Overcast sky conditions occur when clouds cover all or most of the sky and cause low visibility conditions. This makes the sky look dull and gray and it doesn't necessarily mean that precipitation will fall, though the chances for rain or snow do increase on overcast days.
How Meteorologists Define Overcast Skies
In order to classify the sky as overcast, 90 to 100 percent of the sky needs to be covered by clouds. It does not matter which types of clouds are visible, just the amount of the atmosphere that they cover.
Meteorologists use a scale to define cloud cover and "oktas" are the unit of measurement. This weather station model is represented by a pie chart divided into eight slices and each slice represents one okta. For an overcast sky, the pie is filled with solid color and the measurement is given as 8 oktas.
The National Weather Service uses the abbreviation OVC to indicate overcast conditions. Typically, individual clouds are not seen in an overcast sky and the penetration of sunlight is noticeably less.
Although fog can cause low visibility on the ground, overcast skies are those that are higher in the atmosphere. Other conditions can lead to low visibility as well. These include blowing snow, heavy rain, smoke, and ash and dust from volcanoes.
Is It Cloudy or Overcast?
Even though it may seem like overcast is just another way to describe a cloudy day, there are distinct differences. That is why a weather forecast says the day will be partly cloudy, mostly cloudy or overcast.
The weather station model is used to distinguish cloudy from overcast skies. Mostly cloudy (or broken) is classified as 70 to 80 percent cloud cover or 5 to 7 oktas. This is less than the 90 to 100 percent (8 oktas) used to define overcast skies. On mostly cloudy days, you will be able to see separation in the clouds; on overcast days, the sky looks like one big cloud.
Does Overcast Mean It's Going to Rain?
Not all clouds lead to precipitation and certain atmospheric conditions must be present to produce rain or snow. This means that it's not necessarily going to rain just because the sky is overcast.
Overcast Skies Can Warm You up in Winter
In the winter, an overcast sky does have its advantages. It may look dreary outside, but the clouds act as a blanket and will actually help warm up whatever's underneath. This is because the clouds prevent the heat (infrared radiation) from escaping back into the atmosphere.
You can really notice this effect on winter days when the winds are calm. One day may be bright and sunny with no clouds in the sky though the temperatures can be really cold. On the next day, clouds may roll in and even though the winds have not changed, the temperature will rise.
It is a bit of a give and take with winter weather. We like the sun in the middle of winter because it feels nice, yet it might be too cold to be outside. Likewise, an overcast day can be dreary, but you can probably stand to be outside longer, which can be nice as well.