為其麼天氣愈潮濕便會愈悶熱？

You feel muggy in a humid weather is because your sweat cannot evaporate rapidly into the air. Since rapid evaporation of sweat requires the absorption of latent heat, thus bringing away the heat from your body.
The rate of evaporation of water (or sweat) depends much on the humidity of the surrounding air. Evaporation takes place because there are molecules in water that have sufficiently high energy against the intermolecular attractive force between water molecules, thus could escape into the air.
But at the same time, water molecules in the water vapour present in the humid air could also re-enter into the water. The more humid the air is, the more water molecules would re-enter into the water.
Therefore, the difference between the number of water molecules escaping from the water and the number of water molecules re-entering into the water from the air gives the net number of molecules evaporating from the water. In normal circumstances, the no. of molecules re-enter into the water from the wet air is smaller than the no. of molecules escape from the water, thus evpaoration takes place.
However, if the air becomes more humid, there are more water molecules present in the air, the no. of molecules re-enter into the water from the air increases. The makes the net no. of molecules leaving the water to decrease, thus reducing the evaporation rate.
When the air is completely saturated, the no. of molecules re-enter into the water equals the no. of molecules escaping from the water. The net loss of molecules from the water now becomes zero. The water (sweat) could not longer evpaorates, and hence could not bring away the heat from your body by the process of latent heat absorption. You would have a muggy feeling.

(上面錯，唔好信- -，你就算唔濕都係周圍都熱ga啦- -。)

其實係因為人主要係用汗水黎調節體温，而由於出面太濕，好難將水份蒸發/排走，所以唔能夠好快咁排熱，所以你會覺得好悶熱。

因為潮濕即係有水份

有太陽會將水份蒸發

令周圍都會悶熱