I'm looking foward to go to Beijing and Shanghai, but don't know when..

So, as according to my question, I'm a Portuguese plane spotter (for some who don't know what's this, is taking photos to the planes :P), which means I need a time to get some good weather, sunny or even some clouds, not to hot or cold (I mean, temperatures low around 15 it's ok and maximum around 24/25) because of the heat distortion on the photos. I'm thinking around March or October, as it depends also on the tickets prices..any help will be appreciated :)