Guam has a tropical climate with two seasons: a dry season from January to June, and a wet season from July to December. For hiking around Cetti Bay — the overlook, Mount Lamlam, and the trails down to Fouha Point — the dry season is considerably more pleasant. Trails become slippery and overgrown during the wet season, and the views from the overlook are often obscured by cloud and haze after heavy rain.
The best time to visit is early morning in the dry season (February to May). The light is better for photography, the heat is manageable before midday, and there is less chance of afternoon cloud rolling in from the hills. Avoid midday in any season — the exposed ridgeline on the Mount Lamlam trail and the open platforms at the overlook offer no shade, and temperatures regularly exceed 30°C.
Typhoon season overlaps with the wet season and peaks between August and November. Typhoons are infrequent but can be severe; check forecasts if travelling in this period.