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 2017-09-17.

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%
15847
21%
46885
12%
15800
45%
15800
26%
29566

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
19%
2447
6%
895
30%
895
21%
1672
January16th to endNight
19%
960
21%
2496
6%
960
30%
960
22%
1704
February1st to 15thNight
19%
866
15%
2275
3%
863
26%
863
17%
1629
February16th to endNight
16%
631
20%
1893
6%
631
29%
631
21%
1240
March1st to 15thNight
19%
614
18%
2054
7%
614
30%
614
18%
1366
March16th to endNight
20%
576
14%
2061
7%
576
30%
576
14%
1433
April1st to 15thNight
26%
539
21%
1797
10%
539
42%
539
24%
1077
April16th to endNight
30%
540
21%
1800
13%
540
43%
540
24%
1116
May1st to 15thNight
37%
540
17%
1658
11%
539
45%
539
25%
1065
May16th to endNight
42%
563
19%
1497
11%
561
48%
561
30%
993
June1st to 15thNight
57%
540
20%
1440
13%
540
61%
540
41%
856
June16th to endNight
56%
540
23%
1440
12%
540
62%
540
43%
844
July1st to 15thNight
64%
510
19%
1360
15%
510
66%
510
39%
828
July16th to endNight
71%
564
22%
1541
20%
531
73%
531
41%
861
August1st to 15thNight
73%
539
23%
1642
26%
536
75%
536
40%
907
August16th to endNight
72%
573
24%
1910
27%
573
75%
573
37%
1025
September1st to 15thNight
60%
540
26%
1800
26%
540
64%
540
31%
1032
September16th to endNight
57%
589
31%
1802
22%
589
65%
589
34%
1026
October1st to 15thNight
47%
660
29%
1980
20%
660
55%
660
35%
1131
October16th to endNight
29%
704
25%
2112
13%
704
41%
704
29%
1258
November1st to 15thNight
33%
700
28%
2253
14%
700
49%
700
30%
1250
November16th to endNight
24%
799
22%
2419
11%
799
36%
799
24%
1519
December1st to 15thNight
22%
900
16%
2520
5%
900
32%
900
20%
1770
December16th to endNight
18%
960
17%
2688
5%
960
28%
960
16%
1964

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%
47894
Night
12%6%25%424446%45%
47071
TransparencyDay
40%6%12%23%14%2
47583
Night
38%5%11%24%16%2
46887
SeeingDay
15%10%27%26%16%4
5877
Night
17%11%30%26%12%1
15847

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%38%5%23436%40%
1489
16th to endDay
11%8%46%425%45%6%38%
1728
February1st to 15thDay
13%8%47%5%345%46%36%
1588
16th to endDay
11%10%47%5%25%445%38%
1580
March1st to 15thDay
16%8%46%6%24346%36%
1917
16th to endDay
18%10%46%5%24446%31%
2161
April1st to 15thDay
14%7%345%34435%42%
2151
16th to endDay
19%7%25%5%25%446%35%
2160
May1st to 15thDay
18%6%35%5%2445%8%36%
2288
16th to endDay
12%5%35%6%35%5%5%7%40%
2617
June1st to 15thDay
7%425%5%35%5%6%8%44%
2520
16th to endDay
5%313445%5%7%13%45%
2520
July1st to 15thDay
9%313435%6%7%9%45%
2520
16th to endDay
5%2145%35%5%8%10%45%
2518
August1st to 15thDay
7%315%5%36%6%6%9%43%
2307
16th to endDay
7%5%25%435%5%5%7%46%
2292
September1st to 15thDay
12%5%1442446%7%46%
2160
16th to endDay
9%414425%5%5%6%48%
1849
October1st to 15thDay
13%5%24423345%49%
1650
16th to endDay
12%5%2442333453%
1760
November1st to 15thDay
13%5%2341225%6%51%
1604
16th to endDay
17%7%25%42235%6%43%
1539
December1st to 15thDay
16%10%46%324345%38%
1440
16th to endDay
16%10%37%424345%36%
1536
January1st to 15thNight
14%7%36%5%33435%42%
2447
16th to endNight
12%8%45%5%24446%41%
2496
February1st to 15thNight
13%10%46%6%24346%36%
2265
16th to endNight
14%8%5%6%41233444%
1893
March1st to 15thNight
18%9%345%2333440%
2054
16th to endNight
18%8%45%425%5%5%7%31%
2061
April1st to 15thNight
11%6%35%423447%46%
1797
16th to endNight
16%6%25%423347%43%
1800
May1st to 15thNight
14%5%34434447%44%
1658
16th to endNight
11%425%435%45%7%44%
1496
June1st to 15thNight
7%42342445%7%53%
1440
16th to endNight
7%2134235%6%10%52%
1440
July1st to 15thNight
9%3233334310%51%
1440
16th to endNight
441343446%10%51%
1648
August1st to 15thNight
5%31333446%9%53%
1652
16th to endNight
5%2145%235%5%9%54%
1910
September1st to 15thNight
12%5%13324445%51%
1800
16th to endNight
6%5%2432435%6%54%
1802
October1st to 15thNight
10%31442435%6%54%
1980
16th to endNight
15%5%2432333452%
2112
November1st to 15thNight
10%5%24312335%55%
2253
16th to endNight
15%8%2432334447%
2419
December1st to 15thNight
18%9%36%5%2445%6%34%
2520
16th to endNight
19%10%37%433335%34%
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
45%410%23%14%1
1490
16th to endDay
43%5%16%23%10%1
1728
February1st to 15thDay
48%5%12%22%10%
1589
16th to endDay
46%5%11%22%12%1
1580
March1st to 15thDay
49%49%21%12%2
1917
16th to endDay
54%5%12%17%9%
2163
April1st to 15thDay
43%5%9%23%16%2
2151
16th to endDay
49%5%10%20%12%1
2160
May1st to 15thDay
44%8%12%22%11%
2288
16th to endDay
42%8%13%18%14%2
2621
June1st to 15thDay
34%8%12%26%15%1
2520
16th to endDay
27%8%18%25%16%3
2520
July1st to 15thDay
32%9%14%27%12%3
2371
16th to endDay
30%9%17%26%14%1
2362
August1st to 15thDay
36%8%12%25%16%1
2293
16th to endDay
35%7%12%20%18%4
2292
September1st to 15thDay
35%6%10%23%19%4
2160
16th to endDay
33%6%8%29%18%3
1849
October1st to 15thDay
35%49%24%19%5%
1650
16th to endDay
33%49%29%19%3
1760
November1st to 15thDay
33%311%22%22%5%
1604
16th to endDay
42%410%27%14%1
1539
December1st to 15thDay
49%313%23%10%
1440
16th to endDay
49%5%11%20%10%2
1536
January1st to 15thNight
44%410%23%15%
2447
16th to endNight
43%5%13%23%13%
2496
February1st to 15thNight
48%5%10%20%12%1
2275
16th to endNight
43%412%23%14%1
1893
March1st to 15thNight
48%39%22%14%1
2054
16th to endNight
50%7%11%21%8%
2061
April1st to 15thNight
37%5%11%25%17%3
1797
16th to endNight
40%5%9%26%15%2
1800
May1st to 15thNight
39%7%15%20%15%1
1659
16th to endNight
38%7%16%18%15%3
1498
June1st to 15thNight
28%5%13%29%19%2
1440
16th to endNight
25%7%14%27%19%5%
1440
July1st to 15thNight
30%6%11%31%15%4
1360
16th to endNight
24%7%16%31%18%1
1541
August1st to 15thNight
25%7%13%29%20%2
1642
16th to endNight
26%6%13%23%23%5%
1910
September1st to 15thNight
34%48%23%23%5%
1800
16th to endNight
30%5%9%25%25%3
1802
October1st to 15thNight
31%410%25%23%5%
1980
16th to endNight
35%38%26%19%5%
2112
November1st to 15thNight
31%411%24%21%7%
2253
16th to endNight
40%39%27%17%1
2419
December1st to 15thNight
48%5%12%20%12%
2520
16th to endNight
52%5%9%19%10%2
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
25%34%32%5%2
90
16th to endDay
17%36%33%12%
96
February1st to 15thDay
22%25%38%13%
90
16th to endDay
22%29%30%16%1
81
March1st to 15thDay
31%24%35%7%1
90
16th to endDay
29%21%33%14%1
147
April1st to 15thDay
21%12%34%23%7%
270
16th to endDay
24%15%32%18%8%
270
May1st to 15thDay
20%15%32%19%10%1
372
16th to endDay
16%12%33%24%8%4
517
June1st to 15thDay
10%5%29%35%15%4
495
16th to endDay
8%7%32%33%13%4
495
July1st to 15thDay
12%324%33%19%7%
475
16th to endDay
5%319%32%34%5%
515
August1st to 15thDay
12%213%34%30%6%
460
16th to endDay
9%122%29%27%8%
332
September1st to 15thDay
12%6%14%29%28%8%
315
16th to endDay
9%6%20%28%24%9%
215
October1st to 15thDay
15%8%37%25%10%2
90
16th to endDay
16%10%43%26%3
96
November1st to 15thDay
11%7%30%25%16%8%
90
16th to endDay
40%24%16%14%31
90
December1st to 15thDay
21%17%42%11%43
90
16th to endDay
31%25%33%9%1
96
January1st to 15thNight
21%21%35%17%4
900
16th to endNight
20%20%36%19%2
960
February1st to 15thNight
22%19%34%20%2
866
16th to endNight
21%18%40%18%1
631
March1st to 15thNight
26%14%37%18%3
614
16th to endNight
21%14%38%20%4
576
April1st to 15thNight
17%19%34%22%5%
539
16th to endNight
20%8%37%26%6%
540
May1st to 15thNight
16%9%32%29%11%
540
16th to endNight
13%6%32%30%16%1
563
June1st to 15thNight
9%324%40%20%1
540
16th to endNight
7%227%39%20%1
540
July1st to 15thNight
12%115%41%25%3
510
16th to endNight
5%114%40%35%2
564
August1st to 15thNight
6%110%41%36%2
539
16th to endNight
5%115%41%32%3
573
September1st to 15thNight
14%415%32%26%6%
540
16th to endNight
10%5%20%31%25%6%
589
October1st to 15thNight
11%7%30%26%20%3
660
16th to endNight
18%13%36%22%8%
704
November1st to 15thNight
15%15%33%24%11%
700
16th to endNight
22%18%33%20%5%
799
December1st to 15thNight
27%15%32%18%5%
900
16th to endNight
27%16%34%20%1
960
Page generated at 2017-09-18 07:38:23. Version ac04

Copyright 2017 A. Danko


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