Map from Google - El_Salvador

El Salvador, a small country in Central America, has a tropical climate, hot all year round along the coasts and the lowlands below 1,000 metres (3,300 feet), called tierras calientes, with little difference in the temperature (only a few degrees) throughout the year. The real difference between the seasons is found in rainfall and humidity: the rainy season, from late April to October, is definitely muggy. Rainfall occurs mainly as downpour or thunderstorm in the afternoon, so that the amount of sunshine hours, which is great in the dry months, remains acceptable even during the rainy season.
In the capital San Salvador 1,700 millimetres (67 inches) of rain per year fall. From June to September it rains a lot, about 300 millimetres (12 inches) per month, distributed in 20 days; on the other hand, from December to March it almost never rains. Here is the average precipitation.
Average precipitation - San Salvador
San SalvadorJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (mm)5293615029031531035021535101735

The city is located at 700 metres (2,300 feet) above sea level, so it's still in the tierras calientes, but the altitude makes certainly the heat more bearable, in fact the maximum temperature ranges from 29 degrees Celsius (84 °F) from September to November, to 32 °C (90 °F) in March and April. The minimum drops a few degrees below 20 °C (68 °F) during the period from November to March. Anyway, the rainy season, from May to October, is still sultry.
Here are the average temperatures in San Salvador.
Average temperatures - San Salvador
San SalvadorJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)161718192020191919191817
Max (°C)303032323130303029292930
Min (°F)616364666868666666666463
Max (°F)868690908886868684848486

Along the coast of the Pacific Ocean, the heat is more intense, so much so that the night temperatures range from 23 to 25 °C (73 to 77 °F) and the maxima from 30 to 31 °C (86 to 88 °F). However, there can be very hot days, in which highs can reach 35 °C (95 °F). During the rainy season (which occurs in the same period), the heat is annoying because it's sultry. In inland valleys at low altitude, the distance from the sea makes the heat more uncomfortable, due to the less windy conditions.
Average temperatures - Acajutla
Min (°C)232325252525242423242323
Max (°C)313131313130313130313131
Min (°F)737377777777757573757373
Max (°F)888888888886888886888888

For a beach holiday, the sea is warm throughout the year, ranging from 28 °C to 30 °C (82 to 86 °F).
Sea temperature - Acajutla
Sea (°C)282829303029292929292929
Sea (°F)828284868684848484848484

In inland areas there are volcanoes, plateaus and mountains, and the weather gets cooler with altitude. At intermediate altitudes a cloud forest grows; at high altitudes, above 2,000 metres (6,500 feet), it can get cold in the winter nights. The highest point is Cerro El Pital, although the summit, at 2,730 metres (8,957 feet), is located in Honduras; from November to March on the top you can see the snow.
From December to March, sometimes El Salvador is reached by the Norte, a cool wind from the United States that in the plains can bring at most a little cool in the evening and at night, while in San Salvador it can bring a bit of cold at night, with lows around 10 °C (50 °F).
In the period from June to November, El Salvador can be affected by hurricanes, especially between August and October.
El Salvador

When to go

The best time to visit El Salvador, whether you intend to visit the cities and make excursions, or to go to the beach, runs from December to mid-April. In the plains, December and January are preferable because they are less hot.

What to pack

All year round, light summer clothes in the plains; a sweatshirt or sweater for the evening in San Salvador. From May to October, possibly a light raincoat for thunderstorms.
For the mountains: hiking shoes, jacket.