The climate of the region has a considerable maritime influence. Due to the location (10° latitude) it is determined by the circulation of trade winds that, loaded with moisture, enter Costa Rica from the northeast, moving from the Caribbean Sea to the Pacific.
On its way, the winds cross the Cordillera de Tilarán, which acts as a barrier for air masses loaded with moisture. These are cooled when climbing over the mountain range causing orographic rains, and pass to the Pacific slope warming up as they descend.
According to these characteristics, two climate zones can be differentiated in general terms, that of the Caribbean slope and that of the Pacific. The first one has rainfall throughout the year and the second one is less rainy, with a marked period of drought in March.
Due to the pronounced variation in elevation above sea level (200 to 1,800 m), undergo abrupt changes in very short distances, together with equally abrupt changes in the total precipitation, in the strength of the wind, solar brightness and the presence of fog at ground level, to such an extent that the combined effects of local climate change are often drastic. Precipitation in the region in general terms is high, between the 2,000 Mm. and 3,600 mm.
The cloudiness is exceptionally high in parts such as Arenal Viejo, which has an average of 4.4 hours of direct sunshine per day a year. The cloudiness is especially frequent from June to January but it occurs almost all year round and causes drizzles with the action of the wind. Regarding relative humidity, the region is greatly humid; the Caribbean side has an average relative humidity of 89% and the Pacific side of 79%. Humidity follows the rain pattern and is inversely related to temperature and directly related to height.
Wind plays a very important role within the climatic conditions of the region. Northeaster winds predominate almost all year round, which come from the Caribbean, dragging the cloud masses and hitting the mountains, causing orographic rains. The average force of the wind in the area of the Arenal reservoir is 23 km/h., but it is much higher in the highest parts of the region and lower in the areas protected by the conditions of the relief, this has an important influence over the vegetation.