AEROGRAPHER'S MATE 3 & 2 



hot climates over surfaces that are warmed by 

 insolation, such as sandy plains. 



Mirages may occur in any region, even on 

 a city street. They are generally of three types: 

 the inferior mirage, the superior mirage, and 

 the lateral mirage. These depend, respectively, 

 on whether the spurious image appears below, 

 above, or to one side of the true position of 

 the object. The inferior mirage is the most 

 common and is responsible for the illusion of a 

 body of water in a desert or for the illusion of 

 a wet highway on a hot summer day. The superior 

 mirage, more spectacular but less frequent, 

 causes distant objects, trees, ships, mountains, 

 etc., to appear inverted in the sky. Multiple 

 or complex mirages have been observed. 



Crepuscular Rays 



These rays are beams of light apparently 

 diverging from the sun, seen both before and 

 after sunrise and sunset, especially in a hazy 

 or humid atmosphere. The beams are rendered 

 luminous by dust or water vapor. They are 

 especially striking when they shine through rifts 

 in the clouds. They are actually parallel, and 

 their apparent divergency is a result of 

 perspective. 



Halos, Coronas, Rainbows, Fogbows 



These are also optical phenomena; however, 

 since they are discussed in the next chapter 

 under photometeors, we will omit discussion 

 of them at this time. 



Cloud Iridescence 



Cloud iridescence is actually nothing more 

 than a portion of coronas. It is caused by the 

 diffraction of light from the sun or moon. When 

 light waves encounter an obstruction which is 

 small in comparison with their wavelength, the 

 light waves spread out and produce spectral 

 colors due to interference; this is diffraction. 



PROPERTIES OF SOUND 



Sound as related to meteorology and ocean- 

 ography has become of increasing importance 

 to the Aerographer's Mate; consequently, it is 

 necessary that he become familiar with some 

 of the fundamental concepts concerning the 

 properties of sound. 



WHAT SOUND IS 



Sound is the physical cause of your sensation 

 of hearing. Anything that you hear is a sound, 



Sound travels in the form of waves, which 

 vary in length according to their frequency. 

 A complete wavelength is called a cycle, and 

 the number of cycles per second is the sound's 

 frequency. Frequencies are measured in the 

 Hertz system, 1 hertz (Hz) being equal to 1 

 cycle per second. Frequencies of 1000 cps or 

 more are measured in kilohertz (kHz) .Normally, 

 sounds below 20 Hz or above 15 kHz are beyond 

 the human hearing range. Between these two 

 frequencies is the average human audible range. 

 More information of the characteristics of 

 sound is given later in this chapter. 



Before sound can be produced, three basic 

 elements must be present, (See fig, 12-21,) 

 These elements are a source of sound, a 

 medium to transmit the sound, and a detector 

 to hear it. If there is no source to generate 

 a noise, then there can be no sound. The same 

 theory holds true for the other required 

 elements. 



You may recall the experiment in which a 

 bell was placed inside a jar containing a 

 vacuum. You could see the beU ringing, but 

 you could hear nothing because there was no 

 medium to transmit sound from the beU to 

 you. What about the third element, the detector ? 

 You may see a source (such as an explosion) 

 apparently producing a sound, and you know the 

 medium (air) is present, but you are too far 

 away to hear the noise. So far as you are 

 concerned, there was no detector and, therefore, 

 no sound. 



209.22 

 Figure 12-21.— The three elements of sound. 



284 



