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 2017-04-24.

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
51%
15043
24%
45594
17%
15002
59%
15002
30%
27683

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%
522
25%
1653
17%
522
59%
522
33%
956
May1st to 15thNight
54%
495
23%
1485
15%
495
61%
495
32%
894
May16th to endNight
67%
513
22%
1538
16%
513
69%
513
33%
908
June1st to 15thNight
75%
495
19%
1485
17%
495
76%
495
32%
904
June16th to endNight
81%
495
18%
1485
15%
495
82%
495
37%
856
July1st to 15thNight
72%
465
15%
1395
14%
465
72%
465
29%
904
July16th to endNight
76%
522
16%
1469
13%
489
76%
489
30%
928
August1st to 15thNight
76%
494
20%
1510
21%
491
76%
491
29%
955
August16th to endNight
69%
525
18%
1750
19%
525
70%
525
23%
1147
September1st to 15thNight
70%
644
21%
1799
19%
644
70%
644
28%
1189
September16th to endNight
60%
657
23%
1815
18%
657
63%
657
31%
1166
October1st to 15thNight
59%
660
33%
1818
26%
660
64%
660
36%
1081
October16th to endNight
56%
704
34%
2112
27%
704
66%
704
41%
1141
November1st to 15thNight
53%
700
32%
2262
27%
700
64%
700
39%
1192
November16th to endNight
44%
720
28%
2340
19%
720
57%
720
31%
1328
December1st to 15thNight
40%
720
24%
2340
18%
720
50%
720
26%
1420
December16th to endNight
33%
768
21%
2496
16%
768
42%
768
20%
1664

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%41%
46003
Night
12%5%2442445%7%45%
45793
TransparencyDay
38%5%12%20%18%4
45704
Night
36%410%20%20%6%
45596
SeeingDay
14%416%21%24%18%
4795
Night
14%5%21%30%23%4
15047

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%
2163
May1st to 15thDay
11%4245%245%5%8%44%
2136
16th to endDay
10%425%5%336%5%8%45%
2223
June1st to 15thDay
8%425%6%35%6%7%11%37%
2145
16th to endDay
32145%26%7%7%12%46%
2145
July1st to 15thDay
10%5%35%6%48%8%7%11%28%
2145
16th to endDay
6%435%6%48%7%10%11%30%
2283
August1st to 15thDay
7%5%25%5%47%7%9%12%32%
2106
16th to endDay
13%6%36%6%46%6%7%9%29%
2100
September1st to 15thDay
12%5%25%5%45%6%6%9%36%
1818
16th to endDay
16%425%5%23346%44%
1815
October1st to 15thDay
12%413323346%54%
1812
16th to endDay
12%31332333458%
1760
November1st to 15thDay
12%323323345%54%
1593
16th to endDay
13%5%33424345%48%
1620
December1st to 15thDay
16%6%35%4235%5%6%40%
1620
16th to endDay
23%7%26%5%23335%35%
1728
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%
1653
May1st to 15thNight
8%435%43446%7%46%
1484
16th to endNight
7%415%435%6%48%48%
1538
June1st to 15thNight
6%313446%5%8%12%42%
1485
16th to endNight
3214437%7%8%12%44%
1485
July1st to 15thNight
6%5%25%5%36%7%8%13%34%
1485
16th to endNight
4325%6%46%8%8%12%37%
1577
August1st to 15thNight
6%3245%46%7%9%11%38%
1521
16th to endNight
11%5%35%5%35%6%7%11%34%
1750
September1st to 15thNight
7%5%25%5%45%7%6%9%39%
1799
16th to endNight
14%5%15%5%3435%6%44%
1815
October1st to 15thNight
11%414324346%52%
1818
16th to endNight
10%413322345%59%
2112
November1st to 15thNight
12%312322246%58%
2262
16th to endNight
13%5%23423346%51%
2340
December1st to 15thNight
13%6%25%424346%46%
2340
16th to endNight
20%7%25%5%23346%36%
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
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%23%24%3
2163
May1st to 15thDay
36%6%14%21%18%2
2136
16th to endDay
34%6%14%27%15%1
2227
June1st to 15thDay
36%8%19%26%8%1
2145
16th to endDay
26%7%18%28%16%2
2145
July1st to 15thDay
43%10%18%18%7%2
2006
16th to endDay
40%10%20%19%7%1
2128
August1st to 15thDay
39%9%18%21%9%1
2093
16th to endDay
47%9%14%16%10%1
2100
September1st to 15thDay
41%8%14%18%14%3
1818
16th to endDay
40%49%19%21%3
1815
October1st to 15thDay
32%38%20%26%8%
1812
16th to endDay
30%26%17%32%10%
1760
November1st to 15thDay
32%35%17%31%10%
1593
16th to endDay
37%35%18%26%8%
1620
December1st to 15thDay
42%37%22%15%7%
1620
16th to endDay
51%28%17%13%5%
1728
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%20%9%
1653
May1st to 15thNight
35%5%14%22%20%2
1485
16th to endNight
31%7%13%28%15%3
1540
June1st to 15thNight
31%9%20%26%10%1
1485
16th to endNight
26%9%19%23%17%2
1485
July1st to 15thNight
36%9%20%23%7%2
1395
16th to endNight
33%10%23%19%11%1
1469
August1st to 15thNight
34%9%15%25%11%2
1510
16th to endNight
41%8%18%17%12%1
1750
September1st to 15thNight
37%9%15%18%14%4
1799
16th to endNight
38%5%12%20%17%6%
1815
October1st to 15thNight
32%5%9%18%26%8%
1818
16th to endNight
27%35%17%32%14%
2112
November1st to 15thNight
28%25%17%31%13%
2262
16th to endNight
35%26%18%27%10%
2340
December1st to 15thNight
39%38%21%18%7%
2340
16th to endNight
48%37%16%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
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
12%322%26%28%6%
243
May1st to 15thDay
12%422%23%21%15%
228
16th to endDay
11%11%20%30%25%
267
June1st to 15thDay
9%16%16%30%35%
255
16th to endDay
4423%24%42%
255
July1st to 15thDay
10%1414%28%40%
245
16th to endDay
6%310%35%44%
270
August1st to 15thDay
13%415%33%31%
253
16th to endDay
16%1420%31%26%
268
September1st to 15thDay
12%17%15%31%30%
255
16th to endDay
20%111%22%32%11%
254
October1st to 15thDay
18%116%28%25%10%
255
16th to endDay
13%5%16%22%31%10%
272
November1st to 15thDay
28%7%22%18%15%5%
101
16th to endDay
12%11%17%31%21%6%
90
December1st to 15thDay
24%340%20%12%
90
16th to endDay
33%13%22%20%8%1
96
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%32%22%2
522
May1st to 15thNight
10%422%32%26%3
495
16th to endNight
9%113%30%36%8%
515
June1st to 15thNight
7%17%33%40%10%
495
16th to endNight
45%28%48%13%
495
July1st to 15thNight
8%1422%47%14%
465
16th to endNight
5%1322%51%15%
522
August1st to 15thNight
7%1424%48%13%
494
16th to endNight
13%1325%45%11%
525
September1st to 15thNight
10%6%24%46%11%
644
16th to endNight
18%18%34%34%2
657
October1st to 15thNight
13%318%32%26%6%
660
16th to endNight
11%322%33%24%4
704
November1st to 15thNight
14%422%31%24%2
700
16th to endNight
15%8%29%30%15%
720
December1st to 15thNight
15%7%29%34%11%
720
16th to endNight
24%8%28%30%8%
768
Page generated at 2017-04-25 07:48:19. Version ac04

Copyright 2017 A. Danko


Fernbank Observatory Clear Sky Chart | Charts Home