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 2017-03-19.

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
76%
15018
37%
45186
33%
14979
81%
14979
54%
21490

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
79%
720
37%
1980
31%
720
83%
720
62%
937
April1st to 15thNight
73%
637
35%
1791
27%
637
78%
637
54%
895
April16th to endNight
78%
495
43%
1584
39%
495
84%
495
69%
621
May1st to 15thNight
79%
494
42%
1484
36%
494
84%
494
69%
617
May16th to endNight
84%
513
38%
1540
33%
513
86%
513
72%
625
June1st to 15thNight
88%
495
37%
1485
33%
495
89%
495
69%
622
June16th to endNight
90%
495
32%
1485
29%
495
91%
495
73%
602
July1st to 15thNight
81%
465
25%
1395
21%
465
81%
465
46%
724
July16th to endNight
83%
522
28%
1467
24%
489
83%
489
42%
803
August1st to 15thNight
83%
494
32%
1476
29%
491
83%
491
46%
765
August16th to endNight
84%
563
32%
1704
31%
563
85%
563
43%
917
September1st to 15thNight
80%
660
35%
1815
31%
660
81%
660
51%
955
September16th to endNight
83%
656
42%
1815
33%
656
86%
656
63%
848
October1st to 15thNight
77%
660
42%
1904
35%
660
82%
660
64%
858
October16th to endNight
84%
704
44%
2112
40%
704
88%
704
67%
902
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%67%
45629
Night
4212213346%67%
45382
TransparencyDay
18%1312%25%38%
45324
Night
19%1311%24%38%
45188
SeeingDay
6%417%35%27%8%
4706
Night
5%19%39%36%6%
15028

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%
1974
April1st to 15thDay
322212337%70%
1827
16th to endDay
21111222380%
2043
May1st to 15thDay
11112235%78%
2136
16th to endDay
11111211281%
2221
June1st to 15thDay
11111122481%
2145
16th to endDay
11111122480%
2145
July1st to 15thDay
22235%245%6%10%54%
2145
16th to endDay
5%215%436%7%8%11%42%
2285
August1st to 15thDay
3213335%5%7%9%53%
2145
16th to endDay
321443446%9%54%
2143
September1st to 15thDay
421221345%6%63%
1815
16th to endDay
3111223477%
1815
October1st to 15thDay
21121244473%
1724
16th to endDay
11211123478%
1760
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
3111312235%72%
1980
April1st to 15thNight
5%113124345%66%
1791
16th to endNight
2111222381%
1584
May1st to 15thNight
111212247%75%
1484
16th to endNight
2111212481%
1540
June1st to 15thNight
11111222380%
1485
16th to endNight
11112225%80%
1485
July1st to 15thNight
11145%246%5%11%53%
1485
16th to endNight
2214435%7%8%14%44%
1575
August1st to 15thNight
22244345%7%8%53%
1485
16th to endNight
2313435%6%7%9%51%
1704
September1st to 15thNight
221441345%7%60%
1815
16th to endNight
21221233474%
1815
October1st to 15thNight
311221223474%
1904
16th to endNight
12121223378%
2112
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%31%37%
1974
April1st to 15thDay
16%117%35%37%
1827
16th to endDay
11%17%24%53%
2043
May1st to 15thDay
10%117%26%53%
2136
16th to endDay
11%6%25%55%
2224
June1st to 15thDay
10%1315%68%
2145
16th to endDay
9%11414%68%
2145
July1st to 15thDay
24%37%15%19%28%
2006
16th to endDay
31%413%24%19%7%
2130
August1st to 15thDay
23%48%20%18%24%
2130
16th to endDay
24%36%21%24%18%
2143
September1st to 15thDay
21%25%16%26%27%
1815
16th to endDay
11%116%28%50%
1815
October1st to 15thDay
14%2210%26%44%
1724
16th to endDay
11%128%26%49%
1760
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%31%38%
1980
April1st to 15thNight
20%17%31%37%
1791
16th to endNight
10%16%25%56%
1584
May1st to 15thNight
10%116%28%51%
1485
16th to endNight
10%6%20%60%
1541
June1st to 15thNight
11%1313%69%
1485
16th to endNight
9%12411%71%
1485
July1st to 15thNight
24%310%15%17%29%
1395
16th to endNight
26%412%23%23%8%
1467
August1st to 15thNight
23%49%21%15%25%
1476
16th to endNight
25%47%18%24%19%
1704
September1st to 15thNight
22%26%20%21%25%
1815
16th to endNight
13%216%27%48%
1815
October1st to 15thNight
14%1112%23%46%
1904
16th to endNight
11%119%21%53%
2112
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
310%30%30%19%5%
238
April1st to 15thDay
7%11%33%31%10%5%
209
16th to endDay
412%37%30%10%3
210
May1st to 15thDay
312%29%33%18%2
228
16th to endDay
3425%40%23%2
266
June1st to 15thDay
5%223%38%24%5%
255
16th to endDay
317%48%30%9%
255
July1st to 15thDay
410%40%33%11%
245
16th to endDay
5%232%39%19%
271
August1st to 15thDay
3127%46%20%
255
16th to endDay
21427%46%18%
268
September1st to 15thDay
10%6%34%36%12%
255
16th to endDay
6%114%43%28%5%
255
October1st to 15thDay
5%316%31%32%10%
164
16th to endDay
41333%44%13%
96
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
328%44%34%5%
720
April1st to 15thNight
5%213%50%25%2
637
16th to endNight
2214%49%26%5%
495
May1st to 15thNight
2213%46%30%5%
495
16th to endNight
229%41%37%6%
515
June1st to 15thNight
21445%44%2
495
16th to endNight
1333%54%6%
495
July1st to 15thNight
3423%56%10%
465
16th to endNight
4113%66%14%
522
August1st to 15thNight
3112%65%15%
494
16th to endNight
3119%60%14%
563
September1st to 15thNight
315%28%50%10%
660
16th to endNight
217%38%38%10%
656
October1st to 15thNight
4112%39%37%5%
660
16th to endNight
27%40%38%9%
704
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 2017-03-20 05:18:33. Version ac04

Copyright 2017 A. Danko


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