The Marshall Islands have a tropical maritime climate year-round, with temperatures hovering between 27°C and 30°C regardless of season. There is no cold season to avoid.
The drier, calmer months run roughly from November through April. Seas are gentler and rainfall less frequent, making this the better time for diving and any lagoon activity. The wetter season from May through October brings more frequent rain and occasional rough weather, though conditions inside the lagoon remain relatively sheltered compared to exposed ocean.
One scheduling consideration unique to Kwajalein: the US Army conducts ICBM test launches at irregular intervals, with missiles arriving from Vandenberg Air Force Base in California. Tests are not publicly scheduled. When they happen at night, the incoming warheads and their burning debris can light up the sky in a spectacular and slightly eerie display. There is no way to time a visit around these tests.
Wreck diving conditions in the lagoon are generally good throughout the year, given how protected the water is from ocean swells.