PHOTOMETEORS

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