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The Frost Point
Water vapor in the stratosphere, never, never at a level you would call humid, is difficult to measure. An instrument-carrying balloon on its decent back to earth usually does it. A surface on which frost can form is cooled until frost forms just like on he windshield of your just-avoided-pre-Clinton-100x-tariff Lexus on a clear night. The temperature at which this happens is the frost point temperature, a cool and icy cousin of the dew point temperature. As the balloon sinks in the atmosphere adiabatic warming causes the surface where the frost forms to warm and the frost sublimes. Ice becomes gas. Now the plate can be cooled again until frost forms again and the frost point temperature can be determined at the new, lower altitude. This process is repeated at half-minute intervals and a profile of frost point temperatures is the product. Now you can't do that on the way up because it gets colder as you go up and once the frost forms it is there for the rest of the ride. The device that does this measurement is called the frost-point hygrometer. Knowing the frost point temperature and the pressure altitude, the grams of water in the air per kilogram of air can be calculated. That is how the water content of the stratosphere is measured. It is a tricky business and it just is not done in a lot of places around the world. Our article of faith is that the air in the stratosphere is well mixed in each hemisphere and a measurement in one place is likely to be the same in the next palace at the same altitude. You need a bit of faith and a lot of trust in your understanding of the physical laws you love and hold dear.
Now to the wonderful world of what you can do with small numbers. A gram or two or three of water per kilogram of air are common numbers. Change from 2 to 3 g/kg is a 50% increase while a drying from 3 to 2 g/kg only a 33% decline. My wife used to complain that her middle school students couldn’t get fractions or percentages. Her problem was resolved with het retirement. And then there was relative humidity’s dependence on temperature. Life is tough.
Photo: Daniel Schwen, Creative Commons Attribution-Share Alike 3.0 Unported license.