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 2005-11-05 to 2017-04-26.

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
33%
14680
20%
45617
10%
14649
44%
14649
23%
29126

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
22%
720
20%
2506
9%
716
38%
716
18%
1569
January16th to endNight
18%
768
22%
2559
7%
768
37%
768
20%
1587
February1st to 15thNight
18%
704
17%
2240
3%
701
34%
701
18%
1443
February16th to endNight
17%
631
27%
1893
7%
631
40%
631
20%
1198
March1st to 15thNight
21%
720
20%
2127
7%
720
36%
720
20%
1383
March16th to endNight
22%
768
22%
2010
8%
768
33%
768
25%
1344
April1st to 15thNight
22%
718
17%
1796
4%
718
30%
718
25%
1220
April16th to endNight
28%
704
19%
1760
5%
704
35%
704
26%
1204
May1st to 15thNight
30%
460
18%
1450
9%
460
39%
460
20%
965
May16th to endNight
44%
342
15%
1369
12%
342
51%
342
19%
856
June1st to 15thNight
38%
330
15%
1320
12%
330
48%
330
17%
849
June16th to endNight
57%
330
16%
1320
21%
330
61%
330
28%
732
July1st to 15thNight
50%
310
13%
1240
16%
310
55%
310
21%
782
July16th to endNight
57%
348
15%
1306
16%
326
64%
326
24%
761
August1st to 15thNight
60%
538
19%
1519
13%
536
62%
536
32%
947
August16th to endNight
58%
700
16%
1750
9%
700
60%
700
30%
1173
September1st to 15thNight
50%
660
24%
1650
13%
660
55%
660
30%
1096
September16th to endNight
51%
657
25%
1793
18%
657
56%
657
34%
1110
October1st to 15thNight
41%
660
22%
1952
16%
660
50%
660
26%
1231
October16th to endNight
31%
704
23%
2174
13%
704
45%
704
26%
1344
November1st to 15thNight
33%
700
23%
2277
13%
700
48%
700
24%
1388
November16th to endNight
28%
720
19%
2398
10%
720
41%
720
19%
1543
December1st to 15thNight
21%
720
16%
2520
6%
720
34%
720
16%
1654
December16th to endNight
19%
768
20%
2688
6%
768
37%
768
16%
1747

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%5%2443445%7%40%
46049
Night
15%6%2442445%6%43%
45787
Cloud CoverDay
15%6%25%42445%7%40%
46058
Night
15%6%24424446%43%
45793
TransparencyDay
42%6%11%21%15%2
45737
Night
40%5%11%22%16%2
45617
SeeingDay
19%11%26%24%13%5%
5208
Night
18%12%30%27%9%
14681

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 15thDay
15%6%35%5%345%5%6%39%
1432
16th to endDay
15%6%24425%437%41%
1662
February1st to 15thDay
16%6%35%424446%40%
1621
16th to endDay
15%7%45%5%345%5%5%39%
1580
March1st to 15thDay
17%6%26%6%34347%38%
1822
16th to endDay
18%5%24433335%43%
2214
April1st to 15thDay
18%6%24423348%41%
2152
16th to endDay
15%42442446%7%42%
2112
May1st to 15thDay
19%7%245%34447%36%
2178
16th to endDay
15%5%245%35%46%8%37%
2394
June1st to 15thDay
16%5%25%5%35%5%5%10%34%
2310
16th to endDay
12%425%5%35%6%6%8%38%
2310
July1st to 15thDay
11%5%26%5%46%6%7%10%35%
2310
16th to endDay
9%6%36%5%35%6%6%8%38%
2458
August1st to 15thDay
11%5%25%5%446%6%8%39%
2087
16th to endDay
14%5%25%4346%5%7%39%
2100
September1st to 15thDay
14%424424447%46%
1980
16th to endDay
15%4245%34447%43%
1837
October1st to 15thDay
17%7%14423345%45%
1676
16th to endDay
17%5%23313446%46%
1697
November1st to 15thDay
17%6%14423345%46%
1581
16th to endDay
17%42432335%6%45%
1560
December1st to 15thDay
16%7%34315%45%8%40%
1440
16th to endDay
20%6%34334447%37%
1536
January1st to 15thNight
17%5%24434347%42%
2506
16th to endNight
16%7%34424436%43%
2559
February1st to 15thNight
14%8%35%434446%41%
2239
16th to endNight
15%6%2432445%5%46%
1893
March1st to 15thNight
18%5%24325%25%6%43%
2114
16th to endNight
17%7%24423335%45%
2010
April1st to 15thNight
16%5%243245%5%7%41%
1796
16th to endNight
17%6%2442346%6%42%
1760
May1st to 15thNight
17%8%25%42435%7%39%
1450
16th to endNight
15%5%245%25%348%41%
1367
June1st to 15thNight
17%6%25%435%5%46%39%
1320
16th to endNight
9%4145%3436%10%45%
1320
July1st to 15thNight
9%5%25%5%35%6%6%10%38%
1320
16th to endNight
6%3245%45%6%8%8%43%
1402
August1st to 15thNight
9%425%435%5%6%7%46%
1529
16th to endNight
13%5%25%4345%6%7%41%
1750
September1st to 15thNight
15%415%424446%46%
1650
16th to endNight
12%6%24413446%48%
1793
October1st to 15thNight
16%5%34313446%46%
1952
16th to endNight
17%5%1332335%5%48%
2174
November1st to 15thNight
16%6%25%32324449%
2277
16th to endNight
17%5%345%2435%6%39%
2398
December1st to 15thNight
20%6%25%5%2445%5%37%
2520
16th to endNight
18%7%34425%446%40%
2688

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
14%6%345%35%45%6%40%
1432
16th to endDay
15%6%24335%446%42%
1662
February1st to 15thDay
16%6%25%5%24346%40%
1621
16th to endDay
15%7%45%5%345%46%39%
1580
March1st to 15thDay
17%5%35%6%34347%38%
1832
16th to endDay
19%5%24424236%43%
2213
April1st to 15thDay
18%6%24413347%42%
2152
16th to endDay
15%4144345%5%7%42%
2112
May1st to 15thDay
18%7%25%5%35%446%36%
2178
16th to endDay
15%5%25%435%5%5%7%38%
2394
June1st to 15thDay
16%5%24436%6%6%9%35%
2310
16th to endDay
12%425%5%46%6%6%8%38%
2310
July1st to 15thDay
11%5%25%5%36%6%8%9%35%
2310
16th to endDay
8%6%26%5%36%6%6%9%38%
2458
August1st to 15thDay
11%5%35%6%345%6%8%39%
2087
16th to endDay
14%5%245%25%5%6%7%39%
2100
September1st to 15thDay
14%5%2442345%7%45%
1980
16th to endDay
15%4245%34447%42%
1837
October1st to 15thDay
16%8%14323345%45%
1676
16th to endDay
18%424322446%46%
1697
November1st to 15thDay
17%7%24323345%46%
1581
16th to endDay
16%425%31335%7%44%
1560
December1st to 15thDay
16%7%34415%35%7%41%
1440
16th to endDay
19%7%3432346%6%38%
1536
January1st to 15thNight
17%5%23424346%42%
2506
16th to endNight
16%7%24323446%43%
2559
February1st to 15thNight
14%8%35%433446%41%
2232
16th to endNight
15%6%24323446%45%
1893
March1st to 15thNight
18%5%24324346%43%
2127
16th to endNight
16%7%34422335%45%
2010
April1st to 15thNight
16%5%24325%45%7%41%
1796
16th to endNight
17%6%1442345%7%42%
1760
May1st to 15thNight
16%7%35%5%24347%39%
1450
16th to endNight
14%6%335%3445%8%40%
1367
June1st to 15thNight
17%5%35%4245%48%38%
1320
16th to endNight
9%415%43346%9%46%
1320
July1st to 15thNight
9%5%26%5%26%6%6%9%40%
1320
16th to endNight
6%3245%35%7%7%9%42%
1402
August1st to 15thNight
10%4244345%6%7%46%
1529
16th to endNight
12%5%344345%5%8%41%
1750
September1st to 15thNight
14%424434346%46%
1650
16th to endNight
13%5%2432345%6%48%
1793
October1st to 15thNight
15%6%25%313445%46%
1952
16th to endNight
17%5%23422335%48%
2174
November1st to 15thNight
15%5%25%32323449%
2277
16th to endNight
17%6%35%434446%40%
2398
December1st to 15thNight
20%6%25%42345%6%37%
2520
16th to endNight
18%7%35%424445%39%
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
42%6%13%21%11%3
1433
16th to endDay
42%5%11%24%13%3
1662
February1st to 15thDay
45%5%11%22%13%2
1624
16th to endDay
45%410%25%13%1
1580
March1st to 15thDay
46%413%21%11%2
1832
16th to endDay
44%39%21%17%3
2214
April1st to 15thDay
42%410%23%14%4
2152
16th to endDay
39%6%10%16%23%3
2112
May1st to 15thDay
47%6%9%18%16%1
2178
16th to endDay
43%7%13%19%14%2
2398
June1st to 15thDay
44%8%13%21%10%
2310
16th to endDay
40%9%13%21%12%1
2310
July1st to 15thDay
42%8%11%19%15%2
2161
16th to endDay
39%8%13%22%12%3
2291
August1st to 15thDay
39%7%13%20%16%1
2073
16th to endDay
41%7%10%19%18%3
2100
September1st to 15thDay
36%7%10%20%23%1
1980
16th to endDay
40%5%8%18%23%2
1837
October1st to 15thDay
40%5%13%20%18%1
1676
16th to endDay
38%5%10%25%17%2
1697
November1st to 15thDay
41%412%20%19%2
1581
16th to endDay
39%312%25%16%2
1560
December1st to 15thDay
42%5%16%21%12%1
1440
16th to endDay
45%5%14%20%12%
1536
January1st to 15thNight
42%5%12%22%14%3
2506
16th to endNight
41%411%25%14%1
2559
February1st to 15thNight
42%5%13%21%14%2
2240
16th to endNight
39%5%8%24%19%3
1893
March1st to 15thNight
42%5%12%21%15%3
2127
16th to endNight
42%410%19%18%4
2010
April1st to 15thNight
40%6%11%20%17%3
1796
16th to endNight
40%6%8%18%23%3
1760
May1st to 15thNight
45%5%10%17%18%2
1450
16th to endNight
41%8%15%16%16%3
1369
June1st to 15thNight
45%7%12%18%15%
1320
16th to endNight
33%6%15%29%14%1
1320
July1st to 15thNight
39%9%14%20%13%2
1240
16th to endNight
32%11%16%24%12%2
1306
August1st to 15thNight
33%8%15%24%16%2
1519
16th to endNight
38%7%11%20%20%2
1750
September1st to 15thNight
38%6%10%21%22%1
1650
16th to endNight
35%47%23%25%2
1793
October1st to 15thNight
39%5%10%24%17%2
1952
16th to endNight
38%410%25%18%1
2174
November1st to 15thNight
39%310%20%19%5%
2277
16th to endNight
44%411%24%13%2
2398
December1st to 15thNight
46%5%13%20%12%1
2520
16th to endNight
45%5%12%22%12%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
21%26%31%13%5%1
60
16th to endDay
25%20%35%16%2
85
February1st to 15thDay
26%24%32%16%
137
16th to endDay
17%21%37%19%4
239
March1st to 15thDay
23%19%27%24%4
270
16th to endDay
21%18%35%15%5%2
288
April1st to 15thDay
23%14%35%14%10%2
270
16th to endDay
22%11%31%20%9%5%
254
May1st to 15thDay
30%12%22%18%12%2
228
16th to endDay
17%8%20%30%19%2
267
June1st to 15thDay
18%7%30%24%16%2
255
16th to endDay
17%425%33%13%5%
255
July1st to 15thDay
16%418%31%22%7%
245
16th to endDay
10%323%34%21%7%
270
August1st to 15thDay
12%316%38%20%8%
253
16th to endDay
14%211%30%25%15%
268
September1st to 15thDay
13%6%23%23%20%11%
255
16th to endDay
17%5%20%25%14%16%
254
October1st to 15thDay
22%5%20%26%17%7%
255
16th to endDay
16%14%32%23%13%
261
November1st to 15thDay
20%12%27%22%14%2
235
16th to endDay
17%16%35%18%11%1
180
December1st to 15thDay
25%26%21%11%15%
60
16th to endDay
35%18%31%12%1
64
January1st to 15thNight
19%18%35%22%4
720
16th to endNight
20%18%38%21%1
768
February1st to 15thNight
19%19%38%21%
704
16th to endNight
18%22%38%18%2
631
March1st to 15thNight
24%18%34%19%3
720
16th to endNight
20%14%37%20%6%
768
April1st to 15thNight
19%16%38%21%4
718
16th to endNight
18%12%36%25%6%
704
May1st to 15thNight
20%10%33%28%6%
460
16th to endNight
16%426%39%10%1
343
June1st to 15thNight
20%5%27%36%10%
330
16th to endNight
8%225%49%13%
330
July1st to 15thNight
10%227%41%17%
310
16th to endNight
7%224%44%20%
348
August1st to 15thNight
13%118%44%21%1
538
16th to endNight
14%215%40%24%3
700
September1st to 15thNight
16%5%22%36%17%1
660
16th to endNight
16%320%31%24%3
657
October1st to 15thNight
19%8%25%29%15%1
660
16th to endNight
20%12%31%27%7%
704
November1st to 15thNight
18%13%29%24%12%1
700
16th to endNight
19%16%29%25%8%
720
December1st to 15thNight
24%15%31%19%7%
720
16th to endNight
21%16%36%23%2
768
Page generated at 2017-04-27 17:49:03. Version ac04

Copyright 2017 A. Danko


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