Figure 1-11. Pressure Change vs Indicated and MSL Altitude

AVIATION WEATHER

CHAPTER ONE

This updating is accomplished via radio throughout the flight. Usually, when switching to a

different air traffic controller, about every 50 - 100 miles, an updated altimeter setting will also be

passed to the aircrew. This ensures all aircraft in a given area are flying at the correct altitudes

(up to FL180). A change in pressure of 0.10 in-Hg will change the altimeter reading 100 feet.

Therefore, it is imperative to receive a current altimeter setting at your destination prior to

landing. If the altimeter is not adjusted and your flight path takes you into an area of lower MSL

pressure the aircraft will be lower than the altimeter indicates. Conversely, if your flight path

takes you into an area of higher MSL pressure, the aircraft will be higher than the altimeter

indicates. These events are summarized by a set of rhymes, as well as by Figure 1-11.

PRESSURE CHANGE

ALTIMETER

ACTUAL MSL ALTITUDE

Flying toward lower MSL

Indicates higher than actual

Lower than indicated by the

pressure

altimeter

Flying toward higher MSL

Indicates lower than actual

Higher than indicated by the

pressure

altimeter

Figure 1-11 Pressure Change vs Indicated and MSL Altitude

RULE: High to Low, look out below

The aircraft is lower than indicated, thus the indicated altitude is higher than the aircraft.

RULE: Low to High, plenty of sky

The aircraft is higher than indicated, thus the indicated altitude is lower than the aircraft.

It is quite common to confuse these two concepts. Remember, if you have a one inch decrease in

barometric pressure your altimeter (i.e., indicated) will read 1000 feet higher. If this difference

occurred gradually between two points you would fly to a 1000 foot lower true/MSL altitude

because of your altimeter error.

Temperature

Pressure changes have a far greater effect on an altimeter error than temperature. Nevertheless,

when the margin for error is low (extreme low altitude flight), temperature change needs to be

taken into account.

When accounting for altimeter error attributed to temperature use the rules shown for pressure

above (High to Low). For example, when traveling from point A with a temperature of 70F to

point B with a temperature of 90F, without updating the altimeter setting, you can expect to be

higher than your altimeter is showing ("low to high, plenty of sky").

General Structure of the Atmosphere, and Atmospheric Temperature and Pressure 1-13