The atmospheric pressure decreases by 11% for every 1km of height above the Earth's surface from a pressure of 1005 hPa at sea level

Y=1005*0,89^x

x stands for the Number of kilometers

It's an exponential function because it's 11!%! You Always start with the value at x=0 (Here 1005) and multiply it by the percentage with which it increases/decreases.