Weather instruments give you the real story
| Figure 1: Automated surface observing system, or ASOS |
| Figure 2: Wind vane and anemometer |
| Figure 3: Sonic anemometer |
| Figure 4: Ceilometer |
| Figure 5: Visibility sensor |
| Figure 6: Freezing rain detector |
Chances are that the airport you use has a collection of instruments much like those in Figure 1. If so, the next time you taxi past it, nod and say, "Thanks." Without such systems pilots (and everyone else) would have less information about what the weather is doing now, and less accurate forecasts of future weather.
The collection of weather-measuring instruments in Figure 1 is an automated surface observing system (ASOS). While these systems don't receive the publicity of their more glamorous cousins, weather satellites and Doppler radar, they (and similar systems) are the source of much of the data about ground-level weather used in the computer models that produce almost all weather forecasts. They also supply most of the data in the "current conditions" part of your weather briefing.
Automated systems continuously collect weather data. At many airports, they feed data not only to the National Weather Service, but also to broadcast weather updates you can hear over your aircraft radio as you prepare to land at a small airport that doesn't have someone to answer your radio call for information.
The first thing you need as you approach the airport is the altimeter setting. In the old days someone would read a mercury or aneroid barometer, calculate the altimeter setting, and broadcast it.
Today, most airports and weather stations use what is becoming a common kind of weather instrument: a device with electrical properties, such as resistance, that an atmospheric property such as pressure, temperature, or humidity changes. A microprocessor converts this electrical change into a pressure, temperature, or humidity reading. Automated stations use such an electronic barometer to read the airport's atmospheric pressure, which the barometer's microprocessor converts to the altimeter setting you hear.
With the altimeter correctly set, you listen for information about wind speed and direction to learn if the winds are safe for your landing, and if so, which runway to plan on using. Electro-mechanical wind vanes and anemometers are still generally used to supply this data.
The vane in Figure 2 is today's version of the wind vane often seen on barns. The rod points into the wind. As the vane turns, electrical current goes from it to contacts around its base, sending a signal showing where it's pointing. The faster the wind blows, the faster the spinning-cup anemometer on the right spins to send an electrical signal to the wind-speed readout. (The device in front of the vane and anemometer in the photograph is a precipitation-type sensor.)
The National Weather Service is replacing vanes and spinning cups with devices like the one in Figure 3, a sonic anemometer. Each of the three legs sends out sound, which the other two legs receive. The speed of sound through air depends on the air's temperature and this information is fed to the anemometer's processor, which uses the time sound takes to travel between each pair of legs to calculate wind speed and direction. (The math is the same that you use to calculate a wind triangle on an E6B or electronic flight computer.)
Before approaching to land you need the ceiling and visibility. Figures 4 and 5 show how these are measured. A ceilometer (Figure 4) sends an infrared laser beam straight up. If a cloud is above, the beam is reflected back, and the processor uses the round-trip time at the speed of light to calculate the cloud bottom's height. The processor uses the last 30 minutes of data on when clouds were overhead to calculate cloud cover.
The visibility sensor (Figure 5) transmits light from one side, which anything in the air--mist, fog, rain, or snow--scatters. The processor uses the light reaching the other side to calculate how far you would be able to see horizontally at the ground--the visibility.
As a pilot, you also want to know whether precipitation is falling. The precipitation-type sensor, seen in Figure 2 with the wind vane and anemometer, answers that question by detecting differences in the way slowly falling snow scatters light from the way falling rain drops scatter it.
If rain is falling and the temperature is cold, you want to know if it's freezing rain or drizzle. The freezing rain detector (Figure 6) answers this question with the probe at the upper left that vibrates at a steady frequency--unless ice forms on it. The processor reports a change in vibration frequency as freezing rain and heats the probe to melt the ice. The heat is turned off, and the cooling fins below the probe quickly dissipate the heat to allow the probe to collect more ice.
With the NWS ASOS system and the FAA's automated weather observing system (AWOS), the odds are very high that the airport you want to visit has an automated system.
In fact, at almost all of these, you can access the automated data even before you head for the airport. The FAA's Web site has a page with listings of ASOS and AWOS stations; select the airport you want and you'll likely find a Web link to up-to-date data, a telephone number to call to hear the data, and the frequency to use to receive reports in the air.
Jack Williams is coordinator of public outreach for the American Meteorological Society. An instrument-rated private pilot, he is the author of The USA Today Weather Book and The Complete Idiot's Guide to the Arctic and Antarctic, and co-author with Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.
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