Stephen F. Wessling Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stephen F. Wessling Observatory 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
34%
24970
12%
81111
9%
24879
39%
24879
16%
59759

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
18%
941
5%
4394
3%
926
21%
926
5%
3700
January11th to 20thNight
17%
945
3%
4434
2%
930
19%
930
4%
4007
January21st to endNight
19%
1037
2%
4879
2%
1023
20%
1023
4%
4366
February1st to 10thNight
19%
831
8%
2477
4%
831
22%
831
9%
2046
February11th to 20thNight
21%
740
10%
2219
5%
740
25%
740
11%
1760
February21st to endNight
19%
628
9%
1884
3%
628
25%
628
13%
1465
March1st to 10thNight
28%
760
15%
2280
8%
760
33%
760
19%
1601
March11th to 20thNight
31%
760
20%
2165
10%
760
38%
760
24%
1437
March21st to endNight
32%
734
16%
2169
10%
734
38%
734
19%
1547
April1st to 10thNight
29%
540
13%
1800
8%
540
36%
540
18%
1242
April11th to 20thNight
31%
533
20%
1726
11%
533
41%
533
21%
1161
April21st to endNight
35%
540
17%
1620
10%
540
41%
540
21%
1102
May1st to 10thNight
35%
540
14%
1620
8%
540
40%
540
22%
1115
May11th to 20thNight
39%
531
19%
1541
11%
531
48%
531
25%
1031
May21st to endNight
48%
583
15%
1552
9%
582
53%
582
30%
1024
June1st to 10thNight
50%
540
15%
1440
9%
540
55%
540
34%
900
June11th to 20thNight
50%
540
16%
1440
11%
540
53%
540
34%
908
June21st to endNight
55%
540
16%
1440
10%
540
59%
540
34%
916
July1st to 10thNight
67%
525
17%
1400
13%
525
69%
525
41%
823
July11th to 20thNight
63%
522
20%
1392
14%
522
66%
522
41%
811
July21st to endNight
61%
591
19%
1487
15%
558
64%
558
38%
910
August1st to 10thNight
66%
537
20%
1534
18%
534
68%
534
37%
908
August11th to 20thNight
67%
534
24%
1598
19%
534
72%
534
37%
908
August21st to endNight
60%
594
22%
1948
20%
594
64%
594
31%
1128
September1st to 10thNight
61%
540
21%
1800
21%
540
64%
540
29%
1061
September11th to 20thNight
58%
713
28%
1973
22%
713
63%
713
39%
1148
September21st to endNight
49%
720
20%
1980
14%
720
52%
720
29%
1291
October1st to 10thNight
47%
720
22%
1980
17%
720
51%
720
30%
1282
October11th to 20thNight
30%
720
13%
2226
8%
720
36%
720
17%
1614
October21st to endNight
26%
792
12%
2574
8%
792
33%
792
16%
1878
November1st to 10thNight
32%
740
18%
2406
14%
740
39%
740
18%
1628
November11th to 20thNight
23%
760
11%
2470
8%
760
27%
760
12%
1910
November21st to endNight
22%
764
9%
2474
5%
764
26%
764
11%
1932
December1st to 10thNight
15%
950
3%
2660
2%
950
17%
950
7%
2298
December11th to 20thNight
16%
949
5%
3268
3%
949
19%
949
7%
2772
December21st to endNight
11%
1036
3%
4861
1%
1026
15%
1026
4%
4129

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
15%11%5%8%7%35%45%6%32%
78681
Night
15%12%5%8%6%4445%6%30%
81372
TransparencyDay
53%5%11%19%10%1
78382
Night
55%5%11%18%9%1
81192
SeeingDay
18%13%29%24%11%4
14936
Night
19%9%29%31%11%1
25006

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
14%26%12%14%8%433349%
2527
11th to 20thDay
14%17%9%17%15%7%6%3336%
2530
21st to endDay
20%22%11%12%10%442239%
2783
February1st to 10thDay
11%15%9%11%13%6%6%435%17%
1699
11th to 20thDay
11%16%8%13%11%5%5%445%18%
1848
21st to endDay
12%16%8%12%8%45%45%5%20%
1570
March1st to 10thDay
14%12%5%9%7%36%46%5%28%
1900
11th to 20thDay
13%10%49%7%35%45%6%34%
2014
21st to endDay
18%11%48%7%36%45%6%29%
2357
April1st to 10thDay
18%14%5%6%5%3445%6%29%
2160
11th to 20thDay
20%11%36%5%3445%6%32%
2181
21st to endDay
17%10%36%6%35%45%7%34%
2340
May1st to 10thDay
18%7%26%5%3446%7%38%
2340
11th to 20thDay
17%7%35%5%3446%8%38%
2353
21st to endDay
13%5%25%5%25%6%6%8%42%
2725
June1st to 10thDay
11%5%36%5%35%5%5%7%45%
2520
11th to 20thDay
13%6%25%6%3445%9%43%
2520
21st to endDay
13%5%26%5%35%5%5%8%43%
2520
July1st to 10thDay
7%424425%5%6%9%51%
2520
11th to 20thDay
10%425%5%345%6%9%48%
2520
21st to endDay
8%435%5%245%6%10%47%
2743
August1st to 10thDay
10%5%25%5%35%6%7%10%43%
2395
11th to 20thDay
12%425%5%35%5%6%8%45%
2303
21st to endDay
12%6%25%5%345%6%8%45%
2405
September1st to 10thDay
13%5%26%6%46%45%7%42%
2147
11th to 20thDay
13%5%35%6%25%45%7%46%
1980
21st to endDay
13%7%37%6%35%5%5%7%40%
1980
October1st to 10thDay
17%8%36%5%34445%40%
1980
11th to 20thDay
24%10%48%8%45%5%5%6%22%
1732
21st to endDay
25%12%5%8%6%44446%22%
1782
November1st to 10thDay
15%13%46%5%35%45%7%33%
1672
11th to 20thDay
15%19%7%10%7%34446%20%
1710
21st to endDay
23%17%7%9%7%33435%20%
1704
December1st to 10thDay
14%21%13%14%5%45%245%13%
1520
11th to 20thDay
15%19%10%14%9%4434412%
1918
21st to endDay
22%23%9%11%7%444438%
2783
January1st to 10thNight
15%21%12%13%7%4333415%
4428
11th to 20thNight
13%17%9%17%13%9%43349%
4440
21st to endNight
19%22%10%13%9%443339%
4884
February1st to 10thNight
12%14%8%12%12%5%6%44418%
2477
11th to 20thNight
13%12%7%12%10%45%5%5%6%20%
2218
21st to endNight
16%16%49%7%45%5%5%6%23%
1884
March1st to 10thNight
16%9%46%7%345%5%7%33%
2280
11th to 20thNight
12%11%36%5%33448%40%
2165
21st to endNight
18%10%47%6%35%45%7%33%
2169
April1st to 10thNight
17%10%46%6%35%45%7%33%
1800
11th to 20thNight
19%9%45%42435%6%37%
1726
21st to endNight
16%9%36%5%345%5%7%37%
1620
May1st to 10thNight
15%7%36%6%45%5%5%9%35%
1620
11th to 20thNight
16%6%26%5%35%46%8%39%
1540
21st to endNight
13%6%46%5%3445%7%44%
1551
June1st to 10thNight
13%5%444245%7%8%45%
1440
11th to 20thNight
11%5%345%245%6%9%46%
1440
21st to endNight
9%5%35%435%5%6%9%46%
1440
July1st to 10thNight
7%3234345%7%8%54%
1440
11th to 20thNight
6%324445%5%6%9%52%
1440
21st to endNight
8%326%425%5%6%7%51%
1575
August1st to 10thNight
8%5%244445%6%9%50%
1543
11th to 20thNight
8%3144345%6%8%52%
1598
21st to endNight
10%5%35%3245%5%7%50%
1948
September1st to 10thNight
9%5%26%5%245%5%6%50%
1800
11th to 20thNight
12%5%2432345%6%54%
1973
21st to endNight
14%6%26%5%445%5%7%43%
1980
October1st to 10thNight
19%6%34423348%45%
1980
11th to 20thNight
22%9%38%6%35%35%6%32%
2226
21st to endNight
25%11%36%5%34347%30%
2574
November1st to 10thNight
15%11%36%5%35%46%6%36%
2406
11th to 20thNight
16%16%5%9%7%45%6%6%6%21%
2470
21st to endNight
19%19%6%8%6%3445%6%22%
2474
December1st to 10thNight
16%17%10%14%9%45%44415%
2660
11th to 20thNight
16%20%7%13%7%45%445%15%
3279
21st to endNight
23%22%8%11%6%34345%12%
4884

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
82%48%6%1
2528
11th to 20thDay
86%45%41
2530
21st to endDay
84%28%5%1
2783
February1st to 10thDay
70%5%8%14%3
1699
11th to 20thDay
69%49%12%6%
1858
21st to endDay
66%5%12%12%5%
1570
March1st to 10thDay
57%5%13%16%8%1
1900
11th to 20thDay
51%6%13%17%11%2
2014
21st to endDay
55%5%11%20%8%1
2357
April1st to 10thDay
55%5%10%20%10%
2160
11th to 20thDay
52%5%9%19%13%2
2181
21st to endDay
49%6%12%18%13%2
2340
May1st to 10thDay
45%6%12%24%13%2
2340
11th to 20thDay
44%6%13%20%15%2
2357
21st to endDay
39%6%14%25%16%1
2725
June1st to 10thDay
38%5%11%27%17%2
2520
11th to 20thDay
38%5%13%22%21%1
2520
21st to endDay
39%6%13%23%17%2
2520
July1st to 10thDay
28%6%14%30%18%4
2451
11th to 20thDay
32%6%13%28%19%2
2445
21st to endDay
31%7%14%27%18%3
2591
August1st to 10thDay
33%7%15%29%14%1
2380
11th to 20thDay
35%7%14%27%15%2
2303
21st to endDay
36%6%13%26%17%3
2405
September1st to 10thDay
40%6%13%24%14%3
2147
11th to 20thDay
37%5%13%31%12%2
1980
21st to endDay
43%5%13%27%12%
1980
October1st to 10thDay
46%5%10%25%14%1
1980
11th to 20thDay
62%5%12%15%5%
1732
21st to endDay
63%6%12%14%5%
1782
November1st to 10thDay
50%6%12%22%9%
1672
11th to 20thDay
67%412%12%4
1710
21st to endDay
69%413%11%3
1704
December1st to 10thDay
77%48%9%2
1520
11th to 20thDay
76%313%7%1
1915
21st to endDay
81%5%7%6%1
2783
January1st to 10thNight
74%49%11%2
4428
11th to 20thNight
82%47%5%2
4440
21st to endNight
81%48%5%1
4884
February1st to 10thNight
71%48%12%6%1
2477
11th to 20thNight
64%5%10%15%6%
2219
21st to endNight
61%5%13%14%6%
1884
March1st to 10thNight
51%5%14%21%8%1
2280
11th to 20thNight
44%5%15%21%13%2
2165
21st to endNight
52%5%11%22%9%2
2169
April1st to 10thNight
51%411%20%13%1
1800
11th to 20thNight
49%38%22%15%3
1726
21st to endNight
47%6%11%20%14%4
1620
May1st to 10thNight
46%6%13%20%13%2
1620
11th to 20thNight
44%6%11%20%16%4
1541
21st to endNight
40%5%10%27%15%3
1552
June1st to 10thNight
35%6%12%29%16%3
1440
11th to 20thNight
34%7%13%24%20%2
1440
21st to endNight
34%8%15%26%15%3
1440
July1st to 10thNight
25%9%13%28%21%4
1400
11th to 20thNight
27%7%15%30%18%3
1392
21st to endNight
30%7%15%27%17%3
1487
August1st to 10thNight
31%6%15%32%14%1
1534
11th to 20thNight
29%6%15%31%18%1
1598
21st to endNight
33%6%11%26%21%3
1948
September1st to 10thNight
33%8%11%28%18%2
1800
11th to 20thNight
31%412%37%14%2
1973
21st to endNight
40%5%14%29%11%1
1980
October1st to 10thNight
40%410%28%16%1
1980
11th to 20thNight
54%5%13%20%7%1
2226
21st to endNight
56%5%14%18%8%
2574
November1st to 10thNight
48%6%12%22%11%1
2406
11th to 20thNight
62%5%14%13%5%
2470
21st to endNight
65%416%13%21
2474
December1st to 10thNight
74%5%10%8%3
2660
11th to 20thNight
72%5%11%9%2
3281
21st to endNight
77%410%6%2
4884

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%14%38%20%7%1
270
11th to 20thDay
22%18%39%17%4
270
21st to endDay
29%13%33%21%4
297
February1st to 10thDay
19%26%31%18%42
270
11th to 20thDay
18%26%37%15%3
270
21st to endDay
21%31%36%10%2
225
March1st to 10thDay
21%25%39%11%4
270
11th to 20thDay
22%17%29%21%10%1
384
21st to endDay
27%13%30%23%5%2
490
April1st to 10thDay
24%16%34%18%7%1
420
11th to 20thDay
28%22%22%17%8%3
412
21st to endDay
24%15%27%24%10%1
420
May1st to 10thDay
23%17%29%21%8%3
423
11th to 20thDay
19%13%28%24%11%4
499
21st to endDay
15%14%27%27%13%5%
679
June1st to 10thDay
12%9%36%28%9%5%
630
11th to 20thDay
17%9%30%30%11%3
630
21st to endDay
16%8%33%29%10%4
630
July1st to 10thDay
8%5%25%37%17%9%
621
11th to 20thDay
8%7%29%35%16%6%
618
21st to endDay
9%5%31%31%16%8%
684
August1st to 10thDay
9%5%26%26%23%11%
514
11th to 20thDay
13%5%17%29%22%13%
439
21st to endDay
15%6%20%27%22%9%
495
September1st to 10thDay
16%6%24%29%18%8%
450
11th to 20thDay
10%6%23%30%23%8%
450
21st to endDay
16%13%23%26%16%7%
450
October1st to 10thDay
20%12%24%24%17%3
450
11th to 20thDay
31%14%28%23%4
332
21st to endDay
34%13%34%11%5%2
297
November1st to 10thDay
24%19%31%17%7%2
270
11th to 20thDay
22%23%39%12%4
270
21st to endDay
31%17%35%12%4
270
December1st to 10thDay
27%28%26%15%5%
270
11th to 20thDay
23%26%35%13%3
270
21st to endDay
29%23%33%11%31
297
January1st to 10thNight
21%12%33%26%6%1
950
11th to 20thNight
24%12%33%24%6%
950
21st to endNight
22%9%33%30%7%
1045
February1st to 10thNight
18%13%35%30%4
831
11th to 20thNight
21%11%34%28%5%1
743
21st to endNight
24%13%36%23%3
628
March1st to 10thNight
19%10%36%27%8%1
760
11th to 20thNight
17%14%33%28%8%1
760
21st to endNight
21%10%29%31%8%1
734
April1st to 10thNight
22%10%33%29%6%1
540
11th to 20thNight
26%8%30%27%7%1
533
21st to endNight
20%9%31%30%9%1
540
May1st to 10thNight
18%9%31%34%7%1
540
11th to 20thNight
17%6%31%34%11%
531
21st to endNight
16%425%35%19%1
584
June1st to 10thNight
12%427%41%14%1
540
11th to 20thNight
14%5%27%36%17%1
540
21st to endNight
10%424%44%18%1
540
July1st to 10thNight
6%317%42%28%3
525
11th to 20thNight
7%319%45%24%2
522
21st to endNight
8%320%44%24%1
591
August1st to 10thNight
9%115%46%27%2
537
11th to 20thNight
9%114%43%28%3
534
21st to endNight
13%220%41%22%2
594
September1st to 10thNight
11%319%42%21%4
540
11th to 20thNight
13%6%18%35%25%3
713
21st to endNight
16%7%23%32%18%4
720
October1st to 10thNight
22%5%21%34%15%2
720
11th to 20thNight
27%9%27%28%8%1
720
21st to endNight
31%9%28%26%6%
792
November1st to 10thNight
22%10%30%30%9%1
740
11th to 20thNight
23%15%31%25%6%
760
21st to endNight
27%12%30%26%4
764
December1st to 10thNight
24%14%33%24%4
950
11th to 20thNight
25%14%35%20%41
950
21st to endNight
30%12%34%22%1
1045
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