The climate on Ertholmene is drier and sunnier than in the rest of Denmark. The average air temperatures are the same as the rest of the country, but the monthly distribution is somewhat different. The temperature of the sea has a significant influence on the temperature of the air, and on Ertholmene, the sea temperature is somewhat higher between October and January and somewhat lower from March to June.

Because of the lower sea temperatures in spring, many plants and trees come into leaf two or three weeks later than for example on Zealand.

Christiansø’s climate and nature are strongly influenced by the surrounding sea and by the granite that makes up the islands. Both trap in the heat during the summer months. This means that the air temperature stays relatively warm far into autumn, while the cold sea temperatures mean that spring is cooler and arrives later than in the rest of Denmark.