Our precious atmosphere
Where weather happens
On the occasion of World Meteorological Day, we shine a light on an often-unobserved part of the atmosphere.
In these images from 25 January 2025, a deep convective system with an anvil cloud (the highest central cloud) can be seen above Costa Rica. In the left image, the wide, flat anvil cloud can be seen 10-12km high in colours as seen by the naked eye. On the right, the colours have been adjusted to highlight this cloud. An anvil cloud forms when a highly unstable air mass close to the surface rises and pushes a mass of wet, warm air, upward. The moisture in the displaced air condenses along its way through the increasingly colder air, causing the cloud to grow vertically until it reaches the top of the troposphere. There, the stable temperature stops the upward draft, causing the cloud to spread outward rather than upward. The top of this cloud, therefore, marks the top of the troposphere – this is just about the maximum altitude at which weather events form.
In the image on the right, the horizontal grey band depicts the ozone layer, which can be seen in relation to the anvil cloud in the image on the left. Located in the lower stratosphere 20-25km above the surface, the ozone layer reflects between 97- 99% of the sun’s damaging ultraviolet B radiation. These rays harm humans, other animals, and plants. This is why the appearance of a hole in the ozone layer over Antarctica in the early 1980s was such a wake-up call for policy makers, spurring action to restrict the use of ozone-depleting chlorofluorocarbons globally. Thankfully, because of the decreased use of these damaging chemicals, the ozone hole has stopped growing and may even have begun recovering. Continued monitoring of the ozone layer is essential for identifying and stopping any further anthropogenic depletion.
In this image from the autumnal equinox on 22 September 2024, the sun has just set behind the Earth. The beautiful sunset colours, topped with blue sky, make visible the varying density of the troposphere. Because it is densest lower down, there are more air molecules for sunlight to hit as it travels. In addition, the low angle of the sun at sunset causes light to travel further than it does when directly overhead, also contributing to it hitting more air molecules along its path. This results in the scattering of the shortest wavelength of light – blue – lower in the troposphere, leaving the longer wavelength light – red, orange, and yellow – to pass through. Higher up, the troposphere appears blue because more blue light passes through. Please note, too, the thinness of the troposphere, so essential for our existence.
Author:
Sarah Puschmann
