Bellvale Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bellvale Observatory clear sky chart from 2005-11-06 to 2024-01-13.

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
28%
23061
19%
74638
9%
23019
41%
23019
21%
47834

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
16%
760
22%
2649
5%
757
36%
757
19%
1606
January11th to 20thNight
18%
732
18%
2560
5%
732
33%
732
14%
1718
January21st to endNight
17%
792
23%
2637
7%
792
37%
792
19%
1596
February1st to 10thNight
15%
720
17%
2340
4%
720
31%
720
17%
1516
February11th to 20thNight
11%
703
20%
2114
3%
700
31%
700
14%
1423
February21st to endNight
14%
592
25%
1776
7%
592
37%
592
16%
1186
March1st to 10thNight
18%
720
21%
2160
7%
720
36%
720
19%
1378
March11th to 20thNight
19%
720
20%
2049
7%
720
32%
720
19%
1355
March21st to endNight
18%
792
22%
2008
6%
792
30%
792
20%
1392
April1st to 10thNight
18%
720
18%
1800
3%
720
27%
720
20%
1280
April11th to 20thNight
24%
711
18%
1775
4%
711
32%
711
23%
1240
April21st to endNight
25%
698
18%
1778
4%
698
33%
698
22%
1240
May1st to 10thNight
22%
540
16%
1620
7%
540
31%
540
16%
1142
May11th to 20thNight
26%
354
20%
1416
12%
354
41%
354
16%
891
May21st to endNight
45%
388
15%
1551
14%
388
53%
388
21%
909
June1st to 10thNight
39%
360
17%
1440
13%
360
52%
360
21%
868
June11th to 20thNight
40%
360
15%
1440
14%
360
50%
360
20%
860
June21st to endNight
49%
360
16%
1440
14%
360
59%
360
27%
806
July1st to 10thNight
52%
350
15%
1400
18%
350
57%
350
22%
824
July11th to 20thNight
52%
348
15%
1392
18%
348
56%
348
21%
828
July21st to endNight
51%
394
18%
1486
19%
372
58%
372
26%
834
August1st to 10thNight
52%
447
17%
1511
13%
445
57%
445
27%
910
August11th to 20thNight
50%
708
15%
1772
8%
708
52%
708
27%
1255
August21st to endNight
51%
792
19%
1980
9%
792
54%
792
32%
1312
September1st to 10thNight
45%
720
21%
1800
9%
720
50%
720
31%
1203
September11th to 20thNight
46%
720
27%
1867
17%
720
52%
720
37%
1135
September21st to endNight
43%
720
25%
1980
16%
720
53%
720
31%
1274
October1st to 10thNight
35%
720
17%
2096
11%
720
42%
720
23%
1393
October11th to 20thNight
33%
720
22%
2160
12%
720
47%
720
27%
1341
October21st to endNight
23%
792
20%
2515
10%
792
39%
792
21%
1637
November1st to 10thNight
31%
740
23%
2407
10%
740
47%
740
25%
1439
November11th to 20thNight
24%
760
25%
2470
11%
760
41%
760
20%
1544
November21st to endNight
23%
760
22%
2579
8%
760
41%
760
20%
1573
December1st to 10thNight
19%
760
19%
2660
7%
760
35%
760
15%
1720
December11th to 20thNight
19%
760
18%
3200
6%
759
34%
759
13%
2042
December21st to endNight
14%
828
13%
4810
4%
817
33%
817
9%
3164

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%7%35%5%35%5%5%7%38%
74763
Night
16%7%35%5%3445%7%41%
74971
TransparencyDay
44%6%11%22%15%2
74468
Night
42%5%12%23%15%2
74800
SeeingDay
22%16%30%20%9%3
13805
Night
21%14%31%25%8%1
23070

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
13%8%25%5%46%5%5%7%41%
1517
11th to 20thDay
15%9%37%5%35%6%5%5%35%
1462
21st to endDay
13%8%26%5%45%46%7%39%
1714
February1st to 10thDay
18%8%35%6%46%45%6%35%
1620
11th to 20thDay
16%8%36%5%34448%39%
1732
21st to endDay
18%9%36%435%446%38%
1480
March1st to 10thDay
15%7%35%5%3446%8%40%
1797
11th to 20thDay
15%7%26%6%45%46%7%37%
1908
21st to endDay
20%9%25%4345%5%5%38%
2348
April1st to 10thDay
17%8%36%5%3445%6%39%
2160
11th to 20thDay
17%6%35%5%35%447%41%
2134
21st to endDay
20%6%35%7%3445%7%37%
2182
May1st to 10thDay
22%9%35%5%3446%6%34%
2355
11th to 20thDay
18%6%35%5%35%5%5%8%36%
2478
21st to endDay
14%6%36%5%35%5%6%9%40%
2725
June1st to 10thDay
17%5%25%6%45%5%6%9%36%
2520
11th to 20thDay
15%6%26%5%36%7%8%8%34%
2520
21st to endDay
13%5%36%6%45%6%5%9%39%
2520
July1st to 10thDay
12%6%36%6%46%6%7%9%37%
2520
11th to 20thDay
11%5%25%6%46%6%8%9%38%
2520
21st to endDay
11%6%37%6%46%6%6%9%37%
2744
August1st to 10thDay
11%6%36%6%45%6%7%9%36%
2396
11th to 20thDay
12%6%35%7%46%7%7%8%36%
2126
21st to endDay
14%5%35%5%35%5%6%7%41%
2376
September1st to 10thDay
15%5%35%6%46%5%6%6%41%
2160
11th to 20thDay
14%4245%2445%7%48%
2093
21st to endDay
16%5%36%5%445%5%7%42%
1980
October1st to 10thDay
21%6%35%5%24347%40%
1862
11th to 20thDay
14%7%245%35%46%6%44%
1800
21st to endDay
21%7%25%6%345%47%36%
1839
November1st to 10thDay
14%6%245%33336%51%
1671
11th to 20thDay
17%6%3343445%7%43%
1710
21st to endDay
16%7%35%5%35%5%5%8%40%
1596
December1st to 10thDay
18%8%46%6%3436%6%36%
1520
11th to 20thDay
21%6%3443345%6%42%
1906
21st to endDay
17%9%48%7%45%6%5%7%29%
2772
January1st to 10thNight
15%7%345%245%5%8%42%
2649
11th to 20thNight
18%7%36%6%35%5%5%7%35%
2560
21st to endNight
12%8%35%5%3445%9%42%
2637
February1st to 10thNight
17%9%35%43445%7%39%
2340
11th to 20thNight
16%8%36%5%35%45%6%38%
2112
21st to endNight
18%8%36%5%34445%41%
1776
March1st to 10thNight
16%7%3442446%7%42%
2150
11th to 20thNight
17%8%35%435%45%7%40%
2048
21st to endNight
17%7%35%5%35%5%5%7%38%
2008
April1st to 10thNight
17%8%36%6%35%5%48%35%
1800
11th to 20thNight
18%7%35%5%35%35%7%40%
1775
21st to endNight
20%6%35%42436%6%39%
1778
May1st to 10thNight
22%7%46%5%3445%6%34%
1620
11th to 20thNight
17%7%27%5%35%45%6%39%
1415
21st to endNight
13%6%26%5%35%47%8%42%
1550
June1st to 10thNight
17%5%246%245%6%9%40%
1440
11th to 20thNight
13%6%36%5%346%5%10%39%
1440
21st to endNight
11%5%25%5%35%5%6%9%46%
1440
July1st to 10thNight
12%5%35%5%35%5%6%8%43%
1440
11th to 20thNight
12%5%26%5%46%5%6%10%41%
1440
21st to endNight
11%4245%35%6%7%10%43%
1574
August1st to 10thNight
9%5%26%6%346%7%10%43%
1521
11th to 20thNight
13%7%37%6%46%5%5%7%38%
1772
21st to endNight
14%5%26%5%3445%7%44%
1980
September1st to 10thNight
15%6%35%5%3445%7%45%
1800
11th to 20thNight
13%5%24423337%53%
1867
21st to endNight
15%5%24424447%47%
1980
October1st to 10thNight
20%6%36%423446%42%
2096
11th to 20thNight
15%6%25%5%245%46%46%
2160
21st to endNight
22%7%24423446%41%
2515
November1st to 10thNight
12%6%35%5%3446%6%47%
2407
11th to 20thNight
18%7%25%433346%44%
2470
21st to endNight
15%6%25%5%35%45%8%43%
2579
December1st to 10thNight
19%8%26%43436%7%38%
2660
11th to 20thNight
19%8%345%35%5%5%6%38%
3288
21st to endNight
20%9%36%5%34447%34%
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
41%5%11%27%12%4
1518
11th to 20thDay
48%6%13%18%12%3
1462
21st to endDay
43%5%11%24%15%2
1714
February1st to 10thDay
50%5%11%20%13%1
1620
11th to 20thDay
45%5%8%23%16%3
1741
21st to endDay
48%411%21%13%3
1480
March1st to 10thDay
43%413%23%15%3
1800
11th to 20thDay
45%412%23%15%2
1908
21st to endDay
48%5%7%19%16%4
2348
April1st to 10thDay
45%5%9%21%15%4
2160
11th to 20thDay
43%5%10%21%17%3
2134
21st to endDay
47%5%9%21%14%4
2182
May1st to 10thDay
49%5%9%19%15%3
2355
11th to 20thDay
46%6%10%21%14%3
2482
21st to endDay
41%6%16%21%14%2
2725
June1st to 10thDay
44%7%12%21%14%1
2520
11th to 20thDay
45%7%11%25%11%2
2520
21st to endDay
42%6%12%25%15%1
2520
July1st to 10thDay
43%6%11%21%17%1
2450
11th to 20thDay
38%7%15%24%15%1
2445
21st to endDay
43%7%11%20%16%3
2591
August1st to 10thDay
42%7%13%24%13%1
2382
11th to 20thDay
42%7%14%25%11%1
2126
21st to endDay
40%6%11%25%16%2
2376
September1st to 10thDay
42%5%11%23%16%2
2160
11th to 20thDay
35%5%9%24%25%2
2093
21st to endDay
43%5%10%22%18%3
1980
October1st to 10thDay
46%6%8%23%15%3
1862
11th to 20thDay
39%6%12%25%17%1
1800
21st to endDay
47%5%10%21%14%2
1839
November1st to 10thDay
37%412%27%18%3
1671
11th to 20thDay
40%411%23%19%3
1710
21st to endDay
41%6%13%28%11%2
1596
December1st to 10thDay
49%5%14%19%13%1
1520
11th to 20thDay
43%5%16%23%11%2
1906
21st to endDay
53%8%10%21%7%
2772
January1st to 10thNight
40%412%25%16%3
2649
11th to 20thNight
48%6%13%22%9%2
2560
21st to endNight
40%411%24%19%2
2637
February1st to 10thNight
46%414%23%12%2
2340
11th to 20thNight
46%5%10%19%17%4
2114
21st to endNight
46%46%25%16%3
1776
March1st to 10thNight
41%413%24%14%4
2160
11th to 20thNight
43%410%23%16%3
2049
21st to endNight
44%5%9%19%18%4
2008
April1st to 10thNight
47%5%9%21%16%3
1800
11th to 20thNight
45%5%9%21%20%1
1775
21st to endNight
44%6%8%22%16%4
1778
May1st to 10thNight
51%5%8%19%15%2
1620
11th to 20thNight
45%6%10%18%19%2
1416
21st to endNight
39%6%19%23%11%2
1551
June1st to 10thNight
40%6%11%24%18%1
1440
11th to 20thNight
40%7%15%28%10%1
1440
21st to endNight
33%8%16%28%15%1
1440
July1st to 10thNight
38%7%14%24%14%3
1400
11th to 20thNight
39%7%13%28%11%1
1392
21st to endNight
32%9%18%21%17%2
1486
August1st to 10thNight
37%7%16%24%15%1
1511
11th to 20thNight
44%7%14%24%11%1
1772
21st to endNight
39%5%12%21%21%1
1980
September1st to 10thNight
40%5%11%26%17%1
1800
11th to 20thNight
33%411%25%25%2
1867
21st to endNight
39%49%23%24%1
1980
October1st to 10thNight
44%5%11%22%16%2
2096
11th to 20thNight
37%5%14%28%15%1
2160
21st to endNight
45%412%24%15%1
2515
November1st to 10thNight
37%6%12%22%21%2
2407
11th to 20thNight
42%49%24%18%4
2470
21st to endNight
39%5%14%29%12%1
2579
December1st to 10thNight
46%412%24%13%1
2660
11th to 20thNight
45%6%12%20%15%3
3288
21st to endNight
51%6%11%22%10%
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
32%23%27%16%2
180
11th to 20thDay
34%20%33%11%11
166
21st to endDay
23%32%30%10%23
234
February1st to 10thDay
34%28%30%6%2
240
11th to 20thDay
29%28%33%9%1
400
21st to endDay
30%23%34%10%2
346
March1st to 10thDay
27%25%30%15%3
420
11th to 20thDay
25%25%32%12%41
420
21st to endDay
31%21%30%11%5%2
462
April1st to 10thDay
30%20%30%13%6%1
420
11th to 20thDay
25%23%31%12%8%1
412
21st to endDay
30%19%30%14%6%1
420
May1st to 10thDay
35%18%24%15%6%1
423
11th to 20thDay
23%15%38%17%6%1
447
21st to endDay
19%11%27%28%13%2
485
June1st to 10thDay
17%15%33%23%10%2
450
11th to 20thDay
18%13%31%26%10%1
450
21st to endDay
15%13%30%28%12%2
450
July1st to 10thDay
15%9%28%27%15%6%
445
11th to 20thDay
12%5%30%32%17%4
444
21st to endDay
12%8%33%33%13%2
488
August1st to 10thDay
13%6%28%31%15%7%
446
11th to 20thDay
14%8%26%32%14%6%
439
21st to endDay
14%10%26%27%16%7%
495
September1st to 10thDay
19%12%23%23%16%7%
450
11th to 20thDay
16%10%31%20%12%11%
450
21st to endDay
20%12%29%23%11%6%
450
October1st to 10thDay
28%10%26%19%12%5%
450
11th to 20thDay
17%18%31%21%11%2
450
21st to endDay
27%15%33%16%9%
424
November1st to 10thDay
19%17%28%23%10%3
365
11th to 20thDay
26%22%29%17%5%1
370
21st to endDay
25%23%29%17%42
256
December1st to 10thDay
27%29%28%9%6%1
180
11th to 20thDay
37%22%29%11%2
180
21st to endDay
24%23%37%12%41
198
January1st to 10thNight
19%22%39%17%3
760
11th to 20thNight
24%19%34%20%4
732
21st to endNight
20%21%38%19%2
792
February1st to 10thNight
24%18%37%20%1
720
11th to 20thNight
24%24%37%14%1
703
21st to endNight
28%21%33%15%3
592
March1st to 10thNight
24%22%33%18%3
720
11th to 20thNight
25%20%33%18%31
720
21st to endNight
22%21%33%18%5%1
792
April1st to 10thNight
22%22%34%18%4
720
11th to 20thNight
22%18%33%22%5%
711
21st to endNight
24%16%32%23%5%
698
May1st to 10thNight
26%15%31%23%4
540
11th to 20thNight
21%13%34%25%6%1
354
21st to endNight
16%8%24%39%11%1
389
June1st to 10thNight
18%6%29%35%11%
360
11th to 20thNight
14%7%32%33%14%1
360
21st to endNight
11%431%43%11%
360
July1st to 10thNight
13%5%20%40%20%1
350
11th to 20thNight
14%125%43%16%1
348
21st to endNight
11%427%40%18%1
394
August1st to 10thNight
11%125%44%17%2
447
11th to 20thNight
14%421%37%22%2
708
21st to endNight
15%5%19%39%20%2
792
September1st to 10thNight
16%5%23%38%16%2
720
11th to 20thNight
17%5%25%31%18%4
720
21st to endNight
18%6%26%31%16%2
720
October1st to 10thNight
23%9%27%25%14%2
720
11th to 20thNight
18%13%32%29%8%
720
21st to endNight
25%13%34%21%7%
792
November1st to 10thNight
16%13%35%25%10%1
740
11th to 20thNight
23%17%31%21%7%1
760
21st to endNight
19%19%34%20%7%
760
December1st to 10thNight
27%18%33%18%5%
760
11th to 20thNight
23%18%35%20%3
760
21st to endNight
26%18%37%17%2
836
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