Why does it feel more sticky in summer than winter, and relative humidity is defined as the amount of water vapor in the air compared to what?

What this question is really asking

You need two linked ideas: what “relative humidity” compares, and how humidity changes how well sweat can evaporate from your skin.

Relative humidity: what is it compared to?

Relative humidity (RH) compares the air’s actual water vapor to the saturation level at the same temperature:

$$RH = \frac{\text{actual water vapor (or vapor pressure)}}{\text{saturation water vapor at that temperature}} \times 100\%$$

So, RH answers: “How close is the air to being full of water vapor for this temperature?”

Why summer air makes you feel sticky

Your body cools itself mainly by evaporating sweat. Evaporation removes heat from your skin (latent heat of vaporization).

When RH is high, the air is already closer to saturation, so it cannot accept much more water vapor. That reduces the evaporation rate of sweat. Less evaporation means:

• less cooling, so you feel hotter
• sweat stays on your skin longer, which feels sticky

Why this is less of a problem in winter

Winter air is usually colder and often has lower humidity. Even if the RH is not always low, cold air typically contains less water vapor overall, and indoor heated winter air often becomes very dry. Drier air lets sweat evaporate more easily, so you feel less sticky.

Quick check with an example

If the air can hold (at that temperature) a maximum of 20 units of water vapor but actually contains 10 units, then:

$$RH = \frac{10}{20} \times 100\% = 50\%$$

At a higher RH, evaporation slows down, which is why summer humidity feels muggy.