Alabama Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Alabama Observatory clear sky chart from 2005-11-05 to 2017-09-10.

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%
15232
24%
46895
17%
15189
56%
15189
29%
28659

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
25%
720
24%
2327
9%
716
42%
716
21%
1470
January16th to endNight
24%
768
26%
2322
10%
768
38%
768
22%
1531
February1st to 15thNight
27%
704
25%
2112
9%
701
43%
701
26%
1326
February16th to endNight
25%
631
24%
1893
10%
631
37%
631
25%
1206
March1st to 15thNight
30%
720
21%
2151
11%
720
38%
720
25%
1362
March16th to endNight
36%
713
24%
2057
13%
713
46%
713
29%
1264
April1st to 15thNight
40%
539
24%
1797
16%
539
52%
539
30%
1021
April16th to endNight
45%
540
27%
1800
20%
540
56%
540
34%
982
May1st to 15thNight
52%
540
22%
1635
14%
540
60%
540
30%
1008
May16th to endNight
67%
561
24%
1682
19%
561
71%
561
36%
983
June1st to 15thNight
73%
540
17%
1620
15%
540
74%
540
33%
974
June16th to endNight
76%
540
20%
1620
18%
540
77%
540
37%
941
July1st to 15thNight
75%
510
14%
1530
13%
510
76%
510
29%
985
July16th to endNight
73%
564
17%
1615
17%
531
76%
531
29%
1035
August1st to 15thNight
78%
539
18%
1788
22%
536
78%
536
32%
1050
August16th to endNight
71%
573
20%
1910
24%
573
72%
573
25%
1228
September1st to 15thNight
64%
601
19%
1826
20%
601
66%
601
26%
1197
September16th to endNight
64%
657
27%
1815
22%
657
67%
657
35%
1095
October1st to 15thNight
56%
660
35%
1815
26%
660
63%
660
40%
1043
October16th to endNight
52%
704
33%
2128
28%
704
60%
704
36%
1210
November1st to 15thNight
50%
700
30%
2276
25%
700
61%
700
36%
1211
November16th to endNight
38%
720
27%
2340
18%
720
51%
720
28%
1366
December1st to 15thNight
32%
720
22%
2340
14%
720
43%
720
22%
1490
December16th to endNight
27%
768
20%
2496
14%
768
36%
768
19%
1681

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%35%5%5%7%40%
47726
Night
11%5%2442445%7%46%
47096
TransparencyDay
40%6%10%19%17%4
47430
Night
36%5%10%20%20%6%
46897
SeeingDay
15%6%19%23%22%12%
5145
Night
14%7%23%28%21%4
15234

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
19%7%34423435%42%
1611
16th to endDay
18%6%24323335%45%
1899
February1st to 15thDay
17%8%34413345%42%
1750
16th to endDay
19%6%25%423435%42%
1580
March1st to 15thDay
17%7%35%5%33346%39%
1809
16th to endDay
14%6%345%24447%40%
2176
April1st to 15thDay
12%5%35%424446%44%
2151
16th to endDay
10%5%25%5%25%5%5%7%44%
2160
May1st to 15thDay
12%5%25%425%46%8%42%
2316
16th to endDay
9%5%25%5%46%5%6%9%39%
2431
June1st to 15thDay
7%5%16%8%47%7%7%11%33%
2340
16th to endDay
6%3246%37%6%7%10%42%
2340
July1st to 15thDay
9%426%7%5%8%8%7%10%28%
2340
16th to endDay
7%5%26%7%48%8%9%11%29%
2441
August1st to 15thDay
7%325%7%5%8%8%9%10%30%
2160
16th to endDay
10%7%36%7%5%7%7%7%8%28%
2292
September1st to 15thDay
11%5%26%7%46%6%6%7%36%
2010
16th to endDay
13%5%344245%6%5%44%
1815
October1st to 15thDay
10%41442336%6%51%
1815
16th to endDay
11%42442223458%
1740
November1st to 15thDay
13%5%23213236%54%
1582
16th to endDay
15%5%23324446%47%
1620
December1st to 15thDay
20%7%24424445%39%
1620
16th to endDay
23%8%35%422345%35%
1728
January1st to 15thNight
15%7%34423435%44%
2327
16th to endNight
18%6%344245%45%40%
2322
February1st to 15thNight
16%6%23424335%47%
2103
16th to endNight
16%5%34424245%45%
1893
March1st to 15thNight
16%5%24424447%42%
2151
16th to endNight
12%425%42445%5%47%
2057
April1st to 15thNight
11%5%2342435%8%48%
1797
16th to endNight
8%4243235%47%52%
1800
May1st to 15thNight
9%415%435%5%5%8%45%
1634
16th to endNight
4325%4246%6%10%48%
1682
June1st to 15thNight
42146%36%6%7%13%42%
1620
16th to endNight
32245%25%5%8%12%46%
1620
July1st to 15thNight
6%3246%37%7%7%9%40%
1620
16th to endNight
4425%6%35%7%7%11%39%
1724
August1st to 15thNight
4315%436%6%8%11%43%
1800
16th to endNight
9%5%245%46%5%6%9%40%
1910
September1st to 15thNight
9%425%6%35%7%5%7%41%
1826
16th to endNight
8%325%5%3446%5%50%
1815
October1st to 15thNight
8%414423336%56%
1815
16th to endNight
11%6%1332233358%
2128
November1st to 15thNight
11%423322246%56%
2276
16th to endNight
12%5%24424335%51%
2340
December1st to 15thNight
16%6%35%434346%42%
2340
16th to endNight
21%6%35%5%23345%37%
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
45%36%15%24%4
1612
16th to endDay
41%45%21%20%6%
1899
February1st to 15thDay
44%36%23%17%4
1752
16th to endDay
44%46%15%23%5%
1580
March1st to 15thDay
46%49%14%19%5%
1809
16th to endDay
42%410%21%17%2
2177
April1st to 15thDay
39%410%19%18%7%
2151
16th to endDay
36%5%9%23%22%3
2160
May1st to 15thDay
38%7%11%19%18%4
2316
16th to endDay
39%7%13%23%14%2
2435
June1st to 15thDay
41%9%15%19%12%1
2340
16th to endDay
34%8%15%22%15%3
2340
July1st to 15thDay
43%11%18%17%7%1
2201
16th to endDay
42%12%18%17%8%
2288
August1st to 15thDay
41%12%16%16%10%2
2148
16th to endDay
49%9%13%16%9%2
2292
September1st to 15thDay
43%8%12%18%12%4
2010
16th to endDay
38%6%10%19%20%5%
1815
October1st to 15thDay
32%5%8%19%24%10%
1815
16th to endDay
31%3316%34%10%
1740
November1st to 15thDay
32%25%18%30%10%
1582
16th to endDay
36%46%17%26%8%
1620
December1st to 15thDay
46%48%18%16%5%
1620
16th to endDay
52%37%19%12%4
1728
January1st to 15thNight
42%35%17%24%4
2327
16th to endNight
44%4417%20%7%
2322
February1st to 15thNight
40%35%21%20%8%
2112
16th to endNight
41%35%18%24%6%
1893
March1st to 15thNight
40%410%18%19%7%
2151
16th to endNight
36%5%9%24%21%3
2057
April1st to 15thNight
33%410%22%19%9%
1797
16th to endNight
29%5%11%23%23%7%
1800
May1st to 15thNight
35%7%11%18%23%4
1635
16th to endNight
28%7%14%28%13%6%
1684
June1st to 15thNight
30%9%17%24%16%1
1620
16th to endNight
27%9%17%23%19%2
1620
July1st to 15thNight
34%10%20%21%9%3
1530
16th to endNight
32%12%19%21%12%
1615
August1st to 15thNight
29%10%19%24%13%3
1788
16th to endNight
38%9%15%20%12%3
1910
September1st to 15thNight
38%9%15%17%13%5%
1826
16th to endNight
32%5%9%20%22%8%
1815
October1st to 15thNight
29%47%21%23%13%
1815
16th to endNight
31%3312%33%15%
2128
November1st to 15thNight
29%26%18%27%15%
2276
16th to endNight
36%35%17%25%11%
2340
December1st to 15thNight
43%47%17%20%5%
2340
16th to endNight
48%37%18%16%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%21%38%15%31
90
16th to endDay
26%22%42%5%3
96
February1st to 15thDay
23%32%34%8%1
90
16th to endDay
35%13%34%14%1
81
March1st to 15thDay
29%22%32%12%4
99
16th to endDay
19%10%35%22%11%1
288
April1st to 15thDay
19%9%30%24%15%1
270
16th to endDay
12%10%25%28%20%2
270
May1st to 15thDay
14%7%28%24%15%9%
288
16th to endDay
14%419%25%22%13%
331
June1st to 15thDay
10%111%26%28%21%
315
16th to endDay
9%211%25%26%24%
315
July1st to 15thDay
7%210%22%33%23%
305
16th to endDay
11%6%14%34%32%
326
August1st to 15thDay
13%7%25%31%20%
313
16th to endDay
14%25%27%33%16%
332
September1st to 15thDay
11%210%25%31%17%
294
16th to endDay
14%5%13%31%25%9%
254
October1st to 15thDay
14%323%29%23%5%
255
16th to endDay
16%5%19%32%22%3
167
November1st to 15thDay
21%17%21%32%6%1
90
16th to endDay
18%14%37%17%8%2
90
December1st to 15thDay
23%18%37%15%4
90
16th to endDay
34%12%38%10%4
96
January1st to 15thNight
21%15%34%21%7%
720
16th to endNight
20%14%37%24%2
768
February1st to 15thNight
20%12%35%27%3
704
16th to endNight
19%14%36%26%2
631
March1st to 15thNight
20%12%34%26%6%
720
16th to endNight
14%11%33%27%12%
713
April1st to 15thNight
12%10%33%26%15%1
539
16th to endNight
10%7%30%33%17%1
540
May1st to 15thNight
12%426%28%23%4
540
16th to endNight
6%215%37%30%6%
563
June1st to 15thNight
6%9%33%40%10%
540
16th to endNight
417%27%44%14%
540
July1st to 15thNight
7%6%26%42%15%
510
16th to endNight
6%17%22%43%19%
564
August1st to 15thNight
6%5%28%43%15%
539
16th to endNight
11%15%31%39%10%
573
September1st to 15thNight
11%110%28%37%10%
601
16th to endNight
9%115%38%32%3
657
October1st to 15thNight
11%422%30%25%4
660
16th to endNight
13%7%21%30%24%3
704
November1st to 15thNight
13%7%25%32%18%4
700
16th to endNight
16%12%30%27%12%
720
December1st to 15thNight
21%11%30%26%8%1
720
16th to endNight
25%13%31%22%7%
768
Page generated at 2017-09-11 18:35:26. Version ac04

Copyright 2017 A. Danko


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