PHYSICAL INTERPRETATION OF THE WEATHER ^ 



By Edgab W. Woolabd 

 IJ. S. Weather Bureau, Washington, D. C. 



[With 4 plates] 



In the days of ancient Greece, the word "meteorology" — or, rather, 

 its Greek equivalent, (itTecapoKoyla — was introduced to designate a 

 subject that, even then, was already old, namely, knowledge and lore 

 of the weather and other phenomena of the atmosphere. The word 

 "meteor" was then used as a general term for any atmospheric 

 phenomenon; this still is the preferred meaning given in modern 

 dictionaries, but the more common usage now is for astronomical 

 meteors, which in ancient times were, like comets, thought to be 

 entirely of atmospheric origin and were included under meteorology 

 along with a number of other phenomena not now considered to 

 come within its field. During the centuries since Aristotle wrote the 

 earliest laiown formal treatise on meteorology, the scope of the sub- 

 ject has gradually been narrowed, until the modern tendency is to 

 restrict it to only the phenomena that are directly involved in the 

 weather, excluding even other atmospheric phenomena and drawing 

 more or less of a distinction between meteorology and climatology. 



It is not difficult to show that meteorology, in the preceding mean- 

 ing, is a branch of physics (principally "classical" physics), and it 

 is significant that many of the most notable contributions to meteor- 

 ology in the past have been made by physicists. The state of the 

 weather at a given time and place is specified by the values of six 

 quantities known as the meteorological elements, namely, air temper- 

 ature, barometric pressure, wind velocity (direction and speed), 

 humidity, clouds, and precipitation (rain, snow, hail, etc.). Now, 

 these elements are completely determined by the temperature, pres- 

 sure, density, and three velocity components of the air, and its total 

 solid, liquid, and gaseous water content; clearly, therefore, the 

 weather is essentially a dynamic and thermodynamic phenomenon. 



Unfortunately, formidable difficulties are encountered in attempt- 

 ing to base the systematic development and the practical applications 



^ Reprinted by permission from Journal of Applied Physics, vol. 9, No. 1, January 1938. 



223 



