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What is a Cloud?

Clouds have a funny way of capturing our imagination.

Profile picture for user Martijn.Donkersloot
WeatherPro
29 May 2019

What is a Cloud?

Clouds have a funny way of capturing our imagination.

Profile picture for user Martijn.Donkersloot
WeatherPro
29 May 2019
4 min read

The air that we breathe contains water vapor. Its total amount can range from tiny traces to sometimes as much as four percent of the total volume of an air parcel. Clouds appear when this water vapor condenses or freezes onto other particles in the air – like dust or sea salt – called aerosols.  

The main secret behind cloud formation is a simple physical law: Warm air can hold more water vapor than cold air. As air cools, its relative humidity increases. Condensation (or freezing) starts as soon as the relative humidity reaches 100%. At that point, millions of water droplets (or ice crystals) suddenly grow, forming a cloud.

Air can heat up or cool down due to a variety of reasons:

  • The overall weather circulation forces air to lift. This happens under the influence of low-pressure systems, or when the airflow hits a mountain ridge and needs to ascend. As air rises, the temperature drops, causing the relative humidity to increase and clouds to form.

  • Low-level air can also be warmed by the sun-lit ground. The air expands as it gets warmer, becomes lighter than the surrounding air, and starts to rise. You might have experienced these kinds of thermal lifts in a plane when you got a bumpy ride out of the blue. As the rising air cools down, its relative humidity increases until it finally reaches 100%, and a fair weather cloud develops.

  • Moist, warm air blowing over a cold surface can also cause clouds to form. A typical example is a land breeze blowing over cool sea waters.

  • Air warmed during the heat of the day cools down overnight. Clouds formed in this way are better known as fog. For meteorologists, fog is nothing else than a cloud, resting on the ground.

What causes clouds to change colors?

Clouds are made up of millions and millions of tiny water droplets and/or ice crystals. All of these millions of water molecules capture light from the sun and reflect it, which allows us to see them once there is enough condensation in the air. However, water has no natural color, so the different shades of grey, white, and black come from the density of clouds in the air.

  • If there are few clouds in the sky, then they will appear bright white or light in color.

  • If there is a lot of water in the air and there are many clouds, then they will look darker, ranging from grey to black.

How many different types of clouds are there?

Meteorologists group and name clouds by a scheme that has been developed by Luke Howard, a British chemist and amateur meteorologist in the early 19th century. The German poet laureate Goethe, who tried to develop a cloud naming scheme (and failed) was so impressed by Howard’s work that he dedicated him a poem! Today, the World Organization for Meteorology maintains the cloud classification scheme and the International Cloud Atlas (see https://cloudatlas.wmo.int/home.html).

There are over 100 different types of clouds, each with varying shapes, sizes, and characteristics. But generally, these varying cloud types can be grouped into one of 10 key categories depending on their size and altitude.

About the author

Profile picture for user Martijn.Donkersloot
WeatherPro

At WeatherPro, we believe that good things happen when people step outside and great things happen to those who are prepared. We strive to empower life’s outdoor moments. #stepoutside