A Science for Everyone, Community, Meteorology

I Could Visualize the Adiabatic Lapse Rate

Fall ended, and my winter started in December.  It may be due to a warming global temperature – but in the seventies, when much of my life was dedicated to snow surveys, I would have been explaining it by la nina.  Add the tilde to the second n – the Spanish word for little girl, the situation of the coast of Peru that increases precipitation here in the northwest.

I’m not one to complain about rain – one of the predictable portions of our climate is that early Summer has rain, and we tend to harvest alfalfa later than the optimal 10% bloom because of rain.  After July 4, we’re moving into the dry times that make drying hay easier – even if its a bit late.  You develop an appreciation for rain when your climate gives you long, hot dry spells.

This Fall, I could watch Mount Marston and Stahl Peak as the snow would come and go – I have a good view of their western slopes, and my thermometer lets me watch the difference in temperature.  I live at about 3,000 feet elevation.  The top of those two mountains is about 6,000 feet.  It’s one of the great things of living here – mountains are great, and altitude kind of sucks.  Nothing personal, but I like 3,000 foot valleys and 6,000 foot mountains a lot more than 6,000 foot valleys and 10,000 foot mountains.  My lungs fit better.

Back to the topic – the adiabatic lapse rate.  As you go up, atmospheric pressure goes down.  It is kind of obvious – as you climb the mountain, there is less atmosphere above you.  Less atmospheric pressure means that there are fewer particles of atmosphere – nitrogen and oxygen – in any particular unit of volumetric measurement you care to use.  Colloquially, the air is thinner.

It kind of makes sense – with more space between the molecules, molecules hit each other less frequently.  Fewer molecular collisions correlate with a drop in temperature.  (Physicists might invoke causation here – my training really doesn’t let me offer an explanation, but I can point out a correlation.)

So we need two tools to develop an understanding of the adiabatic lapse rate – the thermometer and the barometer.  Evangelina Torricelli invented the barometer in 1643.  Fahrenheit invented the alcohol thermometer in 1709, and a more useful mercury thermometer in 1714.  Paul Kollsman modified the idea of the barometer and developed a usable altimeter in 1928.

The adiabatic lapse rate is defined as the rate at which the temperature of an air parcel changes in response to the compression or expansion associated with elevation change, assuming no heat exchange occurs between the air and its surroundings.  Aviation, and icing wings gave an impetus to quantifying this rate of temperature change – and the need for weather forecasts provided even more.  The number is 5.2 degrees Fahrenheit for every 1000 vertical feet, or 5 degrees Celsius per 1000 meters.  (in the real world it can vary from 4 to 9 depending on humidity, etc)

So this Fall, with its snows and thaws, left me with elevation contours I could watch on the mountainsides – something that the deep snows of winter do not readily allow in the Spring as things warm up.  Since nobody came along and asked “What’s the temperature half-way up Marston?” it has been a private observation – but it has been fun to watch.

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