Cupertino
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cupertino clear sky chart from 2005-11-06 to 2025-05-26.

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%
25265
27%
112226
25%
25037
69%
25037
31%
54602

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
51%
777
21%
5858
19%
756
56%
756
14%
2691
January11th to 20thNight
53%
777
13%
5588
18%
760
60%
760
14%
2950
January21st to endNight
56%
854
25%
6075
25%
836
61%
836
13%
3445
February1st to 10thNight
55%
800
25%
2600
23%
800
62%
800
29%
1541
February11th to 20thNight
52%
780
24%
2427
22%
780
58%
780
30%
1451
February21st to endNight
53%
660
26%
1980
21%
660
61%
660
34%
1146
March1st to 10thNight
47%
783
16%
4602
16%
764
53%
764
13%
2860
March11th to 20thNight
51%
786
18%
4697
21%
760
57%
760
17%
2394
March21st to endNight
48%
858
13%
4948
12%
836
56%
836
14%
3159
April1st to 10thNight
51%
800
23%
2000
13%
800
56%
800
37%
1258
April11th to 20thNight
52%
791
26%
1979
13%
791
57%
791
39%
1189
April21st to endNight
57%
800
28%
2000
13%
800
62%
800
50%
1093
May1st to 10thNight
58%
742
34%
3039
15%
737
63%
737
44%
1204
May11th to 20thNight
71%
534
29%
2712
23%
532
74%
532
48%
807
May21st to endNight
67%
412
24%
2753
25%
410
75%
410
41%
730
June1st to 10thNight
69%
380
30%
1520
30%
380
78%
380
47%
621
June11th to 20thNight
79%
380
37%
1520
42%
380
86%
380
63%
507
June21st to endNight
82%
380
36%
1520
45%
380
87%
380
62%
529
July1st to 10thNight
90%
360
47%
2064
49%
360
92%
360
73%
452
July11th to 20thNight
89%
538
33%
2157
33%
536
90%
536
62%
760
July21st to endNight
86%
613
34%
2321
35%
580
87%
580
60%
812
August1st to 10thNight
82%
561
37%
2379
29%
555
83%
555
42%
1012
August11th to 20thNight
82%
686
36%
2446
25%
679
84%
679
64%
894
August21st to endNight
82%
820
35%
2924
26%
814
84%
814
61%
1136
September1st to 10thNight
85%
760
46%
1900
32%
760
86%
760
76%
877
September11th to 20thNight
76%
760
40%
1900
30%
760
78%
760
60%
990
September21st to endNight
79%
760
41%
2155
38%
760
84%
760
67%
957
October1st to 10thNight
77%
741
46%
3324
34%
740
83%
740
52%
1111
October11th to 20thNight
70%
744
26%
3333
36%
740
76%
740
38%
1374
October21st to endNight
72%
817
38%
3841
37%
813
78%
813
39%
1519
November1st to 10thNight
66%
780
33%
2370
31%
780
72%
780
44%
1240
November11th to 20thNight
64%
800
27%
2600
26%
800
68%
800
36%
1390
November21st to endNight
66%
800
29%
2730
31%
800
70%
800
37%
1429
December1st to 10thNight
56%
783
35%
4227
22%
780
64%
780
25%
1803
December11th to 20thNight
53%
786
14%
4926
21%
779
58%
779
13%
2971
December21st to endNight
45%
862
13%
6811
17%
839
53%
839
9%
4300

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
5%414333346%63%
118059
Night
6%5%25%33335%7%58%
120129
TransparencyDay
23%38%21%26%20%
117817
Night
27%49%20%23%17%
119961
SeeingDay
7%10%24%30%20%8%
16121
Night
8%5%14%36%31%6%
25582

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 10thDay
9%5%27%43435%8%49%
3912
11th to 20thDay
8%6%37%44434453%
4100
21st to endDay
11%8%36%5%5%5%35%7%42%
4510
February1st to 10thDay
10%8%26%44436%6%45%
1800
11th to 20thDay
10%6%26%5%4446%8%44%
1860
21st to endDay
9%6%26%5%5%5%35%8%46%
1650
March1st to 10thDay
9%7%37%6%6%7%5%7%8%36%
4863
11th to 20thDay
6%5%25%45%5%46%10%48%
5000
21st to endDay
7%7%26%6%5%6%5%7%10%39%
5573
April1st to 10thDay
9%6%26%5%4436%8%48%
2400
11th to 20thDay
7%6%25%333347%57%
2369
21st to endDay
5%5%2333335%8%61%
2400
May1st to 10thDay
5%312212235%74%
4006
11th to 20thDay
3112111125%81%
4284
21st to endDay
3413222235%74%
4886
June1st to 10thDay
4414223347%65%
2660
11th to 20thDay
2211111236%80%
2660
21st to endDay
221222213481%
2660
July1st to 10thDay
11111112388%
4420
11th to 20thDay
1112222235%81%
4112
21st to endDay
111111135%85%
4455
August1st to 10thDay
1214223347%71%
4091
11th to 20thDay
111112225%85%
3796
21st to endDay
112222236%79%
4180
September1st to 10thDay
121211113483%
2280
11th to 20thDay
2313323347%69%
2280
21st to endDay
222221235%78%
2020
October1st to 10thDay
2212223245%75%
3180
11th to 20thDay
3313223246%71%
3161
21st to endDay
231323335%7%67%
3448
November1st to 10thDay
5%5%24435%35%7%56%
1926
11th to 20thDay
6%6%35%4445%6%6%52%
1800
21st to endDay
8%5%25%43435%8%53%
1665
December1st to 10thDay
5%315%44446%10%54%
2620
11th to 20thDay
10%7%28%6%6%5%47%9%36%
3028
21st to endDay
9%8%49%6%6%7%5%7%7%32%
4004
January1st to 10thNight
8%7%25%44436%8%50%
6042
11th to 20thNight
8%8%47%444345%49%
5880
21st to endNight
10%8%37%5%45%346%44%
6468
February1st to 10thNight
9%7%36%445%446%49%
2600
11th to 20thNight
9%7%25%5%45%46%8%45%
2425
21st to endNight
8%7%15%445%347%52%
1980
March1st to 10thNight
10%8%37%5%5%5%5%6%7%39%
5029
11th to 20thNight
9%6%27%444347%51%
5000
21st to endNight
9%9%36%45%5%45%7%43%
5402
April1st to 10thNight
8%6%26%5%4436%8%49%
2000
11th to 20thNight
7%6%2433335%7%56%
1979
21st to endNight
431423445%9%61%
2000
May1st to 10thNight
35%23222245%70%
3188
11th to 20thNight
2312222246%76%
2835
21st to endNight
1313222236%75%
2805
June1st to 10thNight
331433225%8%65%
1520
11th to 20thNight
212122147%78%
1520
21st to endNight
131222222480%
1520
July1st to 10thNight
2111111389%
2540
11th to 20thNight
1214222235%77%
2852
21st to endNight
223222246%75%
3109
August1st to 10thNight
2436%423147%63%
2848
11th to 20thNight
22221146%80%
3048
21st to endNight
1313222235%76%
3498
September1st to 10thNight
1212221238%77%
1900
11th to 20thNight
35%14323146%69%
1900
21st to endNight
2313221235%76%
2155
October1st to 10thNight
2212122236%77%
3800
11th to 20thNight
45%14222236%69%
3777
21st to endNight
3324333347%66%
4121
November1st to 10thNight
5%5%25%32336%7%58%
2370
11th to 20thNight
6%6%26%33436%8%54%
2600
21st to endNight
6%5%2444435%8%56%
2730
December1st to 10thNight
442433445%7%59%
4560
11th to 20thNight
11%8%27%5%5%5%5%7%10%35%
5176
21st to endNight
11%8%38%6%6%6%5%7%7%33%
6952

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
36%417%23%16%4
3912
11th to 20thDay
36%38%14%20%19%
4100
21st to endDay
42%37%14%18%16%
4510
February1st to 10thDay
40%46%22%18%11%
1800
11th to 20thDay
39%310%20%21%7%
1869
21st to endDay
40%47%24%19%7%
1650
March1st to 10thDay
46%310%19%20%2
4863
11th to 20thDay
30%5%14%31%13%7%
5000
21st to endDay
39%6%16%27%11%2
5573
April1st to 10thDay
36%412%23%21%4
2400
11th to 20thDay
29%314%27%22%6%
2372
21st to endDay
25%312%31%24%5%
2400
May1st to 10thDay
16%27%33%29%12%
4006
11th to 20thDay
11%15%37%32%13%
4287
21st to endDay
16%18%35%35%5%
4887
June1st to 10thDay
21%211%28%28%10%
2660
11th to 20thDay
10%27%24%40%18%
2660
21st to endDay
10%2314%33%38%
2660
July1st to 10thDay
4116%16%71%
4351
11th to 20thDay
7%238%19%61%
4042
21st to endDay
4218%38%46%
4315
August1st to 10thDay
11%3213%35%36%
4076
11th to 20thDay
5%119%48%35%
3833
21st to endDay
7%3312%42%32%
4179
September1st to 10thDay
8%1211%33%45%
2280
11th to 20thDay
16%3325%34%18%
2280
21st to endDay
11%2423%37%23%
2020
October1st to 10thDay
13%2412%29%39%
3180
11th to 20thDay
16%27%27%31%17%
3161
21st to endDay
17%26%26%32%17%
3448
November1st to 10thDay
29%48%23%24%12%
1926
11th to 20thDay
30%5%10%24%23%9%
1800
21st to endDay
32%38%18%30%9%
1665
December1st to 10thDay
27%37%24%33%7%
2620
11th to 20thDay
44%5%11%19%16%5%
3028
21st to endDay
48%8%14%17%11%2
4004
January1st to 10thNight
34%419%24%16%3
6042
11th to 20thNight
39%410%13%21%12%
5880
21st to endNight
43%48%12%18%16%
6468
February1st to 10thNight
38%37%20%20%12%
2600
11th to 20thNight
37%310%20%23%7%
2427
21st to endNight
34%310%30%18%5%
1980
March1st to 10thNight
45%412%20%16%3
5029
11th to 20thNight
34%5%11%26%17%7%
5000
21st to endNight
42%7%13%24%12%2
5402
April1st to 10thNight
34%414%25%18%5%
2000
11th to 20thNight
30%311%29%19%8%
1979
21st to endNight
22%412%33%23%7%
2000
May1st to 10thNight
18%310%29%27%14%
3188
11th to 20thNight
12%26%43%25%11%
2836
21st to endNight
14%110%38%32%5%
2805
June1st to 10thNight
18%311%28%29%11%
1520
11th to 20thNight
9%27%21%37%25%
1520
21st to endNight
12%1216%33%36%
1520
July1st to 10thNight
46%18%71%
2500
11th to 20thNight
8%329%21%57%
2799
21st to endNight
8%3110%34%44%
3011
August1st to 10thNight
16%5%211%34%32%
2839
11th to 20thNight
8%218%43%38%
3077
21st to endNight
9%4410%45%29%
3498
September1st to 10thNight
8%2211%30%47%
1900
11th to 20thNight
17%4323%33%19%
1900
21st to endNight
12%2421%34%27%
2155
October1st to 10thNight
12%15%12%26%44%
3800
11th to 20thNight
19%36%25%29%18%
3777
21st to endNight
19%26%24%28%20%
4121
November1st to 10thNight
26%38%26%22%15%
2370
11th to 20thNight
29%410%25%23%9%
2600
21st to endNight
29%39%22%24%13%
2730
December1st to 10thNight
26%37%25%28%11%
4560
11th to 20thNight
44%6%10%17%18%5%
5176
21st to endNight
50%5%16%16%9%3
6952

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
22%13%29%20%13%2
317
11th to 20thDay
14%11%29%29%14%4
400
21st to endDay
13%12%28%28%15%4
440
February1st to 10thDay
16%18%28%23%11%5%
400
11th to 20thDay
16%18%31%23%10%2
449
21st to endDay
12%15%33%28%11%2
414
March1st to 10thDay
17%15%33%25%9%1
500
11th to 20thDay
16%18%26%24%13%4
500
21st to endDay
12%19%33%25%9%2
550
April1st to 10thDay
11%16%32%25%12%4
500
11th to 20thDay
11%16%29%24%14%6%
492
21st to endDay
7%19%26%28%13%7%
500
May1st to 10thDay
8%13%32%27%15%5%
503
11th to 20thDay
8%12%30%27%15%8%
528
21st to endDay
5%11%26%30%21%7%
550
June1st to 10thDay
410%27%30%21%8%
490
11th to 20thDay
39%29%34%19%6%
490
21st to endDay
1423%36%26%10%
490
July1st to 10thDay
1216%39%32%10%
486
11th to 20thDay
1113%40%33%12%
484
21st to endDay
112%38%32%17%
531
August1st to 10thDay
2119%37%28%13%
487
11th to 20thDay
120%38%24%16%
477
21st to endDay
114%40%30%14%
539
September1st to 10thDay
2315%38%30%12%
490
11th to 20thDay
35%25%35%23%9%
490
21st to endDay
27%19%33%29%9%
490
October1st to 10thDay
28%22%30%25%12%
490
11th to 20thDay
5%10%20%27%27%11%
490
21st to endDay
6%7%23%32%25%7%
539
November1st to 10thDay
7%12%22%26%25%8%
462
11th to 20thDay
5%10%21%30%24%10%
300
21st to endDay
7%11%25%29%22%6%
233
December1st to 10thDay
8%9%22%30%22%10%
200
11th to 20thDay
10%14%25%22%21%9%
200
21st to endDay
12%20%27%23%15%4
220
January1st to 10thNight
15%5%17%34%23%6%
800
11th to 20thNight
11%7%15%37%23%8%
800
21st to endNight
15%414%37%24%6%
880
February1st to 10thNight
12%9%15%37%22%5%
800
11th to 20thNight
14%8%17%37%18%5%
784
21st to endNight
12%8%19%38%19%3
660
March1st to 10thNight
13%10%18%36%21%3
800
11th to 20thNight
14%8%18%34%22%5%
800
21st to endNight
10%8%22%38%20%2
880
April1st to 10thNight
11%6%22%37%23%3
800
11th to 20thNight
10%11%18%35%22%4
791
21st to endNight
5%7%25%33%24%6%
800
May1st to 10thNight
4326%36%26%5%
747
11th to 20thNight
3313%39%36%6%
542
21st to endNight
25%15%41%33%3
422
June1st to 10thNight
4317%44%28%4
380
11th to 20thNight
1313%50%32%1
380
21st to endNight
19%43%44%2
380
July1st to 10thNight
15%38%52%4
370
11th to 20thNight
1233%54%11%
545
21st to endNight
1130%58%9%
624
August1st to 10thNight
3334%52%8%
567
11th to 20thNight
110%39%45%5%
698
21st to endNight
117%33%48%9%
836
September1st to 10thNight
17%36%45%10%
760
11th to 20thNight
419%38%39%8%
760
21st to endNight
3210%35%40%10%
760
October1st to 10thNight
4210%38%36%10%
760
11th to 20thNight
5%5%9%31%39%10%
760
21st to endNight
5%410%35%32%14%
836
November1st to 10thNight
6%314%33%34%9%
780
11th to 20thNight
8%314%37%31%7%
800
21st to endNight
9%5%14%40%25%7%
800
December1st to 10thNight
10%5%17%36%26%6%
800
11th to 20thNight
15%6%15%38%22%4
800
21st to endNight
14%8%21%34%19%3
880
Page generated at 2025-05-23 06:21:00. Version ac07

Copyright © 2025 Attilla Danko.


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