Cloudy and overcast both describe skies filled with clouds, but they refer to different amounts of cloud coverage. In meteorology, “cloudy” can describe any sky with significant cloud cover, while “overcast” specifically means the sky is completely covered, with no blue patches visible. The distinction matters more than you might expect, affecting everything from UV exposure to solar panel output.
How Meteorologists Define Each Term
Weather services measure cloud cover in oktas, which divide the sky into eighths. Each category of sky condition corresponds to a specific range:
- Clear or sunny: 0/8 of the sky covered
- Mostly clear: 1/8 to 2/8
- Partly cloudy: 3/8 to 4/8
- Mostly cloudy: 5/8 to 7/8
- Overcast (cloudy): 8/8
The National Weather Service treats “cloudy” and “overcast” as equivalent when the entire sky is covered at 8/8. But in everyday conversation and many forecasts, “cloudy” is used loosely for anything from mostly cloudy (5/8) to fully covered skies. “Overcast” is more precise: it always means 8/8, with greater than 90% of the sky blocked by clouds and little to no blue visible.
This is why a forecast saying “cloudy” can still mean you’ll see patches of sun, while “overcast” tells you not to expect any breaks in the cloud layer.
What Each Sky Actually Looks Like
On a mostly cloudy day, you’ll still see some blue sky between cloud formations. Shadows may come and go as the sun peeks through gaps. The light shifts noticeably as clouds drift, alternating between brighter and dimmer moments.
An overcast sky is uniform. The clouds form a continuous sheet, typically stratus clouds that appear gray or grayish-white. There are no gaps, no visible sun disk, and no shadows on the ground. The light is flat and even in every direction. If the overcast layer is thick enough, it can blot out the sun entirely, making it difficult to tell where in the sky the sun even is.
Overcast conditions can also come from higher cloud types. Altostratus clouds (mid-level sheets) and cirrostratus clouds (high, thin veils) can each cover the entire sky. Cirrostratus layers are thinner and may still let you see a hazy outline of the sun, while thick altostratus and low stratus layers block it completely.
Why Aviation Treats the Difference Seriously
Pilots and air traffic controllers use a more granular system. In aviation weather reports, “broken” means 5/8 to 7/8 cloud cover, and “overcast” means 8/8. This distinction determines whether a cloud ceiling is reported. A ceiling is the height of the lowest cloud layer that qualifies as broken or overcast, and it directly affects whether a pilot can land visually or needs to rely on instruments. A mostly cloudy sky with gaps might still allow visual approaches, but a solid overcast layer at low altitude can close an airport to visual traffic.
UV Exposure Through Different Cloud Levels
One of the most practical reasons to understand the difference is sun protection. Clouds reduce ultraviolet radiation, but the amount depends heavily on how much sky they cover. The U.S. National Weather Service and EPA estimate that scattered clouds still transmit about 89% of UV radiation, broken clouds allow about 73% through, and a full overcast drops transmission to roughly 32%.
That means on a mostly cloudy day, you’re still getting a substantial dose of UV. Even under full overcast, nearly a third of the UV radiation reaches you. Sunburn on an overcast day is genuinely possible, and on a partly or mostly cloudy day, it’s almost as likely as under clear skies.
Effects on Solar Panels
Solar panel output tracks closely with cloud coverage. Under light cloud cover, panels can still produce 70-80% of their rated capacity because enough sunlight penetrates the thinner cloud layer. With moderate, heavier coverage, that drops to 40-60%. Under thick overcast skies or storm clouds, output falls to just 10-25% of normal.
If you have solar panels and your forecast says “mostly cloudy,” you can still expect decent energy production. A day described as “overcast” will produce noticeably less, and heavy overcast with dark storm clouds will cut your output dramatically.
How Light Quality Changes
Photographers and anyone working with natural light will notice a real difference between the two conditions. A mostly cloudy sky produces mixed lighting: directional sunlight through gaps creates contrast and shadows, while clouds soften light in other areas. The color and intensity shift constantly.
An overcast sky produces diffuse, even light with a color temperature around 6,000 Kelvin, which skews slightly cool and blue compared to direct sunlight (around 5,200-5,500K). This flat lighting eliminates harsh shadows, which is why portrait photographers often prefer overcast days. The tradeoff is less contrast and a muted quality that can make landscapes look dull. Camera white balance settings typically include separate presets for “cloudy” and “overcast” to compensate for these differences in color temperature.
The Short Version
“Mostly cloudy” means clouds dominate but some sky is visible, and you’ll still get direct sun at times. “Overcast” means complete cloud coverage with no blue sky and no direct sunlight. In weather forecasts, “cloudy” at its strictest definition equals overcast (8/8 coverage), but it’s commonly used for anything above 5/8. When precision matters for planning outdoor activities, solar energy, or travel, treat “overcast” as the more specific and more limiting condition.