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 2017-08-12.

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
46%
14387
10%
46373
8%
14360
48%
14360
16%
33620

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
30%
748
7%
2686
5%
746
33%
746
8%
2201
January16th to endNight
36%
778
7%
2703
6%
778
38%
778
10%
2143
February1st to 15thNight
34%
696
6%
2288
4%
696
38%
696
10%
1858
February16th to endNight
36%
632
10%
2048
8%
632
40%
632
13%
1555
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
44%
713
9%
1793
4%
713
46%
713
18%
1381
April16th to endNight
46%
528
10%
1608
8%
528
48%
528
17%
1172
May1st to 15thNight
57%
359
13%
1439
19%
359
59%
359
21%
875
May16th to endNight
61%
375
7%
1474
12%
374
62%
374
18%
960
June1st to 15thNight
56%
360
5%
1260
4%
360
57%
360
20%
826
June16th to endNight
61%
360
7%
1260
5%
360
62%
360
24%
789
July1st to 15thNight
76%
340
12%
1190
13%
340
77%
340
40%
600
July16th to endNight
84%
376
17%
1336
24%
354
87%
354
48%
577
August1st to 15thNight
82%
450
18%
1500
21%
448
83%
448
44%
714
August16th to endNight
74%
555
21%
1605
19%
555
75%
555
44%
862
September1st to 15thNight
79%
660
23%
1650
15%
660
80%
660
52%
919
September16th to endNight
58%
656
18%
1818
14%
656
59%
656
25%
1236
October1st to 15thNight
51%
660
11%
1980
10%
660
53%
660
22%
1369
October16th to endNight
41%
704
8%
2182
6%
704
44%
704
13%
1677
November1st to 15thNight
30%
700
6%
2450
5%
700
34%
700
9%
2013
November16th to endNight
34%
720
7%
2520
5%
720
37%
720
9%
2005
December1st to 15thNight
32%
729
5%
2574
3%
729
36%
729
8%
2102
December16th to endNight
22%
800
6%
2880
3%
800
27%
800
7%
2394

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%7%32%
47633
Night
18%8%35%5%3445%6%34%
46527
Cloud CoverDay
16%8%36%5%3445%7%34%
47633
Night
18%8%36%5%3445%6%33%
46530
TransparencyDay
49%10%17%16%5%
47291
Night
50%8%15%16%6%1
46374
SeeingDay
17%314%28%26%9%
6020
Night
21%5%17%25%22%7%
14398

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
28%14%38%6%4443417%
1253
16th to endDay
25%14%6%9%9%24345%13%
1517
February1st to 15thDay
26%14%48%7%24445%15%
1576
16th to endDay
18%12%5%9%6%45%45%6%20%
1425
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%
2366
16th to endDay
7%425%5%3445%9%49%
2244
September1st to 15thDay
7%41441345%8%53%
1980
16th to endDay
17%6%26%5%3445%7%36%
1810
October1st to 15thDay
21%9%37%6%25%5%5%5%27%
1650
16th to endDay
23%12%5%7%5%2345%7%19%
1689
November1st to 15thDay
32%11%5%6%434435%17%
1408
16th to endDay
26%12%49%6%35%436%17%
1440
December1st to 15thDay
24%13%5%7%5%34445%20%
1385
16th to endDay
27%12%5%6%6%35%5%5%6%16%
1344
January1st to 15thNight
28%10%46%5%35%445%21%
2686
16th to endNight
24%10%36%6%3446%6%22%
2703
February1st to 15thNight
23%11%47%5%35%436%23%
2276
16th to endNight
19%10%37%5%45%45%6%27%
2048
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
2213313345%67%
1508
16th to endNight
6%3331235%7%60%
1605
September1st to 15thNight
6%212213345%67%
1650
16th to endNight
15%434434448%41%
1818
October1st to 15thNight
18%7%25%5%2344440%
1980
16th to endNight
25%8%35%6%3445%6%26%
2182
November1st to 15thNight
29%8%36%6%35%5%4420%
2450
16th to endNight
21%10%5%8%6%35%5%5%5%23%
2520
December1st to 15thNight
22%12%46%5%35%45%6%22%
2574
16th to endNight
26%12%47%5%35%44421%
2880

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
29%12%49%5%35%34417%
1253
16th to endDay
25%15%7%8%6%4444413%
1517
February1st to 15thDay
26%14%47%6%3445%5%16%
1576
16th to endDay
18%12%5%8%6%45%45%5%22%
1425
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
7%313323347%60%
2366
16th to endDay
8%2145%2446%8%50%
2244
September1st to 15thDay
7%41442245%8%54%
1980
16th to endDay
16%7%26%5%345%5%6%36%
1810
October1st to 15thDay
20%10%47%5%345%6%5%27%
1650
16th to endDay
23%14%37%5%34348%20%
1689
November1st to 15thDay
32%9%46%6%35%44418%
1408
16th to endDay
24%12%6%8%7%345%36%17%
1440
December1st to 15thDay
25%13%46%6%34445%20%
1385
16th to endDay
25%12%5%7%6%35%5%5%5%16%
1344
January1st to 15thNight
28%10%36%7%25%44422%
2685
16th to endNight
24%10%37%5%3446%6%21%
2703
February1st to 15thNight
23%10%47%6%35%446%23%
2276
16th to endNight
20%10%38%6%35%45%6%26%
2048
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
2213222446%66%
1508
16th to endNight
7%32312348%61%
1605
September1st to 15thNight
6%212213246%66%
1650
16th to endNight
14%435%435%448%41%
1818
October1st to 15thNight
17%7%25%5%23336%40%
1980
16th to endNight
25%7%36%6%44447%26%
2182
November1st to 15thNight
30%9%35%6%34445%20%
2450
16th to endNight
21%10%48%6%45%5%5%5%23%
2520
December1st to 15thNight
22%11%47%5%45%45%5%22%
2574
16th to endNight
26%12%5%6%5%35%445%20%
2880

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%8%3
1253
16th to endDay
73%8%9%6%1
1517
February1st to 15thDay
68%10%8%7%3
1576
16th to endDay
62%8%13%11%3
1425
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%21%30%10%2
2349
16th to endDay
29%10%23%27%7%1
2244
September1st to 15thDay
27%10%21%23%15%1
1980
16th to endDay
47%11%15%16%8%
1810
October1st to 15thDay
55%10%14%13%4
1650
16th to endDay
62%11%15%8%2
1689
November1st to 15thDay
68%7%11%9%21
1408
16th to endDay
67%9%10%9%2
1440
December1st to 15thDay
66%10%9%9%4
1385
16th to endDay
68%9%10%7%2
1344
January1st to 15thNight
65%8%10%9%4
2686
16th to endNight
61%9%12%12%21
2703
February1st to 15thNight
62%9%12%10%4
2288
16th to endNight
57%7%13%16%4
2048
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
17%8%22%31%15%4
1500
16th to endNight
22%7%22%32%11%3
1605
September1st to 15thNight
20%6%20%27%21%4
1650
16th to endNight
41%8%15%21%10%1
1818
October1st to 15thNight
46%6%17%18%9%
1980
16th to endNight
57%7%16%12%4
2182
November1st to 15thNight
64%9%11%9%21
2450
16th to endNight
61%9%12%10%6%
2520
December1st to 15thNight
65%8%12%8%4
2574
16th to endNight
66%9%11%8%3
2880

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
20%6%16%25%31%
60
16th to endDay
41%322%16%15%
106
February1st to 15thDay
31%8%20%27%11%1
236
16th to endDay
23%10%21%23%15%4
213
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%29%
300
16th to endDay
8%17%25%41%14%
268
September1st to 15thDay
9%19%34%29%16%
255
16th to endDay
15%412%31%27%8%
255
October1st to 15thDay
25%415%27%23%3
255
16th to endDay
29%5%20%24%17%3
219
November1st to 15thDay
41%125%26%5%
60
16th to endDay
25%10%13%23%15%13%
60
December1st to 15thDay
40%6%28%20%5%
60
16th to endDay
31%15%23%21%7%
64
January1st to 15thNight
31%10%18%20%16%2
750
16th to endNight
27%6%18%22%20%3
778
February1st to 15thNight
27%9%17%24%17%3
704
16th to endNight
23%8%20%24%17%4
632
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%
450
16th to endNight
9%8%25%41%13%
555
September1st to 15thNight
8%6%26%35%22%
660
16th to endNight
17%211%25%30%12%
656
October1st to 15thNight
23%213%27%22%10%
660
16th to endNight
30%414%26%17%7%
704
November1st to 15thNight
32%7%21%26%11%1
700
16th to endNight
24%10%22%22%15%5%
720
December1st to 15thNight
28%8%20%27%13%1
729
16th to endNight
32%9%29%18%10%
800
Page generated at 2017-08-13 11:16:13. Version ac04

Copyright 2017 A. Danko


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