The cloudless sulphur has a wingspan of about two and one-half inches. The male and female have different appearances. The upperside of the male’s wings is bright yellow with very tiny dark spots at the edge of the forewing. The underside of his wings may be yellow or yellow with red-brown spots. The upperside of the female’s wings is yellow to white with some black spots on the edge of the forewing and a large black spot near the center of the forewing. The underside of her wings has some red-brown markings. The underside of the wings of both sexes shows a dark spot just above the center of each wing.
The cloudless sulfur flies in open fields, in clearings, in forest edges and along rivers, railroads and roads. Its larva eats partridge pea plants (Chamaechrista spp.). The adult feeds on flower nectar and is active from April to November. This species migrates into Illinois from the south each year to recolonize the state. There may be three generations annually in southern Illinois. The cloudless sulphur has been found statewide but is most common in the southern half of the state.