Peralta Skies Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Peralta Skies Observatory clear sky chart from 2005-11-05 to 2017-10-31.

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
77%
15780
37%
47421
33%
15741
81%
15741
55%
22462

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
67%
719
32%
2327
31%
716
73%
716
45%
1130
January16th to endNight
65%
768
36%
2496
33%
768
75%
768
41%
1271
February1st to 15thNight
68%
698
35%
2199
32%
698
77%
698
50%
1044
February16th to endNight
68%
632
38%
1865
37%
632
73%
632
46%
980
March1st to 15thNight
70%
720
40%
1980
33%
720
77%
720
57%
977
March16th to endNight
78%
768
39%
2112
32%
768
83%
768
62%
995
April1st to 15thNight
74%
695
34%
1954
26%
695
79%
695
56%
958
April16th to endNight
77%
540
45%
1728
40%
540
84%
540
70%
669
May1st to 15thNight
79%
539
41%
1619
34%
539
84%
539
69%
674
May16th to endNight
83%
561
41%
1684
35%
561
86%
561
73%
676
June1st to 15thNight
89%
540
35%
1620
31%
540
89%
540
69%
674
June16th to endNight
91%
540
35%
1620
31%
540
91%
540
73%
656
July1st to 15thNight
82%
510
24%
1530
20%
510
82%
510
47%
786
July16th to endNight
81%
564
26%
1593
22%
531
82%
531
40%
889
August1st to 15thNight
83%
539
30%
1611
27%
536
83%
536
46%
833
August16th to endNight
84%
615
33%
1860
32%
615
84%
615
43%
1003
September1st to 15thNight
80%
720
34%
1980
30%
720
81%
720
51%
1042
September16th to endNight
83%
716
44%
1980
35%
716
87%
716
65%
914
October1st to 15thNight
78%
720
41%
2077
34%
720
83%
720
66%
922
October16th to endNight
85%
768
47%
2304
42%
768
89%
768
69%
972
November1st to 15thNight
79%
700
42%
2106
37%
700
84%
700
64%
919
November16th to endNight
73%
720
40%
2340
39%
720
79%
720
54%
1047
December1st to 15thNight
71%
720
38%
2340
40%
720
75%
720
47%
1125
December16th to endNight
67%
768
36%
2496
36%
768
72%
768
40%
1306

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
3212213346%68%
48326
Night
4212213346%67%
47617
TransparencyDay
18%1312%25%38%
48021
Night
19%1311%24%39%
47423
SeeingDay
5%317%34%28%10%
5580
Night
5%19%39%37%6%
15790

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
6%32321324465%
1611
16th to endDay
8%312323245%63%
1728
February1st to 15thDay
4313212346%65%
1661
16th to endDay
8%312323336%60%
1608
March1st to 15thDay
5%213222345%66%
1980
16th to endDay
3212312245%68%
2112
April1st to 15thDay
222222337%69%
1994
16th to endDay
21111222480%
2229
May1st to 15thDay
11212236%77%
2331
16th to endDay
11111222382%
2429
June1st to 15thDay
11111122481%
2340
16th to endDay
11111122481%
2340
July1st to 15thDay
22234245%6%10%56%
2340
16th to endDay
5%215%5%36%7%7%12%41%
2467
August1st to 15thDay
3213335%5%7%9%53%
2340
16th to endDay
32343446%9%56%
2339
September1st to 15thDay
421221345%6%63%
1980
16th to endDay
2111223479%
1980
October1st to 15thDay
21121234475%
1881
16th to endDay
112211235%78%
1918
November1st to 15thDay
1211112236%74%
1750
16th to endDay
421321222471%
1620
December1st to 15thDay
8%312223237%61%
1620
16th to endDay
9%31331335%6%58%
1728
January1st to 15thNight
6%213323335%63%
2327
16th to endNight
10%31321333461%
2496
February1st to 15thNight
4313212335%66%
2188
16th to endNight
7%313323346%59%
1865
March1st to 15thNight
5%212212245%69%
1980
16th to endNight
3111212235%72%
2112
April1st to 15thNight
4112113345%67%
1954
16th to endNight
211222382%
1728
May1st to 15thNight
111112246%75%
1619
16th to endNight
2111212482%
1684
June1st to 15thNight
1111222480%
1620
16th to endNight
112225%80%
1620
July1st to 15thNight
11144246%6%11%54%
1620
16th to endNight
3215%435%7%8%14%42%
1701
August1st to 15thNight
2224435%5%7%9%53%
1620
16th to endNight
13135%35%6%7%9%51%
1860
September1st to 15thNight
321431346%7%60%
1980
16th to endNight
21121223375%
1980
October1st to 15thNight
311121223476%
2077
16th to endNight
12111223379%
2304
November1st to 15thNight
311122235%74%
2106
16th to endNight
421221232471%
2340
December1st to 15thNight
8%212312345%63%
2340
16th to endNight
10%313323336%57%
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
23%1413%26%30%
1612
16th to endDay
24%1216%24%30%
1728
February1st to 15thDay
21%1416%26%30%
1661
16th to endDay
26%116%28%25%
1608
March1st to 15thDay
20%16%31%38%
1980
16th to endDay
19%18%32%36%
2112
April1st to 15thDay
16%117%36%37%
1994
16th to endDay
11%118%25%53%
2229
May1st to 15thDay
10%18%26%52%
2331
16th to endDay
10%6%24%56%
2432
June1st to 15thDay
10%1317%67%
2340
16th to endDay
9%11414%69%
2340
July1st to 15thDay
23%37%17%19%29%
2201
16th to endDay
31%415%24%17%6%
2312
August1st to 15thDay
23%48%19%18%25%
2325
16th to endDay
23%36%20%24%21%
2339
September1st to 15thDay
20%25%16%26%28%
1980
16th to endDay
10%16%29%50%
1980
October1st to 15thDay
13%129%25%47%
1881
16th to endDay
11%28%26%49%
1918
November1st to 15thDay
12%110%34%40%
1750
16th to endDay
18%1110%24%43%
1620
December1st to 15thDay
25%1414%29%24%
1620
16th to endDay
26%1320%29%16%
1728
January1st to 15thNight
23%2310%26%32%
2327
16th to endNight
27%1113%26%29%
2496
February1st to 15thNight
21%1315%25%32%
2199
16th to endNight
26%115%27%28%
1865
March1st to 15thNight
18%218%27%42%
1980
16th to endNight
16%129%32%38%
2112
April1st to 15thNight
19%16%33%38%
1954
16th to endNight
9%116%24%57%
1728
May1st to 15thNight
10%116%29%50%
1620
16th to endNight
9%7%19%61%
1685
June1st to 15thNight
11%314%69%
1620
16th to endNight
9%12311%71%
1620
July1st to 15thNight
23%310%16%16%29%
1530
16th to endNight
28%414%22%21%8%
1593
August1st to 15thNight
23%49%20%15%26%
1611
16th to endNight
26%47%17%23%21%
1860
September1st to 15thNight
22%36%20%22%25%
1980
16th to endNight
12%216%27%49%
1980
October1st to 15thNight
13%1111%22%49%
2077
16th to endNight
11%119%21%54%
2304
November1st to 15thNight
14%1110%29%43%
2106
16th to endNight
18%1110%23%44%
2340
December1st to 15thNight
23%1214%30%27%
2340
16th to endNight
28%1416%28%19%
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
26%116%35%17%2
90
16th to endDay
9%315%39%19%12%
96
February1st to 15thDay
5%323%40%25%1
176
16th to endDay
11%6%26%37%15%1
238
March1st to 15thDay
5%10%25%34%18%4
270
16th to endDay
39%31%30%20%4
288
April1st to 15thDay
5%9%30%33%13%7%
269
16th to endDay
411%35%30%13%4
270
May1st to 15thDay
310%28%31%20%4
288
16th to endDay
2425%40%24%2
330
June1st to 15thDay
5%221%37%26%6%
315
16th to endDay
26%43%33%12%
315
July1st to 15thDay
38%35%37%15%
305
16th to endDay
4228%37%26%
327
August1st to 15thDay
3124%47%22%
315
16th to endDay
2424%45%22%
332
September1st to 15thDay
8%6%34%37%13%
315
16th to endDay
5%115%41%29%6%
315
October1st to 15thDay
4216%32%35%8%
216
16th to endDay
4333%42%15%
144
November1st to 15thDay
4321%35%24%11%
90
16th to endDay
10%121%43%23%1
90
December1st to 15thDay
8%16%40%36%6%
90
16th to endDay
22%317%37%17%1
96
January1st to 15thNight
8%214%45%23%5%
720
16th to endNight
11%213%42%24%4
768
February1st to 15thNight
7%213%46%26%3
704
16th to endNight
10%112%50%23%2
632
March1st to 15thNight
6%216%42%26%5%
720
16th to endNight
429%43%35%5%
768
April1st to 15thNight
4212%50%27%2
695
16th to endNight
2214%48%27%4
540
May1st to 15thNight
2213%45%31%5%
540
16th to endNight
229%41%37%6%
563
June1st to 15thNight
1444%45%2
540
16th to endNight
1231%57%6%
540
July1st to 15thNight
3323%58%9%
510
16th to endNight
4112%64%15%
564
August1st to 15thNight
3112%65%16%
539
16th to endNight
3119%60%14%
615
September1st to 15thNight
415%27%50%11%
720
16th to endNight
217%39%38%9%
716
October1st to 15thNight
411%41%36%5%
720
16th to endNight
27%40%39%9%
768
November1st to 15thNight
4110%41%34%7%
700
16th to endNight
5%112%44%30%5%
720
December1st to 15thNight
10%19%42%32%3
720
16th to endNight
11%110%47%24%4
768
Page generated at 2017-11-01 14:23:31. Version ac04

Copyright 2017 A. Danko


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