Partly Cloudy or Partly Sunny?

I had an interesting chat over the phone the other day with Ted Fathauer at the Fairbanks Forecast Office of the National Weather Service. I was inquiring about the difference between "partly cloudy" and "partly sunny." Ted admitted to a bit of subjectivity on the part of the forecaster in the matter, and revealed that it had a lot to do with what the weather had been like for the past few days. In other words, if there had been a streak of really lousy weather at the time of the forecast, but it looked like it might be clearing up, the nod went to "partly sunny." On the other hand, if it had been nice but it looked like some clouds would be showing up in the evening, the outlook was for "partly cloudy."

The subject then turned to the observation that Fairbanks officially enjoyed only three days of clear weather in August, and I wondered how this could be since it seemed to me the sun had been out more than that. Ted explained that the rating is obtained by measuring the percentage of cloud cover during each hourly interval during a day between sunrise and sunset. At the end of the day, the hourly percentages are averaged, and if the cumulative average cloud cover is between 0 and 30 percent for the day, it is defined as being clear. If it is between 40 and 70 percent, the day is defined as partly cloudy, and 80 percent cloud cover or more means -- you guessed it -- cloudy.