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What happens when weather gets hot and humid?

If you went out for a run on a hot and humid day you have probably noticed that whatever shirt you are wearing it gets much wetter than during the dry months. Why is that?

1. We sweat more

It is all about body thermoregulation. When our bodies feel hot (and they usually do when we run, or bike, or do other exercise) the pores in our skin open up and release water and salts. In other words we sweat. The water carries heat away from the body cooling the body in the process.

When the weather is dry the water evaporates rather quickly and the body cooling machine works pretty fast. But on a humid day – like the 90% plus relative humidity day that is quite common during summer months in Hong Kong – the sweat will take its time to evaporate. That means that our bodies cool down slower and may start sweating even more trying to speed up that cooling.

2. The humidity slows down the evaporation

Having something to get that sweat away is always good idea. And that’s what the quick dry materials were created for.

The way the quick dry garments work is that fabric wicks the moisture away from the skin to the surface of the fabric. This helps to keep our body and the inside of the shirt reasonably dry and comfortable. The top layer of the shirt will probably become wet with all the sweat the fabric transported there from the body. How wet it depends how much cooperation the fabric gets from nature.

Wicking fabric illustration

Wicking fabric illustration

On a nice dry day the moisture that makes it to the top of the fabric will evaporate quickly. On a very humid day when the air is already saturated with water the evaporation will be much slower. It is more less the same as the difference between trying to dry laundry in very humid room or in a room with dehumidifier on full power. Even if it is a humid day the breeze and wind will help to speed up the evaporation. If the weather is humid but cooler the lower temperatures also help as we simply do not need to cool down (sweat) so much. On the other hand the hot and humid day as we know from Hong Kong summers creates the perfect storm – more moisture wicked away by the quick dry fabrics from the skin, slower evaporation to the air from the surface. The specific constructions of the quick dry fabrics may expand the evaporation area and speed up the evaporation process but the hot and humid subtropical conditions are always a formidable adversary.

3. What may help

The good quality quick dry materials will do the job whether it is dry or humid day. They will wick the sweat away from the skin. The big difference is that on a very humid day the nature simply can’t keep up with the performance of the fabric. The construction and texture of the outer layer of quick dry fabric can expand the evaporation area and speed up the sweat evaporation but in the hot and humid subtropical conditions it will not perform miracles.

hot and humid run

Hot day out there

So what to do when we plan to keep on sweating for longer than the garment of our choice can handle? Having a spare shirt to change is always a good idea. On a hot day either very loose clothes (promoting the ventilation) or very tight fitting ones (fabric and skin in direct contact) are the way to go. It may be a good idea though to stay away from those baggy ones on hot and very humid day when the clothes are almost guaranteed to get wet and heavy. Opting for tee or jersey with zipper may also help as opening the zipper helps to increase the ventilation.  And of course we can also move faster to generate more breeze.