Earth from space: Beautiful 'lake of haze' in Himalayan valley has a darker, more sinister source

A giant cloud of haze frequently fills this ancient valley at the foot of the Himalayas, which was once an enormous freshwater lake. Although the stunning spectacle results from a natural phenomenon, this particular example was caused by human activity.

The Kashmir Valley is a large, bowl-like ravine that's around 84 miles (135 kilometers) long and 20 miles (32 km) wide. It is surrounded by mountains on all sides, including parts of the Himalayas to the north, south and west. The floor of the valley is around 6,000 feet (1,800 meters) above sea level, on average, but some of the surrounding peaks tower up to 4,000 feet (1,200 m) above the valley's base.

Until around 4.5 million years ago, the Kashmir Valley was a massive freshwater lake, according to NASA's Earth Observatory. Today, the only evidence of the lake is the bowl-like shape of the valley and the sandy, clay-like sediment across the valley's floor.

But under certain conditions, giant clouds of haze can form there, filling the space once occupied by water. This haze, which is normally made from mist and fog, forms when warm air passes over cool ground, known as a temperature inversion, which traps water vapor between the two. Once it has formed, the haze is contained within the valley by the tall surrounding peaks, sometimes for days on end.

Temperature inversions frequently occur in the winter, when the ground is cooler. They are more likely to happen when snow is on the ground, like the powder visible around the rim of the valley in this photo.

Related: See all the best images of Earth from space

However, in this case, only a small percentage of the haze was mist or fog. Instead, most of the cloud consisted of pollution from nearby infrastructure. Multiple satellites detected high levels of air pollution on the day this photo was taken, according to the Earth Observatory. Most of this pollution likely came from charcoal production factories and biomass-burning power plants, which provide the surrounding area with electricity.