Mostly cloudy, overcast, or chance of rain: what each forecast tells you

Sky condition · How much opaque cloud cover

“Mostly cloudy” is a cloud forecast—not a rain probability

National Weather Service forecast terminology describes the predominant or average portion of the sky expected to be covered by opaque clouds. Opaque clouds block the sun; thin, transparent high clouds do not count in the same way.

The standard wording moves from clear or sunny through mostly clear or mostly sunny, partly cloudy or partly sunny, mostly cloudy, and cloudy. “Overcast” is commonly used for a completely covered sky. The categories summarize sky coverage over a period; they do not by themselves tell you whether rain will reach your exact location, how hard it may rain, or whether a storm will be dangerous.

Day and night wording · Same planning question

“Partly sunny” and “partly cloudy” are companion descriptions

NWS sky-condition tables pair “partly sunny” with “partly cloudy” in the same middle cloud-cover category. Daytime wording can emphasize the sun; nighttime wording cannot. Likewise, “mostly sunny” is the daytime companion to “mostly clear.”

Treat the words as an estimate of average sky coverage, not a promise that every hour will look the same. Clouds can increase or decrease during the period, and a single forecast icon cannot show every short-lived change. For a walk, game, drive, or photo outing, open the hourly forecast and compare the hours that matter.

Chance of rain · A separate probability

Read the percentage for your point and forecast period

NWS defines probability of precipitation, or PoP, as the probability that the forecast grid point will receive at least 0.01 inch of precipitation during the stated period. A 40% chance is not a claim that rain will fall for 40% of the period or over exactly 40% of the area.

PoP answers whether measurable precipitation reaches the point. It does not state the expected amount, intensity, duration, or hazard by itself. A mostly cloudy period can stay dry, while a partly sunny period can still include a brief shower. Read the detailed forecast for timing, amounts, thunder, wind, and other context.

Forecast icon · A compact summary

An icon can combine sky, precipitation, wind, and timing

NWS point-forecast icons are an at-a-glance summary. Some can be split to show two different conditions within one forecast period, but even a dual icon cannot carry every detail in the written forecast.

Read the icon label and precipitation percentage, then open the detailed and hourly forecasts. If the icon says “mostly cloudy” while the text mentions a late shower, both can be true: one describes the overall sky and the other describes a precipitation possibility within the period.

A practical outdoor check

Use the sky description as one layer of the plan

1. Start with the exact place and time. A citywide summary or nearby airport observation may not match your trail, field, beach, or destination.
2. Read the sky words. Use clear, partly cloudy, mostly cloudy, or cloudy to understand expected opaque cloud coverage.
3. Read precipitation separately. Check the chance, timing, type, and any forecast amount rather than inferring rain from the cloud label.
4. Check other exposure. Clouds do not automatically remove UV risk, heat, cold, wind, lightning, poor air quality, or low visibility.
5. Check active alerts last—and let them outrank the icon. Follow the area, timing, severity, and protective action in the official message.

Different products · Different decisions

Sky cover never replaces an outlook, forecast, watch, or warning

Outlook

An earlier, broader signal that a weather pattern may support a hazard. Use it to identify days that need closer checking; it is not a local warning.

Forecast

The expected conditions for a place and period. Sky cover, rain chance, timing, wind, and temperature can change as new observations arrive.

Watch

Conditions are favorable for the named hazard in or near an area. Review the plan, route, shelter, and alert methods now.

Warning

The named hazard is occurring, imminent, or detected in the warned area. Follow the warning’s location, timing, and protective instructions immediately.

A pleasant-looking cloud label does not cancel a warning, and a gloomy-looking sky does not create one. The official alert is written for a particular hazard, area, and time.