Weather Rules of Thumb

All of these have different scales, ranging from hours and a few miles with the mountain breeze to thousands of miles and days to weeks with trade winds. Knowing which circulation normally affects your weather and which is dominant at the moment is what I would call one of the secrets to understanding the forecast. If the wind direction is normally 150 degrees on September afternoons, seeing 120 degrees tells you the wind is driven by something different.

Tim Vasquez

Sunday, September 16, 2018 at 12:00 AM ET

Verified

Key Takeaways:

Pilots must differentiate between true winds (from METAR/TAF) and magnetic winds (from ATC) and understand the various meteorological drivers behind wind patterns for better aviation insights.
Atmospheric boundaries, such as fronts, old squall lines, or thunderstorm outflows, are key indicators for cloud and thunderstorm development, with storm dissipation often signaled by outflow moving 40-50 miles away.
Predict fog and low clouds using rules like expecting radiation fog after rain with clear, light-wind nights, or advection fog/stratus when local temperatures are cooler than upstream dewpoints, often seen with Colorado Lows.
Other valuable rules include estimating surface system movement at 50% of 18,000 ft wind speed, using the 5400-meter thickness line to separate rain from snow, and expecting daytime showery precipitation near cold upper-level lows.

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We’re getting into some of the most predictably pleasant flying conditions in the U.S. That’s not to say we can’t get the occasional hurricane bearing down, or a cloud-filled clipper sweeping down from the Dakotas. But as autumn arrives, days are getting shorter, the broad-scale heat-driven cyclone in the western United States is much less intense, and the onshore wind component between the oceanic highs on both coasts and the inland areas dies down.

Reduced surface heating means that diurnally-driven thunderstorms are less aggressive. And for most of the western and southern U.S., this means lots of VFR conditions, especially early in the day.