Page 30 - Volume 15 Number 2

an hour or two ago, but are more dependent upon the height of the tops in the next three or four hours. For that, you must rely on near-term forecast guidance.
For those pilots that want miracles, it is incredibly difficult, if not fundamentally impossible to consistently predict the tops of clouds with any accuracy more than 12 hours in advance.
The Big Picture
Before you start diving in to determine the height of the clouds, take the time to get a sense of the “big picture” or what meteorologists refer to as the synoptic-scale weather. A front moving through your route of flight may produce tops that are changing by the minute.
On the other hand, upslope flow in the central and high Plains may produce a very flat-topped and highly capped stratus deck that changes little throughout the day. Also pay close attention to the diurnal changes. The afternoon sun can often deepen cumuliform clouds or erode a low-level stratus deck.
Pilot Reports
Pilot reports, or more simply PIREPs, are perhaps the best source for tops when they are available and near your proposed route of flight. Of course, early in the morning or later in the evening pilot reports are few in number. Tops reports are even scarcer. However, they still represent the best observational data available to pilots.
While pilot reports are available through Lockheed Martin Flight Services and DUATS, the best free online source is the Aviation Digital Data Service (ADDS). ADDS provides an interactive PIREPs Java tool (http://aviationweather.gov/ adds/pireps/java) that includes the ability to filter on the type of report. Choosing “sky and weather” should include all of the tops reports available within the last two hours
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