The altitude of departing aircraft over a particular area depends on a number of factors. Aircraft climb rates and flight profiles vary considerably. New, modern aircraft (e.g., Airbus A320, Boeing 757) have higher-thrust engines and improved wing designs compared to older aircraft (e.g., Boeing 727, DC-9), which results in a superior climb rate. Even though newer aircraft have more thrust, they also have much quieter engines than older aircraft. Temperature and air density also play a role in aircraft performance and climb rate. As the temperature increases, air density decreases which reduces engine thrust and results in a longer takeoff distance and a lower climb rate. Wind is another important factor in determining aircraft altitude. A strong headwind (coming toward the front of the aircraft) will reduce an aircraft takeoff and landing distance, and increase the climb rate. Air Traffic Control must also consider safety and separation requirements of inbound/outbound aircraft. Combined, these factors heavily influence the operational characteristics of aircraft and play an important role in determining the altitude of aircraft departing over any given area.