Grasslands Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grasslands Observatory clear sky chart from 2005-11-05 to 2017-12-04.

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
71%
15859
35%
47828
28%
15820
77%
15820
50%
23280

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
69%
718
32%
2327
31%
716
75%
716
49%
1100
January16th to endNight
62%
768
34%
2496
30%
768
72%
768
39%
1295
February1st to 15thNight
66%
697
36%
2205
32%
696
77%
696
50%
1016
February16th to endNight
58%
632
39%
1812
31%
632
69%
632
48%
914
March1st to 15thNight
60%
720
40%
1980
28%
720
71%
720
55%
968
March16th to endNight
65%
768
38%
2112
24%
768
76%
768
55%
1038
April1st to 15thNight
59%
676
35%
1935
22%
676
71%
676
53%
937
April16th to endNight
61%
540
44%
1800
32%
540
78%
540
63%
685
May1st to 15thNight
64%
539
38%
1644
26%
539
76%
539
67%
655
May16th to endNight
70%
561
43%
1684
32%
561
79%
561
71%
674
June1st to 15thNight
78%
540
35%
1620
29%
540
80%
540
66%
685
June16th to endNight
82%
540
28%
1620
25%
540
82%
540
55%
761
July1st to 15thNight
80%
510
15%
1530
13%
510
80%
510
36%
880
July16th to endNight
79%
564
12%
1593
11%
531
79%
531
25%
1059
August1st to 15thNight
85%
539
21%
1611
21%
536
85%
536
34%
943
August16th to endNight
80%
579
26%
1881
27%
579
80%
579
34%
1070
September1st to 15thNight
74%
720
25%
1980
21%
720
75%
720
38%
1202
September16th to endNight
82%
716
42%
1980
31%
716
88%
716
65%
926
October1st to 15thNight
76%
720
38%
2102
29%
720
83%
720
65%
931
October16th to endNight
81%
768
44%
2304
37%
768
87%
768
67%
987
November1st to 15thNight
75%
760
41%
2280
33%
760
83%
760
62%
987
November16th to endNight
67%
780
41%
2444
36%
780
74%
780
50%
1158
December1st to 15thNight
68%
736
36%
2392
35%
736
74%
736
46%
1149
December16th to endNight
63%
768
34%
2496
32%
768
69%
768
41%
1260

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
3313424446%61%
48653
Night
2213323446%65%
48030
TransparencyDay
22%46%11%27%26%
48348
Night
18%47%14%28%26%
47836
SeeingDay
7%11%25%32%18%5%
5644
Night
3415%33%34%7%
15871

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
22145%23445%62%
1611
16th to endDay
35%2331336%7%60%
1728
February1st to 15thDay
4312212335%67%
1649
16th to endDay
3423413346%63%
1660
March1st to 15thDay
2213313446%65%
1980
16th to endDay
2213212245%71%
2112
April1st to 15thDay
1112323346%70%
2012
16th to endDay
111111121480%
2160
May1st to 15thDay
11221223477%
2302
16th to endDay
1111212381%
2429
June1st to 15thDay
111213225%77%
2340
16th to endDay
311232445%6%64%
2340
July1st to 15thDay
4425%6%5%8%8%9%10%34%
2340
16th to endDay
8%7%39%8%5%8%7%7%9%23%
2467
August1st to 15thDay
6%6%37%7%47%7%7%7%32%
2340
16th to endDay
4416%6%48%6%6%9%40%
2317
September1st to 15thDay
6%325%7%46%6%6%6%42%
1980
16th to endDay
3212323446%64%
1980
October1st to 15thDay
2112212345%72%
1857
16th to endDay
112212236%75%
1920
November1st to 15thDay
112212346%72%
1909
16th to endDay
3312214235%70%
1843
December1st to 15thDay
431341335%7%61%
1649
16th to endDay
341332235%7%60%
1728
January1st to 15thNight
2313313445%65%
2327
16th to endNight
4313425%45%6%57%
2496
February1st to 15thNight
1212323346%67%
2200
16th to endNight
121333445%5%64%
1812
March1st to 15thNight
221242225%6%68%
1980
16th to endNight
121222333471%
2112
April1st to 15thNight
1212314335%70%
1935
16th to endNight
1111122481%
1800
May1st to 15thNight
111445%78%
1644
16th to endNight
111212383%
1684
June1st to 15thNight
211111122479%
1620
16th to endNight
21232345%6%67%
1620
July1st to 15thNight
23245%45%7%9%12%44%
1620
16th to endNight
3325%5%48%7%10%15%31%
1701
August1st to 15thNight
2214336%6%8%11%48%
1620
16th to endNight
5%4135%35%6%6%10%48%
1881
September1st to 15thNight
5%3245%245%5%9%49%
1980
16th to endNight
111112337%73%
1980
October1st to 15thNight
11221223476%
2102
16th to endNight
11211235%78%
2304
November1st to 15thNight
112212246%73%
2280
16th to endNight
2312413235%68%
2444
December1st to 15thNight
423213446%63%
2392
16th to endNight
3314333446%60%
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
23%36%12%29%25%
1612
16th to endDay
24%35%16%27%23%
1728
February1st to 15thDay
20%2313%30%29%
1649
16th to endDay
23%2413%30%26%
1660
March1st to 15thDay
19%337%36%28%
1980
16th to endDay
16%239%37%29%
2112
April1st to 15thDay
15%238%40%29%
2012
16th to endDay
10%128%30%46%
2160
May1st to 15thDay
11%138%29%44%
2302
16th to endDay
9%227%32%45%
2432
June1st to 15thDay
11%225%25%52%
2340
16th to endDay
20%35%8%21%39%
2340
July1st to 15thDay
36%13%14%13%12%8%
2201
16th to endDay
51%15%16%10%4
2312
August1st to 15thDay
45%10%11%14%14%3
2325
16th to endDay
36%10%14%15%14%8%
2317
September1st to 15thDay
37%7%11%15%18%9%
1980
16th to endDay
20%37%13%32%21%
1980
October1st to 15thDay
14%46%12%31%31%
1857
16th to endDay
12%2410%31%38%
1920
November1st to 15thDay
13%2413%37%28%
1909
16th to endDay
18%2311%30%32%
1843
December1st to 15thDay
23%35%15%33%19%
1649
16th to endDay
22%28%18%31%15%
1728
January1st to 15thNight
19%35%12%31%26%
2327
16th to endNight
26%45%16%28%18%
2496
February1st to 15thNight
17%35%15%26%31%
2211
16th to endNight
20%35%14%32%23%
1812
March1st to 15thNight
17%2411%31%33%
1980
16th to endNight
17%2410%36%28%
2112
April1st to 15thNight
17%3310%40%24%
1935
16th to endNight
9%129%30%45%
1800
May1st to 15thNight
8%2410%33%41%
1645
16th to endNight
7%238%35%42%
1685
June1st to 15thNight
11%12427%51%
1620
16th to endNight
15%45%9%22%41%
1620
July1st to 15thNight
25%10%23%17%13%10%
1530
16th to endNight
35%14%28%15%41
1593
August1st to 15thNight
25%12%15%21%20%4
1611
16th to endNight
28%8%17%17%18%9%
1881
September1st to 15thNight
29%7%13%19%21%8%
1980
16th to endNight
12%37%15%39%21%
1980
October1st to 15thNight
11%26%17%30%29%
2102
16th to endNight
11%2413%29%38%
2304
November1st to 15thNight
12%2419%32%28%
2280
16th to endNight
20%2312%29%32%
2444
December1st to 15thNight
20%35%16%33%20%
2392
16th to endNight
24%48%18%27%17%
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
8%21%22%37%10%
90
16th to endDay
5%14%23%38%12%5%
96
February1st to 15thDay
9%15%24%32%15%3
164
16th to endDay
10%33%25%22%7%
238
March1st to 15thDay
8%22%32%19%12%4
270
16th to endDay
5%29%29%20%9%5%
288
April1st to 15thDay
7%21%36%23%8%2
269
16th to endDay
7%23%39%18%9%1
270
May1st to 15thDay
7%19%36%22%10%2
288
16th to endDay
5%15%38%31%7%2
330
June1st to 15thDay
46%41%33%11%3
315
16th to endDay
4125%42%20%4
315
July1st to 15thDay
6%15%48%24%4
305
16th to endDay
12%11%37%26%10%
327
August1st to 15thDay
10%9%33%32%13%
315
16th to endDay
6%18%38%34%11%
332
September1st to 15thDay
9%216%41%24%5%
315
16th to endDay
6%9%23%34%20%5%
315
October1st to 15thDay
28%25%33%20%8%
192
16th to endDay
2420%35%31%6%
144
November1st to 15thDay
6%12%20%38%18%2
135
16th to endDay
5%17%17%34%22%2
135
December1st to 15thDay
9%8%22%30%30%1
100
16th to endDay
12%17%27%32%9%1
96
January1st to 15thNight
3318%40%28%5%
720
16th to endNight
7%6%18%38%25%3
768
February1st to 15thNight
36%19%36%28%4
704
16th to endNight
210%24%40%18%2
632
March1st to 15thNight
39%23%33%25%3
720
16th to endNight
27%19%35%29%4
768
April1st to 15thNight
29%23%41%20%2
676
16th to endNight
210%24%39%19%3
540
May1st to 15thNight
16%26%35%27%2
540
16th to endNight
420%39%31%2
563
June1st to 15thNight
3212%47%31%2
540
16th to endNight
28%34%47%5%
540
July1st to 15thNight
3425%56%10%
510
16th to endNight
5%218%54%18%
564
August1st to 15thNight
3214%57%22%
539
16th to endNight
7%315%52%20%
579
September1st to 15thNight
5%17%21%49%14%
720
16th to endNight
2111%33%41%9%
716
October1st to 15thNight
1314%34%37%7%
720
16th to endNight
1211%32%41%10%
768
November1st to 15thNight
25%13%34%36%7%
760
16th to endNight
5%6%15%33%31%7%
780
December1st to 15thNight
6%7%14%38%27%6%
736
16th to endNight
5%7%17%40%22%6%
768
Page generated at 2017-12-05 08:19:05. Version ac04

Copyright 2017 A. Danko


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