Figure 1-27.—Obscuration.

particles falling through the atmosphere are never classified as obscurations aloft. In the past, you may have looked at the sky and seen thin fog or haze on the horizon that blocked your view of the clouds. Sometimes the portion of the sky that is hidden from view extends only a few degrees above the horizon, while at other times, the phenomenon may extend well above the horizon. When the phenomenon is thin enough to allow the sun, clouds, the moon, or stars to be seen overhead but not seen near the horizon, the phenomenon is termed a partial obscuration (fig. 1-27). If the phenomenon is dense enough to hide even the portion of the sky directly overhead, it is called a total obscuration. An obscuring phenomenon frequently extends around the entire horizon circle, 360° of azimuth, to completely surround the observation site. Figure 1-27.—Obscuration. (A) Total obscuration-sky completely hidden, (B) partial obscuration-higher cloud, sun, moon, or sky may be seen. The best method to determine how much of the sky is hidden by a partial obscuration is to measure the elevation angle by using a clinometer (see chapter 2). The top of the partial obscuration is considered to be the point where the outline of higher clouds, the sun or the moon, or the light from stars is visible. Tables in NAVMETOCCOMINST 3141.2 and NAVMETOC- COMINST 3144.1 are used to convert elevation angle to eighths of sky coverage. Let’s review a few of the important concepts involved when observing layer amounts. When you are observing cumuliform clouds in a layer and blue sky is visible between the elements, the blue sky is not included as part of the layer coverage. For example, if many small cumulus clouds covered the sky from horizon to horizon, but the blue sky visible between each of the cloud cells is about the same size as the cloud cells, the layer coverage would only be 4/8. If however, the cumuliform cloud elements are joined by very thin cloud, even if the thin cloud is transparent and higher clouds can be seen through the thin cloud, the cloud layer is considered to cover the area. This is commonly the situation with altocumulus and cirrocumulus clouds. For example, if half of the sky is covered by a layer of altocumulus cloud with the denser cells being opaque, and the area between the cells is filled with a transparent cloud through which a very pale blue sky or higher clouds may be seen, the layer coverage is the same as the entire portion of the sky covered, or 4/8. The portion of a higher cloud layer visible through lower transparent clouds is treated as if it were not visible, that is, as if the lower cloud layer were opaque. In a METAR/SPECI observation, when observing layers of clouds that are stacked on top of other layers, only count those clouds visible to you in the layer amount. If, for example, a layer of stratocumulus clouds covers half of the sky, and directly above the stratocumulus layer the observer can see only the edges of an altocumulus layer that seems to cover the same area as the stratocumulus layer, the most the observer could report is 4/8 stratocumulus and 1/8 altocumulus. Similarly, if an altocumulus layer covers 6/8 of the sky in a dense sheet, and 1/8 of cumulus is located below the altocumulus layer, the observer "knows" that the altocumulus layer covers the entire layer extending across 6/8 of the sky, even though 1/8 of the layer is hidden from view by the cumulus cloud. The cloud layers must still be reported as 1/8 cumulus and 5/8 altocumulus, since the observer cannot actually see the remaining 1/8 of altocumulus cloud. The maximum number of reportable layers is limited to six. OCONUS 1-26