0 -4
Max (°C) 6 10 15 19 23 27 29 29 24 18 11 7

Precipitation is moderate, since it amounts to about 700 millimetres per year; the rainiest season is summer, also because of afternoon thunderstorms, while the driest season is winter.
Here is the average precipitation.
Average precipitation - Bolzano
Bolzano J F M A M J J A S O N D Year
Prec. (mm) 25 25 35 50 75 85 90 85 70 85 50 35 710
Days 4 3 5 7 9 9 9 8 7 7 5 4 77

Winter, from December to February, is cold, usually with night temperatures below zero degrees, and daytime temperatures a few degrees above freezing. Although it is a dry season, the sky is often cloudy, and at night sometimes fog can form (though much more rarely than in the Po Valley). In periods of good weather, variations in temperature between day and night can be substantial.
Winter is definitely the driest season of the year, so that it might not rain or snow for weeks. Due to the scarcity of precipitation, snow is quite rare, although some snowfall occurs every year. On average, about 30 centimetres of snow per year fall, as a sum of several light snowfalls; occasionally, however, a more abundant snowfall may occur, around 20-30 cm. However, it usually snows with the wind coming from the south: during cold spells, the city receives a wind falling from the mountains, dry and relatively mild.
Even though night temperatures around -5/-7 °C are quite normal, they drop below -10 °C only during cold spells. Here are some values recorded during the coldest winters: January 1961: -18.5 °C; February 1956: -16 °C; January 1985: -17.5 °C.

In spring, daytime temperatures become rapidly milder, while nights are still cold, at least in the first part. In fine weather, the valley of Bolzano easily heats up, so already in April (and sometimes even in March) highs can exceed 20 °C, and sometimes they can reach 25 °C. Atlantic disturbances, able to bring rains, however, are quite frequent. In late April or May, you experience the first heat thunderstorms.

Summer, from June to August, is hot during the day, while nights are generally quite cool. During periods of good weather, the Bolzano basin gets hot, and light wind conditions make the heat unpleasant. In these periods, temperatures can reach 34/35 °C, and sometimes even higher values (for example, in July 1983 and August 2003 a peak of 39 °C has been recorded). However, because of its northern location, the city is reached with some frequency by the tail of Atlantic fronts, which can bring a bit of cloudiness, and even rain, while in the afternoon or in the evening a thunderstorm may break out. For this reason, the sunshine amount in summer is not very high.

Autumn, from September to November, is initially pleasant, then gradually becomes cooler, and later cold and gray. Temperatures gradually decrease, and at night it starts to get pretty cold quite early, while daytime temperatures are still mild, especially on sunny days, and even warm in September. The first night frosts are recorded typically in early November, but sometimes already in the second half of October. The rains are frequent, and from mid-October, the sun begins to show up quite rarely.

The amount of sunshine in Bolzano is pretty mediocre all year round, at least when compared to the Italian standard: the sun can shine all year round, but in winter there are many gray days, and in spring and autumn Atlantic fronts can pass often enough, while in summer, along with the passage of some Atlantic fronts, cloudiness can form in the afternoon, which may result in some heat thunderstorms. On average there are 3 hours of sunshine a day in December, 7.5 in July and 7 August.

Bolzano, the Christmas markets

When to go

The best times to visit Bolzano are spring and early summer, particularly from mid-April to mid-June. Also September is a good month. You should however take into account some rainy days in both periods.
The summer can be hot, although there may be some pleasant or at least acceptable periods, especially in June. To take into account, in summer and sometimes in late spring, some thunderstorms in the afternoon or in the evening.

In December, Bolzano receives several tourists who come to visit the Christmas markets, while in winter and in summer, you can take a break here as part of a holiday in the mountains.