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> Sent: Sunday, Octobe
Hi Steve,
Thanks for the correction. I dug out an old physics and sure enough the
angle of incidence is the departure from a line normal to the surface and I
was using incidence to mean the departure from the tangent of the surface.
Oddly enough I could find no term for the angle (90-incidence angle).
Dave
----- Original Message -----
From: "Stephen Shaw" <srshaw@Dal.Ca>
To: <naturens@chebucto.ns.ca>
Sent: Monday, February 29, 2016 2:08 AM
Subject: RE: [NatureNS] Fw: Reconsideration; ground frost
> Agreed, for the sunbeam element D units wide that strikes the earth's
> surface at near grazing incidence close to sunrise, at angle A° (say 1°),
> illuminating a corresponding surface area element S, the relative
> insolation D/S = sin A. The angle of incidence is usually defined,
> though, as the angle made relative to a radial line, therefore normal to
> the surface. Incident angle then would be (90-1)°, thereafter falling to
> 0°, if the sun were later to reach the zenith.
>
> The dawn sunbeam is (almost) a tangent that is (90-A°) to the normal to
> the surface, so the angle of incidence as usually defined at that time
> would be (90-1) = 89°. So if you define the angle as usual as that made
> relative to the radius, not to the tangent line, it's cos(90-A°) = D/S =
> sin(A°). It's a cosine relationship if the incident angle is defined
> conventionally.
>
> This ignores a large effect which is the absorbing and scattering effect
> of the thickness of the atmosphere, very large at the ends of the day and
> minimal at noon.
>
> On an earlier question relating to the odd colour of a drake mallard's
> neck, someone asked what the effect of the seasons on light colour might
> be. The main effect on relative skylight composition is not seasonal, but
> is whether the sun happens to be contributing to the incident light, or
> whether illumination is solely from skylight, because of cloud cover or of
> occluding obstacles like mountains. It is discussed in terms of the
> effective 'equivalent colour temperature' ECT of the sky. With the sun in
> evidence, the ECT is usually below 5000K and more reddish, but without it,
> in northern latitudes, it can exceed 20,000K with a more steely blue cast,
> and a more even proportion of UV to Blue. As Planck first analyzed, when
> a perfectly black object is heated up, it radiates a well-defined, broad
> light/heat spectrum with a distribution that depends upon its absolute (K)
> temperature, peaking dull red to blue-white from hot to very hot.
> Skylight has extra glitches and doesn't exactly copy black body radiation,
> but colour specialists have made an equivalent scale (ECT), partly to
> allow creation of standard lighting conditions. This is important, for
> instance for matching cloth colour batches in the textile industry.
> I checked the bible for this, 'Colour Science' (1972, 2nd ed) by G.
> Wyszecki (a Canadian) & W.S. Stiles, and they mention in passing that
> there may be minor variations throughout the season, but it doesn't sound
> that they think these are important. Speculating, presumably the reason
> is that contaminants in the atmosphere are generally more or less colour
> neutral in the visible, so act like grey 'neutral' filters, affecting
> intensity (irradiance) of course, but not much the spectral distribution.
> Particles from forest fires and volcanic emissions might provide partial
> exceptions, and water vapour is important in the infrared.
> Steve
>
>
> ________________________________________
> From: naturens-owner@chebucto.ns.ca [naturens-owner@chebucto.ns.ca] on
> behalf of Hebda, Andrew J [Andrew.Hebda@novascotia.ca]
> Sent: Sunday, February 28, 2016 10:54 PM
> To: naturens@chebucto.ns.ca
> Subject: RE: [NatureNS] Fw: Reconsideration; ground frost
>
> David
>
> I think I am missing something here.
>
> I can see the relationship with the sine of the angle of incidence.. but
> until light strikes the surface, the effective angle of incidence is zero
> (as it is during the dark period), so am no sure how it can approach zero
> if it is there already.... or have I missed something fundamental here?
>
> Andrew
>
> ________________________________________
> From: naturens-owner@chebucto.ns.ca [naturens-owner@chebucto.ns.ca] on
> behalf of David & Alison Webster [dwebster@glinx.com]
> Sent: February-28-16 8:59 AM
> To: NatureNS@chebucto.ns.ca
> Subject: [NatureNS] Fw: Reconsideration; ground frost
>
> Dear All, Feb 28, 2016
> I overlooked another aspect to the question of frost shortly after
> sunrise; effective interception of solar radiation. This would apply to
> some extent year round in all terrain and locations given calm clear
> weather. On a global scale, the area of sunlight intercepted by the earth
> is a disk equal to the cross-sectional area of the globe. A portion of a
> recent private e-mail on this aspect is pasted below.
> START OF PASTE\\\\\\\\\\\\
> As one approaches the edge of this disk (sunrise) the effective area of
> insolation realtive to the area of earth insolated (the interception of
> radiation per unit area) approaches zero. Maximum insolation will be when
> the sun is directly overhead. Elsewhere the incoming radiation per unit
> area will be proportional to the sin of the angle of incidence.
> An object will continue to cool until incoming radiation exceeds
> outgoing radiation.
> Sometimes it take a while to notice the obvious.
> END OF PASTE
> Yt, Dave Webster, Kentville
> ----- Original Message -----
> From: David & Alison Webster<mailto:dwebster@glinx.com>
> To: NatureNS@chebucto.ns.ca<mailto:NatureNS@chebucto.ns.ca>
> Sent: Tuesday, October 13, 2015 7:15 AM
> Subject: Reconsideration; ground frost
>
> Hi Paul, Andrew & All Oct 13, 2015
> This didn't get posted the first time; perhaps because it had become
> too large so I have pasted the original with several earlier exchanges
> clipped.
>
> START OF PASTE\\\\\\\\\\
> Hi Paul, Andrew & All,