Final answer:
The normal pressurization above 18,000 ft is maintained to ensure safety and comfort in an aircraft by keeping cabin pressure equivalent to a lower altitude. As altitude increases, barometric pressure decreases, leading to lower boiling points for water at high altitudes.
Step-by-step explanation:
The normal pressurization above 18,000 ft refers to the aircraft cabin pressure maintained at higher altitudes to ensure a comfortable and safe environment for passengers and crew. As an aircraft climbs, the ambient atmospheric pressure decreases, but aircraft are designed to maintain a cabin pressure equivalent to the atmospheric pressure at a lower altitude, typically around 6,000-8,000 ft. This is to avoid hypoxia (a deficiency in the amount of oxygen reaching the body's tissues), which can occur if the cabin pressure is too low.
Barometric pressure changes as altitude increases, affecting various activities such as boiling point calculation of water, oxygen availability, and pressurization systems. For instance, water boils at a lower temperature at high altitudes due to the decrease in atmospheric pressure: at 1500 m above sea level (approximately 5000 ft), the atmospheric pressure is about 8.59 x 104 N/m2, and water boils at a temperature lower than 100°C. Similarly, at 3000 m or approximately 10,000 ft, where the atmospheric pressure is 7.00 x 104 N/m2, water boils even at a lower temperature.