The twin island state of Antigua and Barbuda lies in the eastern arc of the Leeward Islands of the Lesser Antilles, separating the Atlantic Ocean from the Caribbean Sea. Antigua is 650 km southeast of Puerto Rico and Barbuda lies 48 km due north of Antigua. Low-lying and volcanic in origin, they are part of the Leeward Islands group in the northeast Caribbean. Antigua has an area of 108 sq miles and Barbuda 62 sq miles. The terrain is mostly low-lying limestone and coral islands, with some higher volcanic areas. The sandy soil on much of the islands has only scrub vegetation. Some parts of Antigua are more fertile–most notably the central plain–due to the volcanic ash in the soil. These areas support some tropical vegetation and agricultural uses The climate is characterized as tropical maritime with little seasonal temperature variation. Rainfall averages 990 mm (39 in) per year, with the amount varying widely from season to season. The islands generally experience low humidity and recurrent droughts.