Standeford and Andreas Observatories
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Standeford and Andreas Observatories clear sky chart from 2005-11-06 to 2024-02-15.

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
35%
24944
16%
80357
9%
24850
42%
24850
21%
54312

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 10thNight
27%
939
8%
4412
5%
926
35%
926
10%
3500
January11th to 20thNight
28%
946
11%
4201
5%
930
32%
930
10%
3159
January21st to endNight
21%
1034
7%
4488
4%
1023
26%
1023
9%
3447
February1st to 10thNight
26%
950
14%
2470
4%
950
31%
950
20%
1808
February11th to 20thNight
26%
902
17%
2333
6%
898
33%
898
23%
1636
February21st to endNight
27%
706
16%
1890
5%
706
36%
706
22%
1357
March1st to 10thNight
29%
551
13%
1991
8%
551
35%
551
19%
1315
March11th to 20thNight
29%
540
17%
1980
10%
540
39%
540
18%
1313
March21st to endNight
30%
594
14%
2178
10%
594
38%
594
16%
1496
April1st to 10thNight
27%
540
15%
1812
9%
540
36%
540
20%
1196
April11th to 20thNight
28%
533
18%
1741
14%
533
37%
533
18%
1217
April21st to endNight
28%
540
18%
1620
10%
540
39%
540
20%
1117
May1st to 10thNight
35%
540
14%
1593
9%
540
39%
540
24%
1050
May11th to 20thNight
36%
531
19%
1417
9%
531
43%
531
28%
939
May21st to endNight
40%
584
14%
1552
5%
582
44%
582
27%
1045
June1st to 10thNight
49%
540
17%
1440
6%
540
54%
540
38%
881
June11th to 20thNight
45%
540
15%
1440
6%
540
49%
540
33%
915
June21st to endNight
53%
540
18%
1440
7%
540
56%
540
42%
831
July1st to 10thNight
66%
525
18%
1400
9%
525
68%
525
47%
783
July11th to 20thNight
63%
522
23%
1392
11%
522
67%
522
47%
772
July21st to endNight
64%
591
24%
1596
16%
558
67%
558
51%
826
August1st to 10thNight
66%
537
21%
1601
19%
534
69%
534
36%
923
August11th to 20thNight
64%
534
24%
1600
23%
534
70%
534
39%
899
August21st to endNight
58%
594
25%
1873
23%
594
66%
594
41%
995
September1st to 10thNight
57%
540
23%
1800
24%
540
62%
540
37%
950
September11th to 20thNight
47%
540
26%
1872
19%
540
60%
540
33%
1029
September21st to endNight
41%
579
22%
2019
13%
579
53%
579
28%
1147
October1st to 10thNight
37%
720
21%
2160
13%
720
44%
720
30%
1333
October11th to 20thNight
31%
720
22%
2160
10%
720
41%
720
25%
1377
October21st to endNight
30%
792
20%
2376
9%
792
39%
792
21%
1595
November1st to 10thNight
29%
864
22%
2492
9%
864
41%
864
27%
1572
November11th to 20thNight
25%
950
17%
2660
6%
950
34%
950
20%
1904
November21st to endNight
25%
950
17%
2660
7%
950
34%
950
22%
1816
December1st to 10thNight
25%
950
9%
2660
3%
950
31%
950
18%
1957
December11th to 20thNight
21%
949
11%
3209
3%
948
28%
948
13%
2343
December21st to endNight
18%
1037
7%
4829
4%
1026
26%
1026
6%
3869

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
13%8%47%6%35%5%5%7%37%
78325
Night
12%9%48%6%3445%6%39%
80808
TransparencyDay
46%7%14%22%10%1
78032
Night
45%6%13%22%12%1
80621
SeeingDay
17%10%30%27%13%3
14890
Night
16%9%32%31%10%1
24979

By Month

Percentage Cloud Cover SE (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 10thDay
10%17%7%16%9%345%46%18%
2527
11th to 20thDay
7%13%5%10%9%6%8%7%9%8%18%
2690
21st to endDay
9%18%9%15%9%45%445%19%
3135
February1st to 10thDay
8%11%6%10%9%5%7%6%6%7%25%
1710
11th to 20thDay
10%10%5%9%8%46%6%5%7%29%
1610
21st to endDay
14%9%49%8%45%45%8%30%
1369
March1st to 10thDay
16%11%5%8%6%35%45%7%29%
1980
11th to 20thDay
16%9%47%6%45%447%34%
1980
21st to endDay
20%11%47%6%35%45%6%29%
2178
April1st to 10thDay
18%10%47%4345%6%7%32%
2146
11th to 20thDay
20%11%46%5%34446%33%
2165
21st to endDay
24%9%46%6%3445%6%30%
2340
May1st to 10thDay
19%9%35%6%35%5%5%6%33%
2366
11th to 20thDay
18%7%35%5%245%5%9%37%
2479
21st to endDay
17%6%36%5%35%5%5%7%38%
2725
June1st to 10thDay
12%5%25%4345%5%9%46%
2520
11th to 20thDay
13%5%25%5%46%5%6%8%42%
2520
21st to endDay
10%425%6%345%6%9%44%
2520
July1st to 10thDay
6%42443446%10%52%
2520
11th to 20thDay
7%42442447%9%53%
2520
21st to endDay
7%3145%35%6%7%9%50%
2625
August1st to 10thDay
9%436%6%35%5%7%8%43%
2328
11th to 20thDay
12%6%35%5%345%5%7%45%
2301
21st to endDay
9%4245%3445%8%53%
2481
September1st to 10thDay
14%5%25%5%2346%7%48%
2160
11th to 20thDay
12%5%14434447%51%
2085
21st to endDay
14%6%26%42445%7%45%
1922
October1st to 10thDay
16%7%25%5%35%45%5%44%
1800
11th to 20thDay
16%8%34433446%44%
1800
21st to endDay
19%9%45%423446%39%
1980
November1st to 10thDay
11%9%46%5%2435%7%43%
1582
11th to 20thDay
10%12%6%11%6%3445%7%32%
1520
21st to endDay
15%9%46%5%35%5%5%8%36%
1520
December1st to 10thDay
12%11%49%10%5%6%5%5%6%26%
1520
11th to 20thDay
11%12%7%10%6%45%45%6%30%
1918
21st to endDay
20%13%7%16%7%34435%18%
2783
January1st to 10thNight
8%13%8%18%8%4445%5%22%
4428
11th to 20thNight
6%10%5%13%10%48%8%8%6%20%
4278
21st to endNight
11%18%8%12%7%335%45%23%
4532
February1st to 10thNight
8%12%5%10%7%45%437%34%
2470
11th to 20thNight
9%10%47%7%45%45%7%38%
2332
21st to endNight
12%9%48%8%45%5%47%35%
1890
March1st to 10thNight
13%10%47%5%3445%7%38%
1991
11th to 20thNight
14%11%36%6%35%347%37%
1980
21st to endNight
19%10%36%6%3445%7%34%
2178
April1st to 10thNight
15%10%35%5%3445%6%39%
1812
11th to 20thNight
20%10%46%5%34445%35%
1741
21st to endNight
18%10%46%5%2445%7%37%
1620
May1st to 10thNight
15%7%36%5%2445%7%42%
1593
11th to 20thNight
15%5%25%5%35%448%45%
1416
21st to endNight
15%5%35%5%35%5%5%7%42%
1551
June1st to 10thNight
10%423435%5%5%8%51%
1440
11th to 20thNight
10%5%45%5%345%7%7%46%
1440
21st to endNight
8%32342445%9%57%
1440
July1st to 10thNight
6%3123235%6%10%57%
1440
11th to 20thNight
4213425%5%5%8%63%
1440
21st to endNight
43143235%6%9%60%
1690
August1st to 10thNight
7%5%24435%6%5%9%51%
1611
11th to 20thNight
9%41442435%8%56%
1600
21st to endNight
7%4245%23347%60%
1873
September1st to 10thNight
11%424323346%58%
1800
11th to 20thNight
12%41442346%8%52%
1872
21st to endNight
11%5%3442345%6%52%
2019
October1st to 10thNight
15%6%25%423445%49%
2160
11th to 20thNight
17%9%35%325%45%6%43%
2160
21st to endNight
18%11%36%32335%6%40%
2376
November1st to 10thNight
12%7%35%42445%6%46%
2492
11th to 20thNight
12%11%5%8%6%35%45%438%
2660
21st to endNight
14%9%37%5%3445%7%40%
2660
December1st to 10thNight
11%10%5%10%7%45%5%5%7%32%
2660
11th to 20thNight
12%11%5%9%8%45%5%5%6%29%
3279
21st to endNight
22%12%6%13%6%45%445%20%
4884

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 10thDay
67%6%11%12%4
2528
11th to 20thDay
59%5%13%17%6%
2690
21st to endDay
68%6%12%11%3
3135
February1st to 10thDay
56%7%13%15%7%1
1710
11th to 20thDay
53%8%14%18%6%1
1619
21st to endDay
53%8%12%18%10%
1369
March1st to 10thDay
54%6%14%18%6%1
1980
11th to 20thDay
51%7%13%22%8%
1980
21st to endDay
56%7%13%19%5%1
2178
April1st to 10thDay
50%7%13%20%9%1
2146
11th to 20thDay
53%6%11%20%10%2
2165
21st to endDay
55%7%11%20%7%1
2340
May1st to 10thDay
51%8%13%20%8%1
2366
11th to 20thDay
44%8%13%23%11%1
2483
21st to endDay
45%7%13%21%12%1
2725
June1st to 10thDay
35%8%15%27%14%1
2520
11th to 20thDay
38%9%14%22%16%1
2520
21st to endDay
34%9%17%27%12%2
2520
July1st to 10thDay
27%8%18%33%14%1
2451
11th to 20thDay
26%9%16%27%18%4
2445
21st to endDay
27%9%17%30%14%3
2479
August1st to 10thDay
36%9%15%27%10%3
2314
11th to 20thDay
36%8%15%29%12%
2301
21st to endDay
29%7%14%31%17%2
2481
September1st to 10thDay
36%6%12%29%16%
2160
11th to 20thDay
33%7%17%30%12%1
2085
21st to endDay
39%6%13%30%11%1
1922
October1st to 10thDay
42%6%10%27%14%1
1800
11th to 20thDay
41%6%13%25%14%1
1800
21st to endDay
47%5%14%22%11%1
1980
November1st to 10thDay
41%6%14%20%15%3
1582
11th to 20thDay
53%6%13%19%10%
1520
21st to endDay
46%7%19%20%8%1
1520
December1st to 10thDay
57%8%15%12%7%1
1520
11th to 20thDay
55%6%20%14%5%
1915
21st to endDay
68%5%11%11%31
2783
January1st to 10thNight
63%6%11%14%5%
4428
11th to 20thNight
59%7%12%16%6%
4278
21st to endNight
63%6%12%14%6%
4532
February1st to 10thNight
52%5%13%21%8%1
2470
11th to 20thNight
45%6%14%23%10%2
2333
21st to endNight
48%6%11%20%12%2
1890
March1st to 10thNight
45%7%17%22%7%1
1991
11th to 20thNight
48%5%11%22%12%1
1980
21st to endNight
50%7%13%21%7%2
2178
April1st to 10thNight
45%6%14%22%12%1
1812
11th to 20thNight
50%6%9%16%16%3
1741
21st to endNight
47%6%10%21%14%2
1620
May1st to 10thNight
42%6%15%21%15%1
1593
11th to 20thNight
38%7%14%23%14%3
1417
21st to endNight
40%8%14%21%15%2
1552
June1st to 10thNight
31%7%15%29%16%2
1440
11th to 20thNight
35%7%14%22%19%3
1440
21st to endNight
26%7%16%31%18%3
1440
July1st to 10thNight
22%7%14%35%19%2
1400
11th to 20thNight
20%8%15%32%21%4
1392
21st to endNight
19%6%16%36%19%4
1596
August1st to 10thNight
29%8%15%29%16%3
1601
11th to 20thNight
27%7%16%30%18%2
1600
21st to endNight
25%6%15%30%20%4
1873
September1st to 10thNight
29%413%31%21%1
1800
11th to 20thNight
29%7%16%30%17%1
1872
21st to endNight
33%7%12%30%18%1
2019
October1st to 10thNight
37%5%11%27%17%3
2160
11th to 20thNight
41%5%12%24%16%2
2160
21st to endNight
46%5%12%23%13%1
2376
November1st to 10thNight
39%5%15%23%15%2
2492
11th to 20thNight
48%7%12%20%12%1
2660
21st to endNight
45%6%16%23%9%1
2660
December1st to 10thNight
52%7%16%18%7%
2660
11th to 20thNight
54%7%16%18%5%
3281
21st to endNight
66%7%10%14%4
4884

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 10thDay
16%14%34%24%10%2
270
11th to 20thDay
21%13%35%24%6%1
270
21st to endDay
15%11%39%26%8%2
297
February1st to 10thDay
17%15%43%22%4
270
11th to 20thDay
15%16%41%22%5%
252
21st to endDay
25%14%34%26%2
198
March1st to 10thDay
21%13%42%20%4
240
11th to 20thDay
27%10%34%23%5%1
240
21st to endDay
30%12%27%21%8%1
264
April1st to 10thDay
28%13%32%23%4
406
11th to 20thDay
30%11%24%24%10%1
443
21st to endDay
28%14%25%21%9%2
600
May1st to 10thDay
23%11%34%22%7%2
603
11th to 20thDay
22%11%29%23%12%4
625
21st to endDay
19%7%27%30%15%2
679
June1st to 10thDay
12%7%26%33%18%5%
630
11th to 20thDay
13%8%34%31%12%2
630
21st to endDay
11%8%32%30%14%4
630
July1st to 10thDay
8%5%23%32%25%8%
621
11th to 20thDay
9%6%27%34%18%6%
618
21st to endDay
7%327%37%20%5%
684
August1st to 10thDay
10%221%34%27%6%
626
11th to 20thDay
13%422%31%23%7%
615
21st to endDay
10%5%27%34%19%5%
600
September1st to 10thDay
18%8%26%29%16%3
450
11th to 20thDay
13%6%31%30%16%4
375
21st to endDay
20%14%29%25%8%4
270
October1st to 10thDay
23%10%27%25%11%4
270
11th to 20thDay
19%19%33%23%6%1
270
21st to endDay
30%15%34%13%7%
297
November1st to 10thDay
21%17%33%21%6%2
270
11th to 20thDay
17%21%41%16%6%
270
21st to endDay
23%15%40%18%4
270
December1st to 10thDay
17%10%44%23%6%
270
11th to 20thDay
18%14%40%22%6%
270
21st to endDay
26%16%34%20%3
297
January1st to 10thNight
12%12%38%30%8%1
950
11th to 20thNight
15%12%34%32%6%1
950
21st to endNight
19%12%33%29%7%
1045
February1st to 10thNight
15%13%37%30%5%1
950
11th to 20thNight
15%13%39%26%6%1
902
21st to endNight
16%12%34%29%7%1
706
March1st to 10thNight
17%12%36%25%9%1
551
11th to 20thNight
20%9%36%27%7%1
540
21st to endNight
24%9%30%27%10%1
594
April1st to 10thNight
20%11%39%26%5%
540
11th to 20thNight
26%10%30%27%6%1
533
21st to endNight
22%10%33%27%6%1
540
May1st to 10thNight
19%9%32%30%8%1
540
11th to 20thNight
20%7%32%29%11%1
531
21st to endNight
19%5%28%34%14%
584
June1st to 10thNight
12%429%39%15%
540
11th to 20thNight
13%5%32%40%10%
540
21st to endNight
8%429%41%16%1
540
July1st to 10thNight
6%222%39%28%3
525
11th to 20thNight
4425%44%20%2
522
21st to endNight
5%223%49%20%1
591
August1st to 10thNight
8%217%49%23%1
537
11th to 20thNight
10%218%40%25%3
534
21st to endNight
8%224%41%23%2
594
September1st to 10thNight
13%422%43%17%2
540
11th to 20thNight
12%5%32%32%16%2
540
21st to endNight
13%10%32%33%12%1
579
October1st to 10thNight
18%7%32%28%13%2
720
11th to 20thNight
21%12%32%28%6%1
720
21st to endNight
25%12%29%28%6%
792
November1st to 10thNight
16%12%37%26%8%1
864
11th to 20thNight
18%12%39%23%7%
950
21st to endNight
18%14%38%24%7%
950
December1st to 10thNight
16%11%37%29%6%1
950
11th to 20thNight
21%8%37%27%7%1
950
21st to endNight
26%11%33%25%5%
1045
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