Windsor
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Windsor clear sky chart from 2005-11-06 to 2024-03-28.

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
37%
24260
15%
80981
10%
24160
43%
24160
19%
56744

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
19%
942
11%
4383
5%
926
27%
926
9%
3242
January11th to 20thNight
18%
901
7%
4343
3%
885
23%
885
6%
3488
January21st to endNight
23%
830
6%
4510
6%
814
28%
814
6%
3658
February1st to 10thNight
21%
760
9%
2470
3%
760
27%
760
12%
1908
February11th to 20thNight
25%
740
14%
2288
8%
739
31%
739
15%
1698
February21st to endNight
20%
628
14%
1884
5%
628
29%
628
16%
1383
March1st to 10thNight
28%
760
17%
2215
10%
760
34%
760
19%
1577
March11th to 20thNight
30%
760
20%
2090
10%
760
37%
760
24%
1411
March21st to endNight
33%
821
17%
2256
10%
821
38%
821
19%
1628
April1st to 10thNight
31%
656
17%
1916
8%
656
39%
656
22%
1301
April11th to 20thNight
33%
533
21%
1611
9%
533
43%
533
23%
1075
April21st to endNight
37%
540
19%
1620
8%
540
45%
540
24%
1087
May1st to 10thNight
38%
540
12%
1620
7%
540
42%
540
20%
1136
May11th to 20thNight
45%
531
18%
1592
12%
531
51%
531
23%
1086
May21st to endNight
57%
583
16%
1745
12%
582
61%
582
32%
1059
June1st to 10thNight
54%
428
18%
1399
16%
428
61%
428
31%
814
June11th to 20thNight
60%
360
19%
1260
20%
360
65%
360
27%
746
June21st to endNight
57%
463
21%
1363
17%
463
63%
463
34%
822
July1st to 10thNight
66%
525
17%
1400
14%
525
69%
525
39%
842
July11th to 20thNight
66%
522
21%
1400
15%
522
69%
522
39%
833
July21st to endNight
70%
591
19%
1672
16%
558
73%
558
39%
954
August1st to 10thNight
68%
537
20%
1600
17%
534
70%
534
36%
931
August11th to 20thNight
66%
534
23%
1598
19%
534
70%
534
35%
949
August21st to endNight
66%
594
24%
1782
19%
594
69%
594
35%
1054
September1st to 10thNight
59%
668
19%
1921
15%
668
61%
668
31%
1202
September11th to 20thNight
61%
720
28%
1980
21%
720
64%
720
37%
1191
September21st to endNight
51%
720
22%
1980
16%
720
56%
720
29%
1292
October1st to 10thNight
48%
720
21%
2046
17%
720
52%
720
29%
1303
October11th to 20thNight
32%
720
17%
2160
9%
720
42%
720
20%
1474
October21st to endNight
28%
792
17%
2487
11%
792
36%
792
17%
1772
November1st to 10thNight
37%
740
22%
2406
16%
740
46%
740
25%
1508
November11th to 20thNight
24%
760
18%
2470
10%
760
35%
760
17%
1700
November21st to endNight
24%
760
14%
2599
8%
760
31%
760
14%
1866
December1st to 10thNight
18%
760
8%
2660
4%
760
23%
760
11%
2058
December11th to 20thNight
20%
782
9%
3292
4%
781
29%
781
10%
2437
December21st to endNight
16%
1039
4%
4963
2%
1026
20%
1026
5%
4259

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
14%9%47%6%45%5%6%7%33%
78684
Night
16%10%47%6%345%5%7%34%
81310
TransparencyDay
48%6%13%20%11%1
78395
Night
50%6%11%20%11%1
81119
SeeingDay
16%8%27%27%16%6%
14412
Night
18%8%28%31%13%2
24289

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
16%16%8%10%7%46%446%21%
2527
11th to 20thDay
15%16%7%12%9%5%8%6%5%6%12%
2596
21st to endDay
27%17%8%10%6%3434413%
3124
February1st to 10thDay
15%14%5%9%9%5%5%5%5%6%22%
1710
11th to 20thDay
14%11%6%11%8%45%5%5%7%24%
1775
21st to endDay
17%11%5%10%5%45%45%7%27%
1570
March1st to 10thDay
16%11%5%8%6%45%5%5%7%29%
1964
11th to 20thDay
15%12%47%6%3436%7%34%
2090
21st to endDay
18%12%5%8%8%35%445%27%
2255
April1st to 10thDay
15%10%45%6%46%5%6%6%32%
2060
11th to 20thDay
17%9%27%6%445%6%7%34%
2297
21st to endDay
19%9%36%6%35%45%7%34%
2340
May1st to 10thDay
16%8%37%6%46%5%5%7%33%
2340
11th to 20thDay
18%8%36%6%45%5%5%7%34%
2301
21st to endDay
12%6%25%5%35%5%6%8%43%
2531
June1st to 10thDay
10%6%35%6%45%6%6%9%41%
2547
11th to 20thDay
10%6%36%5%45%5%6%8%42%
2700
21st to endDay
9%5%35%5%45%7%8%10%39%
2588
July1st to 10thDay
8%5%25%6%35%6%6%8%48%
2520
11th to 20thDay
9%425%6%45%5%6%10%44%
2508
21st to endDay
8%5%25%6%36%6%7%9%43%
2547
August1st to 10thDay
8%425%5%45%6%7%10%43%
2327
11th to 20thDay
9%5%25%5%5%6%7%7%11%38%
2303
21st to endDay
10%5%25%5%35%5%6%9%44%
2574
September1st to 10thDay
10%6%36%5%46%5%6%8%42%
2015
11th to 20thDay
10%425%435%6%6%9%46%
1980
21st to endDay
15%6%26%5%345%5%7%42%
1980
October1st to 10thDay
12%5%36%6%345%6%8%42%
1913
11th to 20thDay
17%8%36%6%35%45%8%35%
1800
21st to endDay
19%10%36%6%46%5%5%6%30%
1868
November1st to 10thDay
10%7%46%6%35%5%6%8%40%
1672
11th to 20thDay
13%10%46%5%45%5%6%7%34%
1710
21st to endDay
18%12%46%7%45%45%6%29%
1577
December1st to 10thDay
16%12%6%11%7%46%5%6%6%21%
1520
11th to 20thDay
17%13%6%7%7%45%5%6%8%23%
1868
21st to endDay
27%17%7%10%7%44345%10%
2687
January1st to 10thNight
16%16%6%8%6%35%45%6%26%
4428
11th to 20thNight
14%18%8%11%7%46%5%5%6%16%
4369
21st to endNight
25%16%8%10%6%3443418%
4532
February1st to 10thNight
17%15%5%9%8%45%45%5%24%
2470
11th to 20thNight
15%12%5%10%7%45%5%5%6%27%
2288
21st to endNight
19%11%48%6%35%5%45%30%
1884
March1st to 10thNight
17%9%5%7%6%4445%6%34%
2215
11th to 20thNight
12%8%36%6%346%6%8%38%
2090
21st to endNight
18%8%48%5%45%446%34%
2256
April1st to 10thNight
15%9%45%6%3445%7%37%
1916
11th to 20thNight
18%8%25%5%35%46%8%36%
1611
21st to endNight
16%8%35%4345%6%6%39%
1620
May1st to 10thNight
16%8%36%7%45%5%5%7%34%
1620
11th to 20thNight
18%6%35%6%35%45%7%39%
1592
21st to endNight
9%5%345%35%45%10%46%
1744
June1st to 10thNight
12%5%3443446%8%47%
1399
11th to 20thNight
10%425%5%5%45%6%9%44%
1260
21st to endNight
9%435%5%345%5%8%48%
1363
July1st to 10thNight
8%433435%5%5%9%52%
1440
11th to 20thNight
9%325%335%46%9%52%
1448
21st to endNight
6%3245%35%6%7%10%51%
1771
August1st to 10thNight
8%425%435%5%6%8%50%
1611
11th to 20thNight
8%4245%35%5%6%8%50%
1598
21st to endNight
8%425%425%5%6%8%50%
1782
September1st to 10thNight
11%5%25%5%435%6%6%47%
1921
11th to 20thNight
10%4244335%5%7%53%
1980
21st to endNight
13%6%36%43446%6%44%
1980
October1st to 10thNight
13%6%35%6%34447%45%
2046
11th to 20thNight
17%8%27%6%3446%6%37%
2160
21st to endNight
21%8%45%6%34446%33%
2487
November1st to 10thNight
12%7%36%5%345%5%7%44%
2406
11th to 20thNight
16%9%37%6%45%5%6%6%33%
2470
21st to endNight
20%12%5%7%5%35%447%29%
2599
December1st to 10thNight
18%13%6%8%8%45%5%5%6%22%
2660
11th to 20thNight
15%14%49%6%46%6%5%7%26%
3321
21st to endNight
28%19%6%10%6%3344413%
4973

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
64%8%10%12%6%
2528
11th to 20thDay
72%5%12%8%3
2596
21st to endDay
75%5%9%8%4
3124
February1st to 10thDay
60%6%10%15%7%1
1710
11th to 20thDay
58%5%10%18%8%1
1784
21st to endDay
56%5%12%18%7%1
1570
March1st to 10thDay
54%6%14%18%7%1
1964
11th to 20thDay
49%5%12%20%12%2
2090
21st to endDay
57%5%10%20%7%1
2255
April1st to 10thDay
50%6%13%18%12%1
2060
11th to 20thDay
48%6%12%19%14%2
2297
21st to endDay
48%6%13%18%13%2
2340
May1st to 10thDay
49%6%10%21%12%2
2340
11th to 20thDay
49%5%11%19%14%2
2305
21st to endDay
37%6%16%24%16%2
2531
June1st to 10thDay
37%7%13%27%13%2
2547
11th to 20thDay
38%7%14%22%17%3
2700
21st to endDay
35%9%14%23%17%2
2588
July1st to 10thDay
33%6%10%27%20%4
2451
11th to 20thDay
32%7%15%27%18%2
2433
21st to endDay
34%7%15%24%18%2
2405
August1st to 10thDay
31%8%16%31%13%1
2314
11th to 20thDay
36%9%15%27%12%1
2303
21st to endDay
34%8%14%25%18%2
2574
September1st to 10thDay
38%7%16%23%14%2
2015
11th to 20thDay
32%6%14%33%14%1
1980
21st to endDay
40%5%15%26%13%1
1980
October1st to 10thDay
40%6%13%26%13%2
1913
11th to 20thDay
47%7%13%21%12%1
1800
21st to endDay
52%7%13%19%9%1
1868
November1st to 10thDay
43%6%12%25%12%2
1672
11th to 20thDay
48%6%16%20%9%1
1710
21st to endDay
55%6%13%18%7%1
1577
December1st to 10thDay
62%8%11%14%5%
1520
11th to 20thDay
58%8%13%15%6%
1864
21st to endDay
76%6%8%7%3
2687
January1st to 10thNight
60%6%11%16%6%1
4428
11th to 20thNight
67%7%11%11%4
4369
21st to endNight
70%5%9%12%4
4532
February1st to 10thNight
61%6%11%15%6%1
2470
11th to 20thNight
56%5%9%19%9%1
2290
21st to endNight
56%5%12%18%8%2
1884
March1st to 10thNight
51%5%11%23%9%1
2215
11th to 20thNight
41%8%12%21%15%2
2090
21st to endNight
51%6%8%22%11%1
2256
April1st to 10thNight
47%5%11%22%14%2
1916
11th to 20thNight
45%5%11%19%17%2
1611
21st to endNight
45%5%10%20%16%4
1620
May1st to 10thNight
49%6%11%18%13%3
1620
11th to 20thNight
46%5%9%19%18%3
1593
21st to endNight
34%6%14%26%17%2
1745
June1st to 10thNight
35%5%11%29%17%3
1399
11th to 20thNight
35%8%14%24%16%2
1260
21st to endNight
34%6%12%27%21%1
1363
July1st to 10thNight
30%6%11%28%21%3
1400
11th to 20thNight
30%6%13%31%18%2
1400
21st to endNight
27%7%18%24%19%4
1672
August1st to 10thNight
29%8%15%30%17%2
1600
11th to 20thNight
31%7%14%31%14%3
1598
21st to endNight
30%8%13%28%19%2
1782
September1st to 10thNight
35%8%13%26%16%2
1921
11th to 20thNight
31%7%11%30%20%1
1980
21st to endNight
39%6%12%26%15%1
1980
October1st to 10thNight
39%6%11%25%19%1
2046
11th to 20thNight
47%6%11%20%15%1
2160
21st to endNight
52%6%10%21%10%1
2487
November1st to 10thNight
40%6%12%26%16%1
2406
11th to 20thNight
50%6%12%21%10%1
2470
21st to endNight
56%5%13%19%6%
2599
December1st to 10thNight
62%6%11%15%6%
2660
11th to 20thNight
57%8%13%15%7%
3324
21st to endNight
75%5%9%8%2
4973

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
20%16%37%19%7%1
270
11th to 20thDay
19%13%41%20%5%1
270
21st to endDay
24%10%33%29%4
297
February1st to 10thDay
21%13%40%22%31
270
11th to 20thDay
19%17%34%27%3
270
21st to endDay
18%17%46%15%4
225
March1st to 10thDay
21%15%34%25%5%
334
11th to 20thDay
19%10%33%29%7%1
460
21st to endDay
22%14%32%23%9%1
490
April1st to 10thDay
16%11%31%30%10%1
420
11th to 20thDay
23%15%28%20%9%5%
412
21st to endDay
24%7%33%23%11%3
420
May1st to 10thDay
21%8%28%27%11%4
423
11th to 20thDay
20%7%26%25%16%6%
447
21st to endDay
14%420%31%21%10%
485
June1st to 10thDay
11%425%38%17%5%
554
11th to 20thDay
12%426%35%21%3
630
21st to endDay
10%428%33%19%6%
630
July1st to 10thDay
8%418%31%28%12%
621
11th to 20thDay
12%221%31%24%10%
606
21st to endDay
8%317%33%27%12%
488
August1st to 10thDay
9%313%34%24%17%
446
11th to 20thDay
12%214%33%28%12%
439
21st to endDay
11%416%28%26%15%
495
September1st to 10thDay
13%321%26%24%13%
450
11th to 20thDay
10%216%26%31%14%
450
21st to endDay
17%7%21%24%20%10%
450
October1st to 10thDay
12%7%26%23%21%11%
450
11th to 20thDay
18%14%29%27%9%2
450
21st to endDay
25%11%33%24%6%1
383
November1st to 10thDay
14%12%36%23%12%3
270
11th to 20thDay
17%20%32%24%6%1
270
21st to endDay
23%12%39%22%4
207
December1st to 10thDay
19%17%37%17%7%2
180
11th to 20thDay
26%13%41%17%21
180
21st to endDay
26%17%33%20%3
270
January1st to 10thNight
22%13%36%22%6%1
950
11th to 20thNight
26%11%33%25%5%
905
21st to endNight
19%11%35%30%6%
836
February1st to 10thNight
24%11%36%26%4
760
11th to 20thNight
20%10%35%30%5%
743
21st to endNight
24%12%37%24%3
628
March1st to 10thNight
21%9%33%29%7%
760
11th to 20thNight
16%12%36%29%6%1
760
21st to endNight
21%10%29%30%9%1
821
April1st to 10thNight
19%11%32%32%6%
656
11th to 20thNight
19%9%33%28%10%1
533
21st to endNight
21%6%31%32%9%
540
May1st to 10thNight
20%430%33%11%1
540
11th to 20thNight
19%325%36%15%2
531
21st to endNight
11%220%40%23%3
584
June1st to 10thNight
13%223%46%16%1
428
11th to 20thNight
12%319%48%18%1
360
21st to endNight
10%320%45%20%2
463
July1st to 10thNight
8%314%39%31%6%
525
11th to 20thNight
9%216%41%27%5%
522
21st to endNight
6%214%49%26%4
591
August1st to 10thNight
9%213%41%30%5%
537
11th to 20thNight
10%313%37%32%5%
534
21st to endNight
9%316%36%31%5%
594
September1st to 10thNight
12%6%15%35%25%7%
668
11th to 20thNight
10%417%33%30%5%
720
21st to endNight
16%6%19%31%22%5%
720
October1st to 10thNight
15%6%26%31%19%4
720
11th to 20thNight
21%12%30%25%11%1
720
21st to endNight
26%10%30%28%6%
792
November1st to 10thNight
15%10%33%26%15%1
740
11th to 20thNight
21%13%35%21%8%1
760
21st to endNight
25%10%35%23%6%
760
December1st to 10thNight
24%13%36%21%6%1
760
11th to 20thNight
23%13%36%24%4
783
21st to endNight
29%10%36%22%3
1045
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