Flying the Pressure Hills

by H. Clay Gorton

Density altitude is something we all contend with. Before a flight, altimeter settings are adjusted to the airport elevation and station pressure and temperature are taken into account when calculating the length of the takeoff roll.

However, atmospheric pressure and temperature are dynamic meteorological conditions that are constantly changing, and they affect flight performance as well as takeoff and landing conditions. Aircraft operations would be safer and simpler if only airplanes were equipped with altimeters, which they are not. What we call an altimeter is, in reality, an aneroid barometer, which measures atmospheric pressure. When we fly a constant indicated altitude, we are actually flying a constant barometric pressure.

If we could visualize the constant isobaric surface indicated by our onboard “altimeter,” we would see a surface of hills and valleys in fluid motion. If we fly at constant indicated altitude into an area of lower pressure, we fly ‘downhill’ into the pressure valley. If we fly into an area of higher pressure, we climb the pressure hill.

Pressure variations affect us as we fly cross-country from one pressure level to another or in an extended local flight where the pressure wave may pass over the airport.

In local flights of any extended duration, it is always well to get a current altimeter setting before beginning the landing sequence. In cross-country flights, we may not be able to get an en route altimeter setting. Therefore, it would be prudent in a preflight weather briefing to get the en route barometric pressure, especially when flying over mountainous terrain.

But what is the extent of actual normal pressure variations on true altitude? It is not uncommon for the pressure drop into a low pressure area to be as much as 0.5 inches Hg over a distance from as little as 200 miles in a severe front, to as much as 500 miles in an extended cyclonic system.

Since the atmospheric pressure above a point decreases by about 0.1 inch per 100 feet of altitude, the pressure effect can, indeed, be appreciable. The graph at left shows the altitude effect of flying at a constant altimeter setting from a high-pressure area of 30.0 inches to a low-pressure area of 29.0 inches.

Air temperature also affects the density of the air and, consequently, the density altitude. At station altitude, although the air pressure may be constant, as the temperature rises, the air becomes less dense, resulting in a higher density altitude.

The density altitude increases by about 60 feet per Fahrenheit degree increase in temperature. The density altitude vs. temperature at standard pressure for Salt Lake City International is shown below.

Flying from warm air into cold air has the same effect as flying from a high to a low atmospheric pressure. As air cools, it condenses; thus, the same atmospheric pressure level in the colder air would be closer to the ground.

Since the onboard altimeter follows constant pressure levels, as we fly from warm to cold air, we fly into a ‘temperature valley’ in the atmosphere. As mentioned, the magnitude of this effect is about 60 feet per degree Fahrenheit. Thus, a temperature drop of only 10 F translates into a decrease in true altitude of 600 feet.

When flying from a warmer high-pressure area into a cooler low-pressure area— double jeopardy! High to low, look out below.