Manual de Plantas de Costa Rica | The physical environment| 81 
cording to Holdridge et al. (1971), equivalent temperatures occur approximately 330 m 
lower in elevation on the Caribbean than on the Pacific slope. The average maximum 
monthly temperature on Cerro Chirrip6 (3819 m), the highest peak in the country, is ca. 
7.2°C. At these high elevations, where temperatures as low as -11°C have been recorded 
(Gomez P., 1986), frost is frequent; however, there is no permanent ice (Coen, 1983). 
Throughout the country, the coolest months are November through January, while the 
warmest months are February through April or May. Commonly, winter cold fronts that 
penetrate far to the south in the United States are also experienced in Costa Rica. At 
such times, cold, dry air from the north may sweep across the Gulf of Mexico, picking 
up moisture that is deposited in the mountains of Central America (Coen, 1983). This 
process results in temporales (periods of more or less continuous rain typically lasting 
for several days) on the Caribbean slope, and strong, persistent northerly winds (papa- 
gayos) on the Pacific slope. These winds, averaging 10—30 km/hr with gusts to 90 km/ 
hr, are especially pronounced in the Guanacaste region (as around Cafias and Bagaces) 
during January and February (Coen, 1983). 
Snow is officially unknown in Costa Rica, notwithstanding apocryphal sightings 
on the highest peaks of the Cordillera de Talamanca (Holdridge et al., 1971; Coen, 
1983). These sightings may well have involved hail, which does occasionally fall (and 
may accumulate) even in the Central Valley. Rainfall, on the other hand, is quite well 
known. Mainland Costa Rica, as all of Central America, has two distinctly different 
rainfall regimes. The Caribbean slope is, on the whole, more continuously rainy, espe- 
cially from April through January, peaking from November through January. This peak 
is correlated with northern cold fronts, as discussed in the preceding paragraph. On the 
Pacific slope, the rainy season extends from April or May to November, peaking in Sep- 
tember and October, with virtually no rain during the northern winter (Coen, 1983). On 
either slope, a brief, relatively dry period (veranillo) may occur at some point in the 
middle of the rainy season (July-September, depending on the site). 
In both Caribbean and Pacific rainfall regimes, March and April are always the dri- 
est and hottest months in Costa Rica. But even during this period, the La Selva Bio- 
logical Station (in the Caribbean lowlands) averages only ca. 12 consecutive days with- 
out significant (> 5 mm) rainfall annually, with a maximum of just 30 days (Sanford et 
al., 1994). Seasonality on the Caribbean coast intensifies somewhat southward, with 
annual rainfall of more than 5000 mm near Barra del Colorado along the Rio San Juan, 
but just 2000—2500 mm in the Rio Sixaola region (Herrera, 1985). On the contrary, the 
more extended Pacific dry season generally decreases in length southward. Total annual 
rainfall on the Santa Elena Peninsula is the lowest in the country (< 1500 mm), while 
annual rainfall on the Osa Peninsula compares favorably with that of many sites in the 
Caribbean lowlands (despite a more lengthy dry season). 
The seasonal distribution of rainfall is perhaps more important for plants than the 
annual total (Gémez P., 1986). The more pronounced seasonality of the Costa Rican 
