The sky looks to us like a backdrop. Something up there, shifting between blue and grey. But the moment you climb a few hundred metres above the ground, it becomes clear that the sky is not a backdrop at all but a landscape in its own right, with structure, layers and distinct boundaries. Here are some of the most beautiful phenomena you see from up there, and the simple explanation behind each one.
The circular rainbow
We all know the rainbow as a semicircle. That is not its true shape: a rainbow is a complete circle, and we simply never see the lower half because the ground blocks it. From the air, with the sun behind the aircraft and raindrops ahead of it, the circle closes. You see a complete coloured ring, and at its centre, if conditions are just right, the shadow of the aircraft itself.
The phenomenon occurs because every water droplet acts like a tiny prism: light enters, refracts, reflects off the back wall and exits at a fixed angle of about forty-two degrees to the direction of the light. All the droplets sitting at precisely that angle from our eye together make up the ring.
The glory around the shadow
There is an even rarer and more beautiful phenomenon, known as a glory. When flying above a low cloud layer with the sun behind you, small coloured rings sometimes appear exactly around the aircraft's shadow on the clouds. Unlike a rainbow, a glory is produced by backscattering of light in exceptionally tiny cloud droplets, and its size depends on the size of those droplets. Which means that without knowing anything about the cloud, you can infer something about its internal structure.
The sea-breeze line
In the late morning, the sea starts to work. Land heats faster than water, the air above it rises, and cool air from the sea flows inland to take its place. This is the sea breeze, that pleasant westerly that reaches the coast around midday.
From the air you see what happens at its leading edge: the front of cool marine air pushes eastward and lifts the warm air ahead of it. Where that happens, a row of white clouds forms, usually over the hill country. It is a remarkably straight line of cloud in the middle of an otherwise clear blue sky, and it marks the precise boundary between two air masses.
Why the air is smooth in the morning
Pilots love the early morning hours, and there is a simple physical reason. The sun has not yet had time to heat the ground, so no thermals have formed, those columns of warm air that rise like bubbles. Without thermals the air is completely still and the flight is smooth as glass. It is also why most striking aerial photographs are taken in the morning: not only because of the low light and long shadows, but because the camera is steady.
The inversion layer
On a typical summer day you can see from the air a phenomenon that looks almost artificial: a layer of pale haze spread over the region, with an entirely sharp boundary above it, and clear air beyond. This is the inversion. Normally temperature falls as you climb, but sometimes a layer forms where the opposite happens, and it acts like a lid preventing the air beneath from rising. All the dust and moisture are trapped below it. From below it looks like a grey day. From above you see exactly where the lid ends.
The winter that cleans the air
The finest days for long-distance visibility in Israel come, of all times, after a storm. A rain system washes the tiny suspended particles out of the air and brings cold, dry air from the north. The result is exceptional clarity: on such a day you can pick out the northern peaks and the snow-covered ridge from many tens of kilometres away, a view that simply does not exist during the rest of the year.
And why spring sunsets are so red
In the transitional seasons, warm air masses reach Israel from the desert, and at times they carry very fine dust from the Sahara and the Arabian Peninsula. These particles scatter blue light and let red through, so the sun appears as a large orange disc and the whole sky takes on a copper tone. From the air the effect is doubled, because you see the dust layer from the side rather than only through it. These are the most dramatic sunsets of the year.
The sky changes by the hour, and each of these phenomena is a clue to something happening on a far larger scale. Anyone who gets into the habit of looking up discovers that no two days are alike.השמיים משתנים כל שעה, וכל תופעה כזאת היא רמז למשהו שקורה בקנה מידה גדול הרבה יותר. מי שהתרגל להסתכל למעלה מגלה שאין שני ימים זהים.