Dominion Astrophysical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dominion Astrophysical Observatory clear sky chart from 2005-11-05 to 2018-02-22.

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
45%
15191
10%
48861
8%
15164
48%
15164
16%
35445

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 15thNight
31%
823
7%
2911
5%
821
34%
821
8%
2395
January16th to endNight
33%
847
7%
2928
6%
847
36%
847
9%
2351
February1st to 15thNight
32%
756
7%
2483
4%
756
36%
756
10%
2012
February16th to endNight
36%
660
11%
2139
8%
660
41%
660
13%
1624
March1st to 15thNight
32%
720
5%
2042
4%
720
34%
720
10%
1686
March16th to endNight
34%
768
7%
2087
4%
768
36%
768
12%
1706
April1st to 15thNight
43%
713
9%
1793
4%
713
45%
713
18%
1381
April16th to endNight
47%
528
11%
1608
8%
528
49%
528
17%
1172
May1st to 15thNight
56%
359
13%
1439
19%
359
59%
359
21%
875
May16th to endNight
59%
375
7%
1474
11%
374
60%
374
18%
960
June1st to 15thNight
56%
360
5%
1260
4%
360
57%
360
20%
826
June16th to endNight
60%
360
7%
1260
5%
360
61%
360
24%
789
July1st to 15thNight
75%
340
12%
1190
13%
340
77%
340
40%
600
July16th to endNight
83%
376
17%
1336
25%
354
87%
354
48%
577
August1st to 15thNight
81%
462
17%
1536
21%
460
82%
460
44%
735
August16th to endNight
76%
606
23%
1752
20%
606
76%
606
45%
929
September1st to 15thNight
79%
720
23%
1800
15%
720
80%
720
52%
998
September16th to endNight
57%
716
18%
1983
14%
716
59%
716
25%
1358
October1st to 15thNight
52%
720
11%
2160
11%
720
55%
720
24%
1466
October16th to endNight
42%
768
10%
2381
8%
768
45%
768
14%
1802
November1st to 15thNight
29%
760
6%
2660
4%
760
32%
760
8%
2186
November16th to endNight
31%
780
6%
2730
4%
780
34%
780
9%
2191
December1st to 15thNight
35%
794
6%
2789
4%
794
38%
794
9%
2240
December16th to endNight
21%
880
6%
3120
3%
880
26%
880
7%
2586

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
16%8%36%5%3445%6%32%
49423
Night
18%8%35%5%3445%6%34%
49015
Cloud CoverDay
16%8%36%5%3445%7%34%
49423
Night
19%8%36%5%3445%6%33%
49018
TransparencyDay
49%10%17%16%5%
49081
Night
51%8%15%16%7%1
48862
SeeingDay
17%314%27%26%9%
6588
Night
21%5%17%25%22%7%
15202

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 15thDay
29%13%37%6%45%43416%
1358
16th to endDay
25%14%6%9%9%24345%13%
1644
February1st to 15thDay
26%14%48%7%2435%6%15%
1711
16th to endDay
17%12%5%9%6%45%45%6%21%
1480
March1st to 15thDay
24%12%5%10%6%35%45%6%15%
1917
16th to endDay
21%14%5%9%7%35%45%5%16%
2136
April1st to 15thDay
17%10%48%7%5%6%5%6%7%19%
2155
16th to endDay
17%9%47%5%45%5%7%8%23%
2352
May1st to 15thDay
13%6%36%6%45%5%6%7%34%
2511
16th to endDay
15%8%47%5%35%5%5%7%30%
2636
June1st to 15thDay
12%6%36%5%25%5%6%8%36%
2700
16th to endDay
12%6%25%6%45%5%5%7%38%
2700
July1st to 15thDay
6%324323446%58%
2700
16th to endDay
6%213323346%63%
2739
August1st to 15thDay
7%31432345%6%57%
2415
16th to endDay
7%32442345%8%51%
2449
September1st to 15thDay
7%41442345%8%53%
2160
16th to endDay
16%7%26%5%345%5%7%36%
1974
October1st to 15thDay
20%8%37%6%25%5%5%5%29%
1800
16th to endDay
23%12%47%5%3345%7%21%
1842
November1st to 15thDay
31%11%5%7%5%34435%17%
1528
16th to endDay
26%12%49%6%35%436%17%
1560
December1st to 15thDay
23%13%5%6%5%35%5%5%6%21%
1500
16th to endDay
27%13%5%7%6%35%5%5%5%16%
1456
January1st to 15thNight
29%9%36%5%35%45%5%20%
2911
16th to endNight
24%10%37%6%3446%6%21%
2928
February1st to 15thNight
24%11%47%5%35%436%23%
2471
16th to endNight
19%10%37%5%35%446%27%
2139
March1st to 15thNight
23%11%36%7%35%45%6%21%
2042
16th to endNight
20%9%5%7%5%45%5%5%6%23%
2083
April1st to 15thNight
18%7%36%6%25%6%5%6%31%
1793
16th to endNight
19%6%36%5%245%6%8%31%
1608
May1st to 15thNight
11%5%34425%5%5%5%44%
1439
16th to endNight
10%5%35%5%35%6%6%8%38%
1474
June1st to 15thNight
11%6%35%5%345%47%42%
1260
16th to endNight
10%4244335%6%9%44%
1260
July1st to 15thNight
5%212223335%65%
1260
16th to endNight
4112112236%72%
1428
August1st to 15thNight
3213323345%66%
1544
16th to endNight
6%3331335%7%61%
1752
September1st to 15thNight
6%21221334467%
1800
16th to endNight
14%4345%34448%41%
1983
October1st to 15thNight
16%7%25%5%2344442%
2160
16th to endNight
24%7%35%6%3445%6%28%
2381
November1st to 15thNight
29%9%36%6%35%5%4420%
2660
16th to endNight
22%11%5%8%6%35%45%5%22%
2730
December1st to 15thNight
20%11%46%5%36%5%5%6%23%
2789
16th to endNight
28%12%47%5%35%34420%
3120

Percentage Cloud Cover SW (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 15thDay
30%12%48%5%35%34416%
1358
16th to endDay
26%15%7%8%6%44345%13%
1644
February1st to 15thDay
26%14%47%6%3445%6%16%
1711
16th to endDay
18%12%5%8%6%45%45%5%22%
1480
March1st to 15thDay
23%10%5%10%7%345%5%6%15%
1917
16th to endDay
20%12%410%8%45%5%45%18%
2136
April1st to 15thDay
17%9%36%5%46%6%6%10%21%
2155
16th to endDay
16%7%47%5%46%6%7%8%25%
2352
May1st to 15thDay
13%5%25%6%36%5%6%7%37%
2511
16th to endDay
15%8%36%6%35%5%6%6%33%
2636
June1st to 15thDay
12%6%25%6%2445%9%39%
2700
16th to endDay
11%5%26%5%35%5%5%7%41%
2700
July1st to 15thDay
6%323323346%60%
2700
16th to endDay
5%22312336%66%
2739
August1st to 15thDay
6%313323347%60%
2415
16th to endDay
7%21442346%8%52%
2449
September1st to 15thDay
7%41442245%8%54%
2160
16th to endDay
16%6%26%5%345%5%6%35%
1974
October1st to 15thDay
19%10%47%5%345%5%5%29%
1800
16th to endDay
23%14%37%5%34348%22%
1842
November1st to 15thDay
31%10%46%6%35%44417%
1528
16th to endDay
25%12%6%7%7%345%36%17%
1560
December1st to 15thDay
24%13%46%6%35%5%45%20%
1500
16th to endDay
26%13%5%7%6%35%5%5%5%16%
1456
January1st to 15thNight
29%10%35%6%35%445%20%
2910
16th to endNight
25%10%37%5%3446%6%20%
2928
February1st to 15thNight
25%10%46%5%35%446%24%
2471
16th to endNight
20%10%38%6%35%45%5%27%
2139
March1st to 15thNight
24%11%46%6%35%45%5%21%
2042
16th to endNight
21%9%48%7%35%5%5%6%23%
2087
April1st to 15thNight
18%8%35%5%35%5%6%7%30%
1793
16th to endNight
18%6%37%5%25%45%8%31%
1608
May1st to 15thNight
11%425%5%35%5%5%6%45%
1439
16th to endNight
10%5%27%5%45%5%7%8%37%
1474
June1st to 15thNight
12%6%25%5%35%5%5%5%41%
1260
16th to endNight
10%5%24425%6%7%9%42%
1260
July1st to 15thNight
5%313313335%65%
1260
16th to endNight
5%12112325%72%
1428
August1st to 15thNight
3213222447%66%
1544
16th to endNight
6%312312348%62%
1752
September1st to 15thNight
6%212213245%66%
1800
16th to endNight
14%5%35%5%35%447%41%
1983
October1st to 15thNight
16%7%25%5%23335%41%
2160
16th to endNight
24%7%36%5%44447%27%
2381
November1st to 15thNight
30%10%35%6%345%45%20%
2660
16th to endNight
22%11%48%6%445%5%5%22%
2730
December1st to 15thNight
21%11%37%5%45%5%5%5%23%
2789
16th to endNight
28%12%46%5%34445%20%
3120

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 15thDay
71%7%8%7%3
1358
16th to endDay
74%8%9%6%1
1644
February1st to 15thDay
67%10%8%7%3
1711
16th to endDay
61%8%13%11%3
1480
March1st to 15thDay
67%10%11%7%1
1917
16th to endDay
66%9%13%9%1
2136
April1st to 15thDay
56%12%17%12%2
2155
16th to endDay
52%13%17%12%4
2352
May1st to 15thDay
43%11%20%17%6%
2511
16th to endDay
48%11%21%15%2
2640
June1st to 15thDay
41%11%25%14%5%
2700
16th to endDay
40%10%25%18%4
2700
July1st to 15thDay
24%9%22%28%12%2
2541
16th to endDay
18%9%24%33%11%1
2569
August1st to 15thDay
25%9%22%30%10%2
2398
16th to endDay
28%9%25%27%8%1
2449
September1st to 15thDay
28%10%20%24%15%1
2160
16th to endDay
47%11%16%16%7%
1974
October1st to 15thDay
54%10%15%15%5%
1800
16th to endDay
61%10%14%9%4
1842
November1st to 15thDay
69%6%11%9%21
1528
16th to endDay
68%9%10%9%2
1560
December1st to 15thDay
65%10%10%9%4
1500
16th to endDay
69%9%10%7%3
1456
January1st to 15thNight
65%8%11%9%4
2911
16th to endNight
62%9%12%11%21
2928
February1st to 15thNight
62%8%12%10%4
2483
16th to endNight
57%7%13%15%5%
2139
March1st to 15thNight
63%9%11%12%2
2042
16th to endNight
61%10%12%14%1
2087
April1st to 15thNight
51%10%15%17%4
1793
16th to endNight
49%10%14%19%5%
1608
May1st to 15thNight
39%8%17%22%11%1
1440
16th to endNight
41%11%22%19%5%
1474
June1st to 15thNight
42%9%20%21%41
1260
16th to endNight
35%11%22%22%5%1
1260
July1st to 15thNight
22%7%17%31%16%4
1190
16th to endNight
15%6%21%33%19%3
1336
August1st to 15thNight
18%8%22%31%15%4
1536
16th to endNight
21%7%23%32%11%3
1752
September1st to 15thNight
20%6%20%26%21%5%
1800
16th to endNight
42%8%14%21%11%1
1983
October1st to 15thNight
44%6%17%19%10%
2160
16th to endNight
56%7%16%13%6%
2381
November1st to 15thNight
65%9%11%9%21
2660
16th to endNight
62%9%11%9%5%
2730
December1st to 15thNight
63%8%12%9%5%
2789
16th to endNight
66%8%11%8%4
3120

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 15thDay
26%415%24%26%2
90
16th to endDay
40%5%27%14%11%
142
February1st to 15thDay
31%7%23%25%10%1
281
16th to endDay
24%10%23%23%14%4
232
March1st to 15thDay
36%9%26%17%9%1
270
16th to endDay
28%6%21%30%9%2
288
April1st to 15thDay
20%5%17%28%22%6%
269
16th to endDay
20%216%37%17%5%
270
May1st to 15thDay
15%412%33%28%5%
288
16th to endDay
15%112%26%30%13%
351
June1st to 15thDay
10%214%34%29%8%
495
16th to endDay
10%214%33%29%8%
495
July1st to 15thDay
6%16%28%40%16%
475
16th to endDay
317%25%41%20%
408
August1st to 15thDay
7%1222%36%28%
315
16th to endDay
7%16%23%42%18%
332
September1st to 15thDay
9%7%33%35%14%
315
16th to endDay
15%411%29%30%9%
315
October1st to 15thDay
22%317%28%23%4
315
16th to endDay
28%421%22%17%4
276
November1st to 15thDay
34%7%27%23%5%1
90
16th to endDay
31%10%17%22%10%8%
90
December1st to 15thDay
31%423%16%22%2
90
16th to endDay
36%13%22%20%6%
96
January1st to 15thNight
32%9%17%20%17%2
825
16th to endNight
28%7%20%21%19%2
847
February1st to 15thNight
28%9%17%24%16%3
764
16th to endNight
24%8%21%24%17%4
660
March1st to 15thNight
27%9%24%21%13%4
720
16th to endNight
22%9%22%27%14%3
768
April1st to 15thNight
19%421%27%22%5%
713
16th to endNight
21%416%35%18%3
528
May1st to 15thNight
13%217%30%26%9%
360
16th to endNight
10%312%32%32%9%
375
June1st to 15thNight
14%317%28%30%7%
360
16th to endNight
13%115%33%26%9%
360
July1st to 15thNight
6%17%28%44%10%
340
16th to endNight
5%5%28%45%13%
376
August1st to 15thNight
4422%45%22%
462
16th to endNight
8%8%24%43%14%
606
September1st to 15thNight
7%6%26%36%21%
720
16th to endNight
16%211%26%31%11%
716
October1st to 15thNight
21%213%27%23%10%
720
16th to endNight
29%314%26%19%6%
768
November1st to 15thNight
32%7%21%25%10%1
760
16th to endNight
26%10%23%21%14%4
780
December1st to 15thNight
26%8%19%27%15%2
794
16th to endNight
34%8%28%18%9%
880
Page generated at 2018-02-23 08:21:31. Version ac04

Copyright 2018 A. Danko


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