[Marika (OP)]

Angelo wants to know:

In winter, if the temperature reaches 5 degrees C, it almost feels like short sleeve weather.  If that happens in the summer, I'll look for a heavy jacket.  What changes in my body to acclimate between the seasons?

What do you think?

[evan_au]

One change around your body is that you wear a different set of clothes between summer and winter.

[Colin2B]

How warm or cool you feel depends on how quickly your skin loses heat. As @evan_au says you automatically wear heavier clothers in winter.
Also, the climate does affect how quickly we lose heat. We are all familiar with wind chill which makes us feel colder than ambient temperature would suggest, but less familiar with humidity. Higher humidity makes us feel warmer because it reduces the rate of evaporation of sweat from the skin. So both wind and humidity have to be taken into account, and I have often noticed that the winter’s mornings you mention, although quite dry, are often very still with no noticable wind.