Jefferson Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Jefferson Observatory clear sky chart from 2005-11-06 to 2024-02-20.

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
46%
24072
22%
79582
14%
23965
55%
23965
24%
49247

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
22%
756
24%
3999
8%
737
41%
737
12%
2240
January11th to 20thNight
27%
758
27%
3933
10%
740
44%
740
14%
2174
January21st to endNight
24%
829
18%
4417
10%
814
44%
814
13%
2717
February1st to 10thNight
31%
760
26%
2280
12%
760
44%
760
25%
1426
February11th to 20thNight
23%
742
21%
2213
6%
738
38%
738
20%
1455
February21st to endNight
24%
592
22%
1776
10%
592
36%
592
21%
1184
March1st to 10thNight
33%
720
27%
2071
14%
720
44%
720
28%
1287
March11th to 20thNight
32%
720
21%
1980
8%
720
42%
720
26%
1289
March21st to endNight
33%
792
22%
2178
10%
792
43%
792
27%
1411
April1st to 10thNight
40%
720
25%
1980
12%
720
50%
720
31%
1204
April11th to 20thNight
43%
557
26%
1791
16%
557
56%
557
31%
1019
April21st to endNight
50%
540
29%
1620
19%
540
59%
540
34%
940
May1st to 10thNight
49%
540
22%
1620
11%
540
58%
540
31%
970
May11th to 20thNight
52%
531
16%
1593
10%
531
57%
531
25%
1038
May21st to endNight
62%
582
19%
1744
15%
582
64%
582
28%
1119
June1st to 10thNight
64%
540
15%
1620
13%
540
66%
540
27%
1049
June11th to 20thNight
67%
540
16%
1620
13%
540
69%
540
32%
1006
June21st to endNight
69%
540
14%
1620
11%
540
70%
540
30%
1031
July1st to 10thNight
65%
525
11%
1575
10%
525
66%
525
22%
1099
July11th to 20thNight
69%
522
11%
1566
9%
522
70%
522
23%
1072
July21st to endNight
70%
591
15%
1672
13%
558
71%
558
24%
1152
August1st to 10thNight
67%
537
13%
1600
12%
534
67%
534
23%
1097
August11th to 20thNight
63%
534
19%
1661
18%
534
64%
534
23%
1139
August21st to endNight
62%
602
15%
1988
16%
602
63%
602
19%
1392
September1st to 10thNight
66%
720
19%
1980
16%
720
67%
720
29%
1291
September11th to 20thNight
58%
720
22%
1980
18%
720
61%
720
28%
1315
September21st to endNight
57%
720
25%
1980
19%
720
63%
720
33%
1245
October1st to 10thNight
57%
720
27%
1980
21%
720
63%
720
35%
1186
October11th to 20thNight
50%
720
31%
2119
23%
720
61%
720
33%
1245
October21st to endNight
51%
792
33%
2376
25%
792
63%
792
37%
1310
November1st to 10thNight
51%
740
30%
2338
25%
740
61%
740
34%
1306
November11th to 20thNight
41%
760
28%
2470
20%
760
55%
760
31%
1388
November21st to endNight
42%
760
26%
2470
18%
760
53%
760
27%
1481
December1st to 10thNight
32%
760
25%
2470
13%
760
47%
760
20%
1593
December11th to 20thNight
33%
759
28%
2894
15%
758
48%
758
22%
1642
December21st to endNight
28%
831
15%
4408
11%
817
40%
817
11%
2735

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
14%5%25%5%35%5%6%7%42%
78761
Night
15%5%25%5%3445%7%45%
80441
TransparencyDay
39%7%11%19%18%5%
78484
Night
38%7%10%19%20%6%
80237
SeeingDay
18%9%22%23%19%9%
13951
Night
17%8%22%29%20%4
24093

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%8%35%4345%5%6%40%
2836
11th to 20thDay
14%5%24424445%52%
2840
21st to endDay
28%6%2431233444%
3156
February1st to 10thDay
17%7%25%4445%5%6%41%
1900
11th to 20thDay
22%6%24424435%44%
1838
21st to endDay
22%6%35%424346%40%
1480
March1st to 10thDay
15%425%42435%6%49%
1887
11th to 20thDay
19%6%25%5%34446%41%
1980
21st to endDay
16%6%35%5%35%5%6%6%41%
2178
April1st to 10thDay
13%4245%35%5%6%7%46%
1980
11th to 20thDay
13%5%35%5%345%46%47%
2132
21st to endDay
10%4244345%5%8%52%
2340
May1st to 10thDay
10%425%4345%5%8%49%
2340
11th to 20thDay
13%5%36%6%35%6%6%8%40%
2301
21st to endDay
11%5%25%6%5%7%6%6%9%39%
2531
June1st to 10thDay
10%5%36%7%5%7%6%7%10%35%
2340
11th to 20thDay
6%3245%5%7%8%9%11%40%
2340
21st to endDay
7%427%6%47%7%8%11%38%
2340
July1st to 10thDay
10%6%35%7%48%8%8%11%31%
2340
11th to 20thDay
10%5%36%6%5%8%8%10%11%29%
2340
21st to endDay
6%436%8%5%8%9%9%11%31%
2547
August1st to 10thDay
9%6%37%8%5%8%8%8%11%29%
2327
11th to 20thDay
12%5%37%8%47%6%7%9%32%
2235
21st to endDay
13%7%37%7%5%5%7%7%8%31%
2349
September1st to 10thDay
10%6%36%6%46%5%5%8%40%
1980
11th to 20thDay
15%7%25%5%25%5%7%8%40%
1980
21st to endDay
15%6%24424446%49%
1980
October1st to 10thDay
14%5%23323345%55%
1980
11th to 20thDay
13%424322345%57%
1841
21st to endDay
14%424423335%56%
1980
November1st to 10thDay
12%42333334458%
1736
11th to 20thDay
14%424325%446%53%
1710
21st to endDay
15%5%2443435%7%49%
1710
December1st to 10thDay
22%7%36%43446%5%38%
1710
11th to 20thDay
18%5%245%23346%47%
2153
21st to endDay
23%6%25%42345%8%39%
3124
January1st to 10thNight
19%6%24423446%45%
4109
11th to 20thNight
13%7%36%5%3445%5%46%
4120
21st to endNight
25%8%3432333442%
4499
February1st to 10thNight
15%6%35%434446%46%
2280
11th to 20thNight
22%5%35%42343444%
2213
21st to endNight
21%7%25%434446%41%
1776
March1st to 10thNight
15%6%25%3245%35%51%
2071
11th to 20thNight
16%424435%5%5%7%44%
1980
21st to endNight
15%5%35%5%2445%6%46%
2178
April1st to 10thNight
12%42443445%7%51%
1980
11th to 20thNight
13%42342445%8%51%
1791
21st to endNight
11%4144345%48%52%
1620
May1st to 10thNight
10%424435%5%6%9%48%
1620
11th to 20thNight
10%6%36%6%36%5%6%8%42%
1592
21st to endNight
10%6%25%6%35%5%6%9%43%
1744
June1st to 10thNight
8%425%7%46%7%8%12%37%
1620
11th to 20thNight
6%435%5%35%7%8%12%43%
1620
21st to endNight
6%5%25%6%45%6%8%11%42%
1620
July1st to 10thNight
8%6%36%7%47%6%8%11%34%
1620
11th to 20thNight
8%5%26%6%5%8%6%8%14%33%
1620
21st to endNight
7%436%7%5%6%8%9%13%32%
1771
August1st to 10thNight
10%5%25%6%46%8%9%14%31%
1611
11th to 20thNight
11%6%26%6%45%6%6%11%36%
1661
21st to endNight
13%6%36%7%5%6%6%7%9%33%
1988
September1st to 10thNight
10%5%26%6%45%5%6%9%42%
1980
11th to 20thNight
15%6%245%25%5%6%7%43%
1980
21st to endNight
14%415%5%2446%7%48%
1980
October1st to 10thNight
14%424423446%54%
1980
11th to 20thNight
13%5%2432435%455%
2119
21st to endNight
13%323323445%57%
2376
November1st to 10thNight
13%5%1332434458%
2338
11th to 20thNight
16%423333346%54%
2470
21st to endNight
17%4145%3445%6%48%
2470
December1st to 10thNight
19%6%35%5%24346%42%
2470
11th to 20thNight
18%6%24423335%52%
3042
21st to endNight
21%7%26%5%35%45%6%36%
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%6%7%16%20%8%
2837
11th to 20thDay
33%5%8%21%26%8%
2840
21st to endDay
46%3417%21%9%
3156
February1st to 10thDay
44%48%19%20%6%
1900
11th to 20thDay
43%46%18%22%7%
1848
21st to endDay
46%5%8%14%18%9%
1480
March1st to 10thDay
35%410%18%22%11%
1887
11th to 20thDay
45%5%9%18%16%7%
1980
21st to endDay
42%7%10%21%18%4
2178
April1st to 10thDay
36%6%9%20%22%6%
1980
11th to 20thDay
38%5%7%18%22%9%
2132
21st to endDay
30%6%10%28%21%4
2340
May1st to 10thDay
32%7%13%22%22%4
2340
11th to 20thDay
40%9%14%21%14%2
2305
21st to endDay
39%9%12%23%16%2
2531
June1st to 10thDay
41%10%17%19%10%2
2340
11th to 20thDay
33%11%20%23%12%1
2340
21st to endDay
36%12%17%19%12%4
2340
July1st to 10thDay
41%15%16%18%8%2
2275
11th to 20thDay
42%14%20%15%6%3
2271
21st to endDay
39%15%18%20%8%
2405
August1st to 10thDay
45%13%17%20%6%
2314
11th to 20thDay
46%12%16%15%9%2
2235
21st to endDay
47%11%15%15%9%2
2349
September1st to 10thDay
41%9%13%16%16%4
1980
11th to 20thDay
40%9%10%18%21%2
1980
21st to endDay
36%5%11%21%20%7%
1980
October1st to 10thDay
32%5%9%20%28%7%
1980
11th to 20thDay
31%48%23%25%9%
1841
21st to endDay
31%45%19%29%12%
1980
November1st to 10thDay
30%47%15%31%12%
1736
11th to 20thDay
32%48%20%26%10%
1710
21st to endDay
35%6%7%19%26%7%
1710
December1st to 10thDay
48%47%18%16%7%
1710
11th to 20thDay
40%47%17%24%9%
2150
21st to endDay
43%5%9%19%21%3
3124
January1st to 10thNight
40%5%6%17%25%7%
4109
11th to 20thNight
38%5%6%21%21%9%
4120
21st to endNight
46%45%13%25%7%
4499
February1st to 10thNight
40%48%18%22%8%
2280
11th to 20thNight
44%45%17%21%8%
2214
21st to endNight
45%48%16%18%9%
1776
March1st to 10thNight
37%47%17%22%12%
2071
11th to 20thNight
38%7%12%15%21%7%
1980
21st to endNight
39%5%9%22%21%5%
2178
April1st to 10thNight
32%6%11%20%24%7%
1980
11th to 20thNight
32%5%7%22%23%10%
1791
21st to endNight
31%5%11%26%21%6%
1620
May1st to 10thNight
31%9%13%21%22%4
1620
11th to 20thNight
38%9%16%20%16%2
1593
21st to endNight
36%10%11%25%16%3
1745
June1st to 10thNight
35%12%18%20%12%2
1620
11th to 20thNight
30%12%18%25%13%1
1620
21st to endNight
32%12%19%20%12%4
1620
July1st to 10thNight
41%13%19%17%8%2
1575
11th to 20thNight
38%14%21%17%8%2
1566
21st to endNight
37%15%20%21%6%2
1672
August1st to 10thNight
39%14%20%20%6%
1600
11th to 20thNight
41%12%17%18%12%1
1661
21st to endNight
44%12%17%16%9%2
1988
September1st to 10thNight
37%9%16%17%17%4
1980
11th to 20thNight
39%9%11%19%19%3
1980
21st to endNight
35%6%13%18%20%7%
1980
October1st to 10thNight
32%6%10%18%27%7%
1980
11th to 20thNight
33%5%8%21%24%10%
2119
21st to endNight
29%46%20%29%12%
2376
November1st to 10thNight
30%47%19%26%13%
2338
11th to 20thNight
32%47%17%27%12%
2470
21st to endNight
37%5%7%19%24%8%
2470
December1st to 10thNight
44%47%20%18%6%
2470
11th to 20thNight
36%47%18%28%7%
3044
21st to endNight
46%6%7%21%17%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
26%20%37%14%11
270
11th to 20thDay
31%17%29%19%4
270
21st to endDay
26%22%37%12%31
297
February1st to 10thDay
24%20%31%21%2
270
11th to 20thDay
40%16%31%12%1
267
21st to endDay
36%20%31%10%3
198
March1st to 10thDay
24%14%30%23%9%1
327
11th to 20thDay
25%19%31%18%7%1
420
21st to endDay
23%18%28%23%8%
462
April1st to 10thDay
22%13%29%23%11%1
420
11th to 20thDay
16%15%32%21%15%1
412
21st to endDay
16%12%28%23%16%5%
420
May1st to 10thDay
14%14%30%21%15%6%
423
11th to 20thDay
17%7%21%27%17%11%
447
21st to endDay
15%7%18%25%21%15%
485
June1st to 10thDay
12%316%32%26%10%
450
11th to 20thDay
10%416%28%23%19%
450
21st to endDay
10%320%30%22%15%
450
July1st to 10thDay
12%211%24%28%23%
445
11th to 20thDay
11%210%22%36%19%
444
21st to endDay
6%19%26%39%20%
488
August1st to 10thDay
13%5%15%21%27%20%
446
11th to 20thDay
13%210%25%34%16%
439
21st to endDay
17%212%21%29%18%
495
September1st to 10thDay
11%314%25%31%15%
450
11th to 20thDay
16%5%12%24%30%13%
450
21st to endDay
16%417%24%30%10%
450
October1st to 10thDay
19%420%28%19%10%
450
11th to 20thDay
14%9%24%26%22%5%
450
21st to endDay
17%9%24%25%20%4
495
November1st to 10thDay
22%8%23%28%18%2
334
11th to 20thDay
18%20%30%24%6%2
270
21st to endDay
16%12%35%24%11%2
270
December1st to 10thDay
32%14%29%21%3
270
11th to 20thDay
26%13%31%24%5%1
270
21st to endDay
35%16%25%15%6%2
297
January1st to 10thNight
26%16%29%23%6%
760
11th to 20thNight
22%14%31%26%7%
760
21st to endNight
22%16%33%25%3
836
February1st to 10thNight
20%14%31%30%6%
760
11th to 20thNight
25%15%32%24%4
742
21st to endNight
24%14%33%26%3
592
March1st to 10thNight
18%14%32%31%4
720
11th to 20thNight
19%10%34%27%9%1
720
21st to endNight
19%13%30%29%10%
792
April1st to 10thNight
15%13%29%32%11%1
720
11th to 20thNight
15%10%26%32%16%
557
21st to endNight
13%7%26%33%21%1
540
May1st to 10thNight
12%8%25%31%22%1
540
11th to 20thNight
13%5%22%31%24%4
531
21st to endNight
15%212%33%33%5%
584
June1st to 10thNight
10%214%37%31%6%
540
11th to 20thNight
6%216%30%33%13%
540
21st to endNight
9%111%38%32%9%
540
July1st to 10thNight
10%29%26%42%12%
525
11th to 20thNight
11%28%19%48%13%
522
21st to endNight
6%210%26%45%11%
591
August1st to 10thNight
15%17%26%39%12%
537
11th to 20thNight
13%28%26%41%10%
534
21st to endNight
15%111%24%38%11%
602
September1st to 10thNight
11%311%26%41%9%
720
11th to 20thNight
17%313%27%36%5%
720
21st to endNight
15%215%31%33%3
720
October1st to 10thNight
15%5%18%28%28%6%
720
11th to 20thNight
16%6%22%31%22%3
720
21st to endNight
14%8%22%33%22%2
792
November1st to 10thNight
15%7%22%33%21%2
740
11th to 20thNight
18%10%26%31%14%1
760
21st to endNight
19%8%27%31%14%1
760
December1st to 10thNight
22%13%27%30%8%1
760
11th to 20thNight
20%11%29%29%10%
760
21st to endNight
29%11%25%29%6%
836
Page generated at 2024-02-17 06:28:38. Version ac07

Copyright 2024 Attilla Danko


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