It is mostly hot here. Temperatures are 100+ in the summer. There are a couple regions (Nogales and Cananea) where it can get cold, but colder weather is limited to January even in these areas. Bring a few sweaters for these areas. There is not a lot of rain.
Clothing should be light, with breathable fabrics. Serve shirts have been recommended for this mission for the Elders.
Good shoes are VERY important! They do A LOT of walking!! They do not use bicycles. Missionaries are mostly on foot, although sometimes they will use buses or ubers. It is a good idea to establish an uber account before arriving in the mission.
Apartments have washing machines.
They also have filtered water at every apartment. Do not drink the tap water. A good filtered water bottle is something that can come in handy, though.
Bring a hat and sunscreen.
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Serve shirts have a ventilation panel in the side of the shirt making them noticeably more breathable. Missionaries serving in hot or humid places absolutely RAVE about this feature. The most popular shirt for hot-climate missions, for sure.