A quick google ...
http://www.pragueexperience.com/info...nformation.asp
Clothes to wear: As with many European cities, the Prague weather is highly changeable all year round. Even in the summer bring a warm jumper and a waterproof jacket or umbrella in case of a cold spell. Conversely, in warm in the spring, summer and autumn, cool shorts, skirts and dresses can be very welcome. Sunscreen, sunglasses and hats can also be important.
http://weather.yahoo.com/climo/EZXX0...l?woeid=796597
June 69.0 °F 51.0 °F 2.70 in
Although, maybe some advise from those on the ground would be more useful.