Pleasant Hill Starlight Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Pleasant Hill Starlight Observatory clear sky chart from 2005-11-05 to 2017-12-04.

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
46%
15587
22%
47719
16%
15545
53%
15545
27%
29955

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
22%
719
21%
2327
10%
716
35%
716
19%
1552
January16th to endNight
23%
768
21%
2382
9%
768
34%
768
18%
1633
February1st to 15thNight
22%
704
19%
2112
8%
701
32%
701
22%
1414
February16th to endNight
21%
631
21%
1893
9%
631
31%
631
19%
1297
March1st to 15thNight
27%
720
18%
2142
10%
720
35%
720
20%
1453
March16th to endNight
35%
726
19%
2070
12%
726
40%
726
24%
1382
April1st to 15thNight
40%
539
24%
1797
15%
539
52%
539
31%
1022
April16th to endNight
45%
540
24%
1770
17%
540
55%
540
30%
1028
May1st to 15thNight
47%
540
20%
1620
13%
540
54%
540
29%
1031
May16th to endNight
64%
561
19%
1684
14%
561
67%
561
31%
1027
June1st to 15thNight
69%
540
17%
1620
13%
540
72%
540
33%
966
June16th to endNight
78%
540
21%
1513
18%
540
80%
540
42%
866
July1st to 15thNight
71%
510
15%
1448
11%
510
73%
510
30%
918
July16th to endNight
76%
564
18%
1595
17%
531
76%
531
30%
994
August1st to 15thNight
74%
539
19%
1738
19%
536
75%
536
30%
1042
August16th to endNight
71%
573
20%
1910
24%
573
72%
573
27%
1181
September1st to 15thNight
61%
624
20%
1884
19%
624
62%
624
28%
1209
September16th to endNight
63%
717
29%
1980
24%
717
67%
717
37%
1178
October1st to 15thNight
57%
720
33%
1980
26%
720
62%
720
37%
1167
October16th to endNight
48%
768
32%
2361
26%
768
57%
768
35%
1342
November1st to 15thNight
46%
760
27%
2471
23%
760
55%
760
30%
1422
November16th to endNight
38%
780
27%
2535
19%
780
51%
780
28%
1506
December1st to 15thNight
33%
736
20%
2391
13%
736
42%
736
20%
1590
December16th to endNight
26%
768
19%
2496
12%
768
36%
768
17%
1735

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
13%6%25%5%35%5%5%7%39%
48754
Night
12%6%2442445%7%44%
47914
TransparencyDay
42%6%11%19%15%3
48453
Night
38%6%11%21%18%4
47719
SeeingDay
14%7%20%23%21%12%
5612
Night
15%8%23%27%20%3
15590

By Month

Percentage Cloud Cover SE (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
21%8%46%433245%35%
1611
16th to endDay
20%5%36%413236%41%
1841
February1st to 15thDay
17%10%37%5%22235%36%
1750
16th to endDay
16%8%47%6%3435%436%
1580
March1st to 15thDay
19%10%34424435%37%
1818
16th to endDay
15%7%35%435%46%6%36%
2162
April1st to 15thDay
12%6%25%43445%6%43%
2151
16th to endDay
13%5%36%5%3445%8%40%
2189
May1st to 15thDay
15%5%35%5%3445%7%40%
2331
16th to endDay
10%6%25%7%36%6%6%7%37%
2431
June1st to 15thDay
7%5%35%6%46%6%6%9%38%
2340
16th to endDay
6%325%6%46%6%7%11%38%
2438
July1st to 15thDay
10%436%7%37%7%8%9%31%
2415
16th to endDay
7%426%7%5%8%8%7%9%32%
2465
August1st to 15thDay
6%427%6%5%6%7%7%10%35%
2208
16th to endDay
11%5%26%6%5%6%7%7%10%30%
2292
September1st to 15thDay
14%5%36%6%35%46%7%35%
2064
16th to endDay
10%425%5%25%46%7%45%
1980
October1st to 15thDay
13%424422336%50%
1980
16th to endDay
13%423322345%54%
1857
November1st to 15thDay
13%5%14423345%50%
1717
16th to endDay
15%6%23314446%46%
1755
December1st to 15thDay
20%8%26%6%3344435%
1651
16th to endDay
21%10%35%423335%36%
1728
January1st to 15thNight
18%9%35%5%13235%39%
2327
16th to endNight
17%8%36%6%3444436%
2382
February1st to 15thNight
17%9%36%423246%39%
2103
16th to endNight
17%8%35%434445%38%
1893
March1st to 15thNight
17%7%35%42445%7%37%
2142
16th to endNight
13%9%35%5%33446%41%
2070
April1st to 15thNight
12%41332435%8%49%
1797
16th to endNight
11%5%2432446%7%48%
1770
May1st to 15thNight
13%5%2342445%8%45%
1619
16th to endNight
7%31443446%9%47%
1682
June1st to 15thNight
4325%4346%7%12%44%
1620
16th to endNight
2214425%5%7%13%50%
1513
July1st to 15thNight
8%3245%45%5%6%11%41%
1536
16th to endNight
43245%36%6%8%10%41%
1703
August1st to 15thNight
5%3245%36%6%6%11%44%
1749
16th to endNight
7%5%25%5%35%5%6%10%41%
1910
September1st to 15thNight
11%425%6%35%5%5%7%43%
1884
16th to endNight
7%425%5%2345%7%51%
1980
October1st to 15thNight
12%314322346%54%
1980
16th to endNight
13%6%23312235%54%
2361
November1st to 15thNight
12%424323346%51%
2471
16th to endNight
14%7%34323346%47%
2535
December1st to 15thNight
17%8%25%435%446%38%
2391
16th to endNight
22%9%35%424345%35%
2496

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
52%36%17%17%2
1612
16th to endDay
45%39%23%15%2
1841
February1st to 15thDay
50%38%18%16%2
1752
16th to endDay
49%5%8%18%15%3
1580
March1st to 15thDay
49%48%17%14%5%
1818
16th to endDay
46%5%13%20%12%1
2163
April1st to 15thDay
39%5%10%21%19%4
2151
16th to endDay
41%5%9%22%17%3
2189
May1st to 15thDay
43%6%11%20%14%2
2331
16th to endDay
42%8%13%21%11%1
2435
June1st to 15thDay
38%9%14%22%12%1
2340
16th to endDay
35%9%15%19%16%4
2438
July1st to 15thDay
43%11%17%15%8%3
2274
16th to endDay
42%11%17%19%8%
2310
August1st to 15thDay
40%10%14%17%13%2
2195
16th to endDay
46%10%12%18%10%2
2292
September1st to 15thDay
45%8%12%17%11%4
2064
16th to endDay
35%7%11%22%20%3
1980
October1st to 15thDay
34%5%7%21%25%5%
1980
16th to endDay
32%46%19%28%8%
1857
November1st to 15thDay
36%36%19%26%7%
1717
16th to endDay
38%46%21%23%5%
1755
December1st to 15thDay
50%46%20%13%3
1651
16th to endDay
51%411%16%13%2
1728
January1st to 15thNight
48%37%19%17%3
2327
16th to endNight
49%47%19%15%3
2382
February1st to 15thNight
46%310%19%15%3
2112
16th to endNight
47%5%7%21%15%2
1893
March1st to 15thNight
45%5%9%17%16%5%
2142
16th to endNight
43%5%10%24%14%1
2070
April1st to 15thNight
33%412%22%21%4
1797
16th to endNight
33%6%11%23%21%4
1770
May1st to 15thNight
37%6%12%21%20%1
1620
16th to endNight
31%10%18%23%13%2
1684
June1st to 15thNight
29%9%16%27%14%1
1620
16th to endNight
24%9%18%25%18%4
1513
July1st to 15thNight
33%10%19%19%13%3
1448
16th to endNight
31%11%17%23%13%2
1595
August1st to 15thNight
31%10%16%22%16%2
1738
16th to endNight
35%9%16%22%13%2
1910
September1st to 15thNight
38%7%15%19%14%5%
1884
16th to endNight
30%6%10%23%23%6%
1980
October1st to 15thNight
31%48%23%23%8%
1980
16th to endNight
33%25%20%27%11%
2361
November1st to 15thNight
34%46%20%24%10%
2471
16th to endNight
37%37%19%25%6%
2535
December1st to 15thNight
46%5%8%17%18%4
2391
16th to endNight
52%38%17%15%2
2496

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
25%27%25%11%10%
90
16th to endDay
25%23%41%6%3
96
February1st to 15thDay
26%30%32%8%2
90
16th to endDay
27%18%35%17%1
81
March1st to 15thDay
23%23%37%12%3
108
16th to endDay
19%13%30%20%13%1
288
April1st to 15thDay
15%14%27%26%14%2
270
16th to endDay
16%9%25%28%14%4
270
May1st to 15thDay
15%10%27%20%14%10%
288
16th to endDay
12%322%27%17%15%
331
June1st to 15thDay
9%213%21%26%25%
315
16th to endDay
7%316%26%27%18%
413
July1st to 15thDay
12%118%27%22%17%
378
16th to endDay
9%16%16%30%35%
326
August1st to 15thDay
10%8%27%29%23%
313
16th to endDay
16%5%24%33%19%
332
September1st to 15thDay
15%27%22%32%20%
315
16th to endDay
13%212%24%35%11%
314
October1st to 15thDay
16%220%27%25%6%
315
16th to endDay
12%11%21%26%24%4
213
November1st to 15thDay
21%7%34%29%7%
135
16th to endDay
16%13%33%24%11%1
135
December1st to 15thDay
25%18%32%14%10%1
100
16th to endDay
21%23%36%14%3
96
January1st to 15thNight
24%17%32%19%5%
720
16th to endNight
21%15%35%23%2
768
February1st to 15thNight
20%15%37%22%3
704
16th to endNight
23%17%35%21%2
631
March1st to 15thNight
21%15%31%25%5%
720
16th to endNight
16%13%30%25%12%
726
April1st to 15thNight
15%10%33%27%12%1
539
16th to endNight
13%7%30%33%13%1
540
May1st to 15thNight
15%427%27%22%2
540
16th to endNight
9%216%38%27%5%
563
June1st to 15thNight
5%115%34%36%7%
540
16th to endNight
310%35%40%10%
540
July1st to 15thNight
8%110%29%40%10%
510
16th to endNight
5%6%26%44%15%
564
August1st to 15thNight
6%5%26%50%9%
539
16th to endNight
9%16%30%42%9%
573
September1st to 15thNight
12%113%27%36%8%
624
16th to endNight
8%216%30%38%3
717
October1st to 15thNight
13%5%18%30%27%4
720
16th to endNight
17%6%23%28%21%2
768
November1st to 15thNight
15%9%23%28%19%3
760
16th to endNight
18%11%28%28%12%1
780
December1st to 15thNight
22%12%28%26%8%1
736
16th to endNight
27%13%28%25%4
768
Page generated at 2017-12-05 09:19:04. Version ac04

Copyright 2017 A. Danko


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