Well, there are indeed missions that use OFP's weather effects to produce fog. However, assuming you're having this problem with every mission you play, it may the video settings you specified under OFP's OPTIONS menu.
OFP reg Mister Frag says that it may possible be caused by your maximizing both OFP's framerate and visual quality settings. This is especially true if your PC's processor and video components are not up to coping with the required resources needed to support these settings at their maximum.
Mister Frag says you should make sure you experiment with the framerate and visual quality settings to find a good balance between them that is reasonable for your system.
Setting Visual Quality to maximum and Frame Rate to minimum provides the best visuals, regardless of how long it takes to draw them. If your system isn't powerful enough, framerates can really suffer, which may make OFP's gameplay lag or stutter.
Setting Visual Quality to minimum and Frame Rate to maximum means that OFP will attempt to reach a framerate of at least 40fps, and will allow OFP to increase the visual quality as much as it can while meeting the frame rate goal.
If you have a leading-edge system, you can still exceed the 40fps goal with full visual quality.