Morrison Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Morrison Observatory clear sky chart from 2005-11-05 to 2017-09-17.

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
34%
15927
16%
46816
10%
15885
40%
15885
20%
32599

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 15thNight
15%
900
9%
2506
3%
895
20%
895
11%
2013
January16th to endNight
16%
960
8%
2688
3%
960
21%
960
11%
2183
February1st to 15thNight
18%
712
10%
2156
3%
712
24%
712
12%
1652
February16th to endNight
20%
631
13%
1893
5%
631
27%
631
12%
1464
March1st to 15thNight
24%
720
13%
2151
8%
720
29%
720
18%
1534
March16th to endNight
29%
705
17%
2049
8%
705
37%
705
21%
1427
April1st to 15thNight
29%
539
14%
1797
9%
539
37%
539
17%
1243
April16th to endNight
31%
540
20%
1675
11%
540
40%
540
22%
1106
May1st to 15thNight
34%
540
14%
1620
7%
540
40%
540
21%
1113
May16th to endNight
46%
562
16%
1567
11%
561
50%
561
29%
1002
June1st to 15thNight
50%
540
17%
1440
11%
540
55%
540
37%
887
June16th to endNight
53%
540
15%
1440
10%
540
57%
540
35%
912
July1st to 15thNight
63%
510
18%
1360
14%
510
65%
510
41%
804
July16th to endNight
65%
564
21%
1417
17%
531
67%
531
40%
839
August1st to 15thNight
68%
539
22%
1601
18%
536
71%
536
38%
899
August16th to endNight
64%
573
21%
1878
20%
573
68%
573
33%
1059
September1st to 15thNight
59%
597
27%
1857
23%
597
65%
597
34%
1066
September16th to endNight
57%
665
27%
1837
20%
665
61%
665
36%
1113
October1st to 15thNight
43%
660
27%
1826
18%
660
51%
660
31%
1148
October16th to endNight
23%
704
15%
2288
9%
704
32%
704
16%
1581
November1st to 15thNight
31%
700
20%
2276
15%
700
41%
700
20%
1495
November16th to endNight
24%
720
12%
2340
9%
720
29%
720
12%
1785
December1st to 15thNight
14%
846
8%
2466
2%
846
19%
846
10%
2011
December16th to endNight
14%
960
8%
2688
2%
960
19%
960
8%
2263

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%8%46%5%3445%7%34%
47965
Night
15%9%46%5%34446%34%
46985
Cloud CoverDay
14%8%46%5%3445%7%34%
47974
Night
15%9%46%5%34446%34%
46990
TransparencyDay
48%412%20%11%1
47652
Night
49%411%20%12%1
46816
SeeingDay
16%10%27%24%14%6%
5933
Night
20%9%28%28%12%1
15931

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 15thDay
18%17%6%10%7%5%434416%
1432
16th to endDay
13%17%11%10%8%44345%13%
1536
February1st to 15thDay
13%16%7%11%6%45%445%19%
1702
16th to endDay
15%11%6%11%7%45%46%6%19%
1580
March1st to 15thDay
19%11%5%8%6%24447%25%
1799
16th to endDay
19%9%36%5%34446%31%
2184
April1st to 15thDay
20%10%35%424436%32%
2151
16th to endDay
19%7%24424347%37%
2283
May1st to 15thDay
19%9%35%424447%33%
2331
16th to endDay
12%6%25%5%35%45%7%40%
2546
June1st to 15thDay
10%425%6%35%5%5%7%43%
2520
16th to endDay
9%425%5%447%6%9%40%
2520
July1st to 15thDay
7%425%5%45%5%6%8%45%
2520
16th to endDay
7%325%5%35%5%7%10%43%
2655
August1st to 15thDay
6%425%6%45%5%5%10%43%
2348
16th to endDay
11%424435%46%9%42%
2323
September1st to 15thDay
13%425%5%35%45%8%43%
2091
16th to endDay
11%4345%345%5%8%41%
1829
October1st to 15thDay
15%6%35%42445%7%39%
1802
16th to endDay
22%7%25%5%3346%7%29%
1584
November1st to 15thDay
14%11%345%3335%6%37%
1582
16th to endDay
19%15%47%6%35%335%23%
1620
December1st to 15thDay
18%16%8%10%7%3433418%
1491
16th to endDay
21%17%7%10%7%3334415%
1536
January1st to 15thNight
21%14%6%8%6%34345%20%
2506
16th to endNight
14%15%8%10%8%44445%19%
2688
February1st to 15thNight
16%14%6%8%7%24445%25%
2154
16th to endNight
17%13%6%9%6%35%435%24%
1893
March1st to 15thNight
17%10%5%7%434345%33%
2138
16th to endNight
17%7%35%435%5%47%34%
2049
April1st to 15thNight
18%8%35%6%245%5%6%33%
1797
16th to endNight
18%6%25%434345%39%
1675
May1st to 15thNight
18%7%36%6%24447%34%
1620
16th to endNight
12%5%245%24447%44%
1565
June1st to 15thNight
9%425%42436%7%49%
1440
16th to endNight
8%425%4346%8%11%41%
1440
July1st to 15thNight
5%3245%345%6%8%51%
1440
16th to endNight
7%413325%5%6%11%49%
1513
August1st to 15thNight
5%32443446%9%50%
1611
16th to endNight
10%3135%2445%8%49%
1878
September1st to 15thNight
13%423323346%51%
1857
16th to endNight
13%5%24323448%49%
1837
October1st to 15thNight
15%5%23423346%46%
1826
16th to endNight
24%8%36%433335%35%
2288
November1st to 15thNight
15%10%46%423446%38%
2276
16th to endNight
19%15%5%7%5%34345%24%
2340
December1st to 15thNight
20%15%9%8%6%2434418%
2466
16th to endNight
22%16%6%9%7%44445%14%
2688

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 15thDay
18%17%5%10%8%45%34417%
1432
16th to endDay
13%17%11%10%9%5%4435%13%
1536
February1st to 15thDay
13%16%8%10%6%35%445%19%
1702
16th to endDay
15%10%7%11%8%445%6%5%19%
1580
March1st to 15thDay
18%12%48%6%23447%25%
1809
16th to endDay
19%9%45%5%44445%32%
2183
April1st to 15thDay
20%10%36%424446%32%
2151
16th to endDay
18%8%34424347%37%
2283
May1st to 15thDay
19%9%35%5%25%447%32%
2331
16th to endDay
11%6%36%5%3445%8%40%
2546
June1st to 15thDay
10%435%6%35%448%43%
2520
16th to endDay
9%425%5%45%6%6%10%39%
2520
July1st to 15thDay
7%425%445%5%6%8%44%
2520
16th to endDay
6%3146%35%5%7%10%43%
2655
August1st to 15thDay
5%425%6%35%5%6%10%44%
2348
16th to endDay
10%5%25%5%35%5%5%9%42%
2323
September1st to 15thDay
13%4245%345%5%7%43%
2091
16th to endDay
11%4245%35%45%8%41%
1829
October1st to 15thDay
14%7%25%42345%7%39%
1802
16th to endDay
22%6%35%5%2445%8%29%
1584
November1st to 15thDay
14%10%346%2345%6%37%
1582
16th to endDay
19%16%46%6%35%435%23%
1620
December1st to 15thDay
18%16%8%10%6%3434418%
1491
16th to endDay
21%17%7%10%7%3434415%
1536
January1st to 15thNight
21%14%5%9%6%44345%21%
2506
16th to endNight
14%14%8%10%8%4444419%
2688
February1st to 15thNight
16%13%6%8%7%24445%24%
2147
16th to endNight
17%12%6%8%6%46%435%24%
1893
March1st to 15thNight
17%10%47%5%23346%32%
2151
16th to endNight
16%7%45%5%345%5%6%34%
2049
April1st to 15thNight
19%8%36%5%25%45%6%32%
1797
16th to endNight
18%6%25%5%24346%39%
1675
May1st to 15thNight
18%7%36%5%34447%34%
1619
16th to endNight
12%6%345%2445%7%44%
1565
June1st to 15thNight
9%43442445%8%48%
1440
16th to endNight
8%425%5%246%7%11%40%
1440
July1st to 15thNight
43245%345%7%8%51%
1440
16th to endNight
6%423325%46%11%50%
1513
August1st to 15thNight
5%3244345%6%9%50%
1611
16th to endNight
9%3135%3445%8%49%
1878
September1st to 15thNight
12%423323446%51%
1857
16th to endNight
13%5%24323348%49%
1837
October1st to 15thNight
15%6%22423346%47%
1826
16th to endNight
22%9%36%42333435%
2288
November1st to 15thNight
14%11%35%5%23446%38%
2276
16th to endNight
19%15%5%8%5%34346%24%
2340
December1st to 15thNight
20%16%8%8%6%25%345%18%
2466
16th to endNight
22%15%6%9%7%445%3415%
2688

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 15thDay
69%312%9%4
1433
16th to endDay
71%49%9%4
1536
February1st to 15thDay
66%49%13%6%
1704
16th to endDay
63%411%15%4
1580
March1st to 15thDay
59%313%15%6%
1809
16th to endDay
54%313%22%5%1
2184
April1st to 15thDay
53%39%19%12%
2151
16th to endDay
46%313%19%12%4
2283
May1st to 15thDay
51%412%22%8%
2331
16th to endDay
41%5%12%25%13%2
2550
June1st to 15thDay
38%5%10%25%18%1
2520
16th to endDay
37%6%15%21%17%1
2520
July1st to 15thDay
36%5%11%23%19%3
2371
16th to endDay
33%6%14%26%15%3
2488
August1st to 15thDay
35%5%13%25%17%2
2334
16th to endDay
38%412%25%15%3
2323
September1st to 15thDay
38%412%23%18%3
2091
16th to endDay
38%49%28%17%1
1829
October1st to 15thDay
43%211%24%16%1
1802
16th to endDay
52%414%19%8%
1584
November1st to 15thDay
46%49%21%15%1
1582
16th to endDay
62%411%17%3
1620
December1st to 15thDay
70%38%13%3
1491
16th to endDay
72%311%8%3
1536
January1st to 15thNight
65%410%13%6%
2506
16th to endNight
66%412%13%3
2688
February1st to 15thNight
60%410%16%6%1
2156
16th to endNight
62%39%18%5%
1893
March1st to 15thNight
53%313%17%10%1
2151
16th to endNight
48%413%22%8%1
2049
April1st to 15thNight
51%39%19%15%
1797
16th to endNight
46%39%20%15%3
1675
May1st to 15thNight
49%412%22%10%1
1620
16th to endNight
38%5%14%24%14%2
1567
June1st to 15thNight
33%411%30%17%1
1440
16th to endNight
34%5%18%23%16%1
1440
July1st to 15thNight
29%414%27%19%3
1360
16th to endNight
27%7%14%23%21%4
1417
August1st to 15thNight
29%5%12%27%22%2
1601
16th to endNight
32%5%12%25%20%4
1878
September1st to 15thNight
32%5%9%27%21%4
1857
16th to endNight
34%38%28%22%2
1837
October1st to 15thNight
38%38%27%20%1
1826
16th to endNight
51%311%23%9%1
2288
November1st to 15thNight
46%39%23%14%2
2276
16th to endNight
61%311%17%5%
2340
December1st to 15thNight
68%39%14%3
2466
16th to endNight
70%49%10%3
2688

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 15thDay
22%25%30%16%5%
90
16th to endDay
25%15%42%14%11
96
February1st to 15thDay
28%33%27%6%3
90
16th to endDay
25%13%38%17%4
81
March1st to 15thDay
33%24%22%19%1
99
16th to endDay
29%11%26%21%10%
288
April1st to 15thDay
28%12%31%20%5%1
270
16th to endDay
22%12%30%18%12%4
270
May1st to 15thDay
28%12%25%19%9%4
288
16th to endDay
15%14%30%22%11%5%
446
June1st to 15thDay
13%6%31%32%11%5%
495
16th to endDay
11%9%33%25%15%4
495
July1st to 15thDay
9%5%30%29%15%10%
475
16th to endDay
8%325%32%19%11%
515
August1st to 15thDay
9%215%30%25%16%
321
16th to endDay
14%218%27%25%11%
332
September1st to 15thDay
13%5%18%30%21%10%
315
16th to endDay
13%6%15%26%29%7%
259
October1st to 15thDay
15%12%21%28%17%4
246
16th to endDay
27%17%37%16%1
96
November1st to 15thDay
6%20%27%31%13%1
90
16th to endDay
30%22%35%10%2
90
December1st to 15thDay
24%34%27%12%1
90
16th to endDay
25%32%36%42
96
January1st to 15thNight
28%16%31%19%3
900
16th to endNight
21%15%35%24%3
960
February1st to 15thNight
25%13%34%21%4
715
16th to endNight
23%14%32%23%6%
631
March1st to 15thNight
23%11%33%22%8%
720
16th to endNight
19%11%33%28%6%
705
April1st to 15thNight
22%13%29%26%6%
539
16th to endNight
20%8%34%25%8%1
540
May1st to 15thNight
23%6%28%30%9%
540
16th to endNight
15%6%25%37%15%
563
June1st to 15thNight
10%329%41%14%
540
16th to endNight
9%325%38%22%
540
July1st to 15thNight
7%120%45%23%2
510
16th to endNight
7%117%38%31%3
564
August1st to 15thNight
7%113%45%27%4
539
16th to endNight
12%114%35%31%5%
573
September1st to 15thNight
14%317%35%24%4
597
16th to endNight
15%416%35%25%2
665
October1st to 15thNight
19%6%26%29%16%1
660
16th to endNight
26%14%30%20%7%
704
November1st to 15thNight
20%13%29%25%10%1
700
16th to endNight
28%13%27%23%6%
720
December1st to 15thNight
29%14%35%17%3
846
16th to endNight
31%15%30%19%3
960
Page generated at 2017-09-18 08:38:18. Version ac04

Copyright 2017 A. Danko


Morrison Observatory Clear Sky Chart | Charts Home