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Why is the sky blue, how cold is the air 15 kilometres up, and why does the ozone layer thicken and thin? K. R. Ramanathan worked on all three questions. He began as a physicist in C. V. Raman's laboratory and became a central figure in Indian atmospheric science. This post covers his career, a worked example on light scattering, and what students can learn from how he noticed things.
Who was K. R. Ramanathan?
His Indian National Science Academy biographical memoir records that he was born on 28 February 1893 in Kalpathy, Palghat, now in Kerala. He studied at Victoria College, Palghat, and then Presidency College, Madras, where he took a BA (Honours) in physics in 1914 and went on to teach at Maharajah's College, Trivandrum.
There he showed an early habit that defined his career. The memoir describes how he noticed that the humidity values his students calculated did not match those published by the local observatory. He walked in and found the observers were using the wrong tables. The college principal, who also ran the observatory, put Ramanathan in charge of it. He went on to prepare the first complete rainfall map of Travancore State.
What did he do with C. V. Raman?
At the end of 1921 he joined Raman in Calcutta to study how molecules scatter light. In 1922 he published The molecular scattering of light in vapours and in liquids and its relation to the opalescence observed in the critical state in the Proceedings of the Royal Society. The memoir says he published ten papers on scattering in less than a year and received the first DSc awarded by the University of Madras.
The memoir also tells a story about an anomaly. Working on light scattered by water in 1923, he saw a weak glow he took to be fluorescence from impurities. It persisted however carefully the water was purified. Raman later showed that such light was a property of the substance itself, the effect announced in 1928. Our post on C. V. Raman and scattering explains that discovery.
Worked example: why is blue light scattered more?
The 1922 paper's abstract quotes Einstein's formula for scattering by density fluctuations, in which the scattered fraction has wavelength to the fourth power in the denominator. For small molecules the scattered intensity goes as 1 divided by wavelength to the fourth power.
Compare blue light at 450 nanometres with red light at 700 nanometres:
Ratio of intensities = (700 / 450) to the fourth power.
700 / 450 = 1.556.
1.556 squared = 2.42, and 2.42 squared = 5.86.
So blue light is scattered almost six times as strongly as red. That is why the daytime sky looks blue. A common mistake is to use the ratio itself, 1.56, and forget the fourth power. Our guide to scattering of light and rainbows has more practice.
How did he study the upper atmosphere?
In 1925 he joined the India Meteorological Department. In 1929 he published Distribution of temperature in the first 25 kilometres over the Earth in Nature. The INSA memoir calls this the first published diagram of upper-air temperatures over the world up to 25 kilometres in summer and winter, and notes that it showed the tropopause is low over the poles and high over the tropics.
The tropopause is the boundary above which temperature stops falling with height. A high tropopause over the tropics means the weather layer of the atmosphere is deeper there than near the poles.
What did he find about ozone?
In the early 1940s, the memoir says, he arranged ozone observations at Kodaikanal, Pune and Delhi using Dobson spectrophotometers, instruments that measure how much ultraviolet sunlight ozone absorbs. He retired from the department at 55 and in 1948 became the first director of the Physical Research Laboratory in Ahmedabad.
In 1963 he published Bi-annual variation of atmospheric ozone over the tropics in the Quarterly Journal of the Royal Meteorological Society. Its abstract states that over equatorial, subtropical and lower middle latitude stations "a year of high ozone being followed by one of low ozone", and suggests a link with the two-year cycle of stratospheric winds in low latitudes.
The memoir notes that international colleagues called him "Mr Ozone". He served as president of the International Union of Geodesy and Geophysics (1954-57) and of the International Ozone Commission (1960-67), and received the International Meteorological Organization Prize (1961), the Padma Bhushan (1965) and the Padma Vibhushan (1976). Under his directorship the laboratory became, in the memoir's words, the cradle of space research in India, with rocket studies from Thumba. He died on the night of 30-31 December 1984. Our post on Mario Molina and ozone covers the chemistry of the ozone layer.
What can a student take from how he worked?
Check numbers against each other. His career turned on spotting two numbers that should have agreed and did not.
Keep records over years. The ozone rhythm only appears in long, steady observations.
Carry physics into new fields. He used ideas from light scattering to study the sky.
If wave ideas like scattering feel hard to picture, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read.
This week, redo the worked example for violet light at 400 nanometres against red at 700, and check that you get a ratio a little over nine.
