How does atmospheric pressure change in the troposphere?
Atmospheric pressure in the troposphere drops by about 1 inch of mercury for every thousand feet of altitude above sea level.
Gravity of the earth holds our atmosphere close to the surface, so the density (and pressure) of air gets gradually lower as you go to higher altitude. At sea level, atmospheric pressure is about 30 inches of mercury, and this decreases at a rate of approximately 1 inch for every thousand feet of altitude above sea level, so at an altitude of 4000 feet, the pressure is approximately 26 inches of mercury.
The troposphere is where the effects of weather are most important, and the actual pressure depends on temperature and the presence of high and low-pressure weather systems. Temperature changes with altitude in complicated ways. That's why the 1 inch per thousand feet figure is only approximate.
When airplanes fly in the troposphere, pilots receive barometric pressure corrections from controllers (these are called altimeter settings), which compensate for local changes in temperature and pressure, so that planes can fly at consistent altitudes. The altitudes are not always accurate at high altitudes, but are designed to be highly accurate near the ground so that landings are safe.