How to Observe Cloud Cover

  1. Take the cloud cover measurements at the same site and time as the cloud-type measurement.
  2. Consider the sky to be a hemisphere (or bowl) centered over your position. This is called the celestial dome.
  3. Estimate the total fraction of the celestial dome covered by clouds of any thickness (translucent or not), rounded to the nearest one-eighth, or okta. That is, less than 1/2 okta gets rounded to zero; more than 7.5 oktas gets rounded to 8, etc.
  4. Cloud cover should then be reported according to the following cloud-cover classification definitions:
    • Clear: Zero oktas. Since a “clear” sky can include some clouds (as long as it’s less than 1/2 okta), it is possible to report cloud type even when you report a clear sky!
    • Scattered Clouds: 1–4 oktas. In other words, if the total sky cover is one-half or less, but not zero.
    • Broken clouds: 5–7 oktas.
    • Overcast: 8 oktas, even if breaks are present (up to 1/2 okta).
  5. When both cloudy and clear areas of the sky are visible, use the following simple rule of thumb: if you see more clear blue sky than clouds, or if the amounts are about equal, then the cloud cover is considered to be scattered . If you see more clouds than you do blue sky, then the cloud cover is broken.