The weather is pretty temperate year round. However, it can get chilly, so missionaries will need a jacket and some sweaters. Rain is not a very big issue.
This is a walking mission, so bring comfortable shoes!
Their store that is similar to Walmart is called Lider and they have pretty much everything they need.
They do have tampons, but often poor quality and they can be pretty expensive, so you might want to consider bringing a mission's supply of feminine hygiene products you are used to or using a disc or cup instead.
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Most parents send their Elders with at least 1 suit (most countries in South America only wear suits to church and meetings) and around 8–10 golf pants. Families are sending more shirts because of heat and humidity — 12–14 is the average. Note for Sisters serving in Brazil: feminine hygiene products are expensive there and tampons are often considered taboo. Be prepared to send a good supply with your Sister Missionary.
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.