In a message dated 2006/10/25, tchannel@.............. writes:

> Subj:weather 
> Hi All,   I was so happy with my sensor, over the last few day. I had a 
> major problem with thermos, which disappeared with my insulated box. This 
> question has to do with weather changes, and why that effects noise.
> A day ago the trace was quiet at night and a little bit of noise during the 
> day time and city activities.
> Many hours ago a cold front came thru, arriving at 8:00 pm with winds. The 
> trace went from quiet to noisy several hour ahead of the front. You could look 
> at the trace and see the change as the front got closer.  
> I don't understand, what is stimulating the sensor.
> 1.  Does it respond to barometric pressure changes?

Hi Ted,

       Horizontal seismometers have a relatively low sensitivity to 
baromatric pressure changes. Vertical sensors are effected directly and may show x100 
the noise.

> 2.  When wind is part of the storm, and the wind moves the trees, is this 
> transferred thru the ground to the sensors as earth movements?

       A front / storm has quite violent local turbulence. The quite small 
absolute pressure changes effect large ground areas. 
       The wind noise is right across the spectrum and it effects everything 
from the phone wires to whole mountains. Tall isolated trees pick up wind 
noise very well. So do tall buildings. The frequency of the noise is proportional 
to the wind velocity, but inversely proportional to the effective width of the 
tree, building etc. 

> 3.  Would a bolt of lightening be displayed on the trace?

       This is likely to be a sharp transient - you may also see a second 
sound signal. 

> As a front approaches it is like the atmosphere starts to fill with 
> electromagnet noise.  I'm just trying to understand what creates it.
> Inside the garage and inside the box it seem so isolated, no wind, no air 
> current, no vibrations or physical movements. Yet the sensor can see a front 
> hours before it arrives.

       A sheet of writing paper may be 0.1 mm thick. You seismometer can 
sense movements which are less than 1/1000 of this over many seconds. If your 
garage was not dead quiet, your seismometer would be way off scale.  
       A storm front has huge eddies of air weighing hundreds to thousands of 
tons churning about. Air is quite heavy stuff.

       Regards,

       Chris Chapman
In a me=
ssage dated 2006/10/25, tchannel@.............. writes:



Subj:weather 

Hi All,   I was so happy with my sensor, over the last few day. I=20=
had a major problem with thermos, which disappeared with my insulated box. T=
his question has to do with weather changes, and why that effects noise.
A day ago the trace was quiet at night and a little bit of noise during the=
 day time and city activities.

Many hours ago a cold front came thru, arriving at 8:00 pm with winds. The=20=
trace went from quiet to noisy several hour ahead of the front. You could lo=
ok at the trace and see the change as the front got closer.  

I don't understand, what is stimulating the sensor.

1.  Does it respond to barometric pressure changes?




Hi Ted,



       Horizontal seismometers have a relative=
ly low sensitivity to baromatric pressure changes. Vertical sensors are effe=
cted directly and may show x100 the noise.



2.  When wind is part of=20=
the storm, and the wind moves the trees, is this transferred thru the ground=
 to the sensors as earth movements?




       A front / storm has quite violent loca=
l turbulence. The quite small absolute pressure changes effect large ground=20=
areas. 

       The wind noise is right across the spec=
trum and it effects everything from the phone wires to whole mountains. Tall=
 isolated trees pick up wind noise very well. So do tall buildings. The freq=
uency of the noise is proportional to the wind velocity, but inversely propo=
rtional to the effective width of the tree, building etc. 



3.  Would a bolt of light=
ening be displayed on the trace?




       This is likely to be a sharp transient=
 - you may also see a second sound signal. 



As a front approaches it is li=
ke the atmosphere starts to fill with electromagnet noise.  I'm just tr=
ying to understand what creates it.

Inside the garage and inside the box it seem so isolated, no wind, no air c=
urrent, no vibrations or physical movements. Yet the sensor can see a front=20=
hours before it arrives.




       A sheet of writing paper may be 0.1 mm=20=
thick. You seismometer can sense movements which are less than 1/1000 of thi=
s over many seconds. If your garage was not dead quiet, your seismometer wou=
ld be way off scale.  

       A storm front has huge eddies of air we=
ighing hundreds to thousands of tons churning about. Air is quite heavy stuf=
f.



       Regards,



       Chris Chapman