Amazing Nature

The Revelation of Clouds

Originally published in Green Country, Issue 115 (Aug 2015)
Author: Green Power
The top of cumulus is in a dome shape, while the bottom is flat
The common type of cloud seen in Hong Kong's summer is cumulus. The top is in a dome shape, while the bottom is flat. It is usually accompanied by showers. If it develops into cumulonimbus, there will be rainstorms.

Look up at the sky, and you could see clouds of all shapes. Some are like candy floss, some resemble sheep, others look like cream puffs. In primary school General Studies lessons, we learned that clouds are formed by the condensation of water vapour, which in turn fall as rain. This is the water cycle. Clouds also act as the harbingers of weather to come. From the shapes of clouds, we can predict weather changes. In fact, we can tell the air quality through these clouds.

Clouds are comprised of suspended water droplets and/or ice crystals in the sky, and are formed by the condensation of water vapour. In order for water vapour to condense and form clouds, air must be cooled so that it is saturated to let water vapour convert to liquid state. The process also requires the tiny “aerosol” particles. When you hear the term “aerosol”, the first thing that may come to your mind is a type of artificial chemical, or finely dispersed pesticide or hair spray in compressed gas sprays. In fact, aerosols are suspended solid or liquid particles in the air. They may occur naturally, such as from volcanic debris, or from soil dust in the air, like pollen, salt particles and bacteria. Aerosols can also be produced artificially, for example in compressed gas sprays, or from burning fossil fuel. Aerosols can prompt water vapour to condense (and are then known as condensation nuclei), so that water droplets become suspended in the sky, forming clouds.

The water droplets in a cloud scatter sunlight, making it white
The water droplets in a cloud scatter sunlight, making it white.

The more polluted, the whiter

Clouds are formed by water. Water is transparent, but why are clouds white? This is because water droplets scatter sunlight, making the clouds look white. The more and tinier the water droplets, the bigger the total surface area, so more sunlight is scattered, and we see whiter and shinier clouds. Air pollution is one of the major sources of aerosols. To calculate the Air Quality Health Index, the Environmental Protection Department measures PM10 and PM2.5, which are also aerosol pollutants. Therefore, clouds look whiter and shinier rather than dirtier in places when they contain more air pollutants.

The water droplets in a cloud scatter sunlight
The lower the concentration of aerosols, the bigger the water droplets, and the lower the ability to scatter sunlight, hence it is more likely to rain; and vice versa. (The above one is a cloud formed by low concentration of aerosols, the one below is formed by high concentration of aerosols.)
The water droplets in a cloud scatter sunlight

As early as in 1974, research from the University of Arizona showed that clouds with air pollutants can scatter more sunlight back to space, which can somewhat ease global warming. However, more meteorologists point out that as such clouds collect more aerosols, water in the air is dispersed into more and finer water droplets. This hinders the coalescence of small water droplets and hence the formation of bigger droplets into clouds. Therefore, air pollution affects the amount of rainfall and in turn the entire water cycle and water supply.

Are there any other mysteries behind these clouds? What can they reveal to humans in future? The answers are for scientists to discover!