Mountain View Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mountain View Observatory 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
40%
23659
21%
79899
12%
23561
50%
23561
24%
49174

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
21%
753
25%
4125
8%
737
41%
737
16%
2121
January11th to 20thNight
25%
759
27%
3949
10%
740
41%
740
15%
2153
January21st to endNight
23%
830
16%
4439
10%
814
42%
814
15%
2655
February1st to 10thNight
25%
760
19%
2470
8%
760
39%
760
22%
1585
February11th to 20thNight
15%
719
20%
2313
5%
719
34%
719
19%
1467
February21st to endNight
19%
592
24%
1761
8%
592
37%
592
21%
1155
March1st to 10thNight
25%
720
25%
1980
10%
720
41%
720
28%
1224
March11th to 20thNight
25%
720
20%
1980
9%
720
36%
720
23%
1314
March21st to endNight
24%
792
20%
2175
7%
792
35%
792
25%
1440
April1st to 10thNight
27%
720
18%
1800
5%
720
36%
720
27%
1218
April11th to 20thNight
37%
711
22%
1775
7%
711
43%
711
33%
1153
April21st to endNight
38%
720
20%
1800
8%
720
44%
720
30%
1211
May1st to 10thNight
39%
558
17%
1638
9%
558
45%
558
24%
1088
May11th to 20thNight
41%
488
16%
1549
10%
488
47%
488
23%
1029
May21st to endNight
56%
388
16%
1551
18%
388
60%
388
22%
943
June1st to 10thNight
59%
360
19%
1440
20%
360
64%
360
25%
822
June11th to 20thNight
59%
360
16%
1440
18%
360
63%
360
22%
882
June21st to endNight
66%
360
18%
1440
20%
360
71%
360
27%
824
July1st to 10thNight
61%
350
16%
1400
19%
350
64%
350
21%
849
July11th to 20thNight
73%
500
15%
1544
13%
500
75%
500
31%
943
July21st to endNight
66%
591
17%
1672
13%
558
69%
558
31%
1041
August1st to 10thNight
66%
537
15%
1600
12%
534
67%
534
26%
1077
August11th to 20thNight
61%
580
17%
1644
12%
580
62%
580
25%
1122
August21st to endNight
61%
792
18%
1980
11%
792
62%
792
31%
1342
September1st to 10thNight
58%
720
21%
1800
13%
720
61%
720
31%
1223
September11th to 20thNight
61%
720
29%
1812
21%
720
64%
720
37%
1153
September21st to endNight
46%
720
19%
2074
15%
720
51%
720
25%
1392
October1st to 10thNight
49%
720
19%
2160
18%
720
54%
720
28%
1362
October11th to 20thNight
44%
720
26%
2160
20%
720
55%
720
31%
1303
October21st to endNight
39%
792
27%
2376
18%
792
52%
792
30%
1445
November1st to 10thNight
41%
740
28%
2255
18%
740
55%
740
32%
1308
November11th to 20thNight
36%
760
26%
2518
15%
760
51%
760
29%
1465
November21st to endNight
36%
760
24%
2660
13%
760
51%
760
28%
1505
December1st to 10thNight
28%
760
20%
2660
8%
760
43%
760
20%
1663
December11th to 20thNight
30%
760
22%
3171
12%
759
44%
759
20%
1876
December21st to endNight
31%
827
14%
4788
12%
817
45%
817
13%
2821

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
14%5%25%5%35%5%6%8%42%
78391
Night
15%6%25%43445%7%44%
80680
TransparencyDay
39%8%12%23%16%3
78106
Night
38%7%12%23%17%4
80485
SeeingDay
17%11%25%25%16%6%
13942
Night
17%8%29%31%14%1
23686

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
18%5%24425%5%46%46%
2709
11th to 20thDay
14%9%37%5%345%46%39%
2840
21st to endDay
18%8%35%5%24445%39%
3124
February1st to 10thDay
13%8%26%6%35%46%7%40%
1710
11th to 20thDay
19%6%25%43445%6%41%
1628
21st to endDay
18%7%26%6%2345%6%39%
1492
March1st to 10thDay
13%5%245%34446%50%
1980
11th to 20thDay
16%7%25%44446%6%41%
1980
21st to endDay
16%7%36%6%345%5%7%38%
2179
April1st to 10thDay
12%5%26%435%5%5%7%46%
2160
11th to 20thDay
15%5%245%3345%6%48%
2134
21st to endDay
18%5%35%5%3445%6%40%
2160
May1st to 10thDay
17%6%34435%5%7%9%38%
2335
11th to 20thDay
18%6%35%5%35%5%6%8%37%
2340
21st to endDay
15%5%26%4345%6%9%41%
2725
June1st to 10thDay
11%4245%35%5%7%9%43%
2520
11th to 20thDay
12%5%26%6%35%6%7%9%39%
2520
21st to endDay
9%3245%45%6%8%10%44%
2520
July1st to 10thDay
9%5%36%5%36%5%7%9%41%
2520
11th to 20thDay
6%325%5%36%7%8%12%43%
2358
21st to endDay
9%5%36%6%47%7%7%10%36%
2547
August1st to 10thDay
10%5%25%6%47%6%9%9%35%
2327
11th to 20thDay
12%426%6%47%7%7%9%37%
2237
21st to endDay
10%5%25%5%5%5%5%6%9%43%
2376
September1st to 10thDay
14%6%245%4445%6%45%
2160
11th to 20thDay
11%5%345%3445%8%49%
2144
21st to endDay
20%5%35%435%45%7%39%
1886
October1st to 10thDay
17%5%2443445%8%46%
1800
11th to 20thDay
17%5%244345%5%5%48%
1800
21st to endDay
15%5%24434446%50%
1980
November1st to 10thDay
12%5%24424346%53%
1820
11th to 20thDay
14%5%2443445%6%50%
1658
21st to endDay
16%4243345%45%49%
1520
December1st to 10thDay
16%5%25%4345%7%7%42%
1520
11th to 20thDay
17%5%34434347%47%
1910
21st to endDay
18%6%26%5%35%5%6%7%37%
2772
January1st to 10thNight
14%7%2442345%6%49%
4233
11th to 20thNight
18%8%24323345%47%
4120
21st to endNight
20%8%25%424345%43%
4532
February1st to 10thNight
15%7%26%5%446%5%6%41%
2470
11th to 20thNight
20%7%25%42335%6%43%
2311
21st to endNight
17%8%26%5%34445%42%
1761
March1st to 10thNight
13%4244345%5%6%50%
1980
11th to 20thNight
18%7%2343445%7%42%
1980
21st to endNight
14%7%26%5%345%5%7%43%
2175
April1st to 10thNight
14%6%36%5%2445%6%44%
1800
11th to 20thNight
12%5%245%3445%6%49%
1775
21st to endNight
17%5%25%335%46%7%44%
1800
May1st to 10thNight
17%6%25%5%345%5%8%40%
1638
11th to 20thNight
19%6%24425%5%5%7%39%
1549
21st to endNight
13%5%26%5%345%5%9%43%
1550
June1st to 10thNight
11%5%345%346%8%9%44%
1440
11th to 20thNight
12%6%25%436%5%6%8%42%
1440
21st to endNight
9%325%5%45%6%7%9%45%
1440
July1st to 10thNight
11%5%25%5%36%5%7%11%40%
1440
11th to 20thNight
5%325%6%46%6%6%12%45%
1596
21st to endNight
8%5%36%436%6%8%11%40%
1771
August1st to 10thNight
9%5%25%7%47%7%8%9%38%
1611
11th to 20thNight
11%5%36%6%45%6%7%8%38%
1644
21st to endNight
10%6%36%7%35%6%6%10%39%
1980
September1st to 10thNight
16%6%24435%5%5%7%44%
1800
11th to 20thNight
9%6%26%5%245%48%49%
1812
21st to endNight
19%7%35%43435%6%42%
2074
October1st to 10thNight
18%6%24433445%46%
2160
11th to 20thNight
16%5%25%323446%50%
2160
21st to endNight
14%5%2443445%6%49%
2376
November1st to 10thNight
12%6%25%423345%54%
2255
11th to 20thNight
14%6%233245%5%6%49%
2518
21st to endNight
14%414435%5%5%7%49%
2660
December1st to 10thNight
17%7%35%5%3445%6%42%
2660
11th to 20thNight
17%6%245%3445%5%45%
3285
21st to endNight
17%6%25%5%35%5%5%7%39%
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
37%5%9%30%14%4
2710
11th to 20thDay
43%6%10%19%15%6%
2840
21st to endDay
46%6%10%22%14%2
3124
February1st to 10thDay
42%6%12%23%13%3
1710
11th to 20thDay
43%5%11%21%16%4
1637
21st to endDay
45%5%13%16%16%4
1492
March1st to 10thDay
36%5%11%23%21%4
1980
11th to 20thDay
43%6%10%17%18%6%
1980
21st to endDay
45%6%11%18%16%5%
2179
April1st to 10thDay
36%7%12%20%19%7%
2160
11th to 20thDay
36%6%11%21%20%6%
2134
21st to endDay
42%7%11%21%14%4
2160
May1st to 10thDay
41%9%14%19%14%2
2335
11th to 20thDay
44%9%12%20%12%2
2344
21st to endDay
38%8%16%25%11%1
2725
June1st to 10thDay
36%8%15%24%16%1
2520
11th to 20thDay
38%10%14%22%13%2
2520
21st to endDay
32%10%17%24%16%1
2520
July1st to 10thDay
36%12%13%21%16%2
2450
11th to 20thDay
29%12%18%25%14%2
2288
21st to endDay
38%11%13%21%17%1
2405
August1st to 10thDay
39%10%18%22%10%
2314
11th to 20thDay
39%12%14%22%13%1
2237
21st to endDay
36%8%15%24%13%3
2376
September1st to 10thDay
38%8%13%18%18%4
2160
11th to 20thDay
34%6%10%27%20%3
2144
21st to endDay
43%8%11%19%17%2
1886
October1st to 10thDay
38%6%11%26%16%4
1800
11th to 20thDay
36%6%11%24%21%3
1800
21st to endDay
34%6%9%25%22%4
1980
November1st to 10thDay
32%5%11%24%23%5%
1820
11th to 20thDay
35%6%10%21%21%6%
1658
21st to endDay
35%5%11%26%20%3
1520
December1st to 10thDay
40%7%13%25%12%3
1520
11th to 20thDay
38%5%9%31%12%5%
1906
21st to endDay
42%8%14%25%10%1
2772
January1st to 10thNight
34%6%10%26%19%4
4233
11th to 20thNight
39%410%23%20%5%
4120
21st to endNight
43%5%9%24%17%1
4532
February1st to 10thNight
42%6%12%23%15%2
2470
11th to 20thNight
43%49%21%17%6%
2313
21st to endNight
44%411%21%16%5%
1761
March1st to 10thNight
35%6%11%26%20%3
1980
11th to 20thNight
42%6%11%20%18%4
1980
21st to endNight
40%7%11%20%16%5%
2175
April1st to 10thNight
40%6%11%18%21%5%
1800
11th to 20thNight
34%7%11%21%20%6%
1775
21st to endNight
38%8%12%25%12%5%
1800
May1st to 10thNight
42%7%13%21%13%5%
1638
11th to 20thNight
43%7%13%20%14%2
1550
21st to endNight
38%9%16%23%13%1
1551
June1st to 10thNight
34%9%13%27%15%2
1440
11th to 20thNight
37%10%15%22%14%2
1440
21st to endNight
33%8%17%20%18%3
1440
July1st to 10thNight
37%11%16%18%14%4
1400
11th to 20thNight
31%11%19%22%15%2
1544
21st to endNight
32%10%18%25%14%1
1672
August1st to 10thNight
37%12%16%21%12%1
1600
11th to 20thNight
41%10%14%22%13%1
1644
21st to endNight
38%9%16%22%13%2
1980
September1st to 10thNight
38%9%12%20%17%5%
1800
11th to 20thNight
33%7%10%26%21%3
1812
21st to endNight
43%7%11%20%17%3
2074
October1st to 10thNight
40%5%11%23%17%4
2160
11th to 20thNight
36%5%9%26%21%3
2160
21st to endNight
34%6%9%23%22%4
2376
November1st to 10thNight
33%49%26%22%6%
2255
11th to 20thNight
35%5%8%23%22%7%
2518
21st to endNight
34%5%11%25%21%4
2660
December1st to 10thNight
43%5%11%23%14%4
2660
11th to 20thNight
39%5%10%23%18%4
3288
21st to endNight
40%9%16%21%13%1
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
24%22%32%18%4
234
11th to 20thDay
30%24%27%18%2
270
21st to endDay
21%24%39%12%31
297
February1st to 10thDay
22%23%37%14%4
270
11th to 20thDay
34%24%34%8%1
271
21st to endDay
25%22%29%21%3
346
March1st to 10thDay
18%21%35%22%4
420
11th to 20thDay
24%20%32%18%6%1
420
21st to endDay
22%18%34%19%7%1
462
April1st to 10thDay
21%22%25%25%6%1
420
11th to 20thDay
17%16%25%22%17%3
412
21st to endDay
23%12%30%22%11%1
420
May1st to 10thDay
21%14%28%22%13%3
423
11th to 20thDay
18%9%29%25%15%3
447
21st to endDay
21%222%24%22%9%
485
June1st to 10thDay
11%6%23%29%24%7%
450
11th to 20thDay
13%7%24%31%16%10%
450
21st to endDay
9%323%37%21%7%
450
July1st to 10thDay
12%414%30%28%12%
445
11th to 20thDay
4214%31%35%14%
444
21st to endDay
11%317%31%27%11%
488
August1st to 10thDay
12%416%27%30%12%
446
11th to 20thDay
12%315%28%26%15%
439
21st to endDay
9%315%32%23%18%
495
September1st to 10thDay
16%418%27%18%16%
450
11th to 20thDay
13%418%28%25%13%
450
21st to endDay
24%8%21%26%16%5%
450
October1st to 10thDay
20%5%19%29%19%7%
450
11th to 20thDay
16%10%24%30%16%3
450
21st to endDay
17%10%35%25%10%3
495
November1st to 10thDay
16%8%32%26%13%4
437
11th to 20thDay
18%12%33%24%11%2
318
21st to endDay
19%16%33%24%8%1
180
December1st to 10thDay
17%17%41%19%6%1
180
11th to 20thDay
21%13%36%22%9%
180
21st to endDay
23%18%31%19%9%1
198
January1st to 10thNight
20%13%41%23%3
760
11th to 20thNight
21%13%36%27%4
760
21st to endNight
18%13%42%25%2
836
February1st to 10thNight
19%11%41%26%3
760
11th to 20thNight
24%16%41%17%2
722
21st to endNight
23%14%38%23%2
592
March1st to 10thNight
16%16%40%26%3
720
11th to 20thNight
22%13%36%23%5%1
720
21st to endNight
17%13%40%24%6%
792
April1st to 10thNight
16%16%37%27%5%
720
11th to 20thNight
15%9%34%32%9%1
711
21st to endNight
19%7%31%33%8%
720
May1st to 10thNight
19%8%29%30%13%1
558
11th to 20thNight
20%329%33%13%1
488
21st to endNight
15%316%43%23%1
389
June1st to 10thNight
11%220%45%22%1
360
11th to 20thNight
13%415%43%24%1
360
21st to endNight
8%217%51%21%2
360
July1st to 10thNight
11%218%37%28%4
350
11th to 20thNight
6%19%42%37%5%
500
21st to endNight
9%215%40%30%4
591
August1st to 10thNight
9%211%43%33%2
537
11th to 20thNight
12%213%39%31%3
580
21st to endNight
12%313%39%29%5%
792
September1st to 10thNight
18%213%32%31%4
720
11th to 20thNight
11%216%36%32%4
720
21st to endNight
22%321%32%21%1
720
October1st to 10thNight
21%321%30%21%4
720
11th to 20thNight
18%7%26%34%15%1
720
21st to endNight
17%9%32%29%13%1
792
November1st to 10thNight
14%7%31%35%12%1
740
11th to 20thNight
18%7%35%30%9%1
760
21st to endNight
15%10%35%30%9%1
760
December1st to 10thNight
20%10%37%25%8%
760
11th to 20thNight
17%8%39%31%4
760
21st to endNight
20%10%33%33%4
836
Page generated at 2024-02-12 06:35:34. Version ac07

Copyright 2024 Attilla Danko


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