Cyanometry
Nothing but blue skies ...

by Philip Eden


Cyanometry is a word that does not appear in my desk dictionary. If you recognise the two elements of the word you might be able to deduce its meaning, and if you have a little greek you certainly should. From kuanos, meaning dark blue, and metron, meaning measure, cyanometry is the study of the blueness of the sky.


When I first wrote about this subject in 1999 I tried an internet search to discover whether the word has any currency today. There were only five distinct 'hits'. Two gave one-line definitions from technical glossaries, one from the World Meteorological Organisation, the other from NASA. The third appeared as a possible password on a hacker's website, while the fourth was mentioned in an impenetrable treatise on colour in advertising. And the fifth? Well, the fifth 'hit' revealed that Cyanometry was the name of a greyhound which ran at the Lithgow dog-track near Sydney earlier that year. A current Google search, though, gives 725 hits, which tells us something about the internet.


Cyanometry's main proponent amongst professional scientists was a German physicist called Friedrich Linke who published several learned papers between the 1920s and 1940s on the way the sun shines through the earth's atmosphere. His interest in the blueness of the sky was therefore not an aesthetic one; rather, he recognised that the variation in the colour of a cloudless sky has an influence on the quantity of solar radiation reaching the planet's surface. The deeper the blue, the greater proportion of the sun's energy gets through. Meteorologists call this effect turbidity which is defined as "that property of a cloudless atmosphere which produces attenuation of solar radiation". We might call it simply "haziness". Agents of turbidity include dust, smoke, other solid or gaseous pollution, salt and sand particles, volcanic ash, and water vapour.


We know, for instance, that the debris injected into the atmosphere following a major volcanic eruption in the tropics (such as that of Mount Pinatubo in the Philippines in 1991) can reduce the global temperature by between 0.5 and 1.5°C for a period of two or three years.


In an attempt to gather information about geographical and temporal variations in turbidity, Linke devised a 16 point scale for official weather observers to use, and produced a set of eight cards of different standardised shades of blue, numbered 2, 4, 6, and so on to 16, varying from the white of a dust-ridden desert sky to the deep blue of a pure polar air-mass. The observer could use the odd numbers if he judged the sky colour to lie between any of the given shades.


Sadly for those few meteorologists with an artistic streak the study found little favour outside Germany between the wars, and after 1945 more scientific methods of measuring turbidity were introduced. So we will never hear a weather forecast saying something like, "After the clouds disperse tomorrow's sky will be a 12 on the Linke Scale."