For all vehicles traveling through air, the force of drag is proportional to the coefficient of drag ("Cd"), to the square of the airspeed and to the air density. Since drag rises rapidly with speed, a key priority of supersonic aircraft design is to minimize this force by lowering the coefficient of drag. This gives rise to the highly streamlined shapes of SSTs. To some extent, supersonic aircraft also manage drag by flying at higher altitudes than subsonic aircraft, where the air density is lower.