Aircraft use barometric altitude, e.g. FL300 is the barometric altitude of 30,000 ft MSL. In winter however, FL300 is actually lower than the absolute altitude of 30,000 ft because of a cold and therefore compressed atmosphere, and in summer FL300 should be higher than 30,000 ft because the heated up atmosphere stretches itself out.
Does this mean that in summer it would be more difficult for a plane to reach a certain barometric altitude (e.g. FL300) than in winter? Is it more likely for planes to fly at higher barometric altitudes / flight levels in winter because they don't have to fly up a way as long as in summer?