Weather in Costa Rica in June. The average maximum daytime temperature in Costa Rica in June is a warm 25°C (77°F) with low heat & humidity. There are usually 7 hours of bright sunshine each day, which represents 51% of the 13 hours of daylight. Expect 296mm (11.7 inches) of rainfall across the whole of June with 24 days with some rain falling. Maximum UV levels will be extreme (11+ UV index) at midday when the skies are clear. The average night-time temperature is usually a comfortable 17°C (63°F). The weather in Costa Rica in June is represented by San Jose.