In Gen 3, we know that "weather-related" Pokemon like Castform will start (hopefully) appearing in game. There are theories as to how it will be released;
Castform has 4 forms: Normal, Rainy, Snowy and Sunny.
Some theories suggest that you can only find certain types of Castform in certain conditions (e.g finding Rain Castform during a rainstorm).This would obviously be linked to season (you won't find Snow Castform in the summer) and to daily weather trends if Niantic were to release Castform in this way. (This obviously means that Pokemon Go would have to add daily weather conditions in to the game)
But how would this work around the world?
The system of having changing weather patterns would only work in certain regions of the world. Countries that have 4 seasons (or at least have a change between winter and summer) would be places that this system would work in (e.g Canada, USA, UK, Japan, Australia, to name a few). So, you could say that this system "generally" works out for the majority of Pokemon Go's player base.
But what about countries that have odd weather patterns due to their geographical location? Desert countries (e.g Oman, UAE, Qatar), tropical countries (e.g Cuba), African-continent countries (e.g South Africa) who don't receive any snowfall (and in the case of the Middle East, barely any rain either!) How can Niantic design a system that will benefit certain regions and leave out others?
We know that Niantic is on thin ice when it comes to their set up of Pokemon Go in urban and rural areas. People in rural places feel "disconnected" or "left out" from the rest of the community (this is very evident on the Go Hub Forums). We don't want uprising 2.0 about weather Pokemon.
If Niantic is reading this, I hope you come up with a way to distribute weather-based Pokemon in a fair and balanced manner.