Chapel Hill
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chapel Hill clear sky chart from 2005-11-06 to 2024-01-16.

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
42%
24520
22%
74700
13%
24470
52%
24470
27%
46105

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
24%
757
24%
2459
9%
757
42%
757
21%
1479
January11th to 20thNight
27%
744
25%
2417
11%
744
45%
744
21%
1512
January21st to endNight
24%
792
28%
2574
12%
792
45%
792
26%
1501
February1st to 10thNight
26%
720
21%
2340
11%
720
41%
720
24%
1436
February11th to 20thNight
17%
700
20%
2270
6%
699
35%
699
18%
1428
February21st to endNight
23%
592
30%
1628
11%
592
42%
592
26%
1034
March1st to 10thNight
29%
720
28%
1980
12%
720
44%
720
30%
1195
March11th to 20thNight
27%
720
21%
1980
9%
720
39%
720
24%
1303
March21st to endNight
27%
792
21%
2178
9%
792
38%
792
25%
1417
April1st to 10thNight
30%
720
25%
1877
10%
720
41%
720
30%
1176
April11th to 20thNight
34%
711
26%
1775
10%
711
42%
711
35%
1105
April21st to endNight
38%
720
23%
1800
9%
720
44%
720
33%
1157
May1st to 10thNight
38%
560
19%
1640
8%
560
45%
560
30%
1014
May11th to 20thNight
46%
531
17%
1592
11%
531
50%
531
26%
1028
May21st to endNight
60%
582
16%
1744
13%
582
62%
582
29%
1094
June1st to 10thNight
58%
540
16%
1620
14%
540
59%
540
28%
1032
June11th to 20thNight
58%
540
16%
1620
13%
540
61%
540
29%
1027
June21st to endNight
63%
540
17%
1620
13%
540
66%
540
30%
1023
July1st to 10thNight
65%
525
14%
1575
12%
525
66%
525
28%
1016
July11th to 20thNight
69%
522
14%
1566
13%
522
69%
522
30%
989
July21st to endNight
67%
591
17%
1672
13%
558
70%
558
30%
1069
August1st to 10thNight
62%
537
13%
1600
10%
534
63%
534
25%
1074
August11th to 20thNight
63%
572
17%
1636
14%
572
63%
572
26%
1113
August21st to endNight
63%
792
18%
1980
12%
792
65%
792
32%
1330
September1st to 10thNight
65%
720
23%
1800
17%
720
66%
720
36%
1177
September11th to 20thNight
59%
720
25%
1934
20%
720
62%
720
32%
1236
September21st to endNight
47%
720
19%
2009
14%
720
53%
720
24%
1396
October1st to 10thNight
52%
720
25%
2160
22%
720
60%
720
31%
1313
October11th to 20thNight
46%
720
30%
2160
23%
720
58%
720
31%
1271
October21st to endNight
43%
792
32%
2376
20%
792
60%
792
33%
1354
November1st to 10thNight
45%
740
28%
2221
20%
740
56%
740
32%
1303
November11th to 20thNight
39%
760
29%
2301
17%
760
53%
760
29%
1356
November21st to endNight
40%
760
27%
2541
18%
760
54%
760
29%
1422
December1st to 10thNight
31%
760
21%
2660
11%
760
46%
760
19%
1671
December11th to 20thNight
34%
760
26%
3012
16%
758
49%
758
23%
1724
December21st to endNight
30%
828
17%
4383
13%
817
44%
817
14%
2330

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%5%25%5%35%5%6%8%43%
74650
Night
14%5%25%5%3445%7%45%
75155
TransparencyDay
39%412%23%18%4
74372
Night
39%411%22%20%5%
74952
SeeingDay
17%11%25%24%17%6%
14212
Night
17%10%24%30%17%2
24536

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
17%6%26%434335%46%
1706
11th to 20thDay
18%7%3442445%6%45%
1671
21st to endDay
16%6%34324445%48%
1782
February1st to 10thDay
15%7%25%5%35%46%7%43%
1620
11th to 20thDay
22%6%2442334446%
1573
21st to endDay
19%7%36%432446%41%
1627
March1st to 10thDay
14%5%1442445%6%52%
1980
11th to 20thDay
14%7%35%5%25%5%49%42%
1980
21st to endDay
17%7%35%6%34446%41%
2178
April1st to 10thDay
11%42342445%8%55%
2083
11th to 20thDay
11%5%25%4445%5%8%48%
2134
21st to endDay
15%5%34435%46%7%45%
2160
May1st to 10thDay
12%5%25%435%5%7%8%42%
2334
11th to 20thDay
14%6%25%6%36%6%6%8%39%
2301
21st to endDay
12%5%25%5%46%5%6%8%40%
2531
June1st to 10thDay
10%5%25%6%45%7%8%10%39%
2340
11th to 20thDay
9%5%25%6%35%5%7%10%43%
2340
21st to endDay
7%5%35%6%46%7%7%11%40%
2340
July1st to 10thDay
8%5%27%7%5%7%7%8%10%36%
2340
11th to 20thDay
7%5%36%6%46%8%10%13%33%
2340
21st to endDay
7%5%26%7%5%8%7%7%12%34%
2547
August1st to 10thDay
10%5%36%7%46%7%9%11%31%
2327
11th to 20thDay
10%7%38%7%5%6%7%7%8%32%
2245
21st to endDay
11%6%35%6%46%6%7%10%36%
2376
September1st to 10thDay
9%425%5%35%5%7%9%46%
2160
11th to 20thDay
15%5%25%5%35%5%5%8%42%
2021
21st to endDay
21%7%26%42445%6%39%
1949
October1st to 10thDay
15%5%26%423346%50%
1800
11th to 20thDay
13%5%24324345%56%
1800
21st to endDay
12%5%1442334456%
1980
November1st to 10thDay
14%424423345%56%
1857
11th to 20thDay
15%424323346%53%
1877
21st to endDay
14%424423446%51%
1630
December1st to 10thDay
18%7%345%25%447%42%
1520
11th to 20thDay
16%424322346%55%
2077
21st to endDay
17%6%25%43445%7%42%
3124
January1st to 10thNight
16%6%25%433446%47%
2459
11th to 20thNight
15%7%36%434446%44%
2417
21st to endNight
16%5%24424345%50%
2574
February1st to 10thNight
18%5%25%43345%6%45%
2340
11th to 20thNight
20%6%35%324445%44%
2270
21st to endNight
16%6%2443345%6%45%
1628
March1st to 10thNight
13%5%24424445%52%
1980
11th to 20thNight
18%5%245%35%45%6%42%
1980
21st to endNight
15%6%35%43445%7%44%
2178
April1st to 10thNight
14%5%24424447%49%
1877
11th to 20thNight
14%425%5%23446%51%
1775
21st to endNight
12%425%5%26%5%5%7%47%
1800
May1st to 10thNight
10%425%6%35%5%6%9%45%
1640
11th to 20thNight
16%436%5%3445%8%41%
1592
21st to endNight
11%6%25%5%35%5%6%10%42%
1744
June1st to 10thNight
10%6%246%45%6%8%9%40%
1620
11th to 20thNight
10%5%36%6%35%5%8%10%40%
1620
21st to endNight
8%5%35%6%35%5%7%12%40%
1620
July1st to 10thNight
8%5%36%7%45%6%8%11%38%
1620
11th to 20thNight
7%5%25%5%36%7%9%12%38%
1620
21st to endNight
6%426%7%47%7%7%13%37%
1771
August1st to 10thNight
11%5%36%7%45%6%8%12%33%
1611
11th to 20thNight
11%6%35%6%46%6%8%9%35%
1636
21st to endNight
9%5%26%7%45%5%7%11%38%
1980
September1st to 10thNight
12%5%25%43446%8%47%
1800
11th to 20thNight
14%6%25%5%25%46%7%45%
1934
21st to endNight
18%8%36%5%345%6%6%37%
2009
October1st to 10thNight
15%6%35%5%23445%50%
2160
11th to 20thNight
14%6%25%423346%52%
2160
21st to endNight
13%5%24433345%55%
2376
November1st to 10thNight
14%6%25%423335%53%
2221
11th to 20thNight
12%5%25%435%446%51%
2301
21st to endNight
14%4234235%5%6%51%
2541
December1st to 10thNight
17%7%35%6%34445%42%
2660
11th to 20thNight
16%6%2442345%5%49%
3109
21st to endNight
18%6%3442335%7%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
43%27%24%19%5%
1707
11th to 20thDay
42%37%20%24%4
1671
21st to endDay
39%36%22%23%7%
1782
February1st to 10thDay
41%310%22%18%5%
1620
11th to 20thDay
43%26%21%22%6%
1582
21st to endDay
46%39%17%19%6%
1627
March1st to 10thDay
35%38%23%24%7%
1980
11th to 20thDay
41%49%21%18%7%
1980
21st to endDay
46%39%20%18%4
2178
April1st to 10thDay
29%410%25%25%8%
2083
11th to 20thDay
35%310%23%21%8%
2134
21st to endDay
39%310%24%20%4
2160
May1st to 10thDay
38%416%21%17%3
2334
11th to 20thDay
42%5%14%24%12%2
2305
21st to endDay
41%5%15%25%14%1
2531
June1st to 10thDay
38%6%15%25%15%1
2340
11th to 20thDay
35%5%18%24%16%2
2340
21st to endDay
36%7%18%23%15%2
2340
July1st to 10thDay
41%7%16%20%14%3
2275
11th to 20thDay
38%8%20%23%10%2
2271
21st to endDay
40%7%20%25%9%1
2405
August1st to 10thDay
44%6%18%22%9%1
2314
11th to 20thDay
46%7%16%20%9%1
2245
21st to endDay
42%5%17%22%11%3
2376
September1st to 10thDay
34%6%16%21%18%6%
2160
11th to 20thDay
41%411%19%22%4
2021
21st to endDay
46%37%21%17%4
1949
October1st to 10thDay
38%29%23%24%4
1800
11th to 20thDay
32%28%28%22%8%
1800
21st to endDay
33%36%24%25%9%
1980
November1st to 10thDay
33%27%22%28%8%
1857
11th to 20thDay
34%28%22%27%8%
1877
21st to endDay
35%37%24%25%6%
1630
December1st to 10thDay
44%210%21%19%4
1520
11th to 20thDay
33%17%26%24%9%
2074
21st to endDay
42%210%24%20%2
3124
January1st to 10thNight
40%28%20%23%7%
2459
11th to 20thNight
42%49%18%21%5%
2417
21st to endNight
38%25%23%26%6%
2574
February1st to 10thNight
41%39%22%20%6%
2340
11th to 20thNight
44%37%17%21%8%
2272
21st to endNight
40%39%21%20%8%
1628
March1st to 10thNight
35%37%22%24%9%
1980
11th to 20thNight
43%38%21%19%6%
1980
21st to endNight
40%5%9%21%20%5%
2178
April1st to 10thNight
37%39%18%27%7%
1877
11th to 20thNight
35%29%24%20%9%
1775
21st to endNight
37%411%25%19%5%
1800
May1st to 10thNight
37%414%24%18%4
1640
11th to 20thNight
43%413%23%16%2
1593
21st to endNight
37%7%16%25%14%1
1745
June1st to 10thNight
38%7%16%23%14%2
1620
11th to 20thNight
38%6%18%21%15%2
1620
21st to endNight
35%9%17%21%14%3
1620
July1st to 10thNight
39%7%17%21%13%3
1575
11th to 20thNight
34%9%23%22%9%2
1566
21st to endNight
37%7%21%22%12%
1672
August1st to 10thNight
42%8%20%20%9%1
1600
11th to 20thNight
42%8%18%20%9%2
1636
21st to endNight
39%6%19%24%9%2
1980
September1st to 10thNight
35%6%14%22%17%6%
1800
11th to 20thNight
38%5%11%20%23%3
1934
21st to endNight
47%5%11%18%16%4
2009
October1st to 10thNight
38%38%24%24%4
2160
11th to 20thNight
36%37%24%23%8%
2160
21st to endNight
33%36%21%26%11%
2376
November1st to 10thNight
35%37%19%27%9%
2221
11th to 20thNight
36%37%18%25%12%
2301
21st to endNight
34%36%24%26%7%
2541
December1st to 10thNight
45%38%20%20%5%
2660
11th to 20thNight
39%27%21%22%9%
3111
21st to endNight
40%26%26%24%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
27%21%36%14%3
270
11th to 20thDay
26%23%33%13%4
257
21st to endDay
21%23%42%11%3
264
February1st to 10thDay
23%20%41%13%3
240
11th to 20thDay
39%20%33%7%1
240
21st to endDay
26%21%32%17%3
345
March1st to 10thDay
23%19%34%17%7%
420
11th to 20thDay
21%23%39%11%5%1
420
21st to endDay
21%20%36%18%41
462
April1st to 10thDay
22%18%33%18%7%2
420
11th to 20thDay
15%19%30%23%10%4
412
21st to endDay
19%12%30%26%11%2
420
May1st to 10thDay
18%17%31%22%8%4
423
11th to 20thDay
16%9%23%30%18%5%
447
21st to endDay
21%5%18%27%21%8%
485
June1st to 10thDay
12%423%32%22%6%
450
11th to 20thDay
9%8%22%29%25%8%
450
21st to endDay
12%321%33%25%6%
450
July1st to 10thDay
10%211%33%33%11%
445
11th to 20thDay
6%212%31%36%12%
444
21st to endDay
9%215%35%27%12%
488
August1st to 10thDay
15%317%28%26%11%
446
11th to 20thDay
12%213%29%27%16%
439
21st to endDay
12%411%29%27%16%
495
September1st to 10thDay
12%417%25%28%15%
450
11th to 20thDay
17%412%30%27%11%
450
21st to endDay
25%6%22%26%17%3
450
October1st to 10thDay
18%8%21%24%21%8%
450
11th to 20thDay
16%8%25%28%18%5%
450
21st to endDay
15%13%25%29%14%4
495
November1st to 10thDay
19%10%30%24%14%3
455
11th to 20thDay
19%15%28%26%11%1
437
21st to endDay
14%18%31%21%13%2
232
December1st to 10thDay
23%18%38%12%8%1
180
11th to 20thDay
21%18%35%19%7%1
234
21st to endDay
26%17%30%17%9%1
297
January1st to 10thNight
21%18%31%24%5%
760
11th to 20thNight
20%17%31%25%7%
744
21st to endNight
19%16%37%24%3
792
February1st to 10thNight
22%13%35%27%4
720
11th to 20thNight
25%20%34%20%1
703
21st to endNight
19%16%36%24%4
592
March1st to 10thNight
17%16%35%27%5%
720
11th to 20thNight
22%16%31%21%9%1
720
21st to endNight
18%14%37%26%5%
792
April1st to 10thNight
18%17%31%26%8%
720
11th to 20thNight
15%14%33%25%12%1
711
21st to endNight
16%9%33%34%8%
720
May1st to 10thNight
14%10%33%33%9%
560
11th to 20thNight
17%5%24%34%18%1
531
21st to endNight
14%313%40%28%3
584
June1st to 10thNight
13%316%41%25%2
540
11th to 20thNight
12%415%37%27%5%
540
21st to endNight
10%314%39%31%3
540
July1st to 10thNight
8%113%33%41%4
525
11th to 20thNight
8%29%38%36%7%
522
21st to endNight
6%313%40%34%5%
591
August1st to 10thNight
13%29%36%35%4
537
11th to 20thNight
13%39%33%34%7%
572
21st to endNight
12%39%31%38%7%
792
September1st to 10thNight
15%210%33%33%7%
720
11th to 20thNight
17%314%30%31%5%
720
21st to endNight
25%416%33%22%1
720
October1st to 10thNight
18%5%17%30%25%6%
720
11th to 20thNight
17%9%23%28%20%2
720
21st to endNight
16%10%26%29%18%2
792
November1st to 10thNight
17%8%24%34%14%2
740
11th to 20thNight
15%13%29%32%11%
760
21st to endNight
16%13%27%30%13%1
760
December1st to 10thNight
21%14%29%28%8%
760
11th to 20thNight
19%11%31%31%8%
760
21st to endNight
22%15%26%29%7%
836
Page generated at 2024-01-13 19:28:31. Version ac07

Copyright 2024 Attilla Danko


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