I have been watching the weather forecast for a month and it's been sunny and hot in Rome and Venice. Now I see that the 3 days I am there next week it is supposed to rain and thunderstorm! Is it still fun to walk around in Rome and Venice if the weather is sucky?? Should I pack differently according to weather - as in does it really cool off when it rains there, or does it stay hot and muggy?
I recommend the universal traveler's packing tip: layers, layers, layers. Always prepare for it getting cooler. Venice can get some cool winds if the winds blow in from the sea (which can also cause flooding, but only with very strong winds, which do not normally occur at this time of year). For the most part, it will remain hot and muggy, but the evenings can cool down enough that you will want something to keep you warmer.
As for whether or not it is still fun... absolutely! You will be in two of the best cities in the world, and there are still great indoor things to do anyways. In Venice, you can always just hop on one of the covered vaporetto boats on one of the routes that meanders around the city. Shopping in Venice is also pretty impressive so window shop and duck into as many stores as you want to avoid the rain and check out the fairly unique products for sale.
The worst rain that fell on us in Italy was in Venice. We weren't expecting it and we got a bit wet. It was warm and not bad. The rain didn't last too long and off we went. It didn't take us too long to dry out.