Climate and monthly weather forecast
Guam, USA

Flag of Guam, USA
Flag of Guam, USA

The climate of Guam

Guam has a tropical rainforest type of climate (Köppen climate classification Af) with hot and humid conditions throughout the year. Temperature variations are little across the mainly wet and dry seasons. Guam lies in the region of Micronesia in the Pacific Ocean and is an organized United States territory. The engulfment by the ocean, northeasterly trade winds, and low latitudes mainly influence the climate. The location of Guam in the Western Pacific Ocean corresponds to the most prolific tropical cyclone activity with an average of 25 named storms annually.

Guam, the Land of the Chamorro, is a 30-mile long island with a maximum width of 12 miles and an area of 212 square miles. The largest island in Micronesia has its highest elevation at 407 meters on Mount Lamlam. Guam has ample fresh water and diverse marine ecosystems, with a coral reef surrounding the entire island. The proximity to the equator, just 13°north of it, gives ample rain, warm temperatures, and an equal duration of day and night. The topography ranges from cliffs in the north, a hilly landscape in the center, and mountains toward the southern end.

Guam has two main seasons, with the daily mean temperatures between 80°F (26.7°C) to 82°F (27.8°C) during the year. The dry season is from December to May and contributes to one-fifth of the annual rainfall. Temperatures rarely exceed 90°F (32.2°C) due to the surrounding ocean waters that act as a thermostat. The average temperatures on the island remain in the 75°F (23.9°C) to 88°F (31.1°C) range, with the ocean waters registering a few degrees cooler than the land. Northeasterly trade winds moderate the precipitation during the dry season. The average wind speeds are usually moderate between 10mph (16.1km/h) to 20mph (32.2km/h) with the weakest gusts during August.

The annual rainfall in Guam is a hefty 98" (2489mm) during 255 rainy days in the year. The wet season contributes to 80% of the yearly precipitation with the probability of rain almost every day. Humidity frequently exceeds 84% during the night, but the average monthly humidity is approximately 66%. Guam registers 2048 hours of sunshine annually, with a high degree of ultraviolet radiation. Cloud cover lasts the longest from September to November and is an inherent feature of the climate during the rest of the year. Guam receives volcanic fog from the nearby island of Anatahan in Northern Mariana.

The highest temperature on record for Guam is 96°F (35.6°C), set on April 18, 1971, while the record lowest temperature is 65°F (18.3°C), set on February 8, 1973.

The best time to visit Guam

The best time to visit Guam is during the dry season from January to May when the rainfall is moderate compared to the excess of the wet season. January and February are typically the coldest months of the year, with temperatures between 75°F (23.9°C) to 84°F (28.9°C). March to May is less busy than the early part of the dry season and economical on the pocket. The temperatures are on the pleasant side, and the ocean waters are calm. The drop in relative humidity makes the conditions comfortable in the dry season.

The worst time to visit Guam

The worst time to visit Guam is in the wet season between September to November. The precipitation is substantial, along with high humidity and the possibility of storms. The typhoon season peaks between August and October and poses a constant threat to planned vacations. Humidity makes the heat even worse with a high degree of sweating. Tourists from the American mainland belonging to temperate zones find the conditions uncomfortable. Afternoon thunderstorms are frequent on the island and nearby ocean, which makes outdoor explorations and sea activities challenging. Budget travelers find November to their liking with a dip in rain and storms.

Weather hazards

Guam is prone to thunderstorms, earthquakes, typhoons, and tropical storms. The record for single-day precipitation is 15.4" (391mm) on October 15, 1953. Guam lies on the Mariana Plate and prone to seismic activity due to frequent plate tectonics. Earthquakes occur between magnitudes of 5.0 to 8.7 on the island and the surrounding Mariana. The location of Guam in the Typhoon Alley is the reason for frequent tropical storms. Typhoons are common in the wet season, with the highest probability between August to October, although they occur year-round. Gusts up to 173mph (278.4km/h) during one such typhoon in December 2002 brought large scale devastation. Guam experiences volcanic smog from the neighboring volcanically active islands. Ironically, drought is a severe issue due to small storage capacity and groundwater sources.

Weather in January

January is a pleasant month in Guam as the dry season brings cheer to the population. The average temperatures are in the warm 76°F (24.4°C) to 85°F (29.4°C) range, and humidity is on the wane. The oppressive heat takes a backseat to welcome the New Year with festivals, dance, and excitement. Weather in January »

Weather in February

February is the coldest month of the year in Guam, with the average temperatures in the warm 75°F (23.9°C) to 85°F (29.4°C) range. The daily sunlight lasts for 6 hours, and the conditions are comfortable relative to the usually hot and humid atmosphere. The precipitation averages 4.5" (114mm) and is generally lighter than the wet season so that a rain jacket is optional. Weather in February »

Weather in March

March is the driest month in Guam as the island registers an average of 2.8" (71mm) of rain. The skies tend to be clear, with 7 hours of daily sunshine. The average temperatures are in the 76°F (24.4°C) to 86°F (30°C) range, with relative humidity significantly less than the wet season. Weather in March »

Weather in April

April is a dry month in Guam with the effects of bright sunshine reflecting in the heat. The average high temperature is 87°F (30.6°C), and the daily sunlight lasts for at least 7 hours. Guam has many open beaches where the heat becomes unbearable in the afternoon. Weather in April »

Weather in May

May is the last month of the dry season in Guam, with the average temperatures in the 77°F (25°C) to 88°F (31.1°C) range. The precipitation is to the tune of 4.3" (109mm) with 17 damp days in the month. Afternoons can be hot, but temperatures rarely rise to extreme levels, and the humidity is tenable. Weather in May »

Weather in June

June is a transitional period in Guam from the dry to the wet season. The average temperatures hover in the 77°F (25°C) to 88°F (31.1°C) range, with 6.5 hours of daily sunshine. June is fit for people looking for comfortable weather and affordable rates on flights and hotels. Weather in June »

Weather in July

July is the beginning of the wet season in Guam with the rain gauge registering an impressive 12.1" (307mm). The pickup in humidity is significant with the increased cloud cover resulting in moist conditions. The average temperatures remain in the 77°F (25°C) to 87°F (30.6°C), but the heat is oppressive for people foreign to tropical weather. Weather in July »

Weather in August

August is the wettest month of the year in Guam with an abundant 17.2" (437mm) of rainfall. The wet season and cloudy skies limit bright sunshine to 5 hours every day, although the daylight lasts for 12 hours. Easterly trade winds are at the lowest speeds of the year, and their presence is often nothing more than a gentle breeze. Weather in August »

Weather in September

September lies in the rainy season in Guam as the Pacific island frequently experiences cloudy skies and intense spells of rain. The average temperatures are between 76°F (24.4°C) and 87°F (30.6°C), with humid days and nights. Weather in September »

Weather in October

October is prone to typhoons and thunderstorms in Guam as the island territory receives an average of 11.8" (300mm) of rain. Most of the days are rainy and witness at least a brief shower. The day temperatures scale up to 90°F (32.2°C), but nights rarely drop below 70°F (21.1°C). Weather in October »

Weather in November

November is the last month of the wet season in Guam, with the precipitation decreasing gradually to register 9.1" (231mm). The average temperatures are in the 77°F (25°C) to 87°F (30.6°C) range, with the humidity exceeding 80% during the nights. The tropical storm activity is gradually on the wane as the peak typhoon period is over. Weather in November »

Weather in December

December is a transitional period to the dry season in Guam with a gradual increase in sunshine. The average temperatures are in the 77°F (25°C) to 86°F (30°C) range, but there is a decisive shift in the mood with the frequent breaking of cloud cover. Weather in December »
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