Grand View Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grand View Observatory clear sky chart from 2005-11-05 to 2017-09-24.

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
66%
15691
34%
47002
27%
15651
74%
15651
50%
22791

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
57%
719
32%
2327
27%
716
68%
716
44%
1141
January16th to endNight
55%
768
35%
2496
28%
768
68%
768
41%
1266
February1st to 15thNight
60%
700
34%
2187
27%
700
72%
700
49%
1046
February16th to endNight
51%
632
35%
1895
29%
632
62%
632
43%
998
March1st to 15thNight
57%
720
36%
2005
26%
720
68%
720
50%
1026
March16th to endNight
60%
768
34%
2112
23%
768
69%
768
52%
1075
April1st to 15thNight
49%
715
29%
1974
14%
715
60%
715
48%
1027
April16th to endNight
56%
540
43%
1626
25%
540
71%
540
61%
690
May1st to 15thNight
65%
540
35%
1619
24%
539
72%
539
58%
723
May16th to endNight
61%
561
35%
1684
24%
561
68%
561
59%
744
June1st to 15thNight
68%
540
31%
1620
27%
540
72%
540
64%
687
June16th to endNight
84%
540
34%
1620
31%
540
85%
540
73%
653
July1st to 15thNight
83%
510
24%
1530
22%
510
83%
510
47%
770
July16th to endNight
79%
564
20%
1593
18%
531
80%
531
34%
937
August1st to 15thNight
82%
539
31%
1611
26%
536
83%
536
48%
805
August16th to endNight
83%
651
35%
1836
30%
651
84%
651
50%
960
September1st to 15thNight
80%
720
28%
1980
23%
720
83%
720
48%
1077
September16th to endNight
75%
692
45%
1914
31%
692
84%
692
70%
836
October1st to 15thNight
67%
660
37%
1880
25%
660
77%
660
57%
892
October16th to endNight
71%
704
41%
2112
33%
704
79%
704
59%
942
November1st to 15thNight
67%
700
38%
2205
30%
700
78%
700
55%
984
November16th to endNight
66%
720
39%
2340
34%
720
76%
720
54%
1025
December1st to 15thNight
64%
720
35%
2340
30%
720
74%
720
44%
1150
December16th to endNight
55%
768
31%
2496
26%
768
63%
768
36%
1337

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
4313323346%62%
47925
Night
3312213346%66%
47198
TransparencyDay
23%36%16%27%21%
47620
Night
19%36%18%29%22%
47004
SeeingDay
8%11%24%31%17%6%
5490
Night
5%5%18%31%30%8%
15698

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 15thDay
45%1332223465%
1611
16th to endDay
7%5%12212345%61%
1728
February1st to 15thDay
28%1322335%6%60%
1673
16th to endDay
7%7%13423245%56%
1579
March1st to 15thDay
441221335%6%62%
1953
16th to endDay
4322213246%64%
2112
April1st to 15thDay
431332245%6%62%
1973
16th to endDay
2312312335%70%
2331
May1st to 15thDay
3212323345%67%
2331
16th to endDay
321222333468%
2429
June1st to 15thDay
11221113478%
2340
16th to endDay
11221232477%
2340
July1st to 15thDay
3325%6%45%5%6%6%49%
2340
16th to endDay
6%5%37%7%46%7%7%8%33%
2467
August1st to 15thDay
6%4246%47%5%6%7%45%
2340
16th to endDay
4325%6%35%6%5%7%49%
2364
September1st to 15thDay
43145%25%5%47%54%
1980
16th to endDay
2112212235%73%
1904
October1st to 15thDay
321222224470%
1749
16th to endDay
312212335%72%
1760
November1st to 15thDay
2212212336%71%
1653
16th to endDay
6%41221222472%
1620
December1st to 15thDay
5%7%23322346%59%
1620
16th to endDay
9%7%22322347%54%
1728
January1st to 15thNight
5%42321333465%
2327
16th to endNight
7%5%13212236%61%
2496
February1st to 15thNight
25%13212345%68%
2176
16th to endNight
5%5%13223446%60%
1895
March1st to 15thNight
4412222346%65%
2005
16th to endNight
3312223336%66%
2112
April1st to 15thNight
4312224236%66%
1974
16th to endNight
21112236%75%
1626
May1st to 15thNight
21121224475%
1619
16th to endNight
2212212135%74%
1684
June1st to 15thNight
111121332378%
1620
16th to endNight
1111223480%
1620
July1st to 15thNight
12133235%6%8%60%
1620
16th to endNight
2225%5%36%7%8%11%43%
1701
August1st to 15thNight
221341345%8%61%
1620
16th to endNight
221332445%9%60%
1836
September1st to 15thNight
3213325%5%5%6%60%
1980
16th to endNight
21111112481%
1914
October1st to 15thNight
411213335%71%
1880
16th to endNight
1113212335%73%
2112
November1st to 15thNight
1212212245%71%
2205
16th to endNight
3211212335%71%
2340
December1st to 15thNight
5%5%12213347%63%
2340
16th to endNight
8%5%13313346%56%
2496

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
24%2419%35%13%
1612
16th to endDay
26%28%18%30%14%
1728
February1st to 15thDay
25%37%21%23%18%
1673
16th to endDay
30%25%18%30%12%
1579
March1st to 15thDay
22%25%18%36%14%
1953
16th to endDay
22%25%22%32%15%
2112
April1st to 15thDay
22%35%19%30%18%
1973
16th to endDay
18%2417%34%22%
2331
May1st to 15thDay
19%35%16%31%22%
2331
16th to endDay
19%35%15%33%22%
2432
June1st to 15thDay
11%138%26%48%
2340
16th to endDay
12%135%21%55%
2340
July1st to 15thDay
32%6%8%11%15%25%
2201
16th to endDay
40%10%14%13%14%6%
2312
August1st to 15thDay
34%7%9%13%18%16%
2325
16th to endDay
33%6%7%16%24%12%
2364
September1st to 15thDay
27%5%10%15%20%20%
1980
16th to endDay
15%2414%32%30%
1904
October1st to 15thDay
19%3416%28%28%
1749
16th to endDay
15%35%14%35%25%
1760
November1st to 15thDay
15%25%23%30%22%
1653
16th to endDay
19%1419%31%23%
1620
December1st to 15thDay
26%310%24%25%10%
1620
16th to endDay
31%310%22%23%8%
1728
January1st to 15thNight
23%25%21%32%13%
2327
16th to endNight
25%26%19%28%16%
2496
February1st to 15thNight
20%27%21%27%20%
2187
16th to endNight
25%36%19%30%14%
1895
March1st to 15thNight
20%3418%37%14%
2005
16th to endNight
19%36%21%33%15%
2112
April1st to 15thNight
21%25%17%35%17%
1974
16th to endNight
11%25%18%37%24%
1626
May1st to 15thNight
13%35%17%34%25%
1620
16th to endNight
15%1414%41%23%
1685
June1st to 15thNight
12%127%27%47%
1620
16th to endNight
8%235%20%60%
1620
July1st to 15thNight
18%7%11%15%16%30%
1530
16th to endNight
27%10%17%18%16%9%
1593
August1st to 15thNight
19%6%10%20%23%20%
1611
16th to endNight
20%5%8%23%25%16%
1836
September1st to 15thNight
21%6%11%17%22%19%
1980
16th to endNight
10%1412%35%36%
1914
October1st to 15thNight
15%36%18%26%29%
1880
16th to endNight
14%25%12%31%32%
2112
November1st to 15thNight
15%2419%34%22%
2205
16th to endNight
16%25%19%29%27%
2340
December1st to 15thNight
22%37%26%25%14%
2340
16th to endNight
29%310%21%25%9%
2496

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
24%6%35%20%10%3
90
16th to endDay
10%9%22%34%11%11%
96
February1st to 15thDay
11%11%30%28%13%5%
188
16th to endDay
15%23%28%20%10%1
238
March1st to 15thDay
10%14%29%26%15%3
270
16th to endDay
8%19%28%26%13%3
288
April1st to 15thDay
9%24%31%20%6%7%
269
16th to endDay
7%25%32%23%7%3
270
May1st to 15thDay
7%19%32%25%11%3
288
16th to endDay
6%16%33%26%13%3
330
June1st to 15thDay
313%30%33%13%6%
315
16th to endDay
45%19%40%24%6%
315
July1st to 15thDay
3222%41%20%8%
305
16th to endDay
8%210%39%31%8%
327
August1st to 15thDay
8%112%40%25%10%
315
16th to endDay
10%211%36%27%12%
332
September1st to 15thDay
6%5%21%37%21%7%
315
16th to endDay
5%9%27%28%21%7%
288
October1st to 15thDay
411%32%28%14%7%
189
16th to endDay
8%7%23%33%21%5%
96
November1st to 15thDay
6%10%15%21%34%12%
90
16th to endDay
11%11%35%22%16%3
90
December1st to 15thDay
5%6%22%37%21%6%
90
16th to endDay
17%10%21%31%16%2
96
January1st to 15thNight
8%6%23%31%19%10%
720
16th to endNight
10%6%22%32%20%7%
768
February1st to 15thNight
6%5%23%36%22%6%
704
16th to endNight
8%8%29%31%18%2
632
March1st to 15thNight
6%8%24%31%24%4
720
16th to endNight
47%23%34%24%5%
768
April1st to 15thNight
6%10%30%30%18%4
715
16th to endNight
28%29%34%19%5%
540
May1st to 15thNight
26%22%41%22%4
540
16th to endNight
3426%33%27%4
563
June1st to 15thNight
2322%37%29%5%
540
16th to endNight
1110%36%43%6%
540
July1st to 15thNight
25%30%50%11%
510
16th to endNight
4220%58%14%
564
August1st to 15thNight
3423%54%13%
539
16th to endNight
4320%52%18%
651
September1st to 15thNight
36%25%48%14%
720
16th to endNight
3415%27%37%12%
692
October1st to 15thNight
45%19%35%28%6%
660
16th to endNight
2417%31%31%12%
704
November1st to 15thNight
3322%35%26%8%
700
16th to endNight
5%6%18%37%23%9%
720
December1st to 15thNight
8%5%18%35%25%5%
720
16th to endNight
11%6%22%32%20%5%
768
Page generated at 2017-09-25 06:02:17. Version ac04

Copyright 2017 A. Danko


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