Northeast India is linked to the rest of India by a narrow stretch of land—just 22 kilometres (14 miles) wide—between the border of Nepal on the one side and Bangladesh on the other. The northeast region is made up eight states: Arunachal Pradesh, Assam, Manipur, Meghalaya, Mizoram, Nagaland and Tripura, known as the "Seven Sisters", along with the "brother" state of Sikkim. Northeast India can be divided into three main geographical areas: the mountains and hills of the Eastern Himalaya and Patkai ranges; the Meghalaya and Karbi plateaus; and the plain of the Brahmaputra, which lies between these upland areas.

Mountains and hills

Mountainous or hilly terrain covers around 60% of the total area of the northeast region. It consists of two main parts: the Himalayas and the Patkai. The peaks of the Eastern Himalayas form the region's northern boundary. They extend eastwards from Sikkim through Bhutan to the Siang, the upper reaches of the Brahmaputra in Arunachal Pradesh.
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In the east, running northeast-southwest the length of India's border with Myanmar are the mountains and hills of the Patkai Range. A southern extension of the eastern Himalayas, the peaks are not as high. The Naga Hills and Manipur Hills form part of this range. There are also two offshoots: the Barail range, which runs through southwest Nagaland and into Assam, and the Mizo Hills, which extend north-south through Mizoram.

The Meghalaya (or Shillong) Plateau, which rises sharply on its southern edge from the Sylhet plains in Bangladesh, is made up of the Garo, Khasi and Jaintia Hills. The Karbi Plateau is a circular-shaped plateau that rises to a maximum elevation of 1361 metres (4464 feet) from the Brahmaputra Plain in Assam. To the southeast, the River Dhansiri separates it from the Naga Hills.

Plains

The Brahmaputra Plain is a broad valley running east-west through Assam for a distance of about 660 kilometres. The Himalayas rise to the north, while the Patkai Range, the Karbi Plateau and the Meghalaya Plateau all lie to the south. As with the rest of the Indo-Gangetic Plain—of which the Brahmaputra Plain is part—it was formed at the same time as the Himalayas were being uplifted by the collision of tectonic plates. Where the Indian plate was forced downwards beneath the Eurasian plate, a trough-like feature was created. This was filled by sediments brought down by the Brahmaputra and its numerous tributaries from the mountains, causing it to sink further.

There are other, smaller plains in the northeast region. The Barak and Tripura plains both form part of the Indo-Gangetic Plain in its lower reaches, around the Ganges Delta. The Manipur Basin, often called the Imphal Valley, is an oval-shaped lacustrine plain—the bed of an ancient lake from which the waters have mostly drained, leaving behind a flat area of sediments.

Climate

Much of northeast India has a humid subtropical climate. There are hot, humid summers, heavy monsoon rains and mild winters. Summer lasts between April to May. The monsoon, which accounts for 90% of the annual rainfall to the region, begins in early June and lasts until October. Winter sets in from early November until March.
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The Himalayas, Meghalaya Plateau and the hills of Nagaland, Mizoram and Manipur to the east all have an influence on the local climate. These uplands force the moist southwest monsoon winds upwards, causing them to condense into clouds and release heavy precipitation on their slopes. As a result, the northeast is the rainiest region in the country with many places receiving an average annual rainfall of 200 centimetres (79 inches). Highland areas get even more: 300 centimetres (118 inches) a year. Cherrapunji, on the Meghalaya Plateau, is officially the wettest place on Earth with an annual precipitation of 1178.7 centimetres (463.7 inches).
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The warmest parts of the northeast are in the lowlands of the Brahmaputra and Barak plains. Here, average winter temperatures vary between 16 and 17°C (61–63°F), while average summer temperatures are around 28°C (82°F). Tripura has maximum summer temperatures of 33–35°C (91–95°F). Temperatures are a few degrees lower at higher elevations in upland areas.

​​​​​​​In the hilly areas of Meghalaya, Nagaland, Manipur and Mizoram, winters are cold while summers are cool. At the highest altitudes, in the Great Himalayas of Sikkim and Arunachal Pradesh, there is permanent snow cover.