Big Blue Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Big Blue Observatory clear sky chart from 2005-11-05 to 2018-08-29.

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
36%
17118
21%
50627
12%
17071
45%
17071
26%
31939

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%
975
18%
2652
6%
970
29%
970
21%
1824
January16th to endNight
20%
1040
21%
2704
6%
1040
31%
1040
23%
1839
February1st to 15thNight
19%
940
14%
2469
3%
937
25%
937
17%
1769
February16th to endNight
16%
683
20%
2049
6%
683
28%
683
20%
1368
March1st to 15thNight
18%
665
18%
2225
7%
665
30%
665
19%
1462
March16th to endNight
20%
624
13%
2233
8%
624
29%
624
14%
1575
April1st to 15thNight
25%
584
21%
1947
10%
584
41%
584
22%
1187
April16th to endNight
30%
579
21%
1928
13%
579
44%
579
24%
1191
May1st to 15thNight
38%
585
18%
1797
12%
584
46%
584
26%
1135
May16th to endNight
44%
611
18%
1625
11%
609
50%
609
30%
1073
June1st to 15thNight
59%
585
21%
1560
15%
585
63%
585
42%
915
June16th to endNight
56%
585
22%
1560
12%
585
62%
585
42%
916
July1st to 15thNight
66%
555
20%
1480
16%
555
68%
555
40%
888
July16th to endNight
69%
612
22%
1680
20%
579
72%
579
41%
933
August1st to 15thNight
74%
575
24%
1751
26%
572
76%
572
40%
965
August16th to endNight
70%
615
22%
2050
26%
615
73%
615
36%
1112
September1st to 15thNight
60%
540
26%
1800
26%
540
64%
540
31%
1032
September16th to endNight
58%
636
32%
1944
24%
636
65%
636
34%
1102
October1st to 15thNight
46%
720
28%
2160
19%
720
53%
720
34%
1249
October16th to endNight
30%
768
27%
2304
14%
768
42%
768
29%
1361
November1st to 15thNight
31%
760
27%
2446
14%
760
47%
760
28%
1401
November16th to endNight
25%
866
24%
2621
11%
866
38%
866
25%
1625
December1st to 15thNight
23%
975
17%
2730
5%
975
33%
975
20%
1884
December16th to endNight
17%
1040
17%
2912
4%
1040
28%
1040
16%
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
12%6%25%5%2445%7%42%
51628
Night
12%6%25%424446%45%
50813
TransparencyDay
40%6%12%23%14%2
51317
Night
38%5%11%24%16%2
50629
SeeingDay
15%10%27%26%15%4
7145
Night
17%12%30%26%12%1
17118

By Month

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
12%8%48%5%24346%39%
1614
16th to endDay
11%8%46%5%25%45%7%39%
1872
February1st to 15thDay
13%9%48%5%34446%35%
1724
16th to endDay
13%10%47%5%24446%36%
1710
March1st to 15thDay
15%8%36%6%24346%37%
2077
16th to endDay
20%10%46%5%24345%29%
2341
April1st to 15thDay
14%8%345%34435%41%
2331
16th to endDay
18%7%25%5%25%446%36%
2307
May1st to 15thDay
16%5%35%5%2445%8%36%
2479
16th to endDay
12%5%25%6%35%5%6%7%40%
2841
June1st to 15thDay
7%4245%35%5%6%8%46%
2730
16th to endDay
5%313445%5%6%12%44%
2730
July1st to 15thDay
9%3134346%7%9%46%
2730
16th to endDay
6%315%435%5%7%10%45%
2730
August1st to 15thDay
8%315%5%35%6%6%9%43%
2451
16th to endDay
7%5%25%435%5%5%7%45%
2448
September1st to 15thDay
12%5%1442446%7%46%
2160
16th to endDay
9%4144245%5%6%49%
2001
October1st to 15thDay
14%5%24423345%47%
1800
16th to endDay
12%5%24423335%53%
1920
November1st to 15thDay
14%5%23412247%49%
1741
16th to endDay
16%7%25%422345%45%
1667
December1st to 15thDay
15%9%36%324345%40%
1560
16th to endDay
16%10%37%424345%35%
1664
January1st to 15thNight
14%7%36%5%34435%41%
2652
16th to endNight
11%8%45%5%24446%41%
2704
February1st to 15thNight
13%10%47%6%24446%35%
2459
16th to endNight
16%8%5%6%412335%43%
2049
March1st to 15thNight
17%9%345%2333442%
2225
16th to endNight
20%8%45%425%5%5%6%30%
2233
April1st to 15thNight
11%6%36%423446%45%
1947
16th to endNight
16%6%25%423347%44%
1928
May1st to 15thNight
13%5%24434447%45%
1797
16th to endNight
10%425%435%45%7%45%
1624
June1st to 15thNight
6%42342445%8%53%
1560
16th to endNight
6%21342347%10%52%
1560
July1st to 15thNight
8%313333449%52%
1560
16th to endNight
431343446%10%52%
1787
August1st to 15thNight
5%21333445%9%54%
1761
16th to endNight
5%2145%235%5%9%53%
2050
September1st to 15thNight
12%5%13324445%51%
1800
16th to endNight
6%5%2442435%6%55%
1944
October1st to 15thNight
10%314424346%53%
2160
16th to endNight
14%5%2432334452%
2304
November1st to 15thNight
11%6%24412336%52%
2446
16th to endNight
14%8%24323345%47%
2621
December1st to 15thNight
17%8%35%5%24446%36%
2730
16th to endNight
18%11%47%433335%33%
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
46%410%22%13%1
1615
16th to endDay
42%6%16%23%11%1
1872
February1st to 15thDay
49%5%11%22%9%
1725
16th to endDay
48%410%21%12%2
1710
March1st to 15thDay
48%5%9%21%13%2
2077
16th to endDay
56%5%12%17%8%
2343
April1st to 15thDay
43%49%23%15%2
2331
16th to endDay
48%5%10%21%12%1
2307
May1st to 15thDay
43%8%12%23%11%
2479
16th to endDay
41%8%13%19%14%2
2845
June1st to 15thDay
33%8%12%26%16%1
2730
16th to endDay
28%8%17%26%15%3
2730
July1st to 15thDay
30%9%14%28%12%4
2581
16th to endDay
31%9%17%26%14%1
2574
August1st to 15thDay
35%8%11%25%16%1
2437
16th to endDay
35%7%13%20%17%4
2448
September1st to 15thDay
35%6%10%23%19%4
2160
16th to endDay
33%6%8%29%18%2
2001
October1st to 15thDay
37%49%24%18%5%
1800
16th to endDay
33%49%28%20%3
1920
November1st to 15thDay
35%312%22%21%5%
1741
16th to endDay
41%39%28%15%1
1667
December1st to 15thDay
46%413%24%11%
1560
16th to endDay
49%5%11%20%10%1
1664
January1st to 15thNight
45%410%23%14%1
2652
16th to endNight
42%5%12%24%13%
2704
February1st to 15thNight
48%6%10%19%13%1
2469
16th to endNight
45%411%21%15%1
2049
March1st to 15thNight
47%39%21%15%2
2225
16th to endNight
51%7%10%21%8%
2233
April1st to 15thNight
38%5%10%24%16%3
1947
16th to endNight
40%5%8%26%16%2
1928
May1st to 15thNight
37%7%15%21%16%1
1798
16th to endNight
37%8%16%19%15%3
1626
June1st to 15thNight
27%5%14%29%19%2
1560
16th to endNight
25%7%15%28%18%4
1560
July1st to 15thNight
28%6%12%31%15%5%
1480
16th to endNight
24%7%15%32%18%1
1680
August1st to 15thNight
25%7%13%30%20%2
1751
16th to endNight
27%6%13%24%22%5%
2050
September1st to 15thNight
34%48%23%23%5%
1800
16th to endNight
29%6%8%26%25%3
1944
October1st to 15thNight
32%410%24%22%5%
2160
16th to endNight
34%48%26%21%5%
2304
November1st to 15thNight
33%410%24%19%7%
2446
16th to endNight
39%39%27%18%1
2621
December1st to 15thNight
46%412%21%12%
2730
16th to endNight
53%5%10%19%10%2
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
25%28%35%8%2
135
16th to endDay
13%32%37%15%
144
February1st to 15thDay
24%26%35%12%
135
16th to endDay
34%23%28%12%1
120
March1st to 15thDay
27%25%38%7%
135
16th to endDay
37%23%27%10%1
199
April1st to 15thDay
21%17%33%20%6%
330
16th to endDay
23%15%31%21%7%
319
May1st to 15thDay
18%16%33%21%9%1
439
16th to endDay
15%11%31%25%11%4
597
June1st to 15thDay
9%427%35%18%5%
570
16th to endDay
8%7%32%34%13%3
570
July1st to 15thDay
11%321%32%22%8%
550
16th to endDay
6%322%32%30%4
595
August1st to 15thDay
12%213%35%30%6%
514
16th to endDay
10%223%30%24%7%
384
September1st to 15thDay
12%6%14%29%28%8%
315
16th to endDay
8%5%20%31%25%8%
266
October1st to 15thDay
22%8%35%25%6%1
135
16th to endDay
20%12%38%24%4
144
November1st to 15thDay
16%8%35%22%11%5%
135
16th to endDay
31%19%26%20%2
135
December1st to 15thDay
18%24%37%12%42
135
16th to endDay
26%28%36%8%
144
January1st to 15thNight
21%21%34%17%5%
975
16th to endNight
20%20%37%20%2
1040
February1st to 15thNight
22%19%35%20%2
940
16th to endNight
24%18%38%18%1
683
March1st to 15thNight
25%14%38%18%3
665
16th to endNight
23%13%37%20%4
624
April1st to 15thNight
18%20%33%22%4
584
16th to endNight
20%8%36%27%6%
579
May1st to 15thNight
15%9%32%29%11%
585
16th to endNight
13%5%30%30%18%1
611
June1st to 15thNight
8%223%40%22%1
585
16th to endNight
7%227%41%19%1
585
July1st to 15thNight
11%114%40%28%5%
555
16th to endNight
5%16%40%33%2
612
August1st to 15thNight
6%110%41%37%2
575
16th to endNight
5%117%40%31%3
615
September1st to 15thNight
14%415%32%26%6%
540
16th to endNight
9%5%21%31%26%5%
636
October1st to 15thNight
12%8%29%26%19%3
720
16th to endNight
18%14%35%22%8%
768
November1st to 15thNight
16%15%33%23%10%
760
16th to endNight
21%18%33%20%6%
866
December1st to 15thNight
25%16%31%19%5%
975
16th to endNight
27%16%35%19%1
1040
Page generated at 2018-08-30 04:34:19. Version ac04

Copyright 2018 A. Danko


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