Temperature from biometrics and things effects you and your body temperature.
Your temp will rise/lower depends where you are and are near to or touch also things like snow, torch and light affects
Heat biometrics raises the temp much, even as much as it hurts and same for coldness (based on wolrd/perlin).
Just keep the bar near to green and you are safe.
The bar are placed under the health/breath to not mess with other hud mods.
It can be a good idea to be in water when its hot, but in winter this contributes with a quick death instead.
Grant someone the no_temperature privilege to spare him from all this mess (relogin to take effect) or check with the thermometer (tempsurvive:thermometer) that are simeply crafted with
[bronze ingot]
[wood]
[glass]
A Mod for masochists
If you spawn i a cold biome you are gonna freeze to death many times before you can get a torch and if you spawn in a hot biome you are gonna be in constant heat unless you live under water. Maybe making it compatible with other mods that adds way to warm yourself and take some cool drinks would be a good way to make it less harsh