Mt. SAC Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mt. SAC Observatory clear sky chart from 2005-11-05 to 2017-09-21.

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
78%
15913
33%
46789
30%
15869
80%
15869
50%
23573

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
71%
716
28%
2460
30%
716
75%
716
41%
1221
January16th to endNight
68%
768
34%
2304
31%
768
73%
768
40%
1263
February1st to 15thNight
74%
697
34%
2102
36%
697
77%
697
48%
1076
February16th to endNight
67%
632
28%
1895
30%
632
69%
632
35%
1104
March1st to 15thNight
75%
720
30%
2160
32%
720
78%
720
47%
1101
March16th to endNight
76%
768
29%
2013
24%
768
78%
768
48%
1123
April1st to 15thNight
69%
719
27%
1799
18%
719
71%
719
48%
1040
April16th to endNight
72%
720
30%
1800
18%
720
74%
720
54%
972
May1st to 15thNight
73%
605
28%
1685
20%
605
73%
605
48%
921
May16th to endNight
76%
561
28%
1685
24%
561
76%
561
51%
856
June1st to 15thNight
77%
507
27%
1587
25%
507
78%
507
48%
807
June16th to endNight
92%
360
37%
1440
46%
360
92%
360
58%
536
July1st to 15thNight
90%
340
32%
1360
40%
340
90%
340
53%
530
July16th to endNight
87%
647
29%
1669
23%
607
87%
607
54%
867
August1st to 15thNight
89%
717
35%
1790
23%
713
89%
713
66%
881
August16th to endNight
91%
764
42%
1910
29%
764
91%
764
70%
926
September1st to 15thNight
87%
720
38%
1839
30%
720
88%
720
66%
902
September16th to endNight
89%
680
43%
1881
42%
680
91%
680
71%
822
October1st to 15thNight
83%
660
36%
1815
35%
660
85%
660
57%
901
October16th to endNight
80%
704
38%
2144
40%
704
82%
704
51%
1050
November1st to 15thNight
84%
700
35%
2275
39%
700
87%
700
54%
1013
November16th to endNight
79%
720
35%
2340
39%
720
81%
720
48%
1117
December1st to 15thNight
71%
720
33%
2340
34%
720
75%
720
40%
1225
December16th to endNight
64%
768
33%
2496
30%
768
71%
768
39%
1319

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
4212212346%68%
48039
Night
5%312212236%66%
46991
TransparencyDay
19%36%17%27%26%
47731
Night
21%37%17%25%25%
46800
SeeingDay
5%6%25%39%17%5%
5944
Night
6%17%35%39%10%
15927

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
7%313313236%62%
1464
16th to endDay
10%423212335%59%
1920
February1st to 15thDay
7%322212335%63%
1751
16th to endDay
12%424213437%53%
1579
March1st to 15thDay
431331345%8%60%
1800
16th to endDay
421343346%7%58%
2208
April1st to 15thDay
6%41232345%7%59%
2149
16th to endDay
5%312212235%69%
2160
May1st to 15thDay
4412212346%65%
2263
16th to endDay
4413223246%64%
2429
June1st to 15thDay
5%5%23211237%64%
2368
16th to endDay
1211122481%
2520
July1st to 15thDay
2112212346%71%
2520
16th to endDay
211121235%6%72%
2378
August1st to 15thDay
1111113481%
2160
16th to endDay
21223479%
2292
September1st to 15thDay
1111111125%79%
2121
16th to endDay
2111124380%
1871
October1st to 15thDay
5%212212236%68%
1815
16th to endDay
321211233472%
1724
November1st to 15thDay
3212312235%71%
1584
16th to endDay
5%312322335%65%
1620
December1st to 15thDay
7%21431335%8%58%
1620
16th to endDay
11%412212246%59%
1723
January1st to 15thNight
7%41331333462%
2460
16th to endNight
10%313312326%60%
2304
February1st to 15thNight
7%312313235%64%
2101
16th to endNight
12%4133235%35%54%
1895
March1st to 15thNight
4324213348%61%
2160
16th to endNight
5%31222336%7%60%
2013
April1st to 15thNight
7%6%22213326%61%
1799
16th to endNight
45%13212236%67%
1800
May1st to 15thNight
5%5%13213236%63%
1685
16th to endNight
35%23212236%66%
1684
June1st to 15thNight
332321235%7%64%
1587
16th to endNight
111212145%76%
1440
July1st to 15thNight
2212212227%71%
1440
16th to endNight
221221345%8%64%
1783
August1st to 15thNight
2111111247%74%
1800
16th to endNight
12111145%77%
1910
September1st to 15thNight
212212135%75%
1839
16th to endNight
1111111125%80%
1881
October1st to 15thNight
3212213235%70%
1815
16th to endNight
4312212245%69%
2144
November1st to 15thNight
3212222225%73%
2275
16th to endNight
5%313213346%65%
2340
December1st to 15thNight
7%324313336%58%
2340
16th to endNight
11%423212235%61%
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%37%12%24%28%
1464
16th to endDay
29%36%13%18%27%
1920
February1st to 15thDay
22%45%15%25%26%
1751
16th to endDay
31%5%6%18%27%10%
1579
March1st to 15thDay
21%5%8%20%21%22%
1800
16th to endDay
23%5%11%24%20%15%
2208
April1st to 15thDay
24%5%9%25%22%13%
2149
16th to endDay
18%38%23%27%17%
2160
May1st to 15thDay
21%49%20%26%17%
2263
16th to endDay
21%48%28%27%10%
2432
June1st to 15thDay
23%27%15%32%18%
2368
16th to endDay
11%128%33%42%
2520
July1st to 15thDay
15%2411%25%40%
2371
16th to endDay
14%37%22%30%21%
2230
August1st to 15thDay
10%1213%36%35%
2146
16th to endDay
9%3211%39%33%
2292
September1st to 15thDay
11%2312%33%36%
2121
16th to endDay
9%2215%33%36%
1871
October1st to 15thDay
18%35%20%25%26%
1815
16th to endDay
17%3413%20%39%
1724
November1st to 15thDay
15%36%16%22%35%
1584
16th to endDay
22%36%12%22%33%
1620
December1st to 15thDay
23%410%17%18%24%
1620
16th to endDay
27%47%16%20%22%
1723
January1st to 15thNight
25%36%13%21%30%
2460
16th to endNight
28%37%15%14%30%
2304
February1st to 15thNight
23%27%16%21%28%
2112
16th to endNight
32%47%16%23%15%
1895
March1st to 15thNight
22%38%20%23%20%
2160
16th to endNight
23%5%11%22%23%15%
2013
April1st to 15thNight
26%310%25%20%14%
1799
16th to endNight
20%39%24%24%17%
1800
May1st to 15thNight
23%410%22%23%16%
1686
16th to endNight
20%311%29%24%10%
1685
June1st to 15thNight
21%39%18%31%14%
1587
16th to endNight
14%1313%32%35%
1440
July1st to 15thNight
16%1314%30%34%
1360
16th to endNight
20%36%21%29%17%
1669
August1st to 15thNight
14%1313%40%25%
1790
16th to endNight
12%1213%38%30%
1910
September1st to 15thNight
14%1412%33%33%
1839
16th to endNight
11%1315%31%35%
1881
October1st to 15thNight
17%26%19%29%24%
1815
16th to endNight
20%3311%22%38%
2144
November1st to 15thNight
16%35%15%22%37%
2275
16th to endNight
20%36%16%18%34%
2340
December1st to 15thNight
26%48%15%19%25%
2340
16th to endNight
28%38%13%24%21%
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
17%222%40%17%
81
16th to endDay
13%9%26%32%15%1
288
February1st to 15thDay
9%7%27%34%17%3
266
16th to endDay
14%13%27%30%10%2
238
March1st to 15thDay
7%13%27%27%18%5%
270
16th to endDay
6%11%35%31%13%2
288
April1st to 15thDay
7%14%37%23%10%4
269
16th to endDay
317%41%28%6%4
270
May1st to 15thDay
7%10%32%34%11%3
288
16th to endDay
7%10%35%30%13%2
330
June1st to 15thDay
7%5%26%40%15%4
315
16th to endDay
218%47%20%9%
315
July1st to 15thDay
2112%49%24%8%
305
16th to endDay
219%51%23%10%
327
August1st to 15thDay
2114%55%20%6%
315
16th to endDay
214%52%21%9%
332
September1st to 15thDay
2124%51%12%6%
315
16th to endDay
1228%43%14%8%
276
October1st to 15thDay
310%30%33%17%3
255
16th to endDay
5%427%36%21%4
236
November1st to 15thDay
2311%31%28%23%
90
16th to endDay
35%28%37%21%3
90
December1st to 15thDay
2311%42%36%4
90
16th to endDay
14%121%35%25%2
95
January1st to 15thNight
8%110%41%29%8%
720
16th to endNight
11%29%38%30%6%
768
February1st to 15thNight
7%27%43%31%8%
704
16th to endNight
13%28%44%25%5%
632
March1st to 15thNight
5%110%45%29%7%
720
16th to endNight
5%29%41%34%5%
768
April1st to 15thNight
9%212%41%29%4
719
16th to endNight
5%311%39%33%6%
720
May1st to 15thNight
7%111%35%37%6%
606
16th to endNight
4111%41%35%5%
563
June1st to 15thNight
411%37%41%4
507
16th to endNight
1126%65%4
360
July1st to 15thNight
313%66%15%
340
16th to endNight
3119%59%15%
647
August1st to 15thNight
2221%56%16%
717
16th to endNight
2114%57%23%
764
September1st to 15thNight
3325%47%19%
720
16th to endNight
3329%49%13%
680
October1st to 15thNight
5%440%40%8%
660
16th to endNight
6%7%30%40%14%
704
November1st to 15thNight
35%38%38%13%
700
16th to endNight
6%16%38%36%11%
720
December1st to 15thNight
9%17%42%30%9%
720
16th to endNight
13%212%39%25%7%
768
Page generated at 2017-09-22 04:35:30. Version ac04

Copyright 2017 A. Danko


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