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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 2018-03-11.

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%
15510
16%
49071
10%
15475
44%
15475
21%
32927

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
20%
778
12%
2687
5%
776
29%
776
12%
2030
January16th to endNight
22%
832
14%
2704
6%
832
32%
832
14%
2015
February1st to 15thNight
19%
761
12%
2482
4%
761
29%
761
13%
1831
February16th to endNight
26%
683
15%
2183
7%
683
37%
683
17%
1548
March1st to 15thNight
24%
764
15%
2101
7%
764
31%
764
18%
1532
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%
720
22%
1806
11%
720
60%
720
38%
1131
September16th to endNight
56%
717
25%
2007
18%
717
60%
717
36%
1204
October1st to 15thNight
42%
720
18%
2160
15%
720
48%
720
24%
1452
October16th to endNight
30%
768
15%
2304
9%
768
39%
768
18%
1652
November1st to 15thNight
37%
760
19%
2584
16%
760
44%
760
20%
1696
November16th to endNight
28%
780
12%
2730
7%
780
36%
780
15%
1953
December1st to 15thNight
19%
780
9%
2730
4%
780
25%
780
11%
2077
December16th to endNight
16%
832
9%
2912
3%
832
24%
832
10%
2271

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%
49525
Night
15%8%36%5%3445%6%35%
49250
Cloud CoverDay
15%7%36%5%35%45%7%33%
49535
Night
15%8%36%5%3445%6%36%
49254
TransparencyDay
49%412%21%11%1
49223
Night
48%312%22%12%1
49071
SeeingDay
17%6%26%26%16%7%
5918
Night
19%6%27%31%13%1
15516

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%
1578
16th to endDay
13%14%7%8%9%47%45%6%17%
1872
February1st to 15thDay
14%15%6%10%8%35%44420%
1710
16th to endDay
17%13%5%8%6%24445%26%
1575
March1st to 15thDay
16%9%5%8%7%45%445%28%
2085
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
14%6%25%5%345%5%7%40%
2151
16th to endDay
13%5%36%5%345%47%40%
1953
October1st to 15thDay
20%7%36%6%34446%33%
1800
16th to endDay
23%9%37%5%35%5%5%5%23%
1920
November1st to 15thDay
15%10%48%5%35%46%7%29%
1602
16th to endDay
17%12%5%8%5%45%6%5%7%21%
1560
December1st to 15thDay
18%12%7%9%7%35%45%6%18%
1560
16th to endDay
20%12%6%9%6%4445%6%18%
1664
January1st to 15thNight
20%13%48%6%34445%22%
2687
16th to endNight
14%12%6%9%7%35%446%24%
2704
February1st to 15thNight
14%14%5%8%7%35%5%45%25%
2480
16th to endNight
14%11%48%6%3445%6%28%
2183
March1st to 15thNight
17%9%47%6%34435%32%
2089
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%32442345%7%47%
1806
16th to endNight
11%425%41445%7%48%
2007
October1st to 15thNight
18%6%26%43435%7%37%
2160
16th to endNight
19%8%45%6%3445%7%29%
2304
November1st to 15thNight
17%7%35%5%3445%6%34%
2584
16th to endNight
20%9%36%6%25%5%5%6%26%
2730
December1st to 15thNight
18%14%5%8%6%445%5%6%21%
2730
16th to endNight
20%12%6%8%8%45%5%5%5%18%
2912

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%10%5%8%7%35%445%25%
1578
16th to endDay
13%13%6%10%7%46%6%5%5%19%
1872
February1st to 15thDay
14%14%6%10%7%45%335%22%
1710
16th to endDay
18%11%5%9%6%34445%27%
1575
March1st to 15thDay
15%9%48%7%35%45%5%29%
2096
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
15%5%245%25%5%5%6%41%
2151
16th to endDay
13%5%26%435%35%7%41%
1953
October1st to 15thDay
20%6%26%6%34446%34%
1800
16th to endDay
23%10%36%6%25%45%5%24%
1920
November1st to 15thDay
15%9%47%6%3435%8%30%
1602
16th to endDay
17%12%5%8%5%25%5%5%8%23%
1560
December1st to 15thDay
18%13%6%9%6%35%5%46%19%
1560
16th to endDay
21%11%6%8%6%35%46%6%19%
1664
January1st to 15thNight
20%13%37%6%34446%23%
2687
16th to endNight
14%11%6%8%7%35%45%5%25%
2704
February1st to 15thNight
14%14%5%8%6%345%5%5%26%
2473
16th to endNight
14%10%48%7%3445%5%30%
2183
March1st to 15thNight
18%8%47%6%4434433%
2101
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
12%31343345%6%48%
1806
16th to endNight
12%41441345%7%48%
2007
October1st to 15thNight
19%6%26%5%3345%6%37%
2160
16th to endNight
20%8%36%6%35%45%6%30%
2304
November1st to 15thNight
18%6%25%425%45%7%35%
2584
16th to endNight
20%9%36%435%5%6%6%27%
2730
December1st to 15thNight
19%13%5%7%5%35%45%6%22%
2730
16th to endNight
20%12%5%9%7%45%5%46%18%
2912

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%14%8%
1579
16th to endDay
65%39%13%6%
1872
February1st to 15thDay
65%28%16%5%
1712
16th to endDay
58%311%18%7%1
1575
March1st to 15thDay
56%311%17%9%
2096
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%11%1
2151
16th to endDay
42%411%30%10%
1953
October1st to 15thDay
51%311%22%10%1
1800
16th to endDay
59%415%15%5%
1920
November1st to 15thDay
51%414%20%7%1
1602
16th to endDay
58%415%15%5%
1560
December1st to 15thDay
64%413%14%3
1560
16th to endDay
64%311%15%4
1664
January1st to 15thNight
62%210%14%8%1
2687
16th to endNight
59%310%18%6%
2704
February1st to 15thNight
59%39%20%7%
2482
16th to endNight
54%212%21%8%
2183
March1st to 15thNight
55%210%21%8%
2101
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
34%313%30%16%1
1806
16th to endNight
34%311%35%14%
2007
October1st to 15thNight
46%49%25%13%
2160
16th to endNight
53%313%19%9%
2304
November1st to 15thNight
48%212%22%12%1
2584
16th to endNight
55%415%18%5%
2730
December1st to 15thNight
62%312%16%5%
2730
16th to endNight
65%211%13%5%
2912

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%38%21%7%
96
16th to endDay
27%11%36%19%5%
144
February1st to 15thDay
20%14%46%17%1
135
16th to endDay
24%14%40%19%1
133
March1st to 15thDay
20%10%34%24%9%
312
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
14%117%25%23%18%
315
16th to endDay
14%415%25%22%16%
314
October1st to 15thDay
20%325%26%16%6%
315
16th to endDay
23%10%30%26%8%
336
November1st to 15thDay
22%8%23%29%14%2
131
16th to endDay
20%12%38%23%5%
90
December1st to 15thDay
16%16%34%22%8%1
90
16th to endDay
23%16%40%17%1
96
January1st to 15thNight
28%10%32%22%6%
780
16th to endNight
19%6%35%30%6%
832
February1st to 15thNight
20%9%37%26%4
764
16th to endNight
18%9%34%30%6%
683
March1st to 15thNight
23%9%32%27%6%
764
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
14%219%30%26%5%
720
16th to endNight
13%316%31%28%5%
717
October1st to 15thNight
21%422%31%17%2
720
16th to endNight
23%6%28%33%8%
768
November1st to 15thNight
22%6%26%29%13%1
760
16th to endNight
26%7%27%27%9%1
780
December1st to 15thNight
26%9%34%23%5%
780
16th to endNight
27%7%36%24%3
832
Page generated at 2018-03-12 13:28:11. Version ac04

Copyright 2018 A. Danko


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