Earth – a marvellous planet … utterly engaging
1. Cumulonimbus cloud over Africa
Perhaps the most impressive of cloud formations, cumulonimbus (from the Latin for “pile” and “rain cloud”) clouds form due to vigorous convection (rising and overturning) of warm, moist and unstable air. Surface air is warmed by the sun-heated ground surface and rises; if sufficient atmospheric moisture is present, water droplets will condense as the air mass encounters cooler air at higher altitudes. The air mass itself also expands and cools as it rises due to decreasing atmospheric pressure, a process known as adiabatic cooling. This type of convection is common in tropical latitudes year-round and during the summer season at higher latitudes.
As water in the rising air mass condenses and changes from a gas to a liquid state, it releases energy to its surroundings, further heating the surrounding air and leading to more convection and rising of the cloud mass to higher altitudes. This leads to the characteristic vertical “towers” associated with cumulonimbus clouds, an excellent example of which is visible in this astronaut photograph. If enough moisture is present to condense and heat the cloud mass through several convective cycles, a tower can rise to altitudes of approximately 10 kilometers at high latitudes and to 20 kilometers in the tropics before encountering a region of the atmosphere known as the tropopause—the boundary between the troposphere and the stratosphere.
The tropopause is characterized by a strong temperature inversion. Beyond the tropopause, the air no longer gets colder as altitude increases. The tropopause halts further upward motion of the cloud mass. The cloud tops flatten and spread into an anvil shape, as illustrated by this astronaut photograph. The photo was taken from a viewpoint that was at an angle from the vertical, rather than looking straight down towards the Earth’s surface. The image, taken while the International Space Station was located over western Africa near the Senegal-Mali border, shows a fully formed anvil cloud with numerous smaller cumulonimbus towers rising near it. The high energy levels of these storm systems typically make them hazardous due to associated heavy precipitation, lightning, high wind speeds and possible tornadoes.
2. Lake Carnegie
Ephemeral Lake Carnegie, in Western Australia, fills with water only during periods of significant rainfall. In dry years, it is reduced to a muddy marsh. This image was acquired by the Landsat 7 satellite on May 19, 1999. This is a false-color composite image made using shortwave infrared, infrared and red wavelengths. The image has also been sharpened using the sensor’s panchromatic band.
3. Icefall
This image shows the world’s largest glacier, Lambert Glacier in Antarctica, and icefall that flows into it. Ice flows like water, albeit much more slowly. Cracks can be seen in this icefall as it bends and twists on its slow descent 1300 feet (400 meters) to the glacier below. The image was taken by the Landsat-7 satellite on December 2, 2000 and is a false-color composite made from infrared, red and green wavelengths. The image has also been sharpened using the sensor’s panchromatic band.
