INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS §7 



DIFFRACTION PHENOMENA 



Coronas. — Coronas consist of one or more sets of rainbow-colored 

 rings, nsually of only a few degrees radius, concentrically surrounding 

 the sun, moon, or other bright objects when covered by a thin cloud 

 veil. They differ from halos in having smaller (except in rare 

 cases), and variable radii, and in having the reverse order of colors; 

 that is, blue near the sun, say, and red farthest away. 



Clearly, then, coronas are caused by diffraction or the distribution 

 of effective (nonneutralizing), quantities of light off the primary 

 path, resulting from the action of cloud particles on radiation from 

 a distant source. 



When coronas are seen in clouds whose temperature is above 0° C, 

 or in which halos do not form, it is certain that they are due to 

 droplets. The most brilliant coronas, however, are formed by very 

 high clouds, whose temperatures often must be far below freezing, 

 from which it has been assumed that these coronas must be due to 

 the diffractive action of ice needles. There are reasons, however, 

 for believing that they are chie instead to very small undercooled 

 Avater droplets of approximately uniform size. 



Iridescent clouds. — Thin and perhaps slowly evaporating cirro- 

 stratus and cirrocumulus clouds occasionally develop numerous iri- 

 descent borders and patches of irregular shape, especially of red 

 and green, at various distances from the sun up to 30° or more. A 

 brilliantly colored iridescent cloud of considerable area is justly re- 

 garded as one of the most beautiful of sky phenomena. Imperfectly 

 explained until recently, it is now believed that these colored patches 

 are only fragments of unusually large and exceptionally brilliant 

 coronas, formed as described in the preceding paragraph. 



MIRAGE 



The mirage is a refraction phenomenon occasioned when the air is 

 calm and the change of density with increase of height unusual. 

 (It includes looming, towering, sinking, and stooping, previously de- 

 scribed under the heading "Refraction jDhenomena.") When the den- 

 sity of the air decreases from the ground upward more rapidly than 

 the normal rate, as it does when the ground is covered with a layer 

 of very cold air, the rays of light are bent more than normally 

 toward the earth and the mirage is seen raised above the object, which 

 may be below the horizon at the time. This form of the phenome- 

 non, in which the mirage appears as if reflected from an overhead 

 plane mirror, is known as the superior mirage. 



If, on the other hand, the density increases from the ground up- 

 ward, as it does over highly heated deserts, the rays are bent upward 

 and the image appears as if reflected from a plane mirror below the 

 observer. This is the inferior mirage, common in flat desert regions, 

 especially during the warmer hours of the day. The phenomenon 

 closely simulates, even to the quivering of the images, the reflection 

 by a quiet bodj'' of water of objects on the distant shore, the "water" 

 being the image of the distant low sky. 



In addition to these simpler forms of mirage, there is one of com- 

 plex displacements and distortions known as the Fata Morgana, 

 which apparently results from the coexistence of the temperature 



