Chapter 12 — THE GOVERNING FUNDAMENTALS OF METEOROLOGY 



sea level pressure of 1,013.25 millibars or 

 29.92 inches of mercury, a temperature lapse 

 rate (decrease) of 0.65° C per 100 meters up 

 to 11 kilometers, and a tropopause and strato- 

 sphere temperature of -56,5° C. 



In the Naval Weather Service, pressure 

 measurements are expressed in millibars and 

 inches of mercury. 



VERTICAL DISTRIBUTION 



Pressure at any point in a column of water, 

 mercury, or any fluid, depends upon the weight 

 of the column above that point. 



Air pressure at any given altitude within 

 the atmosphere is determined by the weight 

 of the atmosphere pressing down from above. 

 Therefore, the pressure decreases with altitude 

 because the weight of the atmosphere decreases. 



It has been found that the pressure decreases 

 by half for each 18,000-foot increase in altitude. 

 Thus, at 18,000 feet one can expect an average 

 pressure of about 500 millibars (7.35 pounds 

 per square inch), and at 36,000 feet a pressure 

 of only 250 millibars (3.68 pounds per square 

 inch), etc. Therefore, it may be concluded that 

 atmospheric pressures are greatest at lower 

 elevations because the total weight of the 

 atmosphere is greatest at these points. 



There is a change of pressure whenever 

 either the mass of the atmosphere or the 

 accelerations of the molecules within the 

 atmosphere are changed. 



Although altitude exerts the dominant con- 

 trol, temperature and moisture alter pressure 

 at any given altitude, especially near the earth's 

 surface where heat and humidity are most 

 abundant. The pressure variations produced by 

 heat and humidity (with heat operating as the 

 senior partner) cause the turbulence and wind 

 that help to make our weather. 



PASCAL'S LAW 



Pascal's law relative to the behavior of 

 FLUIDS under pressure applies, of course, to 

 GASES under pressure, all of which means that 

 a gas transmits undiminished pressure in all 

 directions and on all parts of the enclosing 

 wall. The law states that when an external 

 pressure is applied to any confined fluid at 

 rest, the pressure is increased at every point 

 in the fluid by the amount of external pressure 



applied. This means that the pressure of the 

 atmosphere is exerted not only downward on 

 the surface of an object, but also in all directions 

 against a surface which is exposed to the 

 atmosphere. 



TEMPERATURE 



One of the most important properties of 

 the atmosphere is its ability to absorb and 

 lose heat. The heating and cooling of the 

 atmosphere exert a tremendous influence on 

 the processes that determine the weather. 

 Consequently, temperature is one of the principal 

 concerns of the Aerographer's Mate, It is 

 necessary to know the meaning of temperature, 

 the scales and instruments used in its measure- 

 ment, and the important temperature values. 

 Procedures for observing temperature were 

 discussed in earlier chapters. 



DEFINITION 



Temperature may be regarded as a measure 

 of molecular motion determined from an 

 absolute zero point at which all molecular 

 motion stops. Temperature may be defined as 

 the degree of hotness or coldness, or it may 

 be considered as a measure of heat intensity. 

 This being the case, we may conveniently 

 measure temperature in several different ways. 



One way of measuring temperature is by 

 means of a liquid thermometer. The changes 

 in volume of certain sensitive substances, such 

 as alcohol or mercury, which expand greatly 

 with an increase in molecular activity (tempera- 

 ture increase) and contract greatly when the 

 molecular activity slows down (temperature 

 decreases), are observed. 



Another way to measure temperature is to 

 place substances, usually metals, next to one 

 another and note the difference in their 

 expansion or contraction. Each substance has 

 an expansion coefficient which is based on the 

 amount it expands per degree of temperature 

 increase. One metal alone can be used to 

 measure temperature; but since the expansion 

 is usually small, it is very difficult to get an 

 easy and accurate measurement. Two metals 

 welded together make the measurement easier 

 because the strip with the lesser expansion 

 coefficient bends the one with the greater; the 

 curve of bending is easily magnified, thus 



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