Amateur Astronomers Association of Princeton
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Amateur Astronomers Association of Princeton clear sky chart from 2005-11-06 to 2024-01-10.

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
32%
23179
20%
74574
10%
23135
43%
23135
22%
47461

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
17%
760
23%
2647
8%
757
37%
757
19%
1600
January11th to 20thNight
22%
720
19%
2520
7%
720
39%
720
17%
1626
January21st to endNight
18%
792
23%
2578
7%
792
37%
792
20%
1577
February1st to 10thNight
19%
720
17%
2340
5%
720
34%
720
17%
1532
February11th to 20thNight
14%
703
20%
2113
4%
699
33%
699
16%
1403
February21st to endNight
15%
592
26%
1776
8%
592
37%
592
19%
1161
March1st to 10thNight
21%
720
23%
2160
8%
720
39%
720
22%
1343
March11th to 20thNight
19%
720
21%
2021
8%
720
33%
720
20%
1354
March21st to endNight
20%
792
22%
2026
6%
792
32%
792
24%
1391
April1st to 10thNight
20%
720
19%
1800
4%
720
29%
720
23%
1237
April11th to 20thNight
24%
711
19%
1775
5%
711
31%
711
25%
1224
April21st to endNight
26%
720
20%
1800
5%
720
34%
720
26%
1224
May1st to 10thNight
23%
564
15%
1644
7%
564
31%
564
17%
1158
May11th to 20thNight
27%
365
18%
1427
10%
365
38%
365
16%
919
May21st to endNight
46%
388
15%
1551
14%
388
52%
388
18%
977
June1st to 10thNight
42%
360
17%
1440
15%
360
52%
360
20%
871
June11th to 20thNight
44%
360
15%
1440
15%
360
53%
360
20%
884
June21st to endNight
52%
360
14%
1440
16%
360
57%
360
25%
840
July1st to 10thNight
52%
350
13%
1400
15%
350
57%
350
20%
878
July11th to 20thNight
58%
348
17%
1392
20%
348
62%
348
24%
829
July21st to endNight
55%
394
18%
1486
17%
372
62%
372
25%
854
August1st to 10thNight
58%
517
16%
1581
12%
515
61%
515
27%
1003
August11th to 20thNight
54%
710
16%
1774
9%
710
56%
710
27%
1243
August21st to endNight
54%
792
17%
1980
8%
792
56%
792
29%
1339
September1st to 10thNight
45%
720
20%
1800
9%
720
50%
720
28%
1229
September11th to 20thNight
53%
720
28%
1854
18%
720
58%
720
36%
1151
September21st to endNight
45%
720
25%
1980
17%
720
52%
720
27%
1307
October1st to 10thNight
40%
720
17%
2114
12%
720
47%
720
24%
1377
October11th to 20thNight
38%
720
24%
2160
15%
720
50%
720
28%
1321
October21st to endNight
29%
792
22%
2477
13%
792
43%
792
24%
1570
November1st to 10thNight
36%
740
25%
2407
14%
740
52%
740
29%
1379
November11th to 20thNight
29%
760
25%
2470
14%
760
44%
760
22%
1516
November21st to endNight
26%
760
22%
2563
9%
760
44%
760
22%
1533
December1st to 10thNight
21%
760
19%
2660
7%
760
36%
760
16%
1707
December11th to 20thNight
21%
760
20%
3188
7%
759
38%
759
15%
1938
December21st to endNight
22%
829
13%
4790
8%
817
38%
817
10%
2966

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
15%6%35%5%35%5%5%7%40%
74724
Night
16%6%35%5%3445%7%43%
74940
TransparencyDay
42%7%12%21%16%3
74429
Night
41%6%12%22%16%3
74769
SeeingDay
21%15%29%22%10%3
13864
Night
19%12%31%28%9%1
23187

By Month

Percentage Cloud Cover NE (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
12%7%35%6%45%5%5%6%43%
1509
11th to 20thDay
15%7%46%6%345%5%7%38%
1440
21st to endDay
15%6%25%5%36%447%42%
1773
February1st to 10thDay
18%7%36%5%25%5%47%38%
1620
11th to 20thDay
17%7%36%5%34446%40%
1734
21st to endDay
20%9%45%4245%5%5%37%
1480
March1st to 10thDay
14%7%25%5%34446%45%
1797
11th to 20thDay
16%6%36%7%4445%7%40%
1937
21st to endDay
21%6%35%435%446%40%
2330
April1st to 10thDay
17%7%36%5%3445%6%41%
2160
11th to 20thDay
15%5%346%345%6%9%41%
2134
21st to endDay
18%5%35%5%4446%7%39%
2160
May1st to 10thDay
21%7%36%5%3445%8%35%
2317
11th to 20thDay
20%6%245%35%5%5%7%36%
2478
21st to endDay
14%6%25%6%45%45%8%40%
2725
June1st to 10thDay
14%5%35%5%36%6%6%10%37%
2520
11th to 20thDay
14%6%36%6%35%6%6%9%37%
2520
21st to endDay
12%5%25%6%45%6%5%9%41%
2520
July1st to 10thDay
10%6%25%6%5%7%6%6%10%37%
2520
11th to 20thDay
9%5%25%6%26%6%7%10%41%
2520
21st to endDay
11%7%37%6%45%6%6%8%38%
2744
August1st to 10thDay
9%6%26%6%5%5%7%7%9%38%
2324
11th to 20thDay
12%6%25%5%45%7%7%9%38%
2126
21st to endDay
15%6%25%4346%6%8%41%
2376
September1st to 10thDay
14%6%35%6%245%5%7%44%
2160
11th to 20thDay
13%5%25%424447%50%
2106
21st to endDay
17%6%35%6%35%447%40%
1980
October1st to 10thDay
20%6%2444435%6%42%
1843
11th to 20thDay
15%7%244245%5%6%47%
1800
21st to endDay
20%7%24423447%43%
1877
November1st to 10thDay
14%6%24433346%51%
1671
11th to 20thDay
17%5%2442436%5%47%
1710
21st to endDay
15%5%345%2445%7%46%
1615
December1st to 10thDay
18%7%35%425%45%8%39%
1520
11th to 20thDay
20%6%34425%446%42%
1906
21st to endDay
17%7%36%6%46%5%5%8%34%
2772
January1st to 10thNight
16%6%35%434446%46%
2647
11th to 20thNight
18%6%35%5%3445%7%40%
2520
21st to endNight
14%7%35%434447%45%
2578
February1st to 10thNight
17%9%35%434447%40%
2340
11th to 20thNight
18%8%45%434445%42%
2112
21st to endNight
19%7%25%434346%43%
1776
March1st to 10thNight
16%5%24435%35%7%46%
2150
11th to 20thNight
18%6%36%424445%43%
2020
21st to endNight
18%6%25%5%34347%42%
2026
April1st to 10thNight
17%6%36%4345%6%7%40%
1800
11th to 20thNight
16%7%35%6%3445%6%41%
1775
21st to endNight
20%6%2442446%6%43%
1800
May1st to 10thNight
20%9%36%5%34446%35%
1644
11th to 20thNight
19%7%36%5%25%45%7%38%
1426
21st to endNight
15%6%25%5%35%45%9%40%
1550
June1st to 10thNight
15%6%25%435%5%5%6%43%
1440
11th to 20thNight
13%6%35%6%445%6%9%40%
1440
21st to endNight
10%435%6%35%5%7%10%44%
1440
July1st to 10thNight
10%5%36%5%36%6%7%9%40%
1440
11th to 20thNight
9%5%25%6%46%6%7%9%42%
1440
21st to endNight
9%425%445%6%8%9%43%
1574
August1st to 10thNight
9%6%26%5%36%6%6%9%41%
1591
11th to 20thNight
11%6%36%6%35%6%7%8%38%
1774
21st to endNight
14%6%36%5%346%6%8%41%
1980
September1st to 10thNight
15%6%26%5%34447%44%
1800
11th to 20thNight
13%5%24323447%52%
1854
21st to endNight
17%7%25%5%235%46%43%
1980
October1st to 10thNight
19%6%35%423446%43%
2114
11th to 20thNight
15%5%2442445%6%49%
2160
21st to endNight
19%7%2442335%5%45%
2477
November1st to 10thNight
12%5%2443335%5%53%
2407
11th to 20thNight
19%5%34423345%47%
2470
21st to endNight
14%6%3443436%7%46%
2563
December1st to 10thNight
20%7%25%5%2445%6%40%
2660
11th to 20thNight
18%5%25%4345%5%6%42%
3288
21st to endNight
19%8%45%5%34446%38%
4884

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%5%12%23%14%5%
1510
11th to 20thDay
45%7%13%19%13%3
1440
21st to endDay
42%5%10%25%15%3
1773
February1st to 10thDay
46%6%12%23%11%2
1620
11th to 20thDay
46%5%9%23%15%3
1743
21st to endDay
48%5%9%20%14%4
1480
March1st to 10thDay
41%5%12%25%14%3
1800
11th to 20thDay
44%411%22%15%4
1937
21st to endDay
47%5%8%19%17%5%
2330
April1st to 10thDay
44%6%10%20%16%5%
2160
11th to 20thDay
38%6%13%22%17%4
2134
21st to endDay
43%7%10%18%18%4
2160
May1st to 10thDay
48%8%8%17%17%3
2317
11th to 20thDay
46%6%11%19%15%2
2482
21st to endDay
42%8%13%21%14%2
2725
June1st to 10thDay
40%9%14%22%14%2
2520
11th to 20thDay
43%9%13%21%12%2
2520
21st to endDay
39%9%13%22%16%1
2520
July1st to 10thDay
41%10%12%20%15%2
2450
11th to 20thDay
35%9%15%23%16%2
2445
21st to endDay
41%8%12%20%16%3
2591
August1st to 10thDay
38%10%13%22%15%1
2310
11th to 20thDay
40%8%15%20%14%2
2126
21st to endDay
39%8%11%20%18%3
2376
September1st to 10thDay
39%7%11%20%19%3
2160
11th to 20thDay
34%6%11%23%24%3
2106
21st to endDay
44%7%8%18%21%3
1980
October1st to 10thDay
43%6%12%20%17%2
1843
11th to 20thDay
37%5%10%24%21%2
1800
21st to endDay
42%5%10%23%18%2
1877
November1st to 10thDay
35%414%25%21%2
1671
11th to 20thDay
38%5%9%23%22%3
1710
21st to endDay
37%5%13%29%12%3
1615
December1st to 10thDay
44%6%15%21%13%1
1520
11th to 20thDay
43%5%13%25%12%2
1906
21st to endDay
45%9%13%22%10%1
2772
January1st to 10thNight
40%5%10%25%16%4
2647
11th to 20thNight
45%5%14%22%11%2
2520
21st to endNight
39%5%10%25%18%3
2578
February1st to 10thNight
44%6%13%23%12%2
2340
11th to 20thNight
45%5%9%19%17%5%
2114
21st to endNight
43%5%7%23%18%4
1776
March1st to 10thNight
38%5%12%24%17%5%
2160
11th to 20thNight
43%6%9%20%19%4
2021
21st to endNight
44%5%9%18%19%6%
2026
April1st to 10thNight
42%7%10%19%18%4
1800
11th to 20thNight
43%6%10%18%21%3
1775
21st to endNight
40%6%10%20%19%5%
1800
May1st to 10thNight
49%6%10%17%17%2
1644
11th to 20thNight
45%7%11%16%18%3
1427
21st to endNight
42%9%15%19%13%2
1551
June1st to 10thNight
40%7%12%20%19%2
1440
11th to 20thNight
41%8%14%24%12%1
1440
21st to endNight
34%9%17%24%14%2
1440
July1st to 10thNight
38%12%15%19%14%2
1400
11th to 20thNight
36%9%14%25%13%2
1392
21st to endNight
32%11%17%23%15%3
1486
August1st to 10thNight
36%10%18%21%13%2
1581
11th to 20thNight
40%10%13%22%13%1
1774
21st to endNight
39%9%12%19%19%2
1980
September1st to 10thNight
41%7%11%21%18%2
1800
11th to 20thNight
32%6%10%22%25%4
1854
21st to endNight
40%6%7%22%22%2
1980
October1st to 10thNight
42%6%11%23%16%2
2114
11th to 20thNight
36%6%11%28%17%3
2160
21st to endNight
42%410%24%18%2
2477
November1st to 10thNight
34%412%24%22%4
2407
11th to 20thNight
41%49%23%19%5%
2470
21st to endNight
38%5%13%27%16%2
2563
December1st to 10thNight
45%5%12%23%14%2
2660
11th to 20thNight
41%6%12%21%17%4
3288
21st to endNight
46%6%12%24%11%1
4884

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
28%22%28%19%31
178
11th to 20thDay
33%28%27%11%2
160
21st to endDay
22%28%33%12%4
260
February1st to 10thDay
31%27%30%9%3
240
11th to 20thDay
29%21%36%11%2
402
21st to endDay
31%21%31%15%2
346
March1st to 10thDay
27%22%30%18%3
420
11th to 20thDay
25%26%31%13%31
420
21st to endDay
27%23%30%13%5%1
462
April1st to 10thDay
27%22%32%10%7%1
420
11th to 20thDay
22%21%31%15%10%2
412
21st to endDay
30%20%31%13%42
420
May1st to 10thDay
30%20%28%14%7%2
423
11th to 20thDay
23%15%33%20%8%1
447
21st to endDay
21%10%23%28%16%3
485
June1st to 10thDay
17%11%32%26%13%3
450
11th to 20thDay
18%14%28%27%10%3
450
21st to endDay
16%11%31%29%11%3
450
July1st to 10thDay
13%6%23%30%22%6%
445
11th to 20thDay
9%5%25%34%20%6%
444
21st to endDay
13%8%30%30%15%4
488
August1st to 10thDay
11%6%25%35%16%7%
446
11th to 20thDay
14%422%33%22%5%
439
21st to endDay
15%6%26%29%15%10%
495
September1st to 10thDay
17%8%26%24%16%9%
450
11th to 20thDay
15%8%27%26%15%9%
450
21st to endDay
21%13%25%24%11%6%
450
October1st to 10thDay
27%7%25%23%13%6%
450
11th to 20thDay
18%16%26%27%11%2
450
21st to endDay
22%18%29%19%9%2
444
November1st to 10thDay
19%12%30%23%13%3
365
11th to 20thDay
25%17%32%18%8%1
370
21st to endDay
20%17%36%19%7%1
275
December1st to 10thDay
27%25%30%10%8%
180
11th to 20thDay
38%19%26%12%6%
180
21st to endDay
29%17%32%15%6%2
198
January1st to 10thNight
19%19%39%19%3
760
11th to 20thNight
23%16%34%23%3
720
21st to endNight
19%20%38%21%1
792
February1st to 10thNight
22%19%35%22%2
720
11th to 20thNight
24%22%35%17%1
703
21st to endNight
25%19%38%16%2
592
March1st to 10thNight
23%19%34%21%3
720
11th to 20thNight
24%18%35%18%4
720
21st to endNight
21%14%40%20%5%
792
April1st to 10thNight
22%19%37%18%4
720
11th to 20thNight
20%13%38%23%5%
711
21st to endNight
22%13%35%25%5%
720
May1st to 10thNight
24%16%31%24%5%
564
11th to 20thNight
21%10%37%27%5%
365
21st to endNight
17%7%24%39%13%1
389
June1st to 10thNight
17%6%28%37%13%1
360
11th to 20thNight
12%6%30%40%11%
360
21st to endNight
10%328%46%13%
360
July1st to 10thNight
12%324%41%19%1
350
11th to 20thNight
11%321%43%20%1
348
21st to endNight
10%325%45%18%
394
August1st to 10thNight
12%318%45%20%2
517
11th to 20thNight
13%319%38%24%3
710
21st to endNight
15%418%41%20%2
792
September1st to 10thNight
16%5%25%34%17%2
720
11th to 20thNight
16%5%22%33%21%4
720
21st to endNight
21%7%21%34%17%1
720
October1st to 10thNight
23%8%25%27%16%3
720
11th to 20thNight
17%11%29%32%11%
720
21st to endNight
23%12%31%25%8%1
792
November1st to 10thNight
15%11%33%29%12%1
740
11th to 20thNight
21%16%29%26%7%
760
21st to endNight
17%16%34%24%7%
760
December1st to 10thNight
25%18%30%20%6%1
760
11th to 20thNight
21%16%35%24%4
760
21st to endNight
22%17%33%26%3
836
Page generated at 2024-01-07 10:06:55. Version ac07

Copyright 2024 Attilla Danko


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