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 2017-08-12.

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%
14923
20%
46491
10%
14892
44%
14892
23%
29674

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
23%
720
20%
2506
9%
716
39%
716
18%
1569
January16th to endNight
18%
768
22%
2559
6%
768
37%
768
20%
1587
February1st to 15thNight
18%
704
17%
2240
3%
701
35%
701
18%
1443
February16th to endNight
17%
631
26%
1893
7%
631
40%
631
20%
1198
March1st to 15thNight
20%
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
29%
720
19%
1800
5%
720
36%
720
27%
1229
May1st to 15thNight
28%
503
17%
1583
8%
503
37%
503
19%
1067
May16th to endNight
43%
374
15%
1497
13%
374
51%
374
19%
942
June1st to 15thNight
40%
360
14%
1440
12%
360
50%
360
18%
907
June16th to endNight
55%
360
16%
1440
20%
360
61%
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%
572
18%
1621
13%
570
61%
570
32%
1011
August16th to endNight
59%
700
16%
1750
9%
700
61%
700
30%
1173
September1st to 15thNight
51%
660
24%
1650
13%
660
56%
660
30%
1096
September16th to endNight
52%
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%
2397
10%
720
40%
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
36%
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%
47486
Night
15%6%2442445%6%43%
46661
Cloud CoverDay
15%6%25%43445%7%40%
47495
Night
15%6%2442445%6%43%
46667
TransparencyDay
42%6%11%21%15%2
47174
Night
40%5%11%22%16%2
46491
SeeingDay
19%11%26%24%13%4
5628
Night
18%12%30%28%9%
14924

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%245%35%446%40%
1432
16th to endDay
15%6%24335%437%42%
1662
February1st to 15thDay
16%6%25%5%3445%6%40%
1621
16th to endDay
15%7%45%434446%39%
1580
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
11%5%25%5%35%5%7%8%38%
2229
16th to endDay
14%5%245%35%6%5%8%39%
2100
September1st to 15thDay
14%5%24423447%46%
1980
16th to endDay
15%5%15%5%24447%43%
1837
October1st to 15thDay
16%7%23423345%45%
1677
16th to endDay
18%424322446%46%
1697
November1st to 15thDay
17%7%24323345%46%
1581
16th to endDay
17%425%31335%7%44%
1560
December1st to 15thDay
16%7%34325%35%8%40%
1440
16th to endDay
20%6%3432445%7%38%
1536
January1st to 15thNight
17%5%24324346%42%
2506
16th to endNight
16%7%34323446%43%
2559
February1st to 15thNight
14%8%34433446%41%
2239
16th to endNight
15%6%24323445%45%
1893
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%6%6%8%45%
1631
16th to endNight
12%5%25%4345%6%8%41%
1750
September1st to 15thNight
14%5%14434346%46%
1650
16th to endNight
13%6%24313446%48%
1793
October1st to 15thNight
16%6%24323445%46%
1952
16th to endNight
17%5%14422345%48%
2174
November1st to 15thNight
15%5%25%42323449%
2277
16th to endNight
17%5%35%424346%40%
2397
December1st to 15thNight
20%6%25%5%23446%37%
2520
16th to endNight
17%7%34424445%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%25%5%35%45%6%40%
1432
16th to endDay
16%6%24325%446%42%
1662
February1st to 15thDay
16%7%26%5%24446%40%
1621
16th to endDay
15%6%37%5%2445%5%39%
1580
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%
2229
16th to endDay
13%6%25%5%35%5%6%7%38%
2100
September1st to 15thDay
13%5%25%32435%8%45%
1980
16th to endDay
14%5%25%5%34447%42%
1837
October1st to 15thDay
16%8%23423346%45%
1677
16th to endDay
18%42432335%5%46%
1697
November1st to 15thDay
16%6%24323345%45%
1581
16th to endDay
17%5%2432335%6%45%
1560
December1st to 15thDay
16%7%34315%35%7%40%
1440
16th to endDay
19%6%3432445%7%38%
1536
January1st to 15thNight
17%6%24334446%42%
2506
16th to endNight
16%7%34323446%43%
2559
February1st to 15thNight
14%7%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%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%
1631
16th to endNight
12%5%244345%6%8%41%
1750
September1st to 15thNight
14%5%145%25%436%46%
1650
16th to endNight
13%6%2432235%7%48%
1793
October1st to 15thNight
15%6%2431335%5%46%
1952
16th to endNight
16%5%24422345%48%
2174
November1st to 15thNight
15%5%25%42323449%
2277
16th to endNight
17%6%25%424446%39%
2397
December1st to 15thNight
20%6%25%5%2345%6%37%
2520
16th to endNight
18%7%25%42445%6%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
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
2215
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
1677
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%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
33%8%15%23%16%2
1621
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
2397
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
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
13%317%36%20%8%
297
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%
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
12%118%43%22%1
572
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 2017-08-13 14:16:28. Version ac04

Copyright 2017 A. Danko


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