World Meteorological Day 2017 - Understanding Clouds
Introducing the new on-line edition of the International Cloud Atlas
Understanding Clouds is the theme of World Meteorological Day 2017 to highlight the enormous importance of clouds for weather climate and water. Clouds are central to weather observations and forecasts. Clouds are one of the key uncertainties in the study of climate change: we need to better understand how clouds affect the climate and how a changing climate will affect clouds. Clouds play a critical role in the water cycle and shaping the global distribution of water resources
On the lighter side, World Meteorological Day will provide an opportunity to celebrate the inherent beauty and aesthetic appeal of clouds, which has inspired artists, poets, musicians, photographers and countless other enthusiasts throughout history.
World Meteorological Day marks the launch of a new edition of the International Cloud Atlas after the most thorough and far-reaching revision in its long and distinguished history. The new WMO Atlas is a treasure trove of hundreds of images of clouds, including a few newly classified cloud types. It also features other meteorological phenomena such as rainbows, halos, snow devils and hailstones. For the first time ever, the Atlas has been produced in a digital format and is accessible via both computers and mobile devices.
The International Cloud Atlas is the single authoritative and most comprehensive reference for identifying clouds. It is an essential training tool for professionals in the meteorological community and those working in aviation and shipping. Its reputation is legendary among cloud enthusiasts.
The International Cloud Atlas has its roots in the late 19th century. It was revised on several occasions in the 20th century, most recently in 1987, as a hard copy book, before the advent of the Internet.
Advances in science, technology and photography prompted WMO to undertake the ambitious and exhaustive task of revising and updating the Atlas with images contributed by meteorologists, cloud watchers and photographers from around the world.
The present international system of Latin-based cloud classification dates back to 1803, when amateur meteorologist Luc Howard wrote The Essay on the Modification of Clouds.
The International Cloud Atlas currently recognizes ten basic cloud “genera,” which are defined according to where in the sky they form and their approximate appearance.
High-level clouds typically have a base above about 5 000 metres (16 500 feet); middle-level clouds have a base that is usually between 2 000 and 7 000 m (6 500 to 23 000 feet); and low-level clouds usually have their base at a maximum of 2 000 m (6 500 feet).
Most cloud names contain Latin prefixes and suffixes which, when combined, give an indication of the cloud’s character. These include:
· Stratus/strato - flat/layered and smooth
· Cumulus/cumulo - heaped up/puffy
· Cirrus/cirro - feathers, wispy
· Nimbus/nimbo - rain-bearing
· Alto – mid-level (though Latin for high)
The 10 genera are subdivided into “species,” which describe shape and internal structure, and “varieties,” which describe the transparency and arrangement of the clouds. In total there are about 100 combinations.
The International Cloud Atlas includes a new species Volutus (from the Latin volutus which means rolled) for roll clouds.
It also proposes some new “special clouds,” such as Homogenitus (from the Latin homo meaning man and genitus meaning generated or made). Its common names include Contrails (from aircraft).
A special mention is made of Asperitas (from the Latin meaning wave-like and roughness) – a dramatic formation that looks like an upturned roughened sea surface – which has captured the public imagination in recent years. This cloud is included in the atlas as a “supplementary feature.”
The new International Cloud Atlas is a tribute to the generosity of the Hong Kong Observatory and the dedication and enthusiasm of a special WMO Task Team, which spent nearly three years revising the text and collecting and classifying images and data. It increases and enriches our understanding of clouds and will serve as an invaluable resource for many years to come.
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