Excluding the disputed south sea islands, Hǎinán is the southernmost tip of China. Sānyà, in the south, is roughly on the same latitude as the southern reaches of Hawaii and Hanoi in Vietnam, so it can be relied upon to be warm even when the rest of China is freezing.
Weather is best from about November to early March, when average temperatures of 21°C (69.8°F) prevail; the yearly average is 25.4°C (77.7°F). Not surprisingly, this is also the busiest time. From March/April to October/November is not nearly as busy because it can be so diabolically hot and humid that you’re liable to dissolve into a grease spot. The pay off is in heavily discounted hotel prices.
Hǎinán is hit by at least one typhoon a year, usually between May and October. And while it seldom gets the worst of the winds, even the edge of the typhoon can cripple transport and communication with the mainland.