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 2017-09-20.

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
48%
15567
23%
46968
15%
15523
56%
15523
28%
29143

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
27%
720
23%
2327
10%
716
43%
716
21%
1440
January16th to endNight
26%
768
25%
2484
10%
768
44%
768
21%
1535
February1st to 15thNight
29%
704
27%
2111
10%
700
46%
700
26%
1270
February16th to endNight
25%
631
25%
1893
9%
631
41%
631
25%
1171
March1st to 15thNight
34%
720
24%
2041
12%
720
45%
720
27%
1289
March16th to endNight
37%
768
22%
2112
11%
768
45%
768
30%
1324
April1st to 15thNight
43%
674
25%
1932
15%
674
53%
674
34%
1130
April16th to endNight
49%
540
24%
1674
15%
540
58%
540
31%
1002
May1st to 15thNight
49%
540
19%
1620
10%
540
57%
540
28%
1023
May16th to endNight
61%
561
20%
1683
15%
561
64%
561
30%
1047
June1st to 15thNight
67%
540
16%
1620
14%
540
69%
540
31%
1017
June16th to endNight
71%
540
16%
1620
14%
540
72%
540
33%
993
July1st to 15thNight
70%
510
11%
1530
9%
510
71%
510
25%
1024
July16th to endNight
73%
564
16%
1595
14%
531
75%
531
26%
1059
August1st to 15thNight
67%
539
16%
1609
15%
536
67%
536
25%
1083
August16th to endNight
66%
579
17%
1899
18%
579
66%
579
22%
1293
September1st to 15thNight
63%
720
19%
1980
16%
720
64%
720
27%
1323
September16th to endNight
56%
677
24%
1870
18%
677
61%
677
30%
1207
October1st to 15thNight
52%
660
31%
1845
24%
660
59%
660
34%
1107
October16th to endNight
52%
704
34%
2112
26%
704
66%
704
38%
1158
November1st to 15thNight
53%
700
31%
2235
26%
700
64%
700
37%
1203
November16th to endNight
42%
720
27%
2340
19%
720
54%
720
30%
1338
December1st to 15thNight
37%
720
24%
2340
15%
720
48%
720
24%
1446
December16th to endNight
32%
768
22%
2496
14%
768
43%
768
19%
1661

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%425%5%35%5%5%7%42%
47868
Night
13%42442445%7%45%
47167
TransparencyDay
39%6%11%19%17%4
47569
Night
36%6%11%19%19%6%
46970
SeeingDay
16%6%19%23%22%12%
5434
Night
15%7%21%29%21%3
15569

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 15thDay
17%6%35%32434445%
1611
16th to endDay
18%42431335%6%46%
1740
February1st to 15thDay
15%5%243335%45%46%
1750
16th to endDay
17%5%35%423336%45%
1580
March1st to 15thDay
16%415%32435%6%46%
1917
16th to endDay
16%424424446%44%
2111
April1st to 15thDay
12%4245%25%45%6%45%
2025
16th to endDay
11%4244245%5%7%48%
2286
May1st to 15thDay
11%425%425%5%5%7%44%
2331
16th to endDay
11%325%6%46%6%5%8%39%
2431
June1st to 15thDay
8%425%6%5%7%6%7%10%33%
2340
16th to endDay
5%315%5%46%7%8%11%41%
2340
July1st to 15thDay
10%5%25%5%48%7%8%10%30%
2340
16th to endDay
6%435%7%5%8%9%9%10%30%
2465
August1st to 15thDay
10%5%25%7%47%7%6%9%31%
2340
16th to endDay
13%6%37%6%46%6%7%9%28%
2288
September1st to 15thDay
12%6%26%5%35%5%5%7%38%
1980
16th to endDay
17%5%24324448%41%
1859
October1st to 15thDay
14%42331233455%
1785
16th to endDay
12%31431223459%
1760
November1st to 15thDay
12%32332434456%
1621
16th to endDay
15%5%23323345%50%
1620
December1st to 15thDay
18%6%25%42335%5%41%
1620
16th to endDay
25%6%24423335%38%
1728
January1st to 15thNight
18%6%2441433446%
2327
16th to endNight
17%5%33323345%45%
2484
February1st to 15thNight
14%424323335%50%
2103
16th to endNight
17%414423445%47%
1893
March1st to 15thNight
14%424324436%48%
2041
16th to endNight
13%4345%1345%6%48%
2112
April1st to 15thNight
13%31322345%7%51%
1932
16th to endNight
10%3144245%47%50%
1674
May1st to 15thNight
10%5%25%435%5%6%8%43%
1619
16th to endNight
8%5%145%25%5%6%9%45%
1682
June1st to 15thNight
7%424435%6%8%12%40%
1620
16th to endNight
5%3245%35%6%8%12%42%
1620
July1st to 15thNight
7%436%6%47%5%7%12%35%
1620
16th to endNight
6%3245%47%7%9%14%33%
1703
August1st to 15thNight
11%6%245%35%7%7%13%33%
1620
16th to endNight
11%425%6%45%7%7%9%35%
1899
September1st to 15thNight
10%5%25%5%36%5%6%9%38%
1980
16th to endNight
16%415%41445%6%45%
1870
October1st to 15thNight
14%5%13313345%53%
1845
16th to endNight
10%32332433459%
2112
November1st to 15thNight
11%322323235%59%
2235
16th to endNight
16%413323346%52%
2340
December1st to 15thNight
16%6%24423345%46%
2340
16th to endNight
21%7%25%423346%38%
2496

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%49%15%24%4
1612
16th to endDay
39%37%19%23%7%
1740
February1st to 15thDay
39%48%20%23%4
1752
16th to endDay
41%26%21%19%7%
1580
March1st to 15thDay
38%49%19%20%7%
1917
16th to endDay
40%5%10%22%16%4
2112
April1st to 15thDay
37%49%20%21%6%
2025
16th to endDay
34%6%9%21%22%6%
2286
May1st to 15thDay
37%6%12%20%18%3
2331
16th to endDay
40%8%13%22%13%2
2435
June1st to 15thDay
40%11%19%19%8%1
2340
16th to endDay
32%11%17%22%14%2
2340
July1st to 15thDay
41%12%17%19%6%2
2201
16th to endDay
41%12%17%18%7%1
2310
August1st to 15thDay
45%10%14%18%9%1
2327
16th to endDay
49%11%14%13%9%1
2288
September1st to 15thDay
42%8%11%18%15%3
1980
16th to endDay
39%6%12%19%17%4
1859
October1st to 15thDay
32%49%19%26%7%
1785
16th to endDay
29%35%19%30%12%
1760
November1st to 15thDay
30%37%15%31%11%
1621
16th to endDay
36%46%17%28%7%
1620
December1st to 15thDay
44%38%18%15%8%
1620
16th to endDay
50%37%18%13%7%
1728
January1st to 15thNight
42%27%17%24%6%
2327
16th to endNight
40%45%18%22%8%
2484
February1st to 15thNight
37%36%20%23%9%
2112
16th to endNight
38%37%21%22%6%
1893
March1st to 15thNight
37%49%18%20%9%
2041
16th to endNight
36%410%22%21%5%
2112
April1st to 15thNight
31%410%23%22%7%
1932
16th to endNight
33%5%9%21%19%9%
1674
May1st to 15thNight
36%8%14%19%18%2
1620
16th to endNight
33%9%13%26%13%3
1684
June1st to 15thNight
32%10%20%24%10%1
1620
16th to endNight
30%10%19%23%13%2
1620
July1st to 15thNight
39%10%20%20%6%2
1530
16th to endNight
34%14%19%20%8%2
1595
August1st to 15thNight
39%10%17%20%10%
1609
16th to endNight
40%11%17%17%10%
1899
September1st to 15thNight
39%8%15%17%15%3
1980
16th to endNight
38%6%12%20%16%6%
1870
October1st to 15thNight
33%410%17%26%8%
1845
16th to endNight
28%35%18%30%13%
2112
November1st to 15thNight
28%36%17%30%13%
2235
16th to endNight
33%47%17%26%10%
2340
December1st to 15thNight
40%38%19%18%8%
2340
16th to endNight
46%48%15%18%5%
2496

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
25%12%36%20%41
90
16th to endDay
25%23%36%12%2
96
February1st to 15thDay
16%22%40%21%
90
16th to endDay
27%14%40%14%2
81
March1st to 15thDay
19%14%28%25%11%
207
16th to endDay
18%10%29%26%13%1
288
April1st to 15thDay
19%11%24%20%20%3
270
16th to endDay
11%6%26%29%21%5%
270
May1st to 15thDay
14%11%28%17%16%11%
288
16th to endDay
16%5%18%22%21%16%
331
June1st to 15thDay
11%210%27%27%19%
315
16th to endDay
7%116%23%24%26%
315
July1st to 15thDay
11%10%24%27%23%
305
16th to endDay
7%6%25%36%23%
326
August1st to 15thDay
16%110%24%26%20%
313
16th to endDay
18%29%19%31%18%
332
September1st to 15thDay
14%313%19%33%16%
315
16th to endDay
22%212%22%26%13%
270
October1st to 15thDay
18%421%25%20%9%
255
16th to endDay
12%8%17%23%30%7%
272
November1st to 15thDay
27%12%17%20%19%3
129
16th to endDay
11%12%25%30%15%5%
90
December1st to 15thDay
26%12%33%21%6%
90
16th to endDay
39%16%23%15%31
96
January1st to 15thNight
24%14%27%24%8%
720
16th to endNight
20%17%31%26%3
768
February1st to 15thNight
18%13%32%30%4
704
16th to endNight
18%17%34%26%3
631
March1st to 15thNight
18%12%31%31%5%
720
16th to endNight
16%9%30%29%13%
768
April1st to 15thNight
15%10%29%29%14%
674
16th to endNight
12%5%26%32%21%1
540
May1st to 15thNight
12%6%25%31%21%2
540
16th to endNight
10%315%32%34%3
563
June1st to 15thNight
9%113%32%33%10%
540
16th to endNight
7%10%33%36%11%
540
July1st to 15thNight
10%7%26%42%12%
510
16th to endNight
5%7%29%46%10%
564
August1st to 15thNight
15%16%26%41%9%
539
16th to endNight
14%18%24%40%10%
579
September1st to 15thNight
12%110%25%41%7%
720
16th to endNight
17%214%33%29%2
677
October1st to 15thNight
15%5%21%26%27%4
660
16th to endNight
12%6%23%31%23%2
704
November1st to 15thNight
13%6%22%34%21%2
700
16th to endNight
18%9%26%28%15%1
720
December1st to 15thNight
19%12%24%32%11%
720
16th to endNight
25%10%26%30%6%
768
Page generated at 2017-09-21 07:18:23. Version ac04

Copyright 2017 A. Danko


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