This Universe

[Godoo]

During foggy evenings of winter or cloudy nights of summer, North Western horizon of the sky appears orange in colour (9 to 11 pm). Why?

Even after some darkness has settled on the ground upper reaches of the atmosphere still get some light from the setting sun. This light is filtered through a long path in the atmosphere because of its near tangential passage. It is known that scattering in air is discriminatory between colour constituents of white light. Blue scatters almost 16 times more than the Red end of the spectrum.

Therefore, the tangentially travelling sunrays lose most of their blue and neighbouring colours by the time they hit the fog or the clouds at great heights in the Western atmosphere. The light illuminating them is mostly orange which the water droplets scatter back to you.

Incidentally this is also the reason that makes the setting sun looking orange.


Kindly tell me why clouds don’t fall like a rock to the ground?


Clouds are up there because they are made of water vapour that is near dew point — the vapour has condensed into tiny droplets of water whose motion is controlled more by movements of air in which they are suspended than by downward force of gravity. If the cloud transforms into a thick fog and there are no air currents then it would descend to the ground slowly with a terminal velocity so low that it might take hours to come to the ground.

This is often seen in hill valleys. Tiny particles cannot descend to the ground like a rock because as you get small the ratio of the surface area to mass becomes large. Raindrops are heavier. They are formed by coalescing of fog particles and acquire a diameter as large as a millimeter. Even they cannot fall like stones or bullets because increasing air resistance reduces their terminal velocity.


When a plate of food is heated its energy content and therefore its calories must go up. So does eating hot food make us fatter than eating cold food?


It is good that you recognise that there is some common way of measuring energy content. You would see the fallacy of your hypothesis when you are told to get all the requirement of your food through drinking hot water. You do not get fat through input of energy, unless that energy is a result of metabolism that occurs in the body. Input of heat through hot food or sitting in the sun won’t do. If you have a weight problem do not worry about eating hot food or drinking hot water.

Actually hot water might help you to better metabolise the food you eat and reduce accumulation of fat.