Flying through the monsoon

We flew in bright sunshine at the usual cruising altitude of around 10 kilometers above sea level. Below us the unbroken sea of clouds was interrupted by infrequent towers of cumulonimbus anvils. The towers form as clouds condense into rain. The heat released as water changes from steam to liquid creates a local convection which churns up clouds into these anvils. In mid-August more of India should have had rains, so the scene outside my window told me that rainfall is spotty this year.

Then, as we approached Mumbai, the plane began to lose height. At an altitude of around 6 kilometers above sea level we began a dive into the monsoon clouds. I’ve not paid much attention to it before, but this time around I decided to take a time-lapse video, the one you see above. This condenses a little less than half an hour’s descent into two and a half minutes. I was astounded by the changes in the nature of the clouds as we ploughed through the turbulent layer. Not having paid attention, I’d thought of it always as a mass of grey churn. But now I saw that there is much more structure to it. Interesting.

Through clouds

The monsoon set in as a yearly phenomenon when the Tibetan plateau was lifted up by India crashing into the Asian continental plate. This was about 50 million years ago, when the earth was a hothouse, and the first ice sheets of the Antarctic were still 15 million years in the future. So, when it comes to descriptions of the monsoon, almost anything that can be said about it has been said already. Every so often I’m surprised by the aptness with which millennium old Sanskrit poems describe the monsoon. The one experience that is new, that perhaps the generations living now are seeing for the first time, is of flying through the weather.

Coming back from work recently, I spent an hour in the middle of rainclouds driven by monsoon winds. There is a constant turbulence, little sinking feelings in your stomach that you learn to ignore. Outside the window is a wonderful show of clouds and light. The poets of these sights are probably beginning their careers now.

Dune

I had seen totally unexpected landscape in the Thar desert: dry riverbeds with piles of broken rock, and vast stretches of level ground. Although large parts of the desert landscape was of this kind, there was a significant area full of sand dunes. I saw many dunes which were stabilized by plants specialized to grow in the desert, but there was a stretch of the great shifting dunes that deserts are famous for.

Dunes are formed by wind-blown sand. Sure enough, the air was dusty enough here that it felt comfortable if I pulled my tube scarf up to my nose to form a mask. Three kinds of sand dunes are commonly seen: barchans have horns facing away from the wind, parabolic dunes have horns facing the wind, and transverse dunes are perpendicular to the wind. What was I looking at? The great dune in the featured photo was clearly a transverse dune. I could sometimes see sand coming over its slip face. We were camped leewards of it. The ripples behind it were parallel to the edge. Eddies and gusts had formed smaller dunes, still pretty large, at its base. The photo above shows one of these. This was probably a blowout, or a parabolic dune. The horns at its end were not very long. In the picture above you can see that at the foot of the slip face the wind has tried to form yet another (tertiary) blowout dune. I guess this kind of fractal structure of dunes must be fairly common.

I woke up one morning to try to take photos of the dunes before sunrise, and found clouds blowing in. This was the first time I felt a strong wind. From the direction of the clouds it seemed that my guess was wrong: the clouds were blowing parallel to the dunes. The kind of clouds that you see in the photo are a high layer of cumulus clouds (altocumulus stratiformis). They form when ground-level winds carry moisture up where they freeze and then are carried in a different direction by high-altitude winds. So the direction of the movement of the clouds had nothing to do with the movement of the ground wind. In fact, because the ground wind had to be perpendicular to the movement of the clouds, my guess about the dunes had become more likely to be correct!

I had the warm fuzzy feeling which comes of the conviction of being right.

Across the Bay of Bengal

Ten days after cyclone Vardah travelled from the Andaman Sea over the Bay of Bengal to hit Chennai, we flew backwards along its track. The sky over Chennai was a clear blue. As the plane nosed up,Ocean and sky after a cyclone I saw a few banks of fluffy clouds near the horizon. The sky below us was clear as our Airbus 320 reached cruising height. The flight to the Andaman archipelago would take almost two hours. The sky below me seemed clear; only a few stray fluffy clouds occasionally interrupted my view of the calm seas. But when I looked up, there was a thin layer of cirrus clouds very far above the cruising altitude of the plane. They looked dark against the otherwise clear sky. You can see them in the upper half of this photo. You can also see a few wisps of white cumulus clouds against the blue of the sea in the lower part of the photo.

A coral atoll around one of the Andaman islands

The monotony of flying over the ocean was just beginning to lull me to sleep when the plane began its descent. As we came in over the outlying islands, cloud banks piled up ahead of us. The air was not as clear as it had been before. I realized later that clouds tended to mass up near the islands and coast in this season. As a result, the image of the coral atoll surrounding the island in the photo above was hazy, and I had to tweak the photo quite a bit to see the beautiful patterns in the water around the island. It was lovely to see the surf breaking far from the white sandy beach around the island.

The descent to Port Blair was fast, and I barely clicked a few more photos as we came lower. Almost the last photo I managed to take was a beach surrounded by corals. This is the featured photo. The water is so clear that you can see the coral reefs and the wide tongue of white sandy beach. I knew we were in for a great holiday.