Mt. Chestnut Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mt. Chestnut Observatory clear sky chart from 2005-11-06 to 2024-01-17.

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%
23304
15%
74911
10%
23254
39%
23254
18%
52805

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%
758
12%
2488
4%
757
20%
757
8%
2002
January11th to 20thNight
12%
747
8%
2425
3%
747
17%
747
8%
1971
January21st to endNight
15%
792
10%
2574
5%
792
22%
792
9%
2065
February1st to 10thNight
16%
720
10%
2340
4%
720
23%
720
9%
1864
February11th to 20thNight
11%
702
10%
2275
3%
700
20%
700
10%
1760
February21st to endNight
14%
592
13%
1782
5%
592
23%
592
11%
1388
March1st to 10thNight
19%
720
16%
1980
7%
720
28%
720
16%
1451
March11th to 20thNight
24%
720
19%
1980
10%
720
32%
720
18%
1407
March21st to endNight
21%
792
18%
2178
8%
792
30%
792
18%
1602
April1st to 10thNight
23%
720
14%
1879
5%
720
29%
720
20%
1361
April11th to 20thNight
27%
690
18%
1754
7%
690
32%
690
23%
1235
April21st to endNight
25%
540
19%
1620
7%
540
34%
540
21%
1107
May1st to 10thNight
35%
540
15%
1620
7%
540
41%
540
19%
1133
May11th to 20thNight
39%
531
17%
1592
11%
531
45%
531
23%
1074
May21st to endNight
52%
408
19%
1571
19%
408
60%
408
28%
878
June1st to 10thNight
50%
360
17%
1440
17%
360
58%
360
25%
819
June11th to 20thNight
48%
360
17%
1440
19%
360
56%
360
22%
846
June21st to endNight
52%
360
18%
1440
19%
360
60%
360
24%
834
July1st to 10thNight
58%
379
15%
1429
17%
379
62%
379
25%
855
July11th to 20thNight
62%
522
18%
1566
17%
522
65%
522
34%
939
July21st to endNight
62%
591
17%
1672
15%
558
63%
558
29%
1048
August1st to 10thNight
62%
537
19%
1600
15%
534
65%
534
34%
955
August11th to 20thNight
61%
534
21%
1598
16%
534
64%
534
32%
963
August21st to endNight
55%
690
21%
1878
13%
690
59%
690
34%
1171
September1st to 10thNight
53%
720
24%
1800
14%
720
56%
720
36%
1162
September11th to 20thNight
61%
720
27%
1963
19%
720
65%
720
40%
1147
September21st to endNight
44%
720
21%
1984
15%
720
49%
720
27%
1315
October1st to 10thNight
45%
720
19%
2160
17%
720
50%
720
27%
1371
October11th to 20thNight
31%
720
16%
2160
11%
720
40%
720
22%
1453
October21st to endNight
26%
792
19%
2376
11%
792
36%
792
17%
1702
November1st to 10thNight
35%
740
22%
2301
16%
740
45%
740
25%
1450
November11th to 20thNight
24%
760
16%
2617
11%
760
34%
760
15%
1838
November21st to endNight
20%
760
14%
2660
8%
760
29%
760
13%
1941
December1st to 10thNight
17%
760
8%
2660
5%
760
22%
760
8%
2124
December11th to 20thNight
17%
759
9%
3253
5%
759
23%
759
7%
2529
December21st to endNight
15%
828
6%
4856
5%
817
22%
817
4%
4045

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%10%47%6%35%5%5%7%30%
74646
Night
17%11%47%6%3445%6%32%
75173
TransparencyDay
54%411%20%10%1
74363
Night
53%311%20%11%2
74975
SeeingDay
24%14%28%20%10%4
13547
Night
23%12%26%26%11%1
23318

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%16%10%10%9%35%435%18%
1672
11th to 20thDay
18%20%7%11%8%3334417%
1674
21st to endDay
16%18%8%13%8%44445%15%
1782
February1st to 10thDay
19%17%6%10%10%4433419%
1620
11th to 20thDay
17%13%7%13%8%35%35%6%19%
1573
21st to endDay
21%15%5%10%6%34446%24%
1470
March1st to 10thDay
17%10%6%8%7%3445%7%29%
1980
11th to 20thDay
19%12%5%8%6%34446%28%
1980
21st to endDay
21%11%5%8%6%35%5%46%27%
2178
April1st to 10thDay
17%12%5%6%6%35%5%5%7%31%
2079
11th to 20thDay
20%10%36%5%345%5%6%32%
2169
21st to endDay
23%9%36%5%34446%33%
2340
May1st to 10thDay
19%9%38%6%35%5%6%7%28%
2340
11th to 20thDay
21%8%36%6%45%6%47%29%
2301
21st to endDay
13%6%36%6%36%6%7%9%37%
2704
June1st to 10thDay
16%6%27%7%36%6%6%9%33%
2520
11th to 20thDay
15%7%35%6%36%5%6%9%34%
2520
21st to endDay
15%5%36%5%45%5%6%9%37%
2520
July1st to 10thDay
12%6%35%7%45%5%6%7%40%
2485
11th to 20thDay
13%6%26%6%45%5%7%10%37%
2340
21st to endDay
11%6%36%6%46%6%8%9%35%
2547
August1st to 10thDay
11%6%36%7%46%5%7%9%35%
2327
11th to 20thDay
14%6%38%6%46%6%6%10%33%
2303
21st to endDay
12%5%36%7%46%6%6%9%36%
2458
September1st to 10thDay
17%7%35%43345%8%41%
2160
11th to 20thDay
13%6%25%5%35%6%6%8%41%
1994
21st to endDay
19%7%37%5%4445%6%37%
1976
October1st to 10thDay
17%6%36%5%345%6%7%38%
1800
11th to 20thDay
22%9%36%6%35%5%46%32%
1800
21st to endDay
25%9%46%5%3345%6%29%
1980
November1st to 10thDay
12%8%46%6%35%447%41%
1772
11th to 20thDay
18%14%47%5%34345%31%
1560
21st to endDay
19%17%5%9%6%3345%6%23%
1520
December1st to 10thDay
20%19%10%10%6%35%33416%
1520
11th to 20thDay
20%17%8%12%5%34446%18%
1910
21st to endDay
26%24%8%8%5%2334412%
2772
January1st to 10thNight
18%17%7%10%7%35%5%3419%
2488
11th to 20thNight
20%15%7%10%8%45%43419%
2425
21st to endNight
17%17%7%10%8%45%43420%
2574
February1st to 10thNight
20%14%5%10%8%4444421%
2340
11th to 20thNight
19%13%6%11%7%35%445%22%
2273
21st to endNight
20%13%5%11%6%3444425%
1782
March1st to 10thNight
16%12%5%6%6%45%45%6%31%
1980
11th to 20thNight
19%11%46%434446%34%
1980
21st to endNight
18%11%5%8%6%34345%32%
2178
April1st to 10thNight
18%11%46%5%34347%33%
1879
11th to 20thNight
17%9%35%5%345%5%6%36%
1754
21st to endNight
17%9%46%5%35%46%7%34%
1620
May1st to 10thNight
18%8%46%5%35%5%5%7%33%
1620
11th to 20thNight
17%7%36%6%3346%8%37%
1592
21st to endNight
11%5%25%5%245%8%10%44%
1570
June1st to 10thNight
11%425%5%35%5%6%8%45%
1440
11th to 20thNight
13%5%26%5%45%5%6%8%42%
1440
21st to endNight
11%425%5%45%5%7%9%42%
1440
July1st to 10thNight
8%6%36%6%35%5%5%7%46%
1470
11th to 20thNight
9%425%5%445%6%10%46%
1620
21st to endNight
10%6%35%5%46%5%7%10%40%
1771
August1st to 10thNight
9%5%35%445%5%5%8%47%
1611
11th to 20thNight
10%6%45%5%3346%8%46%
1598
21st to endNight
11%5%35%435%5%6%9%46%
1878
September1st to 10thNight
14%5%245%2446%7%47%
1800
11th to 20thNight
11%4234335%6%8%52%
1963
21st to endNight
16%7%36%434447%41%
1984
October1st to 10thNight
15%6%36%43445%8%44%
2160
11th to 20thNight
18%8%35%6%34446%38%
2160
21st to endNight
25%9%35%4345%46%32%
2376
November1st to 10thNight
13%9%35%5%3445%7%42%
2301
11th to 20thNight
18%12%47%5%35%5%5%7%29%
2617
21st to endNight
18%15%47%5%34447%28%
2660
December1st to 10thNight
22%16%8%9%7%34445%18%
2660
11th to 20thNight
19%17%7%10%7%34445%21%
3285
21st to endNight
26%19%7%9%7%44445%13%
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
69%311%12%5%
1673
11th to 20thDay
72%310%12%3
1674
21st to endDay
72%39%11%41
1782
February1st to 10thDay
71%28%13%6%
1620
11th to 20thDay
68%39%14%7%
1582
21st to endDay
63%29%17%8%1
1470
March1st to 10thDay
55%311%22%8%
1980
11th to 20thDay
58%39%19%10%2
1980
21st to endDay
60%28%18%11%1
2178
April1st to 10thDay
54%311%18%12%2
2079
11th to 20thDay
53%39%18%14%2
2169
21st to endDay
54%38%22%11%2
2340
May1st to 10thDay
54%412%19%10%2
2340
11th to 20thDay
54%411%16%12%2
2305
21st to endDay
42%6%16%24%10%2
2704
June1st to 10thDay
48%412%24%12%1
2520
11th to 20thDay
46%415%22%12%1
2520
21st to endDay
43%5%11%28%12%1
2520
July1st to 10thDay
43%5%12%23%14%3
2416
11th to 20thDay
42%5%12%25%15%1
2271
21st to endDay
44%413%22%16%1
2405
August1st to 10thDay
44%413%29%9%1
2314
11th to 20thDay
46%414%25%10%1
2303
21st to endDay
42%6%11%23%15%3
2458
September1st to 10thDay
42%5%9%23%17%3
2160
11th to 20thDay
40%313%27%15%1
1994
21st to endDay
49%310%24%14%1
1976
October1st to 10thDay
44%412%23%16%1
1800
11th to 20thDay
54%410%22%10%
1800
21st to endDay
57%313%16%10%1
1980
November1st to 10thDay
45%311%22%17%2
1772
11th to 20thDay
57%210%20%10%2
1560
21st to endDay
63%311%17%6%1
1520
December1st to 10thDay
73%29%12%4
1520
11th to 20thDay
69%29%16%3
1906
21st to endDay
78%29%10%2
2772
January1st to 10thNight
70%27%16%5%
2488
11th to 20thNight
70%39%14%4
2425
21st to endNight
68%38%15%5%1
2574
February1st to 10thNight
67%39%15%5%
2340
11th to 20thNight
65%39%16%7%
2275
21st to endNight
63%37%16%9%1
1782
March1st to 10thNight
55%310%21%11%1
1980
11th to 20thNight
52%311%22%10%2
1980
21st to endNight
55%36%19%15%2
2178
April1st to 10thNight
52%39%20%14%2
1879
11th to 20thNight
48%410%19%16%3
1754
21st to endNight
48%49%23%13%3
1620
May1st to 10thNight
50%5%8%21%12%4
1620
11th to 20thNight
45%5%11%20%15%3
1593
21st to endNight
35%5%16%28%14%2
1571
June1st to 10thNight
38%414%27%16%1
1440
11th to 20thNight
40%5%14%24%16%2
1440
21st to endNight
39%5%14%25%17%1
1440
July1st to 10thNight
38%6%10%23%19%4
1429
11th to 20thNight
34%5%14%25%20%2
1566
21st to endNight
39%5%15%24%14%3
1672
August1st to 10thNight
34%5%14%29%17%1
1600
11th to 20thNight
37%416%27%15%2
1598
21st to endNight
35%5%12%28%16%4
1878
September1st to 10thNight
35%5%11%26%19%4
1800
11th to 20thNight
30%414%28%22%1
1963
21st to endNight
44%312%22%18%1
1984
October1st to 10thNight
41%412%25%16%2
2160
11th to 20thNight
48%311%24%13%1
2160
21st to endNight
54%410%20%11%2
2376
November1st to 10thNight
42%311%24%16%3
2301
11th to 20thNight
55%39%21%10%3
2617
21st to endNight
59%311%21%6%1
2660
December1st to 10thNight
69%211%13%5%
2660
11th to 20thNight
68%210%16%4
3288
21st to endNight
76%29%11%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
33%28%24%9%5%1
254
11th to 20thDay
34%29%26%7%21
258
21st to endDay
34%28%29%7%2
264
February1st to 10thDay
38%25%33%4
240
11th to 20thDay
43%23%28%6%
240
21st to endDay
43%23%25%8%
224
March1st to 10thDay
32%22%30%12%3
420
11th to 20thDay
35%17%33%9%5%1
420
21st to endDay
33%20%27%14%5%1
462
April1st to 10thDay
31%19%26%16%7%1
420
11th to 20thDay
27%18%26%18%7%3
412
21st to endDay
31%16%31%15%42
420
May1st to 10thDay
30%15%29%18%43
423
11th to 20thDay
25%12%32%18%9%3
447
21st to endDay
16%7%29%26%16%6%
485
June1st to 10thDay
16%11%30%27%13%3
450
11th to 20thDay
20%11%26%26%13%4
450
21st to endDay
16%8%35%27%11%3
450
July1st to 10thDay
14%7%18%30%20%11%
445
11th to 20thDay
15%5%25%28%19%8%
444
21st to endDay
13%7%25%31%17%8%
488
August1st to 10thDay
13%6%21%29%21%9%
446
11th to 20thDay
15%5%23%31%15%11%
439
21st to endDay
13%6%25%28%18%10%
495
September1st to 10thDay
18%7%22%27%17%8%
450
11th to 20thDay
14%222%31%20%10%
450
21st to endDay
21%12%26%22%15%4
450
October1st to 10thDay
18%9%28%24%17%4
450
11th to 20thDay
26%15%30%21%7%1
450
21st to endDay
35%12%30%17%5%
495
November1st to 10thDay
21%19%31%16%11%3
370
11th to 20thDay
30%20%31%13%33
198
21st to endDay
26%23%38%11%21
180
December1st to 10thDay
37%24%26%8%32
180
11th to 20thDay
35%19%33%12%11
180
21st to endDay
38%20%30%11%1
198
January1st to 10thNight
31%19%34%13%2
760
11th to 20thNight
32%19%31%15%3
747
21st to endNight
29%19%31%18%3
792
February1st to 10thNight
32%18%28%20%3
720
11th to 20thNight
31%20%35%14%1
703
21st to endNight
33%19%29%17%2
592
March1st to 10thNight
27%20%30%21%3
720
11th to 20thNight
29%18%26%21%5%
720
21st to endNight
25%16%33%21%5%1
792
April1st to 10thNight
26%16%30%23%5%
720
11th to 20thNight
24%12%34%22%8%1
690
21st to endNight
24%8%37%27%41
540
May1st to 10thNight
22%11%24%32%9%1
540
11th to 20thNight
21%8%26%34%11%1
531
21st to endNight
11%5%26%33%22%3
410
June1st to 10thNight
10%428%39%18%1
360
11th to 20thNight
13%6%23%39%18%1
360
21st to endNight
11%5%24%39%19%1
360
July1st to 10thNight
8%318%40%25%5%
379
11th to 20thNight
10%317%36%29%5%
522
21st to endNight
13%313%42%27%2
591
August1st to 10thNight
12%413%42%27%3
537
11th to 20thNight
13%410%43%25%5%
534
21st to endNight
12%321%31%27%5%
690
September1st to 10thNight
16%420%33%24%4
720
11th to 20thNight
12%318%35%29%4
720
21st to endNight
20%7%24%26%18%4
720
October1st to 10thNight
20%6%23%31%19%2
720
11th to 20thNight
23%11%31%26%9%1
720
21st to endNight
30%13%27%24%6%
792
November1st to 10thNight
20%13%26%29%11%1
740
11th to 20thNight
29%16%26%22%6%1
760
21st to endNight
28%15%31%19%6%
760
December1st to 10thNight
34%18%26%18%4
760
11th to 20thNight
28%21%28%20%4
760
21st to endNight
33%17%28%20%2
836
Page generated at 2024-01-13 23:56:39. Version ac07

Copyright 2024 Attilla Danko


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