octas

Measuring Cloud Cover
During the day the sun is always shining, so the amount of sunshine reaching the ground depends on the amount and duration of any cloud cover. The amount of cloud cover is usually given in units called oktas.

Each okta represents one eighth of the sky covered by cloud.

Clear sky /1 okta / 2 oktas / 3 oktas / 4 oktas / 5 oktas / 6 oktas / 7 oktas / Overcast

Using a Cloud Mirror

You can use a cloud mirror to simply measure oktas. Divide a large mirror into a grid with 16 equal squares using a dark crayon.

Lay the mirror on the ground somewhere you can see the whole sky.

Count the number of grid squares, or fractions of squares with cloud in them.

Divide that number by two to convert sixteenths into oktas.

http://www.bbc.co.uk/weather/weatherwise/activities/weatherstation/cloud_measuring.shtml

http://www.tutiempo.net/silvia_larocca/Temas/Met15.htm