WX BRIEF

Clues in the clouds

An understanding of what lies ahead may be written in water vapor.




A cirrus cloud is all that remains from a decay­ed cumulonimbus, but its presence above growing cumuli indicate that there is still the potential for additional storms.

Banner clouds form as air crests a mountain peak. The air is cooled as it is forced over the top, resulting in condensation that dissipates as the air flows back down the lee slope.

The resulting cloud often obscures the peak and indicates the potential for waves downwind of the mountain.

A pileus, or cap cloud, is often seen covering the top of a developing cumulonimbus. Such clouds are not long-lasting, and usually mean very rapidly ascending air with increased potential that the thunderstorm will become severe.

While clouds are white when viewed from anywhere but beneath, one type of cloud will often appear grayish from any vantage point. Such clouds are volcanic ash clouds and should be avoided at all cost. Even minute amounts of ash can scour windscreens and fuse to engine hot sections, causing flameouts.

Fortunately, these clouds are fairly well monitored and forecast, and affected regions of the sky can in most cases be avoided.

Developing cumulus congestus produces rain showers near PBI (Intl, West Palm Beach FL). This is a common sight along humid coastlines as a daily sea breeze brings scattered showers onshore by afternoon.

One final type of cloud worth noting is condensation trails, or contrails. These long clouds are caused by the input of very hot and humid engine exhaust into very cold air that is incapable of holding much moisture.

As the air mixes and cools, the water vapor is condensed into contrails. Contrails give great clues about the upper air.

Their movement in the sky shows the wind direction, while the rate at which they are pulled apart is an indicator of windspeed. Often they evolve into cirrus. Contrails will dissipate quickly in dry air, so how long they last will tell you how humid the upper levels already are.

There are 2 important considerations when dealing with clouds. First, flying into one will instantly reduce or eliminate visibility in most directions. Be prepared to transition to instruments and maintain situational awareness with the terrain and other traffic.

The second consideration is that any time you are flying into water in its liquid form, you have the potential to accrete ice if temperatures are near to or below freezing. Whenever entering a cloud, keep an eye on the air temperature and, if it is within the range for ice, make sure you know where warmer temperatures and clear skies can be found should you need them.

Getting a handle on the clouds can be as simple as just keeping one's eyes open and watching how the skies change over time. Pilots also have the advantage of being able to see the bigger cloud picture via satellite imagery.

One snapshot from 22,000 miles up can easily show the high thin clouds, the cloud streets, the cumulus towers poking up from a lower deck, or lenticular clouds, and can even hint at where adverse or deteriorating conditions might be found. And, if you should encounter those conditions, don't forget to file a Pirep.

Karsten Shein is a climatologist with the National Climatic Data Center in Asheville NC. He formerly served as an assistant professor at Shippensburg Unversity. Shein holds a commercial license with instrument rating.


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