VASYLKIV and VINNYTSIA, Ukraine — On a chilly morning in early spring, humid mist and low clouds hang over the air base of Vasylkiv, a city of 36,000 people located 30 kilometers southwest of Kyiv. It is home to the Ukrainian Air Force’s 40th Tactical Aviation Brigade.

By midday, the sky has cleared, and to the great joy of the brigade’s pilots, the base’s flight control center gives the go-ahead for a flight session — for the first time in this week of poor weather.