The weather in Bochum is typical for the Ruhr area: temperate, often cloudy, but rarely extreme. An hourly forecast is ideal for deciding whether it's a day for the German Mining Museum or if you can enjoy a dry evening in the Bermuda3eck (Bermuda Triangle) bar district.
Mornings are usually cool and gray. A rain shower is not uncommon. Check the hourly forecast before you leave to see if the sun has a chance to break through.
In the afternoon, the sun can break through and provide pleasant temperatures. This is the best time for a walk around Lake Kemnade. The hourly forecast will warn you of possible showers.
The evenings cool down moderately. A light jacket is usually sufficient for visiting the many outdoor pubs and restaurants. The weather often remains stable, but a quick look at the hourly forecast never hurts.