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" (2489.2mm) 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.
Guam is prone to thunderstorms, earthquakes, typhoons, and tropical storms. The record for single-day precipitation is 15.4" (391.2mm) 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. Temperatures variations between day and night are little, and the proximity to the equator keeps them of equal duration. The rainfall registers up to 5" (127mm) in January during 18 wet days in the month. Lightweight, casual clothing is suitable, and the umbrella can be given a miss, especially in the evening. The season is busy with tourists roaming the length and breadth of the island and the adjoining seas in the calm atmosphere. Fine sandy beaches like Ypao and coral-filled waters are perfect for vacations with moderate sunshine for a company. A visit to Guam in January is the perfect antidote to the severe winters in the American North.
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" (114.3mm) and is generally lighter than the wet season so that a rain jacket is optional. Hagatna (Agana), the capital in central Guam, sees relative humidity between 73 to 78%, with daily high temperatures rarely touching 90°F (32.2°C). Seawater temperatures are perfect for swimming, while snorkeling and scuba diving are just the right activities to explore the exciting underwater world. Sunset cruises are popular in the peaceful realm as the waters remain calm in the absence of storms. Cozy nights provide a perfect environment for fun-filled entertainment. The lack of heavy rainfall makes February the most desirable month for tourism in Guam and costliest on the wallet.
Weather in March
March is the driest month in Guam as the island registers an average of 2.8" (71.1mm) 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. Wind speeds tend to be at the highest of the year in the 15mph (24.1km/h) to 20mph (32.2km/h) range and chase away cloud cover and rain. The brilliant sea is a paradise for underwater life and marine activities in the clear waters amid beautiful sunshine. The Fish Eye Marine Park is the perfect host during the warm days and comfortable nights with even an observatory to watch the marine life without getting wet. The coral makes the seafloor rough, and it is a good idea to have protection for the feet while walking in the water. High tide is the time of fascinating views of the coral world. March is a nice time to visit Guam.
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. The rainforest is the right place to explore for adventure seekers who wish to escape the intense sun rays. Put on sunscreen if on the beach after morning hours and use a broad-brimmed hat. The rainfall registers an average of 3.6" (91.4mm) and shows signs of a gradual reversal of the dry cycle. The wind is a strong motivation for sailing in the ocean waters to those who wish to try something else than snorkeling and swimming. Expect cloud cover occasionally during the afternoons with follow-up rain. April gives more space on the beaches and resorts in Guam as the tourist season slows down.
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" (109.2mm) with 17 damp days in the month. Afternoons can be hot, but temperatures rarely rise to extreme levels, and the humidity is tenable. Light summer clothing is the order of the day, and nights do not need extra layers. The water is the best remedy for the increasing heat and air-conditioners are in action indoors full time. Punta Dos Amantes is the most famous attraction of Guam with a spending view of the mountains, limestone plateaus, and reefs in the clear skies of May. Expect moderate rainfall even on sunny days. However, it is still possible to be outdoors for long hours without rain protection gear. May receive an average of 220 hours of sunshine, the highest monthly amount of the year in Guam.
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. Additionally, the crowds dwindle with the onset of the wet season by the end of the month. The average rainfall increases to register a significant 7.1" (180.3mm) within 25 wet days in the month. Trade winds, which were strong earlier, lose strength resulting in increasing cloud cover. Mornings and evenings are the best times of the day to avoid a downpour and enjoy the beaches. Sunrays are intense from 11 a.m. to 4 p.m., and it is advisable to avoid open beaches during this time. An umbrella or a rain jacket is useful in Guam as the precipitation occurs almost every day in June.
Weather in July
July is the beginning of the wet season in Guam with the rain gauge registering an impressive 12.1" (307.3mm). 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. The humidity often registers above 80%, and air-conditioners keep whirring to keep the conditions comfortable. Cloudy skies allow only 5 hours of daily sunlight, but the rays are intense whenever the sun breaks the cloud cover. Dolphin watching, paddle boating, parasailing, and beach volleyball are still possible during extended periods of rain breaks. Carry a rain jacket when exploring trails and visiting attractions such as Tarzan Falls. If the rain is heavy, there are plenty of shopping malls, museums, indoor amusement parks that will entertain kids as well as adults. Keep an eye on thunderstorms and heavy rain during July in Guam.
Weather in August
August is the wettest month of the year in Guam with an abundant 17.2" (436.9mm) 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. The humidity takes its toll more than the temperatures, especially near the coast and in the rainforests. The average temperatures are in the range of 76°F (24.4°C) to 86°F (30°C), but the conditions tend to be hot and humid for the majority of the visitors. Sheltered beaches offer lovely views of the ocean and protection from the bright sun when it breaks the cloud cover. The tourist population withers down with the gradual deterioration in conditions. August has volatile weather in Guam as the peak of the typhoon season and extreme rainfall both coincide during the month.
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. The precipitation is characteristic of the tropics, with occasional heavy spells that push the monthly average to 14.2" (360.7mm). Rain jackets and umbrellas become necessary as brief showers lower the heat. September is the month with the least sunshine of 132 hours in the month. Southern Guam is home to cascading waterfalls that offer mesmerizing views in the wet season. Watch out for tropical storms in September in Guam when planning sea excursions. Wear water-proof shoes while venturing out on the trails. Summer clothing is usually sufficient during the day as well as night. There are plenty of indoor options suitable for all ages, even on rainy nights.
Weather in October
October is prone to typhoons and thunderstorms in Guam as the island territory receives an average of 11.8" (299.7mm) 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). Typhoons can be destructive, with wind gusts above 100mph (160.9km/h). However, the stricter enforcement of construction codes have kept homes and hotels safe for the population. The sticky weather results in much perspiration that asks for loose-fitting clothing and sweatshirts for comfort. Water sports are a great way to beat the heat and humidity. The sea temperatures are in the warm range of 83°F (28.3°C) to 85°F (29.4°C), and the waters remain clear and bright. A buildup in storm activity and rainfall is usually harmful to the tourist industry. However, budget travelers revel when accommodation costs drop during the slow season in October in Guam.
Weather in November
November is the last month of the wet season in Guam, with the precipitation decreasing gradually to register 9.1" (231.1mm). 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. However, the good idea is to spend more time on the western side of the island where the Philippine Sea is more tranquil than the other side. The sunrays are bright on the open beaches where people spend time even until noon. Historic sites and museums witness many visitors, especially when the day is wet and damp of outdoor activities. Light rain jackets are useful during most of the outdoor explorations. Gentle breezes make evenings attractive for outings. Guam sees a gradual increase in the tourist population with the reversal of the precipitation cycle by the end of 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. The atmosphere is rarely cold, with nights mostly above 70°F (21.1°C). The perspiring conditions give way to a comfortable and relaxed atmosphere with a drop in humidity. The average rainfall registers 6" (152.4mm) during 22 wet days in the month. The average daily sunshine increases to 142 hours in December, signaling bright days ahead. The Apra Harbor is a favorite spot for diving and snorkeling in the clear waters. Visit the inland jungles to smell the perfume of wildflowers and watch the abundant species of animals and birds in the rainforests. White sandy beaches with gentle waves lap towards the shore. Expect plenty of activity during Christmas and the approaching New Year with comfortable weather on the cards in December.