A bit of playable realism: Season an climatic zone effects

[Ignatus]

In the game a ruleset author may advanse the timeline by year fragments - or, if coded so, day fragments, like in Wesnoth. We can set effects on this fragments like defense or move bonuses but we have limited control over terrain. For example, we can modify tile outputs (in winter less food than in summer), but what we naturally also want about winter:

• Boats without ice-breaking ability can't travel (and, possibly, may perish out of a safe bay);
• Land units use rivers as roads and may enter frozen water (maybe also with a danger);
• The map looks different! (In Wesnoth the view is darkened at night, idk if it is possible to set another effect)

Of course we could rewrite the whole map by Lua, but it's not only lifts traffic, it bugs pathfinding and AI while paths could be found with the regular changes considered.

Also, currently a temperature map is built by map generator but is destroyed after the terrain is placed, so if a polar tile has been once somehow transformed into jungle it has the same ability to stay as one as one on equator. Maybe we could have climate-dependent effect on convert time, and climate-dependent probabilities of GW/NW effective levels ("climate" means latitude plus local/adjacent terrain reqs), as well as aforementioned freezing-thawing happens not in all map (and, for a fine world, in different times in different hemitoroids).

Maybe the normal impact of GW/NW may become smoother, applied each turn to a few tiles rather than many tiles in rare turns.

[Alien Valkyrie]

I've been thinking about the seasons and map temperature thing for a while now – if we make seasonal effects, it would also be neat to be able to have two hemispheres.
An idea I had for this was a "MinLatitude" requirement type, with each tile being assigned an integer latitude – maybe from 0 to 100, or from -90 to 90, or from -100 to 100, or whatever. The open question being how to determine latitude in relation to topology and singlepole.

I think for some of the things you mentioned, there are some ways to implement them:

• Boats can't travel in winter – unit move rate can be increased by effects; I would guess that it could also be decreased the same way
• Land units use rivers as roads – might be possible through a separate "Frozen River" extra that has a spontaneous appearance chance of 100%, but requires a river on the tile and can only exist in winter
• Land units may enter frozen water – might be possible through an "ice floes" extra with a high spontaneous appearance chance that can only exist on water in winter and is native to land units
• The map looks different – might be possible through (you guessed it) extras acting as a partially transparent overlay; either ones that only appear at night, or ones that are only visible at night

I don't think it's currently possible to have terrain transform itself without scripts, so even with some way to determine a tile's latitude, the only current way to make climate-dependent changed would be to eschew terrain types and use – once again – terrain extras.
The ability to just apply warming and cooling effects on a tile-by-tile basis would be super neat though.

[Ignatus]

unit move rate can be increased by effects; I would guess that it could also be decreased the same way

Yes, move bonus may be negative. Just most unit classes have minimal moves limit under which they never get in the beginning of their phase (to have any ability to escape if damaged or something).

The map looks different – might be possible through (you guessed it) extras acting as a partially transparent overlay; either ones that only appear at night, or ones that are only visible at night

Yes, in 3.0 we can make daytime the visibility requirement of a certain extra! (I hope client processes it right.)

Land units use rivers as roads – might be possible through a separate "Frozen River" extra that has a spontaneous appearance chance of 100%, but requires a river on the tile and can only exist in winter

Yes it's possible to avoid scripting this way (but I am not sure AI currently makes sensible decisions in this case). But again, I would like to use go-to paths based on it, with waiting out winters. Maybe we should make a player-aware property of roads for if they are passable (unhardcodes restrictinfra via local "DiplRel" and may set also some year fragments interval). Some changes for the tilesets might replace graphics for seasons client-side (better with a smooth visual effect).