Dordt College Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dordt College Observatory clear sky chart from 2005-11-05 to 2017-09-24.

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
39%
16086
19%
46634
11%
16039
46%
16039
29%
29486

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
32%
900
16%
2377
8%
895
39%
895
24%
1649
January16th to endNight
26%
960
18%
2496
6%
960
35%
960
21%
1802
February1st to 15thNight
26%
880
12%
2288
4%
876
31%
876
22%
1653
February16th to endNight
31%
705
20%
1967
7%
705
40%
705
27%
1293
March1st to 15thNight
34%
573
19%
2013
13%
573
43%
573
24%
1231
March16th to endNight
29%
576
15%
2112
10%
576
38%
576
17%
1420
April1st to 15thNight
30%
539
17%
1831
11%
539
41%
539
23%
1153
April16th to endNight
29%
540
18%
1620
12%
540
39%
540
22%
1109
May1st to 15thNight
37%
540
18%
1620
12%
540
45%
540
29%
1005
May16th to endNight
42%
563
16%
1546
7%
561
47%
561
29%
1001
June1st to 15thNight
50%
540
16%
1391
6%
540
53%
540
35%
871
June16th to endNight
61%
540
22%
1260
7%
540
65%
540
53%
734
July1st to 15thNight
65%
510
20%
1257
7%
510
68%
510
51%
702
July16th to endNight
73%
564
23%
1594
18%
531
74%
531
52%
788
August1st to 15thNight
67%
539
23%
1610
22%
536
70%
536
42%
878
August16th to endNight
61%
573
24%
1729
25%
573
67%
573
39%
956
September1st to 15thNight
57%
540
24%
1887
22%
540
67%
540
37%
974
September16th to endNight
45%
519
28%
1914
21%
519
58%
519
36%
964
October1st to 15thNight
36%
629
27%
1949
17%
629
46%
629
32%
1119
October16th to endNight
39%
721
21%
2129
12%
721
46%
721
29%
1325
November1st to 15thNight
37%
875
24%
2316
10%
875
47%
875
34%
1423
November16th to endNight
31%
900
20%
2520
9%
900
39%
900
28%
1625
December1st to 15thNight
26%
900
13%
2520
5%
900
33%
900
20%
1801
December16th to endNight
26%
960
15%
2688
7%
960
34%
960
17%
2010

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
13%6%35%5%2445%7%41%
48224
Night
10%6%35%424446%46%
46810
Cloud CoverDay
13%6%35%5%2445%7%41%
48234
Night
10%6%35%424446%46%
46813
Cloud CoverDay
12%7%36%5%2445%7%41%
48235
Night
10%6%35%424446%46%
46814
Cloud CoverDay
13%6%35%5%24447%41%
48233
Night
10%6%35%424446%46%
46814
TransparencyDay
42%6%13%24%11%1
47918
Night
38%5%13%25%14%2
46634
SeeingDay
14%7%29%30%15%2
6088
Night
14%7%33%33%10%
16086

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
7%10%6%8%8%46%35%8%31%
1504
16th to endDay
9%13%7%10%7%34446%27%
1728
February1st to 15thDay
12%10%5%10%6%35%346%29%
1575
16th to endDay
11%9%36%6%35%45%7%35%
1515
March1st to 15thDay
13%10%5%7%5%23436%37%
1936
16th to endDay
18%11%36%5%2445%6%31%
2112
April1st to 15thDay
16%6%36%5%24446%39%
2115
16th to endDay
25%8%25%423346%31%
2340
May1st to 15thDay
17%6%345%245%47%37%
2331
16th to endDay
17%6%26%43445%7%37%
2567
June1st to 15thDay
15%5%25%425%45%7%40%
2565
16th to endDay
7%4134245%6%8%50%
2700
July1st to 15thDay
7%22342446%9%50%
2622
16th to endDay
5%2134245%5%8%55%
2466
August1st to 15thDay
9%5%24435%5%6%9%43%
2340
16th to endDay
10%425%324347%50%
2472
September1st to 15thDay
12%5%14323446%49%
2070
16th to endDay
10%313323445%56%
1904
October1st to 15thDay
14%5%33322346%49%
1669
16th to endDay
17%6%15%322435%48%
1741
November1st to 15thDay
9%6%25%43435%6%49%
1541
16th to endDay
14%8%36%4245%47%37%
1440
December1st to 15thDay
14%8%5%10%6%35%445%31%
1440
16th to endDay
15%11%5%9%8%45%436%25%
1531
January1st to 15thNight
7%7%46%7%36%5%5%7%37%
2377
16th to endNight
9%11%5%7%6%345%5%7%34%
2496
February1st to 15thNight
11%11%5%7%6%3444436%
2285
16th to endNight
8%7%36%5%34446%44%
1967
March1st to 15thNight
12%6%3442445%6%44%
2001
16th to endNight
14%10%45%43335%6%36%
2112
April1st to 15thNight
14%6%24435%446%42%
1831
16th to endNight
18%9%35%323435%39%
1620
May1st to 15thNight
13%5%24422437%48%
1620
16th to endNight
13%5%144245%47%45%
1544
June1st to 15thNight
9%5%1432346%9%47%
1391
16th to endNight
42231446%8%60%
1260
July1st to 15thNight
321322345%8%62%
1332
16th to endNight
311221346%11%61%
1702
August1st to 15thNight
4313323449%57%
1620
16th to endNight
7%21342445%7%56%
1729
September1st to 15thNight
9%313223246%58%
1887
16th to endNight
8%42231435%5%58%
1914
October1st to 15thNight
13%424312345%53%
1949
16th to endNight
15%6%24323335%49%
2129
November1st to 15thNight
9%5%24433346%51%
2316
16th to endNight
11%6%35%5%24446%46%
2520
December1st to 15thNight
11%10%5%7%5%34445%35%
2520
16th to endNight
16%10%48%7%34435%30%
2688

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
7%10%5%8%8%45%45%7%32%
1505
16th to endDay
9%12%7%10%8%34446%27%
1728
February1st to 15thDay
13%10%5%10%6%36%346%29%
1575
16th to endDay
11%9%36%6%4445%7%36%
1515
March1st to 15thDay
14%9%5%6%5%23446%38%
1947
16th to endDay
18%10%46%43445%6%31%
2110
April1st to 15thDay
15%7%35%5%24446%39%
2115
16th to endDay
25%8%25%424336%32%
2340
May1st to 15thDay
17%7%25%43445%7%37%
2331
16th to endDay
17%6%26%435%447%37%
2567
June1st to 15thDay
16%5%2442445%8%40%
2565
16th to endDay
7%324425%5%6%9%50%
2700
July1st to 15thDay
7%32342446%9%50%
2622
16th to endDay
5%21342446%8%55%
2466
August1st to 15thDay
9%5%245%35%5%6%9%43%
2340
16th to endDay
10%425%42435%6%51%
2472
September1st to 15thDay
13%413423446%49%
2070
16th to endDay
10%313323436%57%
1904
October1st to 15thDay
14%6%2342334450%
1669
16th to endDay
17%6%15%322345%48%
1741
November1st to 15thDay
9%6%24434446%49%
1541
16th to endDay
13%8%36%435%347%38%
1440
December1st to 15thDay
13%9%5%11%6%24445%31%
1440
16th to endDay
14%11%6%9%8%45%436%25%
1531
January1st to 15thNight
7%8%46%7%35%45%8%38%
2377
16th to endNight
9%10%5%7%6%345%5%6%34%
2496
February1st to 15thNight
12%11%5%7%5%3434436%
2278
16th to endNight
8%7%46%5%24446%44%
1967
March1st to 15thNight
12%7%25%43345%6%44%
2013
16th to endNight
15%9%45%42335%6%38%
2110
April1st to 15thNight
14%7%24434346%42%
1831
16th to endNight
18%8%26%424435%39%
1620
May1st to 15thNight
13%5%24323346%48%
1619
16th to endNight
12%5%25%424447%46%
1545
June1st to 15thNight
10%42332445%10%47%
1391
16th to endNight
41231446%8%61%
1260
July1st to 15thNight
3313313447%62%
1332
16th to endNight
31221346%9%62%
1702
August1st to 15thNight
5%32332345%8%57%
1620
16th to endNight
7%2133235%47%56%
1729
September1st to 15thNight
9%313222346%59%
1887
16th to endNight
8%412323345%59%
1914
October1st to 15thNight
12%424322446%53%
1949
16th to endNight
15%6%24313336%49%
2129
November1st to 15thNight
8%5%25%423346%51%
2316
16th to endNight
10%6%25%424447%46%
2520
December1st to 15thNight
12%10%5%7%6%34446%35%
2520
16th to endNight
15%10%48%7%34445%31%
2688

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
7%10%5%8%8%45%348%31%
1504
16th to endDay
9%13%6%11%7%34446%27%
1728
February1st to 15thDay
12%10%5%10%5%46%446%29%
1575
16th to endDay
11%9%46%6%3445%7%35%
1515
March1st to 15thDay
13%10%47%5%23436%37%
1947
16th to endDay
18%11%36%5%3445%5%31%
2112
April1st to 15thDay
15%6%36%5%25%446%39%
2115
16th to endDay
25%9%25%423346%31%
2340
May1st to 15thDay
17%6%35%5%25%45%7%37%
2331
16th to endDay
17%6%36%43445%6%37%
2567
June1st to 15thDay
15%5%35%42445%7%40%
2565
16th to endDay
6%4144235%6%8%50%
2700
July1st to 15thDay
6%31443446%9%50%
2622
16th to endDay
5%21342446%8%55%
2466
August1st to 15thDay
9%5%24435%5%6%9%42%
2340
16th to endDay
10%425%423446%50%
2472
September1st to 15thDay
12%5%1432345%5%50%
2070
16th to endDay
11%313323345%57%
1904
October1st to 15thDay
13%6%24322436%49%
1669
16th to endDay
17%6%15%312445%47%
1741
November1st to 15thDay
9%6%25%43445%6%49%
1541
16th to endDay
13%8%36%5%25%447%37%
1440
December1st to 15thDay
13%9%5%10%6%34435%31%
1440
16th to endDay
14%11%5%10%8%45%436%25%
1531
January1st to 15thNight
7%7%46%7%35%5%5%7%37%
2377
16th to endNight
9%11%47%6%345%5%7%34%
2496
February1st to 15thNight
11%11%5%7%6%3444436%
2278
16th to endNight
8%7%36%5%34446%44%
1967
March1st to 15thNight
12%6%35%42345%6%44%
2013
16th to endNight
14%10%45%423446%36%
2112
April1st to 15thNight
14%6%24425%45%5%42%
1831
16th to endNight
18%9%35%423435%39%
1620
May1st to 15thNight
13%5%25%413437%47%
1619
16th to endNight
13%5%25%4246%37%45%
1544
June1st to 15thNight
9%5%1432346%9%47%
1391
16th to endNight
411231446%8%61%
1260
July1st to 15thNight
321322345%8%62%
1332
16th to endNight
31221346%10%62%
1702
August1st to 15thNight
5%313313449%56%
1620
16th to endNight
7%213423447%55%
1729
September1st to 15thNight
10%313223246%58%
1887
16th to endNight
8%412314345%58%
1914
October1st to 15thNight
12%5%24413345%53%
1949
16th to endNight
15%6%33323236%48%
2129
November1st to 15thNight
9%5%24433446%51%
2316
16th to endNight
10%6%35%424446%46%
2520
December1st to 15thNight
12%10%5%7%5%34445%35%
2520
16th to endNight
15%10%48%7%34435%30%
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
7%10%5%8%8%45%35%8%31%
1504
16th to endDay
10%13%7%10%7%34446%27%
1728
February1st to 15thDay
12%11%5%10%6%36%446%29%
1575
16th to endDay
11%9%45%6%35%45%7%35%
1515
March1st to 15thDay
14%10%5%7%5%23437%37%
1947
16th to endDay
17%11%36%5%3445%6%31%
2110
April1st to 15thDay
15%6%36%5%24446%39%
2115
16th to endDay
25%8%25%423346%31%
2340
May1st to 15thDay
17%6%245%24447%37%
2331
16th to endDay
17%6%26%424447%37%
2567
June1st to 15thDay
15%5%25%42445%6%40%
2565
16th to endDay
7%4144245%5%8%51%
2700
July1st to 15thDay
7%223425%46%10%50%
2622
16th to endDay
5%2134235%5%8%55%
2466
August1st to 15thDay
9%5%24435%5%6%8%43%
2340
16th to endDay
10%425%324446%50%
2472
September1st to 15thDay
13%5%14333446%49%
2070
16th to endDay
10%323323445%57%
1904
October1st to 15thDay
14%6%33322345%49%
1669
16th to endDay
17%6%15%312435%48%
1741
November1st to 15thDay
9%6%15%43435%6%49%
1541
16th to endDay
14%8%37%434447%37%
1440
December1st to 15thDay
14%8%5%10%6%35%445%31%
1440
16th to endDay
15%11%6%9%8%5%4436%25%
1531
January1st to 15thNight
7%7%46%7%36%5%5%7%37%
2377
16th to endNight
10%10%47%6%345%5%7%34%
2496
February1st to 15thNight
11%11%5%7%6%3344436%
2278
16th to endNight
8%7%36%5%34446%44%
1967
March1st to 15thNight
12%6%35%43345%6%44%
2013
16th to endNight
14%9%45%424447%36%
2112
April1st to 15thNight
14%6%24435%45%6%42%
1831
16th to endNight
19%8%25%423335%39%
1620
May1st to 15thNight
13%5%24423437%47%
1619
16th to endNight
13%5%144245%46%45%
1544
June1st to 15thNight
10%5%1432346%9%47%
1391
16th to endNight
411231436%8%60%
1260
July1st to 15thNight
421331335%8%63%
1332
16th to endNight
311221446%11%62%
1702
August1st to 15thNight
5%313323449%56%
1620
16th to endNight
7%21343345%7%56%
1729
September1st to 15thNight
9%313323346%58%
1887
16th to endNight
9%32232435%5%58%
1914
October1st to 15thNight
13%424412345%53%
1949
16th to endNight
16%6%23323335%49%
2129
November1st to 15thNight
9%5%34423346%51%
2316
16th to endNight
11%6%26%424446%46%
2520
December1st to 15thNight
11%11%47%6%35%345%35%
2520
16th to endNight
15%10%48%7%34445%30%
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
51%416%19%8%
1505
16th to endDay
57%5%12%18%5%
1728
February1st to 15thDay
55%5%14%18%6%
1576
16th to endDay
47%7%13%20%10%
1515
March1st to 15thDay
47%5%15%21%8%
1947
16th to endDay
53%6%14%19%5%
2112
April1st to 15thDay
45%5%12%25%9%1
2115
16th to endDay
54%5%10%19%9%
2340
May1st to 15thDay
45%6%12%23%11%
2331
16th to endDay
45%6%12%23%11%
2571
June1st to 15thDay
42%6%15%22%11%1
2565
16th to endDay
28%8%16%29%14%3
2700
July1st to 15thDay
29%7%16%28%13%3
2468
16th to endDay
25%6%15%30%17%4
2311
August1st to 15thDay
34%8%15%27%12%1
2326
16th to endDay
33%5%12%29%15%4
2472
September1st to 15thDay
34%6%13%27%17%1
2070
16th to endDay
29%5%12%35%17%
1904
October1st to 15thDay
36%411%27%18%2
1669
16th to endDay
38%5%13%27%13%1
1741
November1st to 15thDay
34%5%15%24%19%1
1541
16th to endDay
45%5%16%21%10%
1440
December1st to 15thDay
55%5%13%16%7%1
1440
16th to endDay
61%411%15%6%1
1531
January1st to 15thNight
44%6%13%23%11%
2377
16th to endNight
48%6%15%21%7%
2496
February1st to 15thNight
51%412%23%7%1
2288
16th to endNight
39%6%13%25%11%2
1967
March1st to 15thNight
39%5%14%27%11%1
2013
16th to endNight
48%6%11%24%8%1
2112
April1st to 15thNight
41%5%11%25%14%
1831
16th to endNight
47%412%20%12%2
1620
May1st to 15thNight
36%5%13%25%16%1
1620
16th to endNight
37%7%14%25%14%1
1546
June1st to 15thNight
32%7%17%25%15%2
1391
16th to endNight
19%6%16%30%22%3
1260
July1st to 15thNight
20%6%15%31%19%6%
1257
16th to endNight
15%7%16%32%21%5%
1594
August1st to 15thNight
24%6%14%31%19%2
1610
16th to endNight
27%5%14%27%19%5%
1729
September1st to 15thNight
26%412%30%21%4
1887
16th to endNight
26%411%34%21%1
1914
October1st to 15thNight
33%411%26%21%3
1949
16th to endNight
38%313%26%15%1
2129
November1st to 15thNight
33%413%23%23%2
2316
16th to endNight
38%5%16%24%13%
2520
December1st to 15thNight
50%5%15%22%6%
2520
16th to endNight
55%5%11%17%9%
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
11%10%38%27%12%
90
16th to endDay
26%5%37%29%2
96
February1st to 15thDay
17%11%47%18%4
90
16th to endDay
20%11%32%23%12%
81
March1st to 15thDay
16%14%38%26%3
90
16th to endDay
30%8%31%20%8%1
96
April1st to 15thDay
15%14%32%28%8%
234
16th to endDay
32%11%28%17%8%1
450
May1st to 15thDay
20%11%30%26%9%2
468
16th to endDay
20%7%32%25%10%2
517
June1st to 15thDay
13%6%28%31%16%3
495
16th to endDay
8%5%33%33%15%3
495
July1st to 15thDay
7%324%38%21%4
475
16th to endDay
5%124%42%22%3
515
August1st to 15thDay
6%322%38%25%3
493
16th to endDay
11%323%31%23%6%
512
September1st to 15thDay
12%427%32%17%5%
225
16th to endDay
11%7%34%27%15%4
114
October1st to 15thDay
5%11%35%32%10%5%
90
16th to endDay
17%6%41%23%10%
96
November1st to 15thDay
5%8%22%47%11%4
90
16th to endDay
37%6%32%16%6%
90
December1st to 15thDay
26%17%32%15%6%1
90
16th to endDay
27%19%36%14%2
96
January1st to 15thNight
12%10%37%30%7%1
900
16th to endNight
15%11%37%30%5%
960
February1st to 15thNight
16%10%38%27%5%1
880
16th to endNight
12%8%41%26%9%1
705
March1st to 15thNight
14%7%36%31%8%
573
16th to endNight
20%9%34%29%6%
576
April1st to 15thNight
17%11%36%28%4
539
16th to endNight
22%11%32%25%7%1
540
May1st to 15thNight
14%9%34%30%10%
540
16th to endNight
14%6%32%36%9%
563
June1st to 15thNight
11%5%26%41%14%
540
16th to endNight
5%225%50%15%
540
July1st to 15thNight
4123%45%24%
510
16th to endNight
3118%54%21%
564
August1st to 15thNight
5%119%48%22%1
539
16th to endNight
8%222%39%22%3
573
September1st to 15thNight
11%225%39%18%2
540
16th to endNight
11%5%34%31%15%1
519
October1st to 15thNight
16%8%34%29%10%1
629
16th to endNight
19%8%29%32%11%
721
November1st to 15thNight
11%10%37%31%8%
875
16th to endNight
14%10%38%27%8%
900
December1st to 15thNight
18%9%38%29%4
900
16th to endNight
21%8%33%28%6%1
960
Page generated at 2017-09-25 08:02:21. Version ac04

Copyright 2017 A. Danko


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