What are clouds?
Clouds are the visible aggregate of minute particles of water and/or ice.
They form when water vapor condenses.
They can grow very tall or appear flat as a pancake.
They are typically white in color but also appear in different shades of grey or in brilliant yellow, orange or red.
They can weigh tens of millions of tons yet float in the atmosphere.
How clouds are formed?
There are two ingredients needed for clouds to become visible- water and nuclei.
Nuclei
In one form or another water is always present in the atmosphere. However, water molecules in the atmosphere are too small to bond together for the formation of cloud droplets. They need a “flatter” surface, an object with a radius of at least one micrometer on which they can form a bond. Those objects are called nuclei.
Nuclei are minute solid and liquid particles found in abundance. They consist of such things as smoke particles from fires or volcanoes, ocean spray or tiny specks of wind-blown soil.
These nuclei are hygroscopic meaning they attract water molecules.
Called “cloud condensation nuclei”, these water molecule attracting particles are about 1/100th the size of a cloud droplet upon which water condenses.
Therefore, every cloud droplet has a speck of dirt, dust or salt crystal at its core. But, even with a condensation nuclei, the cloud droplet is essentially made up of pure water.
Temperature’s role
But having water attracting nuclei is not enough for a cloud to form as the air temperature needs to be below the saturation point. Called the dew point temperature, the point of saturation is where evaporation equals condensation.
Therefore, a cloud results when a block of air (called a parcel) containing water vapor has cooled below the point of saturation.
Air can reach the point of saturation in a number of ways. The most common way is through lifting of air from the surface up into the atmosphere.
Why clouds appear and disappear?
The atmosphere is in constant motion.
As air rises drier air is added (entrained) into the rising parcel so both condensation and evaporation are continually occurring. So cloud droplets are constantly forming and dissipating.
Clouds form and grow when there is more condensation on nuclei than evaporation from nuclei.
Conversely, they dissipate if there is more evaporation than condensation.
Thus clouds appear and disappear as well as constantly change shape.
If the sky is blue, why are clouds white?
Unlike Rayleigh scattering, where the light waves are much smaller than the gases in the atmosphere, the individual water droplets that make up a cloud are of similar size to the wavelength of sunlight.
When the droplets and light waves are of similar size, then a different scattering, called ‘Mie’ scattering, occurs.
Mie scattering does not differentiate individual wave length colours and therefore scatters all wave length colors the same.
The result is equally scattered ‘white’ light from the sun and therefore we see white clouds.
Yet, clouds do not always appear white because haze and dust in the atmosphere can cause them to appear yellow, orange or red.
And as clouds thicken, sunlight passing through the cloud will diminish or be blocked, giving the cloud a grey colour.
If there is no direct sunlight striking the cloud, it may reflect the colour of the sky and appear bluish.
PRACTICE QUESTIONS
QUES . Rain-bearing clouds look black because CDS 2012
(a) all light is scattered by them
(b) the large number of water droplets in them absorb all the sunlight
(c) they reflect the sunlight back into the atmosphere
(d) there is a lot of dust condensed on the water vapour in such clouds
(a)
QUES . As the sunlight passes through the atmosphere, the rays are scattered by tiny particles of dust, pollen, soot and other minute particulate matters present there. However, when we look up, the sky appears blue during mid-day, because CDS 2010
(a) blue light is scattered most
(b) blue light is absorbed most
(c) blue light is reflected most
(d) ultra violet and yellow component of sunlight combine
(a)
