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 2017-09-18.

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
48%
16009
22%
47200
15%
15973
56%
15973
29%
29506

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%
716
23%
2327
14%
716
45%
716
24%
1432
January16th to endNight
30%
768
25%
2496
13%
768
47%
768
27%
1499
February1st to 15thNight
28%
701
21%
2287
10%
701
43%
701
26%
1374
February16th to endNight
29%
631
29%
1850
14%
631
46%
631
32%
1098
March1st to 15thNight
37%
720
26%
1980
15%
720
48%
720
32%
1214
March16th to endNight
38%
768
22%
2112
13%
768
46%
768
30%
1355
April1st to 15thNight
39%
718
24%
1829
11%
718
47%
718
34%
1165
April16th to endNight
44%
720
22%
1800
10%
720
50%
720
34%
1177
May1st to 15thNight
45%
594
17%
1674
9%
594
50%
594
29%
1076
May16th to endNight
63%
561
17%
1683
13%
561
65%
561
31%
1041
June1st to 15thNight
62%
540
15%
1620
13%
540
63%
540
27%
1059
June16th to endNight
67%
540
16%
1620
15%
540
69%
540
29%
1044
July1st to 15thNight
71%
510
13%
1530
12%
510
72%
510
28%
1009
July16th to endNight
72%
564
14%
1595
12%
531
73%
531
28%
1019
August1st to 15thNight
67%
539
16%
1609
13%
536
68%
536
26%
1078
August16th to endNight
65%
758
17%
1904
12%
758
66%
758
29%
1301
September1st to 15thNight
68%
720
26%
1800
19%
720
69%
720
33%
1203
September16th to endNight
52%
669
16%
1878
13%
669
55%
669
22%
1323
October1st to 15thNight
53%
660
28%
1980
27%
660
59%
660
31%
1212
October16th to endNight
50%
704
34%
2112
25%
704
65%
704
40%
1163
November1st to 15thNight
50%
700
28%
2101
20%
700
60%
700
36%
1209
November16th to endNight
47%
720
26%
2361
19%
720
57%
720
31%
1347
December1st to 15thNight
40%
720
21%
2520
16%
720
50%
720
24%
1530
December16th to endNight
36%
768
25%
2532
16%
768
48%
768
23%
1578

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%
47574
Night
13%5%2442445%7%44%
47394
Cloud CoverDay
13%5%25%5%2445%7%43%
47584
Night
13%5%2442445%7%44%
47398
TransparencyDay
39%411%21%18%4
47285
Night
38%411%20%19%5%
47201
SeeingDay
16%417%24%22%13%
5777
Night
16%423%33%19%2
16018

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
19%5%34423435%44%
1611
16th to endDay
18%5%2331434447%
1728
February1st to 15thDay
17%424424345%46%
1575
16th to endDay
15%5%24313345%49%
1619
March1st to 15thDay
13%5%1422435%5%50%
1969
16th to endDay
16%6%25%424447%42%
2112
April1st to 15thDay
11%314313448%53%
2119
16th to endDay
12%6%25%42445%7%44%
2160
May1st to 15thDay
13%6%36%5%35%45%7%38%
2286
16th to endDay
9%425%6%35%6%5%8%42%
2431
June1st to 15thDay
9%5%25%5%35%6%6%8%40%
2340
16th to endDay
7%425%6%46%6%7%9%39%
2340
July1st to 15thDay
9%5%26%6%45%7%8%11%32%
2340
16th to endDay
6%5%36%7%47%6%7%11%32%
2465
August1st to 15thDay
10%5%36%6%46%6%7%11%30%
2334
16th to endDay
11%6%36%6%46%7%6%10%31%
2292
September1st to 15thDay
10%4245%45%45%8%44%
2160
16th to endDay
22%7%25%5%24346%33%
1804
October1st to 15thDay
18%6%23422335%48%
1650
16th to endDay
11%31442323458%
1760
November1st to 15thDay
14%422312236%54%
1757
16th to endDay
13%424423336%50%
1596
December1st to 15thDay
16%425%423346%45%
1440
16th to endDay
20%7%2431343442%
1686
January1st to 15thNight
15%6%24423445%45%
2327
16th to endNight
16%5%24323345%47%
2496
February1st to 15thNight
17%5%23423345%47%
2285
16th to endNight
13%31432445%6%48%
1850
March1st to 15thNight
13%424423346%50%
1968
16th to endNight
15%424425%446%45%
2112
April1st to 15thNight
12%6%24323447%47%
1829
16th to endNight
12%423435%5%5%6%46%
1800
May1st to 15thNight
11%5%26%4345%5%8%41%
1674
16th to endNight
11%4245%35%46%8%43%
1682
June1st to 15thNight
11%5%25%6%35%5%7%7%39%
1620
16th to endNight
6%5%26%5%35%6%7%10%40%
1620
July1st to 15thNight
6%5%25%5%36%6%10%11%36%
1620
16th to endNight
6%335%6%45%6%9%12%35%
1703
August1st to 15thNight
8%6%35%6%45%6%7%10%35%
1620
16th to endNight
9%5%26%5%446%8%12%34%
1904
September1st to 15thNight
11%425%4345%5%8%45%
1800
16th to endNight
19%6%36%5%24435%37%
1878
October1st to 15thNight
16%5%24313345%48%
1980
16th to endNight
12%314312345%57%
2112
November1st to 15thNight
13%413323346%52%
2101
16th to endNight
14%423324346%50%
2361
December1st to 15thNight
16%5%2442435%6%43%
2520
16th to endNight
19%6%24323345%42%
2532

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
18%6%25%32333444%
1611
16th to endDay
18%5%2332434447%
1728
February1st to 15thDay
17%5%245%24237%46%
1575
16th to endDay
15%5%23313435%49%
1619
March1st to 15thDay
13%5%1331345%6%50%
1980
16th to endDay
16%5%35%42435%6%43%
2111
April1st to 15thDay
10%31431335%7%53%
2119
16th to endDay
12%5%25%5%25%35%8%44%
2160
May1st to 15thDay
12%6%26%5%2446%8%38%
2286
16th to endDay
9%325%5%35%6%5%8%43%
2431
June1st to 15thDay
9%4345%36%5%7%8%41%
2340
16th to endDay
8%415%6%46%7%6%9%40%
2340
July1st to 15thDay
9%436%7%37%6%7%10%32%
2340
16th to endDay
6%5%25%7%47%7%7%10%34%
2465
August1st to 15thDay
11%5%27%6%46%7%7%9%31%
2334
16th to endDay
11%6%26%6%46%5%8%9%31%
2292
September1st to 15thDay
9%425%5%35%46%8%45%
2160
16th to endDay
22%7%36%5%24445%33%
1804
October1st to 15thDay
18%5%13323335%48%
1650
16th to endDay
11%42432323458%
1760
November1st to 15thDay
14%413322336%53%
1757
16th to endDay
13%5%15%423346%50%
1596
December1st to 15thDay
16%424423346%45%
1440
16th to endDay
20%7%2431333442%
1686
January1st to 15thNight
15%6%24423346%45%
2327
16th to endNight
16%5%15%324346%46%
2496
February1st to 15thNight
17%5%23423345%48%
2278
16th to endNight
13%42342445%6%49%
1850
March1st to 15thNight
13%42432335%6%50%
1980
16th to endNight
15%424424446%45%
2112
April1st to 15thNight
13%5%24323347%47%
1829
16th to endNight
12%324435%5%5%7%46%
1800
May1st to 15thNight
10%6%26%5%245%5%7%41%
1673
16th to endNight
11%4244445%6%7%44%
1682
June1st to 15thNight
10%5%35%6%35%5%6%9%39%
1620
16th to endNight
6%435%5%45%6%8%9%40%
1620
July1st to 15thNight
6%5%35%436%6%9%11%35%
1620
16th to endNight
6%335%5%46%5%8%12%36%
1703
August1st to 15thNight
9%5%26%6%36%7%7%10%35%
1620
16th to endNight
8%5%26%6%45%5%8%11%35%
1904
September1st to 15thNight
11%425%425%5%49%44%
1800
16th to endNight
18%7%36%434436%36%
1878
October1st to 15thNight
16%425%323345%48%
1980
16th to endNight
12%323312345%57%
2112
November1st to 15thNight
12%423333345%53%
2101
16th to endNight
14%423323346%50%
2361
December1st to 15thNight
16%5%1442445%6%43%
2520
16th to endNight
19%6%23423445%42%
2532

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
44%36%18%22%5%
1612
16th to endDay
40%26%21%21%8%
1728
February1st to 15thDay
41%16%20%23%5%
1577
16th to endDay
37%38%22%23%4
1619
March1st to 15thDay
34%310%23%21%6%
1980
16th to endDay
41%49%22%18%4
2112
April1st to 15thDay
30%48%28%21%7%
2119
16th to endDay
39%410%20%18%7%
2160
May1st to 15thDay
43%414%19%15%2
2286
16th to endDay
37%5%16%25%14%1
2435
June1st to 15thDay
38%5%17%22%15%1
2340
16th to endDay
36%7%15%22%14%2
2340
July1st to 15thDay
41%9%15%20%10%3
2201
16th to endDay
40%7%19%20%9%1
2310
August1st to 15thDay
44%7%14%19%11%1
2321
16th to endDay
45%7%13%21%10%1
2292
September1st to 15thDay
36%6%13%20%18%5%
2160
16th to endDay
51%47%16%16%3
1804
October1st to 15thDay
38%39%24%19%4
1650
16th to endDay
30%26%23%29%7%
1760
November1st to 15thDay
33%28%20%28%7%
1757
16th to endDay
37%28%21%25%6%
1596
December1st to 15thDay
39%311%22%17%5%
1440
16th to endDay
46%29%18%19%3
1686
January1st to 15thNight
40%39%15%24%5%
2327
16th to endNight
40%26%22%21%6%
2496
February1st to 15thNight
40%27%18%25%6%
2287
16th to endNight
34%39%24%22%6%
1850
March1st to 15thNight
35%49%21%21%7%
1980
16th to endNight
39%48%23%20%4
2112
April1st to 15thNight
36%39%20%23%6%
1829
16th to endNight
37%410%22%17%7%
1800
May1st to 15thNight
39%5%14%21%16%2
1674
16th to endNight
37%416%23%15%1
1684
June1st to 15thNight
41%6%16%19%14%1
1620
16th to endNight
35%9%16%19%14%3
1620
July1st to 15thNight
36%10%16%21%11%2
1530
16th to endNight
36%8%20%21%11%1
1595
August1st to 15thNight
41%7%17%21%9%2
1609
16th to endNight
39%8%17%23%9%1
1904
September1st to 15thNight
36%7%11%21%17%5%
1800
16th to endNight
49%5%10%14%17%3
1878
October1st to 15thNight
39%28%19%24%5%
1980
16th to endNight
29%27%22%26%10%
2112
November1st to 15thNight
33%27%16%29%10%
2101
16th to endNight
35%28%19%24%9%
2361
December1st to 15thNight
41%39%20%19%5%
2520
16th to endNight
43%29%19%19%5%
2532

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
24%10%33%26%5%
90
16th to endDay
19%20%36%16%6%
96
February1st to 15thDay
22%8%47%20%1
90
16th to endDay
18%10%32%29%9%
206
March1st to 15thDay
14%11%32%31%8%1
270
16th to endDay
18%6%32%29%10%2
288
April1st to 15thDay
15%9%27%25%17%5%
270
16th to endDay
17%5%20%27%24%4
270
May1st to 15thDay
17%6%24%27%14%9%
288
16th to endDay
14%312%21%28%20%
331
June1st to 15thDay
12%28%25%31%19%
315
16th to endDay
9%12%25%30%20%
315
July1st to 15thDay
7%17%24%35%23%
305
16th to endDay
10%1418%34%30%
326
August1st to 15thDay
15%17%16%35%23%
313
16th to endDay
13%16%20%29%29%
332
September1st to 15thDay
15%11%21%24%26%
315
16th to endDay
35%29%15%25%11%
262
October1st to 15thDay
23%414%26%20%9%
255
16th to endDay
11%418%32%23%9%
272
November1st to 15thDay
18%424%25%21%5%
265
16th to endDay
15%5%21%33%20%3
154
December1st to 15thDay
18%8%31%25%11%5%
60
16th to endDay
33%11%24%22%7%
89
January1st to 15thNight
19%10%30%31%7%
720
16th to endNight
20%7%36%29%5%
768
February1st to 15thNight
20%9%38%30%2
704
16th to endNight
15%9%41%29%3
631
March1st to 15thNight
16%8%33%35%5%
720
16th to endNight
17%5%33%34%8%
768
April1st to 15thNight
17%8%30%31%11%
718
16th to endNight
15%430%36%12%
720
May1st to 15thNight
13%5%27%40%13%
594
16th to endNight
14%14%36%31%2
563
June1st to 15thNight
13%112%36%32%2
540
16th to endNight
7%111%40%35%4
540
July1st to 15thNight
8%8%39%38%4
510
16th to endNight
7%27%32%44%6%
564
August1st to 15thNight
10%17%37%39%3
539
16th to endNight
11%16%30%41%8%
758
September1st to 15thNight
13%16%33%39%4
720
16th to endNight
23%211%30%30%1
669
October1st to 15thNight
19%218%31%24%3
660
16th to endNight
14%327%29%21%2
704
November1st to 15thNight
17%324%34%17%3
700
16th to endNight
17%5%25%32%17%1
720
December1st to 15thNight
19%5%26%37%9%
720
16th to endNight
21%6%28%34%8%
768
Page generated at 2017-09-19 07:32:30. Version ac04

Copyright 2017 A. Danko


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