AEROGRAPHER'S MATE 3 & 2 



Table 12-1, — Common prefixes used in the 

 Metric System,, 



Tera 



Giga 



Mega ■ 



Kilo 



Hecto 



Decka' 



■ meaning 10l2 

 ■meaning 10^ 

 ■meaning 10 6 

 ■meaning 10 3 

 •meaning 102 

 ■meaning 10* 



Pico -meaning 10~12 

 Nano -meaning 10"^ 

 Micro -meaning 10"*6 

 Milli -meaning 10" 3 

 Centi -meaning 10~2 

 Deci -meaning 10~1 



NOTE: It then can be seen that a "KILO- 

 METER" equals 1,000 meters and a 

 "MILLIMETER" .001 of a meter, etc. 

 These prefixes can be used with all 

 kinds of units, such as, CENTIMETER, 

 KILOGRAMS, and MICROSECONDS. 



expressed as grams per cubic centimeter. We 

 then have the equation 



D- -S-3 



In the metric system, 1 cubic centimeter of 

 water has a mass of approximately 1 gram; 

 therefore, the density of water is given as 

 1 gram per cubic centimeter or 1 g/cm3. In 

 the English system, the density of water is 

 62.4 pounds per cubic foot, or 62.4 lb/ft 3 . 



Force is measured in dynes, A dyne is the 

 force that will give a mass of 1 gram an 

 acceleration of 1 centimeter per second per 

 second; this is commonly written as gm cm 

 per sec 2 , gm cm/sec/sec, or gm cm/sec 2 . 

 The force necessary to accelerate 1 gram 980.665 

 cm/sec 2 at 45° latitude would be 980.665 dynes. 



Measures of weight in the metric system 

 are formed by adding the Greek and Latin 

 prefixes to the gram. There are approximately 

 454 grams to a pound. The kilogram is 1,000 

 grams and is equivalent to about 2.2 pounds. 



In both the English and the metric systems 

 the second is the standard unit of time. 



For more detailed conversion factors com- 

 monly used in meteorology and oceanography, 

 refer to Smithsonian Meteorological Tables. 



PRESSURE 



Since pressure is one of the most important 

 parameters in meteorology, knowledge of its 



atmospheric structure and the forces which 

 cause it to change is needed to understand 

 how it affects the earth's weather patterns, 



DEFINITION 



Pressure is defined as force per unit area. 

 Atmospheric pressure is the force per unit 

 area exerted by the atmosphere in any part 

 of the atmospheric envelope. Therefore, the 

 greater the force exerted by the air for any 

 given area, the greater the pressure. Although 

 the pressure varies on a horizontal plane from 

 day to day, the greatest pressure variations 

 are with changes in altitude. Nevertheless, 

 horizontal variations of pressure are important 

 in meteorology because they cause or help to 

 cause good and bad weather. 



As previously mentioned, pressure is force, 

 and force is related to acceleration and mass 

 by Newton's second law. This law states that 

 acceleration of a body is directly proportional 

 to the force exerted on that body and inversely 

 proportional to the mass of that body. It may 

 be expressed as 



a = 



m 



where "a" is the acceleration, "F" is the 

 force exerted, and "m" is the mass of the 

 body. This is probably the most important 

 equation in the mechanics of physics dealing 

 with force and motion. It is usually stated in 

 the form (F = ma). NOTE: Be sure to use 

 units of mass and not units of weight when 

 applying this equation. 



STANDARDS OF MEASUREMENT 



Atmospheric pressure is normally measured 

 in meteorology by the use of a mercurial or 

 aneroid barometer. Pressure is measured in 

 many different units. One atmosphere of pressure 

 is 29.92 inches of mercury, 760 millimeters 

 of mercury, 1,013.25 millibars, 14.7 pounds 

 per square inch, or 1,033 grams per square 

 centimeter. These measurements are made under 

 standard conditions. 



STANDARD ATMOSPHERE 



The ICAO (International Civil Aeronautical 

 Organization) Standard Atmosphere assumes a 

 mean sea level temperature of 15° C, a standard 



260 



