Significant Weather and Wind Charts — Page 505, Lesson 483

I want to walk you through how we actually use a wind and temperature chart in practice — specifically, how to extract the data you need for flight planning when your route doesn't line up neatly with the grid lines on the chart. Let's start with Figure 27.4, which is a wind and temperature chart for Flight Level 340. On that chart, wind and temperature values are plotted at intersections of latitude and longitude. But your route won't pass exactly through those intersections, so you'll need to interpolate — that means estimate values between the known grid points. Careful interpolation is required. Now, if you want to find an average wind for a whole route, you can't just take one reading. You must split the route into a number of sections — say, every 10 degrees of latitude or longitude, depending on which direction you're flying. For each section, you find the wind and temperature by interpolation, then find a mathematical average of all those values. There's a trap here. If the winds you find vary through 360 degrees — meaning they come from all around the compass — you have to be careful with the averaging. For example, the average of two winds, 310 degrees at 20 knots and 010 degrees at 30 knots, is 340 degrees at 25 knots. It is not 160 degrees at 25 knots. Why? Because 310 and 010 are both northerly directions; their average is a northerly direction. If you just added 310 and 010 and divided by two, you'd get 160, which is completely wrong — that's southerly. So you must think about the actual compass directions, not just the numbers. There's another important note about this chart. It's a portion of a polar stereographic projection. That means the North Pole is at the centre, and the meridians — the lines of longitude — radiate outward like the spokes of a bicycle wheel. Because of this projection, the direction of north changes depending on where you are on the chart. So when you estimate a wind direction from the chart, you must take care to check the local direction of north at that position. Once you have your average wind — the average wind velocity, or W/V — you need to find the wind component. That means applying the average wind to the mean track for your route, using a representative true airspeed, or TAS, and a navigation computer. For normal subsonic jet transport aircraft flying between 30,000 and 40,000 feet, 480 knots is a reasonable figure for TAS. Let me walk you through a worked example from the book: determining the average wind and temperature for the route from Madrid to Athens. We take wind and temperature readings along the route at suitable intervals, interpolating where necessary. The book notes that we don't need to be precise in wind direction because the averaging process will cancel out minor errors in measurement. Here are the readings they took: At 000 degrees East, by interpolation: 220 degrees at 25 knots, minus 40 degrees Celsius. At 005 degrees East, by interpolation: 250 degrees at 25 knots, minus 39 degrees Celsius. At 010 degrees East, by interpolation: 270 degrees at 25 knots, minus 39 degrees Celsius. At 015 degrees East: 290 degrees at 30 knots, minus 39 degrees Celsius. At 020 degrees East, by interpolation: 310 degrees at 50 knots, minus 41 degrees Celsius. To find the average, you add up all the wind directions: 220 plus 250 plus 270 plus 290 plus 310 equals 1340. Divide by the five readings, and you get 268 degrees. Do the same for wind speeds: 25 plus 25 plus 25 plus 30 plus 50 equals 155, divided by 5 gives 31 knots. For temperature: minus 40 plus minus 39 plus minus 39 plus minus 39 plus minus 41 equals minus 198, divided by 5 gives minus 39.6, which rounds to minus 40 degrees Celsius. The book then rounds these to the nearest 5 degrees for wind direction, 5 knots for speed, and 1 degree for temperature. So the final average is: 270 degrees at 30 knots, minus 40 degrees Celsius. Now, the excerpt also introduces low-level forecast information. The UK Met Office — UKMO — produces forecast weather charts for low-level requirements. These give conditions at six-hourly intervals: 0000Z, 0600Z, 1200Z, and 1800Z. The charts are valid for a 9-hour period, though the excerpt cuts off there. That covers the key technique: interpolating from the chart, averaging winds carefully across route segments, and applying that to your flight planning.