Costa Rica has an astounding number of microclimates, dictated by its many forests and changes in altitude, so the weather varies dramatically for such a small country. The sunniest, driest season in most of the country occurs from roughly mid-December through April, which is the busiest tourist season. For you, that means more advance planning, some flexibility in choosing a hotel, and higher prices. Afternoon showers kick in by May and last through November most everywhere, with a brief dry season in June and July. The Caribbean coast flies in the face of these norms with rains spread out fairly evenly throughout the year, with brief drier seasons February through April and September through October. Costa Rica promotes the rainy season as the "green season," touting lush vegetation, smaller crowds, and lower prices. Showers interrupt your afternoon beach time and remote roads can be washed out during the worst of the rains.
Temperatures generally range between 70°F (20°C) and 85°F (30°C). It's the humidity, not the heat, that causes you discomfort, especially in the dense forest of the Caribbean coast, the northern lowlands, and the Osa Peninsula. The arid North Pacific is Costa Rica's hottest region, with temperatures frequently exceeding 90°F (33°C) in the dry season.
One final point: We typically hear, "I didn't realize the rain forest would be so…rainy." Well, you heard it here: it rains often and heartily in the rain forest. The dense foliage and elaborate root systems rely on frequent showers year-round.