Average high temperatures through October are a warm 87F with the average low temperature still a respectable 74F. In terms of average high and low temperatures things in October are not that different to September but where the difference comes is in the number of days where the temperature exceeds 90F.
September has 17 days, over half the month, with daily temperatures rising above 90F but October has just 7 days where the temperature surpasses ninety, and these will mostly be near the beginning of the month. However, it is still warm enough that on only one day in October will the temperature not reach 80F.
The humidity never really fluctuates much in Cancun and you should expect humidity ranging between 74% and 86% with the highest humidity being in the mornings. The humidity serves the purpose of hot temperatures actually feeling even hotter so don’t worry about feeling cold, it wont happen.
One thing to bear in mind is October is the peak month for hurricanes. By now the sea temperatures have reached their highest and it is usual for hurricanes to form in the Atlantic and affect the Caribbean. The biggest hurricane of recent years, Hurricane Wilma, hit Cancun on 20/21 October 2005 and caused severe devastation.
It should be remembered though that in order to cause severe damage and disruption hurricanes have to score a direct hit. Their zone of influence is usually relatively small, perhaps 100 or so miles across, and the Caribbean is a huge geographical area. The chances of a major hurricane striking Cancun directly is still small but is something that you should bear in mind when traveling to Cancun in October.
October is the second wettest month of the year on average, behind September with an average of 7 rainy October days delivering around 9 inches of rainfall in total.