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WCAC
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the WCAC 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
39%
16016
15%
49240
11%
15978
43%
15978
19%
34761

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%
776
9%
2522
4%
776
23%
776
9%
2035
January16th to endNight
16%
832
10%
2704
4%
832
23%
832
11%
2117
February1st to 15thNight
19%
763
8%
2482
4%
761
25%
761
10%
1964
February16th to endNight
21%
683
10%
2062
5%
683
26%
683
11%
1636
March1st to 15thNight
29%
764
14%
2137
9%
764
34%
764
17%
1610
March16th to endNight
30%
768
16%
2112
11%
768
34%
768
20%
1526
April1st to 15thNight
28%
688
12%
1868
6%
688
33%
688
20%
1381
April16th to endNight
37%
540
19%
1620
9%
540
44%
540
24%
1087
May1st to 15thNight
42%
540
13%
1620
8%
540
46%
540
19%
1161
May16th to endNight
56%
561
15%
1684
14%
561
59%
561
27%
1093
June1st to 15thNight
55%
396
15%
1476
17%
396
59%
396
26%
872
June16th to endNight
57%
427
15%
1507
15%
427
62%
427
26%
922
July1st to 15thNight
63%
510
14%
1530
14%
510
65%
510
30%
972
July16th to endNight
68%
564
16%
1595
14%
531
70%
531
33%
957
August1st to 15thNight
69%
539
20%
1609
17%
536
70%
536
33%
957
August16th to endNight
69%
588
22%
1734
17%
588
71%
588
36%
1034
September1st to 15thNight
62%
720
25%
1932
17%
720
65%
720
36%
1197
September16th to endNight
58%
717
25%
1980
21%
717
61%
717
32%
1250
October1st to 15thNight
49%
720
19%
2115
18%
720
52%
720
26%
1394
October16th to endNight
33%
768
17%
2304
14%
768
39%
768
19%
1632
November1st to 15thNight
41%
760
19%
2467
17%
760
48%
760
23%
1601
November16th to endNight
25%
780
13%
2644
9%
780
31%
780
14%
1934
December1st to 15thNight
20%
780
7%
2730
5%
780
23%
780
10%
2159
December16th to endNight
18%
832
8%
2806
3%
832
23%
832
9%
2270

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%9%47%6%35%45%7%31%
49362
Night
15%9%46%5%3445%6%34%
49433
Cloud CoverDay
16%9%37%6%35%45%6%31%
49372
Night
15%9%46%5%3445%6%34%
49437
TransparencyDay
52%6%11%18%9%1
49073
Night
49%6%11%18%11%1
49240
SeeingDay
20%8%24%21%17%8%
5899
Night
19%8%26%29%14%1
16023

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
20%15%7%10%7%4442419%
1746
16th to endDay
16%17%7%12%8%3444415%
1872
February1st to 15thDay
18%16%5%11%8%34335%19%
1710
16th to endDay
17%15%5%9%6%24335%25%
1695
March1st to 15thDay
19%14%5%7%6%24335%26%
2049
16th to endDay
20%10%46%6%33445%30%
2112
April1st to 15thDay
19%10%36%6%2435%6%31%
2089
16th to endDay
17%8%36%5%34446%35%
2340
May1st to 15thDay
19%7%47%6%35%5%5%6%28%
2331
16th to endDay
11%6%25%5%45%5%6%8%37%
2431
June1st to 15thDay
12%6%35%5%36%6%6%8%35%
2481
16th to endDay
11%6%25%5%46%6%7%10%33%
2447
July1st to 15thDay
12%6%36%6%36%5%5%8%35%
2340
16th to endDay
11%425%6%36%6%7%9%35%
2465
August1st to 15thDay
8%6%26%6%47%6%7%10%33%
2340
16th to endDay
10%425%5%46%6%6%9%38%
2455
September1st to 15thDay
14%6%35%5%3345%7%40%
2025
16th to endDay
16%6%25%5%34447%39%
1980
October1st to 15thDay
17%7%27%5%245%46%35%
1845
16th to endDay
21%10%35%5%24345%34%
1920
November1st to 15thDay
13%11%35%5%35%346%38%
1720
16th to endDay
19%13%47%6%345%45%25%
1644
December1st to 15thDay
20%16%7%11%6%3323419%
1560
16th to endDay
21%18%6%11%7%3423415%
1765
January1st to 15thNight
20%16%6%8%7%3444321%
2522
16th to endNight
17%15%5%8%7%34445%22%
2704
February1st to 15thNight
18%13%6%10%7%3434423%
2480
16th to endNight
17%13%6%9%7%35%435%22%
2062
March1st to 15thNight
18%11%47%6%23446%30%
2125
16th to endNight
16%9%5%5%5%345%46%33%
2112
April1st to 15thNight
17%9%46%5%35%45%6%31%
1868
16th to endNight
13%8%36%424446%40%
1620
May1st to 15thNight
17%7%25%6%35%6%6%6%32%
1620
16th to endNight
10%5%25%4345%6%9%40%
1682
June1st to 15thNight
6%5%36%5%245%5%8%45%
1476
16th to endNight
10%4235%25%6%6%10%41%
1507
July1st to 15thNight
7%416%6%345%49%45%
1620
16th to endNight
6%225%5%345%6%10%46%
1703
August1st to 15thNight
6%415%5%335%6%10%45%
1620
16th to endNight
8%4244245%6%8%48%
1734
September1st to 15thNight
13%323313337%51%
1932
16th to endNight
12%5%2442445%6%47%
1980
October1st to 15thNight
14%6%25%5%34446%41%
2115
16th to endNight
19%8%26%5%3435%5%34%
2304
November1st to 15thNight
16%7%2442445%6%40%
2467
16th to endNight
19%11%47%5%34446%28%
2644
December1st to 15thNight
20%15%5%9%6%24445%21%
2730
16th to endNight
19%16%6%8%6%34345%19%
2806

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
20%14%6%10%7%4443419%
1746
16th to endDay
16%17%7%11%8%3445%416%
1872
February1st to 15thDay
18%16%5%10%8%3433419%
1710
16th to endDay
16%15%5%9%6%24345%25%
1695
March1st to 15thDay
19%14%5%7%6%24345%26%
2060
16th to endDay
20%9%46%5%345%45%30%
2111
April1st to 15thDay
19%10%36%6%2435%6%31%
2089
16th to endDay
17%8%36%5%34346%35%
2340
May1st to 15thDay
19%7%37%6%345%5%6%28%
2331
16th to endDay
12%5%25%5%35%5%7%8%37%
2431
June1st to 15thDay
12%6%35%6%36%6%6%8%35%
2481
16th to endDay
12%6%25%5%46%6%7%9%33%
2447
July1st to 15thDay
12%6%36%6%36%5%5%8%35%
2340
16th to endDay
11%425%7%36%6%7%9%35%
2465
August1st to 15thDay
8%6%26%6%46%6%7%10%33%
2340
16th to endDay
10%426%5%46%6%6%9%38%
2455
September1st to 15thDay
14%6%25%43346%7%40%
2025
16th to endDay
16%6%25%5%24447%39%
1980
October1st to 15thDay
17%7%37%5%245%46%35%
1845
16th to endDay
21%9%25%42335%5%34%
1920
November1st to 15thDay
13%10%36%435%446%38%
1720
16th to endDay
19%13%46%5%345%5%426%
1644
December1st to 15thDay
20%16%7%12%5%3323420%
1560
16th to endDay
21%18%6%11%7%24335%15%
1765
January1st to 15thNight
20%16%5%9%7%3444320%
2522
16th to endNight
17%15%5%8%7%3444423%
2704
February1st to 15thNight
18%13%6%10%7%3334423%
2473
16th to endNight
18%13%6%8%6%45%435%23%
2062
March1st to 15thNight
18%11%47%5%23446%30%
2137
16th to endNight
16%9%46%5%24446%34%
2112
April1st to 15thNight
17%9%46%5%4436%6%31%
1868
16th to endNight
13%9%36%423447%40%
1620
May1st to 15thNight
17%7%25%6%35%5%6%6%32%
1619
16th to endNight
11%435%5%35%5%7%9%40%
1682
June1st to 15thNight
6%5%36%5%345%6%7%46%
1476
16th to endNight
10%4235%35%5%7%10%41%
1507
July1st to 15thNight
7%426%7%245%5%8%45%
1620
16th to endNight
6%225%6%245%6%9%46%
1703
August1st to 15thNight
6%415%5%345%6%10%45%
1620
16th to endNight
8%324325%5%5%9%48%
1734
September1st to 15thNight
13%413413337%51%
1932
16th to endNight
12%5%2442435%6%47%
1980
October1st to 15thNight
14%6%26%424446%42%
2115
16th to endNight
19%8%26%5%34445%34%
2304
November1st to 15thNight
16%7%2442445%6%40%
2467
16th to endNight
19%11%47%5%34346%28%
2644
December1st to 15thNight
20%15%5%8%6%24445%22%
2730
16th to endNight
20%16%6%8%6%34445%19%
2806

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
70%49%10%5%
1747
16th to endDay
70%6%11%9%2
1872
February1st to 15thDay
69%5%8%11%5%
1712
16th to endDay
62%411%14%6%
1695
March1st to 15thDay
60%5%10%17%5%
2060
16th to endDay
55%5%9%18%9%
2112
April1st to 15thDay
53%5%12%16%10%
2089
16th to endDay
49%5%10%18%12%3
2340
May1st to 15thDay
53%8%10%17%8%1
2331
16th to endDay
42%8%14%22%10%1
2435
June1st to 15thDay
43%8%13%23%10%
2481
16th to endDay
43%8%14%20%11%1
2447
July1st to 15thDay
46%8%10%19%13%2
2201
16th to endDay
41%9%14%21%11%1
2310
August1st to 15thDay
42%10%13%20%11%
2327
16th to endDay
39%8%12%23%12%3
2455
September1st to 15thDay
41%7%12%21%15%2
2025
16th to endDay
43%6%9%20%18%1
1980
October1st to 15thDay
48%6%10%18%14%1
1845
16th to endDay
51%413%20%9%
1920
November1st to 15thDay
46%410%22%11%3
1720
16th to endDay
60%5%10%15%7%
1644
December1st to 15thDay
70%49%11%3
1560
16th to endDay
72%5%9%9%2
1765
January1st to 15thNight
67%49%12%5%
2522
16th to endNight
63%5%12%13%4
2704
February1st to 15thNight
64%5%10%13%5%
2482
16th to endNight
63%5%9%14%6%
2062
March1st to 15thNight
55%5%10%19%8%1
2137
16th to endNight
50%6%11%19%9%2
2112
April1st to 15thNight
52%5%11%15%14%
1868
16th to endNight
44%5%9%18%17%4
1620
May1st to 15thNight
49%7%11%18%11%1
1620
16th to endNight
38%8%14%20%14%2
1684
June1st to 15thNight
34%8%13%24%16%1
1476
16th to endNight
35%9%15%19%16%2
1507
July1st to 15thNight
36%8%12%23%18%2
1530
16th to endNight
30%9%16%25%14%4
1595
August1st to 15thNight
30%8%14%25%19%1
1609
16th to endNight
30%7%14%26%16%4
1734
September1st to 15thNight
33%5%11%24%20%3
1932
16th to endNight
36%5%9%24%22%1
1980
October1st to 15thNight
42%6%11%19%18%1
2115
16th to endNight
50%5%11%22%8%1
2304
November1st to 15thNight
43%5%12%20%13%4
2467
16th to endNight
56%5%11%18%7%
2644
December1st to 15thNight
64%5%11%12%5%
2730
16th to endNight
66%6%12%12%2
2806

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
24%19%38%9%5%2
135
16th to endDay
27%22%34%14%1
144
February1st to 15thDay
29%20%40%8%
135
16th to endDay
26%21%40%10%
120
March1st to 15thDay
26%15%28%22%7%
276
16th to endDay
26%13%27%22%8%2
288
April1st to 15thDay
26%9%29%20%11%2
270
16th to endDay
18%8%31%21%12%8%
270
May1st to 15thDay
28%9%26%16%13%6%
288
16th to endDay
15%426%26%16%9%
331
June1st to 15thDay
15%420%31%21%6%
315
16th to endDay
18%6%20%26%19%8%
315
July1st to 15thDay
18%316%26%21%13%
305
16th to endDay
11%114%22%32%16%
326
August1st to 15thDay
16%13%23%28%16%
313
16th to endDay
11%311%20%31%20%
332
September1st to 15thDay
14%215%24%24%18%
315
16th to endDay
17%314%17%27%20%
314
October1st to 15thDay
15%7%23%24%23%6%
315
16th to endDay
24%9%31%21%11%1
336
November1st to 15thDay
19%11%23%24%18%2
138
16th to endDay
22%16%41%15%3
109
December1st to 15thDay
20%15%41%13%6%3
90
16th to endDay
33%21%34%9%
119
January1st to 15thNight
28%13%34%19%4
780
16th to endNight
25%13%37%20%3
832
February1st to 15thNight
25%12%35%23%2
764
16th to endNight
24%11%38%22%3
683
March1st to 15thNight
23%12%28%28%5%
764
16th to endNight
19%8%35%26%8%1
768
April1st to 15thNight
21%13%31%25%6%
688
16th to endNight
17%7%32%28%12%1
540
May1st to 15thNight
18%5%26%36%11%1
540
16th to endNight
14%217%38%22%4
563
June1st to 15thNight
8%126%43%19%
396
16th to endNight
11%122%42%21%
427
July1st to 15thNight
10%214%42%27%2
510
16th to endNight
7%111%40%34%4
564
August1st to 15thNight
9%110%44%30%4
539
16th to endNight
10%13%31%38%5%
588
September1st to 15thNight
15%113%32%31%5%
720
16th to endNight
15%315%28%29%7%
717
October1st to 15thNight
18%421%32%20%3
720
16th to endNight
24%11%25%28%9%
768
November1st to 15thNight
19%6%27%31%13%1
760
16th to endNight
25%12%30%20%8%
780
December1st to 15thNight
27%12%32%22%5%
780
16th to endNight
28%11%35%23%1
832
Page generated at 2018-03-12 13:28:10. Version ac04

Copyright 2018 A. Danko


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