Understanding Clouds (2017)

Understanding Clouds (2017)

Clouds play a pivotal role in weather forecasts and warnings. They help to drive the water cycle and the entire climate system. Throughout history, they have inspired artists, poets, musicians, photographers and countless other enthusiasts.

Around the World

Every year, WMO Members around the world organize local and country-wide celebrations of World Meteorological Day. This year, the theme of “Understanding Clouds”, together with the launch of the new edition of the International Cloud Atlas (the first update in 30 years), sparked the imagination of the general public and attracted a new generation of amateur cloud enthusiasts. A small sampling of some of the national celebrations can be found below.

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. 

Classifying clouds

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. 

Clouds, circulation and climate sensitivity

As one of the main modulators of heating in the atmosphere, clouds control many other aspects of the climate system. Limited understanding of clouds is the major source of uncertainty in climate sensitivity, but it also contributes substantially to persistent biases in modelled circulation systems.

"Clouds, Circulation and Climate Sensitivity" is one of seven Grand Challenges of the WMO World Climate Research Programme. Five main initiatives make up this Grand Challenge:

Message from the Secretary-General

Today scientists understand that clouds play a vital role in regulating the Earth’s energy balance, climate and weather. They help to drive the water cycle and the entire climate system. Understanding clouds is essential for forecasting weather conditions, modelling the impacts of future climate change and predicting the availability of water resources.


Learn how to identify cloud types by using this flow chart from the International Cloud Atlas. Clouds are divided into 10 fundamental types known as genera, depending on their general form. The genera are then further subdivided based on a cloud’s particular shape, structure and transparency; the arrangement of its elements; the presence of any accessory or dependent clouds; and how it was formed.

WMO 2017 Calendar


WMO 2017 Calendar




Other resources

Related Links

Clouds - Met Office (Exeter, United Kingdom)

Ten Basic Cloud Types - National Oceanic and Atmospheric Administration (NOAA) National Weather Service (United States of America)

Cloud Spotter - National Oceanic and Atmospheric Administration (NOAA) National Weather Service  (United States of America)

Clasificación básica de nubes - Meteorología de la República Argentina (Argentina)


Name that Cloud Quiz - Met Office UK


Clouds in the Arts

Cloud Poems by Goethe und die Wolken (Dutch)

Martha Chaudry Photography