From the manufacturer:
Uni Chalk Markers are water-resistant, water-based chalk markers ideal for outdoor use on non-porous surfaces like chalk boards and windows. Producing a vivid, opaque color finish, they are perfect for creating signs and menu boards in shops, cafes and restaurants. The ink is odor-free, does not contain solvents or alcohol, and is specially formulated for easy cleaning with a damp cloth. Available in four tip sizes.