In chapter six of George Orwell's Animal Farm, we begin to understand just how hard the animals are working to make a go of things on Animal Farm. They are working hard, not only on the regular jobs which must be done on the farm but also to finish the windmill. To make matters worse, Squealer implements a cut in the animals' rations, though of course he couches it as a simple "readjustment" rather than the cut, which it obviously is. The building is a long, slow, laborious process, and without the mighty strength of Boxer, progress would have been virtually impossible. Despite the many hardships, the animals still believe that what they are doing will benefit them and are therefore willing to continue their hard and thankless labor.

Several changes happen on the farm in this chapter. First, Napoleon decides to begin trading with neighboring farms, and Mr. Whymper is now a regular presence on the farm. Even more significant, at least for now, is that the pigs take up residence in the farmhouse, something which was forbidden from the beginning, of course. What we see clearly but what is still hidden from the animals is that the pigs are growing more corrupt and human-like in their behaviors. This is not going to bode well for the animals, but for now things are relatively calm. Hard, but calm.

When November comes, the windmill is nearly half finished, and the animals feel good about their progress. One night there is a terrible storm. Tiles are blown off the roof, chickens are frightened by what sounded like a gunshot, a huge tree is uprooted, and the flagstaff has been knocked over. It was a mighty storm, and when the animals survey the damage, they are horrified to discover that "the windmill was in ruins."

All the animals are dismayed, and even Napoleon moves a little more quickly than usual to see what has happened. He immediately begins sniffing the ground around the base of the windmill, and soon he announces that Snowball is the one who has destroyed the windmill. Here is his claim:

In sheer malignity, thinking to set back our plans and avenge himself for his ignominious expulsion, this traitor has crept here under cover of night and destroyed our work of nearly a year. Comrades, here and now I pronounce the death sentence upon Snowball.

This is an obviously outrageous statement that is not in the least true; however, Napoleon knows he has to blame someone so that no one will blame or turn on him. It takes a little time, but eventually most of the animals reluctantly accept Napoleon's claim. He leads the animals in several ceremonial moments, again in an attempt to distract them and rally the weary animals for the daunting and backbreaking task of rebuilding.

We have seen this tactic before. Both Snowball and Mr. Jones are easy targets to be blamed for anything. They are not there to defend themselves, so they make easy scapegoats for Napoleon and his propaganda-master, Squealer. Asking if the animals want Jones back and making Snowball a common enemy serve to divert any blame or suspicion from the true culprits in the animal's difficult lives. 

Snowball had nothing to do with the decimation of the windmill and neither did Napoleon; however, in case the animals decided to rebel against him and to ensure they would rally together to rebuild the windmill, Napoleon falsely blames the innocent Snowball for the act of nature. 

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Towards the end of chapter 6, a violent storm hits the farm and the strong gusts of wind destroy the windmill. In the morning, the animals are devastated when they discover that the windmill has been destroyed and Napoleon immediately blames its destruction on Snowball. Napoleon uses Snowball as a scapegoat to conceal the flaws in the windmill's construction by claiming that Snowball trespassed onto the farm under the cover of night and destroyed the windmill himself.

Napoleon then pronounces a death sentence on Snowball and mysteriously discovers the footprints of a pig not far from the knoll, which he uses as proof of Snowball's treachery. In the next chapter, the humans on the surrounding farms declare that the windmill fell during the rough storm because its walls were too thin. After the animals successfully rebuild the windmill, Mr. Frederick's men blow up the windmill using blasting powder in chapter 8.

In Animal Farm, why does Napoleon blame Snowball for the fall of the windmill when it had actually been destroyed in a storm?

Napoleon's purpose is to demonize Snowball as much as he possibly can. He can use Snowball as the scapegoat for everything that goes wrong on the farm. In so doing, he can present himself as a foil to Snowball's evil. The gullible and unintelligent animals would, therefore, be convinced that he is truly their protector, acting in their best interests. 

Before Snowball's expulsion, he and Napoleon were constantly at loggerheads about the management of the farm. They would constantly argue during meetings since they were in direct opposition to one another. Snowball would almost always gain support for his ideas since he was a quick thinker and quite innovative, whilst Napoleon surreptitiously went around influencing other animals, such as the sheep, to do his bidding. He would then use them during meetings to disrupt Snowball's eloquent speeches by constantly bleating, 'Four legs good, two legs bad.' Napoleon employed this pernicious tactic since he was not Snowball's equal when it came to acting in the best interests of the farm. He had other ideas and wanted to have sole control. Snowball was a thorn in his side and he had to get rid of him. He could do this once he had secretly raised Jessie and Bluebell's nine puppies and trained them to do his bidding. They had grown into ferocious dogs and he used them to chase Snowball off the farm.   

Once Snowball was not there to challenge him any longer, Napoleon could freely go about and assert his authority. He started spreading lies and propaganda about Snowball, using Squealer especially. It was, for example, put out that Snowball had been a traitor from the very start and that he was actually fighting on the side of Mr. Jones during the Rebellion.

... it was given out that fresh documents had been discovered which revealed further details about Snowball’s complicity with Jones. It now appeared that Snowball had not, as the animals had previously imagined, merely attempted to lose the Battle of the Cowshed by means of a stratagem, but had been openly fighting on Jones’s side. In fact, it was he who had actually been the leader of the human forces, and had charged into battle with the words ‘Long live Humanity!’ on his lips. The wounds on Snowball’s back, which a few of the animals still remembered to have seen, had been inflicted by Napoleon’s teeth. 

By sullying Snowball's name, Napoleon also destroyed whatever chance Snowball might ever have of returning to the farm. He also destroyed the hopes other animals might have of ever seeing their comrade again. This systematic propaganda campaign put him in good stead with the other animals. Boxer, for example, who expressed some doubt about the damaging claims made about Snowball, was easily persuaded since he believed that, 'Napoleon is always right.'    

Using Snowball also gave Napoleon the perfect opportunity to rid himself of whatever animals were left to either expose him or threaten his authority. He conducted a purge in which many animals were slaughtered after confessing to having assisted Snowball during his seemingly secret visitations to the farm, apparently to do mischief and destroy the animals' hard work. Snowball was, for example, also blamed for mixing weed seeds in those of some crops that had been planted.    

Napoleon's campaign worked well and the animals soon forgot about the positive role that Snowball had played on the farm. They truly believed that he was out to destroy their hard work. Whenever Napoleon wanted to manipulate the animals, he would use Snowball's name to back up his sentiments. He, for example, claimed that Snowball had been hiding on Frederick's farm when he was friends with Pilkington, and vice versa when he sought Frederick's support.

It now appeared that Snowball was not, after all, hiding on Pinchfield Farm, and in fact had never been there in his life: he was living — in considerable luxury, so it was said — at Foxwood, and had in reality been a pensioner of Pilkington for years past.

The animals were easily swayed by this topsy-turvy state of affairs and were more confused than ever by Napoleon's so-called 'clever tactics.' In the end, though, all memory of Snowball, and the memory of much of everything else, faded away.