This page made possible by the sponsorship of: Richard Seely.

San Jose
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the San Jose clear sky chart from 2005-11-06 to 2024-01-16.

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
63%
23746
27%
74823
24%
23699
67%
23699
37%
41482

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%
756
17%
2545
18%
756
54%
756
23%
1647
January11th to 20thNight
55%
740
21%
2405
17%
740
61%
740
26%
1477
January21st to endNight
54%
792
24%
2574
24%
792
58%
792
24%
1643
February1st to 10thNight
56%
720
26%
2340
25%
720
62%
720
29%
1397
February11th to 20thNight
50%
700
25%
2171
22%
700
56%
700
27%
1336
February21st to endNight
49%
592
25%
1776
20%
592
56%
592
29%
1094
March1st to 10thNight
47%
720
17%
2030
14%
720
51%
720
21%
1422
March11th to 20thNight
50%
720
23%
1980
20%
720
54%
720
28%
1313
March21st to endNight
47%
792
18%
2136
11%
792
53%
792
29%
1397
April1st to 10thNight
50%
720
21%
1800
12%
720
54%
720
32%
1187
April11th to 20thNight
48%
711
27%
1779
13%
711
53%
711
36%
1107
April21st to endNight
55%
720
27%
1800
13%
720
59%
720
44%
1040
May1st to 10thNight
57%
684
26%
1764
15%
684
61%
684
46%
997
May11th to 20thNight
64%
474
28%
1537
23%
474
68%
474
43%
747
May21st to endNight
62%
388
26%
1552
24%
388
69%
388
36%
733
June1st to 10thNight
68%
360
28%
1440
28%
360
76%
360
42%
621
June11th to 20thNight
77%
360
35%
1440
41%
360
82%
360
56%
516
June21st to endNight
80%
360
37%
1440
46%
360
84%
360
64%
494
July1st to 10thNight
84%
350
34%
1400
43%
350
86%
350
71%
446
July11th to 20thNight
90%
500
35%
1544
34%
500
91%
500
73%
609
July21st to endNight
86%
591
37%
1674
33%
558
87%
558
65%
716
August1st to 10thNight
76%
537
32%
1602
27%
534
77%
534
55%
739
August11th to 20thNight
84%
668
38%
1736
24%
668
85%
668
74%
791
August21st to endNight
82%
792
39%
1980
25%
792
83%
792
72%
944
September1st to 10thNight
83%
720
43%
1800
31%
720
84%
720
74%
848
September11th to 20thNight
75%
720
40%
1800
30%
720
78%
720
60%
940
September21st to endNight
77%
720
40%
2036
37%
720
81%
720
66%
912
October1st to 10thNight
75%
720
30%
2160
30%
720
81%
720
54%
1017
October11th to 20thNight
69%
720
34%
2160
36%
720
73%
720
48%
1080
October21st to endNight
71%
792
36%
2376
37%
792
76%
792
49%
1191
November1st to 10thNight
67%
740
30%
2261
30%
740
70%
740
39%
1234
November11th to 20thNight
62%
760
27%
2470
26%
760
67%
760
35%
1369
November21st to endNight
64%
760
29%
2584
30%
760
69%
760
35%
1383
December1st to 10thNight
55%
760
23%
2660
21%
760
62%
760
27%
1560
December11th to 20thNight
50%
760
17%
3244
19%
759
54%
759
16%
2273
December21st to endNight
45%
827
12%
4827
16%
817
52%
817
11%
3262

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%424423346%60%
74723
Night
7%6%35%5%3345%7%54%
75087
TransparencyDay
25%5%7%18%25%20%
74442
Night
29%5%8%17%22%19%
74890
SeeingDay
7%12%24%31%20%7%
14292
Night
8%5%15%34%31%7%
23764

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
15%8%36%5%335%7%8%37%
1618
11th to 20thDay
9%5%345%345%6%7%48%
1666
21st to endDay
13%8%45%43445%8%42%
1782
February1st to 10thDay
14%6%35%434447%46%
1620
11th to 20thDay
13%5%36%445%45%8%44%
1677
21st to endDay
12%7%46%434446%47%
1480
March1st to 10thDay
13%8%37%6%45%5%6%9%35%
1927
11th to 20thDay
14%7%36%6%35%5%6%9%36%
1980
21st to endDay
11%5%36%6%36%5%5%6%43%
2218
April1st to 10thDay
11%7%36%5%3445%8%44%
2160
11th to 20thDay
10%6%24333446%55%
2129
21st to endDay
6%5%2332345%8%58%
2160
May1st to 10thDay
5%424324447%61%
2193
11th to 20thDay
5%414422337%65%
2357
21st to endDay
6%5%24423347%60%
2723
June1st to 10thDay
5%5%23323348%63%
2520
11th to 20thDay
2211112246%78%
2520
21st to endDay
131222222480%
2520
July1st to 10thDay
121221223480%
2520
11th to 20thDay
111222333479%
2359
21st to endDay
1112212345%80%
2546
August1st to 10thDay
1214312235%75%
2328
11th to 20thDay
112212235%81%
2164
21st to endDay
112212235%80%
2376
September1st to 10thDay
211221222482%
2160
11th to 20thDay
2213333347%68%
2160
21st to endDay
3212222235%77%
1919
October1st to 10thDay
3323323336%70%
1800
11th to 20thDay
4323324347%65%
1800
21st to endDay
442333445%6%62%
1980
November1st to 10thDay
7%425%5%35%447%54%
1817
11th to 20thDay
7%5%35%6%35%46%7%51%
1710
21st to endDay
9%4245%23448%53%
1592
December1st to 10thDay
8%6%26%6%45%45%8%45%
1520
11th to 20thDay
17%8%49%6%5%445%6%32%
1928
21st to endDay
12%11%48%6%45%5%6%7%32%
2794
January1st to 10thNight
13%8%46%5%345%7%7%38%
2545
11th to 20thNight
10%6%36%6%35%45%7%45%
2405
21st to endNight
13%6%35%6%345%46%44%
2574
February1st to 10thNight
11%7%35%5%34445%49%
2340
11th to 20thNight
10%8%35%5%345%6%7%44%
2169
21st to endNight
11%7%36%5%3445%6%47%
1776
March1st to 10thNight
14%9%46%5%35%5%5%6%38%
2030
11th to 20thNight
13%9%35%5%2435%8%42%
1980
21st to endNight
11%7%35%5%2445%7%46%
2136
April1st to 10thNight
11%8%445%345%6%8%44%
1800
11th to 20thNight
9%7%25%5%23446%53%
1779
21st to endNight
6%5%23424449%58%
1800
May1st to 10thNight
46%245%2435%8%57%
1764
11th to 20thNight
5%7%2442345%8%55%
1536
21st to endNight
5%436%432448%56%
1552
June1st to 10thNight
45%2332335%8%63%
1440
11th to 20thNight
1312213347%73%
1440
21st to endNight
132321223476%
1440
July1st to 10thNight
1412212235%77%
1440
11th to 20thNight
1112223337%77%
1596
21st to endNight
122332345%8%68%
1773
August1st to 10thNight
25%245%22348%62%
1611
11th to 20thNight
21332235%8%71%
1736
21st to endNight
223323347%71%
1980
September1st to 10thNight
1312322237%74%
1800
11th to 20thNight
3415%423346%66%
1800
21st to endNight
3213322236%73%
2036
October1st to 10thNight
5%322413346%68%
2160
11th to 20thNight
5%325%6%33346%61%
2160
21st to endNight
44245%24346%63%
2376
November1st to 10thNight
7%5%245%2335%8%54%
2261
11th to 20thNight
8%6%245%2346%9%51%
2470
21st to endNight
8%424434447%56%
2584
December1st to 10thNight
10%7%35%5%345%5%6%48%
2660
11th to 20thNight
18%9%5%7%6%3445%6%32%
3276
21st to endNight
12%10%47%8%45%6%6%6%31%
4862

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
45%7%10%20%12%5%
1618
11th to 20thDay
34%7%10%18%17%14%
1666
21st to endDay
42%6%9%13%21%10%
1782
February1st to 10thDay
39%5%6%19%20%11%
1620
11th to 20thDay
38%6%8%17%23%8%
1686
21st to endDay
41%5%7%20%20%7%
1480
March1st to 10thDay
46%9%8%17%16%5%
1927
11th to 20thDay
45%7%8%19%17%4
1980
21st to endDay
40%8%12%19%18%4
2218
April1st to 10thDay
39%7%11%18%19%6%
2160
11th to 20thDay
30%7%11%23%23%7%
2132
21st to endDay
24%6%11%29%24%6%
2160
May1st to 10thDay
22%5%10%25%31%7%
2193
11th to 20thDay
20%47%29%31%9%
2360
21st to endDay
24%5%11%29%25%5%
2724
June1st to 10thDay
21%5%9%25%29%11%
2520
11th to 20thDay
9%38%21%36%22%
2520
21st to endDay
9%2314%31%40%
2520
July1st to 10thDay
7%238%27%54%
2451
11th to 20thDay
8%35%10%25%49%
2288
21st to endDay
6%239%31%49%
2405
August1st to 10thDay
12%2411%29%43%
2312
11th to 20thDay
7%2311%34%43%
2164
21st to endDay
7%1214%35%40%
2376
September1st to 10thDay
8%229%31%48%
2160
11th to 20thDay
14%35%23%33%22%
2160
21st to endDay
11%3421%38%23%
1919
October1st to 10thDay
16%48%17%29%26%
1800
11th to 20thDay
18%5%9%19%23%27%
1800
21st to endDay
21%6%7%18%25%23%
1980
November1st to 10thDay
29%6%7%19%24%14%
1817
11th to 20thDay
30%7%10%19%24%11%
1710
21st to endDay
31%410%15%29%12%
1592
December1st to 10thDay
36%8%9%18%17%12%
1520
11th to 20thDay
53%6%8%13%14%6%
1928
21st to endDay
48%7%10%20%12%4
2794
January1st to 10thNight
41%8%10%20%15%6%
2545
11th to 20thNight
38%7%10%16%15%14%
2405
21st to endNight
40%7%9%11%18%14%
2574
February1st to 10thNight
37%46%17%21%14%
2340
11th to 20thNight
37%6%8%17%22%10%
2171
21st to endNight
36%6%9%25%17%7%
1776
March1st to 10thNight
47%7%10%16%15%6%
2030
11th to 20thNight
41%6%7%19%19%8%
1980
21st to endNight
37%8%11%18%19%7%
2136
April1st to 10thNight
36%7%13%19%18%7%
1800
11th to 20thNight
33%49%25%19%10%
1779
21st to endNight
23%7%9%29%24%8%
1800
May1st to 10thNight
23%6%8%21%28%13%
1764
11th to 20thNight
23%48%24%31%9%
1537
21st to endNight
24%6%11%26%27%7%
1552
June1st to 10thNight
19%5%10%23%29%15%
1440
11th to 20thNight
11%38%14%30%32%
1440
21st to endNight
11%1315%28%41%
1440
July1st to 10thNight
8%128%27%55%
1400
11th to 20thNight
8%338%29%49%
1544
21st to endNight
12%3310%27%45%
1674
August1st to 10thNight
19%1312%23%41%
1602
11th to 20thNight
10%2310%32%43%
1736
21st to endNight
11%2310%35%39%
1980
September1st to 10thNight
10%229%23%54%
1800
11th to 20thNight
17%3421%31%24%
1800
21st to endNight
12%25%18%32%30%
2036
October1st to 10thNight
17%48%17%23%31%
2160
11th to 20thNight
21%47%18%23%28%
2160
21st to endNight
21%45%18%27%25%
2376
November1st to 10thNight
27%6%8%20%21%18%
2261
11th to 20thNight
29%6%9%20%25%11%
2470
21st to endNight
29%5%8%18%23%17%
2584
December1st to 10thNight
36%6%8%17%19%14%
2660
11th to 20thNight
53%5%9%12%16%6%
3276
21st to endNight
48%11%11%14%12%5%
4862

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%19%24%19%13%1
279
11th to 20thDay
13%16%27%28%12%3
356
21st to endDay
14%14%28%23%17%4
374
February1st to 10thDay
14%19%25%23%14%5%
340
11th to 20thDay
17%19%29%25%9%2
381
21st to endDay
13%20%32%21%12%1
346
March1st to 10thDay
17%19%32%23%8%1
420
11th to 20thDay
17%20%25%23%11%4
420
21st to endDay
12%22%32%23%8%2
462
April1st to 10thDay
12%19%32%22%12%2
420
11th to 20thDay
12%20%30%23%12%2
412
21st to endDay
5%22%29%27%11%6%
420
May1st to 10thDay
8%16%31%28%13%4
423
11th to 20thDay
8%15%31%27%14%6%
448
21st to endDay
7%13%26%32%16%7%
485
June1st to 10thDay
5%12%25%30%20%8%
450
11th to 20thDay
311%28%34%18%5%
450
21st to endDay
2424%38%24%9%
450
July1st to 10thDay
2217%40%32%8%
446
11th to 20thDay
1113%42%32%12%
444
21st to endDay
13%40%31%15%
487
August1st to 10thDay
1219%39%29%9%
447
11th to 20thDay
117%39%25%16%
437
21st to endDay
114%39%32%13%
495
September1st to 10thDay
15%14%41%29%11%
450
11th to 20thDay
36%23%39%20%10%
450
21st to endDay
47%20%36%27%7%
450
October1st to 10thDay
310%23%31%24%9%
450
11th to 20thDay
5%12%19%28%28%8%
450
21st to endDay
6%10%23%32%23%6%
495
November1st to 10thDay
7%12%23%24%27%7%
413
11th to 20thDay
311%21%29%27%9%
270
21st to endDay
8%10%20%34%22%6%
214
December1st to 10thDay
7%16%22%27%20%8%
180
11th to 20thDay
9%21%21%24%17%8%
180
21st to endDay
12%25%22%24%13%4
198
January1st to 10thNight
16%8%17%30%22%6%
760
11th to 20thNight
12%7%16%35%21%8%
740
21st to endNight
17%6%15%33%24%6%
792
February1st to 10thNight
13%9%14%34%24%5%
720
11th to 20thNight
14%9%19%34%18%6%
704
21st to endNight
13%10%22%34%18%3
592
March1st to 10thNight
15%11%19%31%20%4
720
11th to 20thNight
15%8%18%31%23%5%
720
21st to endNight
12%10%22%35%19%3
792
April1st to 10thNight
12%7%23%34%21%3
720
11th to 20thNight
10%12%21%31%21%5%
711
21st to endNight
6%8%25%31%23%7%
720
May1st to 10thNight
5%426%33%26%5%
684
11th to 20thNight
6%315%36%34%6%
474
21st to endNight
45%17%37%33%4
389
June1st to 10thNight
5%416%40%29%5%
360
11th to 20thNight
1412%49%33%1
360
21st to endNight
119%41%46%3
360
July1st to 10thNight
15%37%52%4
350
11th to 20thNight
1130%55%13%
500
21st to endNight
1226%59%12%
591
August1st to 10thNight
41334%51%8%
537
11th to 20thNight
18%36%48%7%
668
21st to endNight
216%32%49%11%
792
September1st to 10thNight
118%33%45%12%
720
11th to 20thNight
4210%38%40%8%
720
21st to endNight
4310%33%40%11%
720
October1st to 10thNight
5%312%36%34%11%
720
11th to 20thNight
6%5%11%30%38%11%
720
21st to endNight
6%5%11%32%31%16%
792
November1st to 10thNight
6%5%14%32%33%9%
740
11th to 20thNight
10%316%32%31%8%
760
21st to endNight
9%416%39%25%7%
760
December1st to 10thNight
11%8%17%34%24%6%
760
11th to 20thNight
17%6%19%33%20%5%
760
21st to endNight
15%12%21%30%19%3
836
Page generated at 2024-01-12 22:38:07. Version ac07

Copyright 2024 Attilla Danko


San Jose Clear Sky Chart |Charts Home