Stanford Faculty Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stanford Faculty Observatory clear sky chart from 2005-11-05 to 2018-01-20.

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
67%
15920
26%
48519
23%
15884
71%
15884
37%
27160

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%
776
17%
2570
18%
776
63%
776
27%
1602
January16th to endNight
59%
788
24%
2561
24%
788
62%
788
25%
1622
February1st to 15thNight
57%
700
22%
2288
22%
700
61%
700
25%
1438
February16th to endNight
53%
632
21%
1897
18%
632
59%
632
26%
1233
March1st to 15thNight
56%
720
18%
2001
16%
720
58%
720
28%
1325
March16th to endNight
54%
768
19%
2091
14%
768
58%
768
29%
1372
April1st to 15thNight
52%
719
20%
1799
11%
719
56%
719
32%
1191
April16th to endNight
61%
720
24%
1800
12%
720
64%
720
45%
1049
May1st to 15thNight
70%
617
25%
1697
17%
617
72%
617
48%
910
May16th to endNight
69%
405
23%
1528
20%
405
75%
405
35%
758
June1st to 15thNight
74%
360
24%
1440
26%
360
80%
360
39%
660
June16th to endNight
86%
360
35%
1440
45%
360
89%
360
61%
506
July1st to 15thNight
90%
395
32%
1415
37%
395
92%
395
64%
531
July16th to endNight
87%
564
33%
1593
31%
531
87%
531
57%
714
August1st to 15thNight
85%
556
31%
1629
26%
553
85%
553
52%
770
August16th to endNight
86%
764
36%
1910
24%
764
86%
764
63%
966
September1st to 15thNight
85%
720
37%
1800
27%
720
86%
720
65%
894
September16th to endNight
81%
716
39%
1965
35%
716
83%
716
59%
955
October1st to 15thNight
76%
720
27%
2160
28%
720
79%
720
47%
1101
October16th to endNight
73%
768
32%
2304
34%
768
75%
768
42%
1231
November1st to 15thNight
69%
760
25%
2360
26%
760
71%
760
37%
1329
November16th to endNight
65%
780
23%
2629
25%
780
68%
780
32%
1498
December1st to 15thNight
58%
780
22%
2730
21%
780
64%
780
25%
1648
December16th to endNight
48%
832
20%
2912
17%
832
55%
832
22%
1857

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
6%423323347%58%
49004
Night
6%423323348%55%
48708
TransparencyDay
26%5%10%19%22%14%
48693
Night
28%6%11%19%20%13%
48520
SeeingDay
7%8%23%33%20%6%
6424
Night
8%313%32%33%7%
15930

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
13%5%25%43435%7%44%
1697
16th to endDay
13%6%245%3335%6%44%
1766
February1st to 15thDay
13%6%35%425%45%6%41%
1576
16th to endDay
15%6%35%5%3445%7%39%
1576
March1st to 15thDay
12%5%25%5%35%5%6%10%37%
1957
16th to endDay
10%425%6%45%5%5%7%42%
2132
April1st to 15thDay
12%6%35%324448%44%
2149
16th to endDay
7%423323347%59%
2160
May1st to 15thDay
3413213247%65%
2250
16th to endDay
6%5%24313247%59%
2581
June1st to 15thDay
5%412213347%63%
2520
16th to endDay
111112136%77%
2520
July1st to 15thDay
112112335%76%
2457
16th to endDay
11211336%75%
2468
August1st to 15thDay
312112236%72%
2321
16th to endDay
112212347%73%
2292
September1st to 15thDay
212212235%74%
2160
16th to endDay
3212211235%72%
1992
October1st to 15thDay
5%313312237%64%
1800
16th to endDay
5%413324446%58%
1920
November1st to 15thDay
7%5%244235%5%8%50%
1828
16th to endDay
9%6%25%42435%7%47%
1658
December1st to 15thDay
10%6%37%6%235%5%5%43%
1560
16th to endDay
13%7%46%5%2435%7%39%
1664
January1st to 15thNight
12%6%24425%45%9%41%
2570
16th to endNight
14%5%24423446%45%
2561
February1st to 15thNight
9%6%26%5%35%45%7%43%
2276
16th to endNight
13%5%245%335%5%7%42%
1897
March1st to 15thNight
12%425%5%34447%43%
2001
16th to endNight
9%6%35%334446%47%
2091
April1st to 15thNight
8%5%2443447%8%46%
1799
16th to endNight
6%213314347%61%
1800
May1st to 15thNight
131331335%9%62%
1697
16th to endNight
3424323447%60%
1527
June1st to 15thNight
2213311248%66%
1440
16th to endNight
11112225%79%
1440
July1st to 15thNight
11112238%75%
1500
16th to endNight
11221236%11%64%
1701
August1st to 15thNight
12132223410%65%
1638
16th to endNight
222123410%67%
1910
September1st to 15thNight
121222348%71%
1800
16th to endNight
2212212346%71%
1965
October1st to 15thNight
4323212347%64%
2160
16th to endNight
5%413322347%58%
2304
November1st to 15thNight
6%424323347%55%
2360
16th to endNight
7%5%23434447%51%
2629
December1st to 15thNight
10%5%34424448%46%
2730
16th to endNight
14%6%35%42435%6%43%
2912

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
39%5%13%17%15%8%
1697
16th to endDay
41%8%8%12%18%11%
1766
February1st to 15thDay
42%5%9%17%20%4
1576
16th to endDay
45%5%9%22%12%3
1576
March1st to 15thDay
43%7%11%18%13%5%
1957
16th to endDay
40%8%13%19%15%2
2132
April1st to 15thDay
39%6%15%18%17%2
2149
16th to endDay
26%6%13%26%21%6%
2160
May1st to 15thDay
19%6%12%26%26%7%
2250
16th to endDay
27%6%15%25%21%5%
2585
June1st to 15thDay
22%7%12%21%21%13%
2520
16th to endDay
10%37%18%35%24%
2520
July1st to 15thDay
10%47%14%22%41%
2313
16th to endDay
10%37%13%29%35%
2313
August1st to 15thDay
13%47%15%31%27%
2305
16th to endDay
10%46%19%35%23%
2292
September1st to 15thDay
12%37%18%28%29%
2160
16th to endDay
15%37%25%29%18%
1992
October1st to 15thDay
22%39%18%23%21%
1800
16th to endDay
26%5%10%21%21%14%
1920
November1st to 15thDay
33%6%12%25%15%6%
1828
16th to endDay
37%6%9%18%24%4
1658
December1st to 15thDay
41%5%11%14%17%9%
1560
16th to endDay
43%6%12%15%17%4
1664
January1st to 15thNight
39%5%13%18%15%7%
2570
16th to endNight
40%6%9%12%16%15%
2561
February1st to 15thNight
39%6%10%18%18%7%
2288
16th to endNight
41%6%12%25%11%2
1897
March1st to 15thNight
41%6%13%19%14%5%
2001
16th to endNight
38%7%13%18%17%5%
2091
April1st to 15thNight
35%7%16%20%16%2
1799
16th to endNight
23%7%14%26%20%8%
1800
May1st to 15thNight
19%7%11%24%25%10%
1698
16th to endNight
23%7%16%26%19%5%
1528
June1st to 15thNight
18%8%13%22%22%14%
1440
16th to endNight
10%26%16%35%27%
1440
July1st to 15thNight
8%45%13%28%38%
1415
16th to endNight
12%5%10%16%24%30%
1593
August1st to 15thNight
16%5%7%17%28%24%
1629
16th to endNight
12%5%8%19%29%25%
1910
September1st to 15thNight
12%48%16%26%32%
1800
16th to endNight
14%47%23%28%21%
1965
October1st to 15thNight
21%5%9%19%21%22%
2160
16th to endNight
25%410%20%22%16%
2304
November1st to 15thNight
29%5%14%25%17%7%
2360
16th to endNight
32%6%11%22%19%8%
2629
December1st to 15thNight
37%7%9%15%19%11%
2730
16th to endNight
42%5%12%15%17%6%
2912

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
20%7%22%31%13%5%
235
16th to endDay
14%7%24%27%20%5%
269
February1st to 15thDay
15%12%23%25%17%4
236
16th to endDay
18%12%25%27%14%1
237
March1st to 15thDay
17%14%25%23%15%3
270
16th to endDay
10%15%38%25%7%2
288
April1st to 15thDay
15%19%33%21%8%2
269
16th to endDay
7%21%31%26%10%2
270
May1st to 15thDay
5%13%36%31%10%2
288
16th to endDay
10%12%24%32%14%5%
330
June1st to 15thDay
411%30%35%14%3
315
16th to endDay
1319%45%23%7%
315
July1st to 15thDay
2113%51%25%6%
305
16th to endDay
115%41%31%9%
327
August1st to 15thDay
120%42%25%9%
315
16th to endDay
114%43%29%10%
332
September1st to 15thDay
214%41%29%10%
315
16th to endDay
45%20%34%25%10%
315
October1st to 15thDay
47%22%36%20%6%
315
16th to endDay
7%7%22%31%22%9%
336
November1st to 15thDay
7%424%26%24%12%
244
16th to endDay
8%8%20%31%29%1
112
December1st to 15thDay
6%12%23%24%23%10%
90
16th to endDay
10%12%14%22%31%8%
96
January1st to 15thNight
16%311%32%29%6%
780
16th to endNight
16%413%31%25%9%
788
February1st to 15thNight
11%7%15%34%25%6%
704
16th to endNight
15%6%17%34%21%5%
632
March1st to 15thNight
14%6%15%29%29%4
720
16th to endNight
11%6%21%34%22%3
768
April1st to 15thNight
10%7%22%35%20%3
719
16th to endNight
6%5%22%34%24%6%
720
May1st to 15thNight
3218%39%30%5%
618
16th to endNight
3417%34%33%6%
406
June1st to 15thNight
2214%44%33%2
360
16th to endNight
9%41%45%2
360
July1st to 15thNight
430%55%8%
395
16th to endNight
15%23%53%15%
564
August1st to 15thNight
215%27%52%10%
556
16th to endNight
16%32%48%10%
764
September1st to 15thNight
36%27%49%12%
720
16th to endNight
3110%28%42%13%
716
October1st to 15thNight
5%210%35%36%10%
720
16th to endNight
7%39%29%35%15%
768
November1st to 15thNight
8%13%35%33%8%
760
16th to endNight
10%312%35%29%8%
780
December1st to 15thNight
12%417%32%25%8%
780
16th to endNight
16%5%19%31%21%3
832
Page generated at 2018-01-21 09:19:32. Version ac04

Copyright 2018 A. Danko


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