Princeton Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Princeton Observatory clear sky chart from 2005-11-06 to 2024-01-15.

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
45%
24606
22%
74874
14%
24558
53%
24558
27%
46556

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 10thNight
28%
757
24%
2459
11%
757
44%
757
24%
1483
January11th to 20thNight
32%
737
23%
2393
13%
737
45%
737
22%
1513
January21st to endNight
28%
792
28%
2574
14%
792
47%
792
28%
1521
February1st to 10thNight
28%
720
20%
2340
11%
720
41%
720
24%
1455
February11th to 20thNight
20%
700
20%
2210
8%
699
35%
699
21%
1409
February21st to endNight
29%
592
29%
1682
15%
592
44%
592
27%
1056
March1st to 10thNight
32%
720
27%
1980
14%
720
45%
720
32%
1205
March11th to 20thNight
28%
720
20%
1980
10%
720
38%
720
25%
1322
March21st to endNight
32%
792
21%
2178
10%
792
42%
792
28%
1400
April1st to 10thNight
32%
720
24%
1835
9%
720
41%
720
31%
1177
April11th to 20thNight
38%
711
25%
1775
10%
711
46%
711
35%
1133
April21st to endNight
43%
720
24%
1800
10%
720
49%
720
38%
1136
May1st to 10thNight
40%
654
18%
1734
7%
654
45%
654
33%
1074
May11th to 20thNight
48%
531
18%
1592
9%
531
53%
531
26%
1022
May21st to endNight
63%
582
16%
1744
12%
582
64%
582
30%
1100
June1st to 10thNight
60%
540
15%
1620
13%
540
62%
540
29%
1031
June11th to 20thNight
61%
479
14%
1559
13%
479
63%
479
26%
996
June21st to endNight
64%
540
16%
1620
14%
540
66%
540
26%
1084
July1st to 10thNight
67%
525
13%
1575
12%
525
68%
525
25%
1058
July11th to 20thNight
70%
522
12%
1566
10%
522
71%
522
28%
996
July21st to endNight
67%
591
15%
1672
12%
558
69%
558
27%
1089
August1st to 10thNight
65%
537
12%
1600
10%
534
66%
534
25%
1072
August11th to 20thNight
60%
634
15%
1698
10%
634
62%
634
25%
1207
August21st to endNight
64%
792
16%
1980
11%
792
65%
792
30%
1350
September1st to 10thNight
65%
720
22%
1800
16%
720
67%
720
32%
1210
September11th to 20thNight
59%
720
24%
1854
19%
720
62%
720
31%
1230
September21st to endNight
46%
720
18%
2052
13%
720
52%
720
22%
1424
October1st to 10thNight
58%
720
25%
2160
24%
720
64%
720
33%
1294
October11th to 20thNight
49%
720
30%
2160
25%
720
60%
720
34%
1253
October21st to endNight
50%
792
32%
2376
22%
792
64%
792
38%
1318
November1st to 10thNight
48%
740
27%
2221
21%
740
57%
740
32%
1313
November11th to 20thNight
42%
760
28%
2284
18%
760
54%
760
30%
1362
November21st to endNight
43%
760
27%
2593
20%
760
56%
760
31%
1448
December1st to 10thNight
33%
760
21%
2660
11%
760
46%
760
20%
1679
December11th to 20thNight
38%
759
24%
3169
18%
758
51%
758
23%
1793
December21st to endNight
34%
827
18%
4379
14%
817
47%
817
16%
2343

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%35%5%35%5%6%8%43%
74362
Night
13%6%25%5%345%5%7%45%
75348
TransparencyDay
39%5%12%22%18%4
74083
Night
38%5%11%21%19%5%
75146
SeeingDay
16%8%23%25%20%9%
14244
Night
16%7%25%32%18%2
24624

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 10thDay
14%6%35%5%34445%47%
1706
11th to 20thDay
17%8%3442445%5%44%
1656
21st to endDay
16%6%24434445%48%
1782
February1st to 10thDay
16%5%35%4345%46%44%
1620
11th to 20thDay
20%5%25%42344447%
1634
21st to endDay
18%7%35%5%43445%42%
1570
March1st to 10thDay
14%5%244335%45%51%
1980
11th to 20thDay
15%5%36%5%3445%7%42%
1980
21st to endDay
14%7%37%6%245%5%7%41%
2178
April1st to 10thDay
10%42342435%8%54%
2124
11th to 20thDay
9%5%25%4345%5%7%50%
2134
21st to endDay
13%5%25%43445%7%49%
2160
May1st to 10thDay
11%5%35%5%35%5%6%8%45%
2240
11th to 20thDay
12%6%45%6%46%5%5%8%39%
2301
21st to endDay
11%6%25%6%45%6%8%9%38%
2531
June1st to 10thDay
10%6%36%6%46%6%7%8%38%
2340
11th to 20thDay
9%425%5%45%6%7%10%42%
2340
21st to endDay
9%435%7%37%6%9%10%39%
2340
July1st to 10thDay
8%5%26%7%5%7%7%8%11%34%
2340
11th to 20thDay
9%5%36%7%47%8%8%11%34%
2340
21st to endDay
6%5%36%7%5%7%7%8%11%35%
2547
August1st to 10thDay
11%5%27%8%5%6%7%8%10%31%
2327
11th to 20thDay
11%6%37%7%47%7%8%9%31%
2183
21st to endDay
11%7%36%6%46%6%7%10%34%
2376
September1st to 10thDay
9%6%25%5%45%5%7%9%43%
2160
11th to 20thDay
14%6%25%5%4445%7%43%
2102
21st to endDay
18%8%37%6%4445%5%37%
1907
October1st to 10thDay
16%6%1442445%5%48%
1800
11th to 20thDay
13%43443435%6%53%
1800
21st to endDay
12%425%5%34335%55%
1980
November1st to 10thDay
14%5%24323446%53%
1857
11th to 20thDay
14%5%245%23347%51%
1891
21st to endDay
14%42342435%6%52%
1582
December1st to 10thDay
17%6%345%35%45%8%40%
1520
11th to 20thDay
16%5%23423225%56%
1910
21st to endDay
15%7%36%42345%8%42%
3124
January1st to 10thNight
15%6%345%33446%47%
2459
11th to 20thNight
14%8%36%5%3445%6%42%
2393
21st to endNight
16%5%2442445%5%49%
2574
February1st to 10thNight
17%6%35%42445%6%44%
2340
11th to 20thNight
19%6%35%43435%7%42%
2210
21st to endNight
17%5%243245%5%6%47%
1682
March1st to 10thNight
13%5%34423345%53%
1980
11th to 20thNight
16%6%25%5%345%5%7%40%
1980
21st to endNight
14%6%3443445%7%44%
2178
April1st to 10thNight
12%7%25%5%3345%7%47%
1835
11th to 20thNight
11%425%5%3446%7%49%
1775
21st to endNight
11%5%24425%5%6%7%49%
1800
May1st to 10thNight
9%5%26%5%3447%9%45%
1734
11th to 20thNight
12%6%36%5%345%6%8%41%
1592
21st to endNight
11%5%25%6%347%6%9%43%
1744
June1st to 10thNight
10%5%25%6%46%6%7%11%39%
1620
11th to 20thNight
9%5%36%7%35%7%7%8%41%
1559
21st to endNight
7%5%26%6%45%7%8%11%36%
1620
July1st to 10thNight
8%5%37%6%36%6%8%10%36%
1620
11th to 20thNight
7%326%6%46%7%9%12%38%
1620
21st to endNight
6%6%26%6%46%8%9%11%36%
1771
August1st to 10thNight
10%6%38%6%36%6%7%11%35%
1611
11th to 20thNight
10%5%37%8%47%6%8%9%32%
1698
21st to endNight
10%5%36%6%35%6%8%11%37%
1980
September1st to 10thNight
11%5%345%36%5%6%8%43%
1800
11th to 20thNight
13%5%26%5%25%5%5%8%43%
1854
21st to endNight
20%7%36%6%35%36%6%37%
2052
October1st to 10thNight
14%5%25%43345%7%50%
2160
11th to 20thNight
13%6%24423345%54%
2160
21st to endNight
11%5%14322346%56%
2376
November1st to 10thNight
13%435%5%24345%53%
2221
11th to 20thNight
13%6%25%5%23446%50%
2284
21st to endNight
13%5%24424447%51%
2593
December1st to 10thNight
17%8%35%5%3445%7%41%
2660
11th to 20thNight
17%6%24424446%48%
3281
21st to endNight
15%6%35%42445%6%46%
4532

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 10thDay
42%38%20%20%6%
1707
11th to 20thDay
43%47%19%23%5%
1656
21st to endDay
40%35%21%24%7%
1782
February1st to 10thDay
41%311%20%19%6%
1620
11th to 20thDay
42%36%19%24%6%
1643
21st to endDay
45%38%18%19%7%
1570
March1st to 10thDay
36%48%21%24%7%
1980
11th to 20thDay
41%5%10%21%17%6%
1980
21st to endDay
43%5%11%18%18%4
2178
April1st to 10thDay
30%410%25%24%7%
2124
11th to 20thDay
33%411%24%21%7%
2134
21st to endDay
35%5%11%24%20%5%
2160
May1st to 10thDay
37%5%15%23%17%3
2240
11th to 20thDay
44%5%13%23%13%1
2305
21st to endDay
40%8%15%24%14%1
2531
June1st to 10thDay
41%7%14%22%15%1
2340
11th to 20thDay
36%6%16%25%14%2
2340
21st to endDay
37%9%18%21%13%2
2340
July1st to 10thDay
40%8%16%22%11%3
2275
11th to 20thDay
40%9%19%21%9%2
2271
21st to endDay
39%8%21%22%9%1
2405
August1st to 10thDay
44%8%17%20%10%
2314
11th to 20thDay
45%9%16%19%10%1
2183
21st to endDay
43%7%16%22%9%3
2376
September1st to 10thDay
36%8%16%19%17%4
2160
11th to 20thDay
41%411%20%21%4
2102
21st to endDay
49%49%19%15%4
1907
October1st to 10thDay
38%410%23%23%3
1800
11th to 20thDay
34%38%26%23%6%
1800
21st to endDay
35%35%22%27%9%
1980
November1st to 10thDay
34%36%23%27%7%
1857
11th to 20thDay
35%38%20%26%7%
1891
21st to endDay
34%37%25%24%7%
1582
December1st to 10thDay
43%411%21%17%4
1520
11th to 20thDay
35%17%25%23%9%
1906
21st to endDay
42%28%23%22%2
3124
January1st to 10thNight
40%38%19%25%5%
2459
11th to 20thNight
43%410%18%19%6%
2393
21st to endNight
38%35%22%25%7%
2574
February1st to 10thNight
42%38%20%20%7%
2340
11th to 20thNight
44%38%17%22%7%
2212
21st to endNight
39%49%20%20%9%
1682
March1st to 10thNight
35%38%21%23%9%
1980
11th to 20thNight
43%5%9%18%19%6%
1980
21st to endNight
40%411%21%19%5%
2178
April1st to 10thNight
37%49%17%25%8%
1835
11th to 20thNight
35%410%23%21%7%
1775
21st to endNight
33%5%12%27%19%5%
1800
May1st to 10thNight
35%5%15%24%17%4
1734
11th to 20thNight
41%5%14%22%16%1
1593
21st to endNight
36%8%17%24%13%1
1745
June1st to 10thNight
37%8%18%22%13%1
1620
11th to 20thNight
40%7%17%20%14%2
1559
21st to endNight
39%10%16%20%12%2
1620
July1st to 10thNight
39%11%17%20%10%2
1575
11th to 20thNight
34%12%24%20%9%2
1566
21st to endNight
37%11%22%22%9%1
1672
August1st to 10thNight
42%8%22%22%6%
1600
11th to 20thNight
44%9%18%19%8%1
1698
21st to endNight
40%8%19%23%9%2
1980
September1st to 10thNight
37%9%14%20%15%5%
1800
11th to 20thNight
39%7%12%21%19%3
1854
21st to endNight
49%5%11%17%16%3
2052
October1st to 10thNight
35%5%9%23%24%4
2160
11th to 20thNight
34%39%22%25%7%
2160
21st to endNight
30%38%23%27%9%
2376
November1st to 10thNight
36%36%19%28%8%
2221
11th to 20thNight
36%39%16%25%11%
2284
21st to endNight
34%47%22%26%8%
2593
December1st to 10thNight
45%410%19%18%5%
2660
11th to 20thNight
38%37%20%24%8%
3284
21st to endNight
40%36%24%25%3
4532

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 10thDay
18%18%40%19%5%
270
11th to 20thDay
20%21%35%17%8%
252
21st to endDay
20%19%38%17%5%
264
February1st to 10thDay
20%13%42%20%41
240
11th to 20thDay
31%13%36%16%4
301
21st to endDay
24%18%31%21%6%1
346
March1st to 10thDay
17%15%36%24%6%1
420
11th to 20thDay
20%17%36%19%7%1
420
21st to endDay
20%13%33%23%10%2
462
April1st to 10thDay
16%16%29%25%12%2
420
11th to 20thDay
12%13%30%25%17%4
412
21st to endDay
17%9%27%26%18%3
420
May1st to 10thDay
13%11%32%23%14%7%
423
11th to 20thDay
15%5%21%26%23%9%
447
21st to endDay
20%412%27%24%15%
485
June1st to 10thDay
11%417%28%27%12%
450
11th to 20thDay
13%215%24%25%20%
450
21st to endDay
10%214%29%32%13%
450
July1st to 10thDay
10%19%24%35%21%
445
11th to 20thDay
9%37%22%36%22%
444
21st to endDay
7%29%32%35%15%
488
August1st to 10thDay
11%412%24%30%19%
446
11th to 20thDay
12%39%23%30%23%
439
21st to endDay
11%311%25%28%23%
495
September1st to 10thDay
12%313%22%26%23%
450
11th to 20thDay
16%413%27%26%14%
450
21st to endDay
22%5%18%26%21%8%
450
October1st to 10thDay
19%5%20%21%23%12%
450
11th to 20thDay
14%7%23%28%22%6%
450
21st to endDay
13%7%24%33%17%5%
495
November1st to 10thDay
18%6%26%29%16%5%
455
11th to 20thDay
16%13%29%28%12%1
456
21st to endDay
15%11%26%27%19%2
242
December1st to 10thDay
22%13%33%22%9%1
180
11th to 20thDay
18%10%35%22%12%3
180
21st to endDay
21%12%32%21%12%2
297
January1st to 10thNight
19%13%36%26%6%
760
11th to 20thNight
18%13%31%30%7%
737
21st to endNight
21%9%38%28%4
792
February1st to 10thNight
20%11%36%28%5%
720
11th to 20thNight
22%15%39%21%2
703
21st to endNight
19%14%35%28%4
592
March1st to 10thNight
16%12%36%30%5%
720
11th to 20thNight
19%13%36%25%7%
720
21st to endNight
17%10%36%31%6%
792
April1st to 10thNight
16%12%37%28%7%
720
11th to 20thNight
13%12%34%30%11%1
711
21st to endNight
14%8%31%36%10%1
720
May1st to 10thNight
11%10%33%34%11%
654
11th to 20thNight
15%6%21%41%17%1
531
21st to endNight
13%213%40%30%2
584
June1st to 10thNight
12%219%41%25%2
540
11th to 20thNight
11%315%39%28%4
479
21st to endNight
10%314%41%31%2
540
July1st to 10thNight
9%38%37%38%4
525
11th to 20thNight
8%27%35%43%5%
522
21st to endNight
7%39%38%41%3
591
August1st to 10thNight
12%36%41%34%4
537
11th to 20thNight
14%29%33%36%6%
634
21st to endNight
13%29%32%38%7%
792
September1st to 10thNight
12%210%33%37%6%
720
11th to 20thNight
16%213%34%31%3
720
21st to endNight
22%418%30%24%2
720
October1st to 10thNight
15%316%35%27%5%
720
11th to 20thNight
16%6%23%33%21%2
720
21st to endNight
14%5%27%32%19%3
792
November1st to 10thNight
15%7%24%34%17%3
740
11th to 20thNight
16%12%26%32%13%1
760
21st to endNight
15%7%30%32%15%1
760
December1st to 10thNight
21%10%31%33%6%
760
11th to 20thNight
19%8%29%35%8%1
760
21st to endNight
18%11%31%30%9%
836
Page generated at 2024-01-11 22:33:51. Version ac07

Copyright 2024 Attilla Danko


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