From:	David Woods <david@*******.FREESERVE.CO.UK>
Subject:	Re: thermographic vision-question
Date:	Wed, 3 Feb 1999 21:04:54 +0000

Marc Renouf wrote:
>
> On Wed, 3 Feb 1999, David Woods wrote:
>
> > The surface temperature of a human is very dependent on ambient
> > temperature. In the average run most people will be wearing clothes :).
> > So the surface in contact with the wall will be fairly close to ambient,
> > not blood temp.
>
> Not so, my good man. Your clothes are typically warmer that the
> surrounding environment. Easy example? Grab a shirt out of your closet.
> Now take the shirt you're wearing off. Hand them to someone. They will
> be able to tell you (without even having to smell them! ;) ) which one
> you were wearing, simply because that shirt will be warmer than the
> other (which is at ambient temperature because it's been sitting in your
> closet).

Well, it a difference of degree. Clothes will be warmer than ambient,
but by how much? 2-3, 10 degrees?

> Further, if you're leaning up against a wall, there is no air flow
> between you and your shirt, or between the shirt and the wall. This
> airflow is necessary for convective heat transfer (heretofore
> undiscussed).

Assuming your only wearing a shirt. If your wearing armour it's not
going to be far above ambient.

> Convective heat transfer is what keeps your clothes cooler
> than your body. In the absence of such convection, your shirt will
> rapidly heat up to your body temperature, and will rapidly begin heating
> up the wall. Easy example? Driving your car on a hot summer day. Even
> if the rest of you is cool and dry, your back gets all sweaty because
> there's no cooling convective airflow between you and your seat.

Yes, but what if it's about 12C with a stiff wind and your wearing
insulating clothes plus a flak jacket. You lean up against a brick wall.
How long would it take to see noticable heating of the oposite side?
Would it ever, or would the large heat capacity and rapid conduction
along the wall render it beneath delectability?

> > Imo very little heat would be transferred to the wall. It would be tens
> > of minutes before a noticeable change was seen.
>
> Again, it depends on the wall. In some cases, you might be able
> to discern the change in just a few seconds. You may not be able to tell
> whether the guy hiding behind the wall is an elf or a human, but you know
> there's a guy hiding behind the wall.

I'm not convinced it would ever be seconds. But I might be wrong. GM's
call really.

Regards

- David Woods