What Is the Rules of Fasting for Those Living in Northern Hemisphere?

According to the Islamic laws of Ayatollah Sistani, what is the ruling of fasting in the holy month of Ramadan for those who are living in northern hemisphere?

Question: You are requested to let us know the rules of fasting in the month of Ramadan for those who are living in northern hemisphere keeping in view the following scenarios:
1. Where there is either night or day during 24 hours throughout the month of Ramadan. There is no sunset or sunrise in the entire month.
2. Where there is day or night during 24 hours but there is no dawn which precedes sunrise and is the starting time for fasting. In fact, when then sun disappears behind the horizon, the brightness is still there and it continues until the sun rises once again.
3. Where there is day and night during 24 hours and there is dawn also but the days are very long i.e. it is for twenty hours which make fasting very difficult.

Answer: 1. As for the first case, it is obligatory for a duty-bound [mukallaf] to move during the month of Ramadan to a place with ‘normal’ day and night so that he can start fasting, if not, he should move after that month to fast as qadha (making up the missed fast).
2. When it comes to the second scenario, in case the darkness recedes (decreases) little by little after sunset and then it starts increasing once again – be it after two or three hours –, when it starts increasing, it is then the starting time for fasting. And if the darkness or brightness presumably remains the same from the time of sunset until sunrise, the precaution is to take the middle point as the starting time for fasting and abstention from eating and drinking. The duty-bound can get rid of this precaution by moving to another city or place where regular dawn precedes sunrise. He should observe fast there or else he must go on a journey and observe the qadha of missed fast later.
3. As for the third case, it is still obligatory on the duty-bound [mukallaf] to fast from dawn to dusk, if he has the ability to do so and if it does not put him to difficulty which is normally unbearable. He can also go on a journey so that he may not have to observe fast during the month but he will have to observe the qadha later. If fasting is not possible for him, he should either travel or should make niyyah of fasting at dawn and should abstain from eating and drinking until it becomes extremely difficult to fast in which case it is permissible to eat or drink at the time of extreme hunger and thirst, respectively, and the precaution is that he should suffice to the necessary amount (of food or water). He should continue fasting for the rest of the day, and he should also observe the qadha of that day after the month of Ramadan and he will not be liable to any kaffara. God knows best.

source: sistani.org