What is the difference between partly cloudy and partly sunny?
What is the difference between isolated thunderstorms and scattered thunderstorms?

Above are two very common questions this time of year. In an effort to convey as much information as possible in the shortest amount of time, we sometimes get caught in a rut when it comes to our forecast language. Here are some guidelines:

Sunny (or at night, Clear): Sky is nearly free of clouds… no more than 20% of the sky is covered by clouds.

Mostly sunny (or at night, Partly Cloudy): Clouds cover about 20-40% of the sky.

Partly Sunny: Clouds cover 40-60% of the sky

Mostly Cloudy: Clouds cover 60-80% of the sky

Cloudy: Clouds cover more than 80% of the sky

Overcast: Clouds cover the entire sky… implies no breaks in the clouds at all.

For night forecasts, there are minor modifications made in the terms… so that Partly Cloudy includes up to 50% cloud cover, and Mostly Cloudy is above 50% cloud cover.

Other commonly used phrases:

A mixture of clouds and sunshine implies the amount of cloud cover will vary during the day… but in a gradual and subtle way through time.

Intervals of clouds and sunshine implies the amount of cloud cover will vary more quickly and dramatically during the day.

Scattered thunderstorms (around 30 to 40%) suggest the coverage of thunderstorms across the viewing area will be greater than isolated thunderstorms (around 20 to 30%).

A shower or thunderstorm likely suggests that there is a better that 50/50 chance of any one spot in the area will have a shower or thunderstorm. Showers and thunderstorms likely suggests more than one shower or thunderstorm will occur in most spots during the forecast period.

The chance of rain (in percent) is the chance that any given point in the viewing area will have rain during the forecast period. For example, if the forecast for Central Virginia is for a 70% chance of an afternoon shower or thunderstorm, it means that any particular community in Central Virginia has a 7 out of 10 chance of getting a shower or thunderstorm during the afternoon.

 

Sean Sublette

ABC 13 Meteorologist