Chapter 15 — METEOROLOGICAL ELEMENTS 



ring on the outside. The sky is darker inside the 

 ring than outside. Halos are formed by RE- 

 FRACTION of light as it passes through ice 

 crystals. This means that halos are almost 

 exclusively associated with cirriform clouds. 

 Refraction of light means that the light passes 

 through prisms; that is, ice crystals which act 

 as prisms. Some reflection of light also takes 

 place. 



Halos appear in various sizes, but the most 

 comnon size is the small 22-degree halo. The 

 size of the halo can be determined visually with 

 ease. Technically, the radius of the 22-degree 

 halo subtends an arc of 22 degrees. This simply 

 means that the angle measured from the ob- 

 servation point between the luminous body and 

 the ring is 22°. Halos of other sizes are formed 

 in the same manner. 



CORONAS 



A corona is a luminous ring surrounding the 

 sun (solar) or moon (lunar) and is formed by 

 DIFFRACTION of light by water droplets. It 

 may vary greatly in size, but is usually smaller 

 than a halo. All the spectral colors may be 

 visible, with red on the outside; but frequently 

 the inner colors are not visible. Sometimes the 

 spectral colors or portions of them are repeated 

 several times and are somewhat irregularly 

 distributed. This phenomenon is called irides- 

 cence. 



RAINBOWS 



The rainbow is a circular arc seen opposite 

 the sun, usually exhibiting all the primary col- 

 ors, with red on the outside. It is caused by 

 diffraction, refraction, and reflection of light 

 within raindrops, often with a secondary bow 

 outside the primary one. In this case the colors 

 are reversed. The colors of a rainbow are red 

 to blue and violet. 



FOGBOWS 



A fogbow is a whitish semicircular arc seen 

 opposite the sun in fog. Its outer margin has 

 a reddish tinge; its inner margin has a bluish 

 tinge. The middle of the band Is white. An addi- 

 tional bow, with the colors reversed, sometimes 

 appears inside the first. 



ELECTROMETEORS 



An electrometeor is a visible or audible 

 manifestation of atmospheric electricity. The 

 more important electrometeors are thunder- 

 storms, lightning, and the auroras. 



THUNDERSTORMS 



An Aerographer's Mate must be cognizant of 

 thunderstorm activity in order to advise pilots 

 as to the best possible routes for flight. Since 

 approximately 44,000 thunderstorms occur daily 

 over the surface of the earth, a pilot will some- 

 times fly through a thunderstorm or a thunder- 

 storm area. The turbulence within most thunder- 

 storms is considered one of the worst hazards 

 of flying. 



Ground personnel also need to be advised 

 as to the strong gusty surface winds that are 

 often associated with the thunderstorm. 



Much of the information about thunderstorms 

 in this chapter is based on the findings of the 

 Thunderstorm Project which was conducted at 

 Orlando, Fla., and Wilmington, Ohio, as a joint 

 project of the weather services of the Armed 

 Forces and the National Weather Service. 



Formation 



The thunderstorm represents a violent and 

 spectacular atmospheric phenomenon. The 

 thunderstorm is usually accompanied by lightning, 

 thunder, heavy rain, gusty surface wind, and 

 frequently by hail. A certain combination of at- 

 mospheric conditions is necessary for the for- 

 mation of a thunderstorm. These factors are 

 conditionally unstable air of relatively high hu- 

 midity and some type of lifting action. Before 

 the air actually becomes unstable, it must be 

 lifted to a point where it is warmer than the 

 surrounding air. When this condition is brought 

 about , the relatively warmer air continues to 

 rise freely until, at some point aloft, its tem- 

 perature has cooled to the temperature of the 

 surrounding air. In order to bring the warm 

 surface air to a point where it will continue 

 to rise freely, some type of external lifting action 

 must be introduced. Many conditions satisfy this 

 requirement. For example, an air mass may be 

 lifted by heating, terrain, and fronts or con- 

 vergence. 



365 



