What Is Overcast?
Overcast is a sky condition in which clouds cover nearly all or all of the visible sky. Instead of individual clouds separated by large areas of blue sky, the clouds form a widespread layer overhead.
Overcast skies often appear grey because thick cloud cover blocks much of the direct sunlight reaching the ground. However, the brightness of an overcast sky can vary considerably depending on the thickness and type of clouds present.
Overcast conditions do not automatically mean rain or snow. Some cloud layers produce no precipitation, while others are associated with prolonged rain, snow or drizzle.
Related Conditions
| Condition | Description |
|---|---|
| Low Cloud Cover | Clouds that form relatively close to the ground and can sometimes create widespread grey skies. |
| Cloud Ceiling | The height of the base of the lowest widespread cloud layer covering a significant portion of the sky. |
| Nimbostratus Clouds | Thick, widespread clouds commonly associated with prolonged rain or snow. |
| Thick Cloud Cover | Dense cloud cover that blocks much of the direct sunlight reaching the surface. |
| Diminishing Cloudiness | A period when widespread cloud cover begins to break apart and clearer areas gradually develop. |
Related Alerts
| Alert | Description |
|---|---|
| Rainfall Warning | Warns when significant rainfall is expected to reach warning criteria. |
| Snowfall Warning | Warns when significant snowfall is expected to reach warning criteria. |
| Freezing Rain Warning | Warns when freezing rain is expected to create hazardous ice accumulation. |
| Freezing Drizzle Advisory | Issued when freezing drizzle is expected to create icy and hazardous surfaces. |
| Winter Storm Warning | Warns of a significant combination of hazardous winter weather conditions. |
What Causes Overcast Skies?
Overcast skies develop when enough moisture and rising air are present to create a widespread cloud layer that covers nearly the entire sky.
Large weather systems are a common cause. As air rises around fronts and low-pressure systems, it cools. If the air contains enough moisture, water vapour condenses into cloud droplets or ice crystals and clouds can spread across a large area.
Overcast conditions can also develop when moist air becomes trapped in the lower atmosphere. This can create widespread stratus clouds and low cloud cover that may persist for hours or days.
The type and thickness of the clouds depend on the atmospheric conditions. Thin overcast layers may allow some sunlight to remain visible, while thick clouds can make daytime skies appear considerably darker.
If the cloud layer becomes deep enough and contains sufficient moisture, precipitation may develop. Nimbostratus clouds, for example, can produce widespread and persistent rain or snow.
Overcast skies begin to clear when moisture decreases or atmospheric conditions become less favourable for cloud formation. Breaks may gradually develop in the cloud layer before skies transition to mostly cloudy, partly cloudy or clear conditions.
FAQ
Overcast means nearly all or all of the visible sky is covered by clouds.
Mostly cloudy skies still contain noticeable breaks in the cloud cover. Overcast conditions have little or no clear sky visible.
Cloud droplets and ice crystals scatter sunlight. Thick cloud layers allow less direct sunlight to reach the ground, giving the sky a grey or darker appearance.
Overcast skies clear when the cloud layer breaks apart or moves away. This can happen as drier air arrives, a weather system departs or atmospheric conditions become less favourable for cloud formation.