Swirling Cloud Formation Touches the 'Terminator' Line In This Incredible Image from NASA

This stunning photograph of a swirling cloud formation taken as it approaches the "terminator line" was selected as NASA's Image of the Day on Sunday.

The phenomenon was captured by an astronaut above the International Space Station (ISS) on April 4 as it was nearing the southernmost reaches of its orbit. Here, the clouds are more than 2,000 miles to the east of New Zealand and more than 2,700 miles to the west of South America.

NASA's Image of the Day September 15, 2019
NASA's Image of the Day September 15, 2019 depicts a cloud formation approaching the terminator line. The image was taken on April 4, 2019 by a member of the Expedition 59 crew. Credit: NASA.

The terminator line is also referred to as the "day-night line," the "gray line" or the "twilight zone" and describes the visual transition from day to night. It has a fuzzy-like appearance because the Earth's atmosphere bends the sunlight by half a degree, creating an in-between zone that spans a distance equivalent to 37 miles. While it is commonly thought that as half the planet is covered in sunlight the other half is cloaked in darkness, this is not true. The bending of sunlight actually causes more of the Earth to experience daylight at any one time.

The terminator line adds an extra dramatic dimension to photos because the sunlight can create shadows that amp up the contrast of the different cloud heights, making the circular motion appear more distinct to the eye, writes NASA. And those unusual pinks and peaches you can make out on the cloud's outermost swirls are the result of a local sunrise.

Reddish shades such as these emerge when the Sun is at a lower atmosphere in comparison to the atmosphere and the ISS. When this happens, light waves have to pass through a thicker layer of atmosphere, which boosts the red end of the color spectrum. Hence, those pinks and peaches.

The blurring of the clouds at the top of the photo is caused by the positioning of the astronaut, who was facing away from the day hemisphere and towards the night hemisphere at the time the image was taken.

According to NASA, the astronaut sent a message to the ISS asking if the cloud formation had been declared a tropical cyclone.

Tropical storms form above warm oceans close to the equator, when warm, moist air swells upwards causing an area of low pressure near the surface of the water. In this case, however, the storm did not make landfall and was therefore left unnamed.

To see more spectacular images of the blue planet from space - whether they be flood swamps in Sudan to Hurricane Dorian's movements in the Carolinas - check out NASA Earth Observatory's Image of the Day.

Swirling Cloud Formation Touches the 'Terminator' Line In This Incredible Image from NASA | Tech & Science