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Bright Skies Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bright Skies Observatory clear sky chart from 2005-11-05 to 2017-09-10.

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
77%
15803
33%
46744
29%
15755
80%
15755
51%
22926

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
67%
716
29%
2458
28%
716
73%
716
43%
1182
January16th to endNight
64%
768
35%
2380
31%
768
72%
768
42%
1227
February1st to 15thNight
70%
698
35%
2101
33%
696
76%
696
48%
1057
February16th to endNight
62%
632
28%
1895
27%
632
67%
632
36%
1071
March1st to 15thNight
69%
720
28%
2160
27%
720
75%
720
46%
1085
March16th to endNight
73%
768
29%
1998
22%
768
77%
768
46%
1114
April1st to 15thNight
67%
719
30%
1799
19%
719
71%
719
49%
1015
April16th to endNight
73%
720
32%
1800
16%
720
78%
720
56%
944
May1st to 15thNight
72%
635
28%
1715
18%
635
75%
635
47%
948
May16th to endNight
76%
561
29%
1685
23%
561
78%
561
53%
830
June1st to 15thNight
82%
437
29%
1517
29%
437
84%
437
51%
688
June16th to endNight
94%
360
35%
1440
43%
360
94%
360
64%
496
July1st to 15thNight
91%
340
34%
1360
42%
340
91%
340
57%
507
July16th to endNight
89%
620
30%
1640
25%
578
90%
578
59%
782
August1st to 15thNight
90%
717
34%
1790
22%
713
90%
713
67%
886
August16th to endNight
92%
764
41%
1910
28%
764
92%
764
75%
899
September1st to 15thNight
90%
700
39%
1766
31%
700
90%
700
68%
867
September16th to endNight
92%
656
41%
1815
40%
656
93%
656
70%
795
October1st to 15thNight
81%
660
36%
1845
36%
660
84%
660
56%
923
October16th to endNight
80%
704
39%
2123
40%
704
83%
704
53%
1004
November1st to 15thNight
77%
700
35%
2275
36%
700
83%
700
56%
985
November16th to endNight
78%
720
38%
2340
38%
720
83%
720
49%
1082
December1st to 15thNight
69%
720
35%
2340
33%
720
76%
720
40%
1200
December16th to endNight
62%
768
33%
2592
31%
768
69%
768
36%
1339

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
4212212345%68%
47858
Night
4212212346%68%
46949
TransparencyDay
18%36%17%28%25%
47552
Night
19%38%16%25%26%
46756
SeeingDay
6%7%23%36%21%4
5922
Night
6%28%33%38%10%
15816

By Month

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
6%323322335%62%
1477
16th to endDay
11%413212336%58%
1840
February1st to 15thDay
6%313212347%61%
1751
16th to endDay
11%423323336%54%
1579
March1st to 15thDay
4313313446%62%
1800
16th to endDay
5%22342346%9%56%
2223
April1st to 15thDay
5%312223447%61%
2149
16th to endDay
4212112436%69%
2160
May1st to 15thDay
3312222436%66%
2235
16th to endDay
3313222346%67%
2429
June1st to 15thDay
4413212247%66%
2439
16th to endDay
11111223381%
2520
July1st to 15thDay
12221234473%
2520
16th to endDay
1112212345%72%
2407
August1st to 15thDay
11111223480%
2160
16th to endDay
111123481%
2292
September1st to 15thDay
11111123380%
2081
16th to endDay
1111123379%
1815
October1st to 15thDay
4211322335%69%
1782
16th to endDay
32221333471%
1749
November1st to 15thDay
321232233470%
1584
16th to endDay
4212422336%65%
1620
December1st to 15thDay
7%21322345%9%56%
1620
16th to endDay
10%313314345%57%
1626
January1st to 15thNight
6%313322336%62%
2458
16th to endNight
9%323212236%61%
2380
February1st to 15thNight
5%324222235%64%
2101
16th to endNight
12%413423345%54%
1895
March1st to 15thNight
4312322347%62%
2160
16th to endNight
4212325%5%5%6%61%
1998
April1st to 15thNight
6%412213335%65%
1799
16th to endNight
321213347%69%
1800
May1st to 15thNight
45%12213447%63%
1715
16th to endNight
2312212236%70%
1684
June1st to 15thNight
122213347%70%
1517
16th to endNight
1112335%79%
1440
July1st to 15thNight
111212326%74%
1440
16th to endNight
21221245%6%69%
1756
August1st to 15thNight
211112336%74%
1800
16th to endNight
11112345%77%
1910
September1st to 15thNight
22121233476%
1766
16th to endNight
111335%80%
1815
October1st to 15thNight
311322333470%
1845
16th to endNight
321121333471%
2123
November1st to 15thNight
32221223473%
2275
16th to endNight
321221335%5%67%
2340
December1st to 15thNight
7%313313246%60%
2340
16th to endNight
10%423212235%61%
2592

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
25%35%13%23%28%
1477
16th to endDay
27%47%16%17%27%
1840
February1st to 15thDay
21%46%15%26%26%
1751
16th to endDay
30%47%17%28%12%
1579
March1st to 15thDay
22%39%20%22%22%
1800
16th to endDay
24%410%24%20%15%
2223
April1st to 15thDay
21%5%10%24%24%13%
2149
16th to endDay
16%48%23%28%18%
2160
May1st to 15thDay
18%5%9%21%27%18%
2235
16th to endDay
18%49%27%30%10%
2432
June1st to 15thDay
20%38%17%32%17%
2439
16th to endDay
10%15%8%35%38%
2520
July1st to 15thDay
15%1412%28%36%
2371
16th to endDay
14%38%21%30%21%
2261
August1st to 15thDay
10%1413%36%34%
2146
16th to endDay
8%1412%41%32%
2292
September1st to 15thDay
10%2413%32%37%
2081
16th to endDay
10%2313%34%34%
1815
October1st to 15thDay
17%3419%27%26%
1782
16th to endDay
16%4413%22%39%
1749
November1st to 15thDay
18%2414%24%35%
1584
16th to endDay
21%36%12%23%32%
1620
December1st to 15thDay
24%5%8%15%20%24%
1620
16th to endDay
28%37%16%22%21%
1626
January1st to 15thNight
24%27%13%22%29%
2458
16th to endNight
25%27%15%17%31%
2380
February1st to 15thNight
22%37%14%23%28%
2112
16th to endNight
32%27%16%25%15%
1895
March1st to 15thNight
22%310%17%24%21%
2160
16th to endNight
22%6%11%22%21%16%
1998
April1st to 15thNight
23%38%27%22%14%
1799
16th to endNight
16%411%23%25%19%
1800
May1st to 15thNight
21%7%8%20%26%15%
1716
16th to endNight
15%6%10%28%26%12%
1685
June1st to 15thNight
15%6%11%19%30%15%
1517
16th to endNight
10%210%12%32%32%
1440
July1st to 15thNight
13%15%15%26%36%
1360
16th to endNight
16%310%24%25%20%
1640
August1st to 15thNight
13%26%10%37%28%
1790
16th to endNight
10%7%12%37%30%
1910
September1st to 15thNight
13%27%9%30%37%
1766
16th to endNight
10%25%13%31%36%
1815
October1st to 15thNight
17%45%17%27%27%
1845
16th to endNight
18%2412%22%38%
2123
November1st to 15thNight
15%26%15%21%38%
2275
16th to endNight
19%35%16%21%33%
2340
December1st to 15thNight
26%37%15%21%26%
2340
16th to endNight
27%36%14%25%21%
2592

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
18%5%42%27%4
105
16th to endDay
12%10%34%30%11%
275
February1st to 15thDay
9%9%32%31%13%3
266
16th to endDay
14%15%35%23%10%
238
March1st to 15thDay
8%15%31%24%17%1
270
16th to endDay
8%13%33%33%9%1
288
April1st to 15thDay
10%13%39%23%10%3
269
16th to endDay
5%15%42%27%6%2
270
May1st to 15thDay
5%14%30%33%13%3
288
16th to endDay
5%7%33%36%14%2
330
June1st to 15thDay
7%6%21%43%17%3
315
16th to endDay
1112%44%29%10%
315
July1st to 15thDay
16%43%37%9%
305
16th to endDay
25%42%38%10%
327
August1st to 15thDay
319%45%33%6%
315
16th to endDay
8%40%38%10%
332
September1st to 15thDay
317%47%26%4
294
16th to endDay
1122%47%21%5%
255
October1st to 15thDay
37%28%42%14%3
255
16th to endDay
5%5%23%42%18%4
261
November1st to 15thDay
35%8%30%35%16%
90
16th to endDay
5%8%23%33%28%
90
December1st to 15thDay
5%8%13%36%34%1
90
16th to endDay
18%10%22%26%20%1
79
January1st to 15thNight
9%312%41%26%7%
720
16th to endNight
11%312%38%27%5%
768
February1st to 15thNight
7%212%37%31%9%
704
16th to endNight
14%5%10%36%26%5%
632
March1st to 15thNight
6%413%40%27%7%
720
16th to endNight
6%213%41%30%5%
768
April1st to 15thNight
8%414%39%28%4
719
16th to endNight
36%13%38%31%6%
720
May1st to 15thNight
7%213%37%34%5%
636
16th to endNight
3214%40%36%3
563
June1st to 15thNight
310%37%43%4
437
16th to endNight
127%63%6%
360
July1st to 15thNight
2111%70%13%
340
16th to endNight
316%60%19%
620
August1st to 15thNight
2220%56%17%
717
16th to endNight
1115%58%23%
764
September1st to 15thNight
3120%54%20%
700
16th to endNight
1227%51%15%
656
October1st to 15thNight
417%35%40%10%
660
16th to endNight
5%26%30%38%16%
704
November1st to 15thNight
4210%33%33%15%
700
16th to endNight
5%38%35%34%12%
720
December1st to 15thNight
9%48%38%31%8%
720
16th to endNight
13%7%12%38%21%6%
768
Page generated at 2017-09-11 16:35:24. Version ac04

Copyright 2017 A. Danko


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