This page made possible by the sponsorship of: Chris Webb, Mark Manner.

Long Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Long Mountain Observatory clear sky chart from 2005-11-06 to 2024-01-16.

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
44%
23615
21%
74599
15%
23557
51%
23557
24%
48066

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
21%
759
19%
2459
9%
757
33%
757
16%
1695
January11th to 20thNight
18%
741
16%
2406
8%
741
28%
741
12%
1799
January21st to endNight
21%
792
22%
2466
9%
792
34%
792
18%
1682
February1st to 10thNight
22%
720
18%
2160
8%
720
31%
720
17%
1531
February11th to 20thNight
17%
703
16%
2098
7%
699
25%
699
15%
1533
February21st to endNight
18%
592
18%
1776
8%
592
27%
592
15%
1298
March1st to 10thNight
27%
720
20%
2160
11%
720
35%
720
21%
1440
March11th to 20thNight
26%
720
17%
2009
9%
720
32%
720
19%
1432
March21st to endNight
31%
788
17%
2174
9%
788
36%
788
21%
1506
April1st to 10thNight
37%
540
26%
1800
16%
540
50%
540
27%
1068
April11th to 20thNight
36%
533
27%
1775
17%
533
50%
533
28%
1040
April21st to endNight
44%
540
23%
1702
16%
540
52%
540
27%
1058
May1st to 10thNight
44%
540
21%
1620
12%
540
51%
540
29%
1011
May11th to 20thNight
50%
531
18%
1593
12%
531
53%
531
27%
1016
May21st to endNight
63%
582
18%
1745
14%
582
66%
582
28%
1098
June1st to 10thNight
63%
540
17%
1620
14%
540
65%
540
29%
1009
June11th to 20thNight
75%
540
19%
1620
17%
540
77%
540
45%
866
June21st to endNight
68%
540
17%
1620
15%
540
69%
540
33%
968
July1st to 10thNight
70%
525
12%
1575
12%
525
70%
525
26%
1032
July11th to 20thNight
72%
522
17%
1566
17%
522
73%
522
34%
943
July21st to endNight
74%
591
18%
1672
17%
558
75%
558
29%
1066
August1st to 10thNight
72%
537
19%
1619
18%
534
73%
534
31%
985
August11th to 20thNight
71%
534
15%
1776
16%
534
71%
534
27%
1088
August21st to endNight
71%
594
19%
1980
23%
594
71%
594
28%
1186
September1st to 10thNight
69%
607
21%
1867
21%
607
69%
607
31%
1147
September11th to 20thNight
60%
720
24%
1980
19%
720
62%
720
30%
1282
September21st to endNight
62%
720
29%
1980
21%
720
66%
720
38%
1169
October1st to 10thNight
59%
720
33%
1980
28%
720
63%
720
41%
1117
October11th to 20thNight
44%
720
28%
2024
19%
720
53%
720
31%
1250
October21st to endNight
43%
792
29%
2507
24%
792
53%
792
28%
1504
November1st to 10thNight
49%
740
27%
2406
25%
740
57%
740
31%
1367
November11th to 20thNight
36%
760
27%
2470
19%
760
49%
760
28%
1457
November21st to endNight
34%
760
24%
2470
16%
760
45%
760
23%
1564
December1st to 10thNight
28%
760
18%
2470
13%
760
36%
760
15%
1766
December11th to 20thNight
31%
759
21%
2982
14%
759
41%
759
16%
1962
December21st to endNight
25%
833
12%
4472
10%
817
34%
817
9%
3131

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%8%36%6%45%5%6%7%37%
74881
Night
13%8%36%5%3445%7%41%
74926
TransparencyDay
44%6%11%20%15%3
74604
Night
42%6%11%21%17%4
74722
SeeingDay
19%12%23%22%16%8%
13530
Night
18%10%21%28%19%4
23622

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
17%10%5%7%5%35%45%5%34%
1706
11th to 20thDay
21%12%5%8%5%3444430%
1665
21st to endDay
17%9%45%5%2345%5%40%
1889
February1st to 10thDay
20%12%5%7%6%35%24432%
1800
11th to 20thDay
20%12%48%6%3345%5%31%
1748
21st to endDay
20%11%46%5%35%445%33%
1480
March1st to 10thDay
13%10%46%5%345%5%6%38%
1800
11th to 20thDay
19%10%37%6%345%5%5%33%
1949
21st to endDay
15%11%46%6%36%5%6%6%33%
2196
April1st to 10thDay
13%6%35%6%345%5%8%43%
2160
11th to 20thDay
14%8%25%5%35%45%6%43%
2134
21st to endDay
14%7%36%5%35%5%5%7%39%
2255
May1st to 10thDay
14%6%26%6%345%5%8%41%
2340
11th to 20thDay
14%6%36%7%45%6%6%6%36%
2301
21st to endDay
9%5%35%8%46%6%7%8%37%
2531
June1st to 10thDay
10%5%36%7%46%6%7%8%39%
2340
11th to 20thDay
6%426%5%46%6%9%10%43%
2340
21st to endDay
6%435%7%46%7%9%11%37%
2340
July1st to 10thDay
9%7%28%7%47%6%9%10%30%
2340
11th to 20thDay
11%5%26%7%5%7%7%8%10%31%
2340
21st to endDay
9%5%36%7%5%8%7%7%10%31%
2547
August1st to 10thDay
9%6%35%8%5%6%7%10%10%31%
2304
11th to 20thDay
9%6%36%8%5%7%7%7%9%33%
2125
21st to endDay
12%7%36%6%6%7%7%7%9%30%
2376
September1st to 10thDay
13%6%36%6%45%7%6%7%37%
2074
11th to 20thDay
13%7%26%6%45%5%5%7%39%
1980
21st to endDay
12%5%26%435%5%5%6%47%
1980
October1st to 10thDay
11%5%245%4345%5%53%
1980
11th to 20thDay
14%5%2343335%7%52%
1936
21st to endDay
14%7%244345%5%6%46%
1846
November1st to 10thDay
12%5%23324446%55%
1672
11th to 20thDay
12%8%35%43345%7%47%
1710
21st to endDay
15%8%25%4345%5%6%43%
1710
December1st to 10thDay
19%11%47%5%34446%33%
1710
11th to 20thDay
14%13%37%6%45%45%5%34%
2153
21st to endDay
15%12%6%6%6%35%5%5%7%29%
3124
January1st to 10thNight
17%10%48%5%24335%38%
2459
11th to 20thNight
20%13%6%7%6%3444429%
2406
21st to endNight
15%12%47%5%33445%38%
2466
February1st to 10thNight
20%9%48%5%3445%6%33%
2160
11th to 20thNight
23%10%47%6%3334433%
2098
21st to endNight
23%12%35%5%3445%6%30%
1776
March1st to 10thNight
17%9%44425%46%6%40%
2160
11th to 20thNight
18%10%36%5%35%45%6%35%
2009
21st to endNight
15%9%36%5%35%5%5%7%37%
2174
April1st to 10thNight
12%6%245%345%5%8%47%
1800
11th to 20thNight
15%5%3432435%7%49%
1775
21st to endNight
14%5%26%4345%6%8%43%
1702
May1st to 10thNight
11%6%245%35%5%5%8%45%
1620
11th to 20thNight
12%6%26%5%35%46%8%43%
1592
21st to endNight
7%5%26%5%36%6%7%11%41%
1744
June1st to 10thNight
5%6%46%7%45%6%6%9%44%
1620
11th to 20thNight
431343447%13%53%
1620
21st to endNight
5%425%6%36%6%7%11%45%
1620
July1st to 10thNight
8%5%36%5%36%6%9%12%36%
1620
11th to 20thNight
7%415%6%35%5%7%14%43%
1620
21st to endNight
7%5%246%5%6%7%9%11%38%
1771
August1st to 10thNight
5%425%6%46%5%8%12%42%
1630
11th to 20thNight
7%6%25%6%45%7%7%9%43%
1776
21st to endNight
9%5%25%435%6%8%11%41%
1980
September1st to 10thNight
10%6%25%5%345%7%7%46%
1867
11th to 20thNight
10%6%25%6%35%5%6%8%44%
1980
21st to endNight
9%425%43445%7%53%
1980
October1st to 10thNight
11%424423345%59%
1980
11th to 20thNight
16%5%25%433445%50%
2024
21st to endNight
15%7%24423436%49%
2507
November1st to 10thNight
12%7%24423446%52%
2406
11th to 20thNight
13%7%2442335%7%48%
2470
21st to endNight
15%9%444245%46%43%
2470
December1st to 10thNight
20%11%47%5%35%446%32%
2470
11th to 20thNight
16%11%47%5%2435%5%38%
3042
21st to endNight
14%15%5%8%7%45%5%5%7%26%
4532

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
52%47%18%16%3
1707
11th to 20thDay
59%46%15%13%3
1665
21st to endDay
46%49%22%18%2
1889
February1st to 10thDay
56%37%15%17%2
1800
11th to 20thDay
54%5%7%17%13%3
1758
21st to endDay
54%5%7%13%16%6%
1480
March1st to 10thDay
47%5%7%17%18%6%
1800
11th to 20thDay
53%5%9%16%14%3
1949
21st to endDay
51%5%11%20%12%1
2196
April1st to 10thDay
40%6%11%22%18%3
2160
11th to 20thDay
42%5%8%19%23%3
2134
21st to endDay
43%5%9%27%15%1
2255
May1st to 10thDay
42%6%11%26%13%3
2340
11th to 20thDay
48%8%13%20%12%
2305
21st to endDay
42%9%13%21%13%2
2531
June1st to 10thDay
41%7%15%23%12%1
2340
11th to 20thDay
32%9%19%23%14%2
2340
21st to endDay
37%9%19%20%12%3
2340
July1st to 10thDay
44%10%15%18%9%3
2275
11th to 20thDay
45%9%18%15%10%3
2271
21st to endDay
44%10%15%22%8%1
2405
August1st to 10thDay
43%11%20%18%7%1
2291
11th to 20thDay
46%8%15%19%11%1
2125
21st to endDay
47%8%15%16%11%2
2376
September1st to 10thDay
44%6%13%18%15%4
2074
11th to 20thDay
43%7%10%20%18%2
1980
21st to endDay
38%5%10%23%19%5%
1980
October1st to 10thDay
33%5%8%22%25%6%
1980
11th to 20thDay
33%5%9%22%24%8%
1936
21st to endDay
37%5%7%21%25%5%
1846
November1st to 10thDay
30%49%23%28%5%
1672
11th to 20thDay
39%36%23%23%6%
1710
21st to endDay
42%47%22%21%4
1710
December1st to 10thDay
54%46%20%14%3
1710
11th to 20thDay
52%48%16%18%3
2150
21st to endDay
52%6%10%18%12%3
3124
January1st to 10thNight
51%48%18%16%3
2459
11th to 20thNight
59%46%14%14%3
2406
21st to endNight
50%37%22%16%3
2466
February1st to 10thNight
52%49%17%16%2
2160
11th to 20thNight
57%35%18%13%4
2099
21st to endNight
55%5%8%15%13%4
1776
March1st to 10thNight
45%5%9%15%19%7%
2160
11th to 20thNight
50%5%8%20%15%2
2009
21st to endNight
46%6%10%23%13%2
2174
April1st to 10thNight
37%411%23%21%3
1800
11th to 20thNight
35%5%9%22%22%6%
1775
21st to endNight
38%6%11%28%15%1
1702
May1st to 10thNight
37%7%12%23%19%2
1620
11th to 20thNight
39%7%17%23%13%1
1593
21st to endNight
35%10%16%23%14%3
1745
June1st to 10thNight
36%8%15%26%12%2
1620
11th to 20thNight
22%7%23%32%14%2
1620
21st to endNight
32%8%17%27%12%4
1620
July1st to 10thNight
36%12%18%20%11%3
1575
11th to 20thNight
30%10%21%22%13%5%
1566
21st to endNight
34%11%18%24%11%2
1672
August1st to 10thNight
33%9%19%29%10%
1619
11th to 20thNight
35%10%22%17%15%2
1776
21st to endNight
34%8%18%24%13%2
1980
September1st to 10thNight
35%7%15%22%16%4
1867
11th to 20thNight
36%8%11%22%21%1
1980
21st to endNight
32%5%9%25%24%6%
1980
October1st to 10thNight
28%48%26%25%9%
1980
11th to 20thNight
37%48%22%21%8%
2024
21st to endNight
37%4422%25%7%
2507
November1st to 10thNight
35%36%23%25%8%
2406
11th to 20thNight
37%47%20%25%8%
2470
21st to endNight
44%36%22%21%5%
2470
December1st to 10thNight
54%5%7%18%14%3
2470
11th to 20thNight
49%47%16%21%4
3044
21st to endNight
55%6%8%17%12%2
4532

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
27%29%30%10%4
270
11th to 20thDay
33%30%22%11%31
255
21st to endDay
26%30%33%7%3
264
February1st to 10thDay
29%31%29%9%2
240
11th to 20thDay
42%26%28%31
240
21st to endDay
38%25%30%6%1
198
March1st to 10thDay
26%29%33%10%2
240
11th to 20thDay
27%26%26%15%5%1
389
21st to endDay
26%19%32%16%6%1
462
April1st to 10thDay
19%20%29%19%12%
420
11th to 20thDay
21%16%33%20%8%2
412
21st to endDay
22%15%29%20%11%2
420
May1st to 10thDay
19%13%33%19%11%4
423
11th to 20thDay
14%9%30%24%14%9%
447
21st to endDay
14%6%19%29%18%14%
485
June1st to 10thDay
12%5%21%27%22%13%
450
11th to 20thDay
6%6%15%30%25%17%
450
21st to endDay
8%6%20%29%20%17%
450
July1st to 10thDay
12%413%25%26%20%
445
11th to 20thDay
12%311%23%29%23%
444
21st to endDay
11%39%26%30%20%
488
August1st to 10thDay
14%411%27%26%19%
446
11th to 20thDay
12%311%36%30%9%
439
21st to endDay
16%48%23%30%19%
495
September1st to 10thDay
12%311%26%33%14%
450
11th to 20thDay
15%5%12%27%28%13%
450
21st to endDay
14%6%17%30%26%8%
450
October1st to 10thDay
15%5%21%30%22%8%
450
11th to 20thDay
18%10%29%26%15%3
450
21st to endDay
23%11%30%23%10%3
361
November1st to 10thDay
20%11%33%26%8%2
270
11th to 20thDay
17%21%39%20%3
270
21st to endDay
21%16%37%17%8%1
270
December1st to 10thDay
31%17%32%14%5%1
270
11th to 20thDay
29%20%28%17%6%
270
21st to endDay
27%25%30%14%3
297
January1st to 10thNight
25%20%29%21%5%
760
11th to 20thNight
29%22%27%18%4
741
21st to endNight
22%20%34%21%3
792
February1st to 10thNight
25%18%30%23%3
720
11th to 20thNight
30%20%30%18%2
703
21st to endNight
33%20%26%19%2
592
March1st to 10thNight
24%16%30%27%4
720
11th to 20thNight
24%16%29%22%8%
720
21st to endNight
22%16%26%26%9%
788
April1st to 10thNight
18%14%27%30%11%1
540
11th to 20thNight
20%10%30%28%10%2
533
21st to endNight
17%8%27%35%12%1
540
May1st to 10thNight
15%7%28%30%18%1
540
11th to 20thNight
15%5%21%34%21%5%
531
21st to endNight
9%214%35%34%5%
584
June1st to 10thNight
8%214%36%34%6%
540
11th to 20thNight
4112%36%36%11%
540
21st to endNight
7%213%33%37%8%
540
July1st to 10thNight
10%26%28%43%11%
525
11th to 20thNight
10%27%26%43%13%
522
21st to endNight
9%2432%42%12%
591
August1st to 10thNight
7%1430%47%10%
537
11th to 20thNight
8%25%36%42%7%
534
21st to endNight
11%16%27%45%9%
594
September1st to 10thNight
14%26%31%39%9%
607
11th to 20thNight
15%312%27%37%6%
720
21st to endNight
11%314%34%33%5%
720
October1st to 10thNight
13%5%16%31%26%8%
720
11th to 20thNight
20%8%23%28%20%2
720
21st to endNight
19%9%24%31%14%3
792
November1st to 10thNight
17%10%20%34%18%2
740
11th to 20thNight
17%15%28%27%12%1
760
21st to endNight
21%14%26%28%10%1
760
December1st to 10thNight
29%14%25%25%6%
760
11th to 20thNight
23%15%27%28%6%1
760
21st to endNight
27%18%25%24%5%
836
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