The weather, which we tend to think of generally as that thing which we can't predict that may sometimes annoy us, has had a profound effect on the great questions of history. While we may not yet be able to tell if we live in a deterministic universe, the effects of weather on the plans of mankind appear to comprise a stochastic system and, at least to premodern eyes, represent an arbitrary (and powerful) enough occurrence that we may can nominate it as the candidate for most likely to be seen as random.
Here are some events that were unduly influenced by the weather, to the point where the weather itself determined the outcome of major historical crossroads. Upon considering these, one should keep in mind the sheer magnitude of weather's scale in a pre-modern world.
1) The Wreckage of the White Ship. The White Ship, a vessel bearing the only male heir of Henry I of England, son of William the Conqueror, sank out of Calais one fateful night. This would result in the eventual outbreak of the Anarchy and the contest between Henry's nephew Stephen and his daughter, the Empress Matilda, over the English crown.
2) The Destruction of the Spanish Armada. The greatest fleet ever assembled in Europe. The mightiest army to be picked up from recent conquests in the Flemish countryside. The Protestant Wind. The Spanish Armada, a fleet nearly invulnerable to the miserable long cannon of the English, was poised to deposit an overwhelming army on England's shores... until a storm smashed most of the fleet and drove the rest off course.
3) Napoleon and Russia. You know this one.
4) Themistocles and the Persians. And if you don't know this one, the Greeks turned the tides and winds of a narrow channel against the vast and overwhelming numbers of the Persian fleet.
There are countless times that weather has influenced the outcome of events in pre-modern warfare. Mud and rain have spelled the death of many knights charging over ground they thought solid. Fog has led them into marshes, where their horses drowned, and mail-clad knights with them.
The weather is a force to be reckoned with. Do not discount it's power.