Williams Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Williams Observatory clear sky chart from 2005-11-06 to 2024-02-20.

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
53%
23143
18%
80368
18%
23038
56%
23038
22%
52419

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
30%
753
7%
4387
9%
736
34%
736
6%
3420
January11th to 20thNight
38%
755
12%
4177
15%
740
44%
740
9%
3126
January21st to endNight
42%
834
12%
4484
18%
814
46%
814
9%
3454
February1st to 10thNight
42%
760
15%
2470
15%
760
45%
760
18%
1715
February11th to 20thNight
40%
739
14%
2396
14%
738
43%
738
16%
1721
February21st to endNight
40%
592
13%
1831
11%
592
44%
592
15%
1357
March1st to 10thNight
35%
720
10%
2003
8%
720
38%
720
16%
1504
March11th to 20thNight
40%
720
13%
1980
10%
720
43%
720
17%
1505
March21st to endNight
34%
792
8%
2172
5%
792
36%
792
14%
1717
April1st to 10thNight
36%
720
11%
1800
6%
720
38%
720
17%
1405
April11th to 20thNight
42%
711
15%
1779
8%
711
43%
711
20%
1327
April21st to endNight
49%
595
12%
1675
7%
595
51%
595
23%
1147
May1st to 10thNight
54%
540
15%
1620
11%
540
56%
540
28%
1025
May11th to 20thNight
57%
355
16%
1417
19%
355
59%
355
18%
875
May21st to endNight
55%
389
11%
1552
12%
388
59%
388
20%
956
June1st to 10thNight
57%
360
12%
1440
17%
360
60%
360
22%
871
June11th to 20thNight
60%
360
17%
1440
23%
360
65%
360
29%
741
June21st to endNight
78%
360
26%
1440
39%
360
81%
360
46%
600
July1st to 10thNight
84%
350
26%
1400
40%
350
86%
350
53%
536
July11th to 20thNight
85%
348
34%
1392
47%
348
89%
348
59%
499
July21st to endNight
90%
563
38%
1648
38%
532
93%
532
76%
636
August1st to 10thNight
88%
537
34%
1602
31%
534
89%
534
64%
703
August11th to 20thNight
88%
534
39%
1602
32%
534
90%
534
69%
668
August21st to endNight
82%
751
40%
1939
28%
751
83%
751
72%
901
September1st to 10thNight
80%
720
40%
1800
26%
720
82%
720
72%
857
September11th to 20thNight
76%
720
33%
1836
25%
720
77%
720
53%
1002
September21st to endNight
67%
720
28%
2043
26%
720
70%
720
42%
1134
October1st to 10thNight
63%
720
22%
2160
20%
720
69%
720
39%
1189
October11th to 20thNight
63%
720
25%
2160
27%
720
66%
720
34%
1270
October21st to endNight
55%
792
23%
2376
23%
792
58%
792
28%
1492
November1st to 10thNight
45%
740
16%
2356
17%
740
47%
740
18%
1678
November11th to 20thNight
40%
760
12%
2660
11%
760
44%
760
14%
1918
November21st to endNight
43%
760
14%
2660
13%
760
47%
760
19%
1794
December1st to 10thNight
43%
760
18%
2660
16%
760
48%
760
19%
1759
December11th to 20thNight
33%
760
12%
3214
10%
759
37%
759
10%
2450
December21st to endNight
27%
833
8%
4797
11%
817
30%
817
7%
3467

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
8%8%36%6%35%45%7%46%
78432
Night
9%9%36%6%45%5%5%7%42%
80771
TransparencyDay
38%7%11%20%17%6%
78145
Night
41%8%11%19%14%6%
80579
SeeingDay
14%15%27%26%13%3
13956
Night
13%10%19%31%24%4
23163

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
10%17%6%11%9%5%6%5%5%7%19%
2539
11th to 20thDay
16%13%49%7%5%6%5%5%6%23%
2746
21st to endDay
11%12%6%10%8%5%6%5%5%7%25%
3146
February1st to 10thDay
6%11%5%9%7%5%7%7%7%7%29%
1710
11th to 20thDay
9%11%47%7%46%6%6%7%34%
1657
21st to endDay
9%16%5%8%8%46%5%5%6%28%
1423
March1st to 10thDay
9%14%5%10%9%5%7%5%6%7%24%
1952
11th to 20thDay
8%12%48%8%47%7%7%7%27%
1980
21st to endDay
11%13%5%10%8%47%6%7%8%22%
2182
April1st to 10thDay
11%13%5%9%10%5%5%5%47%26%
2160
11th to 20thDay
14%12%5%7%6%35%5%5%8%30%
2129
21st to endDay
10%10%48%6%345%5%9%38%
2281
May1st to 10thDay
10%8%36%6%3345%8%45%
2340
11th to 20thDay
11%8%36%6%35%46%7%42%
2472
21st to endDay
13%9%46%5%3445%8%38%
2723
June1st to 10thDay
11%7%35%5%35%45%8%44%
2520
11th to 20thDay
8%5%3443445%6%54%
2520
21st to endDay
4313323335%69%
2520
July1st to 10thDay
2212212235%78%
2520
11th to 20thDay
321221223378%
2520
21st to endDay
121221232381%
2570
August1st to 10thDay
2112213345%77%
2328
11th to 20thDay
111221223479%
2298
21st to endDay
3212212235%78%
2417
September1st to 10thDay
3212212235%77%
2160
11th to 20thDay
43233245%5%7%60%
2121
21st to endDay
5%315%43335%7%60%
1916
October1st to 10thDay
8%5%2432345%7%56%
1800
11th to 20thDay
6%5%26%5%35%5%5%7%50%
1800
21st to endDay
9%6%27%6%45%5%48%45%
1980
November1st to 10thDay
14%9%47%7%45%5%5%8%31%
1720
11th to 20thDay
11%9%38%6%47%7%8%8%28%
1520
21st to endDay
9%11%36%7%47%7%7%7%33%
1520
December1st to 10thDay
8%7%37%7%46%7%6%7%37%
1520
11th to 20thDay
12%11%49%7%5%6%6%6%9%24%
1928
21st to endDay
11%13%5%10%10%6%7%7%6%7%18%
2794
January1st to 10thNight
12%14%5%10%9%5%6%7%7%6%19%
4406
11th to 20thNight
14%13%5%10%7%5%5%5%5%7%25%
4205
21st to endNight
13%14%5%10%9%5%5%5%5%6%23%
4521
February1st to 10thNight
8%11%47%7%46%6%7%8%32%
2470
11th to 20thNight
10%12%47%7%36%5%6%6%33%
2394
21st to endNight
7%13%5%8%7%46%6%7%7%31%
1831
March1st to 10thNight
9%12%48%7%5%6%6%7%7%29%
2003
11th to 20thNight
8%12%48%8%5%7%5%7%8%29%
1980
21st to endNight
9%13%6%8%8%5%6%6%6%7%26%
2172
April1st to 10thNight
10%13%46%8%5%5%6%7%8%28%
1800
11th to 20thNight
9%11%47%7%35%5%6%7%34%
1779
21st to endNight
11%10%36%4445%6%8%39%
1675
May1st to 10thNight
8%9%35%5%2345%7%47%
1620
11th to 20thNight
8%9%36%5%45%6%5%8%42%
1417
21st to endNight
13%8%245%2446%9%43%
1552
June1st to 10thNight
9%7%35%4345%5%8%46%
1440
11th to 20thNight
9%5%34323346%58%
1440
21st to endNight
421322222474%
1440
July1st to 10thNight
2212223336%75%
1440
11th to 20thNight
311221223380%
1440
21st to endNight
12111212385%
1745
August1st to 10thNight
211221323478%
1611
11th to 20thNight
212212235%81%
1602
21st to endNight
2111212245%80%
1939
September1st to 10thNight
2212211235%80%
1800
11th to 20thNight
432331345%7%65%
1836
21st to endNight
6%5%23433446%60%
2043
October1st to 10thNight
8%414423445%61%
2160
11th to 20thNight
7%6%15%5%35%45%6%54%
2160
21st to endNight
9%7%25%5%35%5%5%6%47%
2376
November1st to 10thNight
11%9%48%6%5%5%5%5%7%33%
2356
11th to 20thNight
11%8%46%9%5%7%6%7%8%28%
2660
21st to endNight
11%10%36%6%46%6%6%7%36%
2660
December1st to 10thNight
9%8%38%6%46%6%7%8%36%
2660
11th to 20thNight
9%12%5%10%9%5%7%6%6%8%23%
3276
21st to endNight
11%14%5%8%8%47%6%6%7%25%
4862

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
62%10%10%14%32
2539
11th to 20thDay
60%8%7%12%8%5%
2746
21st to endDay
54%11%11%13%8%3
3146
February1st to 10thDay
50%11%13%14%11%2
1710
11th to 20thDay
49%9%12%19%9%3
1666
21st to endDay
58%8%8%15%9%2
1423
March1st to 10thDay
59%9%11%14%7%
1952
11th to 20thDay
53%10%11%16%9%2
1980
21st to endDay
58%9%14%13%6%1
2182
April1st to 10thDay
58%8%12%13%8%2
2160
11th to 20thDay
53%8%14%14%8%4
2132
21st to endDay
44%8%15%20%12%1
2281
May1st to 10thDay
39%6%15%28%11%1
2340
11th to 20thDay
41%7%13%27%10%2
2475
21st to endDay
44%7%18%24%6%
2724
June1st to 10thDay
40%7%16%28%10%
2520
11th to 20thDay
30%7%12%35%14%2
2520
21st to endDay
19%5%9%34%26%8%
2520
July1st to 10thDay
11%410%28%30%17%
2451
11th to 20thDay
13%36%22%35%21%
2445
21st to endDay
9%3420%45%19%
2427
August1st to 10thDay
11%49%32%32%12%
2312
11th to 20thDay
10%38%25%43%12%
2298
21st to endDay
12%38%26%39%12%
2417
September1st to 10thDay
11%4424%34%22%
2160
11th to 20thDay
22%7%9%25%28%9%
2121
21st to endDay
22%7%9%25%22%15%
1916
October1st to 10thDay
28%6%12%19%23%12%
1800
11th to 20thDay
31%8%11%19%23%8%
1800
21st to endDay
38%7%11%17%17%10%
1980
November1st to 10thDay
49%8%12%15%13%4
1720
11th to 20thDay
49%10%15%12%10%5%
1520
21st to endDay
48%9%9%14%13%8%
1520
December1st to 10thDay
41%9%12%18%17%3
1520
11th to 20thDay
55%10%13%12%8%2
1928
21st to endDay
58%14%7%14%5%2
2794
January1st to 10thNight
61%11%10%11%42
4406
11th to 20thNight
57%9%8%12%8%5%
4205
21st to endNight
60%8%7%12%8%4
4521
February1st to 10thNight
47%9%14%16%12%3
2470
11th to 20thNight
50%9%10%18%10%2
2396
21st to endNight
50%12%14%13%9%2
1831
March1st to 10thNight
53%8%14%17%8%
2003
11th to 20thNight
51%11%11%16%11%1
1980
21st to endNight
57%10%15%13%41
2172
April1st to 10thNight
53%11%14%15%7%1
1800
11th to 20thNight
47%11%14%17%8%3
1779
21st to endNight
41%11%19%21%7%1
1675
May1st to 10thNight
37%8%16%27%10%2
1620
11th to 20thNight
41%9%15%28%5%2
1417
21st to endNight
40%9%22%23%5%
1552
June1st to 10thNight
38%9%17%30%6%
1440
11th to 20thNight
31%5%17%34%12%1
1440
21st to endNight
17%5%11%38%23%6%
1440
July1st to 10thNight
14%410%31%25%17%
1400
11th to 20thNight
12%36%25%37%17%
1392
21st to endNight
7%2424%49%14%
1648
August1st to 10thNight
11%48%35%33%9%
1602
11th to 20thNight
9%37%32%36%13%
1602
21st to endNight
9%49%31%37%11%
1939
September1st to 10thNight
9%36%25%36%21%
1800
11th to 20thNight
19%7%12%29%27%7%
1836
21st to endNight
26%5%11%23%18%16%
2043
October1st to 10thNight
26%6%11%19%25%12%
2160
11th to 20thNight
31%7%11%21%19%10%
2160
21st to endNight
35%9%12%18%15%11%
2376
November1st to 10thNight
49%9%11%15%11%5%
2356
11th to 20thNight
50%11%12%13%9%6%
2660
21st to endNight
45%9%11%12%15%8%
2660
December1st to 10thNight
43%10%9%19%15%4
2660
11th to 20thNight
55%11%8%14%10%1
3276
21st to endNight
55%11%10%15%7%2
4862

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
24%27%24%17%5%2
180
11th to 20thDay
25%15%27%17%14%2
308
21st to endDay
21%16%27%21%14%1
407
February1st to 10thDay
18%21%28%22%11%
370
11th to 20thDay
24%22%25%21%8%1
363
21st to endDay
25%24%26%18%6%
321
March1st to 10thDay
25%25%25%18%7%
420
11th to 20thDay
20%24%23%24%10%1
420
21st to endDay
25%21%29%19%5%
462
April1st to 10thDay
21%24%28%18%8%1
420
11th to 20thDay
23%25%26%17%7%2
412
21st to endDay
17%23%30%19%10%2
420
May1st to 10thDay
20%15%31%20%10%3
423
11th to 20thDay
21%19%25%22%9%3
448
21st to endDay
19%17%26%23%12%4
485
June1st to 10thDay
15%18%27%22%13%4
450
11th to 20thDay
13%20%31%24%8%4
450
21st to endDay
49%31%37%13%5%
450
July1st to 10thDay
310%33%34%15%4
446
11th to 20thDay
28%36%32%17%5%
444
21st to endDay
1430%40%18%7%
487
August1st to 10thDay
35%28%36%19%8%
447
11th to 20thDay
26%24%39%20%9%
437
21st to endDay
48%29%35%19%5%
495
September1st to 10thDay
5%7%27%37%18%6%
450
11th to 20thDay
9%11%27%33%18%3
450
21st to endDay
10%13%27%28%19%3
450
October1st to 10thDay
12%15%24%28%18%3
450
11th to 20thDay
12%12%25%26%22%4
450
21st to endDay
15%15%27%24%14%5%
495
November1st to 10thDay
20%15%24%24%16%1
328
11th to 20thDay
16%13%24%28%17%2
180
21st to endDay
16%17%23%29%16%
180
December1st to 10thDay
19%14%25%25%12%4
180
11th to 20thDay
24%13%26%24%10%3
180
21st to endDay
25%16%25%25%8%2
198
January1st to 10thNight
18%14%29%25%11%2
760
11th to 20thNight
18%14%25%27%12%4
760
21st to endNight
18%12%22%30%16%2
836
February1st to 10thNight
16%14%20%35%14%1
760
11th to 20thNight
18%15%22%29%15%2
742
21st to endNight
13%16%24%32%13%1
592
March1st to 10thNight
19%18%22%26%13%2
720
11th to 20thNight
15%17%19%30%16%2
720
21st to endNight
16%18%25%27%13%1
792
April1st to 10thNight
20%14%24%25%16%2
720
11th to 20thNight
15%16%22%26%18%4
711
21st to endNight
17%9%19%31%22%2
595
May1st to 10thNight
13%9%18%31%26%4
540
11th to 20thNight
13%7%15%32%28%4
355
21st to endNight
16%8%15%31%27%3
389
June1st to 10thNight
12%6%19%28%32%3
360
11th to 20thNight
11%5%19%36%26%3
360
21st to endNight
6%410%35%44%1
360
July1st to 10thNight
316%38%48%4
350
11th to 20thNight
318%43%43%3
348
21st to endNight
21430%55%9%
563
August1st to 10thNight
4429%53%10%
537
11th to 20thNight
21428%53%10%
534
21st to endNight
3111%38%41%6%
751
September1st to 10thNight
3312%32%44%6%
720
11th to 20thNight
5%410%33%39%9%
720
21st to endNight
9%6%12%34%30%8%
720
October1st to 10thNight
10%417%34%30%4
720
11th to 20thNight
10%8%15%32%30%5%
720
21st to endNight
13%8%17%33%22%7%
792
November1st to 10thNight
18%9%21%32%16%4
740
11th to 20thNight
17%12%25%33%12%2
760
21st to endNight
16%12%24%31%17%1
760
December1st to 10thNight
16%14%22%32%15%2
760
11th to 20thNight
18%14%29%26%11%2
760
21st to endNight
28%15%24%24%7%2
836
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