San Jose
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the San Jose clear sky chart from 2005-11-05 to 2018-02-22.

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
65%
16056
27%
48941
24%
16020
69%
16020
38%
27004

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
59%
776
18%
2582
18%
776
62%
776
27%
1590
January16th to endNight
57%
832
23%
2704
23%
832
61%
832
25%
1701
February1st to 15thNight
58%
760
26%
2474
26%
760
64%
760
27%
1514
February16th to endNight
50%
660
24%
1979
20%
660
57%
660
26%
1277
March1st to 15thNight
53%
720
18%
2013
17%
720
56%
720
26%
1349
March16th to endNight
50%
768
20%
2083
15%
768
55%
768
29%
1360
April1st to 15thNight
48%
719
21%
1799
11%
719
53%
719
29%
1210
April16th to endNight
53%
720
24%
1800
11%
720
57%
720
40%
1075
May1st to 15thNight
63%
635
27%
1715
18%
635
67%
635
52%
884
May16th to endNight
65%
392
26%
1515
24%
392
71%
392
38%
706
June1st to 15thNight
71%
360
27%
1440
29%
360
78%
360
43%
609
June16th to endNight
85%
360
37%
1440
47%
360
88%
360
66%
486
July1st to 15thNight
91%
382
33%
1402
40%
382
92%
382
69%
488
July16th to endNight
86%
564
35%
1593
32%
531
87%
531
63%
696
August1st to 15thNight
82%
568
34%
1641
27%
565
83%
565
60%
746
August16th to endNight
87%
764
40%
1910
26%
764
87%
764
73%
909
September1st to 15thNight
87%
720
40%
1800
30%
720
88%
720
70%
875
September16th to endNight
78%
716
42%
1959
37%
716
81%
716
62%
930
October1st to 15thNight
74%
720
29%
2160
29%
720
78%
720
47%
1088
October16th to endNight
72%
768
34%
2304
36%
768
75%
768
46%
1203
November1st to 15thNight
65%
760
26%
2373
26%
760
70%
760
36%
1324
November16th to endNight
62%
780
25%
2613
26%
780
66%
780
31%
1496
December1st to 15thNight
58%
780
23%
2730
21%
780
64%
780
26%
1621
December16th to endNight
49%
832
20%
2912
18%
832
55%
832
20%
1867

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
7%423323346%59%
49311
Night
7%424323347%55%
49128
TransparencyDay
26%47%18%24%18%
48999
Night
28%47%17%22%18%
48942
SeeingDay
9%11%24%31%17%5%
6512
Night
9%412%30%32%10%
16066

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
12%5%3443246%7%43%
1685
16th to endDay
14%7%34424347%43%
1872
February1st to 15thDay
13%6%35%435%347%43%
1720
16th to endDay
15%6%37%434446%40%
1641
March1st to 15thDay
12%7%36%5%35%45%10%36%
1944
16th to endDay
12%5%35%6%25%5%5%6%39%
2139
April1st to 15thDay
14%7%35%433347%42%
2149
16th to endDay
8%5%13322347%57%
2160
May1st to 15thDay
4312222337%65%
2233
16th to endDay
7%5%24412246%59%
2596
June1st to 15thDay
5%412212247%63%
2520
16th to endDay
12111112480%
2520
July1st to 15thDay
1121222381%
2472
16th to endDay
1111123480%
2468
August1st to 15thDay
132222477%
2309
16th to endDay
1212225%79%
2292
September1st to 15thDay
11212225%78%
2160
16th to endDay
3212212235%71%
1998
October1st to 15thDay
3213323237%65%
1800
16th to endDay
5%41332445%5%59%
1920
November1st to 15thDay
8%425%424446%51%
1815
16th to endDay
10%5%345%24347%47%
1674
December1st to 15thDay
11%5%27%6%34335%44%
1560
16th to endDay
14%8%36%43345%7%39%
1664
January1st to 15thNight
12%5%2442446%7%43%
2582
16th to endNight
12%5%45%423446%45%
2704
February1st to 15thNight
10%6%25%42445%7%45%
2462
16th to endNight
13%7%25%424446%42%
1979
March1st to 15thNight
13%6%45%424446%42%
2013
16th to endNight
11%6%35%42435%9%44%
2083
April1st to 15thNight
11%6%344245%5%8%42%
1799
16th to endNight
7%423323348%57%
1800
May1st to 15thNight
2413323348%62%
1715
16th to endNight
5%424322348%57%
1514
June1st to 15thNight
3412213349%63%
1440
16th to endNight
111211113479%
1440
July1st to 15thNight
11112127%79%
1485
16th to endNight
112213347%69%
1701
August1st to 15thNight
213311249%67%
1650
16th to endNight
112322238%71%
1910
September1st to 15thNight
121112238%73%
1800
16th to endNight
2213211226%71%
1959
October1st to 15thNight
5%213313346%65%
2160
16th to endNight
5%314434346%58%
2304
November1st to 15thNight
7%42442335%8%53%
2373
16th to endNight
8%5%145%24448%50%
2613
December1st to 15thNight
10%6%344235%46%47%
2730
16th to endNight
14%7%35%42435%5%42%
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%6%10%18%15%8%
1685
16th to endDay
42%5%7%13%18%12%
1872
February1st to 15thDay
40%47%18%20%7%
1720
16th to endDay
45%5%7%21%15%3
1641
March1st to 15thDay
44%7%8%17%15%6%
1944
16th to endDay
43%7%10%18%16%3
2139
April1st to 15thDay
41%6%11%18%19%2
2149
16th to endDay
27%5%11%23%23%8%
2160
May1st to 15thDay
19%48%28%31%7%
2233
16th to endDay
26%410%26%25%6%
2600
June1st to 15thDay
21%49%22%24%17%
2520
16th to endDay
8%2416%33%33%
2520
July1st to 15thDay
8%137%25%53%
2327
16th to endDay
7%249%27%48%
2313
August1st to 15thDay
11%1311%29%42%
2293
16th to endDay
7%1314%38%35%
2292
September1st to 15thDay
9%2315%34%35%
2160
16th to endDay
15%3424%32%19%
1998
October1st to 15thDay
19%49%19%23%23%
1800
16th to endDay
23%5%8%19%24%17%
1920
November1st to 15thDay
32%6%9%22%21%7%
1815
16th to endDay
35%5%9%16%27%5%
1674
December1st to 15thDay
42%5%8%15%17%10%
1560
16th to endDay
44%5%10%14%18%6%
1664
January1st to 15thNight
37%6%11%18%17%8%
2582
16th to endNight
38%7%8%12%16%16%
2704
February1st to 15thNight
38%46%17%22%9%
2474
16th to endNight
40%5%9%23%14%5%
1979
March1st to 15thNight
42%5%9%18%15%7%
2013
16th to endNight
39%6%12%17%18%7%
2083
April1st to 15thNight
38%7%11%21%17%3
1799
16th to endNight
26%6%11%24%21%10%
1800
May1st to 15thNight
19%38%22%31%13%
1716
16th to endNight
23%310%24%29%8%
1515
June1st to 15thNight
18%49%20%22%24%
1440
16th to endNight
10%1314%29%41%
1440
July1st to 15thNight
8%115%25%56%
1402
16th to endNight
12%339%24%46%
1593
August1st to 15thNight
17%210%26%42%
1641
16th to endNight
10%129%35%38%
1910
September1st to 15thNight
11%2212%27%43%
1800
16th to endNight
15%1321%31%26%
1959
October1st to 15thNight
21%47%18%21%26%
2160
16th to endNight
23%47%19%26%19%
2304
November1st to 15thNight
29%5%10%21%23%9%
2373
16th to endNight
33%5%8%20%22%10%
2613
December1st to 15thNight
37%5%8%15%20%13%
2730
16th to endNight
43%6%10%13%18%8%
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
21%12%29%21%13%2
223
16th to endDay
17%12%27%22%16%3
304
February1st to 15thDay
16%15%24%24%15%3
283
16th to endDay
20%17%28%21%10%1
264
March1st to 15thDay
18%20%25%20%11%3
270
16th to endDay
16%20%32%20%8%1
288
April1st to 15thDay
17%27%28%17%8%
269
16th to endDay
11%27%31%20%8%1
270
May1st to 15thDay
10%16%35%26%8%2
288
16th to endDay
10%14%23%29%16%4
330
June1st to 15thDay
6%14%31%31%12%3
315
16th to endDay
6%24%41%20%7%
315
July1st to 15thDay
217%45%27%6%
305
16th to endDay
14%50%24%9%
327
August1st to 15thDay
219%39%29%8%
315
16th to endDay
116%42%25%12%
332
September1st to 15thDay
1217%45%24%8%
315
16th to endDay
6%7%19%39%21%5%
315
October1st to 15thDay
5%13%25%33%17%4
315
16th to endDay
8%9%25%32%18%5%
336
November1st to 15thDay
9%7%21%25%27%8%
231
16th to endDay
7%10%27%35%16%2
116
December1st to 15thDay
11%21%16%18%25%6%
90
16th to endDay
11%17%12%30%17%10%
96
January1st to 15thNight
16%312%31%26%9%
780
16th to endNight
16%5%12%29%24%11%
832
February1st to 15thNight
14%7%12%31%25%8%
764
16th to endNight
16%8%16%32%19%6%
660
March1st to 15thNight
15%9%15%26%26%6%
720
16th to endNight
14%8%19%29%23%4
768
April1st to 15thNight
13%10%20%29%21%3
719
16th to endNight
8%9%23%26%23%8%
720
May1st to 15thNight
4320%33%31%6%
636
16th to endNight
6%414%32%35%6%
393
June1st to 15thNight
3414%39%33%5%
360
16th to endNight
18%39%45%5%
360
July1st to 15thNight
1227%57%10%
382
16th to endNight
119%56%20%
564
August1st to 15thNight
2226%56%11%
568
16th to endNight
1326%54%13%
764
September1st to 15thNight
3428%48%15%
720
16th to endNight
327%32%39%14%
716
October1st to 15thNight
5%49%33%33%13%
720
16th to endNight
8%38%27%33%19%
768
November1st to 15thNight
9%213%31%30%12%
760
16th to endNight
10%413%32%28%10%
780
December1st to 15thNight
13%6%14%30%25%8%
780
16th to endNight
17%9%15%31%20%5%
832
Page generated at 2018-02-23 08:21:39. Version ac04

Copyright 2018 A. Danko


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