Weather: India, Afghanistan, Bangladesh, Pakistan

As troublesome as it may be, the monsoon is a member of the family here. India simply wouldn't be the same without it. It's a dramatic sight, to be sure: after days of torrid heat and humidity, with only an occasional storm at best, the skies suddenly darken, the southwest winds kick in, and before long the skies open up. And open up they do: on 26 July 2005, Mumbai set a new national record for 24-hour rainfall with an astounding 942mm (37.1in). The dates on the map below were first calculated in 1940, and they're still good ballpark figures for monsoon arrival. In about half of all years the monsoon is at least three days early or late, but rarely by more than a week. The rains end quickly across the northwest half of India between mid-September and mid-October, but they lag across the south until December in what's known as the northeast (winter) monsoon.

It doesn't rain constantly during the monsoon. About once each summer the rains shift north to the Indian/Nepalese foothills for a week or two and leave the rest of the subcontinent dry. The most likely time for such a “break monsoon” is the middle of August. Even when the monsoon is healthy, squalls come and go, and there's enough sunshine in between so that the air will feel hot and humid to any visitor just arriving, especially toward the south. In fact, India south of the Himalayas is almost never bitingly cold. Even New Delhi is hard-pressed to muster a frost during the dry winter, when sunny and hazy skies are the rule across the north. When cold waves do strike, they can claim many lives due to the scarcity of cold-weather shelter and clothing.

Northwest and Ganges Plain

One of the world's great weather puzzles is why India's rainfall varies so much from northwest to northeast. Parts of the Thar Desert average under 5cm/2 in, a year, whereas Cherrapunji experienced the rainiest twelve months ever recorded on Earth in 1860–61 with 26,470mm/1041in. Whatever the reason for the disparity, visitors should plan on progressively wetter conditions towards the east. In the spring, low-pressure centres help whip up Aandhi – blinding dust storms across the Thar that may be accompanied by thunder but little or no rain. As the storms move east, they often become more intense.

From Patna east to Bangladesh is one of the few corridors outside the US where strong tornadoes occur (albeit rarely) in the weeks before the less violent monsoon rains kick in. Tropical cyclones out of the Bay of Bengal are a threat anytime from March to December along the east Indian coast north of Sri Lanka and, in particular, across low-lying Bangladesh, which has suffered catastrophic cyclone flooding time and again (over 100,000 people were killed in 1970 and a similar number in 1991). Mid-winter temperatures are comfortable, by and large, but bouts of chilly fog may strike the upper Ganges, and smoggy haze is widespread. Dust is added to the equation in April and May, along with pressure-cooker heat that can stay above 30°C/86°F all night and climb to 45°C/113°F by day.

The period after monsoon departure, from mid-October to early December, is prime time for seeing the Ganges Plain: vegetation is lush, the heat less intense, and winter's pollution and dust have yet to arrive. The Thar Desert extends into eastern Pakistan, where the conditions are forbiddingly dry and hot, with only slight relief near the sea. Average highs in May (the hottest month) range from 35°C/95°F on the coast at Karachi to 42°C/108°F at Hyderabad, and Jacobabad has hit 52°C/126°F, Asia's all-time high. Pakistan's northernmost lowlands are prone to frost and persistent cold-season fog; like nearby Kashmir, they experience a wet season in winter as well as in summer. The high mountains of Kashmir and elsewhere across the far northwest get a later start to the monsoon than does Nepal, but they have a better chance of winter snowfall (more so on the peaks, less so in the valleys). The mountains just to the south of Afghanistan block the monsoon and help produce a dry highland climate. The tail ends of Asian storms produce cool winter rains and, at elevation, a decent amount of snow across the country. Afghan temperatures are similar to those across the southwest US mountains, with vivid seasonal contrasts and large day-to-night ranges.

South

As the subcontinent narrows south of 20°N, India's climate becomes increasingly sultry year round, with a few surprises thrown in by the Ghat Mountains. The Western Ghats intercept the southwest monsoon flow and squeeze out much of its moisture. Thus, parts of the south-central peninsula, including Bangalore, get less than half of the monsoon rain observed further north and west (although it rains on almost as many days – just not as hard or as long). Due to its elevation of close to 1000m/3300ft, Bangalore enjoys some of the coolest summer nights of any big Indian city, often dropping below 21°C/70°F.

Further north, Pune is notably cooler and drier than Mumbai, just on the other side of the Ghats. The coast from Mumbai to Trivandrum (the monsoon's traditional entrance point) is among the wettest places in all of India. Much of the Western Ghats is cloaked in fog and relentless rain through the summer. Interestingly, this strip is much less prone than the Bay of Bengal coast to tropical cyclones. Mumbai was spared any serious cyclones through most of the twentieth century, although it's hardly immune: a surge of cyclone-related high water killed some 100,000 people in 1882. Nearly all of Mumbai's rain occurs during the monsoon, but southward, the coast toward Trivandrum gets frequent spring showers well before the official monsoon begins – about every third day as early as April. The monsoon rains across the southeast intensify with autumn as the winds turn to northeast. The rainiest month in Chennai is actually November, although the city is oppressively steamy for months beforehand. With the peak of Nuwara Eliya near its centre, much of Sri Lanka gets drenched all year. Autumn is the wettest period; another rainy season affects all but the island's east coast as the monsoon moves north during April and May. Even mid-winter brings rain every two or three days in Colombo. The lowlands covering the north half of Sri Lanka are less wet overall, with little rain in mid-summer, but virtually every place on the island sees at least 1000mm/39in a year.