I use
Manos Environment Pro made by Makman94. It is a 20km environment mod which makes heavy fog conditions more suitable for hunting: even in poor visibility your crew can sight units from a distance of 1.5 - 2 km, well above minimal torpedo firing range.
You can download its latest version from here:
http://www.subsim.com/radioroom/showthread.php?t=229333
I have an other mod,
Realism- and gameplay-related hardcode fixes for sh3.exe by h.sie. Among many other useful features, it includes a bad weather fix which consists of two counters, one for the stormy sea and the other for the foggy weather. If either of these counters reach a certain value - which could be adjusted by the player at his discretion - the fix forces a reduction of wind speed and/or clouds and thus also fog.
Of course there's no guarantee that you'll never encounter days or even weeks of stormy and foggy weather but at least with this mod it won't stick at those atrocious conditions and stay for the entire patrol.
You can download it from here:
http://www.subsim.com/radioroom/showthread.php?t=174225