Map from Google - Nicaragua

Nicaragua, a country of Central America located just north of the Equator, has a tropical climate, hot all year round.
Temperatures are quite stable, however there is a relatively cool winter from November to January, a hot spring from March to May, and a sultry and rainy summer from May to October.
Then there is a difference between the two sides: the west coast, facing the Pacific Ocean, is hotter and has a very pronounced dry season from December to April, while the eastern one, overlooking the Caribbean Sea, is more humid and rainy. In fact, the latter, being exposed to the north-east trade winds, receives rainfall even in winter, so that its climate can be defined as equatorial.
Consequently, rainfall is abundant on the east coast, where it even exceeds 4,000 millimetres (160 inches) per year, while it's much less abundant in the central plains (where the capital Managua and the lakes Nicaragua and Managua are located) and along the Pacific coast.
In Managua, 1,150 mm (45 in) of rain per year fall, and the dry season, which runs from December to April, is very pronounced, so much so that it almost never rains. In the rainy season, from May to mid-November, the wettest months are June, September and October, with more than 200 mm (8 in) of rain per month. Here is the average precipitation.
Average precipitation - Managua
ManaguaJanFebMarAprMayJunJulAugSepOctNovDecYear
ManaguaJFMAMJJASONDY
Prec. (mm)1055813022514513521522040101140
Prec.(in)0.40.20.20.35.18.95.75.38.58.71.60.444.9
Days4222102220172019105133

Located at 50 km (30 mi) from the sea, and near a lake that provides moisture, Managua is a particularly hot city, especially in spring, before the rains, and in May, the first rainy month, in which, however, the rains slowly begin to lower the temperature. In the period from June to November, the maximum temperatures are around 31 °C (88 °F) and the humidity is higher. Here are the average temperatures.
Average temperatures - Managua
ManaguaJanFebMarAprMayJunJulAugSepOctNovDec
ManaguaJFMAMJJASOND
Min (°C)212122232423232322222121
Max (°C)323334353532313131313131
Min (°F)707072737573737372727070
Max (°F)909193959590888888888888

The sun in Managua often shines in the dry period, while in the rainy season it shines for a few hours a day.
Sunshine - Managua
ManaguaJanFebMarAprMayJunJulAugSepOctNovDec
ManaguaJFMAMJJASOND
Sun (hours)788764566677

On the west coast the rainfall pattern is the same, but the temperature is a bit lower than in Managua, due to sea breezes; however, we can notice a certain temperature increase in spring here as well.
Here are the average temperatures in San Juan del Sur.
Average temperatures - S.Juan del Sur
S.Juan del SurJanFebMarAprMayJunJulAugSepOctNovDec
S.Juan del SurJFMAMJJASOND
Min (°C)212122222322222222222221
Max (°C)303132323231313130303030
Min (°F)707072727372727272727270
Max (°F)868890909088888886868686

On the Caribbean coast, on the contrary, it rains a lot also from November to January. In Bluefields, precipitation amounts to 4,000 mm (160 in) per year; it rains a lot and almost every day from May to December, with a peak in July, with as high as 665 mm (26 in) of rain. March and April are the only months in which the rains give a little break, dropping to around 80 mm (3 in) per month, in both cases in 12 days.
Here is the average precipitation in Bluefields.
Average precipitation - Bluefields
BluefieldsJanFebMarAprMayJunJulAugSepOctNovDecYear
BluefieldsJFMAMJJASONDY
Prec. (mm)26513080753455006655453103453904004050
Prec.(in)10.45.13.1313.619.726.221.512.213.615.415.7159.4
Days211612122223242421212222240

It goes a bit better further to the north, in Puerto Cabezas, where 3,000 mm (120 in) of rain per year fall, and already in February rainfall drops to less than 100 mm (4 in). So here and in general along the Caribbean coast, there is no really dry month; due to the rains and the trade winds, the temperatures are a bit lower than on the west coast, but relative humidity is consistently high.
Here are the average temperatures in Bluefields.
Average temperatures - Bluefields
BluefieldsJanFebMarAprMayJunJulAugSepOctNovDec
BluefieldsJFMAMJJASOND
Min (°C)222222232423232323222222
Max (°C)282930313130292929292928
Min (°F)727272737573737373727272
Max (°F)828486888886848484848482

The sun in Bluefields shines less often than in Managua and on the west coast, as the rains are more frequent and the rainy season is longer.
Sunshine - Bluefields
BluefieldsJanFebMarAprMayJunJulAugSepOctNovDec
BluefieldsJFMAMJJASOND
Sun (hours)567754345555

East of Bluefields we find two islands, called Corn Islands (Big Corn Island or Isla Grande del Maíz, and Little Corn Island or Isla Pequeña del Maíz), which have a similar climate, with a less rainy period from February to April.

The sea in Nicaragua is warm all year round, in both coasts, as we can see from the water temperature at Bluefields.
Sea temperature - Bluefields
BluefieldsJanFebMarAprMayJunJulAugSepOctNovDec
BluefieldsJFMAMJJASOND
Sea (°C)272727282828282829292828
Sea (°F)818181828282828284848282

Nicaragua is known as the land of lakes and volcanoes. Some volcanoes rise from the plains, such as San Cristobal, other from lakes, such as Concepción, which emerges from Lake Nicaragua. In the north-central inner region, there is a mountain range, called Cordillera Chontaleña: in the highlands the weather becomes mild above 1,000 metres (3,300 feet), in the so called tierras templadas.

Lake Nicaragua

Nicaragua is located in the path of hurricanes, which may hit the country in the period from June to November, coming from the east, ie the Caribbean Sea, but are most likely from August to October. A particularly intense hurricane was Joan-Miriam, which caused serious damage throughout the country, and in particular to Bluefields, in late October 1988.

When to go

The best time to visit Nicaragua is from December to April in the central and western part (in Managua you may prefer December and January, which are a little less hot), while it's limited to March and April along the east coast, due to the rains (but in the end you can also go in February, especially in the north, in Puerto Cabezas, where it rains less).

What to pack

For the tierras templadas and the slopes of volcanoes: all year round, light clothing, a sweatshirt or sweater for the evening; raincoat or umbrella.
For plains and coasts: light clothing, a sweater or sweatshirt for the evening from November to March, a hat for the sun; in the rainy season (May to mid-November in Managua, even throughout the year on the east coast), a light raincoat for showers and thunderstorms.