Published on March 1, 2024

Weather Terms: What Does Partly Cloudy Mean?

Paying attention to the weather is critical when planning your day. Surveys show that about half of all Americans read, watch, or listen to the weather forecast at least once daily. 20% also say that they check it multiple times a day.

When checking the weather, you may find yourself dealing with some jargon and vocabulary you won’t find elsewhere. Partly sunny, mostly sunny – but what does partly cloudy mean? These terms can feel vague to people unfamiliar with them, adding to the confusion.

If you’re curious about understanding weather conditions, we’re here to help guide you. Read on for a brief overview of the terms you’ll encounter when checking the weather.

What Does Partly Cloudy Mean?

Let’s get right into it: partly cloudy is one of the many terms that meteorologists will use to describe the weather. The confusion comes from the vagueness of “partly.” Is only part of the sky cloudy, and are those clouds storm clouds?

“Partly cloudy” is defined as 3/8 to 5/8 of the sky being covered by clouds. Put another way, this means that about 37% to 62% of the sky is covered in clouds at these moments. It isn’t a full block out of the sky with storm clouds, and it isn’t a clear sunny day, fitting in between.

What About Partly Sunny?

Another term you may hear is “partly sunny,” which seems to indicate the same thing. If 5/8ths of the sky is covered by clouds, doesn’t that also mean it’s partly sunny with the other 3/8ths?

“Partly sunny” is a term that only works during the day. You wouldn’t refer to a midnight sky with a full moon as “partly sunny,” given that the sun is wholly absent.

But at night, “partly cloudy” can still apply. As such, “partly sunny” is used during the day, while “partly cloudy” is used at all times. During daytime hours, “partly cloudy” can also mean that there’s a chance of storms.

That said, these terms are also often used interchangeably, so feel free to take one as the other. You also may see terms like “mostly sunny.” That may indicate that it’s partly cloudy but with fewer clouds.

Effects of a Cloudy Day

Why does it matter if the sky is partly cloudy? If the forecast calls for rain, we know to expect storms; and if the forecast is calling for a bright, sunny day, we know we can make plans outdoors. How will a cloudy day affect your plans?

Solar Power

Solar power is one of the first things we often consider. Solar panels need direct sunlight to work properly. If you’ve invested tens of thousands of dollars into a solar array, having it rendered less effective by some clouds is frustrating.

Understanding cloudy days and solar power isn’t clear-cut. Some solar panels may have better efficiency and features that let them perform well during cloudy days.

In general, cloudy days will reduce how much energy your panels generate. The clouds can block the sun, reducing how many rays hit your panels. As the clouds pass, the power generation will return.

But did you know that, for a brief moment, clouds can improve your solar panels’ generation? The Cloud-Edge effect occurs when the sun reflects off the edge of a passing cloud. When it does so, this can magnify the beams, increasing the power.

Level of Precipitation

Strictly speaking, a cloudy day does not necessarily mean a stormy day. You can have a fully overcast day without there being any more precipitation than on a clear, sunny day.

You shouldn’t take a partly cloudy day to mean that you’ll encounter storms. However, when a storm moves in, the storm system can often cause the weather to turn cloudy. As such, we suggest always checking your weather report to see if the cloudy day is indicative of an incoming storm.

Other Weather Conditions

We’ve looked at partly cloudy and sunny, but what are some other types of weather you should expect? Most terms are rather self-explanatory, which makes them easy to understand. Here are some of the most common weather terms.

Clear or Sunny

The terms “clear” and “sunny” are often used interchangeably. However, you’ll more often see “clear” as a term for the late evening or nighttime. As with “partly sunny”, this is because describing nighttime as “sunny” simply doesn’t work.

Using the scale of eighths, this indicates a cloud coverage of 1/8th or less. It is the “bottom” of the scale in terms of cloudiness.

Mostly Clear or Sunny

Next on the scale is “mostly clear” or “mostly sunny.” The term picked depends on the time of day.

A mostly clear day indicates that there are very few clouds in the sky, but that they are present. On the eighth scale, this indicates a cloud coverage of 1/8 to 3/8.

Mostly Cloudy

Skipping over partly cloudy, a mostly cloudy day indicates that the majority of the sky is covered in clouds. The term is used regardless of the time of day and isn’t a term that you pair with “sunny.”

The term “mostly cloudy” means that the sky is at least 5/8ths obstructed. It can go as high as 7/8ths.

Cloudy

The last term for the sky condition is “cloudy.” As you may suspect, this is the last term on the scale and can mean that the sky is fully obstructed.

It’s also important to note that a cloudy sky condition is usually related to a high level of precipitation. If there’s a high chance of rain, the forecast often leaves out that it’s cloudy. That’s because cloudy conditions are inferred by a rainstorm.

Understanding Weather Conditions

What does partly cloudy mean, and how does it affect your day? A partly cloudy sky condition means that the sky has some clouds, but that it’s mostly uncovered. Higher cloud density can also relate to a higher level of precipitation, which means it’s important to pay attention to the changing weather conditions.

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