Which light is stronger, cloudy or sunny?

Recently, many readers are puzzled by our frequent forecast of cloudy days. Looking up the data, the difference between sunny and cloudy days is often not very intuitive, but the difference of cloud cover. Clouds in clear sky are less than 1%, partially cloudy clouds account for 20-30%, cloudy clouds account for 40-70%, and more than 80% are considered cloudy. The judgment of cloud cover in the sky often depends on the observation of experienced forecasters. In the forecast, there are a few cloudy days, and there is little difference between a few cloudy and sunny days. It turns out that many people think that in hot summer, cloudy weather clouds may absorb some ultraviolet rays, making the sun less dazzling. However, some studies have also said that ultraviolet rays are stronger on cloudy days. Some foreign researchers have done experiments to measure the light intensity on sunny and cloudy days on an island. The experimental results show that the light intensity in sunny days is 900 watts per square meter, while that in cloudy days reaches 1400 watts per square meter. The reasons are analyzed. On a cloudy day, there are cumulus clouds like cotton candy in the sky. Although the sky is full of holes, cumulus clouds reflect the upper sunlight like a mirror, which is twice or even several times as much as the sunlight passing through the holes in the clouds. As a result of this theory, people in subtropical areas may get sunburn more easily than people in tropical areas. Because there are many cumulus clouds in subtropical areas in summer, it gives people the illusion that ultraviolet rays are not strong, so that they forget to apply sunscreen, open an umbrella and other necessary sun protection measures, and are psychologically paralyzed by the sky. In summer in tropical areas, there are many thunderstorm clouds in the sky, which completely cover the sun. Without reflection, there is nothing, and ultraviolet rays are completely weak. This conclusion reminds me of an example I experienced personally. Military training began in college, and there were white clouds in the sky every day. In private, I told my classmates that this weather is excellent for military training. Unexpectedly, a summer passed, and all the students were as tanned as people in the equatorial region, only to find that a group of freshmen were all blindsided by God.