Cruise Altitude Selection Against a Cold Low and a Jet Stream
You are planning a 4-hour eastbound flight at FL350 in a jet. The 500 hPa chart shows a deep cold low north of the route with a 500 hPa temperature of -38 C, and the tropopause is analysed at FL300 in that region. The 250 hPa wind is 280/120 kt at the core of a jet streak crossing your route. Your aircraft's optimum altitude at the planned weight is FL370; maximum altitude is FL410. Forecast SAT at FL350 is -62 C. The route passes within 150 NM of the low centre, where significant CAT is forecast between FL280 and FL380.
1. Explain what the tropopause height and the 500 hPa temperature tell you about the vertical structure along the route, and how that affects your choice of level.
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A 500 hPa temperature of -38 C is cold (ISA at 500 hPa is about -21 C), indicating a deep, cold low with a low tropopause. The analysed tropopause at FL300 means that at FL350 you will be flying in the stratosphere, above the tropopause. That has two consequences: the air is very stable above the tropopause, so CAT is less likely in the stratosphere itself, but the tropopause is the boundary where CAT is most likely, and flying just above a low tropopause puts you close to that boundary.
Because the tropopause is at FL300, the temperature at FL350 will be close to the tropopause temperature and may not fall much further with height. A SAT of -62 C at FL350 is consistent with a cold, low tropopause. Climbing to FL370 or FL410 would keep you in the stratosphere, which is generally smooth, but the proximity of the jet streak and the low means the tropopause will be displaced and folded, so the smooth-layer assumption is not reliable. The practical point: FL350 is a reasonable level, but the CAT forecast between FL280 and FL380 means you should expect the tropopause to be disturbed along the route and plan for a possible level change.
2. The wind at 250 hPa is 280/120 kt. What does that imply for your eastbound cruise, and how would you use it in the flight plan?
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A 250 hPa wind of 280/120 kt is a strong jet. At FL350 the wind will be somewhat weaker than at 250 hPa but still strong. On an eastbound track the 280 wind is a tailwind component, roughly 120 x cos(angle between 280 and your track). If your track is about 090, the angle is about 170 degrees, so the tailwind component is small to moderate (about 20 to 30 kt) unless the jet core is aligned with your route. The key point is that the jet streak is crossing your route, so the wind will change rapidly along track and you should expect a wind shift and possible CAT at the jet entrance and exit regions.
Flight planning: use the forecast winds for each segment rather than a single average; expect the strongest tailwind near the jet core and plan the step climb to FL370 after the jet crossing, where the wind may be more favourable. Because the jet streak is associated with CAT, brief the cabin and consider a seatbelt sign through the crossing region. If the tailwind is less than expected, the fuel plan should include a contingency for the reduced groundspeed.