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The following stories from the March 5, 2010, edition of AOPA ePilot were provided to AOPA members who expressed an interest in the particular subject areas. Any AOPA member can receive information tailored to their areas of interest by updating their preferences online.



Forecasting change

Some aviation weather forecasts are straightforward. You know at a glance whether to fly. Clear skies with light and variable winds spanning the entire forecast period say “go.” Low clouds, fog, and precipitation from a warm front limping into town say “no.”


What if one kind of weather is giving way to another? Forecast fine print helps pin down the time and extent of change. That’s when forecasters employ shorthand terms like TEMPO, PROB, and BECMG in terminal aerodrome forecasts (TAFs). There’s no guarantee, of course, that the weather will oblige. Your job is to study the information and decide. How to weigh your decision was the subject of vigorous discussion in this thread in the AOPA Aviation Forum.


Portions of a TAF for Pittsburgh International Airport (KPIT) that is presented in Chapter 7 of the Aeronautical Information Manual (7-1-30) show the shorthand in action: “FM 1930 30015G25KT 3SM SHRA OVC015; TEMPO 0920/0922 1/2SM +TSRA OVC008CB.”


The AIM explains: Temporary (TEMPO) changes are expected for less than one hour and in total, less than half of the period between the two-digit hour beginning and two-digit hour ending the time period, in this case from 2000Z to 2200Z.


Later in the forecast appears this information: “PROB30 1004/1007 1SM RA BR; FM101015 18005KT 6SM SHRA OVC020; BECMG 1013/1015 P6SM NSW SKC.” There’s a 30 percent probability of the forecasted conditions between 0400Z and 0700Z. From 1015Z visibility is improving, and conditions are becoming (BECMG) clear with greater than six statute miles visibility and no significant weather (NSW) between 1300Z and 1500Z.


To evaluate any forecast, it helps to understand how meteorologists construct the big picture using instruments sent aloft on weather balloons. “Pressure measurements enable meteorologists to map areas of high and low pressure at altitude, which determine wind strength and direction. The temperature and humidity data enable them to calculate the atmosphere’s stability, which is used to predict the weather over the next several hours,” explained meteorologist Jack Williams in “The Weather Never Sleeps” in the March 2010 AOPA Flight Training.


In flight, keep tabs on weather changes as prescribed in the Aug. 22, 2003, Training Tip.


And here’s another tip about NSW: You’ll only see the term in information blocks with BECMG or TEMPO.


Windsock iPhone app

As a student pilot, you’re likely well tuned in to the crosswinds on the day of your training flight. Make determining the crosswind component easy and quick with Windsock, a new iPhone application. Simply select the runway in use, and spin the dial to find the headwind, tailwind, or crosswind component. It works for winds aloft too. Visit the Web site or purchase from the iTunes app store for less than a dollar.


Note: Products listed have not been evaluated by ePilot editors unless otherwise noted. AOPA assumes no responsibility for products or services listed or for claims or actions by manufacturers or vendors.


Question: I’m planning my first cross country flight and I’m wondering what the difference is between a sectional chart and a world aeronautical chart (WAC).


Answer: Both charts are used for VFR navigation. They also provide similar topographical relief information and aeronautical information. The main difference between the two is the scale. For a sectional chart the scale is 1:500,000, which means that every inch depicted on the chart represents about seven nautical miles (500,000 inches). The scale on a world aeronautical chart is even greater: at 1:1,000,000. The smaller scale also means there are some details available on the sectional chart that are not found on the WAC. Since they lack the detail of other charts, WACs are not recommended for exclusive use by pilots of low-speed, low-altitude aircraft. Sectional charts or terminal area charts are much better options. To find out more about the charts available to pilots see Chapter 9 of the Aeronautical Information Manual. Also see the AOPA Flight Training article “ Chart Basics.”


Got a question for our technical services staff? E-mail [email protected] or call the Pilot Information Center, 800/872-2672. Don’t forget the online archive of “Final Exam” questions and answers, searchable by keyword or topic.

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