Bellevue Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bellevue Observatory clear sky chart from 2005-11-06 to 2024-01-27.

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
32%
22610
8%
74913
5%
22570
36%
22570
9%
60796

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
20%
760
5%
2649
3%
757
23%
757
4%
2319
January11th to 20thNight
20%
760
4%
2660
2%
760
22%
760
5%
2330
January21st to endNight
25%
820
7%
2868
5%
820
29%
820
7%
2367
February1st to 10thNight
20%
720
5%
2362
2%
720
24%
720
6%
2013
February11th to 20thNight
25%
703
8%
2188
3%
700
31%
700
8%
1811
February21st to endNight
27%
592
10%
1776
4%
592
34%
592
8%
1496
March1st to 10thNight
25%
720
9%
2160
6%
720
29%
720
8%
1805
March11th to 20thNight
26%
720
9%
2082
3%
720
32%
720
9%
1730
March21st to endNight
32%
792
14%
1990
6%
792
37%
792
14%
1621
April1st to 10thNight
30%
720
10%
1800
3%
720
33%
720
12%
1492
April11th to 20thNight
30%
711
11%
1775
3%
711
35%
711
13%
1444
April21st to endNight
28%
586
12%
1666
6%
586
33%
586
12%
1336
May1st to 10thNight
30%
404
11%
1484
7%
404
37%
404
10%
1163
May11th to 20thNight
38%
354
13%
1416
11%
354
47%
354
10%
1068
May21st to endNight
38%
388
11%
1551
11%
388
44%
388
11%
1134
June1st to 10thNight
38%
360
5%
1440
5%
360
44%
360
8%
1151
June11th to 20thNight
43%
360
9%
1440
11%
360
50%
360
10%
1076
June21st to endNight
42%
360
7%
1440
10%
360
47%
360
9%
1110
July1st to 10thNight
52%
350
8%
1400
11%
350
57%
350
12%
1045
July11th to 20thNight
54%
348
9%
1392
13%
348
57%
348
11%
1026
July21st to endNight
56%
394
9%
1486
10%
372
59%
372
11%
1115
August1st to 10thNight
58%
358
10%
1422
10%
356
61%
356
13%
1024
August11th to 20thNight
53%
540
13%
1604
9%
540
56%
540
18%
1168
August21st to endNight
52%
792
10%
1980
5%
792
54%
792
17%
1575
September1st to 10thNight
52%
720
8%
1800
4%
720
53%
720
16%
1433
September11th to 20thNight
53%
720
13%
1867
8%
720
55%
720
18%
1435
September21st to endNight
46%
720
11%
1980
8%
720
48%
720
14%
1558
October1st to 10thNight
37%
720
11%
2110
7%
720
41%
720
12%
1698
October11th to 20thNight
33%
720
10%
2186
6%
720
38%
720
10%
1788
October21st to endNight
27%
792
8%
2574
5%
792
31%
792
9%
2098
November1st to 10thNight
29%
740
9%
2407
6%
740
33%
740
9%
1941
November11th to 20thNight
25%
760
12%
2550
8%
760
31%
760
9%
2063
November21st to endNight
21%
760
6%
2660
4%
760
25%
760
5%
2263
December1st to 10thNight
16%
760
3%
2660
1%
760
19%
760
4%
2362
December11th to 20thNight
16%
760
8%
3246
2%
759
22%
759
5%
2701
December21st to endNight
16%
826
3%
4842
1%
817
19%
817
3%
4037

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
17%8%36%6%35%5%5%7%34%
74869
Night
19%10%47%6%3445%6%32%
75168
TransparencyDay
34%37%9%14%7%1
74574
Night
37%36%8%13%6%1
74997
SeeingDay
20%10%27%25%13%4
13483
Night
23%8%29%30%10%1
22621

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
18%16%7%9%9%34445%22%
1517
11th to 20thDay
20%14%5%9%7%34445%25%
1520
21st to endDay
12%13%49%7%46%6%47%27%
1632
February1st to 10thDay
15%15%6%9%7%446%46%26%
1593
11th to 20thDay
14%10%5%9%7%45%5%6%6%28%
1658
21st to endDay
18%14%6%8%6%34445%28%
1480
March1st to 10thDay
16%11%5%8%7%46%445%30%
1797
11th to 20thDay
17%10%37%7%45%5%5%6%32%
1876
21st to endDay
19%9%46%5%35%446%35%
2365
April1st to 10thDay
21%9%47%6%34446%32%
2160
11th to 20thDay
20%7%36%6%34447%37%
2134
21st to endDay
21%9%37%6%34446%32%
2293
May1st to 10thDay
18%7%36%6%45%5%5%7%35%
2492
11th to 20thDay
18%7%26%5%345%5%7%39%
2478
21st to endDay
17%6%36%6%3446%8%36%
2725
June1st to 10thDay
21%7%36%6%35%5%6%8%32%
2520
11th to 20thDay
14%5%36%6%35%5%5%9%39%
2520
21st to endDay
19%7%36%5%45%5%5%7%33%
2520
July1st to 10thDay
11%5%35%6%45%5%6%8%43%
2520
11th to 20thDay
12%5%25%5%45%6%7%10%38%
2520
21st to endDay
12%5%35%6%35%6%7%10%39%
2744
August1st to 10thDay
12%5%36%6%45%6%6%9%39%
2506
11th to 20thDay
10%435%5%45%6%6%8%43%
2274
21st to endDay
13%5%246%345%7%9%43%
2376
September1st to 10thDay
12%5%36%5%25%6%6%9%42%
2160
11th to 20thDay
14%5%244345%5%6%49%
2093
21st to endDay
16%5%246%35%45%7%42%
1980
October1st to 10thDay
21%7%36%5%4446%5%36%
1847
11th to 20thDay
18%8%36%8%5%6%6%5%6%29%
1772
21st to endDay
25%10%35%6%2446%6%29%
1782
November1st to 10thDay
17%8%45%5%25%6%6%7%34%
1671
11th to 20thDay
18%10%5%8%5%35%5%5%6%28%
1626
21st to endDay
22%10%47%6%5%6%5%5%7%24%
1520
December1st to 10thDay
26%12%6%8%6%346%5%6%19%
1520
11th to 20thDay
21%13%5%8%6%335%5%5%26%
1906
21st to endDay
29%13%5%10%6%3433420%
2772
January1st to 10thNight
18%16%7%10%6%45%45%5%20%
2649
11th to 20thNight
18%15%6%9%6%45%445%23%
2660
21st to endNight
14%12%6%9%8%45%446%30%
2868
February1st to 10thNight
15%15%5%11%7%46%5%6%5%22%
2362
11th to 20thNight
15%10%48%6%5%6%5%5%6%30%
2186
21st to endNight
17%13%6%7%6%445%35%32%
1776
March1st to 10thNight
17%12%5%8%7%45%45%5%29%
2150
11th to 20thNight
17%13%47%5%44446%32%
2081
21st to endNight
19%10%37%5%3335%438%
1990
April1st to 10thNight
24%8%45%5%35%45%6%31%
1800
11th to 20thNight
22%7%36%5%34446%36%
1775
21st to endNight
22%8%36%5%3445%6%33%
1666
May1st to 10thNight
22%7%37%5%34447%35%
1484
11th to 20thNight
20%6%26%6%3445%5%39%
1415
21st to endNight
16%5%35%5%35%5%47%41%
1550
June1st to 10thNight
21%8%35%6%35%47%7%32%
1440
11th to 20thNight
16%6%36%5%345%5%8%39%
1440
21st to endNight
17%6%37%6%45%5%6%7%35%
1440
July1st to 10thNight
12%5%25%5%36%48%8%43%
1440
11th to 20thNight
11%5%25%6%36%5%7%10%41%
1440
21st to endNight
11%6%25%5%35%46%9%43%
1574
August1st to 10thNight
12%6%245%25%46%10%44%
1432
11th to 20thNight
13%5%25%5%445%5%8%44%
1604
21st to endNight
13%5%25%5%345%6%8%44%
1980
September1st to 10thNight
12%6%36%6%345%6%7%43%
1800
11th to 20thNight
16%6%25%32335%7%48%
1867
21st to endNight
17%5%35%5%35%5%5%7%43%
1980
October1st to 10thNight
18%6%26%6%35%5%5%6%37%
2110
11th to 20thNight
21%7%37%6%35%446%35%
2186
21st to endNight
24%8%36%6%45%446%30%
2574
November1st to 10thNight
19%9%47%6%3445%7%32%
2407
11th to 20thNight
19%10%47%6%35%5%5%5%33%
2550
21st to endNight
23%13%47%6%45%345%26%
2660
December1st to 10thNight
22%15%5%9%6%35%5%45%21%
2660
11th to 20thNight
23%15%5%7%6%34445%24%
3288
21st to endNight
30%15%5%8%5%2334422%
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
42%40%7%10%1
1518
11th to 20thDay
41%39%6%9%5%
1520
21st to endDay
39%37%10%9%6%
1632
February1st to 10thDay
41%36%8%11%31
1593
11th to 20thDay
38%37%10%10%31
1667
21st to endDay
42%35%8%11%4
1480
March1st to 10thDay
38%36%8%12%5%1
1800
11th to 20thDay
35%37%11%12%5%1
1876
21st to endDay
31%36%8%13%10%2
2365
April1st to 10thDay
36%36%5%14%7%1
2160
11th to 20thDay
31%36%6%14%12%1
2134
21st to endDay
36%36%6%12%8%3
2293
May1st to 10thDay
32%37%6%12%11%2
2492
11th to 20thDay
29%38%6%15%10%3
2482
21st to endDay
28%37%9%13%11%2
2725
June1st to 10thDay
34%38%8%12%7%
2520
11th to 20thDay
29%37%7%16%9%2
2520
21st to endDay
35%38%10%12%5%1
2520
July1st to 10thDay
26%39%9%19%8%
2450
11th to 20thDay
27%38%10%18%7%
2445
21st to endDay
26%40%12%16%6%
2591
August1st to 10thDay
30%37%11%17%5%
2492
11th to 20thDay
23%37%13%20%7%
2274
21st to endDay
26%37%11%16%9%1
2376
September1st to 10thDay
27%36%9%19%8%
2160
11th to 20thDay
26%36%8%19%10%
2093
21st to endDay
31%37%9%18%6%
1980
October1st to 10thDay
34%37%8%15%6%
1847
11th to 20thDay
38%37%9%11%5%
1772
21st to endDay
37%37%9%12%5%1
1782
November1st to 10thDay
28%40%10%15%6%
1671
11th to 20thDay
37%39%8%8%7%
1626
21st to endDay
39%40%9%10%1
1520
December1st to 10thDay
45%41%7%6%2
1520
11th to 20thDay
44%33%10%11%2
1906
21st to endDay
56%25%8%8%2
2772
January1st to 10thNight
43%40%7%8%2
2649
11th to 20thNight
43%40%6%8%3
2660
21st to endNight
40%36%8%10%6%1
2868
February1st to 10thNight
43%36%7%9%4
2362
11th to 20thNight
38%37%9%13%3
2188
21st to endNight
41%36%6%12%5%
1776
March1st to 10thNight
41%35%7%12%5%1
2160
11th to 20thNight
37%37%8%13%5%
2082
21st to endNight
32%37%6%14%9%3
1990
April1st to 10thNight
36%36%7%13%7%2
1800
11th to 20thNight
34%35%6%16%8%1
1775
21st to endNight
34%36%5%15%8%3
1666
May1st to 10thNight
35%35%311%11%5%
1484
11th to 20thNight
30%37%5%13%12%3
1416
21st to endNight
27%37%8%13%13%2
1551
June1st to 10thNight
35%38%9%12%6%1
1440
11th to 20thNight
29%37%9%15%9%2
1440
21st to endNight
32%38%9%14%8%
1440
July1st to 10thNight
26%39%9%17%8%
1400
11th to 20thNight
26%39%10%17%8%
1392
21st to endNight
26%40%10%15%8%
1486
August1st to 10thNight
26%36%13%18%7%
1422
11th to 20thNight
23%36%13%18%10%
1604
21st to endNight
28%37%11%16%7%1
1980
September1st to 10thNight
29%37%7%17%9%
1800
11th to 20thNight
26%36%8%20%9%
1867
21st to endNight
30%37%8%16%9%
1980
October1st to 10thNight
34%37%8%14%7%
2110
11th to 20thNight
37%36%9%13%5%
2186
21st to endNight
37%37%9%11%5%1
2574
November1st to 10thNight
32%41%8%13%6%
2407
11th to 20thNight
33%39%9%11%6%1
2550
21st to endNight
40%39%8%10%3
2660
December1st to 10thNight
45%39%7%7%1
2660
11th to 20thNight
45%32%7%13%3
3288
21st to endNight
56%24%9%10%2
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%13%29%23%11%1
180
11th to 20thDay
27%16%28%22%7%1
180
21st to endDay
20%14%35%22%8%1
188
February1st to 10thDay
30%19%33%15%2
228
11th to 20thDay
23%12%39%22%3
325
21st to endDay
28%11%29%30%2
346
March1st to 10thDay
23%12%34%24%8%
420
11th to 20thDay
19%15%33%25%7%1
420
21st to endDay
26%11%29%20%11%2
462
April1st to 10thDay
25%14%27%21%10%2
420
11th to 20thDay
24%15%24%22%12%3
412
21st to endDay
26%13%27%23%10%2
420
May1st to 10thDay
21%9%31%25%11%3
423
11th to 20thDay
17%9%32%26%12%4
447
21st to endDay
17%8%32%25%14%4
485
June1st to 10thDay
22%10%26%27%12%4
450
11th to 20thDay
17%8%24%32%17%3
450
21st to endDay
20%7%25%25%18%4
450
July1st to 10thDay
10%7%24%31%21%7%
445
11th to 20thDay
11%5%27%31%20%6%
444
21st to endDay
13%5%23%34%20%5%
488
August1st to 10thDay
13%422%31%22%7%
446
11th to 20thDay
11%5%20%31%21%12%
439
21st to endDay
13%7%26%24%20%10%
495
September1st to 10thDay
13%6%24%29%17%10%
450
11th to 20thDay
14%8%22%24%20%12%
450
21st to endDay
17%8%26%25%18%7%
450
October1st to 10thDay
21%11%25%23%15%6%
450
11th to 20thDay
20%12%31%22%13%2
435
21st to endDay
33%11%25%23%8%
396
November1st to 10thDay
26%14%23%26%11%1
365
11th to 20thDay
23%15%33%19%8%1
286
21st to endDay
27%14%29%22%7%1
180
December1st to 10thDay
34%7%24%24%8%2
180
11th to 20thDay
23%13%39%18%41
180
21st to endDay
32%13%30%17%7%1
198
January1st to 10thNight
26%11%30%27%6%
760
11th to 20thNight
27%9%34%24%5%1
760
21st to endNight
19%9%35%28%8%1
820
February1st to 10thNight
22%8%37%26%6%1
720
11th to 20thNight
22%9%35%28%5%
703
21st to endNight
23%8%33%32%4
592
March1st to 10thNight
23%12%31%28%6%
720
11th to 20thNight
26%10%31%25%7%
720
21st to endNight
23%8%29%31%9%1
792
April1st to 10thNight
27%10%27%30%6%
720
11th to 20thNight
25%11%29%28%6%1
711
21st to endNight
26%9%30%26%9%
586
May1st to 10thNight
24%7%29%28%9%1
404
11th to 20thNight
20%8%28%35%9%1
354
21st to endNight
18%5%31%34%12%
389
June1st to 10thNight
25%5%25%33%11%1
360
11th to 20thNight
18%426%36%14%1
360
21st to endNight
17%428%33%17%1
360
July1st to 10thNight
12%325%39%20%1
350
11th to 20thNight
11%223%45%17%1
348
21st to endNight
12%419%46%18%2
394
August1st to 10thNight
16%314%45%22%1
358
11th to 20thNight
14%321%34%25%3
540
21st to endNight
15%322%36%22%2
792
September1st to 10thNight
13%423%42%18%1
720
11th to 20thNight
19%5%18%35%21%2
720
21st to endNight
18%5%25%30%19%2
720
October1st to 10thNight
20%7%29%30%14%1
720
11th to 20thNight
23%7%29%29%11%1
720
21st to endNight
28%8%30%29%6%
792
November1st to 10thNight
25%10%28%27%9%
740
11th to 20thNight
23%12%33%21%10%1
760
21st to endNight
30%11%28%23%8%1
760
December1st to 10thNight
29%11%33%21%6%
760
11th to 20thNight
29%13%31%21%5%1
760
21st to endNight
34%12%29%21%41
836
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