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fresh fall of snow, since the reflection on the sky will be whiter from 

 snow than from ice. With a cloudless sky there can be no iceblink, 

 though there may be a yellow or white haze or glare to indicate the 

 presence of ice. However, with a cloudless sky there may be abnormal 

 refraction, which raises the horizon and enables the observer to see the 

 ice at a greater distance than would normally be possible. The image 

 of the ice or areas of open water, or a mixture of the two, may be seen 

 as an erect or inverted image, or both images may be seen at once, one 

 above the other. In this case the erect image is the higher of the two, 

 which are usually in contact. Allowance must be made for the fact 

 that refraction causes the apparent dimensions of ice to increase, some- 

 times so as to make bergy bits appear like icebergs. Where there is 

 open water there will be seen a dark blue color, toward which the 

 vessel should steer. 



ABNORMAL REFRACTION 



Deceptions of vision at sea are produced by abnormal refraction 

 of light which in the more extreme cases gives rise to false images of 

 land, ships, or other objects. Generally speaking, abnormal refrac- 

 tion at sea is due to an inversion of temperature in a layer of air, the 

 variations in density thus produced causing the light rays to be bent 

 considerably in excess of normal conditions. 



The most frequent and most favorable conditions for excess refrac- 

 tion, under which most of the more fantastic forms of mirage and 

 distortion take place, occurs when a layer of warm air is in contact 

 with cooler water. The air next to the surface of the sea is cooled, and 

 consequently the upper layers are warmer than the lower so that in- 

 stead of the usual decrease there is an increase of temperature with 

 height. Most refraction phenomena are formed at the boundary be- 

 tween this cold, dense layer of air at the surface of the sea and the less 

 dense warm air above. This condition is identical with that which is 

 responsible for the formation of most sea fog ; therefore, the presence 

 of fog is an indication that excessive refraction is likely to be en- 

 countered. 



Similar inversions may be caused by the presence of cold air over 

 warm water. A marked difference between air and sea temperatures 

 is therefore another guide to the presence of excessive refraction. 



Although abnormal refraction is not restricted to particular geo- 

 graphical areas, certain regions of the globe are so situated with re- 

 spect to general meteorological conditions as to be more favorable than 

 others for the occurrence of abnormal refraction phenomena. The 



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