In the late 18th and early 19th centuries, intrepid European artists took to studying the skies, traveling from the Baltic coast to the Swiss Alps to capture atmospheric effects across the continent. French Impressionists in particular sought the magical ephemerality of painting en plain air (in the open air). The 100 oil sketches in this exhibition display the many ways artists capture the outdoors—meticulously, roughly, romantically, minimally. Among those included are Jean-Baptiste-Camille Corot, John Constable, Simon Denis, Jules Coignet, and André Giroux. —E.C.