Using an Aircraft Altimeter as a Barometer

An altimeter is a device designed to estimate altitude above mean sea level (MSL). It does

so solely on the basis of atmospheric pressure. In fact, all altimeters have TWO dials--one

for altitude and one for pressure. An altimeter is basically a mechanical calculator which

translates a change in atmospheric pressure into a change in altitude based on the idea of

a 'standard' atmosphere.

This hypothetical atmosphere starts at 'mean sea level' with a temperature of 15 C

and 29.92 inches of mercury (inHg). It decreases in pressure at a rate of about 1 inHg per

1000 ft elevation and decreases in temperature at a rate of about 2 C per 1000 ft elevation.

Due to the unequal heating of the earth's surface by the sun, however, the real

atmosphere is far from 'standard', and if an altimeter were not adjustable, its reading would

be dangerously inaccurate. To account for the natural variation in barometric pressure

around the world, the aviation community arrived at the idea of an 'altimeter setting'.

All altimeters have a small knob at the bottom left which allows the altimeter to be set

based on the local atmospheric conditions. A pilot can adjust this knob while on the ground

at an airport until the altimeter reads the airport's known elevation. In addition, thousands of

airports around the world have automated stations which determine and broadcast over the

radio the exact 'altimeter setting' (in inHg) which should be used at their location to have an

altimeter on the runway read the correct airport elevation. Pilots flying below 18,000 ft MSL

listen to these broadcasts and every 100 miles or so readjust their altimeters in flight so that

they read appropriately for the local atmospheric conditions. At 18,000 ft MSL and above,

pilots simply ignore local conditions and set their altimeter to 29.92 inHg. This action

ensures both a high probability of being at the indicated altitude when landing and a low

probability of two aircrafts colliding due to non-standard atmospheric conditions when

cruising at high altitude.

The idea behind using an altimeter as a barometer is this: When adjusted to read

true altitude, altimeters display in their altimeter setting window the hypothetical barometric

pressure at mean sea level assuming a standard atmosphere. On the other hand, when

adjusted to read 0 ft altitude, they display in their altimeter setting window the true

barometric pressure. As the true barometric pressure drops, they indicate a rising altitude,

as though in a climbing aircraft. At the same time, the pressure read in the altimeter setting

window remains constant, as though based at an airport below. However, if they are

readjusted to read 0 ft altitude, the pressure they display will drop to reflect the true drop in

atmospheric pressure.

Thus, to use an altimeter as a barometer, simply adjust it to read 0 ft altitude and read

the true pressure in inches of mercury in the altimeter setting window.

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