Particles of rain and snow cross the roofs of buildings when falling.

There is something done as a temporary hack for alien base, but nothing for base defense; check [http://ufoai.org/bugs/ufoalieninvasion/issues/5446#comment_9963 comment #5] from DarkRain in issue #5446

Check CP_CreateBattleParameters() in src/client/cgame/campaign/cp_missions.cpp lines 341 to 345

<source lang="cpp" line start="341">
/* Hack: Alienbase is fully indoors (underground) so no weather effects, maybe this should be a mapdef property? */
if (mission->category == INTERESTCATEGORY_ALIENBASE)
cgi->Cvar_Set("r_weather", "0");
else
cgi->Cvar_Set("r_weather", "%s", cgi->csi->terrainDefs.getWeather(color));
</source>

and consider if it should be extended for INTERESTCATEGORY_BASE_ATTACK, while not implemented a mapdef property.