Decatur Area Astronomy Club
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Decatur Area Astronomy Club clear sky chart from 2005-11-05 to 2017-12-04.

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
39%
16208
20%
47816
13%
16163
46%
16163
25%
31326

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
22%
900
16%
2506
6%
895
31%
895
19%
1799
January16th to endNight
21%
960
14%
2586
5%
960
28%
960
18%
1930
February1st to 15thNight
23%
790
13%
2199
3%
788
30%
788
18%
1595
February16th to endNight
25%
631
19%
1893
8%
631
34%
631
21%
1264
March1st to 15thNight
26%
720
15%
2160
7%
720
33%
720
19%
1524
March16th to endNight
29%
593
15%
2000
9%
593
37%
593
18%
1369
April1st to 15thNight
28%
539
20%
1797
11%
539
42%
539
22%
1142
April16th to endNight
35%
540
19%
1788
15%
540
44%
540
22%
1190
May1st to 15thNight
38%
540
17%
1620
9%
540
45%
540
22%
1113
May16th to endNight
49%
563
19%
1497
12%
561
55%
561
31%
988
June1st to 15thNight
59%
540
19%
1440
12%
540
64%
540
39%
866
June16th to endNight
57%
540
20%
1440
14%
540
61%
540
37%
891
July1st to 15thNight
63%
510
19%
1360
15%
510
65%
510
38%
824
July16th to endNight
75%
564
26%
1417
22%
531
77%
531
47%
794
August1st to 15thNight
76%
539
22%
1667
23%
536
78%
536
41%
910
August16th to endNight
71%
573
22%
1910
24%
573
75%
573
33%
1067
September1st to 15thNight
63%
540
28%
1800
28%
540
68%
540
32%
1013
September16th to endNight
62%
708
32%
1971
25%
708
66%
708
39%
1144
October1st to 15thNight
52%
720
28%
2086
20%
720
59%
720
34%
1242
October16th to endNight
31%
768
23%
2400
14%
768
42%
768
27%
1477
November1st to 15thNight
34%
760
25%
2471
18%
760
46%
760
26%
1471
November16th to endNight
29%
790
22%
2545
13%
790
40%
790
22%
1629
December1st to 15thNight
23%
920
12%
2575
5%
920
29%
920
17%
1946
December16th to endNight
19%
960
12%
2688
3%
960
27%
960
13%
2138

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%7%36%5%3445%7%38%
48685
Night
13%6%35%5%24446%42%
47985
Cloud CoverDay
13%7%35%5%3445%7%38%
48693
Night
14%6%35%5%24446%42%
47990
TransparencyDay
44%6%11%22%13%2
48371
Night
42%5%11%23%15%2
47817
SeeingDay
14%7%25%26%19%7%
6159
Night
17%8%28%30%14%1
16209

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
16%12%6%8%7%5%5%345%25%
1432
16th to endDay
14%13%6%8%7%24435%29%
1635
February1st to 15thDay
13%11%6%9%6%35%446%29%
1663
16th to endDay
13%12%48%6%35%45%6%28%
1580
March1st to 15thDay
17%10%48%5%33446%31%
1790
16th to endDay
16%11%5%6%6%3435%6%30%
2233
April1st to 15thDay
16%8%36%5%2345%5%38%
2151
16th to endDay
19%7%25%5%245%5%6%34%
2172
May1st to 15thDay
19%7%36%435%45%6%32%
2331
16th to endDay
12%6%26%6%35%5%6%7%38%
2615
June1st to 15thDay
9%425%5%3445%8%45%
2520
16th to endDay
8%425%6%35%5%7%8%41%
2520
July1st to 15thDay
9%5%25%6%35%5%5%9%43%
2520
16th to endDay
5%414435%6%8%10%44%
2655
August1st to 15thDay
7%3145%35%6%7%9%45%
2281
16th to endDay
7%4146%26%6%6%8%45%
2292
September1st to 15thDay
13%424434446%46%
2157
16th to endDay
10%41442445%8%48%
1980
October1st to 15thDay
15%5%245%24446%42%
1872
16th to endDay
16%7%245%24345%43%
1824
November1st to 15thDay
15%6%24423347%46%
1717
16th to endDay
17%7%45%423335%41%
1742
December1st to 15thDay
20%9%5%7%5%34347%26%
1467
16th to endDay
19%12%7%8%424447%25%
1536
January1st to 15thNight
14%9%47%6%35%346%34%
2506
16th to endNight
14%11%6%7%5%35%335%32%
2586
February1st to 15thNight
14%10%6%7%6%34346%32%
2197
16th to endNight
15%8%37%5%24345%38%
1893
March1st to 15thNight
17%10%46%5%24436%35%
2147
16th to endNight
17%7%45%434447%35%
2000
April1st to 15thNight
14%6%35%4335%46%41%
1797
16th to endNight
17%6%245%245%45%40%
1788
May1st to 15thNight
17%7%35%42445%5%38%
1620
16th to endNight
9%5%26%5%34449%44%
1496
June1st to 15thNight
10%323225%45%8%50%
1440
16th to endNight
7%235%5%3446%10%46%
1440
July1st to 15thNight
8%32442446%9%47%
1440
16th to endNight
331343345%9%57%
1513
August1st to 15thNight
421442446%10%53%
1676
16th to endNight
6%325%5%24447%53%
1910
September1st to 15thNight
10%314425%346%52%
1800
16th to endNight
7%42442345%7%54%
1971
October1st to 15thNight
12%4143235%46%51%
2086
16th to endNight
16%6%24423345%46%
2400
November1st to 15thNight
12%5%25%424335%49%
2471
16th to endNight
15%8%45%423336%41%
2545
December1st to 15thNight
21%10%47%5%23345%30%
2575
16th to endNight
22%12%48%6%2443425%
2688

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%12%5%7%8%45%335%25%
1432
16th to endDay
15%13%6%8%6%34435%29%
1635
February1st to 15thDay
14%10%6%8%6%25%445%30%
1663
16th to endDay
13%12%48%6%35%45%6%29%
1580
March1st to 15thDay
17%10%48%5%2345%6%31%
1800
16th to endDay
16%11%46%6%35%35%6%30%
2231
April1st to 15thDay
17%7%46%5%2335%6%38%
2151
16th to endDay
19%7%26%5%3445%6%34%
2172
May1st to 15thDay
20%7%35%5%35%45%6%32%
2331
16th to endDay
13%5%35%5%35%5%5%7%38%
2615
June1st to 15thDay
9%425%5%3445%8%45%
2520
16th to endDay
9%425%5%35%5%7%8%41%
2520
July1st to 15thDay
9%425%6%35%5%5%9%43%
2520
16th to endDay
6%424435%7%7%10%44%
2655
August1st to 15thDay
7%324435%6%6%9%46%
2281
16th to endDay
7%4235%35%6%6%8%45%
2292
September1st to 15thDay
13%5%24424446%46%
2157
16th to endDay
10%42443345%7%49%
1980
October1st to 15thDay
16%5%34434446%43%
1872
16th to endDay
16%6%25%42335%5%43%
1824
November1st to 15thDay
15%6%245%23347%46%
1717
16th to endDay
17%7%45%422336%41%
1742
December1st to 15thDay
20%10%47%5%3435%7%26%
1467
16th to endDay
20%12%6%8%324447%25%
1536
January1st to 15thNight
14%10%46%6%35%345%34%
2506
16th to endNight
14%11%6%7%5%34335%32%
2586
February1st to 15thNight
14%10%5%7%5%34346%32%
2190
16th to endNight
15%8%37%5%23345%39%
1893
March1st to 15thNight
18%9%36%434345%35%
2160
16th to endNight
18%8%35%5%24447%35%
2000
April1st to 15thNight
15%6%35%5%245%45%41%
1797
16th to endNight
18%5%25%5%24445%40%
1788
May1st to 15thNight
17%6%25%43445%6%38%
1619
16th to endNight
10%426%5%3445%8%44%
1496
June1st to 15thNight
10%323325%45%8%50%
1440
16th to endNight
7%336%43346%10%46%
1440
July1st to 15thNight
8%32442446%10%48%
1440
16th to endNight
331342436%9%56%
1513
August1st to 15thNight
5%21442446%11%53%
1676
16th to endNight
6%225%5%34448%53%
1910
September1st to 15thNight
11%314424447%52%
1800
16th to endNight
8%31442345%7%54%
1971
October1st to 15thNight
12%413323445%51%
2086
16th to endNight
17%5%25%423345%46%
2400
November1st to 15thNight
12%5%25%423335%49%
2471
16th to endNight
16%8%45%423336%41%
2545
December1st to 15thNight
21%10%5%7%424335%30%
2575
16th to endNight
23%11%48%6%25%33426%
2688

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
61%49%15%9%
1433
16th to endDay
57%312%18%6%
1635
February1st to 15thDay
56%412%18%7%
1665
16th to endDay
54%5%11%19%7%
1580
March1st to 15thDay
53%5%12%21%6%
1800
16th to endDay
54%5%13%20%5%1
2233
April1st to 15thDay
46%5%11%23%12%1
2151
16th to endDay
48%6%12%18%11%2
2172
May1st to 15thDay
50%7%13%19%9%
2331
16th to endDay
42%8%12%18%15%2
2619
June1st to 15thDay
35%8%12%24%17%2
2520
16th to endDay
37%8%13%21%16%3
2520
July1st to 15thDay
38%7%11%24%15%3
2371
16th to endDay
29%9%17%27%14%1
2488
August1st to 15thDay
30%9%15%26%15%2
2266
16th to endDay
32%10%11%24%14%6%
2292
September1st to 15thDay
39%5%7%21%20%5%
2157
16th to endDay
32%5%9%27%22%2
1980
October1st to 15thDay
40%5%9%25%16%2
1872
16th to endDay
42%410%24%17%1
1824
November1st to 15thDay
38%412%23%18%3
1717
16th to endDay
46%410%25%13%
1742
December1st to 15thDay
57%5%12%17%8%
1467
16th to endDay
59%6%10%15%6%1
1536
January1st to 15thNight
51%410%20%12%
2506
16th to endNight
54%413%19%7%
2586
February1st to 15thNight
53%412%18%9%1
2199
16th to endNight
46%413%25%9%
1893
March1st to 15thNight
51%5%10%20%10%1
2160
16th to endNight
47%7%12%21%9%1
2000
April1st to 15thNight
43%49%24%15%2
1797
16th to endNight
45%5%9%21%13%5%
1788
May1st to 15thNight
46%6%12%22%12%
1620
16th to endNight
37%7%14%19%17%3
1498
June1st to 15thNight
30%5%12%27%18%4
1440
16th to endNight
32%6%14%23%19%4
1440
July1st to 15thNight
30%6%11%28%18%4
1360
16th to endNight
22%6%15%31%20%2
1417
August1st to 15thNight
24%7%14%28%22%2
1667
16th to endNight
28%8%11%25%19%6%
1910
September1st to 15thNight
32%5%7%23%23%7%
1800
16th to endNight
28%5%8%27%25%4
1971
October1st to 15thNight
32%5%9%26%20%5%
2086
16th to endNight
40%39%26%18%2
2400
November1st to 15thNight
37%48%24%18%5%
2471
16th to endNight
44%310%25%15%
2545
December1st to 15thNight
55%5%10%17%10%
2575
16th to endNight
62%47%15%7%1
2688

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
30%15%35%13%5%
90
16th to endDay
15%18%39%20%5%
96
February1st to 15thDay
20%16%42%18%2
90
16th to endDay
16%18%32%25%6%1
81
March1st to 15thDay
28%21%33%12%4
90
16th to endDay
23%15%30%22%6%1
223
April1st to 15thDay
19%13%32%21%12%
270
16th to endDay
22%7%28%26%11%4
270
May1st to 15thDay
20%11%27%27%9%3
288
16th to endDay
14%8%30%27%13%5%
515
June1st to 15thDay
9%325%29%25%5%
495
16th to endDay
10%5%32%29%14%7%
495
July1st to 15thDay
13%219%29%21%12%
475
16th to endDay
5%116%30%30%14%
515
August1st to 15thDay
8%10%27%36%15%
388
16th to endDay
6%213%29%35%12%
332
September1st to 15thDay
12%413%23%35%10%
315
16th to endDay
12%314%29%25%14%
314
October1st to 15thDay
21%5%26%22%19%5%
207
16th to endDay
19%10%35%27%42
144
November1st to 15thDay
14%8%28%23%14%9%
135
16th to endDay
28%11%33%20%6%
135
December1st to 15thDay
13%17%36%22%7%5%
100
16th to endDay
25%17%43%12%1
96
January1st to 15thNight
18%15%38%23%4
900
16th to endNight
20%15%35%25%3
960
February1st to 15thNight
19%12%38%26%3
791
16th to endNight
20%12%37%27%3
631
March1st to 15thNight
22%9%34%28%4
720
16th to endNight
20%10%34%24%9%
593
April1st to 15thNight
17%12%37%23%8%
539
16th to endNight
21%7%31%27%10%
540
May1st to 15thNight
20%5%29%31%11%1
540
16th to endNight
11%328%33%19%1
563
June1st to 15thNight
12%120%44%20%
540
16th to endNight
7%224%40%23%1
540
July1st to 15thNight
10%17%39%29%2
510
16th to endNight
412%43%35%4
564
August1st to 15thNight
7%8%43%36%4
539
16th to endNight
6%12%36%39%4
573
September1st to 15thNight
12%115%37%27%5%
540
16th to endNight
9%318%33%32%3
708
October1st to 15thNight
15%423%34%19%2
720
16th to endNight
21%9%34%24%9%
768
November1st to 15thNight
15%10%34%28%10%1
760
16th to endNight
20%14%32%25%7%
790
December1st to 15thNight
26%10%34%21%6%
920
16th to endNight
29%11%32%23%3
960
Page generated at 2017-12-05 09:19:00. Version ac04

Copyright 2017 A. Danko


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