Weather: Costa Rica,Nicaragua
For all the renowned ecological diversity of this small nation, its meteorological diversity is worth noting as well. This is a country where micro-climates reign supreme. The combination of an irregular coastline, the presence of two contrasting bodies of water nearby, and the extremely varied topography leads to a plethora of local wind and rainfall patterns that are nearly impossible to summarize. For what it's worth, you can expect pleasantly mild conditions at moderate altitudes (such as San Jose), with nights a touch cool in the dry season, but afternoons seldom very hot, even in the wet mid-summer. The lowlands are, unsurprisingly, warmer and more humid. The heat is a bit more intense on the Pacific side than on the Atlantic, sometimes topping 35°C/95°F in the spring across Guanacaste and Puntarenas.
Although some pockets of Costa Rica, such as the Valley of the General, are drier than others, “dry” is a relative term in this well-watered country. You can expect frequent afternoon downpours across most of the country between May and early November. The canicula dries things out in many areas for a couple of weeks at some point in July or August. On the Pacific slopes and plains and near the Caribbean coast, the rains usually diminish in September and October, then pick up again in November and continue into January or beyond. Indeed, almost every month sees ample rainfall near the Caribbean; much of it falls at night, though. It's dry nearly everywhere else from February through April, with more sunshine than in the summer. The heat gets intense across the lowlands as spring approaches.
To the north, Nicaragua has its own share of climatic regimes. Heat and humidity rule the eastern lowlands: ample rain falls year-round, and warm-season deluges are especially common on the southeast coast (Hurricanes are also a definite risk along the Nicaraguan coast, especially from September into November.) The northwest highlands provide weather much like that of the Guatemalan sierra, albeit with the temperature turned up a notch.
Nicaragua's touristed west, including Managua, is reliably sultry, with a marked canicula in July or August and a bone-dry season from December through April, with windy and coolish weather common in December and January.