For rain/storms:
@BFT2020 is correct (as far as i know) in that normal XML modding exposes the weather as "probabilities" amd you have no control over duration or when it starts/ends. Fog is similar but there is a second fog configuration you can tweak for just "night" and "day" settings (fog occurs during one or both of these times). See
https://7daystodiemods.com/night-fog/
for the additional fog setting.
now, donovans modlet doesn't mean "no fog on the day" (im 90% sure) think of it as "extra fog is added at the night".
also: there is a "global weather" setting that you cannot access via XML (but maybe C#? As suggested already) that actually dictates some core weather conditions for the world/map. I believe it fluctuates variables (temp, sun, clouds, etc) and the XML changes/mods add/subtract to this, and the extra fog setting adds to it as well.
im playing a modified donovans night fog mod im tweaking in an Undead Legacy game and theres always "fog" now (as the mod sets some base levels of fogginess and i added fog for both night and day) but the global weather and Undead Legacy weather XML make it go from "impossible to see" to "omg i can see about 25 blocks out", but that's a rare daytime condition and even some days its just a wall of fog. Its exactly what i want (deep fog that has chances of clearing up a little) So you likely have to edit multiple weather XML places to get a more persistent/solid fog.... though a youtuber named jawoodle had an a19? game with "all dense fog all the time" as someone or him had to have made a special mod for it that i have not seen posted.
I think for a19
@khzmusik made a "seasons" mod so there is some ability to tweak the weather by time/day and have it "stay" but i think he made a dll mod (c# mods were not supported then) and i believe it was updating the weather constantly (like every second) to override/reset the global weather to he what he wanted on that day/time.
I made a "punishing weather" mod
https://7daystodiemods.com/punishing-weather-core/
That is supposed to make the weather weather (and visual effects) "nastier" but allow for some chance of clearer weather. Check out the XML to see how it was done compared to the probabilities/settings of the vanilla game XML. I made it mostly for a "stormy and windy and visuals obscured a bit" effect most of the time. I would assume you could make changes to this mod to crank up whatever weather probability you want..... but beware the global weather will still do whatever it wants.
If im wrong about any of the details, anyone please correct me

. Im really hoping when the game goes gold the weather will be a lot more user/mod configurable (like colored fog, lightening strikes, etc) than it is today.