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How Is ASOS Weather Different Than ADS-B?

Whether you're flying 50 miles or 500 miles, there's something that you, like all pilots, do as you're approaching your destination. Get the weather.

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But where are you getting your weather from? And how up-to-date is it?

Whether it's summertime thunderstorms or wintertime snow, weather can change quickly. And weather changes of even 20-30 minutes can have a significant impact on your ability to get into your destination.

So when you're picking up the weather on your arrival, where is it coming from, and how old is it?

What Enroute Weather Sources Are You Using?

ADS-B has had a profoundly positive impact on flying in the past several years. And one of those impacts is your ability to get weather reports and forecasts directly on your flight displays and tablet while you're in-flight.

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But there's a catch. If you're looking for the current weather observation for your destination, ADS-B transmits the latest METAR from the airport you choose.

The problem? METARs are old data from the second they are released. METARs are issued for airports once per hour, typically between 50 minutes past the hour, to the top of the hour.

If the weather changes significantly at an airport, a SPECI, which is a special observation, can be issued. But the criteria for issuing SPECIs is fairly significant (we'll cover that in another article). Simply put, the weather has to change a lot for a SPECI to be issued, and weather changes that don't meet SPECI criteria could be enough to go past your personal limits.

On top of that, there's an update cycle problem. ADS-B updates text reports approximately every 5 minutes. So if you're using your MFD or your tablet to get the weather through ADS-B, you could be facing a 5 minute (or even longer) delay from when the SPECI or latest METAR was issued.

What's Going To Give You The Best Picture?

Luckily, there's a much more up-to-date weather source that you can use as you approach your destination: ASOS. That's right, that familiar, repetitive voice that constantly reports the latest weather over, and over, and over.

So how up-to-date is ASOS when you tune your radio to it? It's updated every minute.

Every 60 seconds, the ASOS station checks its sensors, and then encodes the data it collects into a METAR. Then, the computer generated voice message reads off the METAR, as well as any remarks, over the radio frequency.

So when you tune in to ASOS, you're getting the most up-to-date weather you can get your hands on, down to the minute.

How Far Out Can You Get ASOS?

So how far out can you get ASOS weather? Typically you can pick it up within 50 miles of your destination. Since ASOS is transmitted over VHF, it is limited to line-of-sight, which means in mountainous terrain, you're going to be more limited. But if you're out in the open, you can sometimes get it even further than 50 miles.

Either way, you can typically pick up the weather well in advance of your arrival, giving you a good picture of what's happening at the airport.

Putting It All Together

ADS-B has given pilots the ability to get more weather information in the cockpit than ever before. But to use it, you need to know its limits.

Knowing how old your data is lets you make informed decisions on your flight. When you're enroute to your destination, using ADS-B weather is a great way to monitor weather trends, but remember that what you're reading is up to an hour old.

As you're approaching your destination, listen to the airport ASOS. Combine up-to-the-minutes ASOS with your ADS-B weather enroute, and you'll have the best weather picture possible from takeoff to landing.

Colin Cutler

Colin is a Boldmethod co-founder, pilot and graphic artist. He's been a flight instructor at the University of North Dakota, an airline pilot on the CRJ-200, and has directed development of numerous commercial and military training systems. You can reach him at colin@boldmethod.com.

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