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Posted: Mar 29 2005, 07:58 PM
Joined: 29-March 05
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Global shortwave irradiance measurements in high mountains (3400m) suggest that the share of diffuse irradiance might, under rare conditions, equal maximum direct irradiance. Is that possible? I'm posting my question in a general pysics forum instead of discussing this with collegues, because most geographers like me lack the advanced principles of physics and formalism to substantiate their suspicions.
General information and observation:
Global irradiance is the sum of diffuse and direct solar irradiance incident on a horizontal plane. A high summer sun in a clear (blue) sky at noon in the mid latitudes (~45° eg.) can produce 1000 watts/square meter (w/m2) of which then ~950 W/m2 (95%) is direct irradiance (coming from the disc of the sun) and ~50 w/m2 (5%) of diffuse origin.
Thin clouds are excellent diffusors and their presence can boost diffuse radiation to values of 40-50% of the direct beam! (In the example given before global irradiance then may reach 1500 w/m2). Under conditions we can find in high mountains: extreme low turbidity of the air, otherwise low clouds surrounding the sensor but leaving the disc of the sun uncovered, frequently more (60-70%) and on few occasions >80% has been measured.
My curiosity is: which is the theoretical upper limit and especially if this is exactly 100%?
May be some useful hints/proposal of an explanation model.
In my opinion, the question derived from this specific problem would about be: if a punctiform light source (sun) emits light on a plane with infinite extension and this plane is a perfect diffusor (thin cloud cover), then the sum of diffuse irradiance at any point on a distant parallel plane behind that diffusor (earth's surface) is of the same magnitude as if there were no diffusor and the light would arrive directly from that light source (sun).
Probable analog test arrangement: black room: light bulb once directly over light sensor and once with large, thin milk glass in between.
I'm not sure if the parallel plane with the sensor must be or is best described by: a) a partial reflector (<100% is reflected) or a perfect reflector (100%) b) another perfect diffusor (50% is reflected back) or even if this property can be neglected. Observation is that in cases of a high contribution of diffuse irradiance, the main source seems to be cloud parts that are close to the sun; they turn extremely bright.
Thanks for any suggestion, explanations or even answers.