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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 2020-06-03.

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
76%
19508
34%
57733
30%
19460
80%
19460
51%
28542

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%
896
29%
3076
29%
896
73%
896
41%
1499
January16th to endNight
65%
960
36%
2975
32%
960
73%
960
43%
1519
February1st to 15thNight
69%
878
36%
2641
33%
876
76%
876
48%
1329
February16th to endNight
62%
792
30%
2375
29%
792
68%
792
38%
1306
March1st to 15thNight
65%
900
26%
2700
25%
900
71%
900
40%
1458
March16th to endNight
70%
960
29%
2497
21%
960
76%
960
46%
1406
April1st to 15thNight
64%
899
30%
2249
18%
899
70%
899
45%
1308
April16th to endNight
73%
892
32%
2230
16%
892
78%
892
55%
1184
May1st to 15thNight
73%
794
27%
2144
18%
794
75%
794
47%
1192
May16th to endNight
72%
705
30%
2117
23%
705
75%
705
50%
1066
June1st to 15thNight
83%
517
31%
1791
31%
517
85%
517
51%
806
June16th to endNight
89%
420
33%
1680
41%
420
89%
420
56%
627
July1st to 15thNight
91%
400
36%
1600
43%
400
91%
400
57%
596
July16th to endNight
89%
726
29%
1938
23%
684
89%
684
59%
931
August1st to 15thNight
90%
825
36%
2060
23%
821
90%
821
68%
1010
August16th to endNight
92%
892
40%
2230
27%
892
92%
892
73%
1057
September1st to 15thNight
88%
840
40%
2120
32%
840
89%
840
67%
1049
September16th to endNight
90%
836
41%
2310
40%
836
91%
836
70%
1018
October1st to 15thNight
82%
840
36%
2348
36%
840
85%
840
56%
1167
October16th to endNight
81%
896
42%
2702
43%
896
85%
896
57%
1228
November1st to 15thNight
79%
880
37%
2860
38%
880
84%
880
57%
1232
November16th to endNight
75%
900
37%
2925
37%
900
81%
900
47%
1394
December1st to 15thNight
67%
900
34%
2925
32%
900
74%
900
39%
1527
December16th to endNight
63%
960
34%
3240
31%
960
71%
960
38%
1633

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
4212212346%67%
58652
Night
4212212346%66%
57938
TransparencyDay
19%36%17%27%25%
58346
Night
20%37%17%25%25%
57745
SeeingDay
7%7%22%35%22%5%
9571
Night
6%29%34%37%9%
19521

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%323422346%60%
1849
16th to endDay
9%413212336%59%
2300
February1st to 15thDay
7%312212346%62%
2201
16th to endDay
9%423323436%57%
1979
March1st to 15thDay
6%414324446%57%
2250
16th to endDay
5%3134245%5%9%55%
2779
April1st to 15thDay
6%313323447%59%
2689
16th to endDay
4312112336%68%
2665
May1st to 15thDay
4313222447%63%
2796
16th to endDay
3313322346%64%
3053
June1st to 15thDay
3413212247%66%
2873
16th to endDay
12111223477%
2940
July1st to 15thDay
1212212345%72%
2940
16th to endDay
111221235%5%72%
2813
August1st to 15thDay
11111223480%
2473
16th to endDay
1111235%80%
2676
September1st to 15thDay
1111123479%
2500
16th to endDay
12111223478%
2310
October1st to 15thDay
321131223471%
2268
16th to endDay
21221233474%
2226
November1st to 15thDay
321222333470%
1989
16th to endDay
4313422437%62%
2025
December1st to 15thDay
7%31332345%8%56%
2025
16th to endDay
9%313324346%57%
2033
January1st to 15thNight
6%313322337%60%
3076
16th to endNight
8%313312246%61%
2975
February1st to 15thNight
6%323212235%65%
2641
16th to endNight
10%413422345%57%
2375
March1st to 15thNight
7%42332335%8%56%
2700
16th to endNight
5%2123245%6%7%58%
2497
April1st to 15thNight
6%413313346%61%
2249
16th to endNight
321121345%7%68%
2230
May1st to 15thNight
3412213447%62%
2144
16th to endNight
3412212236%68%
2116
June1st to 15thNight
1212213347%69%
1791
16th to endNight
21222335%75%
1680
July1st to 15thNight
112212336%73%
1680
16th to endNight
21221245%7%68%
2054
August1st to 15thNight
211211346%74%
2070
16th to endNight
11212345%76%
2230
September1st to 15thNight
12122223475%
2120
16th to endNight
1111335%79%
2310
October1st to 15thNight
311322333470%
2348
16th to endNight
321121223474%
2702
November1st to 15thNight
222212335%72%
2860
16th to endNight
421221345%6%64%
2925
December1st to 15thNight
7%324323247%58%
2925
16th to endNight
9%412212335%61%
3240

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
26%35%15%24%25%
1849
16th to endDay
26%37%16%19%26%
2300
February1st to 15thDay
22%35%14%26%27%
2201
16th to endDay
27%47%18%26%15%
1979
March1st to 15thDay
27%38%20%19%20%
2250
16th to endDay
24%5%10%22%21%15%
2779
April1st to 15thDay
23%5%11%22%23%13%
2689
16th to endDay
17%37%23%31%16%
2665
May1st to 15thDay
20%5%9%22%26%15%
2796
16th to endDay
20%5%8%26%30%8%
3056
June1st to 15thDay
20%38%18%32%16%
2873
16th to endDay
12%15%10%34%35%
2940
July1st to 15thDay
16%1413%28%35%
2791
16th to endDay
14%38%23%30%19%
2667
August1st to 15thDay
10%1313%36%35%
2459
16th to endDay
8%1314%41%30%
2676
September1st to 15thDay
10%2313%33%36%
2500
16th to endDay
11%2215%35%31%
2310
October1st to 15thDay
17%2418%25%31%
2268
16th to endDay
13%3312%24%42%
2226
November1st to 15thDay
17%2415%23%35%
1989
16th to endDay
22%37%13%22%30%
2025
December1st to 15thDay
25%47%16%19%25%
2025
16th to endDay
27%37%17%24%20%
2033
January1st to 15thNight
24%27%15%22%26%
3076
16th to endNight
25%27%16%18%29%
2975
February1st to 15thNight
22%26%15%24%28%
2652
16th to endNight
30%27%16%25%17%
2375
March1st to 15thNight
27%310%16%22%18%
2700
16th to endNight
23%6%11%22%21%14%
2497
April1st to 15thNight
25%49%25%20%14%
2249
16th to endNight
17%310%23%27%16%
2230
May1st to 15thNight
22%7%9%22%24%14%
2145
16th to endNight
18%6%10%29%25%10%
2117
June1st to 15thNight
15%6%11%22%30%13%
1791
16th to endNight
14%19%13%31%29%
1680
July1st to 15thNight
13%15%16%28%34%
1600
16th to endNight
16%310%25%24%18%
1938
August1st to 15thNight
13%26%11%36%30%
2060
16th to endNight
11%6%14%38%27%
2230
September1st to 15thNight
14%26%10%30%36%
2120
16th to endNight
10%2415%32%34%
2310
October1st to 15thNight
17%3417%26%30%
2348
16th to endNight
16%2312%21%43%
2702
November1st to 15thNight
15%26%16%21%37%
2860
16th to endNight
21%35%16%21%30%
2925
December1st to 15thNight
27%37%16%20%25%
2925
16th to endNight
25%36%14%27%22%
3240

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
15%5%33%27%14%2
207
16th to endDay
11%9%30%32%14%1
447
February1st to 15thDay
10%11%28%28%15%5%
446
16th to endDay
11%13%35%29%10%
398
March1st to 15thDay
13%17%30%24%13%1
450
16th to endDay
10%13%32%30%11%1
480
April1st to 15thDay
12%11%32%26%13%3
449
16th to endDay
6%13%34%29%10%4
438
May1st to 15thDay
8%10%26%35%14%3
468
16th to endDay
7%8%31%34%14%4
522
June1st to 15thDay
6%418%41%21%6%
444
16th to endDay
6%111%42%28%10%
435
July1st to 15thDay
35%40%38%11%
425
16th to endDay
2440%39%12%
455
August1st to 15thDay
29%41%35%10%
419
16th to endDay
17%40%39%10%
460
September1st to 15thDay
4116%44%26%6%
435
16th to endDay
2220%45%22%5%
435
October1st to 15thDay
46%26%41%17%3
435
16th to endDay
35%22%35%24%8%
450
November1st to 15thDay
3313%33%35%10%
225
16th to endDay
8%6%21%31%27%5%
225
December1st to 15thDay
8%8%15%37%26%3
225
16th to endDay
11%10%20%32%22%3
198
January1st to 15thNight
9%312%43%24%7%
900
16th to endNight
11%313%37%28%5%
960
February1st to 15thNight
8%312%36%29%9%
884
16th to endNight
12%413%39%25%5%
792
March1st to 15thNight
10%312%42%25%6%
900
16th to endNight
7%214%42%27%5%
960
April1st to 15thNight
8%414%39%28%4
899
16th to endNight
45%13%39%30%6%
892
May1st to 15thNight
6%212%37%34%5%
795
16th to endNight
5%215%39%34%2
707
June1st to 15thNight
39%37%44%5%
517
16th to endNight
2127%60%7%
420
July1st to 15thNight
2111%69%14%
400
16th to endNight
316%59%19%
726
August1st to 15thNight
2121%56%17%
825
16th to endNight
1115%59%21%
892
September1st to 15thNight
4222%52%17%
840
16th to endNight
2330%49%13%
836
October1st to 15thNight
416%39%38%9%
840
16th to endNight
5%27%30%37%16%
896
November1st to 15thNight
329%36%36%12%
880
16th to endNight
7%38%35%33%12%
900
December1st to 15thNight
10%410%38%29%7%
900
16th to endNight
12%6%12%38%22%6%
960
Page generated at 2020-06-04 00:18:56. Version ac04

Copyright 2020 A. Danko


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