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 2005-11-05 to 2018-03-11.

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
35%
16790
13%
49084
9%
16748
39%
16748
19%
35404

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 15thNight
17%
975
5%
2716
2%
970
19%
970
8%
2353
January16th to endNight
19%
1040
5%
2912
3%
1040
21%
1040
9%
2484
February1st to 15thNight
19%
810
8%
2427
2%
810
23%
810
9%
1991
February16th to endNight
23%
683
9%
2049
5%
683
27%
683
12%
1620
March1st to 15thNight
31%
764
14%
2280
9%
764
35%
764
19%
1597
March16th to endNight
34%
711
17%
2055
11%
711
40%
711
22%
1409
April1st to 15thNight
29%
539
14%
1797
10%
539
36%
539
18%
1258
April16th to endNight
35%
540
20%
1646
11%
540
43%
540
24%
1084
May1st to 15thNight
37%
540
15%
1620
9%
540
43%
540
22%
1115
May16th to endNight
43%
562
16%
1520
8%
561
50%
561
29%
995
June1st to 15thNight
50%
540
13%
1440
8%
540
54%
540
34%
904
June16th to endNight
53%
540
14%
1440
8%
540
57%
540
33%
932
July1st to 15thNight
66%
510
17%
1360
13%
510
68%
510
42%
787
July16th to endNight
61%
564
17%
1417
12%
531
65%
531
39%
853
August1st to 15thNight
67%
539
22%
1566
18%
536
70%
536
39%
889
August16th to endNight
61%
573
22%
1830
19%
573
65%
573
32%
1079
September1st to 15thNight
62%
597
24%
1857
22%
597
65%
597
35%
1059
September16th to endNight
58%
717
24%
1980
17%
717
62%
717
35%
1198
October1st to 15thNight
45%
720
19%
2025
15%
720
49%
720
27%
1347
October16th to endNight
28%
768
12%
2496
8%
768
35%
768
16%
1791
November1st to 15thNight
31%
760
16%
2471
13%
760
37%
760
17%
1734
November16th to endNight
22%
783
8%
2538
5%
783
26%
783
10%
2006
December1st to 15thNight
16%
975
3%
2730
2%
975
17%
975
7%
2404
December16th to endNight
14%
1040
6%
2912
2%
1040
18%
1040
7%
2515

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%35%45%6%33%
49557
Night
15%10%47%6%34446%32%
49252
Cloud CoverDay
15%9%47%6%3445%6%32%
49566
Night
15%10%47%6%34446%32%
49257
TransparencyDay
51%5%11%19%10%1
49243
Night
52%5%12%19%9%1
49084
SeeingDay
18%9%27%25%12%5%
6564
Night
19%7%28%31%11%1
16794

By Month

Percentage Cloud Cover NE (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 15thDay
17%15%9%13%11%5%443410%
1552
16th to endDay
12%18%11%12%11%5%6%4439%
1664
February1st to 15thDay
11%16%9%11%12%5%6%33414%
1763
16th to endDay
12%15%8%11%9%46%5%4417%
1710
March1st to 15thDay
15%12%38%6%35%45%5%27%
1907
16th to endDay
17%7%47%6%24446%33%
2179
April1st to 15thDay
21%11%45%5%24436%31%
2151
16th to endDay
18%8%25%5%25%45%6%35%
2313
May1st to 15thDay
19%5%35%5%2336%7%36%
2331
16th to endDay
12%425%5%25%5%6%8%42%
2594
June1st to 15thDay
12%5%36%5%3445%8%39%
2520
16th to endDay
11%425%445%45%7%43%
2520
July1st to 15thDay
7%324435%45%8%49%
2520
16th to endDay
10%4235%2446%10%45%
2655
August1st to 15thDay
8%424446%5%7%8%43%
2384
16th to endDay
13%5%25%42445%7%43%
2369
September1st to 15thDay
14%424434446%45%
2091
16th to endDay
11%436%6%35%5%5%7%41%
1980
October1st to 15thDay
19%7%26%4345%45%35%
1934
16th to endDay
23%9%47%6%34446%24%
1728
November1st to 15thDay
17%10%37%5%23447%32%
1717
16th to endDay
20%17%5%8%7%34335%19%
1751
December1st to 15thDay
16%19%12%13%7%3433312%
1560
16th to endDay
20%20%10%11%8%443348%
1664
January1st to 15thNight
18%15%8%12%8%5%433315%
2716
16th to endNight
12%13%10%12%9%5%6%445%15%
2912
February1st to 15thNight
11%16%7%11%9%5%5%44419%
2425
16th to endNight
16%13%5%7%8%46%5%5%6%20%
2049
March1st to 15thNight
15%9%46%6%34447%34%
2267
16th to endNight
15%7%45%6%2435%7%35%
2055
April1st to 15thNight
19%10%45%5%25%346%32%
1797
16th to endNight
17%7%25%324446%40%
1646
May1st to 15thNight
16%5%36%5%35%5%5%7%35%
1620
16th to endNight
11%5%26%435%45%9%41%
1518
June1st to 15thNight
10%5%25%42446%8%45%
1440
16th to endNight
9%4145%336%8%9%44%
1440
July1st to 15thNight
6%31343345%10%53%
1440
16th to endNight
7%315%425%6%6%8%50%
1513
August1st to 15thNight
5%4235%25%46%8%52%
1575
16th to endNight
11%41432435%6%50%
1830
September1st to 15thNight
10%314324446%53%
1857
16th to endNight
10%315%424446%51%
1980
October1st to 15thNight
20%5%245%23246%41%
2025
16th to endNight
22%8%36%5%34347%31%
2496
November1st to 15thNight
16%10%45%5%2446%6%32%
2471
16th to endNight
18%17%5%7%6%34446%20%
2538
December1st to 15thNight
19%17%8%11%9%35%34313%
2730
16th to endNight
20%18%7%10%7%34345%13%
2912

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 15thDay
17%16%9%13%10%5%442410%
1552
16th to endDay
12%18%12%11%11%6%5%3339%
1664
February1st to 15thDay
12%16%9%11%12%5%5%33414%
1763
16th to endDay
12%16%8%12%9%35%44417%
1710
March1st to 15thDay
15%12%49%6%25%36%6%27%
1917
16th to endDay
16%8%47%6%2445%6%33%
2178
April1st to 15thDay
21%12%34424446%31%
2151
16th to endDay
19%8%25%5%24446%35%
2313
May1st to 15thDay
20%6%25%42436%6%36%
2331
16th to endDay
12%4245%25%5%5%8%42%
2594
June1st to 15thDay
12%6%36%6%3445%8%38%
2520
16th to endDay
11%5%25%5%35%5%48%42%
2520
July1st to 15thDay
8%324425%45%9%48%
2520
16th to endDay
10%4244245%6%10%44%
2655
August1st to 15thDay
8%4245%35%5%7%9%42%
2384
16th to endDay
14%5%25%5%2345%6%42%
2369
September1st to 15thDay
14%425%425%45%6%44%
2091
16th to endDay
11%5%36%6%3445%8%40%
1980
October1st to 15thDay
19%8%25%5%34445%36%
1934
16th to endDay
25%9%47%6%34346%23%
1728
November1st to 15thDay
17%12%46%5%14446%32%
1717
16th to endDay
21%17%6%8%6%23436%18%
1751
December1st to 15thDay
17%19%12%13%7%3423412%
1560
16th to endDay
20%20%10%13%7%433438%
1664
January1st to 15thNight
19%15%9%12%9%4333315%
2716
16th to endNight
13%14%9%13%10%5%5%345%14%
2912
February1st to 15thNight
11%15%8%11%10%45%43419%
2418
16th to endNight
16%14%48%8%45%5%5%6%20%
2049
March1st to 15thNight
15%9%46%6%34448%33%
2280
16th to endNight
14%9%45%6%34347%35%
2055
April1st to 15thNight
20%9%45%5%25%346%32%
1797
16th to endNight
17%7%25%323447%40%
1646
May1st to 15thNight
16%5%25%5%35%5%5%7%35%
1619
16th to endNight
11%5%36%5%2445%8%42%
1518
June1st to 15thNight
12%4334245%6%8%44%
1440
16th to endNight
9%424425%5%6%10%43%
1440
July1st to 15thNight
6%22232446%8%53%
1440
16th to endNight
7%2244345%6%8%50%
1513
August1st to 15thNight
5%4234345%6%8%52%
1575
16th to endNight
11%424324446%50%
1830
September1st to 15thNight
11%324323446%53%
1857
16th to endNight
10%425%334446%51%
1980
October1st to 15thNight
21%5%24423247%41%
2025
16th to endNight
22%8%37%5%34346%31%
2496
November1st to 15thNight
17%11%36%5%3435%6%33%
2471
16th to endNight
19%17%46%5%3445%5%20%
2538
December1st to 15thNight
20%16%8%12%9%3433413%
2730
16th to endNight
20%18%8%11%7%3444412%
2912

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 15thDay
78%5%7%7%1
1553
16th to endDay
80%47%6%2
1664
February1st to 15thDay
73%47%12%1
1765
16th to endDay
69%410%11%4
1710
March1st to 15thDay
57%5%13%15%7%
1917
16th to endDay
50%5%13%22%7%1
2179
April1st to 15thDay
54%49%20%11%
2151
16th to endDay
48%411%18%13%2
2313
May1st to 15thDay
46%412%23%11%1
2331
16th to endDay
37%6%15%25%14%
2598
June1st to 15thDay
42%411%24%15%
2520
16th to endDay
39%6%13%22%16%1
2520
July1st to 15thDay
31%5%12%27%19%2
2371
16th to endDay
34%6%13%26%16%2
2488
August1st to 15thDay
34%7%14%29%12%1
2369
16th to endDay
39%5%11%24%15%2
2369
September1st to 15thDay
38%5%11%29%13%2
2091
16th to endDay
40%414%26%13%
1980
October1st to 15thDay
50%410%23%11%
1934
16th to endDay
61%5%14%13%4
1728
November1st to 15thDay
52%412%21%8%
1717
16th to endDay
68%413%10%3
1751
December1st to 15thDay
78%37%8%1
1560
16th to endDay
81%38%41
1664
January1st to 15thNight
75%39%8%2
2716
16th to endNight
71%5%12%8%2
2912
February1st to 15thNight
69%38%13%4
2427
16th to endNight
63%5%12%13%5%
2049
March1st to 15thNight
49%5%13%18%10%1
2280
16th to endNight
47%413%22%9%1
2055
April1st to 15thNight
53%38%19%14%
1797
16th to endNight
44%410%21%14%4
1646
May1st to 15thNight
46%6%12%20%12%1
1620
16th to endNight
39%5%11%26%14%2
1520
June1st to 15thNight
35%5%14%27%15%1
1440
16th to endNight
33%9%14%24%16%1
1440
July1st to 15thNight
25%7%13%30%21%2
1360
16th to endNight
29%7%17%26%16%2
1417
August1st to 15thNight
28%6%14%33%17%
1566
16th to endNight
33%6%10%26%20%2
1830
September1st to 15thNight
31%5%11%32%18%1
1857
16th to endNight
33%413%33%13%
1980
October1st to 15thNight
44%310%26%13%1
2025
16th to endNight
53%5%15%19%6%
2496
November1st to 15thNight
52%5%12%18%10%
2471
16th to endNight
65%415%11%2
2538
December1st to 15thNight
76%48%7%2
2730
16th to endNight
74%411%8%1
2912

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 15thDay
19%12%40%26%1
135
16th to endDay
27%11%34%20%6%
144
February1st to 15thDay
14%26%40%14%2
135
16th to endDay
23%20%36%15%4
120
March1st to 15thDay
27%19%27%15%9%
133
16th to endDay
27%7%25%26%10%3
288
April1st to 15thDay
30%10%30%19%8%1
270
16th to endDay
25%10%25%22%12%3
270
May1st to 15thDay
26%10%24%23%8%6%
288
16th to endDay
15%13%30%26%10%4
494
June1st to 15thDay
14%8%35%28%9%4
495
16th to endDay
14%8%33%27%10%4
495
July1st to 15thDay
8%6%27%33%16%8%
475
16th to endDay
10%5%26%34%13%9%
515
August1st to 15thDay
7%321%28%24%14%
357
16th to endDay
22%414%24%21%12%
332
September1st to 15thDay
12%317%29%25%11%
315
16th to endDay
10%5%20%29%25%8%
314
October1st to 15thDay
21%9%18%33%14%2
296
16th to endDay
31%8%38%19%
144
November1st to 15thDay
26%9%35%18%6%2
135
16th to endDay
28%22%30%11%6%
135
December1st to 15thDay
31%27%25%13%2
135
16th to endDay
29%30%30%7%1
144
January1st to 15thNight
25%11%31%23%7%
975
16th to endNight
20%9%35%30%5%
1040
February1st to 15thNight
18%11%34%29%4
813
16th to endNight
25%9%31%27%5%
683
March1st to 15thNight
20%9%31%28%8%
764
16th to endNight
17%8%31%29%10%1
711
April1st to 15thNight
24%8%32%27%6%
539
16th to endNight
21%7%31%29%8%1
540
May1st to 15thNight
19%7%30%33%9%
540
16th to endNight
14%5%30%34%15%
563
June1st to 15thNight
13%426%39%15%
540
16th to endNight
10%326%42%16%
540
July1st to 15thNight
6%218%45%24%2
510
16th to endNight
7%221%42%23%2
564
August1st to 15thNight
6%17%47%25%2
539
16th to endNight
13%118%37%26%3
573
September1st to 15thNight
13%316%38%22%5%
597
16th to endNight
12%420%34%24%4
717
October1st to 15thNight
23%323%33%13%2
720
16th to endNight
27%9%27%26%7%
768
November1st to 15thNight
23%9%27%28%9%
760
16th to endNight
27%10%29%25%5%
783
December1st to 15thNight
29%12%30%24%3
975
16th to endNight
28%11%35%21%2
1040
Page generated at 2018-03-12 13:28:03. Version ac04

Copyright 2018 A. Danko


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