Page 31 - Volume 15 Number 2

including reports of unknown tops
(TOPUNKN).
PIREP tops are always reported in reference to mean sea level (MSL). By the way, don’t forget to look at the remarks section for more nuggets of information about the tops. Here’s an example of a PIREP from the pilot of a Cessna Caravan that includes a report of ragged tops at 5,000 feet with a higher layer above.
ERI UA /OV ERI270015 /TM 1251 /FL050 / TP C208 /SK OVC026-TOP050 /TA 00 /IC MOD CLR /RM TA 02 040 TOP RAGGED HYR LYR ABV
PIREPs have a very short shelf life, especially when the weather is changing quickly. A report older than an hour may be obsolete by the time you hop in the plane and depart. Keep the age of these reports in mind and always get updates through Flight Watch on 122.0 MHz while en route.
Pilot reports are also available on the datalink satellite weather broadcast assuming your portable or panel mounted unit is capable of displaying PIREPs. You’ll need to check with the manufacturer of your satellite weather receiver to determine if PIREPs can be displayed.
RAOBs
The NWS launches weather balloons, also known as radiosondes, twice a day at 1115 UTC and 2315 UTC at 87 locations throughout the United States. (See www.ofcm.gov/ fmh3/text/append-c.html). Now tracked using GPS, radiosondes measure the actual temperature, dewpoint temperature and wind speed and direction as a function of pressure as the hydrogen-filled balloon ascends skyward well into the lower stratosphere (nearly 100,000 feet).
This data is collectively called a radiosonde observation (RAOB) and when plotted on a thermodynamic chart such as a Skew-T log (p) diagram is colloquially referred
to as a temperature sounding. The latest soundings plotted on a Skew-T log (p) diagram can be found on a clickable map here (asp1.sbs.ohio-state.edu/main. php?pageloc=upperair).
RAOBs are not an analysis or forecast; they represent actual atmospheric data. There’s nothing superior to this kind of data short of a pilot report. The key is to examine the temperature dewpoint spread
or what meteorologists refer to as the dew point depression. As the balloon ascends from below into a non-precipitating warm cloud (cloud top temperature warmer than minus 15 degrees Celsius), the dew point depression normally approaches zero indicating saturation. And when the balloon pops out on top of the cloud into clear air, the dewpoint depression should begin to increase.
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