There’s a powerful aviation weather tool out there that you likely have overlooked: the humble meteogram. A meteogram is simply a set of METAR observations graphed against time; this presentation of the data offers the pilot an instantaneous visualization of weather trends at the station. Unlike a simple METAR snapshot, which only offers point-in-time data, a meteogram visualization can tell the pilot how the weather has behaved recently and offer insights into how it might be changing. Are the winds consistent? Is the barometric pressure rising (or falling) rapidly? What is the sky cover trend? These questions and more can be answered by this valuable tool.
Meteograms come in two flavors: observational and predictive. The former consists solely of observed data, and any predictive value is derived from the pilot’s own intuitions. The latter is something akin to a graphed time series of TAF data, and there are at least two sophisticated meteorological models that inform its construction. The observational meteogram is itself a valuable tool for any aviator, as it uses actual data rather than modeled predictions, so in this article I’ll confine myself to discussing that type.
A meteogram plot may be intimidating at first, but with a little help you’ll be able to decipher one with ease. Let’s take a look at a sample meteogram; this is data for my home airport, Paine Field in Everett, Washington (KPAE):
That’s a complex presentation of data and not all of it is immediately intuitive. Let’s break the meteogram down into more digestible sections and learn how to decode it.
First let’s look at the very bottom of the chart; this is where we are show the time scale of the data:
You’ll note from the bold print that the observations cover 0353Z on October 10 to 0353Z on October 11. The X axis itself is labeled with “day/hour” in UTC, so the tic marks correspond to 0600Z on October 10, 0900Z on October 10, etc. All the data on a meteogram are charted to the same time scale.
Now that we have the time scale, let’s move back up to the top of the meteogram:
This first subgraph charts temperature (TMPF), dewpoint (DWPF), and relative humidity (RELH) against time; humidity is on the right-hand scale. The temperature observations are also called out in numeric form just below the X axis. Here we can see an interesting temperature drop around 1100Z, where observed temperatures fell 10 degrees F over a single hour. Cold front passage, perhaps?
Next on the meteogram come precipitation and visibility impairments (WSYM), wind gusts (GUST) and sky-cover information (SKYK). You’ll note that the symbols used here are the standard Station Model depictions:
Fog was followed by intermittent light and moderate rain, which tapered off. Winds were consistently from the south at first, growing in intensity and getting quite gusty…which then died down and shifted abruptly to the north around the same time that the temperatures abruptly fell. (This indeed appears to be a cold front passage around 1100Z on October 10.) Skies were overcast right up to October 11, when they began to show intermittent clearing.
The visibility and sky coverage data is further expanded in the next section:
Elaborating upon the simple data presentation of the Station Model, here we see the actual cloud levels and their coverage amounts. CHC1, 2 and 3 depict first-level, second-level and third-level cloud coverage, respectively. An “O” depicts overcast; “B” is for broken, “S” is for scattered and “-S” corresponds to few. The Y axis is the cloud altitude in feet. This presentation lets you see, for example, that at 1000Z on the 10th, clouds were scattered at 1000′, broken just above that, and overcast at 3000′. Once the front passed, the overcast at 1000′ lifted linearly to 8000′ or so before clearing.
Visibility (VSBY) is line-graphed against the right-hand scale and is easily deciphered.
Below this graph is a numeric presentation of high and low temperatures observed over the preceding six hours (T6XF and T6NF), as well as observed precipitation for the same time period (PO6I):
Finally, sea-level pressure in millibars (PMSL) is graphed:
The benefits of a meteogram are easily apparent. Imagine flying to a remote or unfamiliar airport where you only have the benefit of a METAR snapshot, but perhaps lack a TAF. (Remember, TAFs are not available everywhere that METARs are.) A meteogram lets you see recent weather trends and derive a better understanding of how it might behave, and as PIC become familiar with “all available information concerning that flight” as required by the FAR.
There are a number of different web sites which generate both observational and predictive meteograms. The meteogram I posted in this article was obtained from the University of Wyoming College of Engineering, Department of Atmospheric Science — I find their presentation to be the easiest to use and read.