Pilot answers: Upper air data and forecasts

By Jack Williams ©2015

Q: I use RUC soundings for flight planning, but I am befuddled by the following: When I enter a station, load the Op40 soundings, and hover with my mouse over the loaded graph I see  bold numbers on the far-left near the Pressure (mb) line, and light numbers appear directly underneath my cursor with a dynamic pink line drawn from the cursor upward. Two questions:

(1) Which is the altitude I should be noting: the bold or the fine light-type numbers and is that altitude above mean sea level?

(2) What on earth is the pink line depicting or drawing?

Madeleine – Miami Beach, Fla.

A: You’re working with some hard-core meteorology here. I’ll start by explaining what this is all about to readers who aren’t as familiar with weather forecasting as you obviously are. “RUC” is the Rapid Update Cycle, which is a National Weather Service (NWS) forecasting model designed for users who need quick updates of forecasts for both the surface and aloft, especially aviation.

The National Oceanic and Atmospheric Administration (NOAA) RUC Web site has a huge amount of information on the model including links to several charts it produces including the RUC Soundings page you ask about.

I’m curious about how you are using the soundings for flight planning. I suspect that some of the other products, especially the maps, on the RUC Web site would be as useful, if not more useful.

Part of the kind of sounding diagram discussed in the question and answer. The numbers asked about are missing from this image. NOAA image.

Also, I  hope you are using these graphs as a supplement to the products found on the Standard Briefing part of the NWS Aviation Weather Center Web site.

Back to definitions. A “sounding” is a set of measurements going up (or down) through the atmosphere, which usually includes the altitude, and the atmospheric pressure, temperature, dew point, wind speed, and wind direction at each altitude listed.

Automated reports from commercial airplanes supply a good deal of the data needed to update this model hourly. I describe where this data comes from in my June 2005 Flight Training column on Piecing together weather.

The graphic that you are asking about — the image to the left is an example of part of such a diagram–is a Skew T, Log P diagram. These graphs of upper air soundings are an important tool for meteorologists.

In brief, the red line graphs the temperatures at each altitude and the blue line shows the dew points.

The pink line that you ask about shows what the temperature at each altitude would be if air at the altitude of the bottom of the pink line were forced to rise. (Meteorologists say it shows what would happen if the air were “lifted.”) The black bar across the pink line shows the altitude at which the air would be cold enough for its humidity to begin condensing into cloud drops; in other words, the bottom of clouds.

However, you shouldn’t use this line as a forecast of the height of cloud bottoms. It’s just some of the information that meteorologists use for forecasts.

As to the different numbers, William R. Moninger of NOAA’s Earth Systems Research Laboratory in Boulder, Colo., the developer of this display, e-mailed me that the bold numbers on the left are the values reported by the instruments on the weather balloon or aircraft, or other equipment used to measure the sounding.

As with any graph, the lines fill in the gaps between the points you use to plot the graph. The light numbers are the values of each point along the line, not the actual measurements. If you move your cursor slowly and carefully along the red line you will see that a particular number on the left will stay the same while those along the line change. In some cases you’ll see only a couple of values in light type before the bold type numbers change.

The altitudes shown are the pressure altitude; that is, what the altitude for the pressure measured would be if the altimeter setting at the sounding’s location were 29.92 inches of mercury. In other words, they are flight levels.

You can use the RUC soundings display to request data at various times after the sounding was recorded. These, of course, are  RUC model forecasts would be found if another sounding were made at the valid time of the forecast.

The February 2004 issue of The Front is a good source of information on Skew T, Log P diagrams. (This is a PDF download.) It explains what these graphs show and how a pilot can use them along with weather maps to find layers of warm air aloft when icing is a concern.

For example, this article explains: The closer the red temperature line and the blue dew point line are to each other, “the higher the relative humidity. When these lines are three degrees C or less apart, expect clouds and IMC. Dry air is present when the temperature and dew point lines are farther apart. The greater the distance, the drier the air.”

The Front is a PDF-format newsletter published by the NWS with the goal of helping pilots learn more about weather.  The index on The Front’s home page lists issues by the month of publication but not by topic. Pilots with a serious interest in weather should spend some time browsing the latest and back issues.

Finally, pilots who want to learn more about weather could start with my latest book, The AMS Weather Book to learn about the basic science, such as in the book’s chapters 6 and 7, which explain the basics of observing and forecasting weather.

For more detailed knowledge, you could get a copy of the  Aviation Weather Handbook by Terry T. Lankford. He notes some of the details about weather products in this book, which was last updated in 2001, are outdated.  But, it’s more up to date that most FAA weather materials and the general principles still apply. Also, Lankford’s book goes beyond the meteorology to discuss strategies pilots can use to cope with various kinds of weather.

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  3. […] the way, if you want to learn more about the Skew T, Log P diagram you can go to my answer to a question by a pilot on using such […]