I am an amateur astronomer living near the Washington, DC area, but I have a
question about reading weather IR satellite maps. First of all, here is the
IR map for my area:

http://weather.unisys.com/satellite/...=cur&region=at

Here's my question: The above map shows IR colors corresponding to the
highest cloud tops. Obviously, as in the description below the map, and
what I've always thought, is that from the light blue color to the yellow
would indicate precipitation, and especially thunderstorms in the dark blue
to yellow color. However, there have been times when the map was still
fully updated (which occurs hourly) that there was a thunderstorm in my area
when not even the lightest blue showed up on the map, only the gray. It's
caught me by surprise several times in the past. Why is this the case?
Other times, I have seen one of the colored areas (other than gray) over my
area and we weren't getting any precipitation at all (and yes the map was
the latest update).

If someone can explain why these anomalies -if it is one- occur, I would
greatly appreciate it. Is there another map or website I can use that shows
precipitation more accurately?

Thanks

On Sat, 6 Aug 2011 11:34:23 -0400,
Goldman Candy , in
wrote:
I am an amateur astronomer living near the Washington, DC area, but I have a
question about reading weather IR satellite maps. First of all, here is the
IR map for my area:

http://weather.unisys.com/satellite/...=cur&region=at

Here's my question: The above map shows IR colors corresponding to the
highest cloud tops. Obviously, as in the description below the map, and
what I've always thought, is that from the light blue color to the yellow
would indicate precipitation, and especially thunderstorms in the dark blue
to yellow color. However, there have been times when the map was still
fully updated (which occurs hourly) that there was a thunderstorm in my area
when not even the lightest blue showed up on the map, only the gray. It's
caught me by surprise several times in the past. Why is this the case?

Well, there are a couple of possibilities.

The classic life cycle of a standard thunderstorm (Byers and Braham,
1948) is on the order of an hour. So a storm could be brewing but not
particularly tall and cold during the first satellite image, but then
dump its rain on your head and be finished by the second pass comes
along and again be neither particularly tall or cold.

The thunderstorm is so small that the IR imager can not detect the
presence of the thunderstorm. It looks like the resolutions are in the
4 and 8 kilometer categories, and Unisys doesn't say which is being
presented. It looks like an 8 KM resolution, not real defined. And an
isolated thunderstorm could slip under the radar.

See what I did there? ;-)

I'd go with the first explanation, as I'm reasonably confident that's
what's happening.

Other times, I have seen one of the colored areas (other than gray) over my
area and we weren't getting any precipitation at all (and yes the map was
the latest update).

While all thunderstorms have cold cloud tops, not all cold cloud tops
are thunderstorms.

This is why one should overlay the satellite imagery with the radar
imagery - even if you're just doing it in your head. Clouds associated
with the jet stream/streaks, for instance are cold, but don't produce
rain. But they do have a distinctive shape and are long and fairly
thin, along the axis of the jet. If you look at today's (9 August
2011, 18 UTC) GEOS West enhanced IR image, there is a nice little jet
streak over southern Arizona/northern Mexico.

If someone can explain why these anomalies -if it is one- occur, I would
greatly appreciate it. Is there another map or website I can use that shows
precipitation more accurately?

I'd go with doppler radar:

http://radar.srh.noaa.gov/

It has a different set of problems, but those are typically less
deceiving.

--
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isn't looking good, either.
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