Portola
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Portola clear sky chart from 2005-11-05 to 2018-01-20.

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
63%
15619
27%
48494
24%
15589
68%
15589
42%
24886

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
57%
777
19%
2716
18%
776
62%
776
28%
1545
January16th to endNight
55%
788
23%
2587
22%
788
60%
788
31%
1468
February1st to 15thNight
52%
697
21%
2262
20%
697
57%
697
29%
1285
February16th to endNight
52%
632
24%
1895
22%
632
59%
632
33%
1087
March1st to 15thNight
50%
720
17%
2097
15%
720
56%
720
30%
1256
March16th to endNight
45%
768
19%
2034
12%
768
51%
768
33%
1230
April1st to 15thNight
50%
719
20%
1799
11%
719
53%
719
33%
1115
April16th to endNight
57%
720
23%
1800
11%
720
61%
720
43%
1035
May1st to 15thNight
57%
520
26%
1600
19%
520
61%
520
45%
787
May16th to endNight
69%
374
20%
1496
25%
374
72%
374
36%
705
June1st to 15thNight
71%
360
25%
1440
31%
360
76%
360
44%
615
June16th to endNight
80%
360
32%
1440
43%
360
85%
360
57%
519
July1st to 15thNight
90%
340
33%
1360
45%
340
92%
340
71%
425
July16th to endNight
88%
424
41%
1460
46%
398
90%
398
72%
496
August1st to 15thNight
86%
580
36%
1653
28%
577
88%
577
67%
737
August16th to endNight
84%
764
45%
1910
28%
764
85%
764
79%
858
September1st to 15thNight
85%
720
41%
1800
28%
720
87%
720
76%
837
September16th to endNight
77%
716
44%
1931
36%
716
82%
716
67%
887
October1st to 15thNight
67%
720
31%
2160
28%
720
75%
720
52%
1026
October16th to endNight
72%
768
36%
2322
36%
768
77%
768
53%
1074
November1st to 15thNight
62%
760
25%
2470
25%
760
66%
760
37%
1308
November16th to endNight
55%
780
25%
2620
23%
780
61%
780
34%
1355
December1st to 15thNight
57%
780
22%
2730
21%
780
63%
780
29%
1522
December16th to endNight
46%
832
23%
2912
20%
832
54%
832
25%
1714

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
3423323346%60%
49042
Night
242342445%7%57%
48672
TransparencyDay
23%310%22%23%16%
48718
Night
23%412%24%21%12%
48495
SeeingDay
10%18%28%23%14%5%
6131
Night
6%8%15%38%26%4
15631

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
3437%7%45%47%8%43%
1552
16th to endDay
49%35%6%35%46%9%40%
1737
February1st to 15thDay
48%345%35%5%5%8%45%
1601
16th to endDay
310%46%5%35%5%5%8%42%
1579
March1st to 15thDay
39%45%5%35%5%7%8%39%
1860
16th to endDay
310%35%435%6%6%8%42%
2190
April1st to 15thDay
5%8%46%5%345%6%8%42%
2149
16th to endDay
5%6%33423336%56%
2160
May1st to 15thDay
3313322347%61%
2347
16th to endDay
5%6%245%24446%51%
2615
June1st to 15thDay
3314324345%62%
2520
16th to endDay
21111112481%
2520
July1st to 15thDay
111223381%
2520
16th to endDay
11111212480%
2606
August1st to 15thDay
11111222479%
2296
16th to endDay
111111386%
2292
September1st to 15thDay
11112235%79%
2160
16th to endDay
3211223476%
2027
October1st to 15thDay
2412323335%66%
1800
16th to endDay
25%2332335%6%61%
1901
November1st to 15thDay
5%4145%25%5%5%7%52%
1719
16th to endDay
27%25%425%6%5%9%48%
1667
December1st to 15thDay
38%45%5%35%5%6%10%43%
1560
16th to endDay
37%38%6%36%6%6%10%39%
1664
January1st to 15thNight
26%26%5%36%6%7%10%42%
2716
16th to endNight
37%35%5%45%5%6%7%46%
2587
February1st to 15thNight
16%25%6%35%7%7%9%43%
2251
16th to endNight
37%35%425%5%8%8%45%
1895
March1st to 15thNight
27%35%5%36%5%7%9%43%
2097
16th to endNight
27%245%26%6%6%8%47%
2034
April1st to 15thNight
37%245%36%7%7%8%42%
1799
16th to endNight
45%2232335%9%58%
1800
May1st to 15thNight
4313323446%63%
1600
16th to endNight
36%2432445%6%57%
1496
June1st to 15thNight
3213313347%64%
1440
16th to endNight
12221222378%
1440
July1st to 15thNight
1112125%81%
1440
16th to endNight
112113382%
1560
August1st to 15thNight
32111223475%
1662
16th to endNight
1111122482%
1910
September1st to 15thNight
1121223479%
1800
16th to endNight
221111222477%
1931
October1st to 15thNight
331322323469%
2160
16th to endNight
231321335%6%65%
2322
November1st to 15thNight
25%244245%5%9%52%
2470
16th to endNight
26%24425%5%5%8%53%
2620
December1st to 15thNight
36%35%435%46%10%47%
2730
16th to endNight
38%25%6%36%5%8%9%39%
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
32%8%18%27%12%1
1552
16th to endDay
36%5%14%22%14%5%
1737
February1st to 15thDay
33%5%16%26%15%2
1601
16th to endDay
36%414%29%12%2
1579
March1st to 15thDay
36%7%15%21%15%3
1860
16th to endDay
33%6%23%21%12%3
2190
April1st to 15thDay
36%5%16%24%15%1
2149
16th to endDay
28%313%29%20%3
2160
May1st to 15thDay
22%39%32%27%4
2347
16th to endDay
32%211%26%21%4
2619
June1st to 15thDay
23%28%23%26%14%
2520
16th to endDay
9%1317%40%27%
2520
July1st to 15thDay
8%129%26%51%
2371
16th to endDay
10%129%23%52%
2443
August1st to 15thDay
10%1211%26%46%
2280
16th to endDay
6%1111%36%42%
2292
September1st to 15thDay
9%2214%36%35%
2160
16th to endDay
12%15%26%36%17%
2027
October1st to 15thDay
18%27%26%32%12%
1800
16th to endDay
21%212%26%23%13%
1901
November1st to 15thDay
29%5%14%27%17%6%
1719
16th to endDay
27%6%14%29%19%2
1667
December1st to 15thDay
33%6%13%26%19%
1560
16th to endDay
37%8%15%22%15%1
1664
January1st to 15thNight
32%7%17%27%13%1
2716
16th to endNight
32%5%16%21%17%6%
2587
February1st to 15thNight
29%8%16%27%14%4
2262
16th to endNight
29%6%16%32%12%2
1895
March1st to 15thNight
32%6%18%24%14%3
2097
16th to endNight
28%8%20%26%13%2
2034
April1st to 15thNight
30%6%19%28%12%2
1799
16th to endNight
23%6%17%32%16%3
1800
May1st to 15thNight
21%313%31%23%5%
1601
16th to endNight
26%218%31%17%4
1496
June1st to 15thNight
19%413%24%24%13%
1440
16th to endNight
12%15%17%36%25%
1440
July1st to 15thNight
7%139%28%50%
1360
16th to endNight
8%1211%25%50%
1460
August1st to 15thNight
13%1313%29%39%
1653
16th to endNight
7%1115%35%38%
1910
September1st to 15thNight
9%1414%35%35%
1800
16th to endNight
12%1424%36%19%
1931
October1st to 15thNight
19%28%29%27%13%
2160
16th to endNight
17%211%29%28%10%
2322
November1st to 15thNight
25%6%14%30%16%6%
2470
16th to endNight
25%5%15%29%19%4
2620
December1st to 15thNight
29%8%14%25%19%2
2730
16th to endNight
34%7%14%24%16%2
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
20%12%17%34%13%2
90
16th to endDay
12%22%23%20%17%4
240
February1st to 15thDay
17%19%24%24%13%1
241
16th to endDay
18%21%28%21%8%
238
March1st to 15thDay
18%29%23%14%10%4
270
16th to endDay
17%29%25%17%9%
288
April1st to 15thDay
17%27%27%18%6%2
269
16th to endDay
17%24%27%19%8%1
270
May1st to 15thDay
11%25%28%21%10%2
288
16th to endDay
13%17%26%18%15%8%
330
June1st to 15thDay
10%17%32%21%12%5%
315
16th to endDay
414%35%23%13%8%
315
July1st to 15thDay
19%35%26%17%8%
305
16th to endDay
110%34%31%12%9%
327
August1st to 15thDay
211%37%23%15%9%
315
16th to endDay
313%29%25%19%9%
332
September1st to 15thDay
213%26%29%20%6%
315
16th to endDay
6%15%28%28%16%3
315
October1st to 15thDay
9%14%32%23%16%3
315
16th to endDay
10%17%24%23%19%4
317
November1st to 15thDay
14%13%16%20%24%10%
135
16th to endDay
9%14%23%32%18%1
115
December1st to 15thDay
13%21%17%23%14%10%
90
16th to endDay
7%12%22%21%29%6%
96
January1st to 15thNight
6%9%17%39%21%5%
780
16th to endNight
7%8%20%37%20%4
788
February1st to 15thNight
8%11%17%39%19%3
704
16th to endNight
12%12%16%37%17%3
632
March1st to 15thNight
8%16%18%32%20%4
720
16th to endNight
8%19%21%30%20%
768
April1st to 15thNight
8%15%19%36%19%1
719
16th to endNight
8%11%19%34%21%4
720
May1st to 15thNight
6%10%20%36%23%2
521
16th to endNight
45%12%39%34%2
375
June1st to 15thNight
6%116%41%32%2
360
16th to endNight
2111%45%36%2
360
July1st to 15thNight
1449%42%2
340
16th to endNight
441%46%5%
424
August1st to 15thNight
3443%42%4
580
16th to endNight
1110%44%36%5%
764
September1st to 15thNight
119%35%44%7%
720
16th to endNight
3312%37%32%10%
716
October1st to 15thNight
6%5%15%37%28%6%
720
16th to endNight
5%6%12%35%29%10%
768
November1st to 15thNight
6%7%17%38%23%5%
760
16th to endNight
6%12%19%36%21%4
780
December1st to 15thNight
7%11%16%40%18%5%
780
16th to endNight
11%11%21%38%15%1
832
Page generated at 2018-01-21 09:19:29. Version ac04

Copyright 2018 A. Danko


Portola Clear Sky Chart |Charts Home