Mountain View
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mountain View clear sky chart from 2005-11-05 to 2017-12-20.

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
68%
15798
27%
48097
25%
15762
71%
15762
40%
26017

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
62%
716
19%
2374
21%
716
65%
716
29%
1418
January16th to endNight
60%
768
24%
2496
25%
768
63%
768
27%
1544
February1st to 15thNight
58%
700
23%
2286
23%
700
63%
700
27%
1400
February16th to endNight
52%
632
23%
1895
19%
632
58%
632
28%
1199
March1st to 15thNight
56%
720
19%
2004
17%
720
59%
720
29%
1303
March16th to endNight
54%
768
20%
2089
14%
768
58%
768
31%
1345
April1st to 15thNight
52%
719
22%
1799
12%
719
55%
719
34%
1166
April16th to endNight
60%
720
25%
1800
13%
720
64%
720
46%
1034
May1st to 15thNight
69%
623
27%
1703
19%
623
72%
623
54%
867
May16th to endNight
70%
402
26%
1525
23%
402
76%
402
40%
703
June1st to 15thNight
73%
360
27%
1440
29%
360
81%
360
46%
597
June16th to endNight
85%
360
37%
1440
47%
360
88%
360
66%
484
July1st to 15thNight
93%
392
34%
1412
39%
392
94%
392
73%
482
July16th to endNight
87%
564
36%
1593
33%
531
88%
531
64%
684
August1st to 15thNight
86%
559
34%
1632
27%
556
86%
556
60%
729
August16th to endNight
86%
764
39%
1910
26%
764
87%
764
69%
925
September1st to 15thNight
87%
720
40%
1800
29%
720
88%
720
70%
862
September16th to endNight
81%
716
41%
1964
38%
716
84%
716
63%
923
October1st to 15thNight
75%
720
29%
2160
30%
720
79%
720
51%
1055
October16th to endNight
72%
768
35%
2304
37%
768
76%
768
43%
1218
November1st to 15thNight
68%
760
26%
2364
27%
760
71%
760
38%
1306
November16th to endNight
65%
780
25%
2623
27%
780
69%
780
33%
1458
December1st to 15thNight
60%
780
23%
2730
22%
780
66%
780
27%
1596
December16th to endNight
49%
787
20%
2754
17%
787
56%
787
23%
1719

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%423323346%59%
48736
Night
6%424323347%56%
48285
TransparencyDay
26%38%19%24%17%
48425
Night
27%39%19%22%16%
48098
SeeingDay
7%9%23%33%19%6%
6342
Night
8%312%34%33%8%
15808

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
11%5%2443346%6%47%
1563
16th to endDay
13%7%23434347%44%
1728
February1st to 15thDay
12%7%35%5%3445%5%41%
1576
16th to endDay
14%7%26%5%34347%39%
1579
March1st to 15thDay
10%5%36%6%36%46%10%36%
1953
16th to endDay
9%426%7%35%45%8%41%
2133
April1st to 15thDay
12%6%35%42345%8%44%
2149
16th to endDay
7%413313347%59%
2160
May1st to 15thDay
3313212348%65%
2244
16th to endDay
6%424313236%60%
2584
June1st to 15thDay
4422213346%64%
2520
16th to endDay
11112125%79%
2520
July1st to 15thDay
111112225%78%
2460
16th to endDay
11112226%77%
2468
August1st to 15thDay
32212235%74%
2316
16th to endDay
12212337%74%
2292
September1st to 15thDay
111212225%75%
2160
16th to endDay
3212211235%72%
1995
October1st to 15thDay
4323313346%64%
1800
16th to endDay
5%41333435%5%58%
1920
November1st to 15thDay
7%5%343245%48%51%
1824
16th to endDay
9%5%345%34447%48%
1664
December1st to 15thDay
10%5%36%5%4445%6%42%
1560
16th to endDay
14%6%37%42435%6%40%
1568
January1st to 15thNight
11%6%2443446%8%43%
2374
16th to endNight
12%5%245%34446%45%
2496
February1st to 15thNight
9%6%35%5%25%5%47%43%
2274
16th to endNight
12%6%35%5%3335%7%42%
1895
March1st to 15thNight
11%5%35%5%34446%44%
2004
16th to endNight
9%6%35%32445%6%47%
2089
April1st to 15thNight
8%5%244245%6%8%45%
1799
16th to endNight
5%313224347%60%
1800
May1st to 15thNight
222331235%8%62%
1703
16th to endNight
342432235%8%60%
1524
June1st to 15thNight
2213321248%67%
1440
16th to endNight
221312480%
1440
July1st to 15thNight
11112237%78%
1496
16th to endNight
112213348%68%
1701
August1st to 15thNight
1212322338%68%
1641
16th to endNight
22213348%69%
1910
September1st to 15thNight
111212337%73%
1800
16th to endNight
2212212246%71%
1964
October1st to 15thNight
4312312346%65%
2160
16th to endNight
4423423346%59%
2304
November1st to 15thNight
6%43431335%8%54%
2364
16th to endNight
7%5%24434447%51%
2623
December1st to 15thNight
9%5%25%435%448%46%
2730
16th to endNight
14%5%35%42345%5%44%
2754

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
37%412%20%15%9%
1563
16th to endDay
41%5%8%13%19%12%
1728
February1st to 15thDay
44%37%19%20%4
1576
16th to endDay
45%48%23%13%3
1579
March1st to 15thDay
44%410%20%14%5%
1953
16th to endDay
42%5%13%19%16%2
2133
April1st to 15thDay
40%413%19%19%2
2149
16th to endDay
27%312%26%22%7%
2160
May1st to 15thDay
19%38%29%30%8%
2244
16th to endDay
26%310%28%25%5%
2588
June1st to 15thDay
22%310%22%24%16%
2520
16th to endDay
10%1417%35%30%
2520
July1st to 15thDay
9%1211%25%50%
2316
16th to endDay
9%239%30%43%
2313
August1st to 15thDay
12%1315%31%36%
2300
16th to endDay
9%1217%39%29%
2292
September1st to 15thDay
10%1416%33%32%
2160
16th to endDay
15%2325%33%18%
1995
October1st to 15thDay
21%38%20%23%22%
1800
16th to endDay
24%310%22%23%15%
1920
November1st to 15thDay
32%5%10%26%18%6%
1824
16th to endDay
37%39%19%25%4
1664
December1st to 15thDay
41%49%16%17%9%
1560
16th to endDay
43%413%15%17%4
1568
January1st to 15thNight
38%311%21%16%8%
2374
16th to endNight
39%5%8%13%16%15%
2496
February1st to 15thNight
40%39%20%19%7%
2286
16th to endNight
41%312%26%13%2
1895
March1st to 15thNight
41%413%19%15%5%
2004
16th to endNight
38%5%13%19%17%5%
2089
April1st to 15thNight
35%5%15%23%17%3
1799
16th to endNight
25%413%26%22%8%
1800
May1st to 15thNight
19%310%24%29%12%
1704
16th to endNight
23%313%27%25%6%
1525
June1st to 15thNight
17%411%22%25%18%
1440
16th to endNight
9%1314%34%36%
1440
July1st to 15thNight
7%129%23%55%
1412
16th to endNight
12%2411%24%44%
1593
August1st to 15thNight
14%3311%28%38%
1632
16th to endNight
12%2212%33%36%
1910
September1st to 15thNight
11%2313%30%37%
1800
16th to endNight
15%1423%32%23%
1964
October1st to 15thNight
20%28%21%22%23%
2160
16th to endNight
25%36%21%26%16%
2304
November1st to 15thNight
29%412%26%20%7%
2364
16th to endNight
32%410%24%21%8%
2623
December1st to 15thNight
36%5%9%16%20%11%
2730
16th to endNight
41%5%12%15%18%6%
2754

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%8%26%28%16%2
191
16th to endDay
15%10%24%26%18%4
256
February1st to 15thDay
12%15%26%27%15%2
236
16th to endDay
18%12%28%26%11%2
238
March1st to 15thDay
14%17%29%21%13%3
270
16th to endDay
11%18%34%26%8%1
288
April1st to 15thDay
15%20%33%19%9%2
269
16th to endDay
7%23%32%23%10%2
270
May1st to 15thDay
6%13%35%29%11%2
288
16th to endDay
10%11%23%30%19%4
330
June1st to 15thDay
412%32%33%14%4
315
16th to endDay
1321%43%21%8%
315
July1st to 15thDay
2113%46%27%8%
305
16th to endDay
11%47%28%10%
327
August1st to 15thDay
117%40%29%10%
315
16th to endDay
114%45%26%12%
332
September1st to 15thDay
115%43%27%11%
315
16th to endDay
5%5%20%33%24%10%
315
October1st to 15thDay
48%24%35%20%5%
315
16th to endDay
7%6%24%32%20%8%
336
November1st to 15thDay
10%422%26%26%9%
240
16th to endDay
8%9%19%32%27%2
114
December1st to 15thDay
7%11%24%24%23%8%
90
16th to endDay
9%13%16%27%22%9%
72
January1st to 15thNight
13%49%36%28%7%
720
16th to endNight
15%411%33%23%11%
768
February1st to 15thNight
11%7%13%35%26%6%
704
16th to endNight
15%6%17%33%21%4
632
March1st to 15thNight
13%6%15%31%27%5%
720
16th to endNight
12%420%36%23%2
768
April1st to 15thNight
11%8%20%34%21%3
719
16th to endNight
7%5%21%35%23%6%
720
May1st to 15thNight
3118%40%29%6%
624
16th to endNight
4213%37%35%5%
403
June1st to 15thNight
3115%43%32%2
360
16th to endNight
19%43%42%2
360
July1st to 15thNight
329%58%7%
392
16th to endNight
1224%57%13%
564
August1st to 15thNight
2430%54%8%
559
16th to endNight
1433%49%9%
764
September1st to 15thNight
25%31%47%12%
720
16th to endNight
38%34%39%12%
716
October1st to 15thNight
5%211%35%35%10%
720
16th to endNight
6%38%30%34%16%
768
November1st to 15thNight
8%112%36%32%9%
760
16th to endNight
9%313%36%28%9%
780
December1st to 15thNight
10%415%35%26%7%
780
16th to endNight
15%6%19%33%20%4
787
Page generated at 2017-12-21 04:04:55. Version ac04

Copyright 2017 A. Danko


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