Science Explains#3: Why do Rain Clouds gets Darker?
When the sky gets darker, it’s a sign to get our umbrellas and rain coats ready for a heavy rain coming. But do you know why it gets darker?
Rain clouds are usually a darker shade of gray because the light doesn't scatter to the base that it appears gray as you look from the ground.
How do clouds form?
Clouds are formed when radiant energy from the sun heats the water, the water molecules gets excited and continuous absorption of energy results to evaporation.
Evaporation happens when water molecules overcomes the atmospheric pressure thus turning liquid water into vapor. While water vapor makes its way to the atmosphere, it experiences cooling and condenses when it reach the troposhere, forming clouds by clumping together with the aid of a condensation nuclei - small particles of NaCl formed from sea spray that acts to form water droplets from atmospheric moisture.
When do cloud gets darker?
As the sun continuously gives off energy, clouds gets thicker and denser as it gathers more water droplets and ice crystals.
Water droplets in the clouds naturally absorbed the light without giving it off fully, therefore making it look darker in the ground level. Since light travels as waves of different lengths giving its own unique color depending on its wavelength.
Clouds are white because their water droplets or ice crystals are large enough to scatter the light of the seven wavelengths (red, orange, yellow, green, blue, indigo, and violet), combining to produce visible white light.
However, rain clouds are gray instead of white because of their thickness, or height — and the thicker it gets, the less light it scatters, consequently, less light penetrates all the way through.
Conclusion
That being the case, the larger the water droplets get, the more it is efficient at absorbing light, rather than scattering it. Explaining why rain clouds gets darker shade of grey.
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References:
http://www.weatherwizkids.com/weather-clouds.htm
https://www.livescience.com/39069-why-are-rain-clouds-dark.html
http://www.atmospheric-phenomena-ap.com/2015/01/the-clouds-from-which-rain-falls-10.html