[NLRS] NO TROPO 15:30- 16:00zz

[jcplatt1@mmm.com]

Hi Bill, sorry to hear no "good" tropo (yet?).   I was watching the Hepburn
website too and wondering if condx's were going to be up or not.   The
Hepburn website is a very good "long term" predictor, but for near real
time predictions (besides getting on and making some noise ! ) I look at
the upper air weather data generated by the NWS.   There are several good
websites for this, here is one

http://www.weather.unisys.com/upper_air/skew/index.html

>From this website, click on the NWS station of your choice and you will get
what is called a Skew-T chart .... it plots upper air temperature and
dewpoint as a function of height.   The data is based on balloon readings
taken twice a day (ya, once an hour would be much better !).   The vertical
axis is air pressure that decreases with height above ground ... they
usually have the equivalent height above ground in meters next to the air
pressure.    The temperature and dew point axis are not at 90 degrees to
the air pressure but are "skewed", hence the name for the graphs .... there
is a meteorological reason for the skew; for our needs it matters not other
than to know that the graphs look a bit funny.   The two lines that go up
thru the graph are the air temperature (with height) and dew point (with
height).    The dew point is never greater than the air temp so the air
temp line is the one on the right side of the graph while the dew point
line is the one on the left.

For tropo, what we want to watch for is a temperature inversion that occurs
"low" in the atmosphere  .... an inversion is indicated by warmer drier air
over the top of cooler wetter air.   On the skew-T this will show up very
strongly where the dew point line (the one on the left) breaks suddenly to
the left (lower dew points) while at the same time the temperature line
(the one on the right) breaks suddenly to the right (higher temperatures)
.... the effect (when it occurs) is usually more pronounced with the dew
point line (ie; drier air).    Where this stratification occurs (the
inversion) the refraction index of the air is changed such that radio waves
are refracted back towards earth .... the more pronounced the change the
more bending of the radio waves.

The other key that is needed for tropo is that the inversion needs to be
"low".    Exactly what low is I'm not sure, but my experience is that if
the inversion is occuring above about 5,000 feet its too high and that over
the years our big tropo events have occurred with the inversion level below
3,000 feet and lower.   When I looked at the skew-T data last night for
Mpls the inversion was there but it was at about 8,000 feet ..... this
morning its the same.   Sometimes I have seen these inversions form like
this (at high elevation) and move downward ; I don't know why this may or
may not happen, but I think that the current inversion is too high.

I'm not sure why we cannot couple into a high inversion ..... I suspect
that it has to do with the angle of intercept ; our RF that is launched off
of our antennas has too steep an approach angle into the inversion ; we
want more of a grazing angle with the inversion, not a steep angle as steep
angle may just punch through.

So my thoughts are is that the Hepburn index is picking up on this
inversion but the inversion is still to high .... it would be interesting
to know if the Hepburn index takes the height of the inversions into
account in its prediction .... I don't know.    The Hepburn website is very
very good and I watch it a lot, but I also look at the weather map and the
upper air data for near real time data to augment it and to see where the
inversion is occuring.

On the down side I've been burned because I've looked at the upper air data
and didn't see anything only to not be around for a tropo opening .... the
problem is that the air data is only taken every 12 hours.    I've learned
to watch what is going on just to the west on Mpls .... I watch the
Aberdeen, SD data as well as our weather typically moves from west to east.
Watching the radars used to be good too but they suppress a lot of that
good "ground clutter" data now.

73, Jon
W0ZQ