Windham Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Windham Hill Observatory clear sky chart from 2005-11-06 to 2024-01-13.

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%
23835
25%
74224
17%
23777
56%
23777
32%
42976

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
31%
948
29%
2458
10%
946
43%
946
33%
1545
January11th to 20thNight
31%
915
26%
2379
9%
915
41%
915
30%
1529
January21st to endNight
30%
990
28%
2574
10%
990
41%
990
31%
1642
February1st to 10thNight
27%
824
23%
2264
8%
824
36%
824
25%
1492
February11th to 20thNight
22%
704
20%
2098
7%
700
33%
700
24%
1332
February21st to endNight
22%
489
27%
1673
9%
489
38%
489
20%
1033
March1st to 10thNight
28%
540
22%
1980
12%
540
40%
540
21%
1233
March11th to 20thNight
29%
540
22%
1980
13%
540
41%
540
21%
1205
March21st to endNight
30%
594
25%
2178
14%
594
46%
594
27%
1231
April1st to 10thNight
28%
540
24%
1980
13%
540
45%
540
24%
1154
April11th to 20thNight
32%
533
24%
1831
13%
533
48%
533
27%
1059
April21st to endNight
32%
540
26%
1800
15%
540
49%
540
30%
1023
May1st to 10thNight
34%
540
23%
1634
15%
540
45%
540
30%
989
May11th to 20thNight
45%
531
23%
1594
20%
531
53%
531
34%
939
May21st to endNight
55%
583
25%
1746
21%
582
65%
582
36%
1001
June1st to 10thNight
72%
540
20%
1620
22%
540
75%
540
42%
867
June11th to 20thNight
69%
540
25%
1620
25%
540
75%
540
53%
782
June21st to endNight
73%
540
23%
1620
26%
540
75%
540
37%
922
July1st to 10thNight
76%
525
20%
1575
23%
525
78%
525
38%
901
July11th to 20thNight
83%
522
25%
1566
28%
522
86%
522
54%
751
July21st to endNight
81%
591
20%
1673
23%
558
85%
558
42%
905
August1st to 10thNight
85%
537
31%
1601
35%
534
86%
534
59%
735
August11th to 20thNight
78%
534
26%
1618
29%
534
79%
534
38%
914
August21st to endNight
79%
594
20%
2048
23%
594
82%
594
34%
1148
September1st to 10thNight
75%
540
22%
1980
25%
540
76%
540
34%
1066
September11th to 20thNight
71%
540
22%
1980
26%
540
72%
540
28%
1154
September21st to endNight
72%
540
31%
1980
33%
540
75%
540
39%
985
October1st to 10thNight
72%
604
35%
2044
31%
604
76%
604
47%
950
October11th to 20thNight
55%
720
34%
2160
21%
720
63%
720
40%
1193
October21st to endNight
46%
792
34%
2376
19%
792
56%
792
38%
1322
November1st to 10thNight
52%
740
30%
2220
20%
740
59%
740
38%
1249
November11th to 20thNight
47%
760
29%
2280
17%
760
55%
760
33%
1316
November21st to endNight
37%
926
27%
2446
13%
926
45%
926
30%
1577
December1st to 10thNight
37%
950
29%
2470
13%
950
46%
950
32%
1587
December11th to 20thNight
32%
949
25%
2870
11%
948
39%
948
23%
1846
December21st to endNight
33%
1040
17%
4308
11%
1026
42%
1026
22%
2399

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%5%25%5%35%5%5%7%46%
74948
Night
11%5%2442345%7%52%
74820
TransparencyDay
37%6%11%20%19%7%
74673
Night
33%5%11%21%20%10%
74615
SeeingDay
13%7%20%27%24%8%
13858
Night
14%8%26%33%17%2
23844

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 10thDay
15%7%24323446%49%
1707
11th to 20thDay
18%7%35%423238%45%
1643
21st to endDay
17%7%25%324335%49%
1782
February1st to 10thDay
21%8%35%423346%42%
1710
11th to 20thDay
21%8%35%433336%40%
1749
21st to endDay
23%5%24423345%44%
1581
March1st to 10thDay
20%8%25%435%45%6%38%
1980
11th to 20thDay
18%7%36%5%24445%41%
1980
21st to endDay
16%7%35%5%3435%6%44%
2178
April1st to 10thDay
14%8%35%5%35%5%5%7%40%
1980
11th to 20thDay
15%6%35%5%445%5%7%41%
2075
21st to endDay
12%5%345%45%5%5%7%45%
2160
May1st to 10thDay
13%6%36%5%45%448%41%
2322
11th to 20thDay
13%6%25%5%35%5%7%7%41%
2302
21st to endDay
11%6%36%6%47%6%7%9%35%
2531
June1st to 10thDay
5%425%6%46%6%7%10%46%
2340
11th to 20thDay
5%3245%46%6%7%10%48%
2340
21st to endDay
5%5%25%6%47%6%8%11%40%
2340
July1st to 10thDay
8%436%5%47%6%8%11%38%
2340
11th to 20thDay
4324437%5%8%12%47%
2340
21st to endDay
4325%5%46%6%8%9%48%
2546
August1st to 10thDay
421243447%10%59%
2328
11th to 20thDay
5%4145%35%6%8%10%48%
2283
21st to endDay
6%325%7%5%7%7%7%10%43%
2303
September1st to 10thDay
8%5%25%6%45%5%5%8%48%
1980
11th to 20thDay
10%6%25%5%45%5%5%8%44%
1980
21st to endDay
7%4245%345%6%7%54%
1980
October1st to 10thDay
6%32442445%6%59%
1895
11th to 20thDay
13%423334445%56%
1800
21st to endDay
15%5%2332323459%
1980
November1st to 10thDay
12%5%35%433446%51%
1858
11th to 20thDay
12%6%24334445%53%
1900
21st to endDay
17%7%36%43435%444%
1729
December1st to 10thDay
15%7%3442335%6%48%
1710
11th to 20thDay
18%7%245%34446%44%
2161
21st to endDay
15%8%35%424345%45%
3135
January1st to 10thNight
15%7%25%312346%52%
2458
11th to 20thNight
19%6%24422445%49%
2379
21st to endNight
16%7%24424345%50%
2574
February1st to 10thNight
18%7%35%5%2333446%
2264
11th to 20thNight
20%7%35%333335%46%
2098
21st to endNight
22%6%24413345%47%
1673
March1st to 10thNight
17%6%35%44445%6%42%
1980
11th to 20thNight
14%6%25%5%245%5%7%44%
1980
21st to endNight
12%6%35%42345%7%51%
2178
April1st to 10thNight
11%5%25%5%3445%8%48%
1980
11th to 20thNight
13%5%35%32445%7%49%
1831
21st to endNight
12%6%2442345%6%52%
1800
May1st to 10thNight
9%6%35%4345%6%9%46%
1634
11th to 20thNight
12%5%2442345%9%50%
1593
21st to endNight
7%3245%245%6%10%50%
1745
June1st to 10thNight
6%31432445%11%57%
1620
11th to 20thNight
411232346%10%64%
1620
21st to endNight
5%3244445%6%10%53%
1620
July1st to 10thNight
43245%26%5%6%12%50%
1620
11th to 20thNight
321332346%12%62%
1620
21st to endNight
3213335%6%8%12%52%
1772
August1st to 10thNight
211222245%12%67%
1611
11th to 20thNight
44245%35%5%6%10%51%
1618
21st to endNight
43145%35%6%8%11%49%
2048
September1st to 10thNight
6%5%245%345%7%9%51%
1980
11th to 20thNight
10%6%245%3445%7%49%
1980
21st to endNight
7%41443345%8%59%
1980
October1st to 10thNight
6%313223346%66%
2044
11th to 20thNight
13%423323345%59%
2160
21st to endNight
14%42332223460%
2376
November1st to 10thNight
11%5%23423346%57%
2220
11th to 20thNight
9%6%3443445%6%53%
2280
21st to endNight
17%6%25%333345%48%
2446
December1st to 10thNight
15%7%25%323446%48%
2470
11th to 20thNight
16%8%35%423335%46%
3036
21st to endNight
16%6%3442334452%
4532

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
38%37%18%24%10%
1708
11th to 20thDay
41%45%17%23%9%
1643
21st to endDay
40%26%15%24%12%
1782
February1st to 10thDay
46%36%15%21%10%
1710
11th to 20thDay
47%47%15%19%9%
1759
21st to endDay
44%46%16%17%14%
1581
March1st to 10thDay
46%5%8%13%21%7%
1980
11th to 20thDay
47%46%13%22%8%
1980
21st to endDay
42%5%7%17%22%7%
2178
April1st to 10thDay
45%5%8%19%14%9%
1980
11th to 20thDay
43%5%9%16%18%9%
2075
21st to endDay
38%6%9%19%23%6%
2160
May1st to 10thDay
42%7%9%20%17%6%
2322
11th to 20thDay
39%8%11%22%19%2
2306
21st to endDay
42%9%13%20%15%1
2531
June1st to 10thDay
30%8%15%30%14%2
2340
11th to 20thDay
27%8%15%30%16%3
2340
21st to endDay
35%10%14%24%15%2
2340
July1st to 10thDay
37%10%20%23%10%
2276
11th to 20thDay
28%10%20%26%16%1
2271
21st to endDay
27%11%20%31%11%1
2405
August1st to 10thDay
19%8%19%35%18%1
2314
11th to 20thDay
28%9%20%27%13%3
2283
21st to endDay
34%11%15%24%13%3
2303
September1st to 10thDay
33%7%14%22%16%8%
1980
11th to 20thDay
38%7%14%19%15%7%
1980
21st to endDay
27%7%11%22%19%13%
1980
October1st to 10thDay
26%5%10%22%22%15%
1895
11th to 20thDay
31%48%15%25%16%
1800
21st to endDay
32%35%15%28%17%
1980
November1st to 10thDay
35%5%8%19%23%10%
1858
11th to 20thDay
34%48%19%23%13%
1900
21st to endDay
44%46%17%19%10%
1729
December1st to 10thDay
38%411%19%19%9%
1710
11th to 20thDay
43%46%15%21%10%
2159
21st to endDay
43%26%13%26%9%
3135
January1st to 10thNight
37%35%19%23%13%
2458
11th to 20thNight
38%47%16%23%12%
2379
21st to endNight
38%35%15%22%16%
2574
February1st to 10thNight
43%45%14%19%16%
2264
11th to 20thNight
43%37%14%20%13%
2098
21st to endNight
43%47%13%15%17%
1673
March1st to 10thNight
43%5%8%13%20%11%
1980
11th to 20thNight
40%6%10%13%23%9%
1980
21st to endNight
35%5%11%17%21%11%
2178
April1st to 10thNight
36%6%9%19%18%11%
1980
11th to 20thNight
35%6%11%19%17%13%
1831
21st to endNight
33%5%9%19%26%8%
1800
May1st to 10thNight
35%6%13%20%20%8%
1634
11th to 20thNight
33%5%12%27%19%3
1594
21st to endNight
29%7%17%30%15%2
1746
June1st to 10thNight
23%8%18%31%19%2
1620
11th to 20thNight
16%6%19%34%20%5%
1620
21st to endNight
28%8%14%28%19%3
1620
July1st to 10thNight
27%9%20%33%11%1
1575
11th to 20thNight
14%8%28%34%16%1
1566
21st to endNight
20%11%27%30%11%1
1673
August1st to 10thNight
12%6%22%38%19%2
1601
11th to 20thNight
28%7%18%31%13%3
1618
21st to endNight
27%10%20%29%11%3
2048
September1st to 10thNight
29%7%16%23%17%8%
1980
11th to 20thNight
34%8%16%18%17%7%
1980
21st to endNight
24%6%12%23%23%13%
1980
October1st to 10thNight
20%411%26%22%17%
2044
11th to 20thNight
29%36%17%25%19%
2160
21st to endNight
30%36%16%23%21%
2376
November1st to 10thNight
30%49%20%24%13%
2220
11th to 20thNight
32%48%19%23%13%
2280
21st to endNight
40%48%15%21%12%
2446
December1st to 10thNight
38%48%20%19%10%
2470
11th to 20thNight
42%36%13%22%14%
3037
21st to endNight
37%36%13%30%11%
4532

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%16%33%20%6%2
270
11th to 20thDay
23%13%37%20%6%1
248
21st to endDay
22%18%39%15%7%
264
February1st to 10thDay
27%18%32%20%4
240
11th to 20thDay
33%20%30%14%3
240
21st to endDay
31%22%36%8%3
198
March1st to 10thDay
26%15%36%17%7%
240
11th to 20thDay
19%23%36%17%5%
240
21st to endDay
19%15%38%20%8%
264
April1st to 10thDay
21%14%41%19%5%
240
11th to 20thDay
21%14%31%21%12%1
353
21st to endDay
15%13%34%29%8%2
420
May1st to 10thDay
16%11%33%30%9%1
585
11th to 20thDay
15%6%25%32%18%3
625
21st to endDay
12%6%22%35%20%5%
679
June1st to 10thDay
5%115%37%30%10%
630
11th to 20thDay
6%216%41%28%8%
630
21st to endDay
8%212%30%35%13%
630
July1st to 10thDay
7%18%31%36%17%
621
11th to 20thDay
35%31%43%17%
618
21st to endDay
4328%51%13%
684
August1st to 10thDay
41425%52%14%
626
11th to 20thDay
7%1329%44%16%
597
21st to endDay
8%2317%43%27%
422
September1st to 10thDay
6%5%26%38%26%
270
11th to 20thDay
6%28%24%40%21%
270
21st to endDay
9%27%30%33%19%
270
October1st to 10thDay
5%417%34%33%7%
270
11th to 20thDay
10%7%27%30%20%6%
270
21st to endDay
24%13%24%21%13%5%
297
November1st to 10thDay
17%6%23%34%17%3
270
11th to 20thDay
17%11%28%29%11%3
270
21st to endDay
20%15%31%23%10%1
270
December1st to 10thDay
17%9%36%21%12%5%
270
11th to 20thDay
27%17%30%20%6%
270
21st to endDay
21%14%36%22%5%1
297
January1st to 10thNight
19%12%35%27%7%
950
11th to 20thNight
23%12%31%27%8%
915
21st to endNight
20%11%35%29%5%
990
February1st to 10thNight
23%13%33%27%4
824
11th to 20thNight
25%13%37%22%3
704
21st to endNight
25%15%36%21%2
489
March1st to 10thNight
21%12%37%25%5%
540
11th to 20thNight
17%10%41%25%6%
540
21st to endNight
13%14%41%26%5%1
594
April1st to 10thNight
13%10%45%26%6%
540
11th to 20thNight
13%13%38%27%9%
533
21st to endNight
13%9%44%23%10%1
540
May1st to 10thNight
11%11%41%29%8%1
540
11th to 20thNight
14%5%32%32%16%
531
21st to endNight
7%5%27%41%18%2
584
June1st to 10thNight
6%112%45%32%3
540
11th to 20thNight
4121%51%21%2
540
21st to endNight
5%113%44%31%6%
540
July1st to 10thNight
4112%44%34%6%
525
11th to 20thNight
317%51%33%5%
522
21st to endNight
318%46%38%5%
591
August1st to 10thNight
37%48%39%4
537
11th to 20thNight
6%1448%36%5%
534
21st to endNight
427%38%42%7%
594
September1st to 10thNight
7%18%40%39%5%
540
11th to 20thNight
12%16%36%40%5%
540
21st to endNight
8%111%37%38%5%
540
October1st to 10thNight
6%215%44%28%5%
604
11th to 20thNight
13%5%23%38%18%2
720
21st to endNight
14%8%28%29%18%2
792
November1st to 10thNight
12%5%27%35%20%2
740
11th to 20thNight
12%10%29%37%13%
760
21st to endNight
19%11%29%26%14%1
926
December1st to 10thNight
19%8%32%29%11%
950
11th to 20thNight
23%11%30%29%6%
950
21st to endNight
20%11%31%30%7%
1045
Page generated at 2024-01-10 18:35:24. Version ac07

Copyright 2024 Attilla Danko


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