Sunset Astronomical Society
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sunset Astronomical Society clear sky chart from 2005-11-05 to 2017-03-31.

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%
15281
15%
45439
9%
15241
42%
15241
21%
31498

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%
898
9%
2506
2%
895
23%
895
12%
1990
January16th to endNight
22%
933
9%
2661
4%
933
26%
933
13%
2104
February1st to 15thNight
21%
701
10%
2252
4%
701
27%
701
13%
1698
February16th to endNight
23%
631
14%
1893
6%
631
32%
631
15%
1432
March1st to 15thNight
31%
720
16%
2100
10%
720
37%
720
21%
1475
March16th to endNight
33%
768
18%
2112
11%
768
40%
768
24%
1459
April1st to 15thNight
31%
505
15%
1636
9%
505
38%
505
20%
1116
April16th to endNight
39%
495
20%
1485
11%
495
48%
495
27%
974
May1st to 15thNight
38%
495
16%
1485
10%
495
44%
495
23%
1011
May16th to endNight
49%
514
16%
1458
11%
513
54%
513
33%
907
June1st to 15thNight
49%
393
15%
1218
12%
393
55%
393
27%
777
June16th to endNight
59%
408
15%
1233
12%
408
63%
408
32%
757
July1st to 15thNight
66%
465
17%
1240
14%
465
68%
465
45%
715
July16th to endNight
63%
522
16%
1325
13%
489
66%
489
39%
794
August1st to 15thNight
64%
494
21%
1474
16%
491
68%
491
37%
837
August16th to endNight
63%
525
22%
1608
17%
525
68%
525
36%
908
September1st to 15thNight
56%
588
24%
1743
19%
588
61%
588
34%
1037
September16th to endNight
57%
657
22%
1815
18%
657
60%
657
34%
1107
October1st to 15thNight
45%
660
21%
1884
16%
660
50%
660
28%
1216
October16th to endNight
26%
704
12%
2244
7%
704
33%
704
15%
1629
November1st to 15thNight
34%
700
20%
2276
15%
700
42%
700
21%
1490
November16th to endNight
25%
720
11%
2397
5%
720
32%
720
13%
1779
December1st to 15thNight
18%
825
7%
2625
2%
825
23%
825
12%
2041
December16th to endNight
17%
960
9%
2769
2%
960
23%
960
11%
2245

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
13%7%36%6%35%5%5%7%34%
45633
Night
15%8%46%5%3445%6%35%
45611
Cloud CoverDay
13%7%36%6%35%5%5%7%34%
45642
Night
14%8%46%5%3445%6%35%
45616
TransparencyDay
48%5%13%21%10%
45323
Night
48%5%12%21%11%1
45439
SeeingDay
16%6%26%25%16%7%
5184
Night
18%6%28%31%12%1
15287

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%13%5%10%8%45%5%6%6%17%
1432
16th to endDay
13%15%8%13%9%46%45%413%
1563
February1st to 15thDay
11%14%7%9%7%46%5%5%6%21%
1609
16th to endDay
12%11%6%9%8%45%6%5%5%22%
1580
March1st to 15thDay
14%8%47%6%46%446%30%
1849
16th to endDay
16%8%37%6%35%5%35%32%
2107
April1st to 15thDay
18%10%46%5%3445%6%30%
1992
16th to endDay
16%6%35%5%25%45%5%38%
2145
May1st to 15thDay
19%6%35%5%35%45%7%33%
2136
16th to endDay
11%5%26%4245%5%8%43%
2303
June1st to 15thDay
13%5%25%5%345%5%8%38%
2406
16th to endDay
10%5%245%36%6%6%8%40%
2392
July1st to 15thDay
5%3245%35%6%6%9%46%
2310
16th to endDay
9%425%6%35%5%6%9%42%
2437
August1st to 15thDay
8%425%5%35%6%6%8%42%
2145
16th to endDay
11%5%25%4335%6%7%43%
2239
September1st to 15thDay
12%4145%3445%8%43%
1876
16th to endDay
11%5%25%425%6%6%9%39%
1815
October1st to 15thDay
17%5%36%6%34345%38%
1746
16th to endDay
21%8%37%6%35%45%6%27%
1625
November1st to 15thDay
14%9%36%5%3445%7%35%
1582
16th to endDay
18%11%47%6%45%5%5%7%22%
1560
December1st to 15thDay
15%13%6%9%7%45%435%22%
1332
16th to endDay
20%13%7%10%7%3445%6%17%
1452
January1st to 15thNight
20%12%6%9%7%34445%22%
2506
16th to endNight
13%12%7%9%8%45%45%6%21%
2661
February1st to 15thNight
13%13%5%9%6%35%446%26%
2250
16th to endNight
14%13%5%8%6%34447%25%
1893
March1st to 15thNight
15%7%37%5%345%45%36%
2087
16th to endNight
15%8%35%6%24346%38%
2112
April1st to 15thNight
19%7%45%424446%35%
1636
16th to endNight
17%6%35%424436%41%
1485
May1st to 15thNight
15%5%36%5%3445%7%36%
1485
16th to endNight
12%325%42435%8%45%
1456
June1st to 15thNight
12%5%35%4235%6%6%43%
1218
16th to endNight
7%5%25%6%25%36%10%45%
1233
July1st to 15thNight
6%32433435%8%53%
1320
16th to endNight
7%32342446%8%52%
1423
August1st to 15thNight
7%3143345%6%7%51%
1485
16th to endNight
10%32331245%8%52%
1608
September1st to 15thNight
12%323323346%52%
1743
16th to endNight
9%5%2443445%7%48%
1815
October1st to 15thNight
16%5%3442345%6%42%
1884
16th to endNight
21%7%37%5%25%346%31%
2244
November1st to 15thNight
13%8%35%6%2445%7%38%
2276
16th to endNight
21%11%46%6%35%45%6%25%
2397
December1st to 15thNight
19%13%6%8%5%34446%23%
2625
16th to endNight
20%14%6%8%6%2445%6%19%
2769

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
16%13%6%9%8%45%5%6%6%17%
1432
16th to endDay
13%14%9%14%8%46%5%5%414%
1563
February1st to 15thDay
11%14%6%9%7%36%5%5%6%21%
1609
16th to endDay
12%11%6%10%9%45%5%5%5%23%
1580
March1st to 15thDay
14%8%48%6%46%447%30%
1859
16th to endDay
16%8%48%6%45%435%32%
2106
April1st to 15thDay
18%10%47%6%2445%6%29%
1992
16th to endDay
16%6%345%25%45%6%37%
2145
May1st to 15thDay
20%6%346%35%45%7%33%
2136
16th to endDay
11%5%25%4345%6%8%43%
2303
June1st to 15thDay
13%5%25%5%35%5%5%8%39%
2406
16th to endDay
10%5%24445%6%6%8%40%
2392
July1st to 15thDay
6%315%5%35%5%6%10%46%
2310
16th to endDay
8%425%5%35%5%6%9%42%
2437
August1st to 15thDay
8%425%5%35%6%6%8%42%
2145
16th to endDay
11%5%25%4345%5%7%43%
2239
September1st to 15thDay
13%4145%3445%8%43%
1876
16th to endDay
11%5%25%5%25%5%7%9%39%
1815
October1st to 15thDay
17%5%36%6%34436%38%
1746
16th to endDay
21%9%37%5%35%46%6%27%
1625
November1st to 15thDay
14%9%36%5%3445%7%34%
1582
16th to endDay
17%12%47%6%45%5%5%7%22%
1560
December1st to 15thDay
15%13%6%9%8%35%445%22%
1332
16th to endDay
20%13%8%10%7%3445%6%16%
1452
January1st to 15thNight
20%11%5%10%6%34445%22%
2506
16th to endNight
13%12%8%9%8%45%5%5%6%21%
2661
February1st to 15thNight
13%13%5%9%7%35%446%26%
2243
16th to endNight
14%13%5%8%6%345%47%25%
1893
March1st to 15thNight
15%8%47%5%34445%36%
2100
16th to endNight
15%8%46%5%24346%37%
2112
April1st to 15thNight
19%7%45%424447%35%
1636
16th to endNight
17%6%35%424436%41%
1485
May1st to 15thNight
15%6%36%6%245%5%7%36%
1484
16th to endNight
12%425%42445%8%45%
1456
June1st to 15thNight
11%5%36%43346%7%43%
1218
16th to endNight
7%425%5%35%45%10%44%
1233
July1st to 15thNight
7%31443435%8%53%
1320
16th to endNight
7%31343446%8%52%
1423
August1st to 15thNight
7%3235%245%6%7%51%
1485
16th to endNight
10%314313448%53%
1608
September1st to 15thNight
12%324323346%52%
1743
16th to endNight
9%5%25%33445%7%48%
1815
October1st to 15thNight
16%5%25%42445%6%42%
1884
16th to endNight
19%9%37%6%24446%31%
2244
November1st to 15thNight
13%9%35%5%2435%7%38%
2276
16th to endNight
20%12%46%5%35%45%6%25%
2397
December1st to 15thNight
19%13%6%8%5%34446%23%
2625
16th to endNight
21%13%6%8%6%2445%6%19%
2769

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
66%5%10%13%4
1433
16th to endDay
70%5%8%11%4
1563
February1st to 15thDay
61%5%13%14%4
1611
16th to endDay
59%5%11%16%6%
1580
March1st to 15thDay
52%5%13%19%7%
1859
16th to endDay
53%5%14%19%6%1
2107
April1st to 15thDay
54%49%20%10%
1992
16th to endDay
46%5%14%17%13%2
2145
May1st to 15thDay
49%5%13%21%9%1
2136
16th to endDay
38%5%14%27%14%
2307
June1st to 15thDay
42%5%12%24%13%
2406
16th to endDay
38%7%15%22%15%
2392
July1st to 15thDay
30%6%13%28%17%2
2161
16th to endDay
37%7%14%22%15%2
2272
August1st to 15thDay
36%8%13%25%14%1
2132
16th to endDay
36%6%12%28%14%2
2239
September1st to 15thDay
38%5%13%30%11%
1876
16th to endDay
38%5%17%26%11%
1815
October1st to 15thDay
47%5%10%23%11%
1746
16th to endDay
56%6%16%15%5%
1625
November1st to 15thDay
48%5%14%21%9%
1582
16th to endDay
58%5%14%14%6%
1560
December1st to 15thDay
62%7%13%13%3
1332
16th to endDay
67%412%12%2
1452
January1st to 15thNight
63%410%13%7%
2506
16th to endNight
62%5%14%13%3
2661
February1st to 15thNight
58%6%10%17%6%
2252
16th to endNight
58%410%16%8%
1893
March1st to 15thNight
50%413%21%10%1
2100
16th to endNight
47%312%22%12%1
2112
April1st to 15thNight
49%410%19%15%1
1636
16th to endNight
43%310%19%18%4
1485
May1st to 15thNight
45%5%12%21%12%1
1485
16th to endNight
36%5%12%26%15%4
1458
June1st to 15thNight
39%6%11%24%17%1
1218
16th to endNight
33%7%15%25%17%
1233
July1st to 15thNight
27%412%32%20%3
1240
16th to endNight
29%6%16%27%16%2
1325
August1st to 15thNight
29%5%12%33%17%1
1474
16th to endNight
29%5%13%28%19%2
1608
September1st to 15thNight
34%411%31%18%
1743
16th to endNight
34%5%14%32%12%
1815
October1st to 15thNight
41%411%27%14%1
1884
16th to endNight
54%5%13%19%6%
2244
November1st to 15thNight
44%5%12%23%12%
2276
16th to endNight
58%5%15%14%5%
2397
December1st to 15thNight
61%5%13%12%6%
2625
16th to endNight
64%5%11%12%4
2769

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
22%7%41%21%7%
90
16th to endDay
25%8%39%21%5%
96
February1st to 15thDay
16%18%48%15%
90
16th to endDay
20%9%34%27%7%
81
March1st to 15thDay
25%11%27%30%4
149
16th to endDay
25%230%25%12%3
286
April1st to 15thDay
24%7%28%22%13%3
210
16th to endDay
18%6%30%20%15%8%
210
May1st to 15thDay
27%8%24%18%13%7%
228
16th to endDay
14%7%23%27%17%8%
347
June1st to 15thDay
15%6%30%25%14%6%
420
16th to endDay
10%6%29%24%20%7%
420
July1st to 15thDay
7%325%30%20%12%
400
16th to endDay
9%225%30%23%8%
423
August1st to 15thDay
8%120%24%25%20%
253
16th to endDay
15%413%24%23%18%
268
September1st to 15thDay
11%320%28%23%14%
255
16th to endDay
14%415%29%25%11%
254
October1st to 15thDay
21%7%24%27%16%3
255
16th to endDay
21%10%37%24%5%
137
November1st to 15thDay
5%16%33%26%12%5%
90
16th to endDay
22%17%33%15%11%
80
December1st to 15thDay
18%20%38%23%
60
16th to endDay
28%18%39%14%
82
January1st to 15thNight
24%11%35%21%6%1
900
16th to endNight
18%7%35%30%7%
933
February1st to 15thNight
19%9%36%28%4
704
16th to endNight
21%8%33%29%6%
631
March1st to 15thNight
20%7%31%31%7%
720
16th to endNight
19%7%31%32%8%1
768
April1st to 15thNight
21%9%32%28%7%
505
16th to endNight
22%5%27%31%11%2
495
May1st to 15thNight
17%5%30%35%10%1
495
16th to endNight
14%225%37%18%1
515
June1st to 15thNight
13%426%37%16%
393
16th to endNight
8%221%44%22%1
408
July1st to 15thNight
8%116%43%26%2
465
16th to endNight
8%120%42%24%3
522
August1st to 15thNight
9%14%45%26%3
494
16th to endNight
11%115%36%31%3
525
September1st to 15thNight
13%421%32%23%5%
588
16th to endNight
12%418%35%23%4
657
October1st to 15thNight
19%6%23%33%15%1
660
16th to endNight
24%11%32%24%7%
704
November1st to 15thNight
17%10%31%27%12%
700
16th to endNight
25%10%31%24%7%
720
December1st to 15thNight
26%8%35%26%3
825
16th to endNight
28%8%35%23%4
960
Page generated at 2017-04-01 08:23:13. Version ac04

Copyright 2017 A. Danko


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