Here in the south in the USA, we have air conditioning, so opening windows is not an option in the heat! And our windows have screens on them for those times in the spring and fall when we do open the windows. You obviously live somewhere else. I don't know if your windows have a channel where a screen would go, but you could probably create something that would fit in there with screening. Unfortunately, cats love open windows and will jump several stories down to the ground.
If you are in the UK, check out this link:
http://www.flat-cats.co.uk