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 2017-12-04.

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
37%
16092
15%
47881
10%
16052
42%
16052
22%
33045

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
32%
551
14%
1785
9%
551
38%
551
19%
1223
April16th to endNight
37%
540
20%
1620
11%
540
46%
540
26%
1073
May1st to 15thNight
38%
540
15%
1620
9%
540
44%
540
23%
1103
May16th to endNight
47%
562
17%
1594
11%
561
53%
561
32%
1004
June1st to 15thNight
49%
429
15%
1329
11%
429
54%
429
28%
839
June16th to endNight
57%
445
16%
1345
13%
445
61%
445
32%
820
July1st to 15thNight
67%
510
16%
1360
13%
510
69%
510
44%
785
July16th to endNight
64%
564
18%
1439
15%
531
67%
531
39%
859
August1st to 15thNight
66%
539
20%
1609
16%
536
70%
536
37%
913
August16th to endNight
62%
573
23%
1755
18%
573
67%
573
36%
999
September1st to 15thNight
57%
642
23%
1902
19%
642
61%
642
35%
1111
September16th to endNight
58%
717
24%
1980
20%
717
60%
717
35%
1195
October1st to 15thNight
44%
720
20%
2055
15%
720
48%
720
27%
1342
October16th to endNight
27%
768
14%
2448
9%
768
34%
768
16%
1759
November1st to 15thNight
34%
760
19%
2471
15%
760
41%
760
20%
1637
November16th to endNight
25%
780
12%
2597
5%
780
32%
780
14%
1902
December1st to 15thNight
19%
841
7%
2679
2%
841
24%
841
12%
2078
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%
48600
Night
15%8%46%5%3445%7%36%
48053
Cloud CoverDay
13%7%36%6%35%5%5%7%34%
48609
Night
14%8%46%5%3445%7%36%
48058
TransparencyDay
47%6%13%21%10%1
48290
Night
47%5%12%21%11%1
47881
SeeingDay
16%6%27%25%16%7%
6178
Night
18%6%28%31%12%1
16098

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%
2112
April1st to 15thDay
19%9%46%5%3445%6%30%
2174
16th to endDay
16%6%35%5%35%5%5%5%37%
2340
May1st to 15thDay
19%6%35%5%3445%6%34%
2331
16th to endDay
11%5%26%5%245%5%8%41%
2518
June1st to 15thDay
12%5%25%5%35%5%5%8%39%
2625
16th to endDay
10%5%245%36%6%7%8%38%
2609
July1st to 15thDay
5%3245%35%6%6%9%47%
2520
16th to endDay
8%4246%35%5%6%9%43%
2631
August1st to 15thDay
8%425%6%45%6%6%8%41%
2340
16th to endDay
11%5%25%5%345%6%7%43%
2444
September1st to 15thDay
11%4145%35%45%8%45%
2046
16th to endDay
10%5%25%425%6%6%9%41%
1980
October1st to 15thDay
17%5%36%6%34445%38%
1905
16th to endDay
21%8%37%6%35%45%6%27%
1773
November1st to 15thDay
15%10%36%5%3445%7%34%
1717
16th to endDay
18%11%47%6%45%5%5%7%23%
1690
December1st to 15thDay
15%13%6%9%7%45%435%23%
1360
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
20%7%45%424447%34%
1785
16th to endNight
16%7%35%424447%40%
1620
May1st to 15thNight
15%5%36%5%3445%7%37%
1620
16th to endNight
12%325%42445%8%44%
1592
June1st to 15thNight
11%5%25%4235%6%7%44%
1329
16th to endNight
8%425%5%25%36%9%45%
1345
July1st to 15thNight
6%32333445%8%54%
1440
16th to endNight
6%31342445%8%52%
1537
August1st to 15thNight
7%3143345%6%7%51%
1620
16th to endNight
10%32331345%8%52%
1755
September1st to 15thNight
11%323323346%54%
1902
16th to endNight
9%5%2443445%7%49%
1980
October1st to 15thNight
17%5%345%2345%6%41%
2055
16th to endNight
20%7%37%5%25%346%31%
2448
November1st to 15thNight
14%8%35%6%2445%7%37%
2471
16th to endNight
20%10%46%5%35%45%7%26%
2597
December1st to 15thNight
19%13%6%8%5%34446%23%
2679
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%
2111
April1st to 15thDay
18%10%46%5%2445%6%30%
2174
16th to endDay
16%6%35%5%35%45%6%37%
2340
May1st to 15thDay
19%6%346%3445%7%34%
2331
16th to endDay
11%5%25%5%345%6%8%41%
2518
June1st to 15thDay
12%5%25%5%35%5%5%8%39%
2625
16th to endDay
10%5%25%445%6%7%8%38%
2609
July1st to 15thDay
5%315%5%35%5%6%9%46%
2520
16th to endDay
8%425%5%35%5%6%9%43%
2631
August1st to 15thDay
8%425%5%46%6%6%8%40%
2340
16th to endDay
11%5%25%4345%5%7%43%
2444
September1st to 15thDay
12%42443445%7%45%
2046
16th to endDay
10%5%245%35%5%6%9%41%
1980
October1st to 15thDay
17%5%36%6%34436%38%
1905
16th to endDay
20%9%36%6%35%46%6%27%
1773
November1st to 15thDay
15%10%36%5%3445%7%33%
1717
16th to endDay
17%11%47%6%45%5%5%7%23%
1690
December1st to 15thDay
15%13%6%9%8%35%445%23%
1360
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
20%7%45%424447%34%
1785
16th to endNight
16%7%35%424446%40%
1620
May1st to 15thNight
15%6%26%6%245%47%37%
1619
16th to endNight
12%425%5%2445%8%44%
1592
June1st to 15thNight
11%5%36%43346%7%44%
1329
16th to endNight
8%425%5%35%45%10%45%
1345
July1st to 15thNight
6%31433435%8%54%
1440
16th to endNight
6%31443446%8%52%
1537
August1st to 15thNight
6%3235%245%6%7%51%
1620
16th to endNight
10%4243135%5%7%52%
1755
September1st to 15thNight
11%323323346%53%
1902
16th to endNight
8%5%25%43445%7%49%
1980
October1st to 15thNight
17%6%25%42345%6%41%
2055
16th to endNight
19%8%37%6%24437%31%
2448
November1st to 15thNight
14%9%35%5%2445%7%37%
2471
16th to endNight
19%11%46%5%35%45%6%26%
2597
December1st to 15thNight
19%13%6%8%5%34446%23%
2679
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
2112
April1st to 15thDay
53%5%10%20%10%
2174
16th to endDay
46%6%14%16%13%2
2340
May1st to 15thDay
48%412%21%10%2
2331
16th to endDay
39%5%14%26%13%
2522
June1st to 15thDay
40%6%12%25%13%
2625
16th to endDay
39%7%15%22%14%
2609
July1st to 15thDay
30%7%13%28%18%2
2371
16th to endDay
37%6%13%23%16%2
2466
August1st to 15thDay
37%8%13%25%13%1
2327
16th to endDay
37%6%12%28%14%2
2444
September1st to 15thDay
37%5%12%31%11%1
2046
16th to endDay
37%5%16%27%12%
1980
October1st to 15thDay
47%5%10%23%11%1
1905
16th to endDay
55%6%16%16%5%
1773
November1st to 15thDay
49%5%14%20%9%
1717
16th to endDay
58%5%14%14%6%
1690
December1st to 15thDay
61%7%13%14%3
1360
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
1785
16th to endNight
44%411%19%17%3
1620
May1st to 15thNight
46%5%11%20%12%3
1620
16th to endNight
36%5%12%26%15%3
1594
June1st to 15thNight
38%6%11%25%17%1
1329
16th to endNight
33%6%14%26%17%
1345
July1st to 15thNight
26%5%12%31%20%2
1360
16th to endNight
29%6%15%27%18%2
1439
August1st to 15thNight
29%5%13%33%16%1
1609
16th to endNight
30%5%13%28%19%1
1755
September1st to 15thNight
32%411%31%18%
1902
16th to endNight
33%5%13%33%12%
1980
October1st to 15thNight
43%411%26%14%1
2055
16th to endNight
53%5%12%20%7%
2448
November1st to 15thNight
46%5%12%23%11%
2471
16th to endNight
56%5%16%15%5%
2597
December1st to 15thNight
61%5%14%12%6%
2679
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
26%229%25%12%3
288
April1st to 15thDay
25%7%29%22%12%2
270
16th to endDay
18%10%32%20%12%6%
270
May1st to 15thDay
24%10%26%19%13%6%
288
16th to endDay
16%8%26%24%16%7%
418
June1st to 15thDay
14%6%30%27%14%6%
495
16th to endDay
10%7%33%24%17%6%
495
July1st to 15thDay
7%325%29%20%12%
475
16th to endDay
9%225%30%23%9%
491
August1st to 15thDay
7%120%24%27%17%
313
16th to endDay
16%414%23%24%16%
332
September1st to 15thDay
9%319%26%24%16%
315
16th to endDay
12%5%14%28%27%11%
314
October1st to 15thDay
22%6%23%26%16%3
315
16th to endDay
20%11%34%28%5%
189
November1st to 15thDay
20%11%32%23%8%3
135
16th to endDay
20%19%36%15%8%
120
December1st to 15thDay
19%17%35%22%4
67
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
22%8%32%28%7%
551
16th to endNight
21%5%30%29%10%1
540
May1st to 15thNight
17%5%30%35%10%1
540
16th to endNight
15%227%35%17%1
563
June1st to 15thNight
12%426%38%16%
429
16th to endNight
8%221%44%21%1
445
July1st to 15thNight
8%116%44%25%3
510
16th to endNight
8%119%43%24%3
564
August1st to 15thNight
9%14%45%25%3
539
16th to endNight
11%116%36%30%3
573
September1st to 15thNight
12%320%32%24%6%
642
16th to endNight
11%418%35%24%5%
717
October1st to 15thNight
20%6%23%33%15%1
720
16th to endNight
23%10%32%25%7%
768
November1st to 15thNight
18%10%30%27%12%
760
16th to endNight
25%10%33%23%7%
780
December1st to 15thNight
25%8%34%27%3
841
16th to endNight
28%8%35%23%4
960
Page generated at 2017-12-05 10:19:02. Version ac04

Copyright 2017 A. Danko


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