On a section of the room drawn to any scale. draw sight-lines from the reference point to the horizontal boundary edges of the patch of sky visible through the window. In this case, the lines are drawn to the head of the window and to the top edge of the external horizontal obstruc- tion. Place protractor 1 on the section, with its centre at the re- ference point and its base along the working plane, and note the two values where the sight-lines intersect the protractor scale. The difference hetween the two readings. 9.0% and 2.9% in this example. gives the sky component for a window of infinite length. i.e. 6.1%.
This sky component must be cor- rected for the finite length of the window by applying the auxiliary
protractor 2 on a plan of the room drawn to any scale. First, however. find the average angle of altitude of the sightlines on the section about 28° in the example. Next. take the plan and draw two
two further sight- lines (XD and XE) from the re- ference point to the vertical boun. dary edges of the patch of sky in this case. the sides of the window opening. Place auxiliary protractor 2 on the plan, with its centre at the reference point and base parallel to the plane of the window. Observe where the sight-lines intersect the semicircular scale on the auxiliary protractor corresponding the average angle of altitude of 28°. The sum of these two readings. 0.30 and 0.15 in this example. gives the cor- rection factor for the length of win dow shown, i.e, 0.45.
ANGLES
OF
AZIMUTH
to
Thus,
From readings on protractor 1. sky component for window of in- finite length = 9.0-2.9 = 6.1% Average angle of altitude of unob structed sky = 28°
From readings on protractor 2. correction factor = 0.30 + 0.15 =0.45
Sky component (uniform sky) for window shown in Fig. 6 6.1 X 0.45 = 2.7%
A correction must
now be made to convert this figure to that cor- responding to overcast sky condi tions for the
average angle of altitude of the patch of sky. From Fig. 2 the appropriate correction factor for a patch of sky at an aver- age angle of altitude of 28°=0.83.
120
901
20
6!
1:8
17
U
14
ค
મ
=
OP
7
3
80%
70*
GO
50°
40
30
15* 10° 5
**
5 10° 15° 20°
30*
40*
50*
GO
70*
80"
90*
20
NOTE PROOP LINES [FOR} [VERTIC
GLAZED APERTURES
DROOP LINES FOR HORIZONTAL EDGES PARALLEL TO PLANE CF WINDOW
90
70
60
50*
2
4
6
18
1
85
ANGLES OF
ALTI
UDE
H+
ון
GO
55
45
19
DROOP LINES FOR
18
HORIZONTAL| EDGES AT
RIGHT ANGLES TO PLANE OF WINDOW
To
30
20" 15" 10'
5 10 15
30°
8
9
10
ון
12
13
14
IS
IC
17
18
פן
50*
60
70
80°
90
20 21
22
23
24 25
يه
לן
16
15
14
13
12
10
Fig. 5 Waldram diagram for C.I.E. Overcast sky-corrected for glass losses
THE HONG KONG & Far East BUILDER-VOLUME 19, NUMBER 2