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
