Principia College Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Principia College Astronomical Observatory clear sky chart from 2005-11-05 to 2017-09-24.

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
43%
15881
21%
46946
14%
15835
50%
15835
28%
29707

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
27%
899
18%
2500
8%
895
36%
895
22%
1701
January16th to endNight
25%
960
19%
2496
6%
960
33%
960
23%
1763
February1st to 15thNight
25%
836
14%
2244
5%
833
31%
833
20%
1632
February16th to endNight
25%
631
20%
1893
7%
631
36%
631
24%
1230
March1st to 15thNight
26%
660
17%
2100
8%
660
35%
660
19%
1430
March16th to endNight
31%
576
13%
2031
9%
576
38%
576
17%
1386
April1st to 15thNight
33%
539
20%
1797
12%
539
46%
539
26%
1084
April16th to endNight
40%
540
22%
1800
17%
540
49%
540
26%
1127
May1st to 15thNight
44%
540
18%
1645
13%
539
50%
539
24%
1073
May16th to endNight
54%
563
20%
1497
14%
561
58%
561
32%
984
June1st to 15thNight
64%
540
21%
1440
15%
540
68%
540
42%
839
June16th to endNight
64%
540
21%
1440
15%
540
67%
540
41%
858
July1st to 15thNight
69%
510
20%
1360
15%
510
70%
510
41%
804
July16th to endNight
74%
564
23%
1480
22%
531
76%
531
42%
852
August1st to 15thNight
78%
539
23%
1673
25%
536
79%
536
39%
907
August16th to endNight
74%
573
24%
1910
28%
573
76%
573
37%
1024
September1st to 15thNight
64%
540
27%
1800
29%
540
68%
540
33%
1010
September16th to endNight
63%
639
33%
1860
25%
639
68%
639
38%
1048
October1st to 15thNight
53%
660
28%
1977
21%
660
60%
660
36%
1136
October16th to endNight
38%
704
25%
2131
17%
704
46%
704
31%
1268
November1st to 15thNight
40%
700
28%
2276
18%
700
53%
700
32%
1255
November16th to endNight
30%
768
22%
2388
13%
768
41%
768
26%
1483
December1st to 15thNight
29%
900
15%
2520
8%
900
36%
900
20%
1819
December16th to endNight
24%
960
15%
2688
6%
960
32%
960
17%
1994

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
12%6%25%5%3445%7%42%
47988
Night
13%6%25%424446%44%
47127
TransparencyDay
40%6%12%23%14%2
47673
Night
39%5%11%23%16%3
46948
SeeingDay
12%5%22%28%21%8%
5908
Night
16%7%27%31%15%1
15882

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
13%8%49%6%34347%34%
1435
16th to endDay
13%9%46%6%33446%37%
1728
February1st to 15thDay
15%7%5%6%433447%36%
1617
16th to endDay
12%10%46%5%3435%5%38%
1580
March1st to 15thDay
16%9%46%424435%37%
1869
16th to endDay
17%8%36%6%35%5%5%7%30%
2189
April1st to 15thDay
15%6%25%5%3445%6%41%
2151
16th to endDay
17%6%26%6%34445%38%
2160
May1st to 15thDay
17%5%35%435%45%7%36%
2301
16th to endDay
11%5%25%5%36%5%5%7%41%
2617
June1st to 15thDay
8%3245%35%5%6%8%46%
2520
16th to endDay
6%314435%6%7%10%45%
2520
July1st to 15thDay
8%3145%46%46%10%44%
2520
16th to endDay
5%214335%6%7%10%49%
2584
August1st to 15thDay
7%325%5%35%6%6%9%42%
2277
16th to endDay
7%425%5%36%5%5%9%43%
2292
September1st to 15thDay
12%5%2431445%6%48%
2160
16th to endDay
8%5%2442435%8%51%
1947
October1st to 15thDay
14%4135%24336%50%
1650
16th to endDay
13%5%24323245%52%
1741
November1st to 15thDay
13%423323346%50%
1582
16th to endDay
17%8%34323336%42%
1572
December1st to 15thDay
17%9%36%4245%45%35%
1440
16th to endDay
17%11%36%5%23445%34%
1536
January1st to 15thNight
14%6%37%5%34445%39%
2500
16th to endNight
12%10%46%5%34346%36%
2496
February1st to 15thNight
13%10%46%5%35%35%435%
2235
16th to endNight
15%7%35%5%23245%44%
1893
March1st to 15thNight
19%8%34425%335%38%
2100
16th to endNight
18%7%35%43446%6%33%
2031
April1st to 15thNight
13%7%24424436%46%
1797
16th to endNight
15%5%26%423447%43%
1800
May1st to 15thNight
16%6%335%23447%42%
1645
16th to endNight
8%4346%2445%8%46%
1496
June1st to 15thNight
6%32432447%8%53%
1440
16th to endNight
7%2144245%7%10%49%
1440
July1st to 15thNight
9%3134245%5%8%51%
1440
16th to endNight
32245%25%37%10%51%
1581
August1st to 15thNight
5%313325%5%6%7%54%
1683
16th to endNight
5%3244245%5%8%53%
1910
September1st to 15thNight
10%425%424348%49%
1800
16th to endNight
8%423413346%56%
1860
October1st to 15thNight
10%413323447%53%
1977
16th to endNight
15%5%1331333451%
2131
November1st to 15thNight
10%5%24323346%54%
2276
16th to endNight
16%6%34323346%45%
2388
December1st to 15thNight
20%9%46%424345%32%
2520
16th to endNight
20%12%36%415%345%32%
2688

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
50%5%10%20%11%
1436
16th to endDay
47%414%23%7%
1728
February1st to 15thDay
47%5%15%22%9%
1619
16th to endDay
47%6%12%22%11%1
1580
March1st to 15thDay
49%5%10%22%11%1
1869
16th to endDay
53%6%13%17%8%
2191
April1st to 15thDay
42%5%9%24%16%1
2151
16th to endDay
46%6%10%22%12%1
2160
May1st to 15thDay
45%7%12%23%10%
2301
16th to endDay
40%8%13%19%15%2
2621
June1st to 15thDay
33%8%14%23%19%1
2520
16th to endDay
29%8%18%23%17%3
2520
July1st to 15thDay
33%7%13%27%13%3
2371
16th to endDay
27%9%16%28%16%1
2423
August1st to 15thDay
34%8%12%25%17%1
2263
16th to endDay
33%8%14%20%17%5%
2292
September1st to 15thDay
34%6%9%25%19%4
2160
16th to endDay
32%5%9%29%20%2
1947
October1st to 15thDay
36%49%24%20%4
1650
16th to endDay
33%310%30%18%3
1741
November1st to 15thDay
34%312%22%21%5%
1582
16th to endDay
44%411%26%11%1
1572
December1st to 15thDay
50%5%14%21%8%
1440
16th to endDay
50%5%12%20%8%2
1536
January1st to 15thNight
46%410%24%13%
2500
16th to endNight
47%413%22%11%
2496
February1st to 15thNight
50%5%10%19%12%1
2244
16th to endNight
43%313%25%12%1
1893
March1st to 15thNight
48%5%9%22%12%2
2100
16th to endNight
49%5%13%21%9%
2031
April1st to 15thNight
38%411%24%18%3
1797
16th to endNight
40%5%8%25%16%2
1800
May1st to 15thNight
42%6%12%22%16%
1646
16th to endNight
36%7%14%20%16%4
1498
June1st to 15thNight
26%7%14%28%21%2
1440
16th to endNight
26%7%14%24%20%6%
1440
July1st to 15thNight
29%5%12%31%16%5%
1360
16th to endNight
26%6%15%31%18%1
1480
August1st to 15thNight
25%7%14%26%23%2
1673
16th to endNight
26%6%13%23%24%5%
1910
September1st to 15thNight
32%49%22%22%8%
1800
16th to endNight
28%5%9%25%27%3
1860
October1st to 15thNight
30%5%9%24%22%6%
1977
16th to endNight
35%49%25%20%4
2131
November1st to 15thNight
32%311%23%20%7%
2276
16th to endNight
40%410%27%17%1
2388
December1st to 15thNight
52%411%20%10%
2520
16th to endNight
54%5%10%17%9%1
2688

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
30%8%40%16%31
90
16th to endDay
11%16%48%19%3
96
February1st to 15thDay
15%16%40%24%3
90
16th to endDay
13%18%34%28%4
81
March1st to 15thDay
25%15%36%15%5%1
90
16th to endDay
21%13%35%21%8%
175
April1st to 15thDay
17%11%31%24%14%
270
16th to endDay
19%9%26%29%11%3
270
May1st to 15thDay
16%9%29%27%14%3
333
16th to endDay
15%5%29%29%13%6%
517
June1st to 15thDay
9%321%31%24%9%
495
16th to endDay
7%227%36%20%5%
495
July1st to 15thDay
10%213%34%29%10%
475
16th to endDay
5%112%28%36%16%
515
August1st to 15thDay
10%10%29%33%15%
430
16th to endDay
9%210%25%37%13%
332
September1st to 15thDay
12%314%23%32%13%
315
16th to endDay
10%413%27%28%15%
287
October1st to 15thDay
6%7%23%33%24%4
90
16th to endDay
12%8%27%36%15%
96
November1st to 15thDay
6%425%25%27%10%
90
16th to endDay
32%16%30%14%5%1
90
December1st to 15thDay
12%13%42%21%6%4
90
16th to endDay
28%12%43%12%3
96
January1st to 15thNight
17%13%37%25%6%
900
16th to endNight
18%12%38%27%2
960
February1st to 15thNight
18%13%35%27%4
836
16th to endNight
18%11%37%29%2
631
March1st to 15thNight
25%8%34%26%5%
660
16th to endNight
21%8%33%29%7%
576
April1st to 15thNight
16%10%37%26%9%
539
16th to endNight
17%5%32%32%10%1
540
May1st to 15thNight
17%6%25%37%12%1
540
16th to endNight
9%226%41%18%1
563
June1st to 15thNight
7%120%44%24%2
540
16th to endNight
8%119%44%25%2
540
July1st to 15thNight
10%10%43%29%4
510
16th to endNight
5%17%39%42%3
564
August1st to 15thNight
7%7%40%40%4
539
16th to endNight
6%10%38%39%5%
573
September1st to 15thNight
12%112%33%32%7%
540
16th to endNight
9%5%15%30%33%5%
639
October1st to 15thNight
13%425%28%24%4
660
16th to endNight
20%7%31%30%9%
704
November1st to 15thNight
12%8%35%30%13%
700
16th to endNight
19%11%34%25%7%
768
December1st to 15thNight
25%10%32%26%5%
900
16th to endNight
26%10%32%26%3
960
Page generated at 2017-09-25 08:02:26. Version ac04

Copyright 2017 A. Danko


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