How do you read an altimeter?

An altimeter is essentially a highly sensitive barometer. In order to use an altemeter to determine your altitude you must first set the altimeter to compensate for local meteorological changes in air pressure. This is done by turning the knob until the current local ambient air pressure is shown in the kollsman window.

The kollsman window is a small square window usually in the face of the altimeter. Standard air pressure is 29.92.

Once the ambient air pressure is set, altitude can be read by using the three indicators on the dial of the gauge, similar to the way a clock is read. The long-skinny arm indicates 100's of feet, the shorter arm indicates thousands, and tens of thousands of feet may be indicated by either an even shorter arm, or by a triangle which floats near the ring of numbers.

For an interactive lesson on altimeter usage follow the link below.

It should be noted that while the barometric-style altimeter is found in nearly all aircraft, there are other ways to determine altitude using radar, lasers, or GPS. These systems, while generally more accurate have not found their way into all aircraft yet, and are usually only found in more sophisticated aircraft such as airliners or executive jets. Radar and GPS altitudinal information is displayed to the pilot in dozens of different ways from aircraft to aircraft and there does not seem to be any single standard system.