Pressure decreases with an increase in altitude. This pressure decrease is not linear.

• At high altitude, the gradient of pressure is less than at sea level.
• At sea level, you lose 1 hPa every 30 ft of altitude. At 30 000 ft, you lose only approximately, 0.4 hPa every 30 ft.

ALTITUDE VS PRESSURE:
5 000 ft = 4/5 of sea level pressure (80%)
8 000 ft = 3/4 of sea level pressure (75%)
18 000 ft = 1/2 of sea level pressure (50%)
36 000 ft = 1/4 of sea level pressure (25%)