The National Weather Service denotes sky condition using the portion of sky covered by opaque cloud cover.
In meteorology, an okta is a unit of measurement used to describe the amount of cloud cover at a given location. Cloud conditions are estimated in terms of how many eighths of the sky are covered with opaque cloud, ranging from 0 oktas (completely clear sky) to 8 oktas (completely overcast). It does not include transparent clouds, such as thin cirrus.
Mostly clear or mostly sunny is reported for coverage from 1 to 3 oktas, while partly cloudy or partly sunny refers to cloud coverage of 3 to 5 oktas. Mostly cloudy is reported if 5 to 7 oktas of the sky are covered with opaque clouds. When the sky is obscured by fog, 9 oktas is reported for total sky cover, indicating it could not be observed.
Trained weather observers make these estimates, and while the cloud cover measurements are subjective, they still provide useful and scientifically valuable data.
So, partly cloudy is the same as partly sunny, with 3 to 5 oktas of opaque cloud cover. Of course, partly sunny cannot be used in reporting nighttime conditions.
At any particular observation time, there may be different types of clouds at different heights above the ground. In such situations the total cloud amount is reported and defined as the fraction of the sky covered by any type of opaque cloud. Weather reports also will include the fraction of the sky covered by each type or layer of cloud as if it was the only type or layer of cloud in the sky.
Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. Send them your questions at firstname.lastname@example.org or email@example.com.