Why Do Rain Clouds look Dark?

Why Do Rain Clouds look Dark?

Ever wondered why do rain clouds look dark while the ‘normal’ clouds appear white?

The reason for this phenomena is very simple. The light from the sun does all the tricks.  To understand why, let’s take a look at both types of clouds. For this we will need to understand concept of ‘dew point’.

What is Dew Point?

Dew point is that temperature at which the air can no longer hold moisture (microscopic water droplets)  and the moisture present turns into gaseous state at this point.

What is a cloud? 

A cloud is a mass of visible moisture cooled below its dew point. Whenever any moisture is cooled below its dew point, it’ll condense on itself and become visible to the eye as fog, frost, vapour or cloud. This explains why we often see cloudy jet of air entering in an aircraft cabin on a hot humid day. The air outside is hot and humid. It carries moisture above its dew point. But the interior of the aircraft is cooler than the dew point and the entering air is immediately turned into ‘cloud stream’.

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Flying out from coastal airports like Mumbai, Kolkata, Chennai etc. one can see this happening often. The mist from the Air conditioner vents is what happens when humid air outside enters the colder cabin and condenses. This phenomenon happens at a large-scale in the atmosphere.

 

This happens at a large-scale in our atmosphere, where the cooling is provided by altitude. Higher we go, the colder it gets. If the humidity in the air at a certain level falls below its dew point as the air ascends, it forms cloud much like in the aircraft cabin.

 

What explains White Clouds?

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Ice crystals reflect the sun light to give a white appearance.

 

White clouds have moisture but lesser vapour content because they primarily consist of more ice-crystals and lesser water droplets. The light from the Sun is white and the ice-crystals within the cloud reflect this white light to the ground giving the cloud it’s ‘white’ appearance.

 

What are Black clouds?

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These clouds have more water content than ice, and hence offer more opacity to the incident sun-light rendering them darker.

 

When the same white cloud begins to gather more and more moisture from the surrounding air, it begins to build up mass of water droplets as the moisture condenses. When these condensed droplets begin to gain mass, the trillions of such water droplets begin to act as opaque objects to the incident sun-light. As more and more white lights gets absorbed by the water particles, the cloud begins to change color from white to darker shades until it appears the usual blackish. The thicker the cloud, the darker it gets.

 

How do Black clouds cause rain-fall?

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As the rain falls, the water content begins to go lower in the cloud and ice contents become relatively higher. The cloud begins to change back to a lighter shade or complete dissipate as per existing environment conditions.

 

If the humidity in the air begins to condense when the temperature falls below its dew-point, the ice crystals begin to melt into water. At first the updrafts of the clouds keep the water from falling off the clouds. But as more and more humid air condenses into water due to altitude induced cooling, the water droplets begin to merge with each to form heavier drops. After a while, these drops become heavy enough to overcome the updrafts of air in the clouds and fall from the air  as rain.

Good simple everyday Science!

Written by Aru Raghuvanshi

Aru Raghuvanshi