Mostly Cloudy (구름 많고)

A sky-condition expression used in Korea Meteorological Administration (KMA) forecasts, referring to a state in which cloud cover is about 6 to 8 tenths.

Mostly Cloudy (구름 많고)

Overview

"Mostly cloudy" (구름 많고) is one of the representative expressions used by the Korea Meteorological Administration (KMA) of South Korea when indicating sky conditions in weather forecasts. It refers to a state in which the amount of clouds distributed across the entire sky is about 6 to 8 tenths. In official forecast items it is written in the noun form "구름많음" (mostly cloudy), but within forecast sentences it is used in a predicate form, as in "구름 많고 소나기가 오겠습니다" ("It will be mostly cloudy with showers"). This corresponds to an intermediate stage between "clear" and "cloudy," meaning a state in which clouds cover a considerable portion of the sky but sunlight is not completely blocked.

Main Content

KMA Sky Condition Classification Criteria

The KMA classifies sky conditions into four levels according to the proportion of the sky covered by clouds (cloud cover, 雲量) when an observer regards the entire sky as 10 tenths.

Clear: cloud cover 0–2 tenths. Most of the sky is open, and sunshine is sufficient.
Partly Cloudy: cloud cover 3–5 tenths. Blue sky and clouds are mixed.
Mostly Cloudy (구름 많고/구름많음): cloud cover 6–8 tenths. More than half of the sky is covered by clouds, but sunlight shines through gaps between clouds or part of the blue sky is visible.
Cloudy: cloud cover 9–10 tenths. The sky is almost completely covered by clouds, and the sun is not visible.

In other words, "mostly cloudy" corresponds to the third level and can be understood as the state just before becoming cloudy. However, since cloud cover involves an observer's subjectivity, it is calibrated using automatic observation equipment (ceilometers), satellite imagery, and numerical model data together.

How Clouds Form

Clouds form when water vapor evaporated from the surface rises on updrafts, cools, and attaches to condensation nuclei, changing into tiny water droplets or ice crystals. Depending on the cause of ascent, they are divided into orographic clouds, frontal clouds, convective clouds, and so on; among these, cumulus, stratocumulus, altostratus, and similar clouds are commonly observed in skies under "mostly cloudy" conditions. Even with the same cloud cover, actual brightness and sunshine duration vary greatly depending on the thickness, height, and type of clouds.

Use in Forecast Wording

"Mostly cloudy" is often used in combination with precipitation and temperature information rather than alone. For example, a sentence such as "전국이 구름 많고 오후 한때 소나기가 오는 곳이 있겠습니다" ("The whole country will be mostly cloudy, with some places receiving showers at times in the afternoon") is a typical form that conveys localized precipitation caused by atmospheric instability. It is also combined with air-quality information, as in "구름 많고 미세먼지 농도는 보통" ("Mostly cloudy, with fine dust concentrations at normal levels").

Effects on Daily Life and Industry

Sunshine and temperature: When there are many clouds, solar radiation reaching the ground during the day decreases, lowering the maximum temperature somewhat; at night, clouds send surface radiation back, tending to raise the minimum temperature.
Solar power generation: When cloud cover is 6–8 tenths, power generation efficiency can fall significantly compared with clear days, so sky-condition information is important for forecasting power output.
Agriculture and tourism: Reduced sunshine hours affect crop growth and also influence outdoor activities and tourism plans.
Aviation and transportation: It can be linked to reduced visibility and the possibility of turbulence, so information is provided to pilots and air traffic control.

Comparison with Similar Expressions

In English-speaking forecasts, conditions are subdivided into "partly cloudy" and "mostly cloudy," and the Korean "구름 많고" generally corresponds to mostly cloudy. Meanwhile, "구름 조금" corresponds to partly cloudy, and "흐림" corresponds to cloudy/overcast.

Recent Trends

In 2024–2025, forecasting technology is rapidly being reorganized around artificial intelligence-based models. The KMA is promoting deep learning-based medium-range forecast correction technology and the introduction of AI forecast models to improve the accuracy of sky conditions and precipitation probabilities; data-driven forecast models such as Google's GraphCast, Huawei's Pangu-Weather, and the European Centre for Medium-Range Weather Forecasts (ECMWF)'s AIFS are also achieving results in cloud-cover prediction comparable to traditional numerical models. In addition, real-time cloud-cover analysis combining satellite imagery and ground observations, and improvements in the spatial resolution of very-short-range forecasts, are continuing. On the climate change side, research on cloud-radiation feedback—whether clouds amplify or suppress warming—is active, and aerosol-cloud particle interactions and the decreasing trend of low-level marine clouds are treated as major issues. In everyday life, application areas are expanding, including renewable energy output forecasting, urban heat island and solar irradiance management, and combined analysis of fine dust and clouds.