Why doesn't it rain on foggy days?

This is because the low ground temperature is a foggy condition, not a rainy condition.

Fog is the condensation phenomenon of water vapor in the air near the ground. There are two basic conditions for the formation of fog, one is that the air near the ground is rich in water vapor, and the other is that there is a low temperature layer near the ground. The water vapor in the low temperature air is saturated because of the low temperature, and then condenses into fog.

The conditions for the formation of rainfall are often high ground temperature and low altitude temperature. The hot and humid air mass on the ground rises because the density is lower than that of the cold air mass, and the temperature drops continuously during the rising process. However, the saturated water content of air decreases with the decrease of temperature. When the temperature of moist air mass rising from the ground drops to air humidity saturation, gaseous water begins to condense into water droplets. If the temperature is low enough, water vapor can directly condense into ice.

As long as the water vapor supply is sufficient and the temperature is low enough, small water droplets will continue to condense and collide to form large water droplets. When the gravity of large water drops exceeds the buoyancy of air, water drops will fall. As long as it evaporates without touching the dry hot air mass halfway, raindrops will always fall to the ground and become rain. So when it fogs, it won't rain again.

In autumn and winter, after the cold air in the north moves southward, with the weather turning fine and sunny, the moisture content in the air gradually increases, which is easy to form radiation fog, so the fog in autumn and winter can often predict the fine weather tomorrow.

The fog in spring and summer is different. Most of it comes from the warm and humid air flow at sea. When it hits the cold ground, the air in the lower layer becomes cold and the water vapor condenses into fog. This fog is called advection fog. It is formed when warm and humid air from the ocean invades the mainland and suddenly encounters cold. When these warm and humid air currents meet the dry and cold air currents in the mainland, it will naturally be rainy. So the fog in spring and summer indicates rainy weather.

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