Cancun has ideal weather throughout the year. Anytime of the year you come down to Cancun or any other part of the Yucatan Peninsula, you can expect to find warm, sunny weather more often than not. During the summer, Cancun sees average high temperatures that reach the upper 80's and average low temperatures that dip down into the upper 70's. It's pretty hot in the summer, but you can also expect a consistent sea breeze coming in from the Caribbean Sea. It rarely feels overwhelmingly hot thanks to the city's positioning right on the coast of the Yucatan Peninsula. The winter months get slightly cooler, but rarely, if ever, does the temperature mark dip down below 60 degrees. The average high in the winter months is in the lower 80's, and the average low temperature in the winter months is in the mid 60's.
This year-round great weather is one of the many reasons why Cancun is one of the world's most visited vacation destinations, and also why the Cancun activities are great any time of the year. Scuba diving excursions, for instance, run consistently throughout the year and, more often than not, both the weather and the water conditions are ideal for diving.
Depending on who you ask, the “rainy” season in Cancun happens some time between July and December. The reason it's hard to pinpoint an actual rainy season in Cancun is because of the nature of rainstorms in Cancun. Here, the rainstorms come and go in less than 30 minutes. Unless you are talking about a major storm, which are extremely rare to begin with, rainstorms can appear during the morning for less than an hour and then the skies can be completely sunny and cloudless by the afternoon. Either way, pack a raincoat or an umbrella for your trip, especially if you will be here between July and December. Also, if you are coming in the winter months, back a jacket or sweater, just in case. The chances are that you will not need to use your umbrella or your jacket, but have them handy just in case.