A pet peeve of mine has long been walking up a 60-block tall hill and seeing ambient temperature drop by 10°C.

In the vanilla game temperature decreases linearly with elevation by about 0.16°C for each block above sea level. While this makes for nice snow-capped mountains, I feel it's not particularly realistic for hills, ridges and highlands to get so cold so quickly. Realism, of course, is impossible to achieve to begin with in a toybox world of very limited height (not to even get into how local terrain affects temperature, e.g. canyons being cooler than the mesas surrounding them), so here's a compromise solution.

What this does

This mod changes the function by which temperature decreases with elevation to follow a sigmoid curve rather than being linear. The temperature decreases very gently at first, then starts decreasing steeply at about 100 blocks above sea level until it intersects the vanilla temperature drop at about 170 blocks above sea level. From there the curve levels back out, but this won't be relevant for most worlds due to world height limitations.

To illustrate:

Below sea level this mod uses the standard linear function.

Or see this calculator or the mod screenshot: https://www.desmos.com/calculator/tlizehxfmy (in this calculator, the variable U controls the ambient base temperature)

New world recommended

While this mod can be dropped into an existing world, certain terrain features such as snow and glaciers on mountaintops will not update to reflect the changes in existing chunks.

The numbers in this mod are tuned for a world height of 320. It will work with other world heights, but shorter worlds will be warmer at peak elevation while taller worlds will be colder. This same limitation applies to the vanilla system though.

Configuration (changing the curve)

To configure the mod, run the game with it once and then edit TemperatureHeightTweak.json (or create it with the following content):

{
  "a": 188.0,
  "b": 0.03,
  "c": 155.0
}

These values correspond to the variables in the equation y = a / (1 + exp(-b(x - c))) - a / (1 + exp(bc))

Use this calculator (or any other graphing calculator) to adjust the variables until the curve has an acceptable shape. Adjusting them blindly is an exercise in madness, so don't try it.

Mod compatibility

This should work with pretty much any mod, including worldgen mods. This will only conflict with other mods that pretty much do the exact same thing, as there is no reason to patch the functions this mod patches except to change temperature elevation scaling.