DaveM wrote:
wrote in message
oups.com...
...

It also occurs to me that I need to better understand this "temeprature"
term!


I'm not sure that "temperature" is your problem. I think you might need to
understand how to read and interpret manufacturers specs, which should be a
required course toward an engineering degree. {:) Just be sure that you
look at the complete spec before you buy (or recommend) an instrument.


Not to disagree too strongly with your comments, Dave, but "temperature"
is his big problem -- or at least understanding the principles of
temperature measurement.

Go back, Suraj, and reread my other post on this subject. There are
fundamental matters that you have to understand and take care of before
you even start reading manufacturers' data sheets. Keep in mind that
all these devices maintain their specifications only under certain
definitely specified conditions, and if you don't use the equipment
under these conditions, you don't get the accuracy in your measurement,
even if the equipment meets its specifications perfectly!

I haven't looked closely at the satellite measurements' techniques and
specifications, but I do know that they get their fractional degree data
by averaging many, many readings over rather wide areas. There is no
such thing as a city with a temperature of 22.7C. There will be a city
whose average temperature at a particular time is 22.7C, with a range of
individual measurements at that time maybe +-5C or more.

The satellite measurements can quote this kind of resolution and
accuracy because their sensors are stable and known to this kind of
resolution and accuracy. So if yesterday's temperature for Rome is
22.7C, and today's is 23.2C, you can be sure that the average of the
readings of many places in Rome is 0.5C higher today than that average
was yesterday. That's all. Your friend is not going to verify the
satellite readings with a handheld, or IR, or any other kind of
ground-based measurement. I doubt seriously he could get an equivalent
measurement with the satellite's own equipment from a balloon!

The best he could do is verify that his location is some temperature
near the satellite's reading. But then, maybe his location is in one of
the extreme places :-). If he had dozens or hundreds of sensors spread
all around the city, he still might not be able to couple his sensors to
their surroundings well enough to get an equivalent reading (remember,
you can never measure the temperature of an object -- only the
temperature of the sensor). IR readings won't help, either, since they
are made from different angles at different distances over different
areas.

Now, if your friend has access to a satellite that he can use to
rendezvous with the weather satellite, and take the same kind of
measurement at the same time, from the same location in orbit, with
sensors calibrated to the same specifications under the same conditions,
then he might have a case for saying reasonably that he's verified or
disputed the satellite's measurement.

John Perry