Jefferson Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Jefferson Observatory clear sky chart from 2005-11-05 to 2018-01-20.

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
42%
15937
18%
48162
13%
15896
48%
15896
23%
32427

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
19%
780
12%
2522
7%
776
26%
776
11%
1953
January16th to endNight
20%
788
13%
2560
8%
788
28%
788
13%
1918
February1st to 15thNight
23%
701
13%
2202
7%
701
29%
701
15%
1611
February16th to endNight
22%
631
13%
1893
6%
631
29%
631
15%
1418
March1st to 15thNight
28%
720
15%
2072
9%
720
34%
720
18%
1489
March16th to endNight
32%
768
16%
2112
8%
768
38%
768
22%
1509
April1st to 15thNight
31%
603
16%
1861
9%
603
40%
603
24%
1234
April16th to endNight
39%
540
21%
1620
9%
540
50%
540
27%
1054
May1st to 15thNight
39%
540
15%
1620
9%
540
45%
540
21%
1139
May16th to endNight
59%
562
17%
1684
14%
561
62%
561
29%
1062
June1st to 15thNight
65%
540
18%
1602
15%
540
69%
540
36%
938
June16th to endNight
64%
540
17%
1440
13%
540
67%
540
35%
878
July1st to 15thNight
63%
510
17%
1396
15%
510
64%
510
33%
898
July16th to endNight
73%
564
21%
1595
18%
531
75%
531
39%
901
August1st to 15thNight
72%
539
22%
1609
19%
536
73%
536
35%
958
August16th to endNight
76%
573
23%
1794
22%
573
78%
573
38%
990
September1st to 15thNight
66%
681
27%
1941
23%
681
69%
681
37%
1140
September16th to endNight
61%
717
30%
1980
24%
717
64%
717
35%
1200
October1st to 15thNight
53%
720
26%
2045
22%
720
58%
720
32%
1269
October16th to endNight
37%
768
23%
2369
18%
768
45%
768
23%
1570
November1st to 15thNight
42%
760
24%
2471
19%
760
51%
760
28%
1498
November16th to endNight
31%
780
19%
2535
13%
780
40%
780
17%
1759
December1st to 15thNight
23%
780
11%
2535
6%
780
29%
780
13%
1907
December16th to endNight
20%
832
11%
2704
5%
832
27%
832
11%
2134

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%8%36%5%3445%7%34%
49355
Night
14%7%36%5%3445%7%38%
48348
Cloud CoverDay
14%8%36%6%35%45%7%34%
49365
Night
13%7%36%5%3445%7%38%
48352
TransparencyDay
47%6%11%20%12%1
49059
Night
45%5%11%21%14%2
48162
SeeingDay
17%6%23%23%18%10%
6032
Night
17%7%25%30%16%2
15941

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
20%11%6%9%7%35%44420%
1746
16th to endDay
14%15%6%11%6%45%445%21%
1768
February1st to 15thDay
16%14%6%9%6%35%335%25%
1659
16th to endDay
14%15%6%8%6%3444426%
1580
March1st to 15thDay
21%10%47%5%3335%6%26%
1876
16th to endDay
19%10%46%5%34348%28%
2112
April1st to 15thDay
17%8%45%5%33446%35%
2097
16th to endDay
18%6%25%5%3445%7%37%
2340
May1st to 15thDay
21%8%445%35%35%7%30%
2331
16th to endDay
10%5%36%5%36%5%5%9%37%
2431
June1st to 15thDay
10%436%5%35%5%6%9%38%
2353
16th to endDay
10%425%5%36%6%6%9%38%
2520
July1st to 15thDay
12%5%25%5%36%5%6%9%36%
2478
16th to endDay
8%425%5%36%6%7%9%39%
2465
August1st to 15thDay
7%425%6%35%6%7%9%40%
2340
16th to endDay
8%425%436%5%6%9%43%
2405
September1st to 15thDay
12%6%24425%46%6%45%
2007
16th to endDay
14%5%245%23347%46%
1980
October1st to 15thDay
13%5%25%5%3445%6%41%
1914
16th to endDay
19%7%35%323347%39%
1854
November1st to 15thDay
12%8%35%423446%44%
1717
16th to endDay
17%10%37%6%23446%33%
1755
December1st to 15thDay
22%12%5%8%7%33436%20%
1755
16th to endDay
21%15%5%9%5%34346%21%
1872
January1st to 15thNight
18%12%6%10%7%34345%22%
2522
16th to endNight
15%13%6%8%6%35%44427%
2560
February1st to 15thNight
18%11%5%7%5%3434432%
2200
16th to endNight
16%12%5%8%6%35%335%28%
1893
March1st to 15thNight
18%6%37%6%34446%32%
2060
16th to endNight
17%8%445%34447%34%
2112
April1st to 15thNight
15%7%35%42445%6%39%
1861
16th to endNight
14%5%25%5%3445%8%40%
1620
May1st to 15thNight
19%6%35%5%35%35%8%33%
1620
16th to endNight
10%434425%6%7%9%41%
1682
June1st to 15thNight
5%43442446%9%50%
1602
16th to endNight
6%224435%5%8%11%45%
1440
July1st to 15thNight
8%425%5%245%6%7%47%
1479
16th to endNight
5%225%43446%9%50%
1703
August1st to 15thNight
6%324435%5%6%10%47%
1620
16th to endNight
6%21343345%10%52%
1794
September1st to 15thNight
11%413423446%53%
1941
16th to endNight
11%4245%23346%52%
1980
October1st to 15thNight
11%5%25%42345%7%47%
2045
16th to endNight
17%7%25%5%23345%40%
2369
November1st to 15thNight
14%5%34423447%44%
2471
16th to endNight
18%9%46%5%24447%33%
2535
December1st to 15thNight
21%12%48%5%34335%27%
2535
16th to endNight
22%14%6%8%5%24346%21%
2704

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
19%11%6%9%8%35%445%19%
1746
16th to endDay
13%14%7%10%7%46%345%21%
1768
February1st to 15thDay
16%13%6%9%7%25%445%25%
1659
16th to endDay
14%14%6%8%6%45%445%26%
1580
March1st to 15thDay
21%10%47%6%3345%6%26%
1887
16th to endDay
19%10%47%5%34348%27%
2111
April1st to 15thDay
16%9%35%5%3445%6%35%
2097
16th to endDay
17%6%26%5%25%45%7%37%
2340
May1st to 15thDay
20%8%35%5%25%35%7%29%
2331
16th to endDay
10%6%36%5%36%5%6%9%37%
2431
June1st to 15thDay
9%6%26%5%35%5%6%9%38%
2353
16th to endDay
9%425%5%36%6%6%10%38%
2520
July1st to 15thDay
12%6%25%6%36%5%6%9%35%
2478
16th to endDay
7%5%25%5%36%6%7%10%37%
2465
August1st to 15thDay
7%5%25%6%45%6%7%9%40%
2340
16th to endDay
7%425%5%35%5%7%9%42%
2405
September1st to 15thDay
12%6%14425%45%6%45%
2007
16th to endDay
13%5%25%5%23347%46%
1980
October1st to 15thDay
12%6%25%6%3445%7%41%
1914
16th to endDay
19%7%25%323346%39%
1854
November1st to 15thDay
11%8%35%423446%44%
1717
16th to endDay
16%10%46%7%23446%33%
1755
December1st to 15thDay
21%13%5%8%8%35%445%20%
1755
16th to endDay
20%14%6%9%6%24446%20%
1872
January1st to 15thNight
17%11%6%10%7%35%445%22%
2522
16th to endNight
14%13%5%8%6%45%345%27%
2560
February1st to 15thNight
18%10%5%8%5%3434432%
2193
16th to endNight
16%12%5%8%6%44345%28%
1893
March1st to 15thNight
18%7%37%6%3445%5%32%
2072
16th to endNight
16%9%35%5%24447%34%
2112
April1st to 15thNight
15%6%45%42445%7%38%
1861
16th to endNight
14%6%25%5%3445%8%40%
1620
May1st to 15thNight
18%7%35%5%3445%8%33%
1619
16th to endNight
9%5%24435%6%8%10%41%
1682
June1st to 15thNight
5%43343445%9%50%
1602
16th to endNight
5%2245%35%48%10%45%
1440
July1st to 15thNight
8%425%425%46%8%46%
1479
16th to endNight
5%32443446%9%50%
1703
August1st to 15thNight
44145%25%5%6%9%47%
1620
16th to endNight
5%21443446%10%52%
1794
September1st to 15thNight
10%423323447%52%
1941
16th to endNight
11%425%423346%51%
1980
October1st to 15thNight
10%6%25%42435%7%47%
2045
16th to endNight
17%7%25%5%2335%5%40%
2369
November1st to 15thNight
14%5%34433447%45%
2471
16th to endNight
17%9%45%5%24446%33%
2535
December1st to 15thNight
21%12%5%8%5%34335%26%
2535
16th to endNight
20%14%5%8%5%34346%21%
2704

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
66%49%14%6%
1747
16th to endDay
64%412%13%3
1768
February1st to 15thDay
62%410%14%8%
1661
16th to endDay
59%5%11%18%4
1580
March1st to 15thDay
58%5%12%17%5%
1887
16th to endDay
54%5%12%20%6%
2112
April1st to 15thDay
49%411%22%10%1
2097
16th to endDay
45%6%10%19%13%3
2340
May1st to 15thDay
53%6%11%19%7%1
2331
16th to endDay
41%9%12%23%11%1
2435
June1st to 15thDay
39%8%12%23%16%
2353
16th to endDay
39%7%14%19%15%2
2520
July1st to 15thDay
43%7%10%20%15%2
2332
16th to endDay
37%9%15%24%11%1
2310
August1st to 15thDay
36%7%13%24%15%2
2327
16th to endDay
35%7%13%22%16%4
2405
September1st to 15thDay
37%6%9%22%20%3
2007
16th to endDay
38%6%8%20%25%1
1980
October1st to 15thDay
41%5%10%22%18%1
1914
16th to endDay
44%313%24%12%1
1854
November1st to 15thDay
40%310%23%17%4
1717
16th to endDay
51%410%21%10%
1755
December1st to 15thDay
64%5%9%14%5%
1755
16th to endDay
64%5%11%15%3
1872
January1st to 15thNight
63%48%15%8%
2522
16th to endNight
59%410%18%6%
2560
February1st to 15thNight
55%410%19%8%1
2202
16th to endNight
58%310%18%8%
1893
March1st to 15thNight
51%5%10%20%8%1
2072
16th to endNight
49%5%12%22%8%1
2112
April1st to 15thNight
44%411%21%15%2
1861
16th to endNight
42%411%20%15%5%
1620
May1st to 15thNight
49%5%11%22%9%2
1620
16th to endNight
35%9%13%24%14%3
1684
June1st to 15thNight
29%8%13%27%20%2
1602
16th to endNight
28%10%18%20%19%3
1440
July1st to 15thNight
35%7%10%23%18%4
1396
16th to endNight
27%6%15%31%16%2
1595
August1st to 15thNight
29%8%15%24%19%2
1609
16th to endNight
26%7%13%26%22%4
1794
September1st to 15thNight
31%5%10%24%23%5%
1941
16th to endNight
33%47%23%28%1
1980
October1st to 15thNight
35%5%9%24%21%2
2045
16th to endNight
45%48%26%14%1
2369
November1st to 15thNight
38%411%21%20%4
2471
16th to endNight
50%5%9%22%11%1
2535
December1st to 15thNight
59%49%17%8%
2535
16th to endNight
64%410%15%4
2704

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
24%14%40%14%6%
135
16th to endDay
25%16%42%14%
109
February1st to 15thDay
23%23%34%15%3
90
16th to endDay
20%11%43%18%6%
81
March1st to 15thDay
29%11%29%22%6%
177
16th to endDay
25%10%33%21%8%
288
April1st to 15thDay
22%12%25%27%11%
270
16th to endDay
20%7%29%20%14%8%
270
May1st to 15thDay
24%9%24%23%11%5%
288
16th to endDay
14%325%27%13%14%
331
June1st to 15thDay
10%217%31%27%11%
328
16th to endDay
12%225%30%20%7%
495
July1st to 15thDay
16%219%27%21%13%
436
16th to endDay
7%12%24%28%26%
326
August1st to 15thDay
11%18%22%30%25%
313
16th to endDay
8%210%23%33%21%
332
September1st to 15thDay
15%212%20%28%20%
315
16th to endDay
16%313%17%30%17%
314
October1st to 15thDay
13%5%20%30%21%8%
315
16th to endDay
19%8%31%25%13%1
270
November1st to 15thDay
20%9%22%22%19%5%
135
16th to endDay
20%10%40%23%5%
135
December1st to 15thDay
18%18%34%17%9%1
135
16th to endDay
29%19%37%12%
144
January1st to 15thNight
22%14%35%21%5%
780
16th to endNight
20%13%37%24%3
788
February1st to 15thNight
24%10%34%28%2
704
16th to endNight
23%9%36%26%3
631
March1st to 15thNight
21%11%32%28%5%
720
16th to endNight
21%7%32%25%12%
768
April1st to 15thNight
16%12%34%28%6%
603
16th to endNight
14%6%32%32%11%1
540
May1st to 15thNight
20%6%27%29%14%1
540
16th to endNight
10%219%35%25%6%
563
June1st to 15thNight
5%119%47%23%1
540
16th to endNight
6%218%42%27%2
540
July1st to 15thNight
10%114%37%29%5%
510
16th to endNight
6%9%38%38%6%
564
August1st to 15thNight
7%10%38%35%6%
539
16th to endNight
7%8%35%41%6%
573
September1st to 15thNight
12%311%32%33%6%
681
16th to endNight
13%315%28%33%5%
717
October1st to 15thNight
14%5%21%30%23%4
720
16th to endNight
22%7%28%28%12%
768
November1st to 15thNight
16%7%28%27%17%1
760
16th to endNight
21%12%29%27%8%
780
December1st to 15thNight
27%9%31%25%6%1
780
16th to endNight
29%10%31%24%3
832
Page generated at 2018-01-21 12:19:32. Version ac04

Copyright 2018 A. Danko


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