Would it make sense to just try it ?
Any volunteers?
What we know:
Air does not / or virtually not transmit IR, so it is not likely that we can
see blue thermals (unless bugs, particles, etc. emit sufficient IR).
In clouds, we can see temperature differentials of 0.5 to 2 degrees on the
active part of a cumulus (presumably the water vapour emitting sufficient IR) -
so we might see which clouds have roughly how strong a thermal (not always easy
to spot with the naked eye, depending on your angle or when old and new clouds
are mixed).
It might only work when looking from the cockpit/ground upwards or forward, but
not down (background noise of warmer surface).
What we need:
A camera capable of detecting temperature differentials of 0.1 to 1.0 degrees
Centigrade with sufficient resolution at say 1 to 20 km ahead.
May or may not need a different software to colour code temperature
differentials, rather than precisely reproduce shapes.
Could be a stripped down digital camera with the IR block coating removed and
an IR only filter added, or a thermal imaging FLIR camera (on loan from work?
New ones cost around 2000 Euros); possibly also a night vision CCR binocular
with an IR only filter. Lenses used must be transparent to IR (otherwise you
measure the temperature of the lens).
Done so far:
I used an infrared remote thermometer (with an 8x lens, costs around 25 Euros)
in flight (through open window) and from the ground to measure the temperature
of clouds between 1-5km away. On the day, the clouds had around 12 degrees C
with a backdrop temperature of around -8 and a ground temperature of 27
degrees. The active part of the clouds were between 0.5 and 2 degrees warmer
than the rest of the cloud. This method only allows spot measurements and you
can of course only guess the spot size - a camera would give a far more precise
understanding.
regards,
Andre
________________________________
From: Luke O'Donnell [mailto:[email protected]]
Sent: 28 August 2010 00:47
To: XCSoar
Subject: Re: [Xcsoar-user] Fwd: Infrared and thermals
Interesting, i didn't know that there would be such a small heat differential.
So i suppose the upshot is that however you look at it, using IR for (blue)
thermal detection at any sort of range that would be useful does not appear to
be plausible.
I do however find the idea of using IR with clouds to be interesting. Andre
mentioned that he found it possible to detect hotspots in clouds. Assuming that
this is the case, i could see how it could be useful in the real world. You
could be approaching a large cloud (or a group of clouds) and have a better
indication of which ones are highly active, and furthermore where the best part
of them are. Of course you can usually determine this by looking at shape and
active development, but it's an interesting concept anyway.
Luke
On 28 August 2010 00:29, Martin Gregorie
<[email protected]<mailto:[email protected]>> wrote:
On Fri, 2010-08-27 at 22:44 +1000, Luke O'Donnell wrote:
> Valid points. I suspect that you wouldn't be necessarily looking for
> just hot spots, but perhaps looking at rapid decreases in temperature.
>
I don't think you'd see them. For starters, ground temp differences are
a lot bigger than air temp differences. This is both because different
surfaces have quite different reflectance and heat capacity (low
reflectance and low heat capacity give a real hot spot (asphalt parking
lot) while high reflectance and low heat capacity give the opposite) and
there's mixing in the air mass, but none in the ground.
As I said, if you're looking down on that sort of patchwork, the
difference in emitted IR from air with its small temp difference between
lift and sink combine with its low emitted energy mean it simply can't
be resolved.
I'm doubtful that a camera could resolve the low contrast between
thermal and non-thermal even looking horizontally so the thermal is seen
against air rather than ground. Don't forget that the air's thermal
background is also uneven and shows similar contrast differences from
distant thermals to what we're trying to use to locate a nearby one.
> Depending on the level of achievable contrast as well as how muchj
> temp trace there is when a thermal moves though, it might be possible
> to visually 'see' the thermal tracking over the ground, cooling it as
> it goes.
>
Again I'm doubtful, simply because the heat capacity of air is very low
compared with any solid. This means that the temp change in a bit of
ground that warms a parcel of air will be proportionately lower than the
tempo change in the air parcel. For a given heat exchange we can write:
dT1 * C1 = dT2 * C2 - (1)
Rearranging (1) gives
dT1 = dT2*C2/C1 - (2)
where dT is the temp change and C is the volumetric heat capacity. I
think this is best used rather than heat capacity per gram for rough
calculations comparing a gas with a solid. Air's volumetric capacity is
0.001297 vs 2.17 for granite (figures from Wikipedia's article on heat
capacity).
Plugging these into equation (2) and we see that a 2C increase in air
temp (0.7%, not 8% since we use degrees K, not degrees C for this type
of calculation) corresponds to a change of 0.007*0.001297/2.17 which is
0.0004184% or 0.0012 degrees C. You'd need a really good lab set-up to
measure that sort of temperature change. It not anything you'd attempt
outdoors with handheld or portable equipment.
Now, how does a 2C temp change affect the radiance of a parcel of air?
This is relevant because its brightness is directly proportional to an
object's radiance. The Stefan-Boltzmann law applies, which says that
the emitted radiation power varies as the 4th power of its absolute
temperature, so if we warm air by 2C at 25C the difference is 300**4 -
298**4 or an increase in brightness of 2.7% - again not a lot of
difference.
By comparison my hand at 28.5C is 23% brighter than ground at night
(15C).
> Still, i would be suprised if it would be nearly as easy to use IR to
> detect changes in temperature of air, as opposed to changes in
> temperature of the ground.
>
Exactly so, and that is even without considering differences in the
amount of radiation the ground emits in comparison with air.
Martin
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