What best describes the standard atmosphere at sea level?

• A. At sea level, standard atmosphere is 29.92 inHg or 1013.2 mb, 15 C; pressure and temperature decrease with altitude; standard lapse rate about 1 inHg and 2 C per 1000 ft; at 3000 ft, 26.92 inHg and 9 C.
• B. It is the weather report for the day.
• C. It is the air's humidity.
• D. It is the total air density at sea level.

Answer: (A)

The standard atmosphere at sea level is a defined reference used in aviation to provide consistent values for pressure and temperature with altitude. At sea level it fixes the pressure as 29.92 inches of mercury (1013.2 millibars) and the temperature as 15 °C. From there, pressure and temperature change with height in a predictable way: pressure decreases about 1 inHg per 1000 feet, and temperature decreases about 2 °C per 1000 feet (the standard lapse rate). For example, at 3,000 feet, the standard pressure would be around 26.92 inHg and the temperature about 9 °C. This model is a reference, not a weather report, and not a measure of humidity or a single density value. Density can be derived from pressure and temperature using the equations of state, but the standard atmosphere itself is defined by the base pressure and temperature and their change with altitude, not by density alone.