Our December 2023 issue introduced some Aviation Weather Center charts and discussed how to work up a complete diagnosis of the weather. Last October, the Aviation Weather Center overhauled their website and introduced a new series of charts. Using these updated graphics, we’ll discuss surface maps in more detail. Hopefully by the end of this article we will make you an expert at reading surface charts and maximizing the information you get from them.
And it’s time well spent, because we’re in the era of digital data, where pilots are exposed to countless different kinds of weather information. The surface chart is a core chart where the weather briefing begins, and it represents the highest level of availability of “hard data” available to both forecasters and pilots. It is easy to understand and is never more than an hour old. Next to radar and satellite, it’s the best tool you can use.
Introducing the Surface Chart
The surface chart is built upon a foundation of what we refer to as “data plots.” These are groups of numbers, symbols, and pictograms representing the actual weather at each individual station. This format of presenting data goes back to some of the earliest weather maps created in 1849 by the Smithsonian Institution. These data plots were standardized in the 1870s in both the United States and Europe, and the arrangement of the individual elements that we use today was implemented by the U.S. Weather Bureau during World War II.
The “classic” weather map is built from “synoptic” data, which is a numerical code form called SYNOP which was developed by the World Meteorological Organization and transmitted over teletype and data circuits. SYNOP reports are only taken at operational weather facilities and, depending on the country, at some of the large airports. Synoptic data is normally observed every six hours, which means these classic synoptic charts are available at 0000, 0600, 1200, and 1800 UTC. Most charts you see in older meteorology textbooks are presentations of these synoptic charts.
METAR data, or “airways code” is readily available in great quantities. These reports are taken at thousands of weather stations in the United States and Canada, and across the globe—almost always at airports, and not by weather agencies unless they are specifically providing aviation support. METAR data by design is available hourly and has provisions for “special” observations when important changes occur.
You may be surprised to know that METAR plots were rarely available before the 1990s, though many local forecast offices at the time began leaning on them heavily for short-term forecasting and they now make up the bulk of charts in the United States. The Aviation Weather Charts presented here are constructed entirely from METAR data. Just go to aviationweather.gov, and click on the “Weather” tab, then on “Observations.” Maps like these will appear. If you click on a station, you can see the actual METAR report from which it was constructed.
It is certainly possible to combine SYNOP and METAR data on one map. This is sometimes necessary for looking at weather in unusual locations such as Africa or Asia, where data is sparse and we need every report we can lay our hands on. A specialized tool such as my own Digital Atmosphere analysis software is typically utilized to plot such maps.
Reading a Surface Plot
Let’s examine a surface chart from Aviation Weather Center above. First, we’ll look at a sample data plot: in this case for Provincetown, Massachusetts, which is zoomed in at the lower right. Of course, this is built from a METAR observation.
The very center of the diagram has a circle that is filled in according to the amount of cloud cover. If it’s filled in all the way, that means overcast conditions. The color is a default color for VFR conditions, in this case green. A de-facto convention in meteorology is that this color is blue for MVFR and red for IFR.
Extending away from it is a wind barb, which points into the wind, i.e. where the wind originates. There is a long barb representing 10 knots and a short barb representing five knots of wind speed. Together these provide the average wind speed. The gust speed, if a gust is present, is written next to the barb prefixed with a “G”. At Provincetown, the wind is 15, gusting 18 knots.
The top left corner is always used for air temperature. This is normally in degrees Fahrenheit, but outside the United States the values will usually be in Celsius. The air temperature is observed in a grassy, exposed area at a height of two meters, or about six feet, in a ventilated enclosure. For this reason, on model products you will sometimes see “2 m temperature,” which is shorthand for a “surface temperature” reading.
The bottom-left corner will indicate the dew point temperature. Again, this is in degrees Fahrenheit in the United States and will be in Celsius elsewhere. The dew point value is proportional to the absolute humidity—the actual amount of water vapor in the air per unit volume. The higher it is, the more likely that a “maritime tropical” air mass exists, and the greater the likelihood of severe convective weather or thunderstorms, assuming there is not an upper-level ridge or a capping inversion suppressing storms.
By subtracting the dew point from the air temperature, you will obtain the “dew point depression.” This is inversely proportional to relative humidity. When the dew point depression is zero, the air is saturated and the relative humidity is 100 percent. This means fog or some sort of precipitation is likely.
Dew point depressions below 10 degrees F are correlated with a relative humidity of 70 percent or higher and are often associated with broken or overcast layers of stratus, stratocumulus, nimbostratus, or cumuliform clouds. Dew point depressions of 20 to 30 degrees or more indicate very dry air and a high chance of clear conditions in the lower troposphere.
The space between the air temperature and dew point temperature is reserved for weather type and visibility. The weather type, technically referred to as “present weather,” is a pictogram established by the World Meteorological Organization. You can find a full list of pictograms by searching online for “ww present weather symbols.” This will bring you many results for charts and tables of all possible symbols. The few pictograms on this map are for light continuous snow. Visibility, if provided, will be in either statute miles or meters.
The top right of the plot is reserved for pressure. This is tricky because it can be QNH, altimeter setting, in units, tenths, and hundredths (i.e. 29.92 is “992”). On other maps it will be QFF, sea level pressure, in tens, units, and tenths (i.e. 1013.2 is “132”). Generally METAR data will use QNH and synoptic data will use QFF, which corrects for nonstandard temperature. But not always, since both pressure types can be reported in METAR data!
Experienced forecasters simply look at the map across a wide area and check the values. If most values in the tropics or in stagnant regions are near “992”, which is standard pressure, this means you’re looking at QNH. If these values are near “132”, this means it is QFF. The example map does not show the tropics or stagnant weather conditions, so many readers will be lost, so I will tell you that this is a QNH plot showing a range of values from values from 30.00 inches to 30.37 inches. Since you will probably be viewing charts online, you can just click on the plots and look at the raw observation to determine where they are getting pressure information from.
The center right and bottom right are commonly used for custom fields. Traditionally they were used to include pressure tendency, last-observation weather, and rainfall. This is still the case on conventional synoptic maps used at national weather centers. However nowadays we often find many different types of data there. In this case we find ceiling at center right in hundreds of feet, and the ICAO station indicator at bottom right. This is much more useful to pilots than past weather or rainfall amount.
Analysis
As we mentioned, these data plots are the most elemental presentation of data on a weather chart. Rather than reading off the plots one by one, it’s possible to print the charts on paper and draw lines that represent equal values of pressure, or of temperature. You can also do things like enclose areas of precipitation to show its approximate location, or outline the extent of red IFR and blue MVFR conditions to show their coverage.
This is referred to as the process of analysis, specifically, surface analysis. Typically meteorologists choose to analyze pressure, drawing isobars every 2 or 4 millibars, or every 0.05 inches. This produces an isobar analysis. After these lines are drawn, low- and high-pressure areas are added, then the forecaster can visualize where air masses are located and how the air is flowing.
Other fields can be analyzed. In the tropics it’s common to draw streamlines that are parallel to the wind barbs. You will see this used all the time at the Honolulu and San Juan forecast offices. Temperature fields can be analyzed to obtain an isotherm analysis, which helps define the locations of warm and cold air masses, and helps find fronts.
Forecasters will sometimes plot the dewpoint fields, producing an isodrosotherm analysis. This is useful in severe weather situations for identifying moisture axes and drylines. Moisture axes in the summertime often correlate to where thunderstorms will be most likely during the afternoon. Give it a try next summer; you will be surprised.
It’s also possible to have computer software plot these lines automatically, producing a machine analysis. This has some significant shortfalls, the biggest shortfall being that the forecaster doesn’t get a chance to study each of the plots and observe the patterns created while drawing lines. Important weather features can be missed this way.
Weather centers like the Storm Prediction Center recognize the problem and encourage human analysis, often called hand analysis. Consequently, their forecast desks have more pencils, crayons, and markers than a grade school art class. But due to time limitations, hand analysis is done only for the most important weather fields, and is never done for model charts, which are left to the computers.
As a pilot you will probably never need to perform a hand analysis since you have plenty of other things on your mind, and you will likely have obtained your legal weather briefing before starting up. But it can certainly give you a better handle on the weather that is taking place. The analysis types I recommend for aviation are quick sketches of IFR/MVFR areas and streamlines. By constructing a few charts over the hours leading up to an important flight and comparing the changes from hour to hour, you will immediately see the changes that are happening in the atmosphere. These are the very types of charts I have done at the forecast counter for my own perusal before preparing flight route packages for B-1B and C-130 crews doing low-level operations.
Other Data
Other layers can be added to the map. Aviation Weather Center offers this with the “Layer” tool at the top-right of the screen. In this case we have chosen radar data to display the snow showers moving through New York and Pennsylvania. The Aviation Weather Center offers three types of radar: lowest level, composite, and 18 dBZ echo top. The first two we have discussed in previous issues. Our example uses composite reflectivity, which is the maximum intensity observed at any level and is a sort of “worst case” picture. It’s always the best one to use, though “lowest level” will give a better visualization of squall lines and severe cell structure without smearing from higher levels.
Satellite data can be added as well, though it is not always easy to read. Flight categories, ceiling, and visibility can be overlaid. These are derived from numerical models, possibly due to great detail in offshore locations where there are no observations. PIREPs, SIGMETs, NWS Warnings, and fronts provide even more information to enhance your use of the Aviation Weather Center surface map.
So dig in and see how these products can help your flight planning. Surface charts are easy to use, easy to read, and provide a wealth of useful information. In coming articles of WX SMARTS we’ll provide more articles about how to use the information you find on these maps.
Tim Vasquez was an aviation forecaster in the U.S. Air Force and trained all the incoming meteorologists at Dyess AFB and at the Korea Theater Forecast Unit at Yongsan Garrison. He now brings his training to IFR, Weatherwise, and YouTube.
