My family and i sailed to northern europe in july/aug 08 and i have to tell you we did not pack hot weather clothes ,big mistake it was beautiful,best suntan ever. it was our best cruise ever .
Think of an English summer. Now allow some variation up and down from that and you will get as close as anyone is willing to guess. So take clothes to suit a good and a bad English summer. .... Neil.
St Petersburg sometimes has its first ground frosts towards the end of August, so take something warm for the evenings.