Eli5: I keep my AC at 21 celsius all year. How come I’m hot in a shirt in summer and cold with a sweather in winter?
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Humidity:
Summer is humid, winter is dry. 21C humid feels hotter than 21C dry, because your body has a harder time cooling itself.
(Also MAYBE direct sunlight, which is radiant heat which would heat you, but not the air around you, so your thermostats reads 21C, but in reality you’re heated much more than that due to sunlight, but I doubt this is applicable to you unless you’re in direct sunlight)
Your body tends to acclimatize to what the temperature is. so if its hot most of the time, you’ll get used to hot temperatures and feel that 21 is cold. If its cold you’ll get used to the cold, and 21 is warm.
that’s why when you go outside in the winter it’s really cold, but then you get used to it. Still cold, but as long as you’re wearing enough layers, you’ll be fine. Same as in the summer, when you go outside from the A/C you don’t sweat right away, but then you start sweating as you get used to it as your body tries to cool your body down.
Most likely, this is due to humidity. In the winter time, humidity is usually much lower because cooler air holds less moisture. In the summer, humidity rises which can make it feel warmer.
Example: 21c (69.8f) at 50% humidity feels like 21c (69.8f)
21c (69.8f) at 80% humidity feels like 22c (71.6f).
Others have posted good answers.
I would like to add that there is a neat “Thermal comfort tool” calculator made by some people at UC Berkeley where you can enter variables and see if the room would be comfortable for a human.
Here you can visualize how more than just the temperature matters.
https://comfort.cbe.berkeley.edu/
Is your AC actually keeping up with the temperature you’ve set?
Your entire house is not at a even 21, technically, only your thermostat (which measures the temperature).
So during summer, rooms away from the thermostat with windows could be warmer, and colder in the winter. In addition, things like your floors can vary in temperature. If you have a house built on concrete slab, the floors you stand on will be colder than the air, transferring the cold into your body through your feet.
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