Pisa (along with its nearby Florence) enjoy a Mediterranean-style climate.  This translates into warm, dry summers, and cold (but mild) and damp winters.  

The hottest months are June though September, when temperatures range from 17 degrees C (62 degrees F) to a high of 29 degrees C (84 degrees F).  Humidity during this time reaches highs of 70 percent during this time.

Bring your raincoat and umbrella if you plan on visiting anytime from September through December.  The heaviest rainfall is during October and November, when precipitation can exceed 15 inches each month.

The best time of year to visit for the most comfortable weather (and lack of crowds) April to late June; September and October.  It is best to avoid July and August (if you can), since temperatures can get quite hot (and crowded) for sightseeing.