IN ARABIC, Ramadan means “the hot month”, from the root ramida, meaning “scorched”. The holy month of fasting—when devout Muslims abstain from food, drink and sex in daylight hours—was a physical trial for the first Muslims living in the sweltering Arabian peninsula 1,400 years ago. Since then, Islam has gone global. As the holy month begins tonight in most places, the vagaries of latitude mean that some will be hungrier and thirstier than others.

Because the Islamic calendar is lunar, the month of Ramadan falls about 11 days earlier each year, completing a full cycle through the seasons every three decades or so. The holy month this year coincides with the solstice, June 21st, the longest day of the year in the northern hemisphere. This leaves Muslims in northern latitudes facing dauntingly long days of fasting which, in fact, last a bit longer than from sunrise to sunset: they extend from fajr, the pre-dawn prayer, to maghrib, the post-dusk prayer.

On the first day of Ramadan, Londoners can thus expect to fast for roughly 18 hours—from about 3am to 9pm. Muslims in Helsinki will have it harder, with 20-hour fasts, though they still have it better than those in Arkhangelsk, who must hold out for about 21 hours a day.

The length of a fast is not the only difficulty. Daytime temperatures can prove as much of an ordeal given that observant Muslims may not drink water during fasting times. In our survey of the world’s cities with populations above 300,000 people Mecca, the holiest city in Islam, with an average temperature of 35°C (and an average high of 44°C) in June, stands out as one of the most punishing cities in which to observe Ramadan.

Muslims adapt to the difficulties in several ways. In hot Muslim-majority countries, many people sleep for part of the day and are less active when they are awake. Economic output tends to suffer as a result. The United Arab Emirates (a rich country with lots of air conditioning) legally limits work during Ramadan to no more than 6 hours a day. In northerly places with long days, some Muslims opt to follow the fasting hours of Mecca.

The ideal trade-off is to find a place with shortish days and moderate temperatures: lucky are those starting the fast in Durban (about 12 hours of fasting and average temperature of 17°C) and Rio de Janeiro (12 hours and 21°C). Ramadan Mubarak!