Campinas has a tropical highland climate — warm and humid in summer, noticeably drier and cooler in winter, but never cold. Its elevation (about 680 metres above sea level) keeps it a few degrees cooler than São Paulo city and considerably more bearable than coastal Brazil in the heat of the year.
November to March is the rainy season. Afternoons frequently bring heavy downpours, and December and January can be relentlessly wet. Temperatures reach 30–32°C. This is also the period when the city is at its most lively — Carnival falls in February, and the Festa de Nossa Senhora da Conceição in December draws large crowds.
April to October is the dry season and the more comfortable time to visit. May through September brings cool, clear days and evenings that drop to 15–18°C — warm enough for a light jacket in the evenings but pleasant for walking. July is the coolest month; frost is not unknown, though it is rare at this altitude.
June is worth timing a trip around if you are interested in the Festa Junina celebrations. Campinas does them well: the forró music, quadrilha dancing, traditional food stalls selling canjica, quentão (warm spiced cachaça), and corn-based sweets create a particular atmosphere that feels genuinely local rather than tourist-facing. Street parties run through the month in different neighbourhoods.
For practical purposes, May to August is the sweet spot: good weather, no flooding, and the city going about its normal business.