San Josef Bay
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the San Josef Bay clear sky chart from 2005-11-05 to 2017-09-20.

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
22%
14523
5%
46822
3%
14488
24%
14488
7%
39631

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
16%
750
7%
2686
5%
746
20%
746
4%
2363
January16th to endNight
16%
800
5%
2748
4%
800
17%
800
4%
2423
February1st to 15thNight
14%
720
5%
2387
4%
717
15%
717
4%
2089
February16th to endNight
25%
632
10%
1993
8%
632
27%
632
9%
1611
March1st to 15thNight
13%
720
2%
2100
1%
720
13%
720
4%
1881
March16th to endNight
17%
768
3%
2112
2%
768
17%
768
5%
1898
April1st to 15thNight
17%
539
3%
1693
2%
539
18%
539
5%
1487
April16th to endNight
17%
540
5%
1620
4%
540
20%
540
6%
1395
May1st to 15thNight
27%
443
5%
1437
4%
443
28%
443
9%
1165
May16th to endNight
31%
375
3%
1309
2%
374
32%
374
7%
1071
June1st to 15thNight
30%
360
0%
1260
0%
360
30%
360
9%
974
June16th to endNight
23%
360
0%
1260
0%
360
23%
360
6%
1061
July1st to 15thNight
36%
340
1%
1190
1%
340
36%
340
11%
913
July16th to endNight
36%
407
3%
1268
2%
383
39%
383
13%
964
August1st to 15thNight
46%
538
5%
1499
4%
535
46%
535
17%
1123
August16th to endNight
30%
573
5%
1719
5%
573
32%
573
9%
1402
September1st to 15thNight
45%
627
10%
1810
8%
627
46%
627
19%
1300
September16th to endNight
26%
675
7%
1868
4%
675
28%
675
9%
1588
October1st to 15thNight
25%
660
3%
1934
3%
660
25%
660
9%
1597
October16th to endNight
18%
704
3%
2288
3%
704
19%
704
4%
2027
November1st to 15thNight
12%
700
4%
2409
3%
700
15%
700
3%
2190
November16th to endNight
15%
742
3%
2652
2%
742
17%
742
5%
2272
December1st to 15thNight
17%
750
6%
2700
4%
750
20%
750
5%
2312
December16th to endNight
12%
800
4%
2880
3%
800
15%
800
4%
2525

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
35%12%46%5%24345%14%
48052
Night
37%12%36%5%2333416%
46969
TransparencyDay
75%5%9%7%2
47703
Night
72%5%9%8%3
46822
SeeingDay
38%16%17%17%8%2
6770
Night
39%19%9%15%12%3
14525

By Month

Percentage Cloud Cover NW (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
51%15%3431222111%
1253
16th to endDay
46%12%36%42223311%
1473
February1st to 15thDay
48%13%3441212312%
1476
16th to endDay
32%15%47%42322320%
1479
March1st to 15thDay
45%15%46%4232229%
1860
16th to endDay
39%15%48%6%2322310%
2112
April1st to 15thDay
39%15%47%6%243239%
2251
16th to endDay
37%14%46%5%3433412%
2340
May1st to 15thDay
33%11%47%5%24445%15%
2511
16th to endDay
32%13%46%5%34345%15%
2804
June1st to 15thDay
32%13%5%7%5%2445%5%13%
2700
16th to endDay
39%11%47%5%35%43410%
2700
July1st to 15thDay
31%11%45%43345%6%18%
2700
16th to endDay
27%13%46%4245%5%7%17%
2813
August1st to 15thDay
18%11%5%7%5%35%5%6%8%22%
2452
16th to endDay
32%11%47%6%445%5%5%12%
2483
September1st to 15thDay
23%9%36%5%3445%8%25%
2148
16th to endDay
31%10%45%5%346%47%16%
1861
October1st to 15thDay
35%11%46%424345%15%
1695
16th to endDay
40%11%36%7%444349%
1584
November1st to 15thDay
50%11%46%5%132338%
1448
16th to endDay
41%14%5%7%42222413%
1305
December1st to 15thDay
38%12%46%6%2343314%
1260
16th to endDay
41%16%45%5%242449%
1344
January1st to 15thNight
46%14%45%31222213%
2686
16th to endNight
45%13%35%5%2322212%
2748
February1st to 15thNight
46%11%35%42332313%
2375
16th to endNight
32%15%45%5%2322422%
1993
March1st to 15thNight
43%15%37%43222211%
2100
16th to endNight
41%16%46%42222211%
2108
April1st to 15thNight
37%16%46%5%2234312%
1693
16th to endNight
35%15%5%8%5%2332315%
1620
May1st to 15thNight
30%12%35%5%34336%19%
1436
16th to endNight
30%11%5%6%5%35%5%45%17%
1309
June1st to 15thNight
30%10%25%425%45%7%22%
1260
16th to endNight
36%13%36%41433416%
1260
July1st to 15thNight
28%10%35%424345%26%
1260
16th to endNight
25%12%25%434346%26%
1354
August1st to 15thNight
19%10%47%4345%5%8%27%
1507
16th to endNight
28%11%37%6%34346%20%
1719
September1st to 15thNight
17%7%37%5%2345%8%32%
1810
16th to endNight
34%9%36%5%3445%5%16%
1868
October1st to 15thNight
35%9%35%5%34335%19%
1934
16th to endNight
42%9%47%5%3433411%
2288
November1st to 15thNight
50%12%36%5%122239%
2409
16th to endNight
42%12%45%41322414%
2652
December1st to 15thNight
38%12%46%5%2333416%
2700
16th to endNight
41%15%36%42332312%
2880

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
82%2345%1
1253
16th to endDay
80%35%5%22
1473
February1st to 15thDay
80%26%5%31
1476
16th to endDay
72%46%10%6%
1479
March1st to 15thDay
84%35%41
1860
16th to endDay
82%37%5%
2112
April1st to 15thDay
82%35%5%1
2251
16th to endDay
79%36%6%2
2340
May1st to 15thDay
73%5%11%7%2
2511
16th to endDay
73%5%10%8%2
2808
June1st to 15thDay
76%5%10%41
2700
16th to endDay
80%5%9%4
2700
July1st to 15thDay
68%6%10%10%3
2541
16th to endDay
66%7%16%8%1
2636
August1st to 15thDay
61%8%14%11%3
2435
16th to endDay
75%6%10%6%
2483
September1st to 15thDay
57%6%16%12%5%
2148
16th to endDay
71%5%10%9%2
1861
October1st to 15thDay
73%5%8%8%3
1695
16th to endDay
80%6%5%42
1584
November1st to 15thDay
83%36%32
1448
16th to endDay
78%36%6%4
1305
December1st to 15thDay
76%5%6%6%31
1260
16th to endDay
79%5%5%6%21
1344
January1st to 15thNight
79%346%41
2686
16th to endNight
79%36%5%22
2748
February1st to 15thNight
78%35%6%41
2387
16th to endNight
68%38%13%5%
1993
March1st to 15thNight
81%38%5%1
2100
16th to endNight
80%28%5%1
2112
April1st to 15thNight
77%47%7%2
1693
16th to endNight
75%36%9%31
1620
May1st to 15thNight
65%7%12%10%4
1437
16th to endNight
66%7%13%9%2
1309
June1st to 15thNight
60%8%17%10%21
1260
16th to endNight
71%6%11%10%
1260
July1st to 15thNight
59%7%12%13%6%
1190
16th to endNight
56%8%20%12%2
1268
August1st to 15thNight
50%10%21%12%31
1499
16th to endNight
65%7%14%11%
1719
September1st to 15thNight
48%7%18%16%7%
1810
16th to endNight
68%7%9%10%2
1868
October1st to 15thNight
69%412%11%3
1934
16th to endNight
77%5%8%5%1
2288
November1st to 15thNight
83%345%21
2409
16th to endNight
75%46%8%41
2652
December1st to 15thNight
74%47%6%6%
2700
16th to endNight
78%45%6%31
2880

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
36%8%20%16%15%3
60
16th to endDay
54%20%6%8%8%1
193
February1st to 15thDay
51%19%10%11%6%1
236
16th to endDay
36%25%13%12%9%1
211
March1st to 15thDay
56%22%12%6%2
250
16th to endDay
52%17%12%11%41
288
April1st to 15thDay
48%19%13%11%5%1
269
16th to endDay
43%22%16%11%4
270
May1st to 15thDay
36%18%20%15%8%1
372
16th to endDay
31%17%21%20%6%1
518
June1st to 15thDay
37%18%17%17%7%1
495
16th to endDay
35%19%19%19%5%1
495
July1st to 15thDay
33%12%20%21%9%2
475
16th to endDay
27%15%19%24%11%2
515
August1st to 15thDay
21%8%16%34%14%4
426
16th to endDay
37%9%22%19%9%
332
September1st to 15thDay
31%13%18%17%16%4
315
16th to endDay
38%12%14%20%11%2
272
October1st to 15thDay
43%14%14%14%7%3
254
16th to endDay
47%15%19%11%6%
240
November1st to 15thDay
53%18%12%11%33
100
16th to endDay
36%16%8%15%13%10%
60
December1st to 15thDay
46%6%11%10%15%10%
60
16th to endDay
45%6%17%23%6%1
64
January1st to 15thNight
48%21%7%10%10%2
750
16th to endNight
47%22%8%9%11%2
800
February1st to 15thNight
48%21%9%12%6%1
721
16th to endNight
39%21%8%12%13%4
632
March1st to 15thNight
47%23%9%10%7%1
720
16th to endNight
43%22%9%12%9%2
768
April1st to 15thNight
39%21%11%12%11%3
539
16th to endNight
36%22%14%15%7%2
540
May1st to 15thNight
33%19%8%17%18%3
444
16th to endNight
27%20%9%21%17%4
375
June1st to 15thNight
28%19%10%15%20%5%
360
16th to endNight
37%20%8%17%14%2
360
July1st to 15thNight
25%13%8%28%18%4
340
16th to endNight
20%18%9%18%23%8%
407
August1st to 15thNight
16%10%5%21%31%14%
538
16th to endNight
32%11%8%21%20%4
573
September1st to 15thNight
20%9%10%21%26%10%
627
16th to endNight
38%13%8%19%14%4
675
October1st to 15thNight
40%13%13%17%11%3
660
16th to endNight
48%14%9%13%10%3
704
November1st to 15thNight
49%19%11%15%4
700
16th to endNight
45%23%9%10%9%1
742
December1st to 15thNight
38%23%11%13%9%2
750
16th to endNight
45%22%14%12%5%
800
Page generated at 2017-09-21 04:18:22. Version ac04

Copyright 2017 A. Danko


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