Wx Watch: Tops Troubles

Where are the tops?

December 1, 2000

Who wants to slog along beneath an overcast, unable to see very well in the turbulence, fog, and haze that so frequently live there, hoping that high terrain or obstacles aren't ahead? No one! On a long cross-country, flying below a low cloud deck can be downright unsafe. Should engine or other troubles beset you, there's less gliding range and less time to set up for an emergency off-field landing.

Whether you fly under visual or instrument flight rules, it's nearly always better to fly on top of any lower cloud layers—or between layers, in clear air. Turbulence is very often less of a factor when flying "on top," and you avoid the risks of most types of icing (although freezing rain can fall from a higher layer into the clearer air below). There's better range and reception on communications frequencies and—best of all—better visibility. This visibility is essential for spotting cumulus buildups or thunderstorms and steering well clear of them.

On-top strategies

For VFR-only pilots the rule is to maintain wide visual separation margins while attempting to climb to on-top conditions. Yes, you can legally cruise on top of a solid undercast, but you simply must make sure that you can descend back through gaps or holes in the undercast—maintaining VFR separation from any clouds—along your arrival route to your destination airport. That said, it's important to emphasize that what's legal may not always be the smartest thing to do. It's one thing to fly in strong high-pressure systems and climb and descend through scattered layers to crystal-clear skies; that's pretty much of a slam dunk for no-risk on-top VFR flying. It's quite another to try to top clouds that are associated with fronts and lows, or climb to on-top conditions in fog- or haze-filled areas. In situations like those, you may find yourself climbing, and climbing, and climbing, and still not reaching the Promised Land. Then you're faced with a descent into the unstable, changeable conditions that created those clouds in the first place, and which could quickly turn your flight into a VFR-into-IMC nail-biter.

Instrument-rated pilots aren't exempt from the problems associated with struggling to climb to on-top conditions, either. In fact, they face even more problems. During a climb in the clouds, temperatures can drop to the icing range. Embedded storm cells and precipitation areas can cause unanticipated encounters with dangerous weather. IFR pilots who have asked for, and received, a clearance to climb to on-top conditions may also find themselves climbing to the limit of their airplane's abilities, and thus unable to comply with the clearance. ATC understands this, and will tack a proviso to any on-top clearance: You'll be issued an altitude clearance to fall back on, should you not be able to comfortably climb clear of clouds.

So VFR or IFR, let's agree that it's best to climb to a cloud-free perch and conduct our flights in weather that lets us see any problems well in advance. There is one potential downside to this strategy: Winds are generally stronger at higher altitudes. That's great if it's a tailwind, not so good if it's 30 kt on the nose. Even so, the advantages of cloud-free flying may very well offset the annoyance of a slow groundspeed and the potential dangers of flying in the clag below.

The trouble with tops

All that said, wouldn't it be nice to know where the cloud tops are, so we'd know before taking off if we had a fighting chance of clearing them? Such information might well be the single most important bit of data we could collect during our preflight or in-flight briefings.

But that's precisely the problem. There's alarmingly little information on the altitudes of the tops of cloud layers. For all the meteorological advances in recent decades, apparently very little effort has been put into technologies that could help us in this regard. So for the near future, anyway, we're left to rely on just a few sources of information about tops.

Tops info

Here are the currently available sources of information on cloud top altitudes:

  • Area forecasts. These give the altitudes of the tops of the highest cloud layers, and may mention the altitudes of any layered clouds. Maximum anticipated altitudes of any thunderstorm tops are also given.
  • Radar summary charts. These charts use radar returns to post the altitudes of the highest thunderstorm and/or towering cumulus buildups. That's great information, but there are two drawbacks. Cumulus growth (or dissipation) can be rapid, and the areas of storm/buildup coverage can change quickly. Also, radar paints only precipitation and the largest water droplets, so the actual cloud tops may be several thousand feet higher than advertised. Run-of-the-mill cloud tops often can't be shown because their smaller-droplet constitution evades radar. We're talking ground-based radar here, and while it can sometimes give us top information, it does a far better job of painting precipitation areas and contouring the precipitation gradients (adjoining areas of progressively higher levels of rainfall) of storm cells. That's great information because it tells us where the worst storms are, and where they're moving. But for the altitudes of all cloud tops, fagedahboutit.
  • Satellite imagery. Again, an inexact science. Weather satellites broadcast in three channels: visible, infrared, and water vapor. All of this imagery can be very useful in preflight planning. Visible images can show all clouds—and sometimes even fog. Water vapor imagery shows the density and patterns of any moisture flows. Infrared images are set up to read cloud top temperatures, not altitudes. Yes, there are logarithms that can be helpful in extrapolating temperature information and turning it into altitude data, but what we see when we surf Internet satellite imagery are lighter and darker shades of cloud coverage. The brightest areas are the coldest clouds, the darker areas are warmer clouds. Translated, this means that the bright clouds are the highest, the darker ones the lowest. Useful information, to be sure, but imprecise, and subject to change. Check the times of any satellite shots to see how old they might be.
  • Atmospheric soundings. Radiosonde weather balloons broadcast temperature, dew point, and humidity readings as they rise, and this information is used to plot the atmosphere's profile for analyzing an air mass' stability and winds aloft, to name a few parameters. In a previous article ("WxWatch: Skew T-Time," September Pilot) we discussed soundings and showed how the narrowing and widening of the temperature/ dew point spread at altitude can be used to report the altitudes of any cloud layers, bases, and tops aloft. One Web site ( www-das.uwyo.edu/upperair/sounding.html) lets you study soundings from around the world. Just look for where the traces merge, then check the altitude along the vertical axis of the chart, and you'll have a good idea of where the clouds are. Caveats? Balloons are only launched twice a day, and they are blown downwind as they rise. Information can be old, and just because a weather balloon's identifier may say it's from JFK International Airport, the soundings you see may have been taken over the North Atlantic.
  • Pireps (Pilot reports). Pireps promise the most accuracy, but please check the dates and times of any pireps. It's not unusual for pireps to be a day old, yet still be posted. The problem with pireps is that most pilots never make them, so we are deprived of good cloud-top information by some of the best weather observers in the world.

Make more pireps!

Why are pilots reluctant to make pireps? Perhaps laziness. Or being too busy with flying chores. Or being unsure of—or nervous about—the radio procedures for calling up flight watch (122.0 MHz) and making the reports. Let's all get over these roadblocks and resolve to make more pireps—and don't forget to mention cloud tops and temperatures aloft. These two bits of information help meteorologists validate their forecasts and update their predictions. But even more important, they can give other pilots some of the most reliable weather information possible.


Links to additional information on cloud tops may be found on AOPA Online ( www.aopa.org/pilot/links/links0012.shtml). E-mail the author at tom.horne@aopa.org.