As in many disciplines, meteorology has a language all its own. But when it comes to communicating weather forecasts to the public, we try to keep it simple.

Here are the formal definitions for some of the terms we use for cloud cover. "Cloudy" or "overcast" means 90% or more of the sky covered by clouds; "mostly cloudy" means 70 to 80% cloud-covered, so "mostly cloudy" does allow for some sunny breaks; "partly cloudy" and "partly sunny" both mean that 30 to 60% of the sky is filled with clouds (yes, you heard me right, partly cloudy and partly sunny mean the same thing); "mostly sunny" means that 10 to 20% of the sky is cloud-covered; and finally "sunny" means that less than 10% of the sky has clouds.

Now strictly speaking, these terms are used for observations - that is, you might say that skies are mostly cloudy at 7 p.m., meaning that 70 to 80% of the sky is filled with clouds at that time. But forecasters stretch the meanings and often use these terms in predictions. So, for example, a forecast of mostly cloudy for tomorrow means that 70 to 80% of the time the sun will be blocked by clouds.