STAR School
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the STAR School clear sky chart from 2005-11-05 to 2017-09-20.

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
64%
15554
34%
46941
25%
15513
73%
15513
53%
21969

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
60%
720
33%
2327
28%
716
70%
716
49%
1075
January16th to endNight
61%
768
36%
2496
31%
768
73%
768
49%
1173
February1st to 15thNight
60%
700
34%
2244
28%
700
72%
700
54%
1007
February16th to endNight
46%
632
39%
1877
26%
632
62%
632
53%
876
March1st to 15thNight
56%
720
36%
1980
26%
720
67%
720
56%
958
March16th to endNight
57%
768
35%
2112
19%
768
70%
768
59%
999
April1st to 15thNight
42%
671
31%
1930
14%
671
57%
671
49%
922
April16th to endNight
46%
540
41%
1695
21%
540
67%
540
60%
680
May1st to 15thNight
55%
540
34%
1619
21%
539
67%
539
59%
723
May16th to endNight
55%
561
33%
1684
20%
561
65%
561
59%
737
June1st to 15thNight
59%
540
32%
1620
23%
540
66%
540
66%
679
June16th to endNight
75%
540
33%
1620
29%
540
79%
540
70%
666
July1st to 15thNight
79%
510
21%
1530
18%
510
81%
510
42%
819
July16th to endNight
75%
564
17%
1593
15%
531
76%
531
30%
966
August1st to 15thNight
80%
539
27%
1611
23%
536
81%
536
41%
851
August16th to endNight
82%
574
34%
1801
32%
574
84%
574
46%
916
September1st to 15thNight
79%
720
30%
1980
25%
720
82%
720
50%
1058
September16th to endNight
77%
675
43%
1869
30%
675
84%
675
72%
810
October1st to 15thNight
67%
660
38%
1917
25%
660
76%
660
61%
879
October16th to endNight
71%
704
42%
2112
32%
704
80%
704
62%
932
November1st to 15thNight
68%
700
38%
2148
28%
700
79%
700
60%
949
November16th to endNight
67%
720
39%
2340
34%
720
76%
720
57%
992
December1st to 15thNight
64%
720
35%
2340
28%
720
74%
720
52%
1050
December16th to endNight
55%
768
33%
2496
26%
768
66%
768
40%
1252

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
221332445%7%62%
47900
Night
2212213346%68%
47137
TransparencyDay
19%5%9%19%27%18%
47595
Night
16%48%20%29%20%
46943
SeeingDay
411%22%29%24%7%
5382
Night
35%23%35%27%5%
15560

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
131332445%8%62%
1611
16th to endDay
3313213347%64%
1728
February1st to 15thDay
1314313347%64%
1617
16th to endDay
121222345%7%66%
1597
March1st to 15thDay
111221236%7%67%
1980
16th to endDay
1323213246%67%
2112
April1st to 15thDay
1213323347%67%
2016
16th to endDay
1212213346%71%
2262
May1st to 15thDay
2212213346%68%
2331
16th to endDay
3212313445%67%
2429
June1st to 15thDay
11121223478%
2340
16th to endDay
1112233476%
2340
July1st to 15thDay
32146%49%7%8%8%42%
2340
16th to endDay
5%425%8%410%9%9%10%28%
2467
August1st to 15thDay
4426%6%37%7%8%9%39%
2340
16th to endDay
33135%46%7%9%9%45%
2398
September1st to 15thDay
33135%36%5%6%8%51%
1980
16th to endDay
212211336%74%
1860
October1st to 15thDay
22222333473%
1713
16th to endDay
1212212235%75%
1760
November1st to 15thDay
11212235%77%
1711
16th to endDay
22121222476%
1620
December1st to 15thDay
1412312349%63%
1620
16th to endDay
131342346%7%60%
1728
January1st to 15thNight
1213312346%68%
2327
16th to endNight
2313313246%66%
2496
February1st to 15thNight
1212222436%71%
2233
16th to endNight
121221235%7%68%
1877
March1st to 15thNight
11231245%6%68%
1980
16th to endNight
1211212346%71%
2112
April1st to 15thNight
121222335%7%67%
1930
16th to endNight
111212335%76%
1695
May1st to 15thNight
1112313235%73%
1619
16th to endNight
212213235%74%
1684
June1st to 15thNight
211212225%78%
1620
16th to endNight
11211235%78%
1620
July1st to 15thNight
223425%6%7%9%54%
1620
16th to endNight
3424437%6%9%11%41%
1701
August1st to 15thNight
323435%5%8%9%53%
1620
16th to endNight
1224245%8%10%56%
1801
September1st to 15thNight
22133235%6%9%58%
1980
16th to endNight
11111235%79%
1869
October1st to 15thNight
122112345%74%
1917
16th to endNight
121221222477%
2112
November1st to 15thNight
211213345%73%
2148
16th to endNight
212213325%75%
2340
December1st to 15thNight
222212348%67%
2340
16th to endNight
333323447%62%
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
19%5%12%27%30%3
1612
16th to endDay
18%413%25%31%6%
1728
February1st to 15thDay
20%412%25%24%12%
1617
16th to endDay
16%411%25%32%9%
1597
March1st to 15thDay
14%46%25%37%12%
1980
16th to endDay
17%46%27%31%13%
2112
April1st to 15thDay
16%37%18%35%17%
2016
16th to endDay
14%36%17%37%20%
2262
May1st to 15thDay
16%26%18%34%20%
2331
16th to endDay
18%36%14%33%23%
2432
June1st to 15thDay
10%147%25%50%
2340
16th to endDay
11%237%20%54%
2340
July1st to 15thDay
32%8%13%12%11%21%
2201
16th to endDay
40%14%16%15%9%3
2312
August1st to 15thDay
35%9%13%14%14%12%
2325
16th to endDay
28%10%13%17%19%10%
2398
September1st to 15thDay
27%7%13%15%16%20%
1980
16th to endDay
12%26%15%34%28%
1860
October1st to 15thDay
15%25%16%35%23%
1713
16th to endDay
13%26%17%41%18%
1760
November1st to 15thDay
10%27%30%35%14%
1711
16th to endDay
14%25%26%33%17%
1620
December1st to 15thDay
19%5%14%30%25%4
1620
16th to endDay
21%6%15%30%22%3
1728
January1st to 15thNight
17%49%27%35%5%
2327
16th to endNight
19%39%24%35%8%
2496
February1st to 15thNight
15%38%27%27%16%
2244
16th to endNight
15%48%24%32%14%
1877
March1st to 15thNight
14%47%21%37%14%
1980
16th to endNight
13%38%23%34%15%
2112
April1st to 15thNight
15%37%19%34%19%
1930
16th to endNight
10%15%17%38%25%
1695
May1st to 15thNight
14%37%18%29%27%
1620
16th to endNight
13%26%13%37%26%
1685
June1st to 15thNight
10%236%23%53%
1620
16th to endNight
10%126%18%60%
1620
July1st to 15thNight
23%7%13%15%14%25%
1530
16th to endNight
31%10%19%18%13%7%
1593
August1st to 15thNight
23%7%13%18%19%17%
1611
16th to endNight
20%7%12%23%22%14%
1801
September1st to 15thNight
20%5%11%19%21%20%
1980
16th to endNight
8%25%15%31%36%
1869
October1st to 15thNight
12%46%19%28%28%
1917
16th to endNight
13%2416%34%28%
2112
November1st to 15thNight
13%37%22%35%17%
2148
16th to endNight
13%25%21%34%21%
2340
December1st to 15thNight
14%5%12%26%31%9%
2340
16th to endNight
21%6%12%29%24%5%
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
6%14%24%37%12%4
90
16th to endDay
413%21%32%21%6%
96
February1st to 15thDay
39%19%36%24%7%
132
16th to endDay
5%26%26%29%8%3
238
March1st to 15thDay
317%25%32%15%5%
270
16th to endDay
421%25%30%15%2
288
April1st to 15thDay
6%21%33%17%15%5%
269
16th to endDay
5%26%28%24%12%1
270
May1st to 15thDay
419%30%28%13%3
288
16th to endDay
416%29%25%19%4
330
June1st to 15thDay
29%33%25%19%8%
315
16th to endDay
15%17%33%33%9%
315
July1st to 15thDay
4116%34%33%9%
305
16th to endDay
6%19%33%32%18%
327
August1st to 15thDay
5%7%27%40%16%
315
16th to endDay
5%12%31%35%15%
332
September1st to 15thDay
6%218%31%32%8%
315
16th to endDay
5%6%20%30%29%6%
272
October1st to 15thDay
19%28%29%22%9%
153
16th to endDay
10%416%39%23%5%
96
November1st to 15thDay
5%5%13%22%41%12%
90
16th to endDay
216%28%27%22%2
90
December1st to 15thDay
7%6%24%28%27%4
90
16th to endDay
5%14%20%41%15%2
96
January1st to 15thNight
45%23%34%24%6%
720
16th to endNight
36%24%37%23%3
768
February1st to 15thNight
1430%37%22%3
704
16th to endNight
210%39%30%16%1
632
March1st to 15thNight
110%28%34%20%4
720
16th to endNight
26%30%38%17%3
768
April1st to 15thNight
216%35%29%13%2
671
16th to endNight
210%39%32%11%4
540
May1st to 15thNight
1435%41%14%2
540
16th to endNight
36%31%34%21%2
563
June1st to 15thNight
2332%43%15%1
540
16th to endNight
2117%51%25%1
540
July1st to 15thNight
37%42%41%4
510
16th to endNight
5%435%46%6%
564
August1st to 15thNight
45%30%50%9%
539
16th to endNight
27%31%47%9%
574
September1st to 15thNight
38%32%46%7%
720
16th to endNight
2215%31%37%11%
675
October1st to 15thNight
2325%36%25%6%
660
16th to endNight
2219%34%32%8%
704
November1st to 15thNight
2323%33%30%6%
700
16th to endNight
26%21%36%25%8%
720
December1st to 15thNight
45%21%35%28%4
720
16th to endNight
5%8%25%35%20%3
768
Page generated at 2017-09-21 04:18:14. Version ac04

Copyright 2017 A. Danko


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