WX Watch: Local Flavor

What characterizes this part of the atmosphere? For one, friction with surface features reduces wind speeds. With lowered wind speeds come reductions in Coriolis force—so any wind movements tend to follow surface pressure gradients, and flow perpendicular to isobars. Widely spaced isobars mean lighter winds and tightly-packed isobars—of the sort that come with frontal passages—indicate strong winds.

But calm isobar patterns don’t mean there isn’t any wind. It’s just that localized, microscale forces hold sway. Chief among them is the sun’s daily heating. By late morning, the Earth has heated enough to create thermals, which peak in strength by mid- to late afternoon. This heating isn’t uniform. Grassy fields don’t radiate as much heat as plowed fields, taxiways, runways, and other dark surfaces, which generate plenty of rising air. This rising air translates into turbulence and wind shear conditions—at just the place where you’ll be taking off or landing.

These surface-based thermals become part of a self-sustaining system. As the heated air rises, adjoining air masses fill the vacuum. In fair weather conditions, surface thermals rise to 5,000 feet or so, cooling as they go. Eventually, the air is cool enough to sink to the surface, where it becomes warmed once more. All this activity adds more turbulence and gusty, shifting wind conditions to the surface boundary layer.

Terrain, nearby bodies of water, and other features can also create their own wind effects. These vary from airport to airport. Part of any good checkout usually includes an instructor passing along the local quirks of an airport’s microscale weather. Perhaps there’s always an area of sinking air on approach to a certain runway, caused by cool air over a river on short final. Or a blast of turbulence after taking off and climbing out over a gravel pit or parking lot.

Here’s another microscale effect that can throw curveballs: mechanical turbulence, caused by air flowing past buildings, tree lines, towers, and other obstructions. Blowing air can be temporarily intensified as it makes its way around and through these features. This can create downwind vortices that can easily disrupt an approach and touchdown, when airspeeds are low and control effectiveness is diminished.

There’s more. A phenomenon called wind shadow can suddenly shut off a wind component as an airplane flies into an area downwind of any buildings, tree lines, or other blockages. One second you’re stabilized, the next you’ve lost precious airspeed.

You’ve probably learned that the sun’s differential heating of the Earth’s surface is the principal cause of all global-scale weather. But the daily cycle of heating and cooling is also a big factor in micrometeorology. Take sea breeze fronts, for example. The sun comes up, the beach heats up, thermals form, and cooler ocean or lake air is drawn inland, setting up a temperature boundary with light onshore winds. Any airports near the coast—and there are plenty—will have onshore winds. When the sun goes down, the winds reverse direction because while the Earth radiates its heat away, the ocean retains the day’s heat for a longer period of time. Ever wonder why so many coastal airports in Florida are aligned east-west? Now you know.

Similar dynamics are at work when airports are situated near valleys. Winds rise up valleys with the heat of the day, then descend as night approaches and cooling sets in.

This all goes to show that an absence of airmets doesn’t necessarily mean that flight through the surface boundary layer will be a picnic. After all, there will always be weather. If it’s not something big and scary like a thunderstorm, then it will be small-scale, and a creature of the immediate environment.<

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