Aging Surface Observations

One of the more common concerns raised by ForeFlight customers is the age of surface observations or METARs shown within the app. They often wonder why the age of a METAR can be 60 or more minutes old in some cases. To understand why this occurs, let’s discuss how routine surface observations are taken throughout the world.

metar-age

The age shown here in the airport popover is based solely on the difference between the current time and the time the METAR was issued.

If you visit most any airport in the U.S., you’ll likely see one of two weather observing systems installed: the Automated Surface Observing System (ASOS) or the Automated Weather Observing System (AWOS). Both of these are capable of generating one or more weather reports each hour. Although these systems observe the weather nearly continuously in time, they will only generate official reports known as an aviation routine weather report or METAR when certain conditions apply.

Routine observations

For an ASOS, only one routine report is issued every hour, which is a key reason for the seemingly excessive age of these observations. If you pay close attention to the issuance time on METARs, you will notice that many routine observations are issued a few minutes before the top of each hour. Starting at 47:20 past the hour, the ASOS begins to make its routine observation. By 53:20, the hourly observation has been prepared and edited and should be ready for transmission. This routine report becomes the official hourly observation for the NWS. That’s the METAR you will see in the ForeFlight Mobile app.

It’s important to understand that the age presented in ForeFlight is based on the issuance time in the METAR regardless of when it was disseminated by the ASOS or AWOS station. Once each minute we pull down those latest observations directly from our interface with NOAA, parse them and add them directly into our database. After the METAR was issued, it is not unusual for several minutes to pass before it becomes available to ForeFlight. ForeFlight doesn’t typically receive and ingest the data until 4 or 5 minutes after this issuance time. Therefore, it’s very common that the routine observations will have an age of 4 or 5 minutes when updated. That means it’s quite normal to see an age of 64 or 65 minutes just before it gets refreshed by the latest hourly observation.

metar-refreshed

When a METAR is refreshed in ForeFlight Mobile an age of 4, 5 or 6 minutes is very common. For example, this METAR for Ellington, the METAR was updated 6 minutes ago.

An AWOS, on the other hand, typically issues three routine observations each hour or every 20 minutes. The typical interval is at 15, 35 and 55 minutes past each hour. However, you will find that these times will vary depending on the location. You may even run across some AWOS stations that operate similar to an ASOS, that is, one routine observation an hour.

SPECIs

If the weather is changing rapidly for the better or worse, special observations (SPECIs) are issued in addition to the routine hourly observations and include operationally significant changes to elements like wind direction, wind speed, ceiling height and visibility just to name a few. Given that the ASOS relentlessly measures the weather and could inundate pilots with more frequent special observations than a human observer, the system is purposely throttled to provide SPECIs only at 5-minute intervals. This is to limit the number of observations that can be transmitted during the hour when the weather is changing rapidly. Like the routine observations, SPECIs will also take several minutes to appear in ForeFlight after it is issued.

1-minute weather

Before you depart or when you approach an airport, it’s common to listen to the local weather broadcast over the dedicated ground-to-air frequency. This broadcast is referred to as the 1-minute weather. You can also get the latest weather by calling the stations dedicated telephone number. In either case, this automated weather is often more up to date than what you’d get over ATIS or via ForeFlight. At the moment, ForeFlight only provides the latest official observations that are disseminated in the form of a METAR or SPECI. In other words, we don’t currently provide the 1-minute weather you’d get over the phone or on the radio broadcast.

airport-wx-freq

You can find the frequency and phone number for the local ASOS or AWOS on the Airports view under Weather and Advisory tab.

Of course, all pilots want the latest and greatest information. However, that does not necessarily mean an hourly observation that’s 30 or more minutes old should be considered stale. In fact, if the weather hasn’t undergone an operationally significant change, the latest observation is likely still very representative of the weather at the airport.

Range of usefulness

You can’t talk about age unless you also wrap in a discussion about the range of usefulness of an observation. It’s not unusual for many pilots to assume that a particular observation is useful as far as 20 or more miles from the airport. That may be the case when the weather is fairly homogeneous across a large region. But in most situations, making that assumption can get you into trouble.

These official surface observations are taken to be representative of the weather within the terminal area. The terminal area is defined as the circular region within 5 statute miles from the center of the airport’s runway complex. In other words, they are point observations. Notice in the table below that many of the parameters reported in a METAR are valid only within 1 to 3 miles of the airport. So there are no guarantees that the weather is similar to what’s shown in the observation as you get outside of the terminal area.

table-validity-asos

This table defines the representative range from the airport of the various weather elements provided by the observing system.

So the next time you look at the age of latest surface observation don’t discount its operational value. When the weather isn’t changing all that rapidly, a single update each hour will be the normal case for many reporting stations throughout the world.