Learning to read weather graphics
Pilots should also "look out the window" as they begin making preflight weather decisions. If the bottoms of the clouds are flirting with the tops of the telephone poles as you drive to the airport, and you see a sea gull beside the road with one wing up trying to thumb a ride, you aren't likely to be flying soon unless you are an instrument-rated pilot. But don't give up too soon. If you see clear sky on the horizon as you face into the wind, better weather could be on the way.
Even when the weather looks perfect, you still need to look out the window before taking off, but not in the literal sense. Instead, you use the wonders of today's technology to extend your view as far as you wish.
If you're based at one of the few airports with an FAA flight service station, you could walk in and ask one of the specialists for a weather briefing. If the briefer isn't too busy, this could be a good chance to learn more about the weather by talking with an expert about what the various maps, reports, and forecasts are saying. Otherwise, sit down at a computer and start surfing for maps showing the latest reported weather.
The weather page in the members-only section of AOPA Online is a good place to begin. On that page, you'll see a Surface Forecasts heading above a pull-down menu. The first item should be the surface analysis. In the weather business, an analysis is a report of what was observed at some time in the past. An analysis map is a snapshot, using symbols to indicate what was going on at the valid time.
Let's look at Figure 1, which is a surface analysis from AOPA Online. At the top, we see that it shows the weather as of 0000Z on February 6 (02/06). The Z stands for Zulu time, which used to be called Greenwich Mean Time, but since 1972 its official name is Universal Time (UT). In this case 0000Z is 7 p.m. Eastern time. (The National Science Foundation has a Web page with information on converting to and from Zulu for all North American time zones, including Alaska and Hawaii.)
The map in Figure 1 gives you the big picture of the weather across the United States. The gray lines swirling around the map are isobars, or lines of equal barometric pressure. The line that runs along the Texas-Louisiana border and then heads across the Gulf of Mexico and Florida is the 1,020-millibar (mb) line, which encircles nearly all of the East. You add 10 to the number on the line to get 1,020. Inside of it is the 1,024-mb line encircling an H which shows it's a center of high pressure.
Heading west from the Texas-Louisiana border, you cross the 1,016-mb, the 1,012-mb, and the 1,008-mb lines to reach the low-pressure center over New Mexico marked with an L.
The winds will generally follow the isobars, but crossing the isobars at a slight angle toward lower pressure, away from higher pressure. Winds go clockwise around high pressure, and counterclockwise around low, in the Northern Hemisphere. The map shows you that the winds over eastern Texas and Louisiana will be generally from the north.
The closer the isobars are, the stronger the winds will be. On this map, the strongest winds would be in northern New England and eastern Canada. You'll notice that the low over southern Minnesota doesn't have an isobar around it, which is a sign that it's a relatively weak low - at least for now.
The blue lines with triangles are cold fronts, where cold air is advancing in the direction in which the triangles are pointing. The red lines with red balls are warm fronts where the warm air is advancing toward the balls.
Surface charts can tell you a lot, but they don't tell you everything. In general, you can expect bad weather around areas of low pressure and along fronts. But, notice the large area of green - which indicates precipitation - in the middle of the country, far from any lows or fronts.
Air movements and pressure patterns in the upper atmosphere were causing this rain, ice, and snow. Since the chart is a surface chart, it shows only what's happening at the surface, not aloft. But it does show the surface results of what's happening aloft. The lesson: A lack of lows or fronts in a geographic area doesn't guarantee good flying weather there.
Before we move on to other kinds of maps, let's glance at Figure 2, the National Weather Service (NWS) surface analysis chart. Figure 2 was cropped from the larger original, which shows Alaska, all of Mexico, most of the Caribbean, and far out into the Pacific. If you live on the West Coast, this chart will show you storms heading your way across the Pacific Ocean. Like all the NWS charts we'll discuss, it is found on the NWS Aviation Weather Center Web site.
Once at this page, go to the Standard Brief section, where you'll find all of the maps and text needed for a standard preflight briefing. When you have the big, general picture, you want to start focusing on the area that you expect to be flying across.
Let's take a closer look at what's going on across the Southern Plains, from Texas and Louisiana north into Kansas and Missouri. Our best choice here is a radar image. Let's start with the Southern Plains image (Figure 3) from AOPA Online. You'll find it in the long, pull-down menu under Radar Imagery on the main weather page. Although it resembles the Doppler weather radar images that you see on local television weather segments, this is not really a Doppler image - despite what your local television weathercaster might call it. The radar images from AOPA Online, and most of those that you see on local television, are from the National Weather Service's network of WSR-88d radars, which have Doppler capability - that is, they can show the direction and speeds of winds. But the only time you're likely to see a Doppler image from these radars - unless you work at a weather office - is when severe thunderstorms with a threat of tornadoes are around. Then the weathercaster might show a Doppler image while carefully explaining what you are seeing.
Figure 3, like almost all of the radar images you see on television, is a base reflectivity product. That is, it indicates how much energy is being scattered back to the radar antenna from raindrops, snow crystals, hailstones, and other tiny objects in the air. The heavier the precipitation, the more energy reflected back. Heavier precipitation correlates well with turbulence and other things that pilots should avoid.
Through the magic of electronics and computers, the radars can also get a good line on whether or not the precipitation is frozen, which is of great importance to pilots. In Figure 3, the green areas, with a tiny area of yellow, indicate rain. Very heavy rain shows up as red on these radar images. The various blue shades indicate snow, while shades of pink show mixed precipitation, which could include freezing rain as well as rain, sleet, or snow. All pilots should avoid the pink areas as well as the green or blue areas near them, because they represent a danger of airframe icing.
Remember, the radar image shows what was happening in the recent past. While knowing what was going on is the beginning of figuring out what will be going on, it's only a small part of the story. By the time you take to the air, you could encounter a thunderstorm that wasn't even a small rain shower when you looked at a radar image.
Weather radar shows mostly precipitation; it's not a guide to poor visibility in clouds or fog. For general information about reported ceilings and visibilities, you need to go to the Weather Depiction chart. Both AOPA Online and the National Weather Service have such charts, but AOPA's is much easier to use, mainly because it's in color. You'll find it on the pull-down menu under Surface Forecasts.
If you look at Figure 4, you'll see that the weather depiction chart shows a wide area of marginal VFR weather (MVFR) in blue over Texas, with an area of IFR weather in red in the middle. MVFR weather means that the ceiling is from 1,000 to 3,000 feet above the ground and the visibility is from three to five miles, or that both conditions exist. IFR weather, which requires pilots to be instrument-rated to land or take off (unless they have obtained a Special VFR clearance), means the ceiling is below 1,000 feet or the visibility is less than three miles.
The letters next to the dots, which represent airports, tell you what's casing the bad weather. Unlike National Weather Service maps, which use symbols, the AOPA maps use letters such as R for rain, GF for ground fog, and L for drizzle. The complete list is below the map on the Web site.
Learning how to use any of the many available weather products takes practice. Fortunately, the World Wide Web makes such practice easy. Any time the weather is so foul for flying that even the sea gulls are walking is a good time to go traveling on your computer. Visit AOPA's weather section and the NWS Aviation Weather Center pages; call up maps and other products, and become comfortable with using them. Then, you'll be able to extend your look out the window before a flight all the way to your destination.
Jack Williams is the weather editor of USAToday.com. 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.
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