Weather

The first solo flight is a big milestone for student pilots. Often it’s three trips alone in the airplane around the airport pattern.

Another milestone is the required five hours of solo cross-country flights, where the student flies over the horizon depending only on his or her skills to safely navigate and manage a flight that’s going somewhere. This management includes using the airplane’s altimeter to fly at required altitudes.

On a solo flight to an airport maybe 50 miles away, a pilot is likely to have to change the altimeter’s setting along the way to reflect changes in atmospheric pressure (see “Weather: It’s Not That Simple,” September 2014 Flight Training).

Atmospheric pressure changes with the weather, affecting altimeters. Atmospheric pressures lower than the surrounding pressure are associated with foul weather, because air is rising from surface areas of low pressure. As the rising air cools, its water vapor condenses to form clouds and maybe precipitation.

Surface high pressure is formed when air is sinking from higher altitudes. These generally have clear skies because sinking air is warmed, evaporating clouds and keeping new clouds from forming.

You adjust your altimeter using a figure that all weather stations report called the altimeter setting, which you can obtain via radio if your airport has an automated weather observing system (AWOS) or a tower with an automatic terminal information service (ATIS). If you’re taking off from an airport without regular weather reports, you should use the nearest available report.

When you set your altimeter on the ground to the correct setting, it should read the elevation of the ramp on which you’re sitting. If it isn't within 75 feet, your altimeter might need calibration. If you’re sure your altimeter is correctly calibrated and a nearby altimeter setting isn’t available, you can set your altimeter to show the airport elevation when you are still on the ground.

Think of the altimeter setting as the atmospheric pressure measured at the bottom of a hole dug to sea level at the airport. In pre-computer days, weather observers calculated the altimeter setting using the weather station’s elevation and the station’s actual barometric pressure, called the station pressure. Today’s automated weather observing systems do these calculations.

SETTING YOUR ALTIMETER IN FLIGHT. When you fly more than a few miles from your takeoff point, you need to set your altimeter to the nearest airport at which you intend to land. In fact, on a trip of more than an hour or so it’s a good idea to set it regularly, using settings from airports along your route. If you receive VFR radar advisories from air traffic control, you’ll be provided a current altimeter setting every time you’re handed off to another controller.

The illustration at right shows what happens when you don’t do this when flying into an area of low atmospheric pressure. To keep it simple, we’re assuming both airports are at sea level. The flight path would be the same if the airports were at different elevations.

At Airport A, on the left, you set your altimeter to 30.74 and, since you’re at sea level, it reads 0 for the altitude. As you climb, the altimeter senses lower and lower pressure until the pressure is 25.74 inches of mercury. The altimeter reads 5,000, and you level off and head for Airport B on the right.

Since the surface pressure is lower at B than at A, the pressure levels aloft slope down from A to B following the orange line labeled “5,000 feet, indicated altitude.”

Let’s assume you’re so busy holding an indicated altitude of exactly 5,000 feet and enjoying the view that you neglect to obtain the altimeter setting for Airport B and do not reset your altimeter. As you arrive over B, your altimeter still reads 5,000 feet. But, how high are you actually above the sea-level airport? When you reset your altimeter to Airport B’s 29.74 reading, you see that you are really 4,000 feet above the ground.

The saying “high to low, look out below” helps you remember what happens when you fly into an area of lower atmospheric pressure. Your airplane will be closer to mountaintops or antennas than you think.

What would happen if, in this example, you reset your altimeter to the airport’s setting of 29.74 and returned to Airport A at an indicated altitude of 4,000 feet, without resetting your altimeter when you arrive back at A?

You would follow the orange line to end up higher than your indicted indicated altitude at A. You could remember what happens in such a case with something like: “Low to high, I head for the sky.”

Flying into an area of higher pressure without changing the altimeter setting isn’t as dangerous as the trip from A to B, but it’s still dangerous. Imagine entering a traffic pattern that’s supposed to be 1,000 feet above the airport without setting your altimeter. You’d be 2,000 feet above the airport on the downwind leg. If while on the downwind leg you realize that things don’t look right, you should leave the pattern, set your altimeter, and reenter the pattern at the correct altitude. Otherwise you could descend onto an airplane flying at the correct pattern altitude.

WARM TO COLD, LOOK OUT BELOW. Flying into an area with lower temperatures, like flying into areas with lower atmospheric pressure, can be hazardous. The chart on page 43 illustrates how, when the air is colder, the molecules of the gases that make up the air are closer together (on the right in the chart) than in warmer air (on the left). The result is that when the air becomes colder, a column of air shrinks—just as most things do when they are chilled. As a result, the altitudes of the various pressures aloft will be lower in the cold air than in the warm air, even though the pressure at the surface is the same.

Surface pressure depends on how much air is above a point. The same amount of air is above all of the points in the chart, but the column of air is higher in the warm air on the left; the air has expanded. This is why the “high to low, look out below” rule applies to temperature as well as to air pressure. Fortunately, unless you are flying into extremely cold weather, you aren’t as likely to run into trouble as from not setting your altimeter correctly to account for atmospheric pressure changes.

This was illustrated during the extremely cold weather (even for Alaska) in early February 1999, when the FAA prohibited instrument landings at some airports because the cold made altimeter readings too unreliable.

Jack Williams is an instrument-rated private pilot, and author of The AMS Weather Book: The Ultimate Guide to America’s Weather.

Generally, the higher the altitude, the lower the air pressure.