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Complicated Explanation

This page is simply for those who are interested in understanding the technical side of temperatures and how it works.
You do not need to read this page to understand temperature.
Knowledge from this page is not required to advance at all in the game, and it does not give any sort of advantage.
It is completely optional to read this, and you only should if such specificity interests you.
Otherwise, reading the main temperature page is more than sufficient.

Temperature is calculated using various sources as listed below:

Core temperature:

The core temperature of a player is 0. Wearing armor can apply a modifier to either hot or cold:
Leather or chainmail armor will add a 0.5 modifier for each piece being worn. A full set of this will apply (4 * 0.5) to the core temperature, resulting in +2 (6+ would cause overheating)
Core temperature can also be modified if the player is underneath a block or exposed to the sky. Being under a block (considered as being in shelter) will normalize a players core temperature by 1, so if cold, the core temperature will increase by 1, if hot, it will decrease by 1.
If a player is on fire, they will also get an extreme core temperature modifier of +10 (but probably have more important things on their mind)
Ambient temperature:
Ambient temperature starts at 0 and can be modified by the players surroundings;
Biomes can provide a modifier based on time of day as outlined below, any not listed is 0:
Badlands (and variants) = +3 during day, +4 around noon, -1 at night
Beach = +1
Cold Ocean = -1 during day, -0.5 around noon, -2 at night
Deep Cold Ocean = -2.5 during day, -3 around noon, -3.5 at night
Deep Frozen Ocean = -2.5 during day, -3 around noon, -3.5 at night
Deep Lukewarm Ocean = -2.5 during day, -3 around noon, -3.5 at night
Deep Ocean = -2.5 during day, -3 around noon, -3.5 at night
Desert = +2 during day, +3 around noon, -1 at night
Desert Hills = +2
Desert Lakes = +2
Frozen Oceans = -2.5
Frozen Rivers = -2.5
Ice Spikes = -2.5 during day, -3 around noon, -3.5 at night
Jungle (and variants) = +1 during day, +1.5 around noon, +2.5 at night
Lukewarm Ocean = +1
Mountains (and variants) = -1
Nether = +4
River = -0.5
Snowy Beach = -3
Snowy Mountains = -3
Snowy Taiga = -3
Swamp = +1.5
Taiga (and variants) = -1
The End (and variants) = -4
The Void = -10
Warm Oceans = +1.5
Wooded Badlands = +1
Elevation can provide a modifier. Being too high can be cold, being too low can be too hot. Based on players Y location.
150+ = -2
81-150 = -1
31-80 = 0
16-30 = -1
<= 15 = +1 (gets warmer due to being close to planet core)
Nearby blocks - Looks at blocks in a 2 block radius around the player, each block in that radius can provide a modifier. Blocks with 1 block radius provides a more extreme modifier. Nearby block modifiers has an upper and lower limit of 10 and -10 respectively. Modifiers are cumulative. So being within 1 block of lava would be +2, but also being within 1 block of more lava would be +4
Lava - Distance of 1 = +2, Distance of 2 = +1
Fire - Distance of 1 = +2, Distance of 2 = +0.5
Campfire (if lit) - Distance of 1 = +5, Distance of 2 = +0.5 (useful for warming up in a cold biome)
Furnace, smoker or blast furnace (if lit) - Distance of 1 = +2, Distance of 2 = +1
Blue ice, packed ice, frosted ice, snow block - Distance of 1 = -1.5, Distance of 2 = -0.5
Ice - Distance of 1 = -0.75, Distance of 2 = -0.25
Snow - Distance of 1 = -0.1, Distance of 2 = -0.05