Dunham Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dunham Hill Observatory clear sky chart from 2005-11-05 to 2017-12-02.

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%
16258
15%
47737
10%
16217
42%
16217
20%
33671

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
17%
900
7%
2506
3%
895
21%
895
10%
2053
January16th to endNight
20%
939
7%
2667
3%
939
23%
939
10%
2206
February1st to 15thNight
18%
701
9%
2214
3%
701
24%
701
10%
1742
February16th to endNight
22%
631
13%
1893
5%
631
29%
631
14%
1450
March1st to 15thNight
30%
720
15%
2118
9%
720
34%
720
19%
1502
March16th to endNight
33%
756
18%
2100
10%
756
40%
756
22%
1459
April1st to 15thNight
29%
539
12%
1797
8%
539
35%
539
18%
1250
April16th to endNight
34%
540
19%
1623
10%
540
43%
540
25%
1060
May1st to 15thNight
34%
540
13%
1620
7%
540
40%
540
22%
1128
May16th to endNight
48%
561
16%
1594
11%
561
52%
561
29%
1024
June1st to 15thNight
52%
540
15%
1440
10%
540
56%
540
33%
917
June16th to endNight
56%
540
15%
1440
10%
540
59%
540
33%
918
July1st to 15thNight
67%
510
17%
1360
14%
510
69%
510
42%
799
July16th to endNight
68%
564
20%
1434
16%
531
69%
531
40%
860
August1st to 15thNight
67%
539
21%
1609
18%
536
70%
536
36%
935
August16th to endNight
62%
573
21%
1797
18%
573
66%
573
33%
1036
September1st to 15thNight
61%
627
25%
1887
22%
627
65%
627
35%
1091
September16th to endNight
59%
717
24%
1980
19%
717
62%
717
33%
1215
October1st to 15thNight
46%
720
19%
2031
14%
720
50%
720
26%
1371
October16th to endNight
28%
768
16%
2463
9%
768
37%
768
17%
1755
November1st to 15thNight
35%
760
18%
2471
16%
760
40%
760
20%
1680
November16th to endNight
25%
780
13%
2535
8%
780
32%
780
14%
1871
December1st to 15thNight
18%
833
6%
2470
2%
833
21%
833
9%
2031
December16th to endNight
15%
960
7%
2688
1%
960
20%
960
7%
2318

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%8%46%6%35%45%7%33%
48714
Night
14%9%46%5%3445%6%34%
47908
Cloud CoverDay
13%8%46%6%35%45%7%34%
48723
Night
15%9%46%5%3445%6%34%
47913
TransparencyDay
49%6%11%20%11%1
48403
Night
49%5%11%20%11%1
47737
SeeingDay
17%7%26%26%15%7%
6209
Night
19%7%28%30%12%1
16263

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
16%14%8%9%9%5%6%5%4414%
1432
16th to endDay
12%18%9%11%10%36%44413%
1556
February1st to 15thDay
11%15%6%12%9%5%7%33419%
1646
16th to endDay
14%13%6%10%6%47%5%5%6%20%
1580
March1st to 15thDay
14%9%47%6%345%5%7%29%
1832
16th to endDay
16%10%47%6%2435%6%32%
2133
April1st to 15thDay
19%11%36%6%33446%30%
2151
16th to endDay
16%6%27%5%25%447%38%
2336
May1st to 15thDay
19%7%36%6%25%446%33%
2331
16th to endDay
12%6%25%436%46%6%39%
2520
June1st to 15thDay
9%5%25%5%4446%9%41%
2520
16th to endDay
9%435%5%35%5%6%11%37%
2520
July1st to 15thDay
7%4245%35%6%6%9%45%
2520
16th to endDay
7%425%436%6%6%10%42%
2636
August1st to 15thDay
8%5%25%5%36%5%6%9%41%
2340
16th to endDay
10%6%245%36%5%6%8%39%
2402
September1st to 15thDay
11%5%145%25%45%7%43%
2060
16th to endDay
11%426%5%345%6%9%40%
1980
October1st to 15thDay
15%7%46%6%23446%38%
1929
16th to endDay
20%10%36%6%35%346%28%
1758
November1st to 15thDay
14%10%36%6%34346%37%
1717
16th to endDay
16%14%47%5%35%347%25%
1755
December1st to 15thDay
14%15%8%13%7%34445%17%
1524
16th to endDay
20%17%9%9%7%34435%14%
1536
January1st to 15thNight
19%14%7%9%8%3344419%
2506
16th to endNight
11%14%9%11%8%5%5%5%46%16%
2667
February1st to 15thNight
15%13%6%9%7%45%44423%
2212
16th to endNight
14%11%5%9%8%35%5%5%6%25%
1893
March1st to 15thNight
15%10%37%5%24346%34%
2105
16th to endNight
14%7%46%435%447%36%
2100
April1st to 15thNight
18%8%36%5%35%445%34%
1797
16th to endNight
17%7%2442345%6%40%
1623
May1st to 15thNight
17%7%25%5%35%446%35%
1620
16th to endNight
11%5%25%4345%6%9%41%
1592
June1st to 15thNight
10%5%25%33345%8%46%
1440
16th to endNight
7%4245%35%5%7%10%41%
1440
July1st to 15thNight
6%31442446%8%53%
1440
16th to endNight
6%32442446%9%52%
1531
August1st to 15thNight
6%3245%245%6%10%48%
1620
16th to endNight
10%415%42335%7%51%
1797
September1st to 15thNight
10%31342445%7%52%
1887
16th to endNight
10%5%25%43445%7%48%
1980
October1st to 15thNight
17%5%35%5%23445%41%
2031
16th to endNight
21%8%36%5%34446%32%
2463
November1st to 15thNight
15%9%35%6%24446%37%
2471
16th to endNight
18%12%47%5%35%446%27%
2535
December1st to 15thNight
19%14%7%10%7%34446%18%
2470
16th to endNight
20%16%9%10%6%36%44413%
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
16%15%7%9%9%6%6%5%35%14%
1432
16th to endDay
13%18%9%11%9%45%445%13%
1556
February1st to 15thDay
11%17%6%11%9%46%335%20%
1646
16th to endDay
14%13%6%10%6%45%6%47%21%
1580
March1st to 15thDay
14%10%5%7%6%3446%7%29%
1842
16th to endDay
17%10%5%6%6%3425%6%32%
2132
April1st to 15thDay
20%10%37%5%34346%30%
2151
16th to endDay
16%6%25%5%24446%38%
2336
May1st to 15thDay
20%6%26%5%34446%34%
2331
16th to endDay
12%6%25%5%35%5%47%40%
2520
June1st to 15thDay
10%5%25%5%4445%9%42%
2520
16th to endDay
9%425%5%35%5%7%10%38%
2520
July1st to 15thDay
7%424435%5%6%9%46%
2520
16th to endDay
7%325%436%6%7%9%42%
2636
August1st to 15thDay
9%425%6%36%5%5%9%41%
2340
16th to endDay
11%4245%35%47%8%40%
2402
September1st to 15thDay
12%5%245%25%45%7%44%
2060
16th to endDay
11%5%25%5%345%6%9%41%
1980
October1st to 15thDay
15%7%27%6%33446%38%
1929
16th to endDay
22%9%36%5%34445%29%
1758
November1st to 15thDay
15%9%35%6%34346%37%
1717
16th to endDay
16%14%48%5%34446%26%
1755
December1st to 15thDay
14%16%8%12%6%35%445%18%
1524
16th to endDay
20%17%9%9%6%3444316%
1536
January1st to 15thNight
19%14%7%10%7%3344420%
2506
16th to endNight
11%14%9%11%8%5%5%45%6%17%
2667
February1st to 15thNight
15%14%5%9%8%45%44424%
2205
16th to endNight
15%11%6%7%7%45%5%5%6%25%
1893
March1st to 15thNight
16%9%47%5%2435%6%34%
2118
16th to endNight
15%7%37%425%447%37%
2100
April1st to 15thNight
18%8%35%5%345%45%34%
1797
16th to endNight
17%7%245%2345%6%40%
1623
May1st to 15thNight
18%7%25%435%5%46%35%
1619
16th to endNight
12%5%24425%5%5%8%43%
1592
June1st to 15thNight
11%425%42445%8%46%
1440
16th to endNight
8%4245%35%6%6%10%42%
1440
July1st to 15thNight
6%31342436%8%53%
1440
16th to endNight
6%3234245%5%9%52%
1531
August1st to 15thNight
6%4144345%6%9%48%
1620
16th to endNight
10%42443235%7%51%
1797
September1st to 15thNight
11%32243335%7%52%
1887
16th to endNight
10%5%2442345%7%49%
1980
October1st to 15thNight
17%6%34433346%41%
2031
16th to endNight
22%8%26%5%24346%32%
2463
November1st to 15thNight
16%9%35%5%34446%37%
2471
16th to endNight
18%13%36%5%345%46%28%
2535
December1st to 15thNight
19%15%7%10%6%3435%419%
2470
16th to endNight
22%15%8%9%6%35%445%14%
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
71%6%9%9%2
1433
16th to endDay
73%410%7%3
1556
February1st to 15thDay
68%47%13%4
1648
16th to endDay
63%5%9%14%6%
1580
March1st to 15thDay
52%7%14%17%7%
1842
16th to endDay
52%5%12%21%7%1
2133
April1st to 15thDay
55%39%19%11%
2151
16th to endDay
45%6%12%17%14%3
2336
May1st to 15thDay
50%5%11%21%9%1
2331
16th to endDay
43%6%11%26%11%
2524
June1st to 15thDay
38%7%13%23%15%
2520
16th to endDay
38%9%12%20%17%1
2520
July1st to 15thDay
32%8%11%25%19%2
2371
16th to endDay
35%8%13%24%16%2
2470
August1st to 15thDay
37%7%13%26%12%2
2327
16th to endDay
38%7%12%24%13%2
2402
September1st to 15thDay
38%5%12%26%16%1
2060
16th to endDay
38%6%13%27%15%
1980
October1st to 15thDay
46%6%10%22%12%1
1929
16th to endDay
56%5%13%17%6%
1758
November1st to 15thDay
48%412%21%10%1
1717
16th to endDay
58%5%13%17%4
1755
December1st to 15thDay
67%6%11%11%2
1524
16th to endDay
72%5%10%8%2
1536
January1st to 15thNight
67%410%11%5%
2506
16th to endNight
67%5%13%9%3
2667
February1st to 15thNight
62%6%9%15%5%
2214
16th to endNight
57%7%12%15%6%
1893
March1st to 15thNight
50%5%12%20%9%1
2118
16th to endNight
47%5%12%23%10%1
2100
April1st to 15thNight
50%5%10%17%14%
1797
16th to endNight
43%412%19%15%3
1623
May1st to 15thNight
48%5%11%20%11%2
1620
16th to endNight
38%6%12%24%15%2
1594
June1st to 15thNight
35%6%11%29%15%1
1440
16th to endNight
34%7%15%24%16%1
1440
July1st to 15thNight
27%6%12%30%20%3
1360
16th to endNight
27%8%14%25%21%3
1434
August1st to 15thNight
29%7%13%28%18%1
1609
16th to endNight
32%5%10%28%18%3
1797
September1st to 15thNight
31%5%11%29%20%1
1887
16th to endNight
35%5%12%28%18%
1980
October1st to 15thNight
43%5%10%22%16%1
2031
16th to endNight
52%5%11%22%7%
2463
November1st to 15thNight
48%5%11%21%12%1
2471
16th to endNight
56%5%13%18%5%
2535
December1st to 15thNight
67%5%11%10%5%
2470
16th to endNight
72%5%9%8%2
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
16%14%43%21%4
90
16th to endDay
29%7%37%19%42
96
February1st to 15thDay
23%22%43%11%
90
16th to endDay
18%12%38%24%41
81
March1st to 15thDay
28%11%30%21%8%
132
16th to endDay
27%6%28%25%10%2
288
April1st to 15thDay
25%7%31%25%7%2
270
16th to endDay
17%10%33%19%12%5%
270
May1st to 15thDay
25%7%26%22%12%4
288
16th to endDay
15%9%27%26%14%7%
420
June1st to 15thDay
13%5%29%30%14%6%
495
16th to endDay
12%6%31%29%12%7%
495
July1st to 15thDay
9%420%31%21%11%
475
16th to endDay
9%324%30%21%10%
496
August1st to 15thDay
9%118%26%23%20%
313
16th to endDay
14%415%24%25%15%
332
September1st to 15thDay
14%214%28%23%17%
315
16th to endDay
13%5%15%23%26%15%
314
October1st to 15thDay
20%7%19%32%16%4
315
16th to endDay
25%8%31%29%4
174
November1st to 15thDay
20%11%23%30%11%2
135
16th to endDay
25%19%34%14%6%
135
December1st to 15thDay
26%20%30%14%6%1
94
16th to endDay
26%26%40%6%1
96
January1st to 15thNight
25%12%33%21%5%1
900
16th to endNight
18%10%34%30%5%
939
February1st to 15thNight
21%14%36%23%3
704
16th to endNight
19%11%34%27%6%
631
March1st to 15thNight
21%10%32%27%7%
720
16th to endNight
18%8%32%29%8%1
756
April1st to 15thNight
21%9%33%27%6%
539
16th to endNight
22%6%33%27%8%1
540
May1st to 15thNight
22%432%30%7%1
540
16th to endNight
14%228%34%18%1
563
June1st to 15thNight
12%225%42%16%
540
16th to endNight
10%124%40%21%1
540
July1st to 15thNight
8%117%44%24%3
510
16th to endNight
7%116%42%29%2
564
August1st to 15thNight
9%14%45%25%4
539
16th to endNight
11%116%36%29%5%
573
September1st to 15thNight
13%315%34%27%5%
627
16th to endNight
14%317%31%28%5%
717
October1st to 15thNight
21%5%23%31%16%1
720
16th to endNight
26%9%31%24%7%
768
November1st to 15thNight
21%8%29%30%9%
760
16th to endNight
24%11%32%24%6%
780
December1st to 15thNight
26%12%32%24%3
833
16th to endNight
27%11%35%21%3
960
Page generated at 2017-12-03 11:16:44. Version ac04

Copyright 2017 A. Danko


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