Question to Shaikh Abdullaah Bin Humaid (rahimahullaah):
Will the Muslims living in hot countries or in hot houses be rewarded for their fasting more than those living in cold countries?
Reward is received in accordance with the level of difficulty (i.e. the difficulty that is faced whilst carrying out legislated duties and obligations. As for innovations they are rejected). The Prophet (sallal-laahu-alayhi-wasallam) said: “Perhaps a fasting person will gain nothing but hunger and thirst from his fast (i.e. little reward).’’ 
The Prophet (sallal-laahu-alayhi-wasallam) said: Whoever does not give up lying speech, (false statements) and acting on them (i.e., telling lies), and evil deeds, and behaving in an ignorant evil way, and speaking bad words to others, then Allah is not in need of his (fasting) leaving his food and drink.” 
But if he (or she) intends to please Allaah through his (or her) fast; safeguards it from what will corrupt and nullify it, and distances him (or herself) from backbiting the people, then indeed the reward will be greater whenever difficulty, thirst and hunger increases. So this person (i.e. the fasting person who is living in a hot country) has more reward than others. It is well known that reward is given in accordance with the level of difficulty and that has to be carried out with a sincere intention. [Source: Al-Fataawaa Wad-Duroos Fil Masjidil Haraam…pages: 497. Slightly paraphrased]
 Reported by Ahmad 8856
 Reported by Bukhaari 6057