Climate: the Mongols and their horses were highly adapted to the dry and cold climate of the steppe and Siberian forests. Heat was tolerable for them， but humidity and wetness were far more troublesome. When the Mongols invaded southern Song，they suffered severely from diseases like dysentery， cholera， and malaria， as well as simply from the heat. Even the Northern Chinese had significant health problems fighting in the South under various Chinese dynasties.
金兀术、完颜亮叔侄俩的渡江之战，虽有一时得手，但最终都以失败告终。同样是面对这条大江，蒙古人却成功了。蒙古人的成功和南宋叛将刘整有关。刘整骁勇善战，很有军事才能，但为同僚所妒，又被贾似道排挤，被迫投降蒙古。
