Question

Why do we sweat more on a humid day than on a hot dry day?

Answer 1

In hot dry summer days the temperature of our body is high, and as a mechanism of cooling our bodies, we sweat and since the atmosphere around are bodies is dry (less amount of water vapour is present), the sweat evaporates, and thus, lowering our body temperature.

In hot humid days, sweating occurs due to high body temperature, but since the air around our bodies already contains water vapour in high amount (humidity), the sweat does not evaporate and we feel that we are perspiring a lot.
Therefore we feel that we are "sweating" more on a humid day