March 25, 2013
BY JACK WILLIAMS (From Flight Training, January 2002.)
A few minutes of surfing the Web can usually give a pilot a pretty good idea about whether going flying today is a good idea. Looking at a few weather charts might make it obvious that the weather where you want to fly is so bad that it's not worth the trouble of obtaining a formal preflight weather briefing.
On the other hand, if what you find on the Web seems to indicate that it's going to be a great day to go flying, both the federal aviation regulations (FARs) and good sense require you to obtain a weather briefing before heading off into the wild blue yonder.
While much of the same information is available on television, using the Web means you can go to the charts you want when you want to go to them, not when the schedule says they'll be shown. You can also copy or print out charts from the Web to study at your leisure. Finally, the Web offers a wider variety of charts -- especially those that mainly pilots are interested in -- than you'll find on television. The place to start examining the weather to see what it's doing now is the kind of map meteorologists call a surface analysis.
Surface, as the name says, tells you that this is a chart of weather at the Earth's surface. And, even though the whole idea of flying is to flee the surly bonds of Earth, taking off from and returning to the Earth are the times when weather can cause pilots the most problems. Also, the precipitation and other potential weather hazards at the surface usually extend upward higher than most general aviation aircraft fly.
For meteorologists, the word analysis is always applied to maps or text that describe the weather at a certain time in the past. A surface analysis is a snapshot of the weather across the area covered by the map at a given time, which is always noted on the map.
Obviously the closer the time of the analysis is to the time you are planning to take off, the more useful it will be.
Figure 1 is a surface analysis for all of North America and the surrounding oceans produced by the National Weather Service's (NWS) Hydrometeorological Prediction Center (HPC). Unless you are a meteorologist or a student of meteorology, all of the detail on the HPC map is probably more confusing than informative. Still, as you learn a little about the weather and how to read weather charts, studying the HPC map is a good way to further your weather education.
Figure 2, which is a simplified surface chart found on the Weather section of AOPA Online -- available to AOPA Flight Training members and trial members -- would be a better choice for most pilots. A quick glance at the map shows that flying should have been pretty good, if not downright delightful, across much of the contiguous 48 states on the late October morning the map represents. It's a snapshot of a generally pleasant fall day across most of the country.
You really don't have to know much about weather to realize that the lack of symbols over all of the country east of the Mississippi River and across large parts of the West was probably good news for anyone who wanted to fly that day -- not much weather could have been going on.
The H's over the Kentucky-Tennessee border and the New Mexico-Colorado border indicate centers of areas of high atmospheric pressure. In general, high pressure means clear skies and no precipitation. But, as the green shading in New Mexico shows, precipitation -- in this case, scattered rain showers -- can fall near an H on a weather map.
The light-colored lines swirling around the map are isobars, which are lines of equal atmospheric pressure. The line that makes a large oval over a large part of the East from Lake Ontario in the north to the Texas-Louisiana border in the south shows where the pressure is 1,032 millibars. (You put "9" or "10" in front of the two-digit numbers on the lines -- whichever will make the number closest to 1,000 -- to get the pressure in millibars. For instance, if the number is 96, you add 9 to get 996 millibars.) All air pressures inside the oval are higher than 1,032 millibars, but not as high as 1,036 millibars because in that case we'd see a line with "36," since the isobars are in four-millibar increments.
While the large area inside the 1,032-millibar line probably has good flying weather with clear skies, you would need to check further to make sure haze or fog isn't creating poor visibility in parts of the area. Pilots planning flights that will end after dark also need to be aware that fog could form as temperatures drop under clear skies.
While the map doesn't show any adverse weather over the area from central Illinois into southeastern Minnesota, the closely packed isobars could mean that strong winds might be a problem, especially for inexperienced pilots facing a crosswind landing.
On the day represented by the map, most of the "weather" in the 48 contiguous states was across the northern Plains and the Northwest. The blue line with triangles from western Ontario in Canada extending across northwestern Minnesota, eastern North Dakota, and South Dakota represents a cold front where cold air is advancing from the northwest into warmer air to the southeast.
The red and blue line stretching across Wyoming, Idaho, corners of Oregon and Nevada, and then northern California to the Pacific Ocean is a stationary front, which divides warm and cold air. The word stationary tells us that neither the warm nor the cold air is clearly advancing, but they could be moving back and forth.
Meteorologists developed the idea of weather fronts during World War I and got the name fronts from the war's battle fronts that were in the news. As with the battle fronts of World War I, weather fronts are where a lot of the action is taking place, and at times they can be downright dangerous places to be.
How much action takes place in the neighborhood of a weather front depends on the temperature contrast across the front -- the greater the contrast, the wilder the weather -- and how much water vapor is available to make rain, ice, or snow.
When the data used for the maps in Figures 1 and 2 were collected, the temperature contrast was small, with readings from the mid-40s to low 50s over North Dakota and in the mid-50s to low 60s across Minnesota. Also, since the northern Plains are far from any ocean, not much humid air was available to create rain. That's why no large area of rain or snow is shown on the surface analysis.
Although a surface analysis gives you a pretty good idea of what the weather is up to, it doesn't directly tell you anything about the ceiling and visibility. If these are below certain values, flying is not safe except for instrument-rated pilots.
Answers to these questions can be found on weather depiction charts, which are a special kind of surface chart with information for pilots. Figure 3 is a weather depiction chart taken from AOPA Online for the same time as the surface analysis of Figure 2.
The big difference is that the weather depiction chart indicates areas of marginal visual flight rules (MVFR) weather in blue and areas of instrument flight rules weather (IFR) in red. The MVFR areas have ceilings -- the heights of the bottoms of clouds that cover at least half of the sky -- between 1,000 and 3,000 feet above the ground or horizontal visibility between three and five miles. The IFR areas have ceilings below 1,000 feet or visibility of less than three miles.
Weather depiction charts produced by the National Weather Service show more detail than those found on AOPA Online. For example, Figure 4 is a close-up of the northern Plains area on the NWS weather depiction chart covering the same time as the chart depicted in Figure 3.
Each circle is a weather reporting station with the amount of shading indicating the amount of cloud cover. The numbers under station circles are the ceilings in hundreds of feet; that is, add two zeros to get the ceiling. Remember, ceiling is always the height of the bottoms of the clouds above the ground, not the altitude above mean sea level.
The areas encircled by solid lines have MVFR weather while the hatched areas have IFR weather. Numbers to the left of weather station circles give the visibility and symbols show the kind of weather causing poor visibility or low ceilings. The symbols are the same as those the NWS uses on surface charts. The most common ones are dots to show rain and asterisks to show snow.
In Figure 4 you see the cold front shown in the other figures, but in Figure 4 it's described as "COLD WK"; that is, a weak cold front. Figure 4 also has a short stationary front from eastern South Dakota across eastern Nebraska into Kansas, which is described as a weak stationary front. We also see two dashed lines, one from southeastern South Dakota into southeastern Colorado and the other from the Utah-Colorado border southwestward into Arizona. Both of these are marked "TROF," to show they are troughs, or elongated areas of low pressure.
The stationary front and troughs bear watching as areas where troublesome weather could develop.
Information on how to read National Weather Service products, including surface charts and weather depiction charts, can be found in Advisory Circular 00-45E, Aviation Weather Services, published jointly by the Federal Aviation Administration and the NWS. Chapters of the complete booklet are available on the Web as PDF files that can be viewed with Adobe Acrobat Reader.
The availability on the Web of huge amounts of weather information, including surface charts and weather depiction charts, makes it easy for a pilot to have a good idea of what to look for when it's time to obtain a weather briefing before a flight. This information is also a good way to become a student of the weather who looks out the window at the real world of weather and compares it with the charts and text that attempt to describe it.
Some weather Web sites
The FAA and NWS booklet Aviation Weather
The AOPA website's weather page (AOPA members, including AOPA Flight Training members and trial members, have access to AOPA Online's weather data.)
The NWS Aviation Weather Center website
The NWS Hydrometeorological Prediction Center website
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