Re: Could air currents reach supersonic speeds? (theoretical)

The speed of sound at 0 degrees C and standard atmospheric pressure is 
1139.4 km/h (usually measured in m/s, but all the data I'm presenting is in 
km/h, so I've converted for comparison).

Ground-level winds frequently exceed 100 km/h - southern Alberta, Canada, is 
a good example of a location where typical isobaric winds can reach that 
velocity. According to the Guinness Book of Records, the highest recorded 
isobaric wind speed is 333 km/h, at the USAF base at Thule, Greenland. To 
explore the Guinness site, go here:
 http://www.guinnessworldrecords.com/record_catagories/
recordhome.asp?RecordID=47490

The highest windspeeds on Earth occur in tornadoes, and the record-holder on 
Earth is the 1999 Oklahoma tornado, where doppler radar clocked the circular 
winds at 509 km/h - less than half the speed of sound. For more on the 
Oklahoma tornado, see:
 http://www.sciencenews.org/sn_arc99/5_15_99/fob1.htm

It is a common misperception that the high-level jet stream is the fastest 
wind known.  Weather services will classify an upper-air flow as a jet 
stream once it exceeds about 110 km/h, though jet stream speeds as high as 
320 km/h have been recorded. For more on the jet stream, see:
 http://www.encyclopedia.com/articles/06635.html

To find faster winds, we must look beyond Earth. Both Venus and Jupiter have 
very high winds. On Venus, top windspeeds occur in an easterly belt that 
flows about 370 km/h. The Galileo Probe experiment measured Jovian 
windspeeds from 30 km above the upper cloud deck to 80 km below. The wind 
speed increased from 300 km/h at the beginning of measurement to 640 km/h 45 
km below the cloud tops, and remained constant for the remainder of the 
probe's descent. Contact was lost at the 80 km level, but it is unlikely 
that speeds below exceeded the maximum recorded. For more on the probe's 
data, see:
  http://www.jpl.nasa.gov/galileo/status970813.html

In short, planetary wind speeds in the most extreme circumstances in our 
solar system do not reach more than about Mach 0.5, and current theory holds 
that to be the maximum obtainable speed under best (or worst, as you would 
have it) conditions on Earth.