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

2017-09-12
.

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
36%
16003
13%
46770
9%
15961
40%
15961
19%
33668

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
16%
900
4%
2506
2%
895
18%
895
8%
2180
January16th to endNight
19%
960
5%
2688
3%
960
21%
960
9%
2305
February1st to 15thNight
19%
746
7%
2237
2%
746
23%
746
9%
1835
February16th to endNight
24%
631
9%
1893
5%
631
28%
631
12%
1502
March1st to 15thNight
30%
720
14%
2148
9%
720
35%
720
19%
1505
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
61%
584
23%
1821
22%
584
65%
584
35%
1044
September16th to endNight
56%
657
21%
1815
15%
657
61%
657
34%
1115
October1st to 15thNight
45%
660
20%
1856
15%
660
50%
660
28%
1230
October16th to endNight
27%
704
11%
2288
7%
704
34%
704
16%
1644
November1st to 15thNight
32%
700
17%
2276
14%
700
38%
700
18%
1573
November16th to endNight
22%
723
8%
2343
5%
723
25%
723
10%
1884
December1st to 15thNight
16%
900
3%
2520
2%
900
17%
900
6%
2231
December16th to endNight
14%
960
6%
2688
2%
960
18%
960
8%
2315

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%8%47%6%35%45%6%33%
47904
Night
15%10%47%6%34446%32%
46938
Cloud CoverDay
15%9%47%6%3445%6%32%
47913
Night
15%10%47%6%34446%32%
46943
TransparencyDay
51%5%11%20%10%1
47590
Night
52%5%12%19%9%1
46770
SeeingDay
18%9%27%26%13%5%
5991
Night
19%7%28%31%11%1
16007

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%16%9%13%9%5%443410%
1432
16th to endDay
12%19%12%12%11%5%6%3329%
1536
February1st to 15thDay
11%16%9%11%11%5%6%33415%
1624
16th to endDay
11%16%8%11%9%46%5%4416%
1580
March1st to 15thDay
16%11%38%6%35%46%5%27%
1802
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%414434447%45%
2047
16th to endDay
12%436%6%35%5%5%7%39%
1815
October1st to 15thDay
18%7%26%4345%45%36%
1773
16th to endDay
23%8%48%6%34446%24%
1584
November1st to 15thDay
16%10%37%5%23447%33%
1582
16th to endDay
21%17%5%9%7%34335%17%
1617
December1st to 15thDay
17%19%11%12%7%3432313%
1440
16th to endDay
20%19%10%11%8%443339%
1536
January1st to 15thNight
19%16%9%12%7%4433315%
2506
16th to endNight
12%14%10%12%9%5%5%445%14%
2688
February1st to 15thNight
11%16%7%11%9%5%5%44419%
2235
16th to endNight
16%13%5%8%8%46%5%5%6%20%
1893
March1st to 15thNight
16%9%36%6%2445%7%34%
2135
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
11%314324446%53%
1821
16th to endNight
11%415%424446%50%
1815
October1st to 15thNight
18%5%245%23246%42%
1856
16th to endNight
21%8%36%5%34347%30%
2288
November1st to 15thNight
15%10%45%5%2436%6%34%
2276
16th to endNight
19%17%5%7%6%3445%6%18%
2343
December1st to 15thNight
20%17%8%11%9%45%33312%
2520
16th to endNight
21%18%7%10%7%24345%13%
2688

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%10%13%9%5%443410%
1432
16th to endDay
13%19%13%11%11%5%5%3339%
1536
February1st to 15thDay
12%16%9%11%11%5%5%32415%
1624
16th to endDay
11%16%8%12%10%45%44417%
1580
March1st to 15thDay
16%11%49%6%25%46%6%26%
1812
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%5%25%45%6%44%
2047
16th to endDay
12%5%36%6%245%5%8%38%
1815
October1st to 15thDay
19%8%25%5%33445%36%
1773
16th to endDay
25%8%47%6%34346%23%
1584
November1st to 15thDay
16%11%46%5%24446%33%
1582
16th to endDay
22%17%6%9%6%24436%17%
1617
December1st to 15thDay
17%19%12%12%7%3423313%
1440
16th to endDay
21%20%10%13%7%333339%
1536
January1st to 15thNight
19%16%9%12%8%3333315%
2506
16th to endNight
13%14%9%13%10%5%5%345%14%
2688
February1st to 15thNight
12%15%8%11%10%45%33419%
2228
16th to endNight
15%14%49%7%45%5%5%6%20%
1893
March1st to 15thNight
16%9%46%5%33448%33%
2148
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%
1821
16th to endNight
11%425%433446%50%
1815
October1st to 15thNight
20%5%25%423347%41%
1856
16th to endNight
22%8%37%5%34346%31%
2288
November1st to 15thNight
16%11%36%5%2435%5%34%
2276
16th to endNight
20%18%5%6%6%3445%5%18%
2343
December1st to 15thNight
21%16%8%11%9%3433312%
2520
16th to endNight
21%18%8%11%6%3444413%
2688

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
1433
16th to endDay
81%36%6%2
1536
February1st to 15thDay
73%47%13%1
1626
16th to endDay
70%5%10%10%3
1580
March1st to 15thDay
57%5%13%15%7%
1812
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
39%5%11%28%12%2
2047
16th to endDay
42%414%25%12%
1815
October1st to 15thDay
49%410%23%10%
1773
16th to endDay
61%5%14%13%4
1584
November1st to 15thDay
51%413%21%8%
1582
16th to endDay
69%412%10%2
1617
December1st to 15thDay
78%38%8%1
1440
16th to endDay
81%38%41
1536
January1st to 15thNight
75%38%8%2
2506
16th to endNight
72%411%8%2
2688
February1st to 15thNight
69%38%13%4
2237
16th to endNight
63%5%12%12%5%
1893
March1st to 15thNight
49%5%14%18%10%1
2148
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%31%17%1
1821
16th to endNight
34%413%33%12%
1815
October1st to 15thNight
43%311%26%13%
1856
16th to endNight
53%5%15%18%6%
2288
November1st to 15thNight
50%5%13%19%10%1
2276
16th to endNight
67%414%10%2
2343
December1st to 15thNight
77%48%6%2
2520
16th to endNight
73%411%8%1
2688

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
17%15%41%25%
90
16th to endDay
34%430%23%6%1
96
February1st to 15thDay
14%26%38%15%4
90
16th to endDay
23%19%32%19%4
81
March1st to 15thDay
32%19%20%17%8%
102
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%418%31%23%10%
299
16th to endDay
13%5%20%29%22%9%
254
October1st to 15thDay
21%8%16%33%16%2
238
16th to endDay
29%8%42%19%
96
November1st to 15thDay
16%12%35%21%10%4
90
16th to endDay
35%16%30%8%8%
90
December1st to 15thDay
31%26%26%12%3
90
16th to endDay
32%33%28%6%
96
January1st to 15thNight
26%11%31%22%7%
900
16th to endNight
20%9%35%30%5%
960
February1st to 15thNight
18%12%33%29%4
749
16th to endNight
24%9%30%28%6%
631
March1st to 15thNight
21%9%31%28%8%
720
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%4
584
16th to endNight
13%419%35%22%4
657
October1st to 15thNight
22%323%34%13%2
660
16th to endNight
27%9%27%26%7%
704
November1st to 15thNight
23%9%27%28%9%
700
16th to endNight
28%10%28%25%6%
723
December1st to 15thNight
30%12%30%22%3
900
16th to endNight
28%10%35%21%2
960
Page generated at 2017-09-13 07:39:26. Version ac04

Copyright 2017 A. Danko


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