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Figure 1-28.Cloud-to-ground lightning under a cumulonimbus cloud base.
PRESSURE

Aerographers Mate, Module 01-Surface Weather Observations
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Figure 1-29.—Cloud-to-ground lightning from the side of cumulonimbus calvus cloud to ground several miles from the cloud base. Auroras Auroras are luminous phenomena that appear in the high atmosphere in the form of arcs, bands, draperies, or curtains. These phenomena are usually white but may have other colors. The lower edges of the arcs or curtains are usually well defined; the upper edges are diffuse.  Polar  auroras  are  caused  by  electrically charged particles ejected from the sun that act on the rarified (select) gases of the upper atmosphere. The particles are channeled by earth’s magnetic field, so auroras are concentratednear the magnetic poles. In the Northern Hemisphere, they are known as the Aurora Borealis, while in the Southern Hemisphere they are known as the Aurora Australis. Another form of the aurora  is  airglow.  Airglow  is  fainter  and  lacks definition, but may be seen in the low and middle latitudes as a faint glow in the sky. Unless remarkably intense or vivid, auroras are not reported in surface aviation observations. Shipboard observers will only report auroras when located north of 45" north latitude or  south  of  45"  south  latitude. PHOTOMETEORS Photometeors consist of a number of atmospheric phenomena attributed to the reflection or refraction of visible  light  in  the  sky  by  liquid  water  droplets,  ice crystals, by the air itself, or by solid particles in the air, such  as  volcanic  ash  or  dust.  Several  types  of photometeor phenomena may be used to assist in the identification of cloud type, such as the halo, corona, irisation, and rainbows. Fogbows are classified as photometeors, as are superior mirages (objects such as buildings, trees, or mountains seen inverted in the sky) and inferior mirages (shimmering wet appearance of hot surfaces such as roads or sand). Other than aiding in the   identification   of   other   phenomena,   these phenomena are not significant in surface aviation weather  observations;  therefore,  they  are  not  reported. Of the weather elements we have discussed to this point, most of the identification of the phenomena is based on the observer’s knowledge and on what the observer sees directly. Sure, instruments are in use to help  the  observer  determine  visual  range  and  cloud height, but the only method to determine cloud type and what type of weather is occurring is by the observer’s classification.   Many of the remainder of the observable elements for a surface aviation weather observation are directly  obtainable  from  instruments,  so  our explanation  should  be  somewhat  easier  for  the remainder of the chapter. Let’s continue with the next section on observing the pressure. 1-37







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