Relative humidity is defined as the ratio of water vapour content to the water vapour capacity of the air at a given temperature. When the realtive humidity is 100%, the air is 'holding' all the vapour that it can at a given temperature. Under these conditions the air is termed saturated, because no more vapour can evaporate into the air. Now, sweating produces cooling because the high velocity (high temperature) molecules have sufficient energy to break free from the surface and evaporate, causing the average speed of the remaining molecules to decrease, and these lower speeds correspond to lower temperatures. Hence in high humidity conidtions, sweat cannot evaporate and clings to ths skin and it feels warmer than it otherwise would in lower humidity conidtions.