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 2017-05-01.

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%
15026
27%
45698
24%
14990
68%
14990
38%
25407

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
60%
716
18%
2383
19%
716
63%
716
28%
1435
January16th to endNight
57%
768
24%
2496
25%
768
60%
768
26%
1565
February1st to 15thNight
56%
700
24%
2280
23%
700
61%
700
25%
1441
February16th to endNight
49%
632
22%
1895
18%
632
55%
632
25%
1235
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
64%
586
29%
1582
18%
586
68%
586
52%
815
May16th to endNight
64%
359
24%
1386
20%
359
71%
359
38%
644
June1st to 15thNight
71%
330
29%
1320
30%
330
79%
330
43%
559
June16th to endNight
84%
330
35%
1320
44%
330
88%
330
64%
451
July1st to 15thNight
90%
348
35%
1278
42%
348
91%
348
67%
453
July16th to endNight
86%
522
33%
1467
30%
489
87%
489
61%
648
August1st to 15thNight
82%
520
36%
1503
29%
517
83%
517
60%
679
August16th to endNight
86%
700
38%
1750
25%
700
87%
700
72%
839
September1st to 15thNight
87%
660
42%
1650
31%
660
88%
660
72%
792
September16th to endNight
78%
656
39%
1795
35%
656
80%
656
60%
867
October1st to 15thNight
73%
660
29%
1980
29%
660
77%
660
44%
1027
October16th to endNight
70%
704
32%
2112
33%
704
74%
704
44%
1123
November1st to 15thNight
65%
700
28%
2186
27%
700
70%
700
38%
1201
November16th to endNight
63%
720
25%
2412
26%
720
67%
720
32%
1363
December1st to 15thNight
56%
720
22%
2520
19%
720
63%
720
24%
1549
December16th to endNight
48%
768
19%
2688
17%
768
54%
768
20%
1727

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%424323346%59%
46064
Night
7%424323347%55%
45885
TransparencyDay
26%47%18%24%17%
45752
Night
28%48%17%22%18%
45699
SeeingDay
10%12%25%32%15%3
5471
Night
9%412%30%32%10%
15036

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%3443236%7%46%
1555
16th to endDay
15%7%35%423346%44%
1728
February1st to 15thDay
15%6%35%435%346%41%
1584
16th to endDay
16%6%37%434446%39%
1579
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
4313322337%64%
2048
16th to endDay
7%5%24322246%58%
2374
June1st to 15thDay
5%412312247%63%
2310
16th to endDay
12111123479%
2310
July1st to 15thDay
11121222380%
2266
16th to endDay
1121223479%
2286
August1st to 15thDay
132122478%
2117
16th to endDay
1212235%79%
2100
September1st to 15thDay
11212225%79%
1980
16th to endDay
3212212335%72%
1832
October1st to 15thDay
4323323337%63%
1650
16th to endDay
5%41332445%5%59%
1760
November1st to 15thDay
7%425%424447%51%
1672
16th to endDay
10%5%346%24346%48%
1545
December1st to 15thDay
12%6%38%6%34335%42%
1440
16th to endDay
15%8%36%433347%38%
1536
January1st to 15thNight
11%5%2442445%7%45%
2383
16th to endNight
13%5%35%423446%46%
2496
February1st to 15thNight
11%6%35%5%245%5%7%43%
2268
16th to endNight
14%7%36%424446%41%
1895
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
3413213249%62%
1582
16th to endNight
4424322349%57%
1385
June1st to 15thNight
3323213349%63%
1320
16th to endNight
1112112135%78%
1320
July1st to 15thNight
21112137%77%
1361
16th to endNight
11221345%8%67%
1575
August1st to 15thNight
213311249%67%
1512
16th to endNight
112322337%71%
1750
September1st to 15thNight
121112138%74%
1650
16th to endNight
2313212226%70%
1795
October1st to 15thNight
5%313313346%62%
1980
16th to endNight
5%3145%34346%57%
2112
November1st to 15thNight
7%42442335%8%54%
2186
16th to endNight
8%5%145%23348%51%
2412
December1st to 15thNight
10%7%45%4335%46%44%
2520
16th to endNight
15%7%35%42435%5%42%
2688

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%6%10%19%16%9%
1555
16th to endDay
43%46%13%18%13%
1728
February1st to 15thDay
43%47%17%20%5%
1584
16th to endDay
46%6%7%21%14%3
1579
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
20%47%27%31%8%
2048
16th to endDay
26%411%26%24%7%
2378
June1st to 15thDay
21%49%22%23%18%
2310
16th to endDay
9%25%17%35%30%
2310
July1st to 15thDay
8%138%28%49%
2121
16th to endDay
8%249%30%44%
2131
August1st to 15thDay
10%1312%29%43%
2101
16th to endDay
7%1314%39%34%
2100
September1st to 15thDay
8%1214%35%37%
1980
16th to endDay
14%2424%31%21%
1832
October1st to 15thDay
21%49%20%24%19%
1650
16th to endDay
24%5%8%20%25%15%
1760
November1st to 15thDay
32%6%8%22%22%8%
1672
16th to endDay
35%5%9%16%27%5%
1545
December1st to 15thDay
44%5%9%15%18%6%
1440
16th to endDay
45%5%11%14%16%6%
1536
January1st to 15thNight
36%6%11%18%18%9%
2383
16th to endNight
38%7%7%12%16%17%
2496
February1st to 15thNight
41%5%7%17%19%8%
2280
16th to endNight
41%5%10%23%14%4
1895
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%48%21%31%14%
1583
16th to endNight
23%411%23%28%8%
1386
June1st to 15thNight
18%5%10%20%20%25%
1320
16th to endNight
10%1314%29%39%
1320
July1st to 15thNight
9%216%27%53%
1278
16th to endNight
12%4310%26%42%
1467
August1st to 15thNight
16%210%26%42%
1503
16th to endNight
11%1210%36%36%
1750
September1st to 15thNight
11%1210%28%45%
1650
16th to endNight
16%2319%30%27%
1795
October1st to 15thNight
23%48%19%22%21%
1980
16th to endNight
24%47%20%26%16%
2112
November1st to 15thNight
28%5%10%21%23%10%
2186
16th to endNight
32%5%8%20%22%11%
2412
December1st to 15thNight
40%5%8%15%20%8%
2520
16th to endNight
43%6%11%13%17%8%
2688

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
20%14%28%22%10%1
183
16th to endDay
19%12%28%21%14%3
256
February1st to 15thDay
19%17%24%23%14%1
237
16th to endDay
21%17%28%21%9%1
238
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
12%13%38%26%6%2
229
16th to endDay
11%16%25%29%14%3
266
June1st to 15thDay
5%12%35%32%11%2
255
16th to endDay
7%29%40%16%5%
255
July1st to 15thDay
2120%47%22%4
245
16th to endDay
113%53%23%7%
271
August1st to 15thDay
222%41%26%5%
255
16th to endDay
220%47%22%7%
268
September1st to 15thDay
1318%47%21%9%
255
16th to endDay
5%7%18%40%21%5%
255
October1st to 15thDay
6%12%25%36%15%3
255
16th to endDay
10%9%29%34%14%
272
November1st to 15thDay
9%8%19%23%29%8%
178
16th to endDay
6%14%32%32%10%3
77
December1st to 15thDay
15%26%23%21%13%
60
16th to endDay
14%18%12%34%12%7%
64
January1st to 15thNight
15%312%30%26%10%
720
16th to endNight
17%5%12%29%23%12%
768
February1st to 15thNight
15%8%12%31%24%8%
704
16th to endNight
17%8%16%32%18%6%
632
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
3320%33%31%7%
587
16th to endNight
6%415%32%34%6%
360
June1st to 15thNight
3314%40%32%5%
330
16th to endNight
18%41%43%4
330
July1st to 15thNight
1329%54%10%
348
16th to endNight
118%57%20%
522
August1st to 15thNight
1228%55%11%
520
16th to endNight
1427%54%12%
700
September1st to 15thNight
3425%50%15%
660
16th to endNight
417%31%39%15%
656
October1st to 15thNight
6%49%31%33%14%
660
16th to endNight
8%38%29%30%18%
704
November1st to 15thNight
9%314%29%30%13%
700
16th to endNight
10%413%32%28%10%
720
December1st to 15thNight
14%6%15%30%24%7%
720
16th to endNight
18%9%15%31%19%5%
768
Page generated at 2017-05-02 05:43:28. Version ac04

Copyright 2017 A. Danko


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