A rainy or hot day may dissuade you from washing your own windows, but is unlikely to deter a professional window cleaner. On hot days water dries very quickly and can leave a mark- so speed and flawless technique is essential. Window cleaners in Sydney are careful when in dangerous winds and electrical storms, however, the weather rarely prevents us from washing windows.
You may also be concerned about water restrictions in the hotter months- expert window cleaners are very thrifty with the water we use, and often have an exemption from water restrictions (not that we need it!).
Dirty windows look worse after a rainy day as the water sticks to the existing dirt. Rain will run straight off clean windows. So don't hold back from getting your windows cleaned if there is a bit of rain about.