Sunset Bay
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sunset Bay clear sky chart from 2005-11-06 to 2024-02-20.

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
26%
22719
13%
80661
8%
22636
34%
22636
14%
57765

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 10thNight
11%
753
7%
4414
2%
737
19%
737
4%
3439
January11th to 20thNight
17%
755
7%
4407
3%
740
24%
740
4%
3293
January21st to endNight
18%
829
7%
4683
6%
814
27%
814
6%
3767
February1st to 10thNight
14%
760
11%
2470
3%
760
23%
760
10%
1889
February11th to 20thNight
12%
740
14%
2399
4%
739
26%
739
12%
1756
February21st to endNight
20%
592
15%
1803
6%
592
31%
592
12%
1336
March1st to 10thNight
17%
720
14%
2117
7%
720
26%
720
13%
1581
March11th to 20thNight
19%
720
16%
1980
6%
720
29%
720
17%
1435
March21st to endNight
24%
792
20%
2091
7%
792
34%
792
19%
1475
April1st to 10thNight
20%
720
16%
1800
4%
720
26%
720
18%
1324
April11th to 20thNight
23%
711
18%
1775
5%
711
30%
711
20%
1283
April21st to endNight
26%
609
16%
1689
7%
609
34%
609
19%
1244
May1st to 10thNight
33%
360
18%
1440
15%
360
43%
360
17%
938
May11th to 20thNight
30%
354
21%
1416
15%
354
47%
354
19%
859
May21st to endNight
36%
389
14%
1551
12%
388
49%
388
24%
897
June1st to 10thNight
34%
360
10%
1440
8%
360
43%
360
16%
917
June11th to 20thNight
37%
360
13%
1440
15%
360
47%
360
20%
858
June21st to endNight
40%
360
10%
1440
13%
360
48%
360
18%
924
July1st to 10thNight
46%
350
15%
1400
18%
350
55%
350
25%
790
July11th to 20thNight
49%
348
16%
1392
22%
348
56%
348
25%
801
July21st to endNight
50%
394
16%
1486
15%
372
57%
372
26%
847
August1st to 10thNight
48%
442
16%
1506
12%
440
55%
440
25%
914
August11th to 20thNight
52%
534
18%
1598
12%
534
57%
534
32%
994
August21st to endNight
43%
768
18%
1956
8%
768
48%
768
32%
1286
September1st to 10thNight
43%
720
17%
1800
6%
720
47%
720
31%
1184
September11th to 20thNight
47%
720
22%
1800
12%
720
52%
720
35%
1160
September21st to endNight
42%
720
21%
2050
15%
720
47%
720
27%
1348
October1st to 10thNight
33%
720
17%
2160
11%
720
41%
720
20%
1469
October11th to 20thNight
22%
720
13%
2160
6%
720
31%
720
16%
1587
October21st to endNight
21%
792
13%
2484
6%
792
31%
792
12%
1855
November1st to 10thNight
23%
740
17%
2435
10%
740
33%
740
17%
1680
November11th to 20thNight
19%
760
15%
2660
9%
760
28%
760
12%
1902
November21st to endNight
17%
760
10%
2660
5%
760
25%
760
9%
2046
December1st to 10thNight
14%
760
8%
2660
3%
760
20%
760
7%
2174
December11th to 20thNight
14%
760
11%
3248
5%
759
23%
759
7%
2489
December21st to endNight
11%
827
5%
4851
2%
817
18%
817
4%
4024

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
16%10%48%6%45%5%5%6%32%
78226
Night
16%11%48%6%45%45%6%31%
80956
TransparencyDay
52%6%11%19%10%1
77931
Night
54%6%11%19%9%1
80785
SeeingDay
24%16%31%19%8%3
13428
Night
23%14%31%24%8%1
22749

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 10thDay
14%14%7%11%9%5%5%5%45%22%
2517
11th to 20thDay
12%14%7%13%9%5%5%5%4421%
2520
21st to endDay
14%21%8%14%7%4443317%
2968
February1st to 10thDay
12%15%5%13%8%5%6%445%22%
1710
11th to 20thDay
11%14%7%10%8%5%45%45%28%
1654
21st to endDay
18%13%49%7%46%5%5%5%24%
1453
March1st to 10thDay
15%12%6%10%7%3445%5%29%
1837
11th to 20thDay
14%13%5%8%9%35%45%430%
1980
21st to endDay
17%11%46%6%34345%37%
2263
April1st to 10thDay
21%13%46%5%34446%30%
2160
11th to 20thDay
21%9%46%5%24445%38%
2134
21st to endDay
21%10%47%5%34445%32%
2285
May1st to 10thDay
19%8%46%6%3445%6%35%
2520
11th to 20thDay
16%8%46%6%3445%7%38%
2478
21st to endDay
15%7%36%6%45%5%5%7%38%
2725
June1st to 10thDay
20%8%46%6%36%5%6%6%31%
2520
11th to 20thDay
15%6%35%5%35%6%6%7%39%
2520
21st to endDay
17%7%36%7%45%5%6%8%33%
2520
July1st to 10thDay
11%5%36%5%46%6%6%9%39%
2520
11th to 20thDay
11%6%36%6%35%7%7%9%38%
2520
21st to endDay
12%5%26%5%36%6%7%10%38%
2744
August1st to 10thDay
11%6%26%7%35%6%7%8%38%
2418
11th to 20thDay
12%5%37%7%45%5%6%8%38%
2303
21st to endDay
14%5%26%5%35%6%6%10%37%
2381
September1st to 10thDay
14%6%25%6%445%6%8%40%
2160
11th to 20thDay
13%5%25%5%345%6%9%44%
2160
21st to endDay
17%6%36%5%35%5%5%7%39%
1907
October1st to 10thDay
20%9%36%6%4445%6%34%
1800
11th to 20thDay
21%11%47%7%45%5%47%27%
1800
21st to endDay
26%11%47%6%345%46%25%
1872
November1st to 10thDay
14%10%47%5%35%5%5%7%35%
1639
11th to 20thDay
17%11%5%8%8%35%45%5%28%
1520
21st to endDay
19%14%6%9%7%5%5%446%20%
1520
December1st to 10thDay
21%18%7%11%7%35%45%5%15%
1520
11th to 20thDay
18%14%6%8%7%34445%25%
1906
21st to endDay
26%17%5%14%6%3332319%
2772
January1st to 10thNight
16%14%8%11%8%45%5%5%5%19%
4429
11th to 20thNight
11%13%5%11%11%5%6%5%5%6%21%
4440
21st to endNight
16%19%7%10%8%6%443420%
4686
February1st to 10thNight
12%15%6%11%9%46%5%5%6%23%
2470
11th to 20thNight
12%13%6%10%8%45%5%45%29%
2397
21st to endNight
15%12%5%8%8%45%5%46%29%
1803
March1st to 10thNight
16%13%5%10%8%44445%29%
2107
11th to 20thNight
14%13%6%8%6%34346%33%
1979
21st to endNight
15%9%46%6%4445%6%37%
2091
April1st to 10thNight
20%10%47%5%34446%33%
1800
11th to 20thNight
19%9%36%5%3435%6%37%
1775
21st to endNight
21%9%46%7%35%446%32%
1689
May1st to 10thNight
18%9%45%5%345%5%5%38%
1440
11th to 20thNight
18%7%25%434447%41%
1415
21st to endNight
12%5%35%5%345%6%9%44%
1550
June1st to 10thNight
16%6%36%6%445%7%8%34%
1440
11th to 20thNight
13%6%36%435%5%5%7%44%
1440
21st to endNight
16%7%35%5%45%5%5%9%36%
1440
July1st to 10thNight
9%6%26%5%35%5%6%8%46%
1440
11th to 20thNight
10%5%25%5%345%6%10%43%
1440
21st to endNight
9%425%6%36%5%8%9%45%
1574
August1st to 10thNight
9%5%25%6%45%5%7%8%43%
1516
11th to 20thNight
8%5%36%5%25%6%6%8%46%
1598
21st to endNight
10%7%26%43446%7%46%
1956
September1st to 10thNight
11%6%25%5%35%6%6%6%45%
1800
11th to 20thNight
15%6%135%3445%6%49%
1800
21st to endNight
15%6%25%5%35%45%6%44%
2050
October1st to 10thNight
18%8%36%5%345%5%6%38%
2160
11th to 20thNight
20%11%37%6%35%45%6%30%
2160
21st to endNight
21%10%39%6%36%45%6%26%
2484
November1st to 10thNight
16%12%46%5%35%45%6%34%
2435
11th to 20thNight
17%11%5%8%7%45%5%5%6%28%
2660
21st to endNight
20%13%5%10%6%4345%6%23%
2660
December1st to 10thNight
19%17%6%11%7%45%5%4418%
2660
11th to 20thNight
19%16%6%9%7%3343425%
3288
21st to endNight
28%17%6%12%6%3333316%
4884

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 10thDay
63%8%13%10%7%
2518
11th to 20thDay
66%5%10%11%7%1
2520
21st to endDay
73%47%10%6%1
2968
February1st to 10thDay
63%5%9%17%41
1710
11th to 20thDay
57%5%11%16%9%1
1663
21st to endDay
62%5%9%12%10%1
1453
March1st to 10thDay
58%410%17%10%1
1840
11th to 20thDay
57%5%9%16%11%3
1980
21st to endDay
51%38%19%14%6%
2263
April1st to 10thDay
54%47%19%13%3
2160
11th to 20thDay
50%47%21%16%2
2134
21st to endDay
54%48%15%15%4
2285
May1st to 10thDay
50%5%8%18%15%3
2520
11th to 20thDay
46%5%8%21%17%2
2482
21st to endDay
45%6%13%21%13%3
2725
June1st to 10thDay
52%6%10%19%12%1
2520
11th to 20thDay
42%7%11%24%13%3
2520
21st to endDay
48%6%13%21%11%1
2520
July1st to 10thDay
39%7%11%28%15%1
2450
11th to 20thDay
38%9%15%26%11%1
2445
21st to endDay
41%8%14%27%10%
2591
August1st to 10thDay
41%8%15%24%11%
2404
11th to 20thDay
42%8%15%25%8%1
2303
21st to endDay
41%7%14%26%11%1
2381
September1st to 10thDay
40%7%14%28%11%
2160
11th to 20thDay
37%7%13%29%14%
2160
21st to endDay
46%5%14%27%9%
1907
October1st to 10thDay
51%6%10%22%11%1
1800
11th to 20thDay
57%5%13%17%7%1
1800
21st to endDay
61%5%10%16%6%1
1872
November1st to 10thDay
49%5%16%22%7%1
1639
11th to 20thDay
57%6%12%14%10%1
1520
21st to endDay
65%5%12%10%7%1
1520
December1st to 10thDay
71%6%10%10%41
1520
11th to 20thDay
61%6%13%14%6%
1906
21st to endDay
73%37%14%4
2772
January1st to 10thNight
65%8%12%11%3
4429
11th to 20thNight
63%6%14%11%41
4440
21st to endNight
70%5%9%10%6%
4686
February1st to 10thNight
61%5%10%16%6%1
2470
11th to 20thNight
57%5%10%20%8%1
2399
21st to endNight
56%5%9%18%11%1
1803
March1st to 10thNight
58%410%16%10%1
2117
11th to 20thNight
52%5%11%19%12%2
1980
21st to endNight
48%5%9%20%15%4
2091
April1st to 10thNight
52%5%7%20%15%2
1800
11th to 20thNight
49%47%23%15%2
1775
21st to endNight
54%5%6%17%16%2
1689
May1st to 10thNight
48%5%7%20%15%5%
1440
11th to 20thNight
42%5%9%24%17%3
1416
21st to endNight
36%7%15%20%19%3
1551
June1st to 10thNight
45%7%12%24%11%1
1440
11th to 20thNight
39%7%11%23%18%2
1440
21st to endNight
44%8%15%20%13%1
1440
July1st to 10thNight
34%7%12%26%17%3
1400
11th to 20thNight
37%6%14%31%12%
1392
21st to endNight
34%8%17%28%13%
1486
August1st to 10thNight
36%9%18%24%13%1
1506
11th to 20thNight
36%6%18%29%10%1
1598
21st to endNight
36%7%15%29%12%1
1956
September1st to 10thNight
36%8%13%28%14%1
1800
11th to 20thNight
36%5%14%34%11%1
1800
21st to endNight
41%5%13%32%8%
2050
October1st to 10thNight
47%6%11%22%15%
2160
11th to 20thNight
53%6%13%19%8%
2160
21st to endNight
57%6%13%16%8%
2484
November1st to 10thNight
50%5%14%21%10%1
2435
11th to 20thNight
54%6%12%18%9%1
2660
21st to endNight
61%6%12%16%5%
2660
December1st to 10thNight
69%6%9%13%3
2660
11th to 20thNight
64%49%15%7%1
3288
21st to endNight
74%47%12%3
4884

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 10thDay
24%19%35%17%5%
180
11th to 20thDay
32%18%35%11%4
180
21st to endDay
23%21%36%14%6%1
254
February1st to 10thDay
30%26%30%11%3
270
11th to 20thDay
30%25%39%41
267
21st to endDay
32%24%33%10%1
319
March1st to 10thDay
27%18%37%14%3
420
11th to 20thDay
29%19%32%16%31
420
21st to endDay
33%20%27%14%5%
462
April1st to 10thDay
32%21%28%13%6%
420
11th to 20thDay
34%17%21%17%8%2
412
21st to endDay
30%18%32%11%7%1
420
May1st to 10thDay
26%15%32%17%7%3
423
11th to 20thDay
22%16%31%22%6%2
447
21st to endDay
20%15%33%19%9%4
485
June1st to 10thDay
25%14%32%20%9%
450
11th to 20thDay
18%14%30%23%12%3
450
21st to endDay
20%12%32%23%12%2
450
July1st to 10thDay
11%7%30%35%13%3
445
11th to 20thDay
14%9%32%27%14%4
444
21st to endDay
15%8%35%28%11%3
488
August1st to 10thDay
16%5%30%30%13%5%
446
11th to 20thDay
13%8%29%29%17%5%
439
21st to endDay
16%12%29%24%14%6%
495
September1st to 10thDay
16%10%30%22%14%7%
450
11th to 20thDay
14%10%28%24%14%10%
450
21st to endDay
20%13%30%17%13%6%
450
October1st to 10thDay
25%16%25%20%9%5%
450
11th to 20thDay
28%20%30%15%5%2
450
21st to endDay
33%19%27%16%5%
441
November1st to 10thDay
26%17%32%17%6%1
333
11th to 20thDay
29%21%33%11%24
180
21st to endDay
29%24%34%8%41
180
December1st to 10thDay
34%16%39%9%2
180
11th to 20thDay
31%27%31%8%31
180
21st to endDay
31%21%32%13%31
198
January1st to 10thNight
26%20%34%17%3
760
11th to 20thNight
27%19%30%18%5%1
760
21st to endNight
20%15%37%24%5%
836
February1st to 10thNight
23%18%37%19%3
760
11th to 20thNight
21%19%42%15%3
742
21st to endNight
24%20%31%21%4
592
March1st to 10thNight
26%18%32%20%3
720
11th to 20thNight
24%21%31%18%5%1
720
21st to endNight
23%16%31%23%7%
792
April1st to 10thNight
27%17%31%21%4
720
11th to 20thNight
25%20%28%20%7%
711
21st to endNight
25%15%28%24%7%1
609
May1st to 10thNight
22%10%29%27%11%1
360
11th to 20thNight
21%10%33%27%8%1
354
21st to endNight
13%8%37%31%12%
389
June1st to 10thNight
16%10%31%34%8%
360
11th to 20thNight
15%11%31%32%10%1
360
21st to endNight
22%7%25%33%13%
360
July1st to 10thNight
9%7%31%38%14%1
350
11th to 20thNight
12%5%29%40%13%1
348
21st to endNight
9%431%42%12%1
394
August1st to 10thNight
12%5%26%38%20%
442
11th to 20thNight
10%425%38%20%3
534
21st to endNight
13%7%30%31%17%2
768
September1st to 10thNight
15%8%29%30%17%1
720
11th to 20thNight
18%6%24%30%19%3
720
21st to endNight
20%10%25%24%20%2
720
October1st to 10thNight
24%11%26%25%12%2
720
11th to 20thNight
28%13%32%19%8%1
720
21st to endNight
28%16%28%22%6%
792
November1st to 10thNight
27%13%31%22%6%1
740
11th to 20thNight
25%19%32%15%8%1
760
21st to endNight
32%17%27%19%5%
760
December1st to 10thNight
31%16%34%16%3
760
11th to 20thNight
28%19%30%19%31
760
21st to endNight
35%16%30%16%3
836
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