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David Dunlap Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the David Dunlap Observatory clear sky chart from 2005-11-05 to 2017-09-10.

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
37%
14782
16%
46735
10%
14747
44%
14747
21%
31362

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
18%
718
11%
2479
5%
716
28%
716
11%
1888
January16th to endNight
22%
768
15%
2496
6%
768
33%
768
14%
1868
February1st to 15thNight
20%
701
12%
2287
4%
701
29%
701
13%
1686
February16th to endNight
27%
631
15%
2017
7%
631
37%
631
17%
1426
March1st to 15thNight
24%
720
15%
1980
7%
720
31%
720
17%
1454
March16th to endNight
32%
768
21%
2112
11%
768
41%
768
26%
1407
April1st to 15thNight
28%
718
16%
1817
5%
718
34%
718
23%
1297
April16th to endNight
39%
564
21%
1644
9%
564
48%
564
29%
1048
May1st to 15thNight
47%
486
18%
1566
12%
486
55%
486
25%
997
May16th to endNight
49%
374
18%
1497
17%
374
58%
374
28%
792
June1st to 15thNight
45%
360
14%
1440
13%
360
54%
360
22%
817
June16th to endNight
54%
360
14%
1356
18%
360
60%
360
28%
764
July1st to 15thNight
63%
340
17%
1354
24%
340
69%
340
37%
661
July16th to endNight
63%
482
17%
1516
17%
452
65%
452
34%
839
August1st to 15thNight
69%
539
19%
1609
15%
536
72%
536
40%
898
August16th to endNight
64%
624
25%
1770
16%
624
69%
624
39%
1015
September1st to 15thNight
55%
700
22%
1755
11%
700
59%
700
37%
1105
September16th to endNight
55%
657
23%
1840
15%
657
59%
657
35%
1118
October1st to 15thNight
42%
660
19%
1980
16%
660
48%
660
24%
1333
October16th to endNight
29%
704
13%
2112
7%
704
37%
704
18%
1535
November1st to 15thNight
38%
700
20%
2380
17%
700
45%
700
21%
1545
November16th to endNight
29%
720
12%
2520
8%
720
35%
720
14%
1817
December1st to 15thNight
19%
720
9%
2520
5%
720
25%
720
11%
1928
December16th to endNight
17%
768
8%
2688
3%
768
23%
768
10%
2124

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
15%8%37%6%35%5%5%7%32%
47860
Night
15%8%36%5%3445%6%35%
46914
Cloud CoverDay
15%7%36%5%35%45%7%33%
47870
Night
15%8%36%5%3445%6%36%
46918
TransparencyDay
49%412%21%10%1
47558
Night
48%312%23%11%1
46735
SeeingDay
17%5%25%27%16%7%
5378
Night
19%6%27%31%13%1
14788

By Month

Percentage Cloud Cover NE (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
17%12%410%7%45%45%6%22%
1456
16th to endDay
12%14%7%8%9%47%45%6%17%
1728
February1st to 15thDay
14%15%6%10%8%35%44420%
1575
16th to endDay
16%13%5%8%6%24445%26%
1455
March1st to 15thDay
16%8%5%8%7%45%445%28%
1969
16th to endDay
16%9%36%6%3445%435%
2112
April1st to 15thDay
20%8%46%43345%6%30%
2131
16th to endDay
18%7%25%434445%38%
2327
May1st to 15thDay
15%6%27%5%25%5%5%7%35%
2385
16th to endDay
12%5%25%5%35%45%8%40%
2618
June1st to 15thDay
16%5%25%425%5%6%9%35%
2520
16th to endDay
14%4245%35%6%7%9%35%
2578
July1st to 15thDay
8%425%5%37%6%6%8%40%
2520
16th to endDay
10%5%26%6%35%5%7%9%34%
2546
August1st to 15thDay
8%5%26%6%35%5%7%9%37%
2340
16th to endDay
11%5%25%5%36%5%6%9%38%
2418
September1st to 15thDay
15%6%25%5%345%5%6%39%
2090
16th to endDay
14%5%36%5%34447%38%
1790
October1st to 15thDay
19%7%26%5%34446%33%
1650
16th to endDay
24%10%47%5%35%5%5%5%22%
1760
November1st to 15thDay
14%10%47%435%46%8%30%
1476
16th to endDay
17%12%6%8%5%45%5%5%6%21%
1440
December1st to 15thDay
19%12%7%10%7%35%45%6%16%
1440
16th to endDay
20%12%7%9%6%4445%6%17%
1536
January1st to 15thNight
20%13%48%6%35%445%21%
2479
16th to endNight
14%12%6%9%7%45%446%24%
2496
February1st to 15thNight
14%14%5%8%6%35%5%45%25%
2285
16th to endNight
14%11%48%6%4445%6%28%
2017
March1st to 15thNight
17%9%48%6%34435%32%
1968
16th to endNight
14%7%36%423335%43%
2112
April1st to 15thNight
19%7%36%5%35%5%45%35%
1817
16th to endNight
12%6%36%42435%6%44%
1644
May1st to 15thNight
15%435%4346%5%8%39%
1566
16th to endNight
12%42332445%7%49%
1495
June1st to 15thNight
13%6%245%3446%9%40%
1440
16th to endNight
10%425%5%3446%8%44%
1356
July1st to 15thNight
5%3133245%5%7%56%
1434
16th to endNight
7%324435%5%6%8%48%
1620
August1st to 15thNight
7%31342446%8%50%
1620
16th to endNight
8%42343445%6%53%
1770
September1st to 15thNight
12%42442345%7%47%
1755
16th to endNight
12%425%31445%7%47%
1840
October1st to 15thNight
18%7%26%434347%37%
1980
16th to endNight
20%8%46%6%3445%7%28%
2112
November1st to 15thNight
17%7%35%5%35%45%6%36%
2380
16th to endNight
21%10%46%5%25%5%5%6%25%
2520
December1st to 15thNight
19%14%5%7%6%345%5%6%21%
2520
16th to endNight
21%12%6%9%8%45%5%5%5%16%
2688

Percentage Cloud Cover SE (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
18%11%5%8%7%35%45%5%25%
1456
16th to endDay
13%13%7%11%8%46%6%5%5%19%
1728
February1st to 15thDay
14%14%6%11%7%45%345%22%
1575
16th to endDay
17%12%5%8%6%34345%27%
1455
March1st to 15thDay
16%9%38%7%35%45%5%29%
1980
16th to endDay
16%8%36%5%3345%5%36%
2111
April1st to 15thDay
21%8%36%42445%6%30%
2131
16th to endDay
18%7%25%434445%39%
2327
May1st to 15thDay
15%6%36%6%25%46%6%35%
2385
16th to endDay
12%5%25%5%345%6%7%40%
2618
June1st to 15thDay
16%6%245%25%5%6%8%36%
2520
16th to endDay
13%415%5%36%5%7%10%36%
2578
July1st to 15thDay
8%425%5%36%5%7%8%40%
2520
16th to endDay
10%426%6%35%5%7%8%37%
2546
August1st to 15thDay
8%5%26%6%45%46%10%39%
2340
16th to endDay
10%5%345%345%6%9%40%
2418
September1st to 15thDay
16%5%245%245%5%6%40%
2090
16th to endDay
14%5%37%5%35%347%39%
1790
October1st to 15thDay
19%6%26%5%34446%34%
1650
16th to endDay
24%10%36%6%35%45%5%23%
1760
November1st to 15thDay
14%8%47%5%35%35%8%31%
1476
16th to endDay
17%13%5%8%5%25%5%5%7%23%
1440
December1st to 15thDay
19%13%6%9%6%345%5%6%18%
1440
16th to endDay
21%11%6%8%6%35%5%6%6%18%
1536
January1st to 15thNight
20%13%37%6%34446%22%
2479
16th to endNight
14%11%6%8%7%35%45%5%25%
2496
February1st to 15thNight
14%14%5%7%6%345%5%5%26%
2278
16th to endNight
14%10%48%7%3445%5%30%
2017
March1st to 15thNight
19%8%48%6%4434432%
1980
16th to endNight
15%8%36%423335%43%
2112
April1st to 15thNight
19%7%35%5%34446%35%
1817
16th to endNight
12%6%25%5%23347%44%
1644
May1st to 15thNight
15%425%4345%5%8%38%
1565
16th to endNight
12%32442346%7%49%
1495
June1st to 15thNight
13%5%25%435%35%9%41%
1440
16th to endNight
11%416%3345%5%8%43%
1356
July1st to 15thNight
5%3143245%5%8%55%
1434
16th to endNight
8%32443447%8%49%
1620
August1st to 15thNight
6%31442446%8%51%
1620
16th to endNight
8%41443445%6%53%
1770
September1st to 15thNight
13%41343345%7%47%
1755
16th to endNight
13%41441345%7%47%
1840
October1st to 15thNight
18%6%26%43345%6%37%
1980
16th to endNight
21%8%36%6%35%45%6%28%
2112
November1st to 15thNight
17%6%25%435%45%6%37%
2380
16th to endNight
21%9%46%435%5%6%6%26%
2520
December1st to 15thNight
20%13%5%7%5%35%446%22%
2520
16th to endNight
21%12%5%9%7%45%6%46%17%
2688

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
61%312%15%7%
1457
16th to endDay
66%38%13%6%
1728
February1st to 15thDay
65%29%16%5%
1577
16th to endDay
58%211%18%7%1
1455
March1st to 15thDay
56%312%18%8%
1980
16th to endDay
49%39%20%15%1
2112
April1st to 15thDay
52%310%20%12%
2131
16th to endDay
47%210%22%15%2
2327
May1st to 15thDay
46%410%23%14%
2385
16th to endDay
40%414%23%14%1
2622
June1st to 15thDay
43%412%25%13%
2520
16th to endDay
41%5%14%23%14%
2578
July1st to 15thDay
36%5%11%27%17%2
2371
16th to endDay
43%5%13%23%12%1
2388
August1st to 15thDay
39%415%23%14%1
2327
16th to endDay
39%413%30%11%1
2418
September1st to 15thDay
41%5%14%26%10%
2090
16th to endDay
44%312%29%9%
1790
October1st to 15thDay
50%311%22%10%1
1650
16th to endDay
60%415%14%4
1760
November1st to 15thDay
50%414%20%8%1
1476
16th to endDay
59%414%15%5%
1440
December1st to 15thDay
65%414%13%2
1440
16th to endDay
65%412%15%3
1536
January1st to 15thNight
63%210%14%7%1
2479
16th to endNight
60%310%19%6%1
2496
February1st to 15thNight
58%39%20%7%
2287
16th to endNight
54%212%21%8%
2017
March1st to 15thNight
55%211%21%7%
1980
16th to endNight
44%28%24%16%2
2112
April1st to 15thNight
49%310%20%15%1
1817
16th to endNight
41%211%23%17%3
1644
May1st to 15thNight
42%410%24%16%2
1566
16th to endNight
34%314%27%16%3
1497
June1st to 15thNight
39%311%27%16%1
1440
16th to endNight
37%312%26%18%1
1356
July1st to 15thNight
25%411%30%24%2
1354
16th to endNight
31%415%28%17%3
1516
August1st to 15thNight
28%419%31%14%1
1609
16th to endNight
30%311%35%16%1
1770
September1st to 15thNight
35%314%30%15%
1755
16th to endNight
35%312%34%13%
1840
October1st to 15thNight
46%410%25%13%
1980
16th to endNight
55%313%19%8%
2112
November1st to 15thNight
47%212%22%13%1
2380
16th to endNight
56%315%17%5%
2520
December1st to 15thNight
63%312%16%4
2520
16th to endNight
67%312%13%4
2688

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
17%14%40%18%9%
64
16th to endDay
29%11%33%20%5%
96
February1st to 15thDay
17%16%46%16%2
90
16th to endDay
21%12%38%24%2
93
March1st to 15thDay
22%10%34%22%10%
270
16th to endDay
21%6%32%26%9%2
288
April1st to 15thDay
23%8%30%26%8%1
270
16th to endDay
21%7%29%23%14%4
270
May1st to 15thDay
20%424%26%13%10%
288
16th to endDay
14%430%27%17%6%
331
June1st to 15thDay
20%5%20%31%20%2
315
16th to endDay
15%324%33%17%4
373
July1st to 15thDay
9%218%31%27%9%
305
16th to endDay
9%319%34%21%11%
326
August1st to 15thDay
9%117%30%24%16%
313
16th to endDay
13%213%25%27%17%
332
September1st to 15thDay
15%119%26%21%15%
294
16th to endDay
17%316%29%20%12%
254
October1st to 15thDay
17%425%27%17%7%
255
16th to endDay
25%9%30%26%7%
272
November1st to 15thDay
15%9%22%32%16%3
95
16th to endDay
25%10%33%25%6%
60
December1st to 15thDay
18%16%38%16%8%1
60
16th to endDay
23%18%43%12%1
64
January1st to 15thNight
28%11%32%21%5%
720
16th to endNight
19%7%34%30%7%
768
February1st to 15thNight
20%10%36%27%5%
704
16th to endNight
18%10%33%30%7%
631
March1st to 15thNight
23%9%32%27%6%
720
16th to endNight
19%8%31%27%11%1
768
April1st to 15thNight
22%9%33%28%6%
718
16th to endNight
15%7%31%33%11%1
564
May1st to 15thNight
17%224%34%18%1
486
16th to endNight
12%326%36%20%1
375
June1st to 15thNight
15%5%25%35%17%
360
16th to endNight
11%321%43%18%1
360
July1st to 15thNight
6%19%48%23%2
340
16th to endNight
8%118%46%23%3
482
August1st to 15thNight
7%11%46%31%2
539
16th to endNight
10%114%37%30%4
624
September1st to 15thNight
15%220%31%25%4
700
16th to endNight
14%317%33%27%4
657
October1st to 15thNight
21%422%31%16%2
660
16th to endNight
24%6%28%32%8%
704
November1st to 15thNight
20%5%28%29%13%1
700
16th to endNight
26%8%26%27%9%1
720
December1st to 15thNight
27%9%34%23%5%
720
16th to endNight
27%6%36%24%4
768
Page generated at 2017-09-11 19:35:22. Version ac04

Copyright 2017 A. Danko


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