Wednesday, September 3, 2014

Thermals - David Holt

A definition is in order.  A solar induced thermal is a column of rising air that is created by the sun and it will henceforth be referred to as simply a thermal.  Thermals are generated by the sun’s heating of the ground in an uneven manner.  As the sun illuminates the earth’s surface, certain areas of that surface are more susceptible to heating than other areas. Therefore certain portions of the ground are superheated when compared to other portions.  The air above those superheated portions of the ground then becomes superheated by conduction.  Now we have a parcel of air that is superheated relative to the surrounding air.

The parcel of superheated air contains molecules that are extremely agitated and vibrate more than those in the cooler surrounding air.  The vibration of the molecules causes them to collide with one another thus knocking them about to the extent that the superheated parcel of air is less dense than the surrounding air.  The less dense parcel of superheated air now begins to rise.  Thus the thermal is born.
It must be stressed that it is the ground that is superheated first and the air above the superheated ground that is superheated second.  The sun cannot superheat parcels of air as the air is uniform in color and texture while the ground isn’t.  In fact the air is transparent to the suns rays.  Therefore the ground with its variations in color, texture, and moisture content is of prime importance in the generation of a thermal.  Just consider that the ground is heated first and the air above it second.
To illustrate the variations in color and texture consider the following.  A freshly plowed area surrounded by fields of dried grasses.  The plowed area is darker than the dried grasses thus allowing it to absorb heat rather than reflect it.  The plowed area will become superheated.  A strip mall with an asphalt parking lot situated in the median of a concrete highway will become superheated due to the dark asphalt absorbing heat while the light concrete roadway reflects heat.  A grassy knoll surrounded by grassy fields will become superheated quicker than the area around it even though the surrounding area is of similar color and texture.  This would be because the knoll has better drainage and will shed moisture quicker than the low-lying ground.  Heavy moisture in the ground will delay the superheating of the ground as it becomes a heat sink.

When that first superheated parcel of air begins to rise, it is replaced by the surrounding cooler air which in turn becomes superheated and it in turn starts to rise.
 Soon you have a column of air rising and it will continue to rise, cooling as it rises until it cools to the temperature of the surrounding air at some altitude above sea level. That altitude will depend upon the vertical stability of the air and the humidity of the air in the thermal.  Also the detection of a thermal will depend upon the relative humidity at the earth’s surface.  If the air is sufficiently humid then a cloud will form capping the thermal when the dew point temperature of the rising air reaches the dew point temperature of the surrounding air.

This cycle of superheating, rising air, and cooling of the air will continue as long as the sun is shining.  But, anyone who has spent some time hawkwatching has noted that by one o’clock PM on a hot and humid day the cumulus clouds spawned by the thermals have begun to amass and at some point amass sufficiently to block the sun from striking the ground.  And, you guessed it, the thermals will stop forming.  This cessation of the thermals can cause a decrease in the number of hawks counted, as they may not find a thermal to soar upon within the purview of the observers at the hawkwatch.

Not to worry, as the thermals may reappear as the clouds caused by the early morning thermals begin to spread and in some cases dissipate allowing the sun to once again illuminate the ground.  If this is the case the cycle will repeat itself albeit in different places until nightfall.  The foregoing has described a model of the birth, life, and death of a solar induced thermal.  But there are some wrinkles to this model that should be addressed for the hawkwatcher stationed in the Blue Ridge Mountains of southwest Virginia.

In September the mountains in southwest Virginia are covered with the rich foliage of deciduous and coniferous forests.
  Stand at your site and look around and you will see a sea of uniform green some fifty to a hundred feet above the ground all around your site.  How is the sun ever going to illuminate the ground unevenly through this canopy of green?  To my knowledge the sun cannot possibly provide a rich source of thermals along the mountainous territory of southwest Virginia and yet many hawks are counted in this venue.  And, during that same time the thermals are the rule while all other soaring means are the exception to the rule.

In the vicinity of Harvey’s Knob there is a black asphalt strip in the middle of that sea of green.  There is an occasional outcrop of rock and a mowed area that resembles a clear cut. The parking lot at Harvey’s Knob is a non-uniformity in color that provides a source for a thermal.  How many hawks have you counted directly overhead in the Broad-winged hawk season?  Those of you who are reading this from other lookouts make the same survey.  Remember all you have to consider is a circle with a two to three mile radius centered on the site.  That will indicate possible thermal sources within your purview thus providing a reason for your counts on those days when the winds are light and variable.

And there are other questions regarding the anatomy of a thermal that I will be glad to field.  But, remember that I am not a meteorologist.  I am merely a hawkwatcher who has concentrated his efforts into understanding why that hawk was in view when I went to some hawkwatching site.  This wasn’t meant to be a scientific paper as it wouldn’t pass muster in scientific circles.  It is a paraphrase of the science of a thermal as I have understood it from the various sources I have researched.  If I have left something out I am sorry, but I do think I have provided something for you to think about during those long hours when no hawks are in sight.  I submit my Email address for anyone who wishes to keep his queries private.
Dave Holt 12/20/06

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