THERE ARE ALWAYS CLOUDS IN THE SKY, YOU JUST CAN’T ALWAYS SEE THEM.
A dry sky is dark blue. The blue color is due to Rayleigh scattering by molecules and very small particles, of which there are of course very many. Rayleigh scattering of blue light is greater than for other colors of light, particularly red, since the shorter the wavelength, the more that color light is scattered. In fact it is an inverse 4th power relationship. This means that if the wavelength is 1/2 as great, that light will scatter 2^4 = 16 times as much.
That’s why the Sun looks yellow. The blue light has been scattered out of the direct path to your eye. It scatters out in every direction and eventually scatters back to your eye from every direction, thus we have a blue sky and a yellow Sun. When recombined on objects around us, our eye interprets the light as “white”.
At sunrise and sunset when the Sun is low on the horizon, if the sky is relatively clear, the blue, green, and yellow light have been mostly scattered out by the long path through the atmosphere, giving us just the beautiful red (and infrared) end of the light spectrum. If there is no Rayleigh scattering, the sky is black, like on the Moon. In the 60’s my father visited Egypt. He noted that the air was so dry, “The sky looked almost black.” For a photographer, bright morning and evening light has that special warm glow that is very pleasing to the eye.
As the humidity goes up, the particles grow larger by condensation and crystallization. We can observe this as the color of the sky becomes a WHITER shade, thus lighter blue. That “white color” is the result of a more balanced scattering of light due to Mie scattering. Unlike Rayleigh scattering, Mie scattering does NOT depend upon the wavelength of light. It scatters light equally, regardless of wavelength. As the diameter of a particle grows larger, it scatters light more effectively, sort of like a curved mirror. As the diameter increases, the “mirror” gets “flatter” and reflects light more effectively.
When water droplets are greater than the wavelength of light, they become very noticeable due to that Mie scattering. At about 2 microns we see them as FOG (a micron is one millionth of a meter, a meter is about 39″ long). In a fog, you are in the middle of a cloud where the droplets are large enough to see as a mass.
In a maritime climate, like the British Isles, the fog droplets can get quite large to the point where you can see individual droplets, when the Sun is down and they are illuminated by a directed light, like a headlight. At that point they can be about 20 microns in diameter and noticeably falling slowly and swirling around revealing the air currents.
As droplets get larger, as in storm clouds, the greater diameter of the droplets, hundreds of microns in diameter, eventually means that less of the light gets scattered down to our eye. The light penetrates deeper into the cloud, thus the clouds look darker and we can expect rain. It is that diminished illuminance and the sudden fall of radiant warmth on our skin when the Sun is blocked by dark clouds that our unconscious mind recognizes and warns us that, “It is about to rain.”
The reason clouds remain aloft is that they are supported by rising air currents from warmer, moist air. The smaller the droplets, the higher their surface area is relative to their weight, thus the easier it is for them to float aloft.
Gliders use those rising currents of air to climb and stay aloft. If you are near an airport with glider activity, you will see gliders towed up to a few thousand feet by a powered airplane, then released. The glider pilot heads toward the nearest, small cumulous cloud as he knows there will be a tilted column of air rising up to the cloud. He then circles under the small cloud and goes up with the rising current of air. That rising column of air is called a “thermal” as it is produced by heat from the Sun. Thermals are strongest on relatively clear mornings. By afternoon they are usually not strong enough to keep a glider aloft.
On a clear day, sunlight penetrates the sky, warming us radiantly. At the same time, we radiate heat back into space at a much lower temperature than the Sun. Radiance is proportional to the 4th power of the absolute temperature. Absolute zero (total absence of heat energy) is about -459.67 degrees Fahrenheit (-273.15 C), so at about 50 F we are about 500 degrees F above absolute zero. On the other hand, the Sun is about 10,000 degrees F above absolute zero. This means that due to its high temperature, the radiance of the Sun is roughly 20^4 = 160,000 times as great as the surface of the Earth. Radiance is also proportional to surface area. This is why we should be glad that the Sun is a very small object in the sky. Meanwhile we radiate in all directions into space over a large area, so we do not heat up to the same temperature as the Sun.
As the Sun sets on a clear night, we continue to radiate, convect, and the surface cools. Those tiny particles condense and grow until they scatter light back down to the Earth more effectively and also glow in the infrared, warming us. Eventually we reach equilibrium and stop cooling off. How warm we stay, depends mostly upon how many particles there are and their diameter, thus the humidity. This is why the deserts are hot during the day and cold at night. It is usually dry in the desert, so the temperatures vary more from day to night.
On a cloudy day, clouds scatter sunlight back up into space. That along with snow cover and water in the oceans keeps us cooler. At the same time, clouds reflect back down the infrared that we radiate outwards. That’s why it doesn’t get as hot or cold in cloudy weather. Clouds also glow at night in the infrared keeping us warm. Venus is closer to the Sun than we are and has very thick cloud cover, so it is no wonder that it is constantly very hot at the surface. It is not due to a “runaway greenhouse effect” as so often dimly proclaimed.
Note all of the above is about water in all its forms. Water is what makes our climate so complex. It is essential to life. We have vast oceans of it that store immense amounts of heat and stabilize temperatures. We have clouds and humidity that profoundly stabilize the Earth’s temperature.
The climate of Mars is very easy to understand and the weather is very predictable. You can predict the wind velocity and direction a year ahead. There are no clouds or weather patterns. No vast oceans of water. There is no water to speak of. Man needs to explore and get to Mars, but it would be a terrible place to live.
We are currently in a warm interval of the Quaternary Ice Age. We have been flipping back and forth between glacial and interglacial periods for 2.5 million years. During glaciation, more of the water on Earth is locked up in glaciers than currently. 13,500 years ago, during the Last Glacial Maximum, the oceans were 400′ lower. The continental shelf was above water. You could walk from Florida to the Bahamas.
This made our atmosphere much drier. The glaciers reflected more of the heat of the Sun back into space. When glaciation takes hold, the Earth dwells in glacial periods about 5 times as long as the interglacial periods. The Ice Age was a Hell on Earth – COLD, DRY, and DUSTY. Our ancestors had to be extremely tough and resourceful to survive.
Note that CO2 doesn’t enter into the above. Water absorbs infrared at the same wavelengths as CO2 and is at vastly higher concentration. In 1909 the physicist, Robert W. Wood, performed the definitive Greenhouse Effect experiment showing that “radiation trapping” was simply NOT significant in a greenhouse. He knew this already, as he was an accomplished physicist working in the area of radiation, but he found it necessary to perform the experiment to demonstrate it to those who do not know how to do the simple calculation. He knew that a greenhouse “gets hot” since the glass prevents hot air from rising – Precisely as any sensible person would expect if not misled in grade school. Arrhenius didn’t understand this. That’s why he came up with his unfounded theory of Anthropogenic Global Warming.
Contrary to the clamoring Global Warming Idiocracy, CO2 at 400 parts per million is just not important. We do not need to let them shake us down with taxes to solve a problem that does not exist.
Lord Rayleigh – 1842-1919 was a British physicist.
Gustav Mie – 1868-1957 was a German physicist.
Robert W Wood – 1868-1955 was an American physicist.
Fergus S. Smith