Heavens View Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Heavens View Observatory clear sky chart from 2005-11-06 to 2024-01-12.

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
33%
24094
16%
74871
10%
24038
40%
24038
20%
52031

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
17%
950
13%
2648
4%
946
25%
946
13%
2061
January11th to 20thNight
17%
908
8%
2541
3%
908
21%
908
11%
2051
January21st to endNight
20%
840
10%
2622
4%
840
25%
840
12%
2044
February1st to 10thNight
16%
720
9%
2340
2%
720
23%
720
9%
1852
February11th to 20thNight
20%
700
14%
2156
6%
700
28%
700
13%
1631
February21st to endNight
17%
592
15%
1776
4%
592
28%
592
14%
1306
March1st to 10thNight
24%
720
17%
2123
9%
720
31%
720
18%
1501
March11th to 20thNight
27%
720
21%
1980
9%
720
36%
720
22%
1353
March21st to endNight
29%
792
17%
2178
9%
792
35%
792
19%
1573
April1st to 10thNight
24%
618
15%
1878
8%
618
31%
618
19%
1291
April11th to 20thNight
28%
533
21%
1625
9%
533
38%
533
22%
1083
April21st to endNight
33%
540
19%
1620
7%
540
43%
540
24%
1065
May1st to 10thNight
34%
540
14%
1620
8%
540
39%
540
22%
1105
May11th to 20thNight
40%
531
19%
1592
12%
531
47%
531
27%
1007
May21st to endNight
54%
583
16%
1727
12%
582
60%
582
33%
1045
June1st to 10thNight
52%
424
18%
1324
15%
424
58%
424
31%
776
June11th to 20thNight
50%
360
20%
1260
20%
360
57%
360
28%
714
June21st to endNight
55%
453
20%
1353
16%
453
62%
453
32%
808
July1st to 10thNight
64%
525
18%
1400
14%
525
67%
525
42%
812
July11th to 20thNight
60%
522
21%
1392
17%
522
64%
522
39%
832
July21st to endNight
64%
591
19%
1610
13%
558
68%
558
40%
915
August1st to 10thNight
67%
537
20%
1600
17%
534
70%
534
38%
918
August11th to 20thNight
62%
534
23%
1598
18%
534
67%
534
35%
925
August21st to endNight
59%
594
24%
1795
17%
594
64%
594
36%
1027
September1st to 10thNight
55%
643
20%
1903
15%
643
59%
643
31%
1151
September11th to 20thNight
58%
720
27%
1980
20%
720
62%
720
37%
1174
September21st to endNight
48%
720
23%
1980
17%
720
54%
720
31%
1265
October1st to 10thNight
42%
720
21%
2031
15%
720
48%
720
28%
1320
October11th to 20thNight
28%
720
16%
2160
7%
720
38%
720
18%
1535
October21st to endNight
23%
792
15%
2527
8%
792
33%
792
15%
1840
November1st to 10thNight
33%
740
22%
2406
15%
740
43%
740
23%
1512
November11th to 20thNight
22%
760
16%
2470
8%
760
32%
760
16%
1728
November21st to endNight
20%
760
14%
2594
8%
760
29%
760
14%
1873
December1st to 10thNight
17%
760
9%
2660
5%
760
23%
760
10%
2064
December11th to 20thNight
14%
893
9%
3427
2%
891
21%
891
9%
2606
December21st to endNight
11%
1039
5%
4975
1%
1026
18%
1026
5%
4268

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
14%10%47%6%45%5%6%7%33%
74599
Night
15%11%47%6%35%45%6%34%
75104
TransparencyDay
50%5%11%21%12%1
74307
Night
50%411%21%12%1
74916
SeeingDay
19%11%29%23%13%5%
14263
Night
20%10%30%28%11%1
24104

By Month

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 10thDay
14%15%7%13%7%45%5%5%6%20%
1517
11th to 20thDay
14%17%8%14%9%5%4446%16%
1448
21st to endDay
15%19%8%13%8%45%44417%
1732
February1st to 10thDay
14%14%7%12%10%46%445%20%
1620
11th to 20thDay
14%13%6%11%10%45%5%5%5%22%
1691
21st to endDay
14%16%7%11%8%3444424%
1480
March1st to 10thDay
15%10%5%8%8%5%45%6%5%29%
1836
11th to 20thDay
12%14%5%8%6%4435%7%31%
1980
21st to endDay
18%13%49%6%45%445%28%
2178
April1st to 10thDay
16%12%5%7%6%4445%6%30%
2096
11th to 20thDay
17%10%47%5%35%46%7%33%
2284
21st to endDay
17%9%47%6%4445%7%33%
2340
May1st to 10thDay
16%9%46%6%45%45%8%33%
2340
11th to 20thDay
19%7%36%5%345%5%7%36%
2301
21st to endDay
13%6%36%5%35%5%6%9%40%
2544
June1st to 10thDay
11%5%36%6%45%5%6%9%41%
2628
11th to 20thDay
11%5%36%6%35%5%6%8%42%
2700
21st to endDay
10%6%36%5%45%6%8%10%38%
2599
July1st to 10thDay
7%5%25%5%45%6%8%9%45%
2520
11th to 20thDay
9%5%25%6%35%7%7%9%43%
2520
21st to endDay
8%6%25%7%47%6%8%10%39%
2609
August1st to 10thDay
9%5%26%6%46%6%7%9%39%
2327
11th to 20thDay
10%6%26%6%46%6%7%8%38%
2303
21st to endDay
11%6%26%6%36%6%5%9%41%
2556
September1st to 10thDay
11%7%27%5%46%5%7%9%37%
2039
11th to 20thDay
12%425%4345%7%9%45%
1980
21st to endDay
16%6%345%46%6%7%9%35%
1980
October1st to 10thDay
14%7%36%6%45%5%5%6%39%
1928
11th to 20thDay
18%10%46%8%45%5%5%6%31%
1800
21st to endDay
20%11%5%7%6%3445%7%28%
1828
November1st to 10thDay
12%8%48%6%346%6%8%35%
1672
11th to 20thDay
13%13%5%8%7%45%5%6%6%29%
1710
21st to endDay
17%14%6%7%5%45%5%47%26%
1582
December1st to 10thDay
14%15%9%12%7%35%45%5%21%
1520
11th to 20thDay
15%17%6%9%7%345%5%6%23%
1728
21st to endDay
22%22%7%12%8%3434411%
2683
January1st to 10thNight
16%13%6%10%7%45%5%5%5%23%
2648
11th to 20thNight
17%16%7%11%7%45%445%20%
2541
21st to endNight
12%14%8%12%9%45%445%22%
2622
February1st to 10thNight
16%16%6%9%8%5%6%445%22%
2340
11th to 20thNight
13%12%6%11%8%45%5%5%5%26%
2155
21st to endNight
18%13%49%8%35%345%29%
1776
March1st to 10thNight
15%11%49%6%35%45%5%34%
2123
11th to 20thNight
13%9%46%5%45%447%38%
1980
21st to endNight
19%9%47%6%35%35%6%33%
2178
April1st to 10thNight
17%10%47%5%345%5%7%34%
1878
11th to 20thNight
18%9%26%5%3445%7%37%
1625
21st to endNight
17%8%25%5%35%46%6%38%
1620
May1st to 10thNight
16%8%47%5%35%46%7%35%
1620
11th to 20thNight
17%7%25%4335%5%7%42%
1592
21st to endNight
9%5%25%5%45%6%6%8%46%
1726
June1st to 10thNight
11%5%26%5%25%36%10%46%
1324
11th to 20thNight
10%5%35%435%5%7%9%45%
1260
21st to endNight
9%6%26%5%34448%48%
1353
July1st to 10thNight
8%4145%245%5%9%54%
1440
11th to 20thNight
9%424435%5%6%9%50%
1440
21st to endNight
6%425%4346%7%10%50%
1705
August1st to 10thNight
7%424435%5%7%10%48%
1611
11th to 20thNight
7%4246%45%6%6%8%48%
1598
21st to endNight
9%5%25%5%245%47%52%
1795
September1st to 10thNight
10%5%36%5%35%47%7%46%
1903
11th to 20thNight
10%425%4345%5%7%52%
1980
21st to endNight
13%8%35%5%3445%7%43%
1980
October1st to 10thNight
13%8%36%5%33446%43%
2031
11th to 20thNight
19%10%47%5%44445%33%
2160
21st to endNight
21%10%37%7%35%446%29%
2527
November1st to 10thNight
14%9%35%5%3445%6%42%
2406
11th to 20thNight
15%10%5%7%7%35%6%6%6%30%
2470
21st to endNight
18%14%5%8%5%3445%8%27%
2594
December1st to 10thNight
16%16%8%10%6%35%45%5%21%
2660
11th to 20thNight
13%14%6%13%7%45%5%46%23%
3465
21st to endNight
24%24%8%10%6%2333412%
4978

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
64%410%14%7%1
1518
11th to 20thDay
70%411%10%5%
1448
21st to endDay
71%36%12%7%1
1732
February1st to 10thDay
66%48%14%7%
1620
11th to 20thDay
63%48%15%9%1
1700
21st to endDay
62%5%8%15%8%2
1480
March1st to 10thDay
56%5%12%18%9%1
1836
11th to 20thDay
52%5%9%19%13%3
1980
21st to endDay
59%410%17%9%1
2178
April1st to 10thDay
54%5%10%20%11%1
2096
11th to 20thDay
50%411%20%13%2
2284
21st to endDay
50%5%13%17%13%1
2340
May1st to 10thDay
50%412%22%10%2
2340
11th to 20thDay
48%5%11%21%14%2
2305
21st to endDay
41%5%14%24%14%2
2544
June1st to 10thDay
38%5%13%27%15%2
2628
11th to 20thDay
39%5%13%24%16%2
2700
21st to endDay
39%7%12%24%16%2
2599
July1st to 10thDay
34%6%11%28%18%3
2451
11th to 20thDay
36%5%13%26%18%2
2445
21st to endDay
37%6%12%23%19%3
2464
August1st to 10thDay
38%6%14%28%13%2
2314
11th to 20thDay
40%6%13%25%14%2
2303
21st to endDay
39%6%10%27%16%2
2556
September1st to 10thDay
42%6%13%25%12%2
2039
11th to 20thDay
35%413%35%13%1
1980
21st to endDay
43%5%13%26%13%1
1980
October1st to 10thDay
43%5%10%26%13%2
1928
11th to 20thDay
52%412%21%10%1
1800
21st to endDay
55%5%13%18%9%
1828
November1st to 10thDay
46%5%11%25%11%1
1672
11th to 20thDay
55%412%19%9%1
1710
21st to endDay
57%412%19%7%1
1582
December1st to 10thDay
64%410%17%4
1520
11th to 20thDay
61%5%14%15%6%
1724
21st to endDay
78%39%7%3
2683
January1st to 10thNight
61%49%18%8%1
2648
11th to 20thNight
66%411%13%6%
2541
21st to endNight
64%410%16%6%1
2622
February1st to 10thNight
65%5%8%13%8%1
2340
11th to 20thNight
59%5%8%17%11%1
2156
21st to endNight
57%311%17%10%1
1776
March1st to 10thNight
52%49%23%11%1
2123
11th to 20thNight
47%49%22%15%2
1980
21st to endNight
53%49%21%11%2
2178
April1st to 10thNight
50%49%22%14%1
1878
11th to 20thNight
46%411%19%18%2
1625
21st to endNight
44%5%10%22%16%3
1620
May1st to 10thNight
48%411%23%12%2
1620
11th to 20thNight
42%411%21%19%4
1593
21st to endNight
34%6%14%27%16%3
1727
June1st to 10thNight
35%5%10%30%18%2
1324
11th to 20thNight
35%5%14%25%18%2
1260
21st to endNight
35%5%13%27%20%1
1353
July1st to 10thNight
30%411%29%23%4
1400
11th to 20thNight
31%5%12%32%18%2
1392
21st to endNight
27%7%18%25%20%3
1610
August1st to 10thNight
29%6%17%32%15%1
1600
11th to 20thNight
31%6%15%32%12%3
1598
21st to endNight
31%5%13%30%19%1
1795
September1st to 10thNight
36%5%13%28%16%2
1903
11th to 20thNight
31%6%12%31%20%1
1980
21st to endNight
40%412%31%13%1
1980
October1st to 10thNight
41%5%10%25%18%1
2031
11th to 20thNight
52%5%11%19%12%1
2160
21st to endNight
56%5%9%21%9%1
2527
November1st to 10thNight
42%5%11%26%15%1
2406
11th to 20thNight
52%5%13%21%8%1
2470
21st to endNight
57%413%19%7%
2594
December1st to 10thNight
65%410%16%5%
2660
11th to 20thNight
61%5%12%14%7%1
3468
21st to endNight
76%410%7%3
4978

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%39%13%5%
270
11th to 20thDay
23%12%47%14%31
243
21st to endDay
29%14%38%13%5%1
264
February1st to 10thDay
29%20%37%14%1
240
11th to 20thDay
26%23%37%13%1
240
21st to endDay
28%29%31%11%2
198
March1st to 10thDay
28%24%32%12%5%
276
11th to 20thDay
24%12%34%23%7%1
420
21st to endDay
30%11%33%19%5%1
462
April1st to 10thDay
19%15%36%20%8%1
420
11th to 20thDay
27%16%30%15%6%5%
412
21st to endDay
26%13%28%22%9%2
420
May1st to 10thDay
25%14%26%23%9%3
423
11th to 20thDay
20%10%29%23%11%6%
447
21st to endDay
17%7%22%29%16%8%
498
June1st to 10thDay
14%7%30%30%16%3
630
11th to 20thDay
15%8%29%26%18%4
630
21st to endDay
13%6%29%30%17%5%
630
July1st to 10thDay
9%5%21%33%21%12%
621
11th to 20thDay
11%6%26%30%19%8%
618
21st to endDay
11%423%33%20%9%
550
August1st to 10thDay
11%322%27%23%15%
446
11th to 20thDay
15%321%25%23%13%
439
21st to endDay
13%5%23%26%21%12%
495
September1st to 10thDay
15%6%23%24%20%12%
450
11th to 20thDay
12%322%25%24%14%
450
21st to endDay
18%12%21%20%20%8%
450
October1st to 10thDay
20%9%26%21%17%7%
450
11th to 20thDay
20%17%30%26%6%1
450
21st to endDay
29%15%37%15%41
343
November1st to 10thDay
21%17%36%19%6%1
270
11th to 20thDay
23%22%34%14%42
270
21st to endDay
25%18%41%13%3
210
December1st to 10thDay
26%22%31%13%7%1
180
11th to 20thDay
27%26%37%7%21
180
21st to endDay
27%25%35%12%1
268
January1st to 10thNight
22%16%36%20%5%1
950
11th to 20thNight
24%14%36%21%41
908
21st to endNight
19%11%38%27%5%
840
February1st to 10thNight
26%15%35%22%2
720
11th to 20thNight
23%15%33%24%5%
703
21st to endNight
28%16%34%19%3
592
March1st to 10thNight
22%13%34%24%7%
720
11th to 20thNight
19%13%36%25%6%1
720
21st to endNight
24%10%29%28%8%1
792
April1st to 10thNight
21%13%36%24%41
618
11th to 20thNight
22%11%35%23%7%2
533
21st to endNight
21%7%33%31%8%
540
May1st to 10thNight
21%5%33%33%7%1
540
11th to 20thNight
21%429%33%12%1
531
21st to endNight
12%324%41%18%2
584
June1st to 10thNight
12%6%23%45%14%
424
11th to 20thNight
11%428%43%13%1
360
21st to endNight
12%422%43%18%1
453
July1st to 10thNight
11%217%39%27%4
525
11th to 20thNight
11%220%42%21%4
522
21st to endNight
7%319%49%20%2
591
August1st to 10thNight
9%215%44%27%3
537
11th to 20thNight
9%318%34%31%5%
534
21st to endNight
11%319%35%29%3
594
September1st to 10thNight
12%5%19%35%22%6%
643
11th to 20thNight
13%418%34%26%4
720
21st to endNight
16%8%20%31%20%5%
720
October1st to 10thNight
20%7%24%29%18%2
720
11th to 20thNight
27%12%28%25%7%1
720
21st to endNight
28%12%32%24%4
792
November1st to 10thNight
20%12%29%26%12%1
740
11th to 20thNight
23%15%35%20%6%1
760
21st to endNight
27%13%35%19%6%
760
December1st to 10thNight
22%16%36%20%5%1
760
11th to 20thNight
22%16%39%20%3
893
21st to endNight
31%12%38%17%1
1045
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