Using the skipTime command sometimes leads to very thick fog after a while, which doesn't go away, even if you skip time even more. Ends up looking like this.
Was not able to alwasy reproduce it, however, it happens somewhat frequently. The time you have to skip for it to happen varies, too.
When you use skiptime, the game "simulates" weather changes that might happen over that period of time (possibly just current rate of change * skipped time?).
Unfortunately this simulation can put the values of overcast and fog well beyond their normal valid range range of 0-1 (eg, fog can equal -1.79, 2.6), and from there they can take a long time to wander back into the valid range where any change becomes apparent.