Guatemala's climate has been described as the "Land of Eternal Spring" - much of the country does enjoy an agreeable climate. However, there are exceptions!
Altitude is the biggest climatic consideration in Guatemala. In the highlands including Antigua, Guatemala City, Lago de Atitlan and Chichicastenango, the altitude is between 4,260-6,890' (1,300-2,100 m) and the daytime climate is usually pleasant between 64-82°F (18-28°C). Humidity is rarely a problem. Be prepared for temperatures to cool off at night - make sure you pack some layers!!
In the coastal areas and the Peten jungle lowlands the heat and humidity can be draining. Be prepared at any time of the year to find temperatures above 85°F (30°C). Tikal owes the lushness of its jungle environment to the climate. Marvel at the ruins but be ready for steamy and humid conditions - as you wander among these towering pyramids, imagine building them under these temperatures!
There are two seasons in Guatemala. Winter, or the rainy season, is from May to October. The skies are normally a mix of sunshine and clouds and you can expect periodic short tropical downpours in the late afternoons, clearing for the evening.
Summer, or the dry season, is between November and April and paradoxically is the time of coolest temperatures. Due to the clear skies the coolest nighttime temperatures occur in December and January, with occasional snowfall and frosts at high altitudes. For most of the country, this is the best time of year to visit Guatemala. April and May are the hottest months; relief from the heat comes with the start of the rains.