The Star Deck Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Star Deck Observatory clear sky chart from 2005-11-05 to 2017-04-24.

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
40%
15347
18%
45517
12%
15305
46%
15305
24%
30349

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
19%
900
14%
2506
6%
895
26%
895
15%
1927
January16th to endNight
22%
891
12%
2619
7%
891
28%
891
15%
1994
February1st to 15thNight
22%
702
12%
2121
5%
701
28%
701
16%
1531
February16th to endNight
26%
631
16%
1893
7%
631
33%
631
17%
1376
March1st to 15thNight
26%
720
15%
2136
9%
720
32%
720
19%
1477
March16th to endNight
31%
731
16%
2075
8%
731
38%
731
21%
1466
April1st to 15thNight
32%
539
19%
1797
11%
539
43%
539
23%
1151
April16th to endNight
36%
522
21%
1632
12%
522
46%
522
26%
1043
May1st to 15thNight
36%
495
16%
1485
9%
495
42%
495
20%
1050
May16th to endNight
56%
513
17%
1524
13%
513
59%
513
30%
961
June1st to 15thNight
60%
495
18%
1320
13%
495
63%
495
36%
819
June16th to endNight
62%
495
17%
1320
12%
495
65%
495
36%
821
July1st to 15thNight
64%
465
17%
1240
13%
465
66%
465
36%
772
July16th to endNight
74%
522
22%
1363
18%
489
76%
489
45%
766
August1st to 15thNight
73%
494
25%
1474
22%
491
75%
491
39%
834
August16th to endNight
72%
525
22%
1739
22%
525
76%
525
38%
919
September1st to 15thNight
66%
565
29%
1720
25%
565
71%
565
36%
984
September16th to endNight
60%
657
27%
1815
20%
657
63%
657
35%
1092
October1st to 15thNight
52%
660
30%
1815
22%
660
59%
660
37%
1079
October16th to endNight
33%
704
21%
2258
15%
704
41%
704
24%
1422
November1st to 15thNight
41%
700
26%
2276
19%
700
52%
700
29%
1345
November16th to endNight
30%
720
18%
2340
13%
720
38%
720
19%
1596
December1st to 15thNight
23%
741
12%
2361
7%
741
29%
741
13%
1791
December16th to endNight
19%
960
12%
2688
4%
960
26%
960
13%
2133

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%7%36%5%3445%7%36%
46093
Night
15%7%36%424446%38%
45692
Cloud CoverDay
15%7%36%5%3445%7%36%
46102
Night
15%8%36%5%24446%38%
45697
TransparencyDay
46%411%22%12%2
45786
Night
46%310%22%14%2
45517
SeeingDay
17%5%21%24%19%10%
5178
Night
19%7%26%30%15%2
15351

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
20%14%5%8%5%2435%5%24%
1432
16th to endDay
13%14%7%9%6%3445%6%24%
1604
February1st to 15thDay
15%13%5%8%6%35%44428%
1738
16th to endDay
16%13%5%8%6%24336%28%
1580
March1st to 15thDay
22%9%5%6%424437%28%
1814
16th to endDay
18%10%36%6%33447%30%
2160
April1st to 15thDay
20%7%24433446%36%
2151
16th to endDay
19%7%26%433336%37%
2187
May1st to 15thDay
21%7%36%6%35%45%6%30%
2136
16th to endDay
12%5%25%5%36%5%6%7%38%
2237
June1st to 15thDay
12%5%145%35%6%6%7%41%
2310
16th to endDay
9%425%5%25%5%5%10%43%
2310
July1st to 15thDay
12%416%5%346%6%8%39%
2310
16th to endDay
7%4245%36%6%7%10%42%
2396
August1st to 15thDay
9%5%2443447%10%43%
2145
16th to endDay
8%415%435%5%6%9%43%
2109
September1st to 15thDay
13%5%25%3345%5%7%44%
1898
16th to endDay
13%5%2432345%7%46%
1815
October1st to 15thDay
13%5%25%5%3445%6%44%
1815
16th to endDay
19%5%26%424346%38%
1612
November1st to 15thDay
11%5%2442345%5%48%
1582
16th to endDay
20%10%45%5%2445%6%31%
1620
December1st to 15thDay
23%12%5%8%5%24345%23%
1596
16th to endDay
22%15%5%8%5%34245%22%
1536
January1st to 15thNight
21%10%48%6%3334426%
2506
16th to endNight
17%13%6%9%5%3444327%
2619
February1st to 15thNight
18%11%6%7%5%23345%30%
2119
16th to endNight
15%13%5%8%5%35%34430%
1893
March1st to 15thNight
17%8%47%5%34445%34%
2123
16th to endNight
17%9%35%5%2445%6%33%
2075
April1st to 15thNight
15%7%35%5%3445%5%40%
1797
16th to endNight
16%6%25%5%33348%40%
1632
May1st to 15thNight
20%8%35%5%2446%7%31%
1485
16th to endNight
13%415%426%5%5%8%41%
1522
June1st to 15thNight
9%5%25%4245%6%9%45%
1320
16th to endNight
9%224435%6%6%9%45%
1320
July1st to 15thNight
9%324435%5%5%7%48%
1320
16th to endNight
7%32332445%8%54%
1464
August1st to 15thNight
6%315%43446%8%51%
1485
16th to endNight
6%3145%245%5%8%52%
1739
September1st to 15thNight
10%41432445%8%50%
1720
16th to endNight
12%42442445%6%49%
1815
October1st to 15thNight
10%3235%1435%6%51%
1815
16th to endNight
19%5%25%333336%42%
2258
November1st to 15thNight
12%6%34324346%48%
2276
16th to endNight
20%10%35%413346%34%
2340
December1st to 15thNight
22%11%5%9%5%2334425%
2361
16th to endNight
24%14%5%7%5%24335%23%
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
21%13%5%8%5%2435%5%24%
1432
16th to endDay
13%15%7%9%6%3345%5%24%
1604
February1st to 15thDay
15%12%5%9%6%34345%28%
1738
16th to endDay
15%13%5%9%5%34436%28%
1580
March1st to 15thDay
22%9%5%7%424447%27%
1824
16th to endDay
17%10%37%6%24347%30%
2159
April1st to 15thDay
19%8%25%5%33346%36%
2151
16th to endDay
19%6%26%433446%36%
2187
May1st to 15thDay
20%7%36%6%35%45%6%29%
2136
16th to endDay
12%435%5%36%6%6%7%38%
2237
June1st to 15thDay
12%5%145%35%6%5%8%40%
2310
16th to endDay
9%4246%25%5%6%10%42%
2310
July1st to 15thDay
12%425%6%246%6%9%38%
2310
16th to endDay
7%3245%35%6%8%10%42%
2396
August1st to 15thDay
8%5%24435%5%6%10%43%
2145
16th to endDay
8%425%5%35%5%6%9%43%
2109
September1st to 15thDay
12%5%25%435%5%5%7%43%
1898
16th to endDay
14%5%24324447%45%
1815
October1st to 15thDay
13%5%245%2445%6%43%
1815
16th to endDay
19%5%25%5%34446%38%
1612
November1st to 15thDay
11%5%335%33445%48%
1582
16th to endDay
20%10%46%424446%30%
1620
December1st to 15thDay
23%12%5%8%5%24445%23%
1596
16th to endDay
22%15%6%7%5%34245%22%
1536
January1st to 15thNight
21%10%5%8%7%3434426%
2506
16th to endNight
17%13%6%9%5%3444426%
2619
February1st to 15thNight
17%11%6%7%5%23345%30%
2112
16th to endNight
15%13%5%8%435%345%30%
1893
March1st to 15thNight
18%8%37%5%24346%34%
2136
16th to endNight
17%9%36%5%3445%6%33%
2075
April1st to 15thNight
15%7%35%5%35%45%6%39%
1797
16th to endNight
15%6%25%5%34347%40%
1632
May1st to 15thNight
19%7%36%5%2446%7%31%
1484
16th to endNight
12%5%15%425%5%5%8%42%
1522
June1st to 15thNight
8%5%15%43445%9%45%
1320
16th to endNight
9%2235%346%7%10%44%
1320
July1st to 15thNight
9%424435%5%6%6%47%
1320
16th to endNight
7%3134245%5%8%54%
1464
August1st to 15thNight
6%314435%46%8%52%
1485
16th to endNight
5%2144345%5%9%52%
1739
September1st to 15thNight
10%31342445%8%50%
1720
16th to endNight
12%324324446%49%
1815
October1st to 15thNight
10%42342435%6%51%
1815
16th to endNight
18%6%25%423336%42%
2258
November1st to 15thNight
12%6%34323346%47%
2276
16th to endNight
20%10%36%5%23346%34%
2340
December1st to 15thNight
22%12%6%9%5%2343425%
2361
16th to endNight
24%14%5%7%5%24235%23%
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
62%28%17%8%
1433
16th to endDay
59%312%17%5%
1604
February1st to 15thDay
58%310%17%9%
1740
16th to endDay
57%211%21%6%
1580
March1st to 15thDay
56%313%18%6%
1824
16th to endDay
53%315%20%5%
2160
April1st to 15thDay
48%311%23%12%1
2151
16th to endDay
48%310%21%11%4
2187
May1st to 15thDay
54%413%18%8%
2136
16th to endDay
42%7%13%19%16%2
2241
June1st to 15thDay
39%413%26%14%1
2310
16th to endDay
36%5%14%22%17%3
2310
July1st to 15thDay
42%410%22%17%2
2161
16th to endDay
33%6%19%27%11%2
2234
August1st to 15thDay
34%5%12%25%19%1
2132
16th to endDay
36%5%10%25%15%6%
2109
September1st to 15thDay
39%410%21%19%5%
1898
16th to endDay
37%37%23%26%1
1815
October1st to 15thDay
39%39%26%19%2
1815
16th to endDay
47%211%24%13%1
1612
November1st to 15thDay
37%39%25%18%4
1582
16th to endDay
54%210%21%10%1
1620
December1st to 15thDay
63%39%16%6%
1596
16th to endDay
64%210%17%3
1536
January1st to 15thNight
60%37%19%8%
2506
16th to endNight
61%210%18%6%
2619
February1st to 15thNight
55%213%19%7%1
2121
16th to endNight
56%211%20%8%
1893
March1st to 15thNight
50%312%22%9%1
2136
16th to endNight
50%312%23%7%1
2075
April1st to 15thNight
45%29%24%15%2
1797
16th to endNight
43%28%22%14%7%
1632
May1st to 15thNight
50%5%12%22%9%
1485
16th to endNight
38%5%14%22%15%3
1524
June1st to 15thNight
35%412%26%19%1
1320
16th to endNight
32%7%14%23%18%3
1320
July1st to 15thNight
35%414%21%19%5%
1240
16th to endNight
26%415%33%17%2
1363
August1st to 15thNight
29%413%26%22%2
1474
16th to endNight
28%413%27%20%6%
1739
September1st to 15thNight
32%39%23%23%6%
1720
16th to endNight
34%38%24%25%3
1815
October1st to 15thNight
32%39%26%24%3
1815
16th to endNight
44%29%27%14%1
2258
November1st to 15thNight
38%28%22%22%5%
2276
16th to endNight
51%29%22%13%
2340
December1st to 15thNight
62%28%16%8%
2361
16th to endNight
65%28%16%6%
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
32%14%28%18%41
90
16th to endDay
15%22%41%16%3
96
February1st to 15thDay
17%17%40%23%1
90
16th to endDay
18%8%48%17%7%
81
March1st to 15thDay
32%11%32%19%4
114
16th to endDay
23%11%32%20%10%1
288
April1st to 15thDay
22%9%28%28%8%1
270
16th to endDay
26%6%24%23%11%7%
243
May1st to 15thDay
24%9%27%22%11%4
228
16th to endDay
17%321%22%19%16%
281
June1st to 15thDay
16%219%30%23%7%
420
16th to endDay
12%320%32%21%9%
420
July1st to 15thDay
18%214%29%22%14%
400
16th to endDay
9%110%24%32%21%
382
August1st to 15thDay
14%17%16%33%26%
253
16th to endDay
11%18%22%32%23%
268
September1st to 15thDay
9%310%23%35%18%
255
16th to endDay
19%311%20%28%16%
254
October1st to 15thDay
11%5%21%31%21%9%
255
16th to endDay
17%8%42%25%5%
124
November1st to 15thDay
12%10%21%15%33%7%
90
16th to endDay
27%8%38%16%6%1
90
December1st to 15thDay
27%13%28%23%42
90
16th to endDay
28%12%44%13%1
96
January1st to 15thNight
26%11%35%22%4
900
16th to endNight
24%11%33%26%3
891
February1st to 15thNight
23%11%32%28%2
704
16th to endNight
22%10%34%28%3
631
March1st to 15thNight
22%11%33%26%6%
720
16th to endNight
21%7%33%29%7%1
731
April1st to 15thNight
17%10%35%27%9%
539
16th to endNight
18%5%35%29%10%1
522
May1st to 15thNight
22%5%26%32%13%
495
16th to endNight
14%218%34%26%3
515
June1st to 15thNight
10%317%48%19%
495
16th to endNight
10%116%40%28%2
495
July1st to 15thNight
11%110%37%34%3
465
16th to endNight
8%28%36%37%6%
522
August1st to 15thNight
6%110%41%33%7%
494
16th to endNight
7%19%33%41%5%
525
September1st to 15thNight
11%312%33%32%6%
565
16th to endNight
14%214%30%33%4
657
October1st to 15thNight
13%422%32%20%5%
660
16th to endNight
21%8%32%27%9%
704
November1st to 15thNight
16%8%30%28%15%1
700
16th to endNight
25%10%28%26%9%
720
December1st to 15thNight
28%8%31%23%7%
741
16th to endNight
31%9%31%21%5%
960
Page generated at 2017-04-25 07:48:22. Version ac04

Copyright 2017 A. Danko


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