Climate in Guatemala varies according to season, geography and altitude. In general, the best time to travel is November through April. The Pacific and Caribbean coasts are by and large hot all year round in the high 80° F to mid-90° F. Expect occasional rain anytime of the year on the tropical Atlantic coast, although storms are possible during hurricane season August-October.
In the northern lowland rainforests of the Petén region, including Tikal National Park, humidity is high and temperatures reach 100° F from May-August. The highlands including Guatemala City, Antigua and Lake Atitlán (Panajachel) are generally 5,000 ft or more above sea level. This region is commonly referred to as the "Land Of Eternal Spring" as it is characterized by pleasantly warm days and cool nights. As you move into the western highlands the altitude increases up to 10,000 ft on the Pan American highway where the temperature decreases sharply in the evening. In December and January there may be frost in the early morning at the highest elevations.