Cloud Cover versus Sun Altitude

The cloud cover statistics presented here do not take into account the slanted line of sight for the Sun at low altitudes. As you approach the horizon, cloud obscuration depends on two parameters: the horizontal and vertical extents of cloud. As you move toward the zenith, the vertical component becomes much less important, until, when it is overhead, the only thing that matters is the horizontal dimension.

Unfortunately, there is no reasonable way to estimate these parameters, since the vertical depth depends on the type of cloudiness. Cloud statistics do exist which could be reconstituted into a horizon cloud index, but it would be very probabilistic in nature - a 10% chance of overcast, 50% chance of some broken amount and so on. Cloud depth is also required, although it would only be a rough estimate.

We plan to investigate this subject in greater detail in the next solar eclipse bulletin.

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