Bon Accord Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bon Accord Observatory clear sky chart from 2005-11-06 to 2024-03-01.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.
Observation TypeMinimum conditions for hour to be usableOut of hours
Clear of CloudsTransparencySeeingSky Brightness
Planetary:50%(no limit)Average(no limit)Nighttime Hours
Deep Sky:70%Average(no limit)5.9Nighttime Hours
CCD Imaging:70%Below AveragePoor(no limit)Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

PlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
Night
41%
23603
11%
80563
7%
23546
45%
23546
17%
55296

By Month

MonthDays of MonthPlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
January1st to 10thNight
35%
935
8%
4692
5%
926
38%
926
9%
3418
January11th to 20thNight
36%
933
19%
4617
7%
929
41%
929
12%
2939
January21st to endNight
28%
963
8%
5053
5%
949
35%
949
9%
3340
February1st to 10thNight
39%
760
10%
2660
7%
760
44%
760
13%
1949
February11th to 20thNight
39%
741
10%
2461
9%
740
43%
740
16%
1751
February21st to endNight
42%
628
14%
1998
12%
628
48%
628
19%
1409
March1st to 10thNight
42%
722
13%
2165
12%
722
45%
722
19%
1579
March11th to 20thNight
41%
720
15%
1999
10%
720
45%
720
22%
1429
March21st to endNight
41%
792
10%
2148
6%
792
43%
792
20%
1585
April1st to 10thNight
45%
624
11%
1704
6%
624
47%
624
21%
1231
April11th to 20thNight
39%
533
14%
1601
11%
533
43%
533
19%
1149
April21st to endNight
44%
540
12%
1620
11%
540
47%
540
23%
1092
May1st to 10thNight
43%
378
8%
1302
8%
378
46%
378
18%
909
May11th to 20thNight
51%
354
6%
1239
4%
354
52%
354
21%
783
May21st to endNight
44%
388
0%
1358
0%
388
44%
388
16%
938
June1st to 10thNight
46%
360
0%
1260
0%
360
46%
360
16%
887
June11th to 20thNight
43%
360
0%
1138
0%
360
43%
360
13%
872
June21st to endNight
58%
360
0%
1080
0%
360
58%
360
23%
714
July1st to 10thNight
56%
350
0%
1122
0%
350
56%
350
24%
733
July11th to 20thNight
55%
348
0%
1218
0%
348
55%
348
19%
800
July21st to endNight
62%
394
4%
1302
3%
372
64%
372
26%
776
August1st to 10thNight
60%
358
7%
1335
8%
356
62%
356
22%
818
August11th to 20thNight
59%
452
10%
1520
12%
452
61%
452
26%
946
August21st to endNight
56%
594
16%
1782
14%
594
58%
594
27%
1162
September1st to 10thNight
50%
571
14%
1660
11%
571
52%
571
27%
1066
September11th to 20thNight
54%
720
17%
1980
12%
720
57%
720
31%
1263
September21st to endNight
48%
720
20%
1980
13%
720
53%
720
30%
1273
October1st to 10thNight
45%
720
16%
2092
11%
720
49%
720
25%
1407
October11th to 20thNight
42%
720
17%
2311
11%
720
49%
720
23%
1516
October21st to endNight
35%
810
15%
2592
9%
810
43%
810
17%
1819
November1st to 10thNight
31%
925
13%
2590
8%
925
36%
925
16%
1987
November11th to 20thNight
30%
950
12%
2821
5%
950
37%
950
15%
2157
November21st to endNight
32%
950
8%
2850
4%
950
36%
950
16%
2142
December1st to 10thNight
33%
950
13%
2850
7%
950
39%
950
16%
2097
December11th to 20thNight
34%
950
11%
3480
7%
949
38%
949
12%
2600
December21st to endNight
38%
1030
7%
4983
5%
1026
43%
1026
12%
2760

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

Occurrence of each ForecastData
Points
Cloud CoverDay
10%8%47%7%46%6%6%8%34%
78211
Night
8%9%48%8%46%6%6%8%33%
81134
TransparencyDay
45%10%18%20%7%
77912
Night
47%9%16%19%8%1
81013
SeeingDay
12%7%24%31%19%6%
14817
Night
13%6%27%36%16%2
23662

By Month

Percentage Cloud Cover NW (2006-)
Color Meanings:
Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
5%12%6%11%12%6%7%7%7%8%21%
2219
11th to 20thDay
5%10%49%7%5%5%8%8%9%32%
2199
21st to endDay
7%11%5%9%6%35%6%6%8%33%
2584
February1st to 10thDay
8%15%6%10%8%5%5%6%7%8%22%
1520
11th to 20thDay
6%11%7%10%7%47%7%8%7%27%
1606
21st to endDay
5%11%6%10%8%5%6%7%7%8%28%
1456
March1st to 10thDay
7%12%7%12%7%46%5%6%9%23%
1801
11th to 20thDay
9%10%410%6%36%6%7%8%31%
1960
21st to endDay
11%13%6%11%7%45%6%7%8%23%
2204
April1st to 10thDay
8%11%48%8%46%6%5%8%32%
2255
11th to 20thDay
10%12%5%7%6%45%6%6%7%33%
2310
21st to endDay
10%9%37%6%46%6%6%8%34%
2340
May1st to 10thDay
11%9%47%7%5%6%7%6%8%30%
2653
11th to 20thDay
8%6%35%6%35%5%7%10%41%
2654
21st to endDay
16%7%36%6%45%5%6%7%36%
2917
June1st to 10thDay
15%8%48%7%46%6%5%8%31%
2700
11th to 20thDay
19%7%38%7%5%6%6%7%8%23%
2817
21st to endDay
11%5%35%7%36%6%8%10%37%
2880
July1st to 10thDay
11%6%25%6%46%5%7%10%38%
2800
11th to 20thDay
11%6%36%5%36%6%7%11%36%
2700
21st to endDay
9%415%5%35%7%7%9%45%
2938
August1st to 10thDay
9%5%15%6%46%7%8%10%38%
2594
11th to 20thDay
8%5%25%5%446%7%9%44%
2379
21st to endDay
9%6%26%6%45%5%5%9%43%
2574
September1st to 10thDay
12%6%35%6%35%5%6%7%41%
2298
11th to 20thDay
14%6%26%6%35%5%6%9%39%
1980
21st to endDay
9%7%25%6%35%46%8%44%
1980
October1st to 10thDay
11%9%38%6%45%5%46%38%
1868
11th to 20thDay
11%8%47%6%345%6%8%38%
1648
21st to endDay
9%10%5%9%8%35%6%6%7%32%
1746
November1st to 10thDay
10%11%6%9%8%5%6%46%7%27%
1488
11th to 20thDay
7%12%5%10%8%45%5%5%9%30%
1359
21st to endDay
5%11%7%12%7%46%5%7%8%30%
1330
December1st to 10thDay
5%12%6%9%8%7%6%5%7%8%26%
1330
11th to 20thDay
7%9%5%10%8%6%8%7%7%8%26%
1682
21st to endDay
46%47%8%5%6%6%7%9%41%
2442
January1st to 10thNight
410%5%13%12%6%7%6%6%7%24%
4726
11th to 20thNight
48%49%10%6%6%6%7%7%34%
4699
21st to endNight
8%12%5%9%8%45%6%6%8%30%
5088
February1st to 10thNight
7%10%5%11%9%6%7%6%6%7%26%
2660
11th to 20thNight
6%11%5%8%9%47%7%7%7%29%
2460
21st to endNight
5%9%6%9%8%47%7%6%8%31%
1998
March1st to 10thNight
6%10%5%10%9%5%5%6%6%8%29%
2165
11th to 20thNight
9%9%48%8%47%6%7%8%30%
1999
21st to endNight
9%13%6%9%7%45%5%6%8%29%
2148
April1st to 10thNight
8%10%38%8%5%7%6%6%6%32%
1704
11th to 20thNight
9%11%5%8%9%5%5%5%5%7%31%
1601
21st to endNight
13%8%37%6%25%5%6%9%36%
1620
May1st to 10thNight
13%8%5%8%7%45%6%7%7%30%
1302
11th to 20thNight
11%7%37%8%35%46%7%39%
1239
21st to endNight
18%8%26%7%45%5%6%8%30%
1358
June1st to 10thNight
15%9%46%6%46%7%6%7%31%
1260
11th to 20thNight
17%8%38%7%48%8%7%8%21%
1138
21st to endNight
8%5%36%6%45%7%8%8%39%
1080
July1st to 10thNight
12%5%36%6%346%6%10%38%
1156
11th to 20thNight
11%5%246%46%5%8%10%38%
1260
21st to endNight
8%426%6%36%5%8%8%44%
1379
August1st to 10thNight
8%436%5%37%6%7%10%42%
1342
11th to 20thNight
9%7%345%45%6%7%8%42%
1520
21st to endNight
10%6%36%6%45%6%6%8%40%
1782
September1st to 10thNight
12%6%36%5%3445%7%45%
1660
11th to 20thNight
12%7%35%43436%8%46%
1980
21st to endNight
10%5%35%6%35%5%6%6%47%
1980
October1st to 10thNight
11%9%37%6%35%46%7%39%
2092
11th to 20thNight
9%7%47%6%35%6%7%8%38%
2311
21st to endNight
8%10%48%8%46%6%6%7%33%
2592
November1st to 10thNight
13%14%6%8%7%5%5%5%5%6%26%
2590
11th to 20thNight
9%13%5%10%8%46%5%7%7%26%
2821
21st to endNight
6%10%6%10%9%46%6%6%8%28%
2850
December1st to 10thNight
8%12%5%10%8%46%5%5%7%29%
2850
11th to 20thNight
7%12%6%11%8%5%6%6%7%8%25%
3510
21st to endNight
37%37%7%46%5%6%9%41%
5214

Transparency (when cloud cover <= 30%)
Color Meanings:
Too cloudy to forecast Poor Below Average Average Above average Transparent
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
57%10%12%17%41
2219
11th to 20thDay
45%9%13%15%17%1
2220
21st to endDay
46%9%20%19%5%
2584
February1st to 10thDay
57%7%15%13%7%
1520
11th to 20thDay
51%11%17%14%7%
1615
21st to endDay
49%10%14%20%7%
1456
March1st to 10thDay
56%9%16%14%4
1801
11th to 20thDay
47%9%16%18%9%1
1961
21st to endDay
57%10%14%16%3
2204
April1st to 10thDay
47%11%18%17%7%1
2255
11th to 20thDay
46%9%16%22%7%
2310
21st to endDay
45%10%16%21%6%2
2340
May1st to 10thDay
49%9%15%18%8%
2653
11th to 20thDay
37%9%14%29%11%1
2658
21st to endDay
46%8%14%22%11%
2918
June1st to 10thDay
49%9%16%19%7%
2700
11th to 20thDay
56%10%17%13%5%
2817
21st to endDay
39%12%22%22%5%1
2880
July1st to 10thDay
39%11%20%24%5%1
2725
11th to 20thDay
40%12%27%20%1
2619
21st to endDay
33%12%28%25%2
2775
August1st to 10thDay
36%14%29%19%3
2576
11th to 20thDay
33%11%26%24%6%
2379
21st to endDay
36%10%22%25%6%
2574
September1st to 10thDay
39%11%19%23%6%2
2298
11th to 20thDay
40%8%20%25%7%
1980
21st to endDay
37%10%18%26%8%2
1980
October1st to 10thDay
47%7%18%20%8%
1868
11th to 20thDay
43%9%18%22%8%
1648
21st to endDay
49%9%13%22%6%
1746
November1st to 10thDay
57%8%14%15%6%1
1488
11th to 20thDay
52%7%17%18%5%
1359
21st to endDay
50%10%16%17%5%1
1330
December1st to 10thDay
54%9%14%16%7%1
1330
11th to 20thDay
51%11%17%15%6%
1684
21st to endDay
38%8%13%26%14%1
2442
January1st to 10thNight
56%10%14%14%6%1
4726
11th to 20thNight
46%8%14%18%14%1
4740
21st to endNight
49%9%19%15%6%
5088
February1st to 10thNight
54%8%14%16%7%1
2660
11th to 20thNight
50%9%16%15%9%1
2461
21st to endNight
48%9%14%20%8%1
1998
March1st to 10thNight
50%9%16%19%6%1
2165
11th to 20thNight
48%7%14%20%10%1
1999
21st to endNight
51%8%16%17%7%1
2148
April1st to 10thNight
46%10%16%17%11%
1704
11th to 20thNight
51%7%12%18%10%2
1601
21st to endNight
43%8%13%21%13%1
1620
May1st to 10thNight
48%9%14%19%9%1
1302
11th to 20thNight
42%7%12%24%14%1
1239
21st to endNight
50%8%12%19%10%2
1358
June1st to 10thNight
49%10%14%18%8%1
1260
11th to 20thNight
55%11%15%14%4
1138
21st to endNight
37%13%22%23%41
1080
July1st to 10thNight
37%12%19%23%6%1
1122
11th to 20thNight
39%14%22%21%4
1218
21st to endNight
32%12%24%28%4
1302
August1st to 10thNight
34%13%24%26%2
1335
11th to 20thNight
35%11%23%25%5%1
1520
21st to endNight
39%9%18%26%8%1
1782
September1st to 10thNight
37%9%19%25%8%2
1660
11th to 20thNight
36%7%18%28%9%1
1980
21st to endNight
35%9%15%26%13%2
1980
October1st to 10thNight
43%7%16%24%10%
2092
11th to 20thNight
42%8%17%24%8%1
2311
21st to endNight
46%11%14%22%7%1
2592
November1st to 10thNight
57%7%12%19%6%
2590
11th to 20thNight
55%9%14%16%6%
2821
21st to endNight
52%10%16%17%41
2850
December1st to 10thNight
53%7%12%17%9%1
2850
11th to 20thNight
55%9%16%14%6%
3507
21st to endNight
37%9%17%20%15%2
5214

Seeing (when cloud cover <= 80%)
Color Meanings:
Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
11%5%26%33%19%6%
180
11th to 20thDay
8%11%24%34%17%6%
180
21st to endDay
11%12%28%31%14%4
198
February1st to 10thDay
12%11%28%30%14%5%
180
11th to 20thDay
11%7%30%29%18%6%
242
21st to endDay
7%6%32%29%18%7%
268
March1st to 10thDay
12%6%26%36%18%3
421
11th to 20thDay
10%324%38%19%5%
420
21st to endDay
14%425%31%22%4
462
April1st to 10thDay
8%8%27%35%18%3
420
11th to 20thDay
13%5%22%33%20%7%
412
21st to endDay
13%8%26%30%18%5%
420
May1st to 10thDay
10%10%24%32%19%4
578
11th to 20thDay
10%5%23%34%20%8%
625
21st to endDay
16%5%21%32%17%10%
679
June1st to 10thDay
16%9%24%31%17%3
630
11th to 20thDay
20%6%21%26%20%7%
747
21st to endDay
11%6%16%30%26%11%
810
July1st to 10thDay
12%319%32%23%9%
720
11th to 20thDay
12%5%23%29%24%7%
618
21st to endDay
8%5%21%30%27%9%
684
August1st to 10thDay
11%6%19%28%23%14%
534
11th to 20thDay
8%6%24%34%22%6%
437
21st to endDay
11%6%22%29%23%9%
495
September1st to 10thDay
15%10%26%28%16%6%
450
11th to 20thDay
16%9%24%30%16%4
450
21st to endDay
11%13%29%27%15%4
450
October1st to 10thDay
16%9%25%28%18%4
338
11th to 20thDay
15%6%30%30%16%3
270
21st to endDay
14%13%36%23%12%3
297
November1st to 10thDay
17%7%23%37%13%3
270
11th to 20thDay
12%7%30%36%11%4
194
21st to endDay
6%8%37%32%9%8%
180
December1st to 10thDay
9%8%29%33%15%5%
180
11th to 20thDay
11%8%24%40%13%3
180
21st to endDay
7%5%22%36%24%7%
198
January1st to 10thNight
11%5%31%36%15%2
950
11th to 20thNight
9%7%32%36%14%1
950
21st to endNight
15%10%33%29%11%2
971
February1st to 10thNight
10%8%25%37%17%4
760
11th to 20thNight
11%5%31%35%17%1
744
21st to endNight
7%5%28%37%19%5%
628
March1st to 10thNight
10%5%27%40%16%3
722
11th to 20thNight
12%5%30%35%15%3
720
21st to endNight
14%425%38%17%2
792
April1st to 10thNight
13%425%36%19%2
624
11th to 20thNight
14%428%38%14%2
533
21st to endNight
15%6%26%35%18%
540
May1st to 10thNight
14%6%25%34%19%2
378
11th to 20thNight
12%6%21%42%18%2
354
21st to endNight
22%5%18%35%20%2
389
June1st to 10thNight
20%7%18%41%13%2
360
11th to 20thNight
21%323%29%22%2
360
21st to endNight
9%419%37%26%4
360
July1st to 10thNight
13%318%40%26%
350
11th to 20thNight
11%322%39%22%3
348
21st to endNight
9%416%42%25%4
394
August1st to 10thNight
9%5%17%32%32%5%
358
11th to 20thNight
11%419%38%26%2
452
21st to endNight
11%420%40%24%1
594
September1st to 10thNight
16%321%37%20%2
571
11th to 20thNight
14%5%21%41%18%2
720
21st to endNight
13%5%27%39%14%2
720
October1st to 10thNight
16%427%38%15%1
720
11th to 20thNight
12%6%29%38%13%2
720
21st to endNight
13%9%31%33%13%
810
November1st to 10thNight
20%6%30%32%10%2
925
11th to 20thNight
13%7%34%32%11%3
950
21st to endNight
10%5%36%36%12%1
950
December1st to 10thNight
14%8%30%31%15%3
950
11th to 20thNight
10%7%33%35%14%2
950
21st to endNight
10%5%27%36%18%4
1045
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