Weather Map Basics

A wealth of information is available

By Jack Williams

SigWx Prognosis Chart come as four maps on a sheet. The two on the left side are for 12 hours from the forecast time; the two on the right for 24 hours ahead.

Figure 1: Convective outlook chart

Figure 2: 12-hour surface forecast

Figure 3: Forecast weather at the surface

Figure 4: Forecast of weather between surface and 24,000 feet

No pilot today should act like it's still 1927 and the airplane waiting on the ramp is a new Boeing 40A loaded with 1,000 pounds of mail waiting to head west from Chicago. Back then the weather briefing consisted of calls to a farmer 100 miles to the west and another 200 miles down the line to ask what their weather is like, combined with a glance at the sky.

One of the reasons that today's pilots can tap into an elaborate system of weather forecasts and predictions is that all too often airmail pilots ran into unforeseen weather. Tapping into the weather data system should commence at least an hour before the planned takeoff. It should begin with a look at what the weather should be doing along your route at the time you want to fly. To help you make sense of all of this, let's get some meteorological definitions out of the way.

An analysis is a map, or maybe text, describing what the weather is actually doing at a particular time.

A prog (for prognosis) is a forecast of what the weather will be doing at a particular time in the future.

Valid time, often abbreviated VT, is the time for which a particular forecast is made and is noted on the map or in the text.

Day 1 on weather products is the day the forecast was made. A forecast for the next day is Day 2, and so forth.

All of the times used on weather products are in Zulu time. This amounts to the standard time in London, England. (It used to be called Greenwich Mean Time or GMT, but that is no longer the name.)

We'll see how all of this works as we look at some products a pilot could consult while deciding if weather is likely to be safe for the flight.

The first product, Figure 1, is the simplest. It's the convective outlook produced by the National Weather Service's Storm Prediction Center in Norman, Oklahoma. You can find these maps — which provide a general look at where thunderstorms are likely — on the Web ( ). The label at the bottom shows that the map was issued on the fourth day of the month, "04" at 12:56 a.m. Zulu time. (Weather products use the 24-hour clock with 00 as 12 a.m.) It's valid between 1 a.m. and 12 noon.

To convert Zulu to local time, if the United States is on standard time, subtract five hours from Zulu to get Eastern time, six hours for Central, seven hours for Mountain, and eight hours for Pacific time. If the country's on daylight savings time, subtract one hour less for each time zone.

To figure out where these maps are showing that thunderstorms will be, imagine you are walking along a line in the direction indicated by the arrow. The storms will be on your right. Figure 1 shows that ordinary thunderstorms are expected over much of Texas and Louisiana, and probably also northern Mexico, but the NWS doesn't forecast for Mexico. The green arrow with slgt indicates a slight risk of severe thunderstorms with winds faster than 50 knots, or tornadoes, or large hail. If you're thinking of flying in the area where thunderstorms are possible, think some more. Thunderstorms won't be going on everywhere in the area for the 12 hours the forecast covers, but you need to ensure that you stay away from them.

Figure 2 is a 12-hour surface forecast. It's one of the several maps you can find in the members-only section of AOPA Online ( Note that while Figure 1 gave a 12-hour period as the valid time, the surface forecast in Figure 2 gives an exact time: 1800Z, or 6 p.m. The difference is that Figure 1 is saying that thunderstorms are possible in the indicated area any time during the 12-hour period; Figure 2 is a snapshot of what the forecasters expect the weather to be at a particular time.

The AOPA maps have a legend below them. This notes that the red area is where thunderstorms are expected at 1800Z, while the green area shows where nonthunderstorm precipitation — rain, snow, or ice — is likely. In fact, this map is predicting freezing rain at the surface along the Tennessee-North Carolina border. The symbol that looks somewhat like an "S" that's fallen over with the dot indicates slight freezing rain. If there were two dots, it would be moderate or heavy freezing rain. The "L" over Louisiana shows the center of a low-pressure area. All in all, this map shows that you probably would not want to fly a small aircraft across a large part of the Southeast on this day.

With weather, as with medical decisions, a second opinion can be a good idea.

The National Weather Service's low-level significant weather prog charts, or SigWX progs, are a good source of such second opinions. Low-level means they cover the weather up to 24,000 feet. High-level charts take over at that altitude. These maps are available on the NWS Aviation Center's Web site in the "Standard Briefing" section ( ).

SigWX progs come as four maps on a sheet (see p. 42). The two on the left side are for 12 hours from the forecast time; the two on the right for 24 hours ahead. This is handy because it shows you how forecasters expect the weather to move. The two bottom charts, for 12 and 24 hours, show the forecast weather right at the surface. One of these — see Figure 3 — covers the same time as Figure 2, and you can see the features are much the same, but with some different kinds of information.

The areas enclosed by the green lines are different types of precipitation. This is much more complicated than such maps usually are; it represents the early stages of a storm that brought rain, freezing rain, sleet, and snow to the South and the Northeast, and these are on the map in different places.

For pilots, the upper panel of the SigWX prog answers some important questions at a glance. Figure 4 is a forecast of what's going to be happening between the surface and 24,000 feet at the same time as the Figures 2 and 3 maps. The wavy blue-green lines outline areas where the ceiling is forecast to be between 1,000 and 3,000 feet, or the visibility to be between three and five miles. This is known as marginal VFR weather; that is, weather that will be marginal for flying under visual flight rules.

The solid red lines outline areas where the ceiling will be below 1,000 feet or the visibility less than three miles. Such conditions require instrument flight rules (IFR) flights. The yellow lines outline areas where moderate or greater turbulence is possible. The numbers under the turbulence symbol, such as in northern Louisiana in Figure 4, give altitudes where turbulence is expected. The 16/ (the slash is hard to see, it's covered by the red line) means the turbulence will extend from the surface to 1,600 feet. The symbol on the Colorado-Kansas border (240/150) shows that turbulence is expected between 15,000 and 24,000 feet.

The solid blue line that runs from the Atlantic over New Jersey, south across the Carolinas, and then snakes around the Southeast, before heading from Texas to the Washington-Oregon border shows where the freezing level is at the surface. The dashed green lines show the freezing levels at different altitudes. For example, the "80" on the dashed green line on the Atlantic Coast at the North Carolina/South Carolina border shows the freezing level is at 8,000 feet. The "120" on the line to the south shows that it's at 12,000 feet.

Student pilots should know how to read the SigWX prog charts — not just because they provide useful information, but because they can be encountered on either the knowledge test or the private pilot practical test.

As we've seen, just a few lines on maps can give you a huge amount of useful information. With all of the weather products easily available on the Internet, pilots have plenty of resources to use in making good weather decisions. If you want to pretend that you're an old-time airmail pilot, don't ignore the weather information available today — instead, buy a white silk scarf and wear it when you go flying with weather knowledge that the airmail pilots could have only dreamed about.

Jack Williams is the weather editor of An instrument-rated private pilot, he is the author of The USA Today Weather Book and co-author with Dr. Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.