I've often noticed when flying with Cathay London - Hong Kong - Sydney, at all seasons, that the flight from London to Hong Kong, across Russia and China is invariably smooth throughout, but the continuation always seems to hit mild turbulence as it crosses the Philippines and Indonesia.

I've always put this down to tropical weather patterns being much more aggressive than in temperate areas. It could also be due to less flexible Air Traffic Control not allowing changes to the route - once over Australia you can notice significant alterations of track on the Airshow as the crew successfully avoid tropical weather, while over the other countries you seem to follow the same fixed route regardless.