Wx Watch: The Big Burn-Off

All pilots crave good weather, so we're all alert for any mention of a large high-pressure system moving across our proposed route of flight. But large high-pressure systems carry some tricks that we should be aware of — tricks that can lead to low-IMC (instrument meteorological conditions). One of them is radiation fog. Here's how it works.

Assuming clear skies during the nighttime hours, the ground begins to radiate huge amounts of heat into the atmosphere as soon as the sun goes down. The result, of course, is a lowering of temperatures near the surface. By the time the predawn hours arrive, a thin layer of air just above the surface reaches its lowest temperatures — hey, it's had all night to cool down! This layer can be knee-high, or it can extend upward as far as 1,000 feet or so. It all depends on the amount of moisture in the air, the presence of any nearby bodies of water, the time of year, and the elevation.

This is the time when the temperature can drop to, or very near, the dew point. When that happens, the air becomes saturated with water vapor and this moisture condenses into the tiny water droplets we know as fog (which is, in truth, a cloud on the ground). And this is what creates the dense radiation fogs that can keep all of us pilots — VFR-only or seasoned instrument-rated pros — stuck on the ground, waiting for the fog to dissipate.

For that to happen, the temperature must rise. This makes the temperature-dew point spread widen, and causes the fog to "burn off." Since our earliest days as student pilots, we've been taught that fog is possible when temperatures come within 5 degrees Fahrenheit (or 2 degrees Celsius) of the dew point, and that's a good rule. When the temperature is "on top" of the dew point — i.e., the temperature and dew point are the same — you can almost guarantee fog.

Top down

But what causes the burn-off? Sometimes, you'll hear pilots explain it by saying that the top of the fog layer is heated by the rising sun, and that the fog is burned off from the top down. Nice idea, but it's not that simple. To understand fog burn-off, you have to look at the temperature lapse rate in the few thousand feet above the surface, and see what happens in the morning hours after sunrise.

On a clear morning, the lapse rate's typical signature shows a warming trend in the air nearest the surface. The air closest to the ground is the coldest, while temperatures gradually increase with altitude — at least for 3,000 feet or so — before resuming a "normal" lapse rate (temperatures decreasing with altitude). Meteorologists call this a "nocturnal temperature inversion."

To burn the fog off, those lowest, coldest temperatures must rise. For that to happen, winds at the top of the inversion have to begin blowing. They don't have to blow very hard — just a few knots — but this wind does an important job. It creates turbulent eddies in the fog layer, eddies that churn the air in the inversion. Air aloft is transported downward to the surface, and from the surface to the top of the inversion.

Never calm

"But what if the wind is calm?" you may ask. The answer is that the wind is never truly calm. Variations in the local terrain, for example, can create air movements too small to detect by a human observer, but which still can impart vertical motions to the lowest levels of the atmosphere.

As the colder air above the inversion descends, it warms by compression, mixes with the foggy low-level air, and causes surface temperatures to slowly rise. Meanwhile, that coldest layer hugging the surface is boosted aloft by this gentle turbulence, where it is in turn sent downward to begin its heating cycle.

As the air heats up at the surface, the fog slowly burns off as it evaporates. The process is accelerated when the fog becomes thin enough to let sunshine through to the ground. Now the surface can warm up and help the process. By this time, those light winds aloft may be noticeable at the surface. And surface heating will cause surface winds to increase as thermals rise and nearby air parcels move in to take their place. The inversion has disappeared, the temperature-dew point spread has greatly widened, and now we can go flying.

For a simplified view of the burn-off process, just think of citrus growers in Florida. When a frost threatens, they start their wind machines at night. This helps bring warm air aloft down to the surface and prevents crops from freezing. That's their way of beating a nocturnal inversion. As for us, we're left cooling our heels, waiting for midmorning. By the time the windsock begins to stir, it'll be takeoff time.

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