Morton Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Morton 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
63%
24294
23%
74840
21%
24256
65%
24256
34%
43800

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
55%
756
13%
2457
14%
756
56%
756
19%
1708
January11th to 20thNight
54%
728
14%
2366
14%
728
56%
728
18%
1652
January21st to endNight
57%
792
15%
2574
16%
792
58%
792
20%
1776
February1st to 10thNight
55%
720
14%
2340
15%
720
57%
720
23%
1536
February11th to 20thNight
55%
700
17%
2206
18%
700
57%
700
26%
1399
February21st to endNight
51%
592
21%
1776
19%
592
55%
592
26%
1167
March1st to 10thNight
51%
720
18%
2029
16%
720
54%
720
27%
1353
March11th to 20thNight
60%
720
24%
1980
21%
720
62%
720
39%
1168
March21st to endNight
53%
792
19%
2178
15%
792
56%
792
31%
1388
April1st to 10thNight
52%
702
20%
1962
16%
702
55%
702
33%
1205
April11th to 20thNight
57%
533
26%
1648
19%
533
63%
533
36%
935
April21st to endNight
59%
540
26%
1620
20%
540
62%
540
37%
922
May1st to 10thNight
57%
540
24%
1620
19%
540
60%
540
35%
933
May11th to 20thNight
68%
531
24%
1595
20%
531
69%
531
41%
852
May21st to endNight
62%
582
19%
1746
18%
582
64%
582
36%
995
June1st to 10thNight
67%
540
24%
1604
22%
540
70%
540
43%
823
June11th to 20thNight
69%
540
29%
1482
26%
540
72%
540
57%
733
June21st to endNight
76%
540
28%
1620
29%
540
78%
540
59%
729
July1st to 10thNight
75%
525
21%
1575
21%
525
76%
525
46%
801
July11th to 20thNight
78%
522
23%
1566
22%
522
80%
522
44%
824
July21st to endNight
76%
591
26%
1674
23%
558
76%
558
42%
898
August1st to 10thNight
76%
537
28%
1602
23%
534
77%
534
45%
815
August11th to 20thNight
78%
534
34%
1602
28%
534
80%
534
50%
798
August21st to endNight
80%
594
37%
1807
35%
594
80%
594
51%
880
September1st to 10thNight
78%
661
33%
1921
28%
661
81%
661
55%
927
September11th to 20thNight
75%
720
35%
1980
30%
720
78%
720
55%
1012
September21st to endNight
74%
720
37%
1980
30%
720
77%
720
55%
1006
October1st to 10thNight
65%
720
30%
2070
22%
720
70%
720
46%
1098
October11th to 20thNight
69%
720
34%
2160
28%
720
74%
720
46%
1107
October21st to endNight
68%
792
34%
2376
29%
792
72%
792
46%
1215
November1st to 10thNight
63%
740
28%
2395
25%
740
68%
740
38%
1291
November11th to 20thNight
63%
760
26%
2470
25%
760
66%
760
35%
1360
November21st to endNight
61%
855
22%
2565
20%
855
63%
855
31%
1579
December1st to 10thNight
57%
950
16%
2660
13%
950
59%
950
30%
1725
December11th to 20thNight
50%
949
14%
3171
11%
948
52%
948
18%
2234
December21st to endNight
43%
836
8%
4463
11%
835
46%
835
10%
2956

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
8%7%35%5%345%5%7%49%
74465
Night
9%8%35%43445%7%49%
75234
TransparencyDay
32%410%22%20%12%
74190
Night
34%411%23%18%10%
75030
SeeingDay
10%415%32%28%10%
13714
Night
12%313%41%28%4
24323

By Month

Percentage Cloud Cover SW (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%12%46%5%3446%7%35%
1708
11th to 20thDay
10%13%5%9%6%34446%37%
1633
21st to endDay
9%16%6%8%6%345%46%32%
1782
February1st to 10thDay
11%13%5%6%6%36%5%5%7%34%
1620
11th to 20thDay
10%14%47%5%345%7%6%35%
1638
21st to endDay
10%15%410%6%3346%7%32%
1480
March1st to 10thDay
9%11%48%5%35%5%7%8%35%
1930
11th to 20thDay
8%11%46%5%35%6%6%8%39%
1980
21st to endDay
12%13%47%6%35%5%6%7%32%
2178
April1st to 10thDay
10%11%37%6%35%45%7%40%
1998
11th to 20thDay
9%10%45%5%445%6%6%41%
2258
21st to endDay
10%8%35%5%3445%7%46%
2340
May1st to 10thDay
8%6%36%5%36%6%7%8%42%
2340
11th to 20thDay
7%5%25%5%35%5%7%9%47%
2301
21st to endDay
8%6%26%436%7%8%8%41%
2530
June1st to 10thDay
4425%3345%6%8%56%
2340
11th to 20thDay
6%41332335%6%63%
2446
21st to endDay
3223323446%69%
2340
July1st to 10thDay
221332445%8%64%
2340
11th to 20thDay
33245%25%5%7%7%58%
2340
21st to endDay
4324435%6%7%9%54%
2545
August1st to 10thDay
3324435%6%7%7%56%
2328
11th to 20thDay
332333445%7%63%
2299
21st to endDay
44133245%5%6%62%
2548
September1st to 10thDay
321332335%6%69%
2034
11th to 20thDay
5%31332345%7%64%
1980
21st to endDay
6%312323445%68%
1980
October1st to 10thDay
11%5%24335%45%5%54%
1890
11th to 20thDay
6%7%24433346%60%
1800
21st to endDay
6%7%244235%5%6%57%
1980
November1st to 10thDay
12%7%36%5%24446%48%
1683
11th to 20thDay
14%7%35%5%34447%46%
1710
21st to endDay
9%11%47%5%45%446%42%
1592
December1st to 10thDay
12%11%47%5%3345%8%39%
1520
11th to 20thDay
14%12%48%6%35%5%5%7%31%
1926
21st to endDay
10%12%5%8%6%35%5%5%7%35%
3128
January1st to 10thNight
13%11%46%5%3446%8%36%
2457
11th to 20thNight
11%11%47%5%245%48%38%
2366
21st to endNight
10%14%46%445%5%5%8%35%
2574
February1st to 10thNight
10%12%45%4345%5%7%40%
2340
11th to 20thNight
9%11%46%6%34445%43%
2205
21st to endNight
8%12%5%9%5%345%6%7%37%
1776
March1st to 10thNight
11%13%46%4345%5%7%36%
2029
11th to 20thNight
9%8%25%5%235%6%9%45%
1980
21st to endNight
11%11%45%4346%5%8%39%
2178
April1st to 10thNight
14%11%34323347%46%
1962
11th to 20thNight
8%8%25%435%5%5%7%48%
1648
21st to endNight
10%6%35%5%3435%7%49%
1620
May1st to 10thNight
10%5%26%5%36%5%6%7%45%
1620
11th to 20thNight
8%5%2442445%8%53%
1594
21st to endNight
9%6%35%5%245%6%8%46%
1746
June1st to 10thNight
6%33432446%7%57%
1604
11th to 20thNight
5%412323345%67%
1482
21st to endNight
421321345%6%69%
1620
July1st to 10thNight
43244345%5%9%56%
1620
11th to 20thNight
34145%46%6%6%8%52%
1620
21st to endNight
44246%346%6%8%53%
1773
August1st to 10thNight
4325%3345%5%8%57%
1611
11th to 20thNight
332443446%8%59%
1602
21st to endNight
4313425%5%6%8%60%
1807
September1st to 10thNight
4213324446%67%
1921
11th to 20thNight
6%31232345%7%63%
1980
21st to endNight
6%413322335%69%
1980
October1st to 10thNight
9%5%23324446%59%
2070
11th to 20thNight
8%5%23324345%61%
2160
21st to endNight
7%5%23324445%61%
2376
November1st to 10thNight
10%7%2432345%8%52%
2395
11th to 20thNight
11%6%2442345%7%52%
2470
21st to endNight
9%9%45%43345%7%48%
2565
December1st to 10thNight
11%9%46%42345%7%45%
2660
11th to 20thNight
15%11%45%5%35%5%6%7%35%
3271
21st to endNight
9%11%5%9%6%35%5%7%7%33%
4552

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
47%10%17%21%41
1709
11th to 20thDay
48%9%16%18%8%1
1633
21st to endDay
51%9%15%19%5%1
1782
February1st to 10thDay
46%8%15%19%9%1
1620
11th to 20thDay
46%413%25%11%1
1647
21st to endDay
48%314%24%11%1
1480
March1st to 10thDay
40%417%26%11%2
1930
11th to 20thDay
38%215%28%14%3
1980
21st to endDay
46%315%23%9%3
2178
April1st to 10thDay
41%312%21%17%6%
1998
11th to 20thDay
37%211%27%19%4
2261
21st to endDay
33%210%29%22%3
2340
May1st to 10thDay
33%310%27%21%6%
2340
11th to 20thDay
29%39%28%25%7%
2304
21st to endDay
33%411%26%21%5%
2530
June1st to 10thDay
23%27%21%32%14%
2340
11th to 20thDay
21%25%18%29%25%
2446
21st to endDay
16%2111%29%41%
2340
July1st to 10thDay
17%3413%25%37%
2276
11th to 20thDay
22%35%15%24%31%
2271
21st to endDay
23%38%17%22%28%
2404
August1st to 10thDay
23%36%19%22%27%
2313
11th to 20thDay
20%2413%29%31%
2299
21st to endDay
20%3419%31%24%
2548
September1st to 10thDay
15%25%18%29%32%
2034
11th to 20thDay
18%37%22%26%25%
1980
21st to endDay
19%2422%32%21%
1980
October1st to 10thDay
29%38%23%27%10%
1890
11th to 20thDay
25%37%31%27%7%
1800
21st to endDay
26%5%14%32%21%3
1980
November1st to 10thDay
37%411%25%20%4
1683
11th to 20thDay
37%5%11%31%15%1
1710
21st to endDay
41%8%13%24%14%2
1592
December1st to 10thDay
45%8%16%21%8%2
1520
11th to 20thDay
53%8%12%22%6%
1924
21st to endDay
48%11%12%18%11%
3128
January1st to 10thNight
48%9%17%22%41
2457
11th to 20thNight
48%8%17%18%7%1
2366
21st to endNight
47%7%19%20%6%1
2574
February1st to 10thNight
44%6%18%23%8%1
2340
11th to 20thNight
41%5%12%26%14%1
2206
21st to endNight
44%5%12%26%12%2
1776
March1st to 10thNight
46%313%27%11%1
2029
11th to 20thNight
33%212%31%18%3
1980
21st to endNight
39%413%28%12%4
2178
April1st to 10thNight
38%311%23%18%7%
1962
11th to 20thNight
33%211%26%23%4
1648
21st to endNight
32%310%28%23%3
1620
May1st to 10thNight
33%310%25%23%5%
1620
11th to 20thNight
28%38%28%25%8%
1595
21st to endNight
32%413%28%19%4
1746
June1st to 10thNight
24%36%22%30%14%
1604
11th to 20thNight
19%16%18%30%25%
1482
21st to endNight
16%2211%29%40%
1620
July1st to 10thNight
24%45%13%19%36%
1575
11th to 20thNight
26%47%14%21%30%
1566
21st to endNight
27%49%16%19%26%
1674
August1st to 10thNight
24%48%20%19%25%
1602
11th to 20thNight
23%26%15%24%31%
1602
21st to endNight
21%36%16%28%27%
1807
September1st to 10thNight
17%3415%26%34%
1921
11th to 20thNight
20%26%19%26%26%
1980
21st to endNight
20%2318%33%24%
1980
October1st to 10thNight
26%28%23%30%10%
2070
11th to 20thNight
25%26%28%30%8%
2160
21st to endNight
25%310%32%25%5%
2376
November1st to 10thNight
30%311%30%21%5%
2395
11th to 20thNight
31%5%14%32%17%3
2470
21st to endNight
37%6%11%31%13%2
2565
December1st to 10thNight
39%6%17%26%10%2
2660
11th to 20thNight
49%7%14%23%8%
3272
21st to endNight
50%8%14%19%8%
4552

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
25%214%33%18%9%
270
11th to 20thDay
14%217%39%21%6%
245
21st to endDay
14%413%43%18%8%
264
February1st to 10thDay
19%5%17%41%14%4
240
11th to 20thDay
15%5%20%37%18%5%
303
21st to endDay
13%5%24%38%17%4
346
March1st to 10thDay
16%419%37%17%6%
420
11th to 20thDay
14%419%34%25%5%
420
21st to endDay
19%6%24%30%18%2
462
April1st to 10thDay
13%7%21%32%19%7%
420
11th to 20thDay
15%9%27%28%17%4
412
21st to endDay
15%8%25%25%21%7%
420
May1st to 10thDay
9%8%22%34%21%7%
423
11th to 20thDay
9%322%28%28%10%
448
21st to endDay
9%5%16%28%28%13%
485
June1st to 10thDay
46%15%30%36%10%
450
11th to 20thDay
7%5%16%33%31%8%
556
21st to endDay
4317%34%34%9%
450
July1st to 10thDay
236%34%42%14%
446
11th to 20thDay
5%27%35%41%9%
444
21st to endDay
316%27%47%16%
487
August1st to 10thDay
5%19%33%40%13%
447
11th to 20thDay
3311%24%43%16%
437
21st to endDay
5%29%28%43%14%
495
September1st to 10thDay
4210%31%37%17%
450
11th to 20thDay
6%414%28%34%15%
450
21st to endDay
7%413%27%35%14%
450
October1st to 10thDay
9%5%17%30%26%13%
360
11th to 20thDay
7%414%30%27%18%
270
21st to endDay
9%319%37%25%7%
297
November1st to 10thDay
19%411%31%23%12%
270
11th to 20thDay
13%314%38%20%12%
270
21st to endDay
14%310%34%28%10%
270
December1st to 10thDay
15%313%35%23%11%
270
11th to 20thDay
16%6%9%41%24%4
270
21st to endDay
23%5%14%35%19%4
297
January1st to 10thNight
18%311%40%23%4
760
11th to 20thNight
16%312%37%27%4
728
21st to endNight
16%411%43%22%4
792
February1st to 10thNight
16%215%48%16%3
720
11th to 20thNight
14%415%46%19%2
704
21st to endNight
13%417%47%17%2
592
March1st to 10thNight
17%416%44%16%3
720
11th to 20thNight
12%315%43%24%3
720
21st to endNight
15%5%16%45%17%2
792
April1st to 10thNight
20%5%16%40%18%2
702
11th to 20thNight
10%421%49%16%1
533
21st to endNight
12%418%39%24%2
540
May1st to 10thNight
10%5%16%45%23%2
540
11th to 20thNight
9%311%40%34%3
531
21st to endNight
10%5%13%37%31%4
584
June1st to 10thNight
9%5%12%39%33%3
540
11th to 20thNight
7%416%43%29%2
540
21st to endNight
5%112%39%39%3
540
July1st to 10thNight
5%28%33%46%7%
525
11th to 20thNight
5%17%41%41%6%
522
21st to endNight
5%2329%57%5%
591
August1st to 10thNight
417%42%42%2
537
11th to 20thNight
416%40%44%6%
534
21st to endNight
426%38%47%3
594
September1st to 10thNight
5%28%37%41%8%
661
11th to 20thNight
7%210%35%40%7%
720
21st to endNight
9%211%31%40%7%
720
October1st to 10thNight
11%315%37%27%7%
720
11th to 20thNight
9%312%39%32%6%
720
21st to endNight
11%214%40%30%4
792
November1st to 10thNight
13%215%37%28%4
740
11th to 20thNight
13%215%44%22%4
760
21st to endNight
13%412%41%23%7%
855
December1st to 10thNight
16%314%43%21%3
950
11th to 20thNight
21%5%11%42%19%2
950
21st to endNight
21%414%44%16%2
854
Page generated at 2024-01-09 14:24:27. Version ac07

Copyright 2024 Attilla Danko


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