Fernbank Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fernbank Observatory clear sky chart from 2005-11-05 to 2017-12-20.

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
52%
15867
24%
48090
17%
15826
59%
15826
30%
29133

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
30%
719
24%
2327
11%
716
45%
716
24%
1419
January16th to endNight
30%
768
25%
2439
12%
768
45%
768
24%
1536
February1st to 15thNight
33%
703
26%
2112
12%
701
49%
701
29%
1286
February16th to endNight
28%
631
25%
1893
10%
631
41%
631
27%
1168
March1st to 15thNight
35%
720
24%
2066
12%
720
44%
720
30%
1269
March16th to endNight
43%
768
23%
2112
13%
768
50%
768
33%
1293
April1st to 15thNight
46%
635
26%
1893
18%
635
55%
635
35%
1079
April16th to endNight
51%
540
26%
1707
18%
540
60%
540
34%
983
May1st to 15thNight
54%
540
22%
1620
15%
540
60%
540
32%
972
May16th to endNight
66%
561
23%
1682
17%
561
69%
561
33%
998
June1st to 15thNight
74%
540
17%
1620
16%
540
75%
540
31%
1003
June16th to endNight
79%
540
17%
1620
15%
540
80%
540
35%
953
July1st to 15thNight
74%
510
14%
1530
13%
510
74%
510
30%
977
July16th to endNight
76%
564
16%
1595
14%
531
77%
531
30%
1007
August1st to 15thNight
74%
539
19%
1649
19%
536
74%
536
28%
1061
August16th to endNight
69%
573
20%
1910
21%
573
70%
573
23%
1234
September1st to 15thNight
68%
703
20%
1963
18%
703
69%
703
29%
1282
September16th to endNight
64%
717
25%
1980
21%
717
66%
717
33%
1233
October1st to 15thNight
60%
720
32%
1983
25%
720
66%
720
37%
1174
October16th to endNight
55%
768
36%
2304
28%
768
66%
768
41%
1238
November1st to 15thNight
54%
760
30%
2456
27%
760
64%
760
38%
1302
November16th to endNight
46%
780
29%
2535
21%
780
59%
780
32%
1422
December1st to 15thNight
40%
780
24%
2535
17%
780
51%
780
27%
1540
December16th to endNight
34%
788
21%
2559
16%
788
43%
788
20%
1704

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
12%5%25%5%345%5%8%42%
48740
Night
11%5%2442445%7%46%
48289
TransparencyDay
38%5%12%20%18%4
48441
Night
35%5%11%20%20%6%
48092
SeeingDay
14%415%21%24%18%
5698
Night
14%5%21%30%24%4
15871

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
16%6%25%434445%43%
1611
16th to endDay
18%6%2442345%6%42%
1782
February1st to 15thDay
15%6%2442445%6%43%
1750
16th to endDay
17%6%34424445%42%
1580
March1st to 15thDay
15%426%5%24448%41%
1893
16th to endDay
15%5%15%4245%5%7%41%
2111
April1st to 15thDay
11%5%25%5%24447%45%
2064
16th to endDay
10%42443445%8%48%
2250
May1st to 15thDay
11%42442445%7%46%
2331
16th to endDay
9%425%5%346%5%8%44%
2431
June1st to 15thDay
8%425%6%35%6%7%11%36%
2340
16th to endDay
42145%36%6%7%11%44%
2340
July1st to 15thDay
9%5%35%5%48%8%7%11%29%
2340
16th to endDay
6%435%6%48%7%9%11%31%
2465
August1st to 15thDay
8%5%25%5%5%7%7%9%11%30%
2297
16th to endDay
13%6%36%6%46%6%7%9%30%
2292
September1st to 15thDay
12%5%245%35%6%6%9%38%
1983
16th to endDay
15%4245%23346%47%
1980
October1st to 15thDay
13%413323346%53%
1977
16th to endDay
12%31342333458%
1920
November1st to 15thDay
13%32332335%5%53%
1729
16th to endDay
13%5%33424345%49%
1755
December1st to 15thDay
17%5%35%4235%46%41%
1755
16th to endDay
23%7%26%5%23435%35%
1764
January1st to 15thNight
17%6%24323346%45%
2327
16th to endNight
16%6%35%423335%44%
2439
February1st to 15thNight
15%5%23423446%47%
2103
16th to endNight
16%434432345%46%
1893
March1st to 15thNight
12%5%24423446%47%
2066
16th to endNight
11%6%24324347%49%
2112
April1st to 15thNight
12%412324447%50%
1893
16th to endNight
10%32442445%8%49%
1707
May1st to 15thNight
8%435%5%3446%6%47%
1619
16th to endNight
7%415%435%6%5%8%48%
1682
June1st to 15thNight
6%414446%5%8%12%41%
1620
16th to endNight
4214437%7%8%12%43%
1620
July1st to 15thNight
6%5%25%5%36%6%8%12%36%
1620
16th to endNight
4325%6%46%8%8%12%37%
1703
August1st to 15thNight
6%325%5%46%7%8%11%37%
1660
16th to endNight
11%5%35%5%35%5%6%10%36%
1910
September1st to 15thNight
8%5%25%5%45%7%6%9%41%
1963
16th to endNight
13%41443336%7%47%
1980
October1st to 15thNight
11%414323346%53%
1983
16th to endNight
9%413322335%60%
2304
November1st to 15thNight
12%312322247%57%
2456
16th to endNight
12%423423445%52%
2535
December1st to 15thNight
14%6%25%424346%45%
2535
16th to endNight
20%7%25%5%23346%36%
2559

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%310%14%25%3
1612
16th to endDay
42%26%19%21%7%
1782
February1st to 15thDay
41%38%20%23%3
1752
16th to endDay
43%26%20%19%6%
1580
March1st to 15thDay
41%39%19%19%4
1893
16th to endDay
40%412%21%17%3
2112
April1st to 15thDay
38%49%22%20%5%
2064
16th to endDay
33%5%9%24%24%3
2250
May1st to 15thDay
36%5%13%21%20%3
2331
16th to endDay
34%7%14%26%14%1
2435
June1st to 15thDay
37%8%18%25%8%1
2340
16th to endDay
29%7%17%27%15%2
2340
July1st to 15thDay
41%10%19%19%6%1
2201
16th to endDay
39%10%19%20%8%1
2310
August1st to 15thDay
41%9%17%20%9%1
2284
16th to endDay
47%9%14%16%11%
2292
September1st to 15thDay
40%8%13%18%15%3
1983
16th to endDay
37%410%21%21%3
1980
October1st to 15thDay
32%48%20%26%7%
1977
16th to endDay
30%25%16%33%11%
1920
November1st to 15thDay
32%37%17%30%9%
1729
16th to endDay
36%35%18%27%8%
1755
December1st to 15thDay
42%37%22%16%7%
1755
16th to endDay
51%28%17%13%5%
1764
January1st to 15thNight
41%27%19%24%5%
2327
16th to endNight
42%25%18%22%7%
2439
February1st to 15thNight
38%26%20%21%10%
2112
16th to endNight
40%26%20%23%6%
1893
March1st to 15thNight
37%39%20%19%8%
2066
16th to endNight
35%310%23%21%5%
2112
April1st to 15thNight
32%38%24%22%8%
1893
16th to endNight
32%5%9%22%21%9%
1707
May1st to 15thNight
35%5%13%22%20%2
1620
16th to endNight
32%7%13%27%15%3
1684
June1st to 15thNight
32%10%20%25%10%1
1620
16th to endNight
28%9%20%22%16%2
1620
July1st to 15thNight
34%9%20%24%7%2
1530
16th to endNight
33%10%23%19%11%2
1595
August1st to 15thNight
36%10%16%24%11%1
1649
16th to endNight
40%8%18%18%12%1
1910
September1st to 15thNight
37%8%15%18%15%4
1963
16th to endNight
35%5%13%21%18%6%
1980
October1st to 15thNight
31%5%9%20%26%8%
1983
16th to endNight
27%35%17%33%14%
2304
November1st to 15thNight
29%27%17%30%12%
2456
16th to endNight
33%26%18%27%10%
2535
December1st to 15thNight
39%39%21%18%7%
2535
16th to endNight
48%37%16%18%5%
2559

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
20%12%27%28%7%3
90
16th to endDay
21%17%39%14%6%
96
February1st to 15thDay
11%21%38%25%3
90
16th to endDay
25%9%39%20%3
81
March1st to 15thDay
17%12%27%32%7%2
183
16th to endDay
16%7%27%29%17%1
288
April1st to 15thDay
16%11%26%18%24%2
270
16th to endDay
13%321%28%26%6%
270
May1st to 15thDay
12%5%23%23%19%14%
288
16th to endDay
12%212%23%27%22%
331
June1st to 15thDay
9%17%15%29%36%
315
16th to endDay
9%6%24%22%36%
315
July1st to 15thDay
8%414%29%42%
305
16th to endDay
7%5%11%35%41%
326
August1st to 15thDay
16%5%17%32%27%
313
16th to endDay
16%1420%31%26%
332
September1st to 15thDay
13%29%16%30%26%
315
16th to endDay
16%110%20%35%15%
314
October1st to 15thDay
18%114%29%24%11%
315
16th to endDay
12%6%16%23%31%9%
336
November1st to 15thDay
25%5%21%19%22%6%
147
16th to endDay
9%9%21%32%20%5%
135
December1st to 15thDay
26%5%31%22%12%
135
16th to endDay
30%12%25%23%7%
108
January1st to 15thNight
21%12%31%26%7%
720
16th to endNight
18%12%33%29%5%
768
February1st to 15thNight
18%9%33%33%5%
704
16th to endNight
18%9%39%28%4
631
March1st to 15thNight
16%10%33%31%7%
720
16th to endNight
15%9%26%32%15%
768
April1st to 15thNight
15%6%28%32%16%1
635
16th to endNight
12%326%33%21%2
540
May1st to 15thNight
11%422%32%26%3
540
16th to endNight
9%113%32%34%8%
563
June1st to 15thNight
8%6%31%41%11%
540
16th to endNight
5%5%28%46%12%
540
July1st to 15thNight
8%1420%49%15%
510
16th to endNight
5%1322%51%15%
564
August1st to 15thNight
8%15%23%49%12%
539
16th to endNight
13%1325%45%10%
573
September1st to 15thNight
11%8%23%44%11%
703
16th to endNight
16%18%32%37%3
717
October1st to 15thNight
12%317%32%27%6%
720
16th to endNight
11%423%32%24%4
768
November1st to 15thNight
13%422%32%24%2
760
16th to endNight
14%8%28%32%15%1
780
December1st to 15thNight
16%7%29%34%11%1
780
16th to endNight
24%8%28%30%8%
788
Page generated at 2017-12-21 07:04:53. Version ac04

Copyright 2017 A. Danko


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