WEATHER By Jack Williams WILEY POST'S high-altitude flights get some credit for discovering the jet stream. JET STREAMS AFFECT WEATHER WHERE YOU FLY hen you’re watching the weather on local television—which helps you see the big picture before you obtain a preflight weather briefing—the meteorologist is likely to show a simplified upper air map to help explain the current or expected weather while pointing to the “jet stream” on the map. THE AIR UP THERE W JET STREAM WIND SPEEDS VARY WITH "JET STREAKS" OF FASTER WINDS EMBEDDED IN SLOWER WINDS. Often the map’s “jet stream” doesn’t meet the strict definition: “a relatively narrow river of very strong horizontal winds (usually 50 knots or greater) embedded in the winds that circle Earth aloft.” That’s OK. For your purposes, all you need to know is generally which way winds are blowing high in the atmosphere. This is because high-altitude winds follow the boundaries between deep layers of warm and cold air. These boundaries are fronts at the surface and are usually locations of potentially dangerous weather. If the “jet stream” you see on television is running generally west to east with maybe a few shallow waves, the weather should remain generally calm for at least the next day or two. Since temperature of the air between the surface and any particular level in the upper atmosphere determines patterns of winds aloft, high-altitude winds that turn toward the south indicate cold air is moving south. The jet stream is above the warm-cold air boundary below. When a jet stream that’s been heading toward the south turns to head north, it shows that warm air below is moving toward the north. On the other hand, upper-level winds blowing generally west to east indicate that large 40 / FLIGHTTRAINING.AOPA.ORGhttp://FLIGHTTRAINING.AOPA.ORG