Dean Memorial Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dean Memorial Observatory clear sky chart from 2005-11-05 to 2018-01-20.

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%
16569
19%
48516
11%
16522
45%
16522
27%
31108

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
29%
975
14%
2690
6%
970
36%
970
23%
1826
January16th to endNight
24%
985
17%
2561
5%
985
32%
985
21%
1840
February1st to 15thNight
23%
880
14%
2288
4%
876
30%
876
21%
1645
February16th to endNight
28%
711
20%
1973
7%
711
37%
711
26%
1294
March1st to 15thNight
31%
570
20%
2010
13%
570
41%
570
24%
1207
March16th to endNight
25%
576
11%
2097
6%
576
33%
576
15%
1447
April1st to 15thNight
27%
539
17%
1797
11%
539
37%
539
22%
1163
April16th to endNight
32%
540
18%
1742
11%
540
43%
540
21%
1170
May1st to 15thNight
36%
540
17%
1551
10%
540
43%
540
26%
1021
May16th to endNight
41%
563
18%
1497
8%
561
48%
561
28%
1012
June1st to 15thNight
55%
540
16%
1440
8%
540
58%
540
36%
892
June16th to endNight
55%
540
18%
1440
9%
540
58%
540
40%
853
July1st to 15thNight
69%
510
17%
1360
10%
510
71%
510
48%
760
July16th to endNight
71%
564
24%
1508
20%
531
73%
531
46%
817
August1st to 15thNight
67%
539
21%
1610
22%
536
70%
536
38%
906
August16th to endNight
63%
573
22%
1839
25%
573
67%
573
34%
1060
September1st to 15thNight
59%
540
27%
1800
27%
540
65%
540
36%
972
September16th to endNight
52%
591
27%
1977
21%
591
62%
591
36%
1072
October1st to 15thNight
38%
720
24%
2160
16%
720
46%
720
31%
1310
October16th to endNight
35%
768
23%
2304
12%
768
45%
768
28%
1412
November1st to 15thNight
34%
815
24%
2500
13%
815
45%
815
29%
1488
November16th to endNight
28%
975
19%
2730
8%
975
37%
975
26%
1783
December1st to 15thNight
21%
975
13%
2730
4%
975
30%
975
19%
1971
December16th to endNight
23%
1040
16%
2912
6%
1040
32%
1040
17%
2187

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%7%36%5%3445%7%38%
49008
Night
12%7%35%424346%44%
48694
Cloud CoverDay
13%7%35%5%3445%7%38%
49017
Night
12%7%35%424346%44%
48698
TransparencyDay
44%5%13%22%12%1
48703
Night
40%413%24%14%2
48517
SeeingDay
15%7%28%28%15%3
6406
Night
15%7%32%31%11%1
16569

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
9%10%49%7%45%346%31%
1575
16th to endDay
12%12%6%8%6%445%5%6%26%
1767
February1st to 15thDay
11%13%5%7%6%345%45%32%
1575
16th to endDay
12%8%47%6%35%446%36%
1498
March1st to 15thDay
13%9%46%6%35%45%8%32%
1948
16th to endDay
19%12%46%5%33436%30%
2127
April1st to 15thDay
19%7%45%424447%34%
2151
16th to endDay
24%9%36%5%24346%30%
2217
May1st to 15thDay
20%7%36%5%3445%6%32%
2400
16th to endDay
15%6%25%5%25%5%5%6%37%
2617
June1st to 15thDay
12%5%25%5%445%5%7%41%
2520
16th to endDay
9%4245%35%5%5%9%43%
2520
July1st to 15thDay
7%415%5%345%6%8%47%
2520
16th to endDay
7%4245%35%6%6%9%45%
2554
August1st to 15thDay
8%4145%36%6%7%8%42%
2340
16th to endDay
12%5%245%3345%7%45%
2363
September1st to 15thDay
11%41432335%6%53%
2160
16th to endDay
11%5%25%42445%7%47%
1968
October1st to 15thDay
16%6%35%413245%45%
1800
16th to endDay
16%7%25%423445%42%
1920
November1st to 15thDay
11%7%25%433447%45%
1684
16th to endDay
16%8%45%422348%39%
1560
December1st to 15thDay
16%12%48%5%24446%28%
1560
16th to endDay
16%14%7%9%7%33335%23%
1664
January1st to 15thNight
11%7%45%6%45%447%37%
2690
16th to endNight
11%12%49%5%24346%35%
2561
February1st to 15thNight
12%11%5%6%5%34446%35%
2285
16th to endNight
12%8%35%423346%43%
1973
March1st to 15thNight
15%7%34422336%47%
1998
16th to endNight
17%9%38%5%35%445%32%
2097
April1st to 15thNight
15%7%45%424345%41%
1797
16th to endNight
19%8%16%5%34435%38%
1742
May1st to 15thNight
14%6%25%33447%8%39%
1551
16th to endNight
12%425%5%35%5%46%43%
1496
June1st to 15thNight
9%43433445%10%47%
1440
16th to endNight
7%3144345%6%10%49%
1440
July1st to 15thNight
3213325%5%5%10%57%
1440
16th to endNight
3224425%35%9%56%
1612
August1st to 15thNight
5%31442345%7%56%
1620
16th to endNight
9%314424446%53%
1839
September1st to 15thNight
9%42422225%6%56%
1800
16th to endNight
10%423323347%55%
1977
October1st to 15thNight
14%5%25%423226%50%
2160
16th to endNight
14%6%24423346%47%
2304
November1st to 15thNight
12%6%25%423346%49%
2500
16th to endNight
15%7%34423345%44%
2730
December1st to 15thNight
14%12%47%434445%33%
2730
16th to endNight
18%11%48%6%2334430%
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
10%9%48%8%5%6%347%30%
1575
16th to endDay
13%11%6%8%7%45%5%5%6%26%
1767
February1st to 15thDay
11%13%5%8%5%3445%5%32%
1575
16th to endDay
12%9%47%5%25%45%6%36%
1498
March1st to 15thDay
14%9%36%6%35%45%8%32%
1959
16th to endDay
19%12%36%5%34446%29%
2125
April1st to 15thDay
19%8%45%424447%34%
2151
16th to endDay
24%9%35%5%23346%30%
2217
May1st to 15thDay
20%7%35%5%35%446%32%
2400
16th to endDay
15%6%35%5%35%45%7%37%
2617
June1st to 15thDay
13%425%5%3446%7%41%
2520
16th to endDay
9%424436%5%5%9%43%
2520
July1st to 15thDay
7%415%5%345%6%8%47%
2520
16th to endDay
7%424435%6%6%10%45%
2554
August1st to 15thDay
8%5%24435%6%7%9%42%
2340
16th to endDay
12%5%25%5%2345%7%45%
2363
September1st to 15thDay
11%42332335%7%52%
2160
16th to endDay
10%5%15%42445%7%48%
1968
October1st to 15thDay
16%6%35%423245%45%
1800
16th to endDay
16%7%25%423445%42%
1920
November1st to 15thDay
11%6%35%324447%45%
1684
16th to endDay
16%8%35%413338%39%
1560
December1st to 15thDay
16%12%5%9%5%25%436%28%
1560
16th to endDay
16%14%7%8%7%33336%23%
1664
January1st to 15thNight
11%8%35%5%45%447%37%
2690
16th to endNight
11%12%48%5%25%346%34%
2561
February1st to 15thNight
12%11%47%434346%35%
2278
16th to endNight
12%8%35%424446%43%
1973
March1st to 15thNight
15%7%35%412237%46%
2010
16th to endNight
16%9%47%6%25%445%32%
2097
April1st to 15thNight
14%8%44424446%41%
1797
16th to endNight
19%7%26%5%24435%38%
1742
May1st to 15thNight
15%6%244245%6%8%39%
1550
16th to endNight
12%5%25%5%35%446%43%
1496
June1st to 15thNight
9%42432445%10%47%
1440
16th to endNight
7%31442446%9%50%
1440
July1st to 15thNight
422332445%10%57%
1440
16th to endNight
321442445%9%56%
1612
August1st to 15thNight
5%41442345%8%55%
1620
16th to endNight
9%324424436%53%
1839
September1st to 15thNight
9%424322246%57%
1800
16th to endNight
10%313323348%55%
1977
October1st to 15thNight
14%5%25%423226%50%
2160
16th to endNight
14%7%1432335%5%47%
2304
November1st to 15thNight
13%6%24423346%49%
2500
16th to endNight
15%7%34423345%44%
2730
December1st to 15thNight
14%11%5%7%5%24346%33%
2730
16th to endNight
18%11%48%5%23345%30%
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
53%412%18%10%
1576
16th to endDay
56%6%14%15%6%
1767
February1st to 15thDay
53%413%21%6%
1576
16th to endDay
48%5%14%19%9%2
1498
March1st to 15thDay
49%5%15%19%8%1
1959
16th to endDay
56%5%13%18%6%
2127
April1st to 15thDay
49%411%25%8%
2151
16th to endDay
55%410%18%9%
2217
May1st to 15thDay
51%6%13%20%8%
2400
16th to endDay
45%7%12%20%11%2
2621
June1st to 15thDay
40%6%15%21%14%
2520
16th to endDay
35%7%14%24%16%1
2520
July1st to 15thDay
34%7%14%26%12%3
2371
16th to endDay
32%6%17%24%17%1
2395
August1st to 15thDay
34%8%14%26%14%1
2326
16th to endDay
38%5%11%25%15%2
2363
September1st to 15thDay
31%413%26%21%2
2160
16th to endDay
34%6%12%31%14%1
1968
October1st to 15thDay
41%49%24%18%1
1800
16th to endDay
43%5%11%25%13%1
1920
November1st to 15thDay
37%413%25%17%1
1684
16th to endDay
44%416%24%9%
1560
December1st to 15thDay
56%413%17%6%1
1560
16th to endDay
63%411%13%6%
1664
January1st to 15thNight
45%416%19%14%
2690
16th to endNight
50%415%20%9%
2561
February1st to 15thNight
49%413%22%9%
2288
16th to endNight
41%413%25%12%1
1973
March1st to 15thNight
40%313%27%13%1
2010
16th to endNight
53%414%18%8%1
2097
April1st to 15thNight
43%410%26%12%2
1797
16th to endNight
47%411%20%12%2
1742
May1st to 15thNight
41%6%14%22%13%
1551
16th to endNight
40%7%11%22%16%2
1498
June1st to 15thNight
32%6%17%25%14%3
1440
16th to endNight
29%5%12%26%21%4
1440
July1st to 15thNight
22%6%19%30%16%4
1360
16th to endNight
23%6%16%31%20%1
1508
August1st to 15thNight
26%7%13%34%14%3
1610
16th to endNight
31%6%11%25%19%6%
1839
September1st to 15thNight
29%48%29%25%2
1800
16th to endNight
29%411%33%19%2
1977
October1st to 15thNight
38%29%24%22%3
2160
16th to endNight
37%413%25%16%2
2304
November1st to 15thNight
36%311%26%18%3
2500
16th to endNight
41%413%28%11%1
2730
December1st to 15thNight
52%412%21%9%
2730
16th to endNight
56%412%16%9%
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
20%11%42%20%31
135
16th to endDay
21%10%34%30%11
109
February1st to 15thDay
14%21%38%18%6%
90
16th to endDay
20%8%40%20%7%1
81
March1st to 15thDay
15%25%37%20%1
90
16th to endDay
37%10%28%17%4
111
April1st to 15thDay
22%10%32%24%10%
270
16th to endDay
31%8%28%19%9%1
327
May1st to 15thDay
22%11%32%23%8%1
468
16th to endDay
17%9%30%27%12%2
517
June1st to 15thDay
11%5%27%32%18%4
495
16th to endDay
9%7%31%31%16%4
495
July1st to 15thDay
6%326%34%20%9%
475
16th to endDay
8%220%36%27%4
515
August1st to 15thDay
9%119%34%28%5%
493
16th to endDay
13%216%34%23%8%
403
September1st to 15thDay
12%317%33%24%8%
315
16th to endDay
7%5%36%28%18%3
189
October1st to 15thDay
20%8%35%24%6%4
135
16th to endDay
18%13%31%25%10%
144
November1st to 15thDay
8%8%37%29%10%4
135
16th to endDay
27%14%37%16%4
135
December1st to 15thDay
19%17%40%14%8%
135
16th to endDay
25%15%40%14%4
144
January1st to 15thNight
16%10%36%28%6%
975
16th to endNight
16%11%37%28%6%
985
February1st to 15thNight
19%9%38%26%6%
880
16th to endNight
16%10%39%26%5%1
711
March1st to 15thNight
18%9%35%24%10%
570
16th to endNight
22%7%38%24%6%
576
April1st to 15thNight
17%11%38%26%5%
539
16th to endNight
22%7%34%26%8%1
540
May1st to 15thNight
17%6%35%32%7%
540
16th to endNight
13%5%33%32%14%
563
June1st to 15thNight
12%421%46%13%
540
16th to endNight
8%229%44%14%
540
July1st to 15thNight
5%18%45%28%1
510
16th to endNight
316%49%26%2
564
August1st to 15thNight
5%214%46%28%2
539
16th to endNight
9%217%40%26%3
573
September1st to 15thNight
12%317%38%22%4
540
16th to endNight
11%427%34%21%1
591
October1st to 15thNight
16%7%32%28%13%1
720
16th to endNight
16%9%34%29%10%
768
November1st to 15thNight
17%9%34%28%8%
815
16th to endNight
19%10%39%21%9%
975
December1st to 15thNight
20%13%37%24%3
975
16th to endNight
24%8%34%26%5%
1040
Page generated at 2018-01-21 11:19:22. Version ac04

Copyright 2018 A. Danko


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