When a weather report uses the term “overcast,” it represents a specific, measurable meteorological condition indicating a complete or near-complete blanket of clouds covering the sky. This condition alters the amount of sunlight reaching the ground, resulting in diffuse light. Understanding the precise definition of overcast is important because it is a professional classification used in weather and aviation reports to convey a consistent and quantifiable measure of sky cover.
Defining Overcast: Sky Coverage
The technical definition of an overcast sky focuses on the percentage of the sky obscured by clouds. Meteorologists use a specific unit of measurement called an okta, which divides the sky into eight equal parts. An overcast condition, abbreviated as OVC in aviation reports, is reported when the cloud cover equals eight oktas.
This translates to the cloud layer covering 90 to 100 percent of the sky, leaving no visible patches of blue. The total coverage must be solid enough to completely hide the sun’s disk, even if the clouds themselves are not particularly thick. An overcast classification is strictly quantitative, focusing on the fraction of the sky covered rather than the type of cloud present.
The Clouds Responsible
The sky condition is usually created by a broad, low-lying cloud formation known as the stratus family. Stratus clouds appear as a flat, uniform, gray sheet resembling fog that has lifted off the ground. Stratus clouds often form when moist air rises gently and cools, creating a consistent layer that can stretch across vast areas.
Sometimes, this uniform layer is a nimbostratus cloud, which is distinguished by its greater vertical thickness and moisture content. Nimbostratus clouds are significantly darker and thicker, completely blocking out the sun and producing prolonged, steady precipitation like continuous rain or snow. While stratus clouds may only produce a light mist or drizzle, nimbostratus signals a more persistent and wet overcast condition.
How Overcast Differs from Other Sky Conditions
Overcast is the final step in a continuum of sky conditions used in weather reporting that quantifies cloud cover in oktas. This scale ranges from “Clear” (0 oktas) to “Overcast” (8 oktas), with several distinct categories in between. For example, “Few” clouds indicates 1 to 2 oktas of coverage, and “Scattered” clouds covers 3 to 4 oktas.
The distinction between “Broken” and “Overcast” is important for aviators and forecasters. Broken clouds, abbreviated as BKN, refer to a sky that is 5 to 7 oktas covered, meaning a majority of the sky is obscured, but distinct breaks of blue sky are still visible. Once the cloud cover reaches 8 oktas, the classification changes to Overcast, indicating that the cloud layer is continuous from horizon to horizon, and no breaks are present.