While it can't account for everything, an explanation to consider would be the pegasi abilities. They can and do manipulate weather, so some unnatural patterns would be expected.

Yeah, I won't deny there are some odd things happening, but Hoi4 weather is really hard to work with, and it is far from perfect even when it works, as you noticed yourself. Still, if you can manage to improve things, feel free to share.

I've gone and done some work to this extent. What I'd like you to do is view the interim stage here:

https://drive.google.com/file/d/1kaOJP-aDNa0FN95_LTg2UaQkKXLkRF7S/view?usp=sharing

One thing you may not have realized, is that HOI4 uses a Miller Cylindrical projection, and that the Equator is at Y=1400 (and not Y=1024 as you would guess). In order to make everything fit on a smaller map, Paradox moved the Americas up north and butchered the shape of South America. (You can see this in-game, it's too close to Africa).

This means the map does not cover the entire planet. It goes up to about 73 degrees (latitude) North, or up to about 40 degrees south. You can infer this from the formulas for the Miller projection and from simply looking at a summer sun celebration or winter night celebration day night/day cover in the far north of the map. The point where there is a polar day or night is the polar circle, located at 66.5 degrees North. Since the width of the map is known, that basically is enough information to get complete lat/lon coordinates for every pixel on it.

In other words, the southernmost tip of equestria is still north of 30 degrees.

I've filled in some ocean currents. I've had to guess at the location of continental shelf vs. oceanic shelf and used the shape of the mountains for these guesses, but there's still obviously room for change.

First, notice the L and H symbols. These indicate latitudes of natural Low and High pressure (due to Hadley/Ferrel cells). The area below the 'H' band naturally has more dry, sunny weather, while the area in between the two bands has more unpredictable, rainy weather (from cold polar air forming a front).

The green arrows indicate prevailing wind directions. The wind in the 30-60 cell is more unstable than that in the polar and equatorial cell. These will influence rain patterns: land areas where the wind blows from warm water have wetter, milder climates. Mountains can form a natural barrier, however.

The thick unbroken lines are the main oceanic currents. The small arrows indicate surface water flow over continental (shallow) seas. These currents tend to have a major effect on local weather patterns:

Cold currents tend to lead to much drier climates. They also shift the transition to colder climate zones south a bit. Land near them tends to sport savannah, deserts, mediterranean, and cold steppes. Land near warm currents on the other hand has ranforest, subtropical forest, temperate rainforest, and taiga.
Coastal climates can also be carried into continents by prevailing winds as long as there are no mountains in the way.
The effect of larger currents is correspondingly (much) greater.

The areas indicated in lime denote an estimate of arctic sea ice extent (high/low estimate, or winter/summer extent).

Some areas actually end up with a very fitting climate. Here's some examples/estimates of how working out the climates using the currents so far will go, what I've got so far. If I'm correct in interpreting all this;

Olenia is likely going to be a lot like Northwestern Europe. So is the northeastern part of Griffonia, as well as parts of western griffonia, around Skyfall.

Mediterranean climates are quite common in this world compared to ours. Southern riverlands (Koppen Csb/Csc), Minotauria, and parts of New Mareland (Csa) would get this climate.

Yakyakistan, Nova Griffonia, and the Polar bear communities are likely to get extremely cold winters (Dfd/Dwd in valleys, EH on mountains) if I work this out further.

Northern Griffonia is likely to have quite a bit of ET (tundra) climate.

Southern equestria and griffonia may also see large areas of monsoon-influenced hot-summer oceanic climate. This type (Cwa) is rare in our world, but caused by the 30-degree ocean band here.

Continental climates dominate the rest of the world. Equestria north of its separating mountain range & crystal empire (Dwb), all of the Changeling lands (Dwc), southeastern griffonia (Dsa/b), and northeastern griffonia (Dfb/c) will have this climate.

Mountain ranges may cause small areas of B-type climate. The Badlands (BSk), bits of southeastern griffonia (BWk/BSk), and perhaps the Palomino desert (BWh/BWk) could get this climate *due to mountain ranges blocking rainfall*.

Also, one thing I would like to know to continue this effort: What are the heights of the peaks/mountain ranges?
I'd like to make a scale (e.g. grey goes from 5A to E7 on the map, how high from sea level is each tint?). This scale doesn't need to be linear, as long as it's strictly increasing.

Example tall mountain tops and plateaus for various countries, listed in order of height, with country/region tags:
I've also added some real life mountain peaks for Earth countries that the local names seem to borrow from for comparison.


*More like a hill than an actual mountain...

Apologies for being silent for so long, I forgot to observe this thread... My bad. I sent you a friend request so we can talk about it more :)