The island of Bali lies 3.2 km (2 mi) east of Java, and is approximately 8 degrees south of the equator. Bali and Java are separated by the Bali Strait. East to west, the island is approximately 153 km (95 mi) wide and spans approximately 112 km (69 mi) north to south; administratively it covers 5,780 km2, or 5,577 km2 without Nusa Penida District, its population density is roughly 750 people/km2.
Bali's central mountains include several peaks over 2,000 metres (6,600 feet) in elevation and active volcanoes such as Mount Batur. The highest is Mount Agung (3,031 m (9,944 ft)), known as the "mother mountain" which is an active volcano rated as one of the world's most likely sites for a massive eruption within the next 100 years. Mountains range from centre to the eastern side, with Mount Agung the easternmost peak. Bali's volcanic nature has contributed to its exceptional fertility and its tall mountain ranges provide the high rainfall that supports the highly productive agriculture sector. South of the mountains is a broad, steadily descending area where most of Bali's large rice crop is grown. The northern side of the mountains slopes more steeply to the sea and is the main coffee producing area of the island, along with rice, vegetables and cattle. The longest river, Ayung River, flows approximately 75 km (see List of rivers of Bali).
The island is surrounded by coral reefs. Beaches in the south tend to have white sand while those in the north and west have black sand. Bali has no major waterways, although the Ho River is navigable by small sampan boats. Black sand beaches between Pasut and Klatingdukuh are being developed for tourism, but apart from the seaside temple of Tanah Lot, they are not yet used for significant tourism.
The largest city is the provincial capital, Denpasar, near the southern coast. Its population is around 491,500 (2002). Bali's second-largest city is the old colonial capital, Singaraja, which is located on the north coast and is home to around 100,000 people. Other important cities include the beach resort, Kuta, which is practically part of Denpasar's urban area, and Ubud, situated at the north of Denpasar, is the island's cultural centre.
Three small islands lie to the immediate south east and all are administratively part of the Klungkung regency of Bali: Nusa Penida, Nusa Lembongan and Nusa Ceningan. These islands are separated from Bali by the Badung Strait.
To the east, the Lombok Strait separates Bali from Lombok and marks the biogeographical division between the fauna of the Indomalayan ecozone and the distinctly different fauna of Australasia. The transition is known as the Wallace Line, named after Alfred Russel Wallace, who first proposed a transition zone between these two major biomes. When sea levels dropped during the Pleistocene ice age, Bali was connected to Java and Sumatra and to the mainland of Asia and shared the Asian fauna, but the deep water of the Lombok Strait continued to keep Lombok Island and the Lesser Sunda archipelago isolated.
Being just 8 degrees south of the equator, Bali has a fairly even climate year round. Average year-round temperature stands at around 30°C with a humidity level of about 85%.
Day time temperatures at low elevations vary between 20–33°C (68–91°F), although it can be much cooler than that in the mountains. The west monsoon is in place from approximately October to April, and this can bring significant rain, particularly from December to March. Outside of the monsoon period, humidity is relatively low and any rain is unlikely in lowland areas.
Bali’s average year-round temperature is around 30 °C (86 °F) with a humidity level of about 85%. as well as during the Easter and Christmas holidays, when the weather is very unpredictable.
Bali lies just to the west of the Wallace Line, and thus has a fauna that is Asian in character, with very little Australasian influence, and has more in common with Java than with Lombok. An exception is the yellow-crested cockatoo, a member of a primarily Australasian family. There are around 280 species of birds, including the critically endangered Bali myna, which is endemic. Others include barn swallow, black-naped oriole, black racket-tailed treepie, crested serpent-eagle, crested treeswift, dollarbird, Java sparrow, lesser adjutant, long-tailed shrike, milky stork, Pacific swallow, red-rumped swallow, sacred kingfisher, sea eagle, woodswallow, savanna nightjar, stork-billed kingfisher, yellow-vented bulbul and great egret.
Until the early 20th century, Bali was home to several large mammals: the wild banteng, leopard and the endemic Bali tiger. The banteng still occurs in its domestic form, whereas leopards are found only in neighbouring Java, and the Bali tiger is extinct. The last definite record of a tiger on Bali dates from 1937, when one was shot, though the subspecies may have survived until the 1940s or 1950s.
Squirrels are quite commonly encountered, less often is the Asian palm civet, which is also kept in coffee farms to produce Kopi Luwak. Bats are well represented, perhaps the most famous place to encounter them remaining is the Goa Lawah (Temple of the Bats) where they are worshipped by the locals and also constitute a tourist attraction. They also occur in other cave temples, for instance at Gangga Beach. Two species of monkey occur. The crab-eating macaque, known locally as "kera", is quite common around human settlements and temples, where it becomes accustomed to being fed by humans, particularly in any of the three "monkey forest" temples, such as the popular one in the Ubud area. They are also quite often kept as pets by locals. The second monkey, endemic to Java and some surrounding islands such as Bali, is far rarer and more elusive and is the Javan langur, locally known as "lutung". They occur in few places apart from the Bali Barat National Park. They are born an orange colour, though by their first year they would have already changed to a more blackish colouration. In Java however, there is more of a tendency for this species to retain its juvenile orange colour into adulthood, and so you can see a mixture of black and orange monkeys together as a family. Other rarer mammals include the leopard cat, Sunda pangolin and black giant squirrel.
Snakes include the king cobra and reticulated python. The water monitor can grow to at least 1.5 m (4.9 ft) in length and 50 kg (110 lb) and can move quickly.
The rich coral reefs around the coast, particularly around popular diving spots such as Tulamben, Amed, Menjangan or neighbouring Nusa Penida, host a wide range of marine life, for instance hawksbill turtle, giant sunfish, giant manta ray, giant moray eel, bumphead parrotfish, hammerhead shark, reef shark, barracuda, and sea snakes. Dolphins are commonly encountered on the north coast near Singaraja and Lovina.
A team of scientists conducted a survey from 29 April 2011 to 11 May 2011 at 33 sea sites around Bali. They discovered 952 species of reef fish of which 8 were new discoveries at Pemuteran, Gilimanuk, Nusa Dua, Tulamben and Candidasa, and 393 coral species, including two new ones at Padangbai and between Padangbai and Amed. The average coverage level of healthy coral was 36% (better than in Raja Ampat and Halmahera by 29% or in Fakfak and Kaimana by 25%) with the highest coverage found in Gili Selang and Gili Mimpang in Candidasa, Karangasem regency.
Among the larger trees the most common are: banyan trees, jackfruit, coconuts, bamboo species, acacia trees and also endless rows of coconuts and banana species. Numerous flowers can be seen: hibiscus, frangipani, bougainvillea, poinsettia, oleander, jasmine, water lily, lotus, roses, begonias, orchids and hydrangeas exist. On higher grounds that receive more moisture, for instance around Kintamani, certain species of fern trees, mushrooms and even pine trees thrive well. Rice comes in many varieties. Other plants with agricultural value include: salak, mangosteen, corn, kintamani orange, coffee and water spinach.
Some of the worst erosion has occurred in Lebih Beach, where up to seven metres (23 feet) of land is lost every year. Decades ago, this beach was used for holy pilgrimages with more than 10,000 people, but they have now moved to Masceti Beach.
From ranked third in previous review, in 2010 Bali got score 99.65 of Indonesia's environmental quality index and the highest of all the 33 provinces. The score measured three water quality parameters: the level of total suspended solids (TSS), dissolved oxygen (DO) and chemical oxygen demand (COD).
Because of over-exploitation by the tourist industry which covers a massive land area, 200 out of 400 rivers on the island have dried up and based on research, the southern part of Bali would face a water shortage up to 2,500 litres of clean water per second by 2015. To ease the shortage, the central government plans to build a water catchment and processing facility at Petanu River in Gianyar. The 300 litres capacity of water per second will be channelled to Denpasar, Badung and Gianyar in 2013.