ATHENS, Greece—A severe snowstorm disrupted road and air traffic Monday in the Greek capital of Athens and in neighboring Turkey’s largest city of Istanbul, while snow blanketed most of Turkey and Greece, including several Aegean islands. The Greek government declared a holiday Tuesday in the greater Athens area, parts of central Greece and the islands to keep people from leaving home. Key branches of the civil service as well as food stores, gas stations and pharmacies will stay open. “We ask citizens to avoid leaving their homes. It will be a difficult night,” Climate Change and Civil Protection Minister Christos Stylianides said. “The height of the snow is unprecedented in some areas.” The storm snarled traffic in Athens and put most public transport out of action. Army and fire service teams were deployed late Monday to extract hundreds of motorists trapped for hours in snowed-in cars, taking them to rented …