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 2018-10-27.

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
41%
17271
19%
51183
13%
17229
47%
17229
24%
33961

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
21%
975
14%
2716
6%
970
28%
970
16%
2066
January16th to endNight
22%
965
13%
2837
7%
965
28%
965
15%
2151
February1st to 15thNight
22%
762
11%
2302
5%
761
27%
761
16%
1679
February16th to endNight
25%
683
15%
2049
7%
683
32%
683
16%
1512
March1st to 15thNight
26%
780
16%
2314
8%
780
33%
780
20%
1583
March16th to endNight
30%
792
17%
2248
8%
792
38%
792
21%
1591
April1st to 15thNight
31%
584
18%
1947
11%
584
42%
584
22%
1268
April16th to endNight
36%
579
21%
1805
12%
579
46%
579
26%
1163
May1st to 15thNight
37%
585
15%
1755
9%
585
42%
585
21%
1218
May16th to endNight
55%
609
17%
1809
13%
609
59%
609
29%
1151
June1st to 15thNight
61%
585
18%
1560
13%
585
64%
585
37%
962
June16th to endNight
63%
585
19%
1560
14%
585
67%
585
37%
956
July1st to 15thNight
68%
555
18%
1480
15%
555
70%
555
39%
901
July16th to endNight
73%
612
23%
1615
19%
579
75%
579
44%
913
August1st to 15thNight
71%
575
24%
1715
21%
572
74%
572
39%
973
August16th to endNight
72%
621
22%
2057
23%
621
75%
621
37%
1089
September1st to 15thNight
64%
668
27%
2033
24%
668
69%
668
35%
1185
September16th to endNight
62%
777
28%
2145
21%
777
65%
777
36%
1287
October1st to 15thNight
51%
780
28%
2145
21%
780
58%
780
36%
1299
October16th to endNight
34%
816
22%
2615
16%
816
42%
816
25%
1627
November1st to 15thNight
40%
760
25%
2471
18%
760
51%
760
28%
1486
November16th to endNight
30%
780
20%
2535
14%
780
39%
780
21%
1698
December1st to 15thNight
24%
803
12%
2558
7%
803
31%
803
15%
1899
December16th to endNight
19%
1040
12%
2912
4%
1040
26%
1040
13%
2304

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%35%5%3445%7%36%
52372
Night
15%7%36%424446%39%
51358
Cloud CoverDay
15%7%35%5%3445%7%36%
52381
Night
15%7%36%5%24446%39%
51363
TransparencyDay
46%411%22%13%2
52065
Night
45%310%22%14%2
51183
SeeingDay
18%5%22%24%19%9%
7303
Night
18%6%25%30%16%2
17275

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%15%5%8%5%2445%5%23%
1552
16th to endDay
13%14%7%8%6%3445%5%25%
1738
February1st to 15thDay
16%13%48%6%34445%27%
1887
16th to endDay
18%13%48%5%24336%28%
1710
March1st to 15thDay
21%9%5%6%5%24437%29%
1966
16th to endDay
20%9%36%6%3335%7%30%
2340
April1st to 15thDay
20%8%35%5%33446%35%
2331
16th to endDay
19%7%26%434336%37%
2430
May1st to 15thDay
19%7%36%6%35%45%6%32%
2526
16th to endDay
12%6%25%5%36%6%6%7%37%
2655
June1st to 15thDay
11%4145%35%6%5%7%42%
2730
16th to endDay
9%425%5%25%6%5%9%43%
2730
July1st to 15thDay
11%415%5%346%6%9%40%
2730
16th to endDay
8%423435%6%7%10%42%
2804
August1st to 15thDay
9%5%24435%47%9%44%
2487
16th to endDay
9%415%5%35%5%6%8%43%
2495
September1st to 15thDay
13%5%25%43445%7%44%
2243
16th to endDay
14%5%2332345%7%47%
2145
October1st to 15thDay
14%6%25%5%3445%6%41%
2145
16th to endDay
19%6%25%424345%40%
1863
November1st to 15thDay
13%6%34433445%47%
1717
16th to endDay
19%10%45%5%1445%6%32%
1755
December1st to 15thDay
22%12%5%8%5%24345%24%
1729
16th to endDay
23%15%5%8%5%34245%22%
1664
January1st to 15thNight
20%10%5%8%6%3334427%
2716
16th to endNight
17%13%6%8%5%3444427%
2837
February1st to 15thNight
19%11%6%7%5%23345%29%
2300
16th to endNight
17%13%5%8%435%34429%
2049
March1st to 15thNight
17%8%36%5%34445%35%
2301
16th to endNight
18%9%35%5%2445%6%33%
2248
April1st to 15thNight
17%7%35%5%3435%5%39%
1947
16th to endNight
15%6%25%5%33348%39%
1805
May1st to 15thNight
18%7%35%5%2446%7%33%
1755
16th to endNight
12%5%15%426%5%5%9%40%
1807
June1st to 15thNight
8%5%25%4245%5%8%47%
1560
16th to endNight
8%2144246%6%10%46%
1560
July1st to 15thNight
8%324335%5%5%8%49%
1560
16th to endNight
7%31342345%8%54%
1716
August1st to 15thNight
5%315%43446%8%51%
1726
16th to endNight
5%3145%245%5%8%52%
2057
September1st to 15thNight
11%5%1432345%8%48%
2033
16th to endNight
11%42443445%6%50%
2145
October1st to 15thNight
11%4245%2435%7%49%
2145
16th to endNight
18%5%25%333336%43%
2615
November1st to 15thNight
13%7%35%324346%46%
2471
16th to endNight
19%9%35%423346%36%
2535
December1st to 15thNight
21%11%5%9%5%2333427%
2558
16th to endNight
23%14%5%7%5%34335%24%
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
21%14%5%8%6%2445%5%23%
1552
16th to endDay
14%15%7%9%6%3345%5%24%
1738
February1st to 15thDay
17%12%5%9%6%34345%27%
1887
16th to endDay
17%13%48%5%24336%28%
1710
March1st to 15thDay
21%9%5%7%5%24437%28%
1976
16th to endDay
19%10%36%6%24347%30%
2339
April1st to 15thDay
19%8%25%5%33346%35%
2331
16th to endDay
19%7%26%434446%36%
2430
May1st to 15thDay
19%7%35%6%3445%6%31%
2526
16th to endDay
12%5%35%5%35%6%6%7%37%
2655
June1st to 15thDay
11%4145%35%6%5%8%42%
2730
16th to endDay
9%4245%25%5%6%10%42%
2730
July1st to 15thDay
11%425%6%246%6%9%40%
2730
16th to endDay
8%3245%35%6%7%10%42%
2804
August1st to 15thDay
8%5%24435%5%6%10%43%
2487
16th to endDay
8%5%25%5%35%5%6%8%43%
2495
September1st to 15thDay
13%5%25%4245%5%7%43%
2243
16th to endDay
14%5%24323347%47%
2145
October1st to 15thDay
14%5%245%345%5%6%41%
2145
16th to endDay
19%5%25%433445%39%
1863
November1st to 15thDay
13%5%345%33445%46%
1717
16th to endDay
19%9%46%424446%32%
1755
December1st to 15thDay
22%12%5%7%6%24445%24%
1729
16th to endDay
22%15%6%7%5%34245%22%
1664
January1st to 15thNight
21%10%5%8%6%3434427%
2716
16th to endNight
17%13%6%8%5%3444427%
2837
February1st to 15thNight
18%11%6%7%5%23345%29%
2293
16th to endNight
17%13%5%8%435%34429%
2049
March1st to 15thNight
17%8%37%5%24446%35%
2314
16th to endNight
19%9%36%43445%6%33%
2248
April1st to 15thNight
17%7%25%5%34345%38%
1947
16th to endNight
15%7%25%5%34347%40%
1805
May1st to 15thNight
18%7%36%5%2446%7%33%
1754
16th to endNight
12%5%16%425%5%5%9%40%
1807
June1st to 15thNight
7%5%25%42445%8%47%
1560
16th to endNight
8%2235%346%7%10%46%
1560
July1st to 15thNight
8%324435%5%6%8%49%
1560
16th to endNight
6%31342345%8%54%
1716
August1st to 15thNight
5%314435%46%8%51%
1726
16th to endNight
5%3144345%5%8%52%
2057
September1st to 15thNight
11%41342445%8%48%
2033
16th to endNight
11%324424446%50%
2145
October1st to 15thNight
11%42443435%7%49%
2145
16th to endNight
17%6%25%423336%43%
2615
November1st to 15thNight
13%6%34423346%45%
2471
16th to endNight
19%9%35%423346%36%
2535
December1st to 15thNight
20%11%5%9%5%2343428%
2558
16th to endNight
23%14%5%7%5%24235%23%
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
62%28%16%8%
1553
16th to endDay
59%312%18%5%
1738
February1st to 15thDay
59%310%16%9%
1889
16th to endDay
58%111%20%7%
1710
March1st to 15thDay
55%312%19%7%
1976
16th to endDay
54%315%20%5%
2340
April1st to 15thDay
49%312%22%11%
2331
16th to endDay
48%310%20%12%4
2430
May1st to 15thDay
52%413%19%9%
2526
16th to endDay
42%7%12%20%15%1
2659
June1st to 15thDay
37%5%12%25%16%1
2730
16th to endDay
36%5%14%23%16%2
2730
July1st to 15thDay
39%412%23%17%2
2581
16th to endDay
32%7%17%26%13%2
2642
August1st to 15thDay
34%5%12%26%18%1
2474
16th to endDay
36%5%10%26%15%5%
2495
September1st to 15thDay
39%411%22%18%4
2243
16th to endDay
36%38%25%24%1
2145
October1st to 15thDay
41%39%25%17%2
2145
16th to endDay
45%211%24%14%
1863
November1st to 15thDay
40%29%25%17%4
1717
16th to endDay
52%29%23%10%1
1755
December1st to 15thDay
61%39%17%7%
1729
16th to endDay
65%210%17%3
1664
January1st to 15thNight
59%36%19%10%
2716
16th to endNight
60%210%18%7%
2837
February1st to 15thNight
56%212%17%8%
2302
16th to endNight
58%210%18%8%
2049
March1st to 15thNight
49%311%22%10%1
2314
16th to endNight
51%311%23%8%1
2248
April1st to 15thNight
47%29%23%15%2
1947
16th to endNight
44%28%21%14%7%
1805
May1st to 15thNight
48%5%12%21%10%1
1755
16th to endNight
39%5%13%22%15%3
1809
June1st to 15thNight
34%412%25%20%2
1560
16th to endNight
30%7%14%26%17%3
1560
July1st to 15thNight
32%415%23%18%6%
1480
16th to endNight
27%414%33%18%2
1615
August1st to 15thNight
29%413%26%22%2
1715
16th to endNight
27%413%27%20%5%
2057
September1st to 15thNight
33%310%24%21%6%
2033
16th to endNight
34%38%25%25%3
2145
October1st to 15thNight
33%310%26%22%3
2145
16th to endNight
43%28%28%15%1
2615
November1st to 15thNight
39%28%22%21%4
2471
16th to endNight
50%29%24%13%
2535
December1st to 15thNight
59%28%18%10%
2558
16th to endNight
64%28%16%6%
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
31%11%27%21%7%
135
16th to endDay
20%18%37%21%2
144
February1st to 15thDay
26%14%35%22%1
135
16th to endDay
27%11%39%15%6%
120
March1st to 15thDay
26%11%40%18%3
161
16th to endDay
27%9%32%19%9%1
352
April1st to 15thDay
22%10%29%27%8%1
330
16th to endDay
26%8%24%21%12%5%
319
May1st to 15thDay
21%9%27%25%11%4
348
16th to endDay
18%223%23%18%13%
411
June1st to 15thDay
13%218%31%26%7%
570
16th to endDay
12%322%31%20%8%
570
July1st to 15thDay
14%114%28%24%17%
550
16th to endDay
9%12%25%32%18%
520
August1st to 15thDay
13%17%21%33%22%
358
16th to endDay
11%211%26%32%16%
396
September1st to 15thDay
14%29%25%31%16%
375
16th to endDay
19%314%17%29%15%
374
October1st to 15thDay
16%5%22%30%20%6%
375
16th to endDay
21%6%38%24%9%
211
November1st to 15thDay
22%8%20%18%22%6%
135
16th to endDay
22%8%40%22%5%
135
December1st to 15thDay
20%18%32%20%5%2
135
16th to endDay
28%14%43%12%
144
January1st to 15thNight
25%10%34%23%5%
975
16th to endNight
23%11%34%26%3
965
February1st to 15thNight
24%10%33%28%2
764
16th to endNight
24%9%34%27%3
683
March1st to 15thNight
21%11%34%27%5%
780
16th to endNight
22%7%32%28%7%1
792
April1st to 15thNight
19%9%35%27%8%
584
16th to endNight
18%6%33%29%10%1
579
May1st to 15thNight
21%427%32%14%
585
16th to endNight
14%217%36%25%3
611
June1st to 15thNight
9%217%48%21%1
585
16th to endNight
9%117%40%28%2
585
July1st to 15thNight
10%110%37%35%4
555
16th to endNight
8%19%37%37%5%
612
August1st to 15thNight
6%110%40%34%6%
575
16th to endNight
7%19%36%39%5%
621
September1st to 15thNight
13%211%33%32%6%
668
16th to endNight
13%214%30%34%5%
777
October1st to 15thNight
14%421%33%21%5%
780
16th to endNight
20%8%32%27%10%
816
November1st to 15thNight
17%8%30%28%14%1
760
16th to endNight
23%9%30%27%9%
780
December1st to 15thNight
26%9%32%23%7%
803
16th to endNight
31%9%32%22%4
1040
Page generated at 2018-10-28 08:41:07. Version ac04

Copyright 2018 A. Danko


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