Gretna City Park Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Gretna City Park Observatory clear sky chart from 2005-11-06 to 2024-02-20.

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
57%
23094
22%
79007
18%
22992
63%
22992
27%
46951

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 10thNight
37%
753
23%
3686
10%
737
48%
737
17%
2158
January11th to 20thNight
42%
752
31%
3619
15%
740
51%
740
21%
1880
January21st to endNight
32%
833
17%
4094
10%
814
44%
814
15%
2516
February1st to 10thNight
35%
760
23%
2280
12%
760
44%
760
25%
1477
February11th to 20thNight
37%
742
20%
2214
9%
739
45%
739
26%
1413
February21st to endNight
32%
592
19%
1776
9%
592
40%
592
23%
1198
March1st to 10thNight
39%
653
24%
2093
15%
653
47%
653
28%
1273
March11th to 20thNight
46%
540
20%
1980
14%
540
55%
540
23%
1183
March21st to endNight
49%
594
26%
2069
20%
594
58%
594
32%
1122
April1st to 10thNight
47%
540
25%
1800
19%
540
55%
540
25%
1078
April11th to 20thNight
44%
533
23%
1775
18%
533
52%
533
27%
1085
April21st to endNight
60%
540
30%
1800
28%
540
68%
540
39%
935
May1st to 10thNight
67%
540
25%
1800
27%
540
72%
540
38%
952
May11th to 20thNight
67%
531
19%
1770
22%
531
70%
531
30%
1042
May21st to endNight
74%
582
21%
1938
23%
582
76%
582
34%
1089
June1st to 10thNight
75%
540
15%
1800
18%
540
75%
540
28%
1065
June11th to 20thNight
80%
540
16%
1746
19%
540
81%
540
34%
993
June21st to endNight
74%
540
13%
1620
14%
540
74%
540
30%
1012
July1st to 10thNight
74%
525
11%
1575
12%
525
74%
525
26%
1022
July11th to 20thNight
84%
522
12%
1668
15%
522
84%
522
26%
1072
July21st to endNight
81%
591
12%
1858
16%
558
81%
558
28%
1124
August1st to 10thNight
86%
537
14%
1778
17%
534
87%
534
36%
982
August11th to 20thNight
79%
534
12%
1776
15%
534
79%
534
25%
1144
August21st to endNight
74%
594
14%
1980
17%
594
74%
594
24%
1278
September1st to 10thNight
76%
540
19%
1800
21%
540
77%
540
27%
1107
September11th to 20thNight
71%
583
22%
1843
21%
583
72%
583
27%
1168
September21st to endNight
72%
720
28%
1980
23%
720
73%
720
36%
1202
October1st to 10thNight
72%
720
30%
2032
24%
720
75%
720
46%
1099
October11th to 20thNight
65%
720
34%
2160
28%
720
71%
720
43%
1134
October21st to endNight
59%
792
30%
2376
26%
792
63%
792
36%
1336
November1st to 10thNight
62%
740
31%
2220
27%
740
67%
740
38%
1242
November11th to 20thNight
55%
760
32%
2328
25%
760
63%
760
35%
1304
November21st to endNight
49%
760
27%
2470
22%
760
57%
760
29%
1465
December1st to 10thNight
45%
760
26%
2470
19%
760
54%
760
26%
1501
December11th to 20thNight
44%
759
22%
2791
16%
758
50%
758
22%
1721
December21st to endNight
36%
832
13%
4042
12%
817
43%
817
14%
2579

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
11%6%36%6%46%6%7%9%38%
79304
Night
11%5%244345%6%8%47%
79859
TransparencyDay
39%10%13%17%16%5%
79043
Night
33%9%14%20%17%7%
79641
SeeingDay
12%7%19%25%24%14%
13559
Night
12%6%18%29%27%8%
23119

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 10thDay
16%5%25%6%35%5%6%8%39%
3156
11th to 20thDay
12%5%2442445%6%52%
3160
21st to endDay
23%8%35%5%35%446%34%
3476
February1st to 10thDay
15%8%36%5%3445%7%40%
1900
11th to 20thDay
15%6%25%5%44448%43%
1838
21st to endDay
16%7%26%6%35%5%47%37%
1480
March1st to 10thDay
13%5%26%5%45%5%6%7%40%
1880
11th to 20thDay
13%7%35%6%345%6%7%41%
1980
21st to endDay
11%5%245%3445%8%48%
2286
April1st to 10thDay
9%5%35%446%6%7%9%42%
2160
11th to 20thDay
14%6%36%6%345%6%8%40%
2134
21st to endDay
8%5%245%345%7%9%47%
2160
May1st to 10thDay
7%425%6%445%7%9%47%
2160
11th to 20thDay
9%5%25%6%46%6%7%9%42%
2124
21st to endDay
6%5%26%7%5%8%7%7%10%38%
2337
June1st to 10thDay
10%6%37%7%48%7%8%11%30%
2160
11th to 20thDay
5%436%8%6%9%8%10%11%29%
2204
21st to endDay
7%6%38%8%6%9%9%10%11%24%
2340
July1st to 10thDay
11%7%47%8%6%8%9%10%10%21%
2340
11th to 20thDay
8%6%38%8%6%10%10%9%12%20%
2228
21st to endDay
8%6%36%8%6%9%9%8%11%26%
2350
August1st to 10thDay
7%5%27%8%5%8%9%9%13%26%
2148
11th to 20thDay
10%7%38%8%6%9%8%9%10%22%
2125
21st to endDay
9%6%48%7%5%6%7%8%11%29%
2376
September1st to 10thDay
8%6%37%8%5%7%8%8%10%30%
2160
11th to 20thDay
10%6%36%6%5%6%7%7%8%36%
2098
21st to endDay
8%426%6%46%7%6%8%44%
1980
October1st to 10thDay
6%3245%35%5%6%9%52%
1923
11th to 20thDay
7%425%4245%6%8%53%
1800
21st to endDay
10%4245%2346%7%53%
1980
November1st to 10thDay
8%4144335%5%7%54%
1858
11th to 20thDay
10%5%24434446%54%
1845
21st to endDay
13%6%245%45%6%6%8%42%
1710
December1st to 10thDay
13%6%345%36%45%8%43%
1710
11th to 20thDay
14%8%36%5%335%5%7%41%
2262
21st to endDay
17%8%26%5%345%5%6%38%
3476
January1st to 10thNight
15%9%25%5%3445%5%44%
3789
11th to 20thNight
15%5%24323346%52%
3800
21st to endNight
23%8%23423347%40%
4180
February1st to 10thNight
15%6%36%6%3445%7%42%
2280
11th to 20thNight
17%7%35%42345%7%43%
2213
21st to endNight
18%7%445%34446%40%
1776
March1st to 10thNight
12%6%25%4245%5%9%45%
2093
11th to 20thNight
12%5%245%45%5%6%8%43%
1980
21st to endNight
11%41442445%7%54%
2069
April1st to 10thNight
13%425%4345%6%7%47%
1800
11th to 20thNight
14%5%35%5%245%5%7%45%
1775
21st to endNight
5%323335%5%5%9%56%
1800
May1st to 10thNight
7%3234345%5%10%54%
1800
11th to 20thNight
6%4245%35%6%7%9%48%
1769
21st to endNight
6%424435%5%7%11%49%
1938
June1st to 10thNight
8%5%245%35%5%7%11%45%
1800
11th to 20thNight
33145%37%5%9%12%48%
1746
21st to endNight
6%5%35%5%35%6%7%11%44%
1620
July1st to 10thNight
5%6%25%5%36%6%8%12%41%
1620
11th to 20thNight
43135%37%7%10%15%41%
1723
21st to endNight
3425%5%37%7%8%13%43%
1968
August1st to 10thNight
22245%446%8%14%50%
1790
11th to 20thNight
6%3245%36%8%10%13%40%
1776
21st to endNight
8%425%6%46%6%7%11%42%
1980
September1st to 10thNight
6%5%35%5%35%7%8%11%43%
1800
11th to 20thNight
7%425%6%45%6%6%11%43%
1843
21st to endNight
9%4244345%5%8%51%
1980
October1st to 10thNight
6%414423448%60%
2032
11th to 20thNight
7%32432335%8%60%
2160
21st to endNight
10%4244345%45%56%
2376
November1st to 10thNight
10%5%24433445%58%
2220
11th to 20thNight
10%42342445%6%55%
2328
21st to endNight
10%5%25%44445%6%48%
2470
December1st to 10thNight
13%7%26%5%2445%7%46%
2470
11th to 20thNight
13%7%35%5%345%5%6%45%
2915
21st to endNight
17%10%36%5%34445%40%
4180

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 10thDay
40%7%9%14%21%9%
3157
11th to 20thDay
31%7%8%17%31%7%
3160
21st to endDay
51%5%7%12%18%6%
3476
February1st to 10thDay
45%6%9%13%17%10%
1900
11th to 20thDay
39%8%8%18%18%9%
1848
21st to endDay
46%7%11%13%17%6%
1480
March1st to 10thDay
39%7%10%20%17%5%
1880
11th to 20thDay
40%9%12%14%19%7%
1980
21st to endDay
34%7%14%22%17%6%
2286
April1st to 10thDay
34%10%14%17%18%8%
2160
11th to 20thDay
40%6%10%20%17%7%
2134
21st to endDay
30%9%12%21%22%6%
2160
May1st to 10thDay
31%8%13%23%18%7%
2160
11th to 20thDay
35%10%14%21%19%2
2128
21st to endDay
36%11%16%21%15%1
2337
June1st to 10thDay
43%10%16%22%9%1
2160
11th to 20thDay
38%13%19%20%9%1
2204
21st to endDay
46%15%18%14%7%
2340
July1st to 10thDay
49%16%19%14%3
2276
11th to 20thDay
47%16%22%12%2
2161
21st to endDay
44%14%21%17%4
2219
August1st to 10thDay
41%15%23%16%4
2136
11th to 20thDay
49%16%20%12%31
2125
21st to endDay
44%14%18%15%7%1
2376
September1st to 10thDay
42%15%16%13%10%4
2160
11th to 20thDay
39%12%15%16%13%4
2098
21st to endDay
33%11%12%21%15%8%
1980
October1st to 10thDay
26%9%13%21%21%11%
1923
11th to 20thDay
28%6%11%20%24%13%
1800
21st to endDay
29%6%9%18%25%14%
1980
November1st to 10thDay
27%7%9%24%24%9%
1858
11th to 20thDay
32%5%7%21%24%11%
1845
21st to endDay
37%8%10%19%18%10%
1710
December1st to 10thDay
39%8%11%20%16%7%
1710
11th to 20thDay
42%6%9%14%21%7%
2260
21st to endDay
44%9%10%14%22%3
3476
January1st to 10thNight
40%7%8%14%23%9%
3789
11th to 20thNight
32%6%7%18%28%9%
3800
21st to endNight
45%5%7%14%20%9%
4180
February1st to 10thNight
42%5%8%17%18%9%
2280
11th to 20thNight
41%5%8%16%19%11%
2214
21st to endNight
42%8%11%15%13%11%
1776
March1st to 10thNight
35%7%11%17%20%9%
2093
11th to 20thNight
37%8%16%15%16%8%
1980
21st to endNight
29%6%14%21%22%7%
2069
April1st to 10thNight
34%9%12%18%16%11%
1800
11th to 20thNight
38%6%11%21%16%8%
1775
21st to endNight
24%8%12%25%25%6%
1800
May1st to 10thNight
25%7%12%29%20%7%
1800
11th to 20thNight
30%11%15%22%19%2
1770
21st to endNight
27%10%17%28%15%2
1939
June1st to 10thNight
31%12%19%27%12%
1800
11th to 20thNight
25%12%23%28%12%
1746
21st to endNight
30%14%26%21%9%
1620
July1st to 10thNight
32%15%29%20%3
1575
11th to 20thNight
26%21%31%18%3
1668
21st to endNight
29%14%30%22%4
1858
August1st to 10thNight
21%14%33%26%6%
1778
11th to 20thNight
29%19%31%17%3
1776
21st to endNight
35%14%22%20%6%3
1980
September1st to 10thNight
32%13%22%18%13%3
1800
11th to 20thNight
33%13%16%18%14%6%
1843
21st to endNight
29%9%14%25%15%8%
1980
October1st to 10thNight
23%6%14%23%21%14%
2032
11th to 20thNight
24%5%11%22%25%13%
2160
21st to endNight
29%6%8%15%27%14%
2376
November1st to 10thNight
29%5%8%22%25%11%
2220
11th to 20thNight
30%5%8%19%26%12%
2328
21st to endNight
35%6%8%21%21%9%
2470
December1st to 10thNight
37%6%10%22%19%6%
2470
11th to 20thNight
39%6%9%15%16%14%
2916
21st to endNight
46%7%9%16%17%6%
4180

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 10thDay
19%14%37%23%6%1
270
11th to 20thDay
13%17%35%29%6%
270
21st to endDay
22%20%35%20%3
297
February1st to 10thDay
22%19%33%20%5%1
270
11th to 20thDay
22%13%36%21%7%
267
21st to endDay
13%20%44%19%5%
198
March1st to 10thDay
14%16%34%31%4
240
11th to 20thDay
16%16%40%23%5%1
240
21st to endDay
14%13%31%30%12%1
372
April1st to 10thDay
13%11%31%30%12%2
420
11th to 20thDay
18%14%28%25%13%2
412
21st to endDay
8%8%29%30%21%4
420
May1st to 10thDay
10%5%22%33%25%4
423
11th to 20thDay
10%217%31%31%9%
447
21st to endDay
7%110%34%32%17%
485
June1st to 10thDay
11%27%27%34%18%
450
11th to 20thDay
9%19%25%36%20%
494
21st to endDay
8%28%26%33%22%
630
July1st to 10thDay
9%28%19%35%27%
621
11th to 20thDay
7%1415%38%36%
508
21st to endDay
10%5%16%35%33%
488
August1st to 10thDay
5%2413%35%41%
446
11th to 20thDay
8%5%11%33%42%
439
21st to endDay
9%38%16%36%29%
495
September1st to 10thDay
9%16%20%37%26%
450
11th to 20thDay
10%211%21%39%17%
450
21st to endDay
8%28%28%39%15%
450
October1st to 10thDay
6%417%39%29%5%
393
11th to 20thDay
5%419%40%26%7%
270
21st to endDay
17%9%20%32%21%2
297
November1st to 10thDay
14%7%15%40%21%3
270
11th to 20thDay
16%10%30%30%12%2
270
21st to endDay
14%12%33%32%9%1
270
December1st to 10thDay
15%9%34%27%11%4
270
11th to 20thDay
17%17%33%26%6%
270
21st to endDay
23%12%28%25%10%1
297
January1st to 10thNight
17%11%31%34%7%1
760
11th to 20thNight
15%11%26%37%10%1
760
21st to endNight
19%11%32%30%7%
836
February1st to 10thNight
18%11%31%33%8%1
760
11th to 20thNight
19%12%29%30%10%
742
21st to endNight
20%13%32%30%6%
592
March1st to 10thNight
15%9%32%31%12%
653
11th to 20thNight
15%7%27%34%16%1
540
21st to endNight
11%10%28%34%17%1
594
April1st to 10thNight
14%9%26%34%16%1
540
11th to 20thNight
16%8%28%30%17%1
533
21st to endNight
8%5%23%40%22%2
540
May1st to 10thNight
6%418%43%28%1
540
11th to 20thNight
6%316%41%30%4
531
21st to endNight
5%112%30%41%11%
584
June1st to 10thNight
9%16%31%44%10%
540
11th to 20thNight
315%26%50%15%
540
21st to endNight
8%16%21%44%22%
540
July1st to 10thNight
6%1413%50%26%
525
11th to 20thNight
4127%47%39%
522
21st to endNight
3314%51%29%
591
August1st to 10thNight
2211%49%36%
537
11th to 20thNight
7%28%46%37%
534
21st to endNight
9%1415%45%26%
594
September1st to 10thNight
8%416%46%26%
540
11th to 20thNight
8%36%22%46%15%
583
21st to endNight
11%18%29%41%10%
720
October1st to 10thNight
7%212%36%37%6%
720
11th to 20thNight
7%417%33%33%6%
720
21st to endNight
10%7%18%34%26%4
792
November1st to 10thNight
11%6%16%37%27%3
740
11th to 20thNight
12%8%20%37%20%3
760
21st to endNight
13%8%24%37%17%2
760
December1st to 10thNight
15%10%24%30%19%2
760
11th to 20thNight
18%6%26%35%13%1
760
21st to endNight
23%10%26%30%10%1
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