Hurricane hunter airplanes normally drop packages of instruments that radio back data such as air pressure as they fall toward the ocean beneath a small parachute. The last air pressure report before the instruments hit the water is the surface pressure. But N43RF didn't have any of these instrument packages, called dropsondes, aboard when it approached Hurricane Gilbert on Sept. 13, 1988, because of a dispute over whether NOAA or the National Weather Service would pay for them.
Even without a direct surface pressure measurement, the scientists aboard N43RF determined that the surface pressure in Gilbert's eye was 888 millibars, or 26.22 inches of mercury - the lowest surface air pressure ever recorded in the Western Hemisphere. How did the scientists arrive at this record low air pressure in Gilbert's eye? With the help of their altimeter.
As N43RF flew into Gilbert's eye, the airplane's pressure altimeter was reading about 12,500 feet while the airplane's radar altimeter was showing that the airplane was actually less than 10,000 feet above the ocean, says Hugh Willoughby, one of the scientists aboard and current director of the NOAA Hurricane Research Division.
Every pilot who's ever read about or sat through a ground school session on altimeters has heard the memory aid: "High to low, look out below." Once a pilot understands what that means, he or she isn't likely to be fooled by the lies that altimeters often tell. The altimeter on N43RF is a good example; it was telling the pilots that the ocean was 12,500 feet below when it was really less than 10,000 feet below. Think about what a 2,500-foot altimeter error would mean if you were heading toward 2,000-foot mountains and the altimeter was indicating 3,500 feet. Of course, you aren't likely to encounter that kind of error unless your altimeter is broken or you are flying into a strong hurricane. Nevertheless, even a much smaller error, say a few hundred feet, can be dangerous.
To understand "high to low, look out below" and how the scientists aboard N43RF calculated the surface pressure in Gilbert's eye, let's begin with the fact that an airplane's pressure altimeter is nothing but a barometer. It's an instrument that measures air pressure and nothing else. But instead of being calibrated in inches of mercury, or the metric measurement millibars, the special barometer we call an altimeter is calibrated in feet above mean sea level.
This is possible because not only does the air's pressure decrease as you go higher into the sky, it decreases, on the average, at a known rate. This makes sense because air pressure is caused by the air above any particular point pressing down on the air at that point. The higher you go, the less air there is above you pressing down. Calibration of an altimeter is based on the correlation between atmospheric pressure and altitude, as shown in a Standard Atmosphere Table. Page 46 shows an abbreviated table showing air pressure and temperatures for each 1,000 feet up to 10,000 feet. Using this table, we can calibrate the altimeter to read 1,000 feet when it senses a pressure of 28.86 inches of mercury; 2,000 feet for 27.82 inches of mercury, and so forth.
By the way, pilots are taught the rule of thumb that pressure drops one inch of mercury for each 1,000 feet of altitude gained. This estimate is pretty close, especially for the first 5,000 feet. The rule of thumb tells you that the pressure for 5,000 feet is 24.92 (29.92 - 5). This is only .03 inches of mercury off, not bad for math you can do in your head while flying an airplane.
Altimeters calibrated to the standard atmosphere would work perfectly if the weather never changed. Unfortunately, the weather does change, sometimes bringing areas of high pressure and other times areas of low pressure - hurricanes are the ultimate in low pressure. To illustrate the problem, imagine that you keep your airplane at a sea-level airport. On a day when the air pressure happens to be exactly 29.92, your altimeter reads 0, accurately reflecting the airport's sea level elevation. Now imagine that you return to the airport a few days later when an area of low pressure has moved in. Now the airport barometer reads 28.86. If the altimeter hasn't been touched since the last time, what does it read as the airplane sits on the ramp? It reads 1,000 feet because the altimeter is calibrated to read 1,000 feet when it senses a pressure of 28.86. What does it read a few days later when high pressure has moved in with good weather and the airport barometer reads about 30.92 inches of mercury? If it could, the altimeter would tell you that you were about 1,000 feet below sea level - to an altimeter, higher pressure means lower altitude.
You can see why altimeters have the knob that you use to dial in the altimeter setting. Let's go back to the day when the barometer at your sea-level airport read 28.86 inches of mercury. To make your altimeter read the correct altitude, you would have used the knob to turn the altimeter setting to 28.86. When you dialed that number into the Kollsman window where the setting is displayed, your altimeter would have read 0 - the airport elevation. When you then climbed to a point where the air pressure was 27.82 inches of mercury, the altimeter would have read 1,000 feet.
So far we've been talking about an airport at sea level. What happens when you go to an airport that's not at sea level? (Remember, the standard atmosphere starts at sea level, not at the ground.) The answer is simple: Whoever takes weather observations, or the automated systems that do this at many airports, figures out what the pressure would be at sea level if the pressure could be measured at sea level, which is below the ground. But you don't need a weather observer to do this for you. One way you can do this yourself is to use an altimeter that you are sure is properly calibrated and working well. While you're still on the ground, just turn the setting knob until the altimeter reads the elevation of where the airplane is sitting. The number in the Kollsman window is the altimeter setting. If you are taking off from an airport with no weather observations, this is how you set your altimeter.
Now that you understand the basic workings of your altimeter, it's time to take a closer look at the saying, "high to low, look out below." Imagine you properly set your altimeter and take off from an airport where the altimeter setting is 29.92. You climb until you reach the altitude where the pressure is 24.86 inches of mercury and your altimeter reads 5,000 feet. Now, imagine that you fly to an airport where the altimeter setting is 28.86 inches of mercury. You concentrate so hard on holding altitude at 5,000 feet - as indicated by your altimeter - that you forget to call Flight Watch along the way to get altimeter settings. Figure 1 shows your path.
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