||Thunderhead with anvil, Australia |
Astronauts aboard the International Space Station (ISS) took this image of a large thundercloud at sunset from a point over Papua New Guinea. The side-on view shows that the cloud was in the vicinity of the horizon more than 1500 km west of the spacecraft, suggesting that the cloud lay over northern Australia or the Gulf of Carpentaria. With enough convective energy, clouds in the lowest layer of the atmosphere, the troposphere, can rise until they bump up against the stable overlying layer, at which point they spread sideways, making a thin upper zone shaped like an anvil, such as the one seen here. Knowing the camera lens focal length and the cloud location near the horizon, this expansive anvil measures approximately 200 km.
The spectacular detail seen in side-on views, backlit by the bright atmosphere, shows no rain falling from most of the cloud. The warm colors of the troposphere, compared with the blues above, are partly a measure of dust and other particulates that reflect red and yellow wavelengths.
Astronauts often comment on the thinness of the Earth's life-supporting envelope, and how it suggests the fragility of our planetary ecosystem. They also note that the number of atmospheric layers they detect with their eyes is much greater than their photographs show.