3B Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the 3B Observatory clear sky chart from 2005-11-05 to 2018-10-08.

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
49%
17428
22%
51387
15%
17392
56%
17392
29%
32128

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
33%
776
23%
2522
14%
776
46%
776
25%
1542
January16th to endNight
32%
832
26%
2704
15%
832
48%
832
28%
1612
February1st to 15thNight
28%
761
20%
2482
10%
761
42%
761
25%
1517
February16th to endNight
30%
683
28%
2003
14%
683
46%
683
31%
1199
March1st to 15thNight
36%
780
26%
2145
15%
780
47%
780
32%
1316
March16th to endNight
37%
832
22%
2288
13%
832
45%
832
30%
1473
April1st to 15thNight
39%
778
24%
1982
11%
778
47%
778
34%
1258
April16th to endNight
44%
771
22%
1927
9%
771
50%
771
35%
1256
May1st to 15thNight
48%
644
19%
1814
10%
644
52%
644
31%
1144
May16th to endNight
61%
609
16%
1827
12%
609
63%
609
28%
1168
June1st to 15thNight
62%
585
15%
1755
14%
585
63%
585
27%
1138
June16th to endNight
68%
585
16%
1755
15%
585
70%
585
29%
1131
July1st to 15thNight
72%
555
14%
1665
13%
555
73%
555
29%
1077
July16th to endNight
68%
612
14%
1739
11%
579
69%
579
26%
1136
August1st to 15thNight
67%
575
16%
1715
13%
572
68%
572
26%
1152
August16th to endNight
67%
821
16%
2063
11%
821
68%
821
30%
1391
September1st to 15thNight
67%
780
24%
1950
18%
780
68%
780
32%
1323
September16th to endNight
54%
777
17%
2180
15%
777
57%
777
23%
1525
October1st to 15thNight
55%
752
27%
2255
27%
752
61%
752
32%
1367
October16th to endNight
50%
768
36%
2304
26%
768
66%
768
41%
1252
November1st to 15thNight
49%
760
27%
2281
20%
760
59%
760
35%
1328
November16th to endNight
48%
780
27%
2558
21%
780
59%
780
32%
1442
December1st to 15thNight
39%
780
22%
2730
15%
780
50%
780
23%
1667
December16th to endNight
37%
832
24%
2743
16%
832
48%
832
23%
1714

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
13%5%25%5%3445%7%42%
51728
Night
13%5%2442445%7%44%
51581
Cloud CoverDay
13%5%25%5%2445%7%42%
51738
Night
13%42442445%7%44%
51585
TransparencyDay
39%5%11%21%17%4
51439
Night
38%411%20%19%5%
51388
SeeingDay
16%417%24%22%13%
7177
Night
15%422%33%20%2
17437

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
18%5%34423435%44%
1746
16th to endDay
18%5%23313345%48%
1872
February1st to 15thDay
18%424423345%45%
1710
16th to endDay
15%5%24323345%48%
1752
March1st to 15thDay
13%5%1432335%5%50%
2134
16th to endDay
16%6%25%324447%42%
2288
April1st to 15thDay
11%314313448%52%
2296
16th to endDay
12%6%25%42445%7%44%
2308
May1st to 15thDay
12%6%36%5%35%5%5%7%39%
2477
16th to endDay
10%5%25%6%35%6%5%8%40%
2639
June1st to 15thDay
9%425%5%35%6%7%8%40%
2535
16th to endDay
7%425%5%36%6%7%9%40%
2535
July1st to 15thDay
9%5%26%6%45%7%8%11%33%
2535
16th to endDay
7%5%36%7%46%7%7%11%31%
2673
August1st to 15thDay
10%5%36%7%46%6%7%11%30%
2481
16th to endDay
11%5%36%6%46%6%6%9%32%
2484
September1st to 15thDay
10%4145%45%5%5%9%43%
2340
16th to endDay
20%6%25%5%35%346%34%
2106
October1st to 15thDay
16%6%13422335%49%
1875
16th to endDay
11%31441222459%
1920
November1st to 15thDay
14%413313346%52%
1907
16th to endDay
12%424423345%52%
1728
December1st to 15thDay
17%425%423346%45%
1560
16th to endDay
20%6%24423435%42%
1827
January1st to 15thNight
14%6%24423445%46%
2522
16th to endNight
16%5%24323345%48%
2704
February1st to 15thNight
18%6%23423345%46%
2480
16th to endNight
14%31432445%7%48%
2003
March1st to 15thNight
14%423423346%50%
2133
16th to endNight
15%424424446%45%
2288
April1st to 15thNight
12%5%24323447%48%
1982
16th to endNight
12%4234345%5%6%46%
1927
May1st to 15thNight
10%5%26%43445%8%43%
1814
16th to endNight
12%4245%35%46%8%41%
1826
June1st to 15thNight
11%425%6%35%5%7%7%39%
1755
16th to endNight
6%5%26%5%35%6%7%10%39%
1755
July1st to 15thNight
6%5%25%5%36%6%9%11%37%
1755
16th to endNight
6%346%6%45%6%9%12%34%
1847
August1st to 15thNight
8%5%35%6%46%6%7%10%34%
1726
16th to endNight
8%5%25%5%45%6%7%12%36%
2063
September1st to 15thNight
11%425%5%345%5%8%43%
1950
16th to endNight
17%6%36%5%34446%36%
2180
October1st to 15thNight
15%5%24323345%49%
2255
16th to endNight
11%314312345%58%
2304
November1st to 15thNight
14%413323346%51%
2281
16th to endNight
13%423324346%51%
2558
December1st to 15thNight
17%5%24424346%43%
2730
16th to endNight
18%6%24423346%42%
2743

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
17%6%25%42433445%
1746
16th to endDay
18%5%13323345%48%
1872
February1st to 15thDay
17%5%245%24237%45%
1710
16th to endDay
15%5%33323435%48%
1752
March1st to 15thDay
14%5%1331345%5%50%
2145
16th to endDay
16%5%3442435%6%43%
2287
April1st to 15thDay
11%31432345%8%52%
2296
16th to endDay
12%5%25%5%25%35%7%43%
2308
May1st to 15thDay
12%6%26%5%3446%8%39%
2477
16th to endDay
10%425%6%35%5%5%8%40%
2639
June1st to 15thDay
9%425%5%35%5%7%8%40%
2535
16th to endDay
7%3145%46%7%6%9%40%
2535
July1st to 15thDay
9%426%7%36%6%7%10%34%
2535
16th to endDay
7%6%25%7%47%7%7%10%32%
2673
August1st to 15thDay
11%5%26%6%46%7%7%9%31%
2481
16th to endDay
11%5%26%6%46%5%8%9%32%
2484
September1st to 15thDay
10%4245%35%5%6%8%43%
2340
16th to endDay
20%7%25%5%24446%34%
2106
October1st to 15thDay
16%5%24323345%49%
1875
16th to endDay
10%41431323459%
1920
November1st to 15thDay
14%413322347%51%
1907
16th to endDay
12%414423346%51%
1728
December1st to 15thDay
18%424423346%45%
1560
16th to endDay
20%7%24423335%42%
1827
January1st to 15thNight
15%6%24423345%45%
2522
16th to endNight
16%5%15%324345%47%
2704
February1st to 15thNight
17%5%23424345%47%
2473
16th to endNight
13%41342445%6%48%
2003
March1st to 15thNight
14%424323346%50%
2145
16th to endNight
15%424424446%45%
2288
April1st to 15thNight
13%5%24323347%47%
1982
16th to endNight
12%324435%5%47%46%
1927
May1st to 15thNight
10%6%26%5%245%5%8%42%
1813
16th to endNight
12%425%5%445%6%7%42%
1826
June1st to 15thNight
10%425%6%35%5%6%9%39%
1755
16th to endNight
6%435%445%6%8%10%40%
1755
July1st to 15thNight
5%5%35%436%6%9%11%37%
1755
16th to endNight
6%436%6%46%6%8%12%34%
1847
August1st to 15thNight
9%5%26%6%36%7%7%10%34%
1726
16th to endNight
8%5%26%5%45%5%8%11%36%
2063
September1st to 15thNight
11%425%435%5%49%42%
1950
16th to endNight
17%6%36%434446%36%
2180
October1st to 15thNight
15%425%323345%49%
2255
16th to endNight
11%313312345%58%
2304
November1st to 15thNight
13%423333345%52%
2281
16th to endNight
13%323323346%51%
2558
December1st to 15thNight
17%5%1442435%5%43%
2730
16th to endNight
19%6%24423446%42%
2743

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
43%36%18%22%5%
1747
16th to endDay
39%26%21%20%8%
1872
February1st to 15thDay
41%27%20%22%5%
1712
16th to endDay
38%38%21%22%4
1752
March1st to 15thDay
35%39%23%22%6%
2145
16th to endDay
41%49%22%18%4
2288
April1st to 15thDay
30%49%28%20%6%
2296
16th to endDay
39%49%19%18%7%
2308
May1st to 15thDay
41%414%20%15%2
2477
16th to endDay
40%5%15%23%13%1
2643
June1st to 15thDay
37%5%17%23%14%1
2535
16th to endDay
35%8%15%23%14%2
2535
July1st to 15thDay
40%8%14%22%10%3
2396
16th to endDay
42%7%19%19%9%1
2518
August1st to 15thDay
44%8%15%19%11%1
2468
16th to endDay
44%7%14%21%10%1
2484
September1st to 15thDay
37%7%13%19%17%4
2340
16th to endDay
49%48%18%16%2
2106
October1st to 15thDay
37%310%24%19%4
1875
16th to endDay
30%25%23%30%7%
1920
November1st to 15thDay
34%28%20%26%7%
1907
16th to endDay
35%28%20%26%7%
1728
December1st to 15thDay
40%311%23%16%5%
1560
16th to endDay
46%29%19%18%3
1827
January1st to 15thNight
40%39%16%24%6%
2522
16th to endNight
39%26%22%21%7%
2704
February1st to 15thNight
41%27%18%24%5%
2482
16th to endNight
35%39%23%21%6%
2003
March1st to 15thNight
35%38%20%22%8%
2145
16th to endNight
39%48%22%20%4
2288
April1st to 15thNight
36%39%21%22%6%
1982
16th to endNight
36%410%21%17%8%
1927
May1st to 15thNight
37%5%14%21%18%2
1814
16th to endNight
39%6%15%22%14%1
1828
June1st to 15thNight
40%6%17%20%13%1
1755
16th to endNight
35%10%16%20%14%3
1755
July1st to 15thNight
35%10%16%22%11%2
1665
16th to endNight
38%8%19%20%10%1
1739
August1st to 15thNight
41%7%17%22%8%1
1715
16th to endNight
38%8%18%23%9%1
2063
September1st to 15thNight
37%8%12%20%16%5%
1950
16th to endNight
47%5%11%15%16%2
2180
October1st to 15thNight
37%39%20%24%4
2255
16th to endNight
28%27%22%28%10%
2304
November1st to 15thNight
34%38%18%26%9%
2281
16th to endNight
34%27%18%25%10%
2558
December1st to 15thNight
42%38%21%19%5%
2730
16th to endNight
43%29%20%18%5%
2743

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%14%32%29%5%
135
16th to endDay
19%18%31%18%11%
144
February1st to 15thDay
22%10%42%22%2
135
16th to endDay
20%9%30%29%9%
255
March1st to 15thDay
17%13%33%27%6%
330
16th to endDay
19%6%34%28%8%2
352
April1st to 15thDay
16%9%27%26%16%4
330
16th to endDay
17%5%21%26%24%4
319
May1st to 15thDay
14%5%22%30%16%9%
348
16th to endDay
18%311%21%26%17%
395
June1st to 15thDay
11%27%25%32%21%
375
16th to endDay
9%11%24%31%22%
375
July1st to 15thDay
7%16%23%35%26%
365
16th to endDay
13%16%19%33%26%
390
August1st to 15thDay
15%18%16%35%22%
358
16th to endDay
12%15%21%28%30%
396
September1st to 15thDay
16%19%18%23%31%
375
16th to endDay
29%28%19%27%13%
374
October1st to 15thDay
20%312%23%25%15%
345
16th to endDay
10%5%20%30%22%10%
336
November1st to 15thDay
19%424%26%20%6%
325
16th to endDay
12%7%20%35%20%4
196
December1st to 15thDay
23%5%32%22%13%3
90
16th to endDay
29%10%27%26%5%
134
January1st to 15thNight
18%9%31%31%8%
780
16th to endNight
20%7%35%29%6%
832
February1st to 15thNight
20%9%37%30%2
764
16th to endNight
16%9%39%29%4
683
March1st to 15thNight
17%9%32%34%5%
780
16th to endNight
17%6%33%34%7%
832
April1st to 15thNight
17%8%30%32%10%
778
16th to endNight
15%429%36%12%
771
May1st to 15thNight
12%426%41%14%
644
16th to endNight
15%13%37%30%2
611
June1st to 15thNight
13%111%36%33%3
585
16th to endNight
7%110%40%36%4
585
July1st to 15thNight
7%8%39%39%4
555
16th to endNight
8%17%32%44%5%
612
August1st to 15thNight
10%17%36%39%4
575
16th to endNight
11%17%30%40%8%
821
September1st to 15thNight
13%26%31%39%7%
780
16th to endNight
22%29%30%32%2
777
October1st to 15thNight
18%217%30%27%4
752
16th to endNight
14%426%29%22%2
768
November1st to 15thNight
17%323%34%18%3
760
16th to endNight
16%5%25%32%19%1
780
December1st to 15thNight
20%6%26%36%9%
780
16th to endNight
21%6%28%35%8%
832
Page generated at 2018-10-09 10:39:09. Version ac04

Copyright 2018 A. Danko


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