Princeton University
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Princeton University clear sky chart from 2005-11-05 to 2018-02-22.

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%
15702
20%
48903
10%
15671
44%
15671
23%
31167

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
21%
780
20%
2716
9%
776
38%
776
18%
1685
January16th to endNight
19%
832
22%
2772
7%
832
38%
832
20%
1719
February1st to 15thNight
17%
764
17%
2431
3%
761
34%
761
18%
1586
February16th to endNight
16%
659
27%
1976
7%
659
39%
659
20%
1262
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
23%
718
17%
1796
4%
718
30%
718
25%
1220
April16th to endNight
28%
720
19%
1800
5%
720
35%
720
27%
1229
May1st to 15thNight
29%
503
17%
1583
8%
503
38%
503
19%
1067
May16th to endNight
43%
374
15%
1497
13%
374
50%
374
19%
942
June1st to 15thNight
41%
360
14%
1440
12%
360
51%
360
18%
907
June16th to endNight
55%
360
16%
1440
20%
360
60%
360
27%
805
July1st to 15thNight
51%
340
12%
1360
15%
340
54%
340
21%
853
July16th to endNight
57%
376
16%
1418
17%
354
64%
354
24%
830
August1st to 15thNight
59%
587
18%
1658
12%
585
61%
585
32%
1038
August16th to endNight
58%
764
18%
1910
10%
764
60%
764
31%
1276
September1st to 15thNight
48%
720
23%
1800
12%
720
53%
720
31%
1189
September16th to endNight
52%
717
26%
1956
19%
717
57%
717
34%
1206
October1st to 15thNight
41%
720
21%
2130
15%
720
49%
720
26%
1350
October16th to endNight
32%
768
25%
2372
15%
768
46%
768
27%
1448
November1st to 15thNight
33%
760
22%
2472
12%
760
47%
760
24%
1511
November16th to endNight
28%
780
21%
2597
11%
780
42%
780
20%
1635
December1st to 15thNight
21%
780
16%
2730
6%
780
34%
780
15%
1811
December16th to endNight
19%
832
20%
2912
6%
832
36%
832
16%
1871

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%2442445%7%40%
49354
Night
15%6%2442445%6%43%
49073
Cloud CoverDay
15%6%25%43445%7%40%
49363
Night
15%6%2442445%6%43%
49079
TransparencyDay
42%6%11%21%16%2
49042
Night
40%5%11%22%16%2
48903
SeeingDay
19%11%27%23%13%4
6241
Night
18%12%30%27%9%
15703

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%5%245%35%346%41%
1552
16th to endDay
16%6%24325%437%42%
1801
February1st to 15thDay
17%6%25%5%34346%40%
1760
16th to endDay
15%7%45%434446%39%
1644
March1st to 15thDay
17%5%35%6%25%347%38%
1821
16th to endDay
19%5%24424336%43%
2214
April1st to 15thDay
18%6%25%313347%42%
2152
16th to endDay
14%4245%3445%7%42%
2160
May1st to 15thDay
19%6%25%5%2445%6%36%
2375
16th to endDay
17%6%245%25%5%5%7%37%
2618
June1st to 15thDay
15%5%245%35%5%5%9%36%
2520
16th to endDay
12%4245%45%6%6%8%38%
2520
July1st to 15thDay
11%5%25%436%5%7%9%35%
2520
16th to endDay
9%6%26%5%35%6%6%8%38%
2654
August1st to 15thDay
12%5%25%5%35%5%7%8%38%
2277
16th to endDay
14%4245%346%5%8%39%
2292
September1st to 15thDay
14%5%24423447%46%
2160
16th to endDay
14%5%15%5%24447%43%
2004
October1st to 15thDay
17%7%2442335%5%44%
1829
16th to endDay
18%424322446%47%
1851
November1st to 15thDay
17%6%24323345%44%
1716
16th to endDay
16%42431335%6%46%
1690
December1st to 15thDay
16%7%34415%35%7%40%
1560
16th to endDay
19%6%343245%5%7%38%
1664
January1st to 15thNight
17%5%24324346%43%
2716
16th to endNight
15%7%34323446%44%
2772
February1st to 15thNight
15%8%34433446%40%
2430
16th to endNight
16%6%24323445%45%
1976
March1st to 15thNight
18%5%14334346%43%
2114
16th to endNight
17%7%34423335%45%
2010
April1st to 15thNight
16%5%24325%46%7%41%
1796
16th to endNight
17%5%2442335%7%42%
1800
May1st to 15thNight
17%7%26%42435%6%38%
1583
16th to endNight
16%6%235%25%448%41%
1495
June1st to 15thNight
17%435%425%447%40%
1440
16th to endNight
9%4145%3447%9%45%
1440
July1st to 15thNight
10%426%5%26%6%6%10%40%
1440
16th to endNight
7%3246%35%6%7%8%43%
1514
August1st to 15thNight
9%424435%5%6%8%45%
1668
16th to endNight
13%5%25%43445%8%42%
1910
September1st to 15thNight
14%414434347%46%
1800
16th to endNight
12%5%24413447%48%
1956
October1st to 15thNight
16%6%24323445%46%
2130
16th to endNight
16%5%14322335%49%
2372
November1st to 15thNight
15%5%25%423345%48%
2472
16th to endNight
16%5%34424346%41%
2597
December1st to 15thNight
20%6%25%5%23446%36%
2730
16th to endNight
17%7%3442445%5%41%
2912

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%5%25%5%3445%6%42%
1552
16th to endDay
16%6%24325%447%42%
1801
February1st to 15thDay
17%7%26%5%23446%40%
1760
16th to endDay
15%6%36%5%2445%5%39%
1644
March1st to 15thDay
16%6%35%6%34346%39%
1831
16th to endDay
19%5%24433336%43%
2213
April1st to 15thDay
18%6%24313346%42%
2152
16th to endDay
14%4245%3446%6%42%
2160
May1st to 15thDay
18%6%25%435%45%6%36%
2375
16th to endDay
16%6%25%435%5%5%7%37%
2618
June1st to 15thDay
14%5%245%35%5%5%9%36%
2520
16th to endDay
11%5%25%5%36%6%6%8%38%
2520
July1st to 15thDay
11%5%25%5%46%6%7%10%34%
2520
16th to endDay
8%6%26%5%36%6%6%8%37%
2654
August1st to 15thDay
11%5%26%5%446%6%8%38%
2277
16th to endDay
13%6%245%25%5%6%8%39%
2292
September1st to 15thDay
13%5%2432435%7%45%
2160
16th to endDay
14%5%25%5%34447%42%
2004
October1st to 15thDay
16%8%23423346%44%
1829
16th to endDay
18%413322346%47%
1851
November1st to 15thDay
17%6%25%323346%44%
1716
16th to endDay
16%42432335%6%47%
1690
December1st to 15thDay
16%7%34315%35%7%40%
1560
16th to endDay
19%7%3432445%7%38%
1664
January1st to 15thNight
17%6%24323446%43%
2716
16th to endNight
16%7%34323446%43%
2772
February1st to 15thNight
15%7%35%4235%46%40%
2423
16th to endNight
16%6%24323446%45%
1976
March1st to 15thNight
18%5%24324346%43%
2127
16th to endNight
16%7%25%232345%45%
2010
April1st to 15thNight
17%5%243245%5%7%41%
1796
16th to endNight
16%5%2442345%6%42%
1800
May1st to 15thNight
17%7%36%434446%38%
1583
16th to endNight
16%6%2344445%7%40%
1495
June1st to 15thNight
16%5%25%42445%7%40%
1440
16th to endNight
8%4145%2447%8%45%
1440
July1st to 15thNight
9%5%25%5%26%6%6%10%39%
1440
16th to endNight
7%3245%46%5%7%9%42%
1514
August1st to 15thNight
9%4244345%6%8%45%
1668
16th to endNight
12%5%2443445%8%42%
1910
September1st to 15thNight
14%414425%447%46%
1800
16th to endNight
12%5%2442245%7%48%
1956
October1st to 15thNight
15%6%2432345%5%46%
2130
16th to endNight
16%5%23422345%49%
2372
November1st to 15thNight
15%5%25%423245%47%
2472
16th to endNight
16%6%25%424446%41%
2597
December1st to 15thNight
19%7%26%5%2345%6%36%
2730
16th to endNight
17%7%25%42445%6%40%
2912

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
41%6%12%22%12%4
1553
16th to endDay
42%5%11%24%14%2
1801
February1st to 15thDay
46%5%11%22%12%2
1763
16th to endDay
45%410%25%13%1
1644
March1st to 15thDay
46%413%21%11%2
1831
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
2160
May1st to 15thDay
47%6%9%17%17%2
2375
16th to endDay
44%7%13%19%13%1
2622
June1st to 15thDay
43%8%13%22%11%
2520
16th to endDay
40%9%12%22%13%1
2520
July1st to 15thDay
42%8%12%18%15%2
2371
16th to endDay
39%8%13%22%12%2
2487
August1st to 15thDay
40%7%13%20%16%1
2263
16th to endDay
39%7%10%20%18%3
2292
September1st to 15thDay
36%6%10%20%22%1
2160
16th to endDay
41%5%8%19%22%2
2004
October1st to 15thDay
41%5%12%19%19%1
1829
16th to endDay
37%49%26%18%2
1851
November1st to 15thDay
41%412%20%17%2
1716
16th to endDay
37%311%25%18%2
1690
December1st to 15thDay
43%5%15%21%13%1
1560
16th to endDay
45%5%14%19%13%
1664
January1st to 15thNight
41%5%11%22%15%4
2716
16th to endNight
41%311%25%15%1
2772
February1st to 15thNight
43%5%13%20%14%2
2431
16th to endNight
40%5%8%23%19%3
1976
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%22%3
1800
May1st to 15thNight
46%5%10%16%18%2
1583
16th to endNight
42%8%14%17%15%2
1497
June1st to 15thNight
43%7%12%19%16%
1440
16th to endNight
34%6%15%28%14%1
1440
July1st to 15thNight
38%10%15%19%13%2
1360
16th to endNight
33%10%16%24%12%2
1418
August1st to 15thNight
34%8%15%23%16%2
1658
16th to endNight
38%7%10%20%20%2
1910
September1st to 15thNight
37%6%10%22%21%1
1800
16th to endNight
34%5%7%23%25%2
1956
October1st to 15thNight
39%5%11%24%17%2
2130
16th to endNight
37%410%27%18%2
2372
November1st to 15thNight
39%411%20%18%5%
2472
16th to endNight
43%310%24%14%2
2597
December1st to 15thNight
47%5%13%20%11%1
2730
16th to endNight
44%5%12%22%13%1
2912

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
22%28%30%12%42
90
16th to endDay
25%18%37%17%1
128
February1st to 15thDay
29%25%31%13%
186
16th to endDay
19%20%39%17%3
265
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
21%11%32%19%9%4
270
May1st to 15thDay
30%16%23%17%9%2
288
16th to endDay
22%7%21%29%16%2
331
June1st to 15thDay
17%8%31%24%15%2
315
16th to endDay
17%8%27%30%11%4
315
July1st to 15thDay
16%423%29%19%5%
305
16th to endDay
11%322%32%22%6%
326
August1st to 15thDay
15%317%35%19%8%
313
16th to endDay
13%217%31%23%12%
332
September1st to 15thDay
13%9%25%23%18%10%
315
16th to endDay
16%7%20%25%14%15%
314
October1st to 15thDay
23%6%20%27%16%7%
315
16th to endDay
16%14%30%21%15%1
319
November1st to 15thDay
21%11%29%22%13%1
280
16th to endDay
16%19%35%17%10%
220
December1st to 15thDay
28%23%25%8%13%
90
16th to endDay
33%23%30%11%1
96
January1st to 15thNight
19%18%35%21%4
780
16th to endNight
20%18%37%21%
832
February1st to 15thNight
19%19%37%21%
764
16th to endNight
19%22%37%18%2
659
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%
720
May1st to 15thNight
21%12%31%28%6%
503
16th to endNight
18%427%37%10%1
375
June1st to 15thNight
19%5%26%37%11%
360
16th to endNight
9%127%48%13%
360
July1st to 15thNight
10%226%42%17%
340
16th to endNight
8%223%44%20%
376
August1st to 15thNight
13%118%42%22%1
587
16th to endNight
14%215%40%23%2
764
September1st to 15thNight
16%5%24%35%16%1
720
16th to endNight
15%420%31%24%3
717
October1st to 15thNight
19%7%25%29%15%2
720
16th to endNight
20%12%31%26%8%
768
November1st to 15thNight
18%12%30%25%12%1
760
16th to endNight
18%16%30%24%8%
780
December1st to 15thNight
24%15%31%20%7%
780
16th to endNight
21%17%35%22%2
832
Page generated at 2018-02-23 11:21:33. Version ac04

Copyright 2018 A. Danko


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