Analysis Code for High Altitude Balloons - FedOA - Università degli ...

ACHAB: Analysis Code for High Altitude Balloons Chapter 3
• Compute Local Hour Angle ( LHA ).
The Local Hour Angle is then:
LHA = θ −
• Convert Local Hour Angle and declination to horizon coordinates.
Now it is possible to compute the local solar elevation angle (ELV) and
RA
azimuth angle using the following relationships:
( Lat)
sinδ
+ cos(
Lat)
cos ( LHA)
sin ELV = sin
δ cos
tan AZ = −
cos
where Lat is the balloon actual latitude.
sin(
LHA)
( Lat)
tanδ
− sin(
Lat)
cos(
LHA)
Using this algorithm it is possible to simulate the day and night cycle.
Twilight.
Before sunrise and again after sunset there are time intervals during which there is
natural light provided by the upper atmosphere, which does receive direct sunlight
and reflects part of it toward the Earth's surface. This kind of illumination is referred
to as twilight.
There are several definitions for twilight according to application. The present model
considers civil twilight, which is defined to begin (in the morning), and to end (in the
evening) when the center of the Sun is geometrically 6 degrees below the horizon 31 .
3.2.3.2 – Atmospheric Transmissivity
Irradiance of both the solar radiation and the longwave radiation is obviously
influenced by the presence of the atmosphere. The transmissivity of a solar beam and
the attenuation of the ground thermal radiation, follow an exponential decay 9 based
on Beer’s Law. All of the following equations are based on those from Ref. 9 and
Ref. 20.
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