W & B Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the W & B Observatory clear sky chart from 2005-11-06 to 2024-02-15.

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
46%
24033
30%
79510
17%
23923
60%
23923
33%
41643

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 10thNight
41%
758
33%
4010
19%
736
60%
736
21%
1987
January11th to 20thNight
38%
756
40%
3928
19%
740
55%
740
26%
1562
January21st to endNight
38%
830
20%
4306
17%
814
57%
814
16%
2584
February1st to 10thNight
36%
760
35%
2319
16%
760
57%
760
36%
1176
February11th to 20thNight
28%
722
31%
2152
13%
718
46%
718
33%
1148
February21st to endNight
23%
592
35%
1776
9%
592
48%
592
29%
925
March1st to 10thNight
30%
720
32%
2062
11%
720
50%
720
30%
1160
March11th to 20thNight
40%
720
30%
1980
12%
720
53%
720
33%
1125
March21st to endNight
30%
712
37%
2098
13%
712
53%
712
36%
1050
April1st to 10thNight
24%
540
37%
1800
12%
540
55%
540
32%
836
April11th to 20thNight
27%
533
33%
1777
13%
533
54%
533
31%
848
April21st to endNight
29%
540
38%
1800
16%
540
59%
540
33%
834
May1st to 10thNight
32%
540
33%
1800
17%
540
58%
540
41%
809
May11th to 20thNight
45%
531
28%
1770
19%
531
61%
531
39%
855
May21st to endNight
40%
583
32%
1783
14%
582
60%
582
44%
839
June1st to 10thNight
55%
540
26%
1440
17%
540
61%
540
40%
826
June11th to 20thNight
54%
540
37%
1440
21%
540
66%
540
56%
724
June21st to endNight
57%
540
21%
1491
16%
540
61%
540
33%
927
July1st to 10thNight
64%
525
18%
1575
15%
525
66%
525
32%
945
July11th to 20thNight
72%
522
21%
1566
18%
522
74%
522
33%
929
July21st to endNight
70%
591
17%
1676
15%
558
71%
558
30%
1026
August1st to 10thNight
74%
537
15%
1779
17%
534
75%
534
29%
1074
August11th to 20thNight
68%
534
20%
1778
20%
534
72%
534
29%
1062
August21st to endNight
70%
594
21%
1980
21%
594
74%
594
31%
1142
September1st to 10thNight
66%
540
23%
1800
21%
540
71%
540
28%
1056
September11th to 20thNight
69%
540
33%
1800
30%
540
75%
540
35%
989
September21st to endNight
64%
540
34%
1978
29%
540
74%
540
41%
922
October1st to 10thNight
52%
710
29%
2150
18%
710
62%
710
38%
1148
October11th to 20thNight
54%
720
38%
2160
20%
720
67%
720
48%
1042
October21st to endNight
54%
792
39%
2376
22%
792
66%
792
46%
1135
November1st to 10thNight
52%
740
40%
2220
21%
740
67%
740
45%
1057
November11th to 20thNight
55%
897
38%
2417
20%
897
66%
897
51%
1217
November21st to endNight
40%
950
34%
2470
15%
950
51%
950
36%
1414
December1st to 10thNight
42%
950
35%
2470
16%
950
55%
950
35%
1421
December11th to 20thNight
31%
949
28%
3087
12%
948
45%
948
26%
1760
December21st to endNight
34%
945
20%
4496
15%
931
49%
931
22%
2089

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
45%25%43445%7%55%
78549
Night
35%2442445%7%58%
80562
TransparencyDay
25%9%7%17%25%16%
78277
Night
21%10%7%17%27%18%
80356
SeeingDay
10%21%27%24%14%3
13694
Night
8%16%25%30%18%3
24052

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 10thDay
28%7%11%42334452%
2858
11th to 20thDay
25%34323245%67%
2860
21st to endDay
5%14%6%6%435%46%444%
3146
February1st to 10thDay
38%23323436%62%
1857
11th to 20thDay
5%7%34323336%60%
1789
21st to endDay
26%24423336%64%
1480
March1st to 10thDay
48%346%2445%6%55%
1894
11th to 20thDay
5%8%25%423346%59%
1980
21st to endDay
25%2443445%7%61%
2256
April1st to 10thDay
242332345%6%65%
2160
11th to 20thDay
35%24422245%68%
2131
21st to endDay
3413313345%70%
2160
May1st to 10thDay
2223424435%70%
2160
11th to 20thDay
4423436%45%8%58%
2125
21st to endDay
221322335%6%71%
2485
June1st to 10thDay
23123345%6%8%62%
2520
11th to 20thDay
321222336%8%67%
2520
21st to endDay
6%4245%46%7%8%8%46%
2461
July1st to 10thDay
9%6%37%7%47%7%8%9%33%
2340
11th to 20thDay
6%5%36%8%5%8%8%8%10%33%
2340
21st to endDay
9%5%36%8%6%9%10%9%10%24%
2541
August1st to 10thDay
9%6%36%8%5%8%7%8%10%30%
2149
11th to 20thDay
8%6%37%8%47%7%9%8%33%
2123
21st to endDay
6%6%37%7%5%7%7%7%9%36%
2376
September1st to 10thDay
10%7%37%6%35%5%7%8%40%
2160
11th to 20thDay
8%5%25%5%345%6%7%51%
2160
21st to endDay
441432445%6%63%
1980
October1st to 10thDay
5%5%25%424346%59%
1806
11th to 20thDay
3313323346%69%
1800
21st to endDay
2312223335%74%
1980
November1st to 10thDay
1323423347%68%
1859
11th to 20thDay
2424323447%66%
1742
21st to endDay
46%2433346%7%57%
1710
December1st to 10thDay
5%7%23325%35%7%59%
1710
11th to 20thDay
48%35%5%335%5%6%53%
1963
21st to endDay
26%34423346%63%
2968
January1st to 10thNight
25%5%9%43444455%
4097
11th to 20thNight
24235%33335%68%
4110
21st to endNight
412%34436%7%6%6%44%
4521
February1st to 10thNight
28%2442335%7%61%
2319
11th to 20thNight
5%7%2322335%6%60%
2152
21st to endNight
36%3432345%6%61%
1776
March1st to 10thNight
48%245%34446%57%
2062
11th to 20thNight
48%2342445%6%58%
1980
21st to endNight
35%23323346%65%
2098
April1st to 10thNight
23233245%6%6%65%
1800
11th to 20thNight
45%23323447%64%
1777
21st to endNight
35%12323345%68%
1800
May1st to 10thNight
3212323347%70%
1800
11th to 20thNight
2323324348%65%
1770
21st to endNight
221232445%6%69%
1783
June1st to 10thNight
32245%36%6%6%8%55%
1440
11th to 20thNight
311222435%8%68%
1440
21st to endNight
4425%7%46%6%6%8%46%
1491
July1st to 10thNight
6%426%445%6%7%11%45%
1620
11th to 20thNight
5%215%437%8%8%12%43%
1620
21st to endNight
44245%45%8%9%15%39%
1775
August1st to 10thNight
4325%7%36%7%11%13%38%
1790
11th to 20thNight
6%325%5%37%6%8%12%42%
1778
21st to endNight
3325%6%36%7%7%10%47%
1980
September1st to 10thNight
5%5%25%5%45%6%7%8%48%
1800
11th to 20thNight
6%3144346%5%8%56%
1800
21st to endNight
33123245%5%7%65%
1978
October1st to 10thNight
34134236%6%9%59%
2150
11th to 20thNight
2323323236%73%
2160
21st to endNight
421331333472%
2376
November1st to 10thNight
231322335%7%68%
2220
11th to 20thNight
232232345%7%68%
2417
21st to endNight
46%25%42445%6%59%
2470
December1st to 10thNight
5%7%2332445%7%58%
2470
11th to 20thNight
26%36%5%34447%54%
3229
21st to endNight
39%34423345%60%
4713

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 10thDay
18%21%410%33%14%
2859
11th to 20thDay
14%11%5%20%35%16%
2860
21st to endDay
27%18%313%25%13%
3146
February1st to 10thDay
21%7%5%16%29%23%
1857
11th to 20thDay
24%8%417%25%23%
1799
21st to endDay
19%7%310%28%32%
1480
March1st to 10thDay
27%9%5%14%27%19%
1894
11th to 20thDay
27%6%413%30%21%
1980
21st to endDay
19%9%416%35%17%
2256
April1st to 10thDay
17%6%5%17%35%20%
2160
11th to 20thDay
19%4313%32%28%
2134
21st to endDay
17%6%317%36%21%
2160
May1st to 10thDay
18%5%314%31%29%
2160
11th to 20thDay
24%8%7%15%28%18%
2128
21st to endDay
16%36%20%30%25%
2485
June1st to 10thDay
19%6%12%21%24%18%
2520
11th to 20thDay
16%45%17%26%32%
2520
21st to endDay
32%9%9%18%20%12%
2461
July1st to 10thDay
44%10%14%13%13%6%
2276
11th to 20thDay
40%11%16%20%10%3
2271
21st to endDay
44%15%20%15%5%
2400
August1st to 10thDay
45%10%16%21%6%1
2136
11th to 20thDay
44%11%15%17%10%2
2123
21st to endDay
42%10%15%20%12%1
2376
September1st to 10thDay
41%8%10%17%17%7%
2160
11th to 20thDay
33%7%9%20%25%6%
2160
21st to endDay
23%6%9%22%30%10%
1980
October1st to 10thDay
25%6%9%18%23%17%
1806
11th to 20thDay
20%4415%29%29%
1800
21st to endDay
15%5%422%35%20%
1980
November1st to 10thDay
17%5%421%34%19%
1859
11th to 20thDay
18%6%5%19%29%24%
1742
21st to endDay
22%8%5%15%30%20%
1710
December1st to 10thDay
21%8%5%23%28%15%
1710
11th to 20thDay
27%10%6%14%27%16%
1961
21st to endDay
17%11%415%26%27%
2968
January1st to 10thNight
18%18%314%30%18%
4097
11th to 20thNight
15%10%421%37%14%
4110
21st to endNight
26%19%313%26%13%
4521
February1st to 10thNight
19%8%5%16%25%26%
2319
11th to 20thNight
20%10%415%27%24%
2152
21st to endNight
19%9%311%28%30%
1776
March1st to 10thNight
24%8%414%28%22%
2062
11th to 20thNight
24%9%413%33%17%
1980
21st to endNight
15%10%5%18%34%19%
2098
April1st to 10thNight
17%7%5%15%37%19%
1800
11th to 20thNight
16%9%5%14%30%24%
1777
21st to endNight
17%6%419%35%18%
1800
May1st to 10thNight
16%5%5%12%34%27%
1800
11th to 20thNight
20%6%7%20%28%19%
1771
21st to endNight
16%5%7%18%29%25%
1784
June1st to 10thNight
24%8%9%19%21%20%
1440
11th to 20thNight
16%4418%21%37%
1440
21st to endNight
34%7%11%17%15%16%
1491
July1st to 10thNight
30%10%17%19%17%6%
1575
11th to 20thNight
27%12%16%28%13%4
1566
21st to endNight
28%14%26%22%9%1
1676
August1st to 10thNight
31%12%22%24%8%2
1779
11th to 20thNight
30%12%21%24%11%2
1778
21st to endNight
29%10%19%23%17%1
1980
September1st to 10thNight
31%10%12%17%20%9%
1800
11th to 20thNight
26%9%9%20%25%11%
1800
21st to endNight
19%7%9%22%33%10%
1978
October1st to 10thNight
22%8%9%19%26%16%
2150
11th to 20thNight
17%5%417%28%29%
2160
21st to endNight
16%5%418%33%24%
2376
November1st to 10thNight
16%5%420%33%22%
2220
11th to 20thNight
14%7%5%18%31%24%
2417
21st to endNight
24%7%416%29%19%
2470
December1st to 10thNight
21%10%5%18%26%19%
2470
11th to 20thNight
25%9%8%15%22%20%
3230
21st to endNight
21%10%413%27%24%
4713

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 10thDay
19%31%24%23%3
270
11th to 20thDay
17%28%24%24%6%1
270
21st to endDay
10%33%30%16%9%
297
February1st to 10thDay
18%29%27%21%6%1
270
11th to 20thDay
18%37%28%13%4
252
21st to endDay
25%39%21%12%31
198
March1st to 10thDay
19%36%30%11%41
334
11th to 20thDay
21%29%30%13%6%1
420
21st to endDay
16%43%30%8%21
462
April1st to 10thDay
10%40%35%12%21
420
11th to 20thDay
15%41%31%11%21
412
21st to endDay
13%40%32%12%3
420
May1st to 10thDay
7%35%43%13%3
423
11th to 20thDay
427%42%20%6%1
448
21st to endDay
425%42%20%8%1
557
June1st to 10thDay
5%9%32%35%17%2
630
11th to 20thDay
410%34%32%18%3
630
21st to endDay
5%5%28%35%24%4
571
July1st to 10thDay
9%222%33%27%7%
446
11th to 20thDay
8%10%35%37%9%
444
21st to endDay
7%19%31%41%11%
487
August1st to 10thDay
8%213%34%34%9%
447
11th to 20thDay
9%419%35%25%8%
437
21st to endDay
8%314%34%33%8%
495
September1st to 10thDay
11%5%21%36%21%6%
450
11th to 20thDay
7%5%30%34%18%6%
450
21st to endDay
411%36%29%13%6%
270
October1st to 10thDay
11%16%28%25%16%4
270
11th to 20thDay
7%19%32%25%13%4
270
21st to endDay
10%24%26%22%14%4
297
November1st to 10thDay
7%35%30%17%8%3
270
11th to 20thDay
7%21%30%25%14%3
270
21st to endDay
14%33%26%19%7%1
270
December1st to 10thDay
16%30%24%22%5%1
270
11th to 20thDay
11%36%29%19%6%
270
21st to endDay
19%34%27%14%5%
297
January1st to 10thNight
8%19%27%29%13%4
760
11th to 20thNight
10%20%28%27%12%3
760
21st to endNight
10%22%26%26%14%1
836
February1st to 10thNight
9%22%30%25%11%2
760
11th to 20thNight
13%23%34%23%7%1
722
21st to endNight
12%32%31%19%5%1
592
March1st to 10thNight
12%28%26%24%9%1
720
11th to 20thNight
12%21%25%29%12%
720
21st to endNight
10%28%31%23%8%1
712
April1st to 10thNight
6%33%34%21%6%
540
11th to 20thNight
10%29%32%22%7%1
533
21st to endNight
10%29%31%24%6%
540
May1st to 10thNight
5%23%38%27%7%
540
11th to 20thNight
419%27%33%14%2
531
21st to endNight
316%37%33%9%1
584
June1st to 10thNight
39%28%42%18%1
540
11th to 20thNight
37%34%37%18%2
540
21st to endNight
6%421%42%24%3
540
July1st to 10thNight
6%219%39%31%4
525
11th to 20thNight
3212%34%40%8%
522
21st to endNight
5%214%33%34%13%
591
August1st to 10thNight
5%19%28%43%15%
537
11th to 20thNight
6%312%27%41%11%
534
21st to endNight
3313%32%41%8%
594
September1st to 10thNight
5%416%32%36%6%
540
11th to 20thNight
7%317%37%32%4
540
21st to endNight
47%21%36%27%4
540
October1st to 10thNight
5%10%28%35%19%3
710
11th to 20thNight
5%10%28%34%19%3
720
21st to endNight
7%16%21%27%22%8%
792
November1st to 10thNight
6%18%21%31%20%5%
740
11th to 20thNight
5%13%24%34%19%4
897
21st to endNight
10%20%24%25%17%4
950
December1st to 10thNight
12%21%22%29%14%2
950
11th to 20thNight
9%25%29%26%10%1
950
21st to endNight
15%20%26%27%10%1
950
Page generated at 2024-02-12 04:35:31. Version ac07

Copyright 2024 Attilla Danko


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