Why is cloudy day worse than sunny day?

I think when you say the sun, you mean very hot. A cloudy day doesn't mean it must be cool, but it may be hotter than a sunny day!

Cloudiness refers to the thick clouds in the sky, which have certain reflection on solar radiation, but it cannot be said that it is completely isolated from solar radiation. Water vapor and carbon dioxide in the air will absorb some solar radiation and some will be absorbed by the earth. On cloudy days, the atmospheric inverse radiation is strong, so the temperature difference will be smaller.

Therefore, even on cloudy days, the temperature in the air is not necessarily low, and it may even feel hotter than when the sun shines.

And because of the heat island effect, the temperature in the city center will be much higher than that in the suburbs. Cloudy days lead to air circulation, and the carbon dioxide produced by the city cannot be well dispersed to the suburbs for oxygen conversion (the suburbs are mostly plants, and plants can produce oxygen through breathing), so you will feel that the weather is very hot and the sun is still shining on you.

Carbon dioxide is the most important cause of global warming. There are many reasons for carbon dioxide, such as the decrease of green space, air pollution in urban factories and so on. Still pay attention to environmental protection and cool the earth!