Do you need to know, when is the best time to go to Guam? Then you should take a look here, what weather you can expect there during the year.
The climate of Guam is tropical marine. You can expect warm and humid weather, temperatures are moderated by northeast winds. There are two seasons, the dry season is from January to June, the rainy season is from July to December. There are only little seasonal variations of temperatures.
When speaking about the usual weather in this state, the information is relevant for example for these popular places: Diving in Guam, Hagåtña, Mangilao, Talofofo, Tamuning, Tumon.
Let's look now what weather you can expect in the area around the capital and in many other cities. The warmest month here is September, the average maximum temperature is 31.8 ℃ (89.24 ℉). The average minimum temperature in this month is 26 ℃ (78.8 ℉). The coldest month here is December, the average maximum temperature is 30 ℃ (86 ℉). The average minimum temperature in this month is 25.2 ℃ (77.36 ℉). 
Typical (usual) weather in Guam in January is this: The average highest temperature in Guam in January is 30.8 ℃ (87.44 ℉). The average lowest temperature is 24.3 ℃ (75.74 ℉). At the beginning of January you can expect lower temperatures, the average highest temperature is around 30.4 ℃ (86.72 ℉). At the end of January you can expect lower temperatures, the average highest temperature is around 30.6 ℃ (87.08 ℉). The average rainfall is 115.7 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:20 (hours and minutes), in the middle of the month 11:24 and at the end of the month 11:32.These numbers above are valid primarily for the capital and the area around it.Typical (usual) weather in Guam in February is this: The average highest temperature in Guam in February is 30.4 ℃ (86.72 ℉). The average lowest temperature is 24.4 ℃ (75.92 ℉). At the beginning of February you can expect higher temperatures, the average highest temperature is around 30.6 ℃ (87.08 ℉). At the end of February you can expect higher temperatures, the average highest temperature is around 30.55 ℃ (86.99 ℉). The average rainfall is 96.6 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:32 (hours and minutes), in the middle of the month 11:41 and at the end of the month 11:52.These numbers above are valid primarily for the capital and the area around it.Typical (usual) weather in Guam in March is this: The average highest temperature in Guam in March is 30.7 ℃ (87.26 ℉). The average lowest temperature is 24.5 ℃ (76.1 ℉). At the beginning of March you can expect lower temperatures, the average highest temperature is around 30.55 ℃ (86.99 ℉). At the end of March you can expect higher temperatures, the average highest temperature is around 31 ℃ (87.8 ℉). The average rainfall is 75 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:52 (hours and minutes), in the middle of the month 12:03 and at the end of the month 12:15.These numbers above are valid primarily for the capital and the area around it.Typical (usual) weather in Guam in April is this: The average highest temperature in Guam in April is 31.3 ℃ (88.34 ℉). The average lowest temperature is 24.9 ℃ (76.82 ℉). At the beginning of April you can expect lower temperatures, the average highest temperature is around 31 ℃ (87.8 ℉). At the end of April you can expect lower temperatures, the average highest temperature is around 31.3 ℃ (88.34 ℉). The average rainfall is 101.7 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:15 (hours and minutes), in the middle of the month 12:26 and at the end of the month 12:36.These numbers above are valid primarily for the capital and the area around it.Typical (usual) weather in Guam in May is this: The average highest temperature in Guam in May is 31.3 ℃ (88.34 ℉). The average lowest temperature is 25 ℃ (77 ℉). At the beginning of May you can expect lower temperatures, the average highest temperature is around 31.3 ℃ (88.34 ℉). At the end of May you can expect higher temperatures, the average highest temperature is around 31.35 ℃ (88.43 ℉). The average rainfall is 152.5 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:36 (hours and minutes), in the middle of the month 12:44 and at the end of the month 12:51.These numbers above are valid primarily for the capital and the area around it.Typical (usual) weather in Guam in June is this: The average highest temperature in Guam in June is 31.4 ℃ (88.52 ℉). The average lowest temperature is 26.1 ℃ (78.98 ℉). At the beginning of June you can expect lower temperatures, the average highest temperature is around 31.35 ℃ (88.43 ℉). At the end of June you can expect lower temperatures, the average highest temperature is around 31.35 ℃ (88.43 ℉). The average rainfall is 164.1 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:51 (hours and minutes), in the middle of the month 12:54 and at the end of the month 12:54.These numbers above are valid primarily for the capital and the area around it.Typical (usual) weather in Guam in July is this: The average highest temperature in Guam in July is 31.3 ℃ (88.34 ℉). The average lowest temperature is 25.9 ℃ (78.62 ℉). At the beginning of July you can expect higher temperatures, the average highest temperature is around 31.35 ℃ (88.43 ℉). At the end of July you can expect higher temperatures, the average highest temperature is around 31.45 ℃ (88.61 ℉). The average rainfall is 268.7 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:54 (hours and minutes), in the middle of the month 12:51 and at the end of the month 12:44.These numbers above are valid primarily for the capital and the area around it.Typical (usual) weather in Guam in August is this: The average highest temperature in Guam in August is 31.6 ℃ (88.88 ℉). The average lowest temperature is 24.9 ℃ (76.82 ℉). At the beginning of August you can expect lower temperatures, the average highest temperature is around 31.45 ℃ (88.61 ℉). At the end of August you can expect higher temperatures, the average highest temperature is around 31.7 ℃ (89.06 ℉). The average rainfall is 344.2 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:44 (hours and minutes), in the middle of the month 12:35 and at the end of the month 12:25.These numbers above are valid primarily for the capital and the area around it.Typical (usual) weather in Guam in September is this: The average highest temperature in Guam in September is 31.8 ℃ (89.24 ℉). The average lowest temperature is 26 ℃ (78.8 ℉). At the beginning of September you can expect lower temperatures, the average highest temperature is around 31.7 ℃ (89.06 ℉). At the end of September you can expect lower temperatures, the average highest temperature is around 31.8 ℃ (89.24 ℉). The average rainfall is 346 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:25 (hours and minutes), in the middle of the month 12:14 and at the end of the month 12:03.These numbers above are valid primarily for the capital and the area around it.Typical (usual) weather in Guam in October is this: The average highest temperature in Guam in October is 31.8 ℃ (89.24 ℉). The average lowest temperature is 25.5 ℃ (77.9 ℉). At the beginning of October you can expect lower temperatures, the average highest temperature is around 31.8 ℃ (89.24 ℉). At the end of October you can expect lower temperatures, the average highest temperature is around 31.3 ℃ (88.34 ℉). The average rainfall is 304.7 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:03 (hours and minutes), in the middle of the month 11:52 and at the end of the month 11:41.These numbers above are valid primarily for the capital and the area around it.Typical (usual) weather in Guam in November is this: The average highest temperature in Guam in November is 30.8 ℃ (87.44 ℉). The average lowest temperature is 24.8 ℃ (76.64 ℉). At the beginning of November you can expect higher temperatures, the average highest temperature is around 31.3 ℃ (88.34 ℉). At the end of November you can expect lower temperatures, the average highest temperature is around 30.4 ℃ (86.72 ℉). The average rainfall is 205 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:41 (hours and minutes), in the middle of the month 11:32 and at the end of the month 11:25.These numbers above are valid primarily for the capital and the area around it.Typical (usual) weather in Guam in December is this: The average highest temperature in Guam in December is 30 ℃ (86 ℉). The average lowest temperature is 25.2 ℃ (77.36 ℉). At the beginning of December you can expect higher temperatures, the average highest temperature is around 30.4 ℃ (86.72 ℉). At the end of December you can expect higher temperatures, the average highest temperature is around 30.4 ℃ (86.72 ℉). The average rainfall is 139.8 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:25 (hours and minutes), in the middle of the month 11:20 and at the end of the month 11:19.These numbers above are valid primarily for the capital and the area around it.