Gretna
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Gretna clear sky chart from 2005-11-05 to 2018-02-13.

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
38%
16959
13%
48811
7%
16916
43%
16916
21%
34409

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%
974
9%
2910
6%
970
34%
970
14%
2230
January16th to endNight
26%
1040
11%
2935
5%
1040
31%
1040
13%
2318
February1st to 15thNight
38%
937
12%
2494
5%
936
42%
936
20%
1848
February16th to endNight
35%
732
12%
1994
6%
732
40%
732
17%
1523
March1st to 15thNight
34%
639
13%
2079
8%
639
40%
639
17%
1498
March16th to endNight
37%
576
12%
2112
10%
576
43%
576
14%
1517
April1st to 15thNight
41%
539
16%
1720
11%
539
50%
539
24%
1105
April16th to endNight
40%
540
16%
1620
12%
540
44%
540
21%
1125
May1st to 15thNight
35%
540
12%
1530
5%
540
38%
540
23%
1050
May16th to endNight
41%
563
12%
1311
5%
561
44%
561
31%
904
June1st to 15thNight
47%
540
5%
1260
3%
540
49%
540
33%
864
June16th to endNight
51%
540
7%
1260
4%
540
54%
540
35%
846
July1st to 15thNight
62%
510
9%
1190
6%
510
63%
510
46%
732
July16th to endNight
63%
564
16%
1293
8%
531
65%
531
47%
756
August1st to 15thNight
60%
539
17%
1555
10%
536
63%
536
39%
895
August16th to endNight
55%
573
22%
1719
18%
573
63%
573
41%
936
September1st to 15thNight
53%
540
18%
1824
16%
540
60%
540
36%
988
September16th to endNight
46%
537
21%
1980
19%
537
54%
537
27%
1156
October1st to 15thNight
33%
672
16%
2112
10%
672
39%
672
21%
1405
October16th to endNight
33%
924
14%
2460
6%
924
38%
924
21%
1759
November1st to 15thNight
31%
950
13%
2616
5%
950
36%
950
19%
1913
November16th to endNight
26%
975
12%
2792
6%
975
32%
975
16%
2104
December1st to 15thNight
28%
975
6%
2925
4%
975
30%
975
11%
2361
December16th to endNight
23%
1040
8%
3120
4%
1040
27%
1040
9%
2576

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%8%36%6%35%5%5%7%34%
49252
Night
11%9%47%6%35%5%5%7%35%
48962
Cloud CoverDay
12%8%37%6%35%45%7%34%
49262
Night
11%9%47%6%35%45%6%35%
48965
Cloud CoverDay
12%8%36%6%35%5%5%7%34%
49263
Night
11%9%47%6%35%5%5%6%35%
48966
Cloud CoverDay
13%8%36%6%35%5%5%7%34%
49261
Night
11%9%47%6%35%45%6%35%
48966
TransparencyDay
47%7%15%21%7%
48920
Night
47%6%14%21%8%
48811
SeeingDay
16%6%27%29%15%3
6698
Night
15%6%29%33%13%1
16964

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
8%12%49%8%46%6%8%7%24%
1358
16th to endDay
11%13%7%12%7%48%6%6%416%
1638
February1st to 15thDay
8%10%5%7%8%49%7%5%6%26%
1634
16th to endDay
9%9%5%13%9%6%6%7%5%8%16%
1491
March1st to 15thDay
12%12%5%9%9%45%5%5%5%22%
1870
16th to endDay
13%11%48%8%46%5%5%7%24%
2112
April1st to 15thDay
11%9%46%44445%7%36%
2225
16th to endDay
17%9%46%5%2445%6%32%
2340
May1st to 15thDay
19%8%35%5%34445%33%
2418
16th to endDay
18%7%25%5%24448%35%
2803
June1st to 15thDay
18%6%25%5%25%5%5%8%33%
2700
16th to endDay
14%6%25%5%36%46%8%37%
2700
July1st to 15thDay
8%4145%35%5%6%9%45%
2700
16th to endDay
6%3133345%6%9%50%
2784
August1st to 15thDay
8%5%25%5%345%5%8%45%
2397
16th to endDay
9%31342345%7%51%
2483
September1st to 15thDay
11%5%1432445%8%46%
2133
16th to endDay
15%42442345%8%43%
1980
October1st to 15thDay
16%9%35%433337%38%
1835
16th to endDay
16%9%36%6%4445%7%31%
1763
November1st to 15thDay
13%11%6%8%6%34436%32%
1572
16th to endDay
13%13%7%9%7%3435%6%24%
1495
December1st to 15thDay
10%16%6%12%10%46%7%45%16%
1365
16th to endDay
13%13%7%12%8%5%5%45%5%18%
1456
January1st to 15thNight
7%12%6%9%9%36%6%7%6%24%
2910
16th to endNight
10%12%6%10%8%45%6%5%6%23%
2935
February1st to 15thNight
7%9%5%8%7%47%6%5%7%30%
2491
16th to endNight
9%9%5%10%8%5%6%5%5%5%28%
1994
March1st to 15thNight
11%10%5%8%6%345%5%7%30%
2067
16th to endNight
13%9%48%7%345%5%7%30%
2112
April1st to 15thNight
9%7%35%5%45%6%5%6%41%
1720
16th to endNight
17%8%45%5%24445%36%
1620
May1st to 15thNight
17%6%25%5%34447%38%
1530
16th to endNight
14%6%25%3235%5%7%42%
1310
June1st to 15thNight
13%5%236%2445%8%43%
1260
16th to endNight
10%424325%47%10%44%
1260
July1st to 15thNight
6%31232446%8%55%
1260
16th to endNight
5%1133245%5%9%57%
1382
August1st to 15thNight
6%31442445%8%52%
1563
16th to endNight
5%313324448%57%
1719
September1st to 15thNight
8%41342345%7%55%
1824
16th to endNight
13%414433447%47%
1980
October1st to 15thNight
14%7%25%5%2446%6%39%
2112
16th to endNight
15%9%36%6%35%447%34%
2460
November1st to 15thNight
13%11%47%6%24446%32%
2616
16th to endNight
11%12%5%8%7%3435%6%29%
2792
December1st to 15thNight
10%14%7%12%9%45%45%5%20%
2925
16th to endNight
15%14%7%12%9%45%45%5%16%
3120

Percentage Cloud Cover NW (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
7%12%5%9%9%46%6%7%6%24%
1359
16th to endDay
10%14%6%11%8%5%8%6%6%416%
1638
February1st to 15thDay
8%11%47%9%49%6%5%7%25%
1634
16th to endDay
9%10%5%12%8%6%7%7%6%7%17%
1491
March1st to 15thDay
12%11%5%10%10%46%5%5%6%22%
1881
16th to endDay
12%11%5%8%8%45%5%5%7%24%
2110
April1st to 15thDay
11%9%46%5%245%6%7%36%
2225
16th to endDay
17%10%35%5%3446%6%32%
2340
May1st to 15thDay
19%8%36%6%34445%33%
2418
16th to endDay
18%6%35%5%3435%7%35%
2803
June1st to 15thDay
17%6%25%5%35%5%5%8%33%
2700
16th to endDay
14%5%25%5%46%46%8%37%
2700
July1st to 15thDay
8%316%5%35%5%6%9%44%
2700
16th to endDay
6%323435%47%9%50%
2784
August1st to 15thDay
8%5%25%5%3346%8%45%
2397
16th to endDay
9%42442436%7%51%
2483
September1st to 15thDay
11%5%2342445%6%47%
2133
16th to endDay
15%425%5%2345%8%42%
1980
October1st to 15thDay
17%9%25%424347%39%
1835
16th to endDay
16%9%36%5%35%45%7%31%
1763
November1st to 15thDay
13%10%5%9%6%25%345%32%
1572
16th to endDay
12%14%7%9%8%3445%6%24%
1495
December1st to 15thDay
8%16%5%13%9%47%6%45%16%
1365
16th to endDay
13%13%7%13%8%45%45%419%
1456
January1st to 15thNight
7%13%6%9%9%47%6%6%5%24%
2910
16th to endNight
10%13%6%10%8%46%6%5%6%22%
2935
February1st to 15thNight
7%9%48%7%48%6%5%7%29%
2484
16th to endNight
9%9%5%9%9%5%6%5%5%5%28%
1994
March1st to 15thNight
11%10%5%9%6%345%5%7%29%
2079
16th to endNight
12%9%48%7%35%45%6%30%
2110
April1st to 15thNight
9%7%345%45%6%6%6%40%
1720
16th to endNight
16%8%36%435%44437%
1620
May1st to 15thNight
17%7%36%5%24446%38%
1529
16th to endNight
13%6%25%32346%7%42%
1311
June1st to 15thNight
13%6%2443435%8%43%
1260
16th to endNight
10%323326%5%5%10%44%
1260
July1st to 15thNight
7%332235%6%9%54%
1260
16th to endNight
421233446%9%55%
1382
August1st to 15thNight
6%3145%2445%8%52%
1563
16th to endNight
5%31332345%8%57%
1719
September1st to 15thNight
7%41442345%7%55%
1824
16th to endNight
13%424423347%47%
1980
October1st to 15thNight
13%7%35%5%2445%6%39%
2112
16th to endNight
15%10%35%6%345%5%7%34%
2460
November1st to 15thNight
12%11%48%6%3435%6%32%
2616
16th to endNight
11%12%5%8%7%34447%29%
2792
December1st to 15thNight
10%14%7%12%8%45%5%5%5%20%
2925
16th to endNight
14%14%7%12%8%45%345%16%
3120

Percentage Cloud Cover SW (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
7%12%49%8%5%6%6%7%7%24%
1358
16th to endDay
11%13%7%13%7%48%6%5%5%16%
1638
February1st to 15thDay
8%11%47%8%49%7%5%7%26%
1634
16th to endDay
9%10%5%13%9%6%6%8%6%8%16%
1491
March1st to 15thDay
12%11%6%9%9%46%45%6%22%
1881
16th to endDay
12%11%48%8%46%5%48%24%
2112
April1st to 15thDay
11%9%36%43446%7%36%
2225
16th to endDay
17%9%46%5%2446%6%33%
2340
May1st to 15thDay
19%8%36%5%34446%33%
2418
16th to endDay
17%7%35%5%34448%35%
2803
June1st to 15thDay
17%7%35%5%25%5%6%8%33%
2700
16th to endDay
14%5%245%36%5%5%8%37%
2700
July1st to 15thDay
8%425%435%5%6%9%44%
2700
16th to endDay
6%3233345%7%9%50%
2784
August1st to 15thDay
8%4245%345%6%9%45%
2397
16th to endDay
8%41443345%7%51%
2483
September1st to 15thDay
11%5%2443445%8%46%
2133
16th to endDay
15%425%42345%8%42%
1980
October1st to 15thDay
16%9%35%423437%38%
1835
16th to endDay
16%8%36%6%445%5%7%31%
1763
November1st to 15thDay
13%11%6%8%5%24446%32%
1572
16th to endDay
13%13%7%10%7%3336%6%23%
1495
December1st to 15thDay
9%15%6%12%10%46%6%5%5%16%
1365
16th to endDay
13%13%6%12%8%5%6%45%5%18%
1456
January1st to 15thNight
7%13%6%9%8%36%7%6%6%24%
2910
16th to endNight
10%12%6%10%8%45%6%5%6%22%
2935
February1st to 15thNight
7%9%48%7%47%6%5%7%30%
2484
16th to endNight
8%9%5%10%8%5%6%5%5%5%28%
1994
March1st to 15thNight
11%10%5%9%6%35%5%5%7%30%
2079
16th to endNight
13%9%49%7%35%45%6%30%
2112
April1st to 15thNight
8%7%35%5%45%6%5%6%40%
1720
16th to endNight
17%8%45%5%25%445%36%
1620
May1st to 15thNight
16%7%25%5%34446%38%
1529
16th to endNight
13%7%243245%5%7%43%
1310
June1st to 15thNight
13%5%236%2445%7%43%
1260
16th to endNight
10%323425%5%6%10%44%
1260
July1st to 15thNight
6%212325%46%9%54%
1260
16th to endNight
5%1123335%6%10%56%
1382
August1st to 15thNight
6%314425%45%8%52%
1563
16th to endNight
5%313314448%57%
1719
September1st to 15thNight
7%41442345%7%55%
1824
16th to endNight
13%5%14434347%47%
1980
October1st to 15thNight
14%7%25%5%3445%7%39%
2112
16th to endNight
15%9%25%6%3445%7%34%
2460
November1st to 15thNight
13%11%47%6%3445%6%32%
2616
16th to endNight
11%12%6%8%7%3435%6%29%
2792
December1st to 15thNight
10%14%6%13%8%45%45%5%20%
2925
16th to endNight
15%14%8%11%9%45%445%16%
3120

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
8%12%5%9%8%47%6%7%6%24%
1358
16th to endDay
11%13%6%13%7%48%6%6%5%15%
1638
February1st to 15thDay
8%10%48%8%49%6%6%6%26%
1634
16th to endDay
9%12%411%10%5%6%7%5%7%18%
1491
March1st to 15thDay
13%11%5%10%9%46%45%6%22%
1881
16th to endDay
12%11%5%8%7%45%5%5%8%24%
2110
April1st to 15thDay
12%10%36%5%2445%7%36%
2225
16th to endDay
18%8%36%5%3346%6%34%
2340
May1st to 15thDay
20%7%35%5%34446%33%
2418
16th to endDay
19%6%345%3435%7%36%
2803
June1st to 15thDay
17%6%35%5%35%47%7%33%
2700
16th to endDay
14%5%24436%5%6%7%37%
2700
July1st to 15thDay
9%325%5%35%5%7%8%44%
2700
16th to endDay
7%4133235%6%9%50%
2784
August1st to 15thDay
9%4245%245%6%8%46%
2397
16th to endDay
9%32442345%7%52%
2483
September1st to 15thDay
11%5%24424447%47%
2133
16th to endDay
15%415%42435%8%43%
1980
October1st to 15thDay
16%9%25%423447%38%
1835
16th to endDay
16%8%46%5%35%5%5%7%31%
1763
November1st to 15thDay
14%11%5%8%5%24435%33%
1572
16th to endDay
13%14%6%10%7%345%46%23%
1495
December1st to 15thDay
9%17%5%12%10%47%5%5%6%15%
1365
16th to endDay
13%14%6%13%9%45%45%5%18%
1456
January1st to 15thNight
8%12%6%9%8%36%6%6%5%24%
2910
16th to endNight
10%12%6%10%8%46%6%5%6%21%
2935
February1st to 15thNight
7%9%48%6%48%6%6%7%29%
2484
16th to endNight
9%9%5%11%7%5%7%5%45%28%
1994
March1st to 15thNight
11%9%6%8%6%35%5%5%6%30%
2079
16th to endNight
12%9%5%8%7%35%5%46%31%
2112
April1st to 15thNight
9%6%35%5%36%5%6%7%40%
1720
16th to endNight
16%8%35%5%35%44437%
1620
May1st to 15thNight
17%8%26%423447%37%
1529
16th to endNight
15%6%243335%6%6%42%
1310
June1st to 15thNight
14%5%23335%35%7%44%
1260
16th to endNight
10%3234346%5%10%43%
1260
July1st to 15thNight
7%3133235%7%8%55%
1260
16th to endNight
5%113315%5%6%9%56%
1382
August1st to 15thNight
6%324434448%52%
1563
16th to endNight
6%31332335%7%57%
1719
September1st to 15thNight
7%41332435%7%55%
1824
16th to endNight
14%5%1342335%7%46%
1980
October1st to 15thNight
14%7%25%5%3435%7%38%
2112
16th to endNight
15%10%25%5%245%5%7%34%
2460
November1st to 15thNight
14%12%47%5%3445%6%32%
2616
16th to endNight
12%11%6%10%6%3435%6%29%
2792
December1st to 15thNight
10%15%7%12%8%46%45%420%
2925
16th to endNight
14%14%7%12%8%46%45%5%15%
3120

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
56%6%13%19%4
1359
16th to endDay
66%6%9%13%3
1638
February1st to 15thDay
55%7%15%15%5%
1635
16th to endDay
63%6%10%13%4
1491
March1st to 15thDay
61%6%13%13%4
1881
16th to endDay
57%7%14%15%3
2112
April1st to 15thDay
46%6%15%23%6%1
2225
16th to endDay
49%5%15%16%10%1
2340
May1st to 15thDay
51%410%23%7%
2418
16th to endDay
47%6%12%21%10%1
2807
June1st to 15thDay
47%7%18%20%6%
2700
16th to endDay
42%8%17%24%5%
2700
July1st to 15thDay
34%7%19%29%8%
2541
16th to endDay
28%9%21%29%10%
2610
August1st to 15thDay
34%8%20%27%7%1
2381
16th to endDay
30%7%16%32%12%
2483
September1st to 15thDay
34%7%20%28%9%
2133
16th to endDay
39%7%16%30%6%
1980
October1st to 15thDay
45%6%14%23%8%
1835
16th to endDay
49%7%19%16%6%
1763
November1st to 15thDay
53%5%15%17%5%1
1572
16th to endDay
60%7%13%13%4
1495
December1st to 15thDay
67%7%11%12%1
1365
16th to endDay
66%7%8%11%5%
1456
January1st to 15thNight
56%8%11%17%5%
2910
16th to endNight
60%6%11%14%6%
2935
February1st to 15thNight
50%7%15%18%7%
2494
16th to endNight
56%6%12%19%5%
1994
March1st to 15thNight
51%7%14%20%5%
2079
16th to endNight
53%7%12%20%5%1
2112
April1st to 15thNight
40%6%14%23%11%2
1720
16th to endNight
48%5%11%20%11%1
1620
May1st to 15thNight
46%5%12%23%11%
1530
16th to endNight
39%7%13%22%16%1
1311
June1st to 15thNight
37%7%19%25%9%1
1260
16th to endNight
32%10%18%29%7%1
1260
July1st to 15thNight
24%7%17%35%12%1
1190
16th to endNight
21%7%18%32%17%1
1293
August1st to 15thNight
29%5%18%34%10%1
1555
16th to endNight
24%6%17%32%17%
1719
September1st to 15thNight
28%6%17%31%14%1
1824
16th to endNight
36%5%16%29%11%
1980
October1st to 15thNight
43%7%15%23%9%
2112
16th to endNight
48%6%16%23%5%
2460
November1st to 15thNight
51%5%14%18%6%1
2616
16th to endNight
55%7%15%16%5%
2792
December1st to 15thNight
64%7%14%12%2
2925
16th to endNight
68%5%9%11%4
3120

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
10%7%26%38%13%3
135
16th to endDay
18%226%38%13%
144
February1st to 15thDay
15%11%38%30%3
126
16th to endDay
19%34%33%12%
81
March1st to 15thDay
13%6%30%38%11%
90
16th to endDay
21%6%22%26%18%4
96
April1st to 15thDay
14%14%29%29%11%
344
16th to endDay
22%9%26%26%12%2
450
May1st to 15thDay
21%9%29%26%11%2
468
16th to endDay
22%7%26%26%13%4
517
June1st to 15thDay
20%7%28%25%13%4
495
16th to endDay
16%6%25%33%14%4
495
July1st to 15thDay
12%325%27%24%6%
475
16th to endDay
8%424%35%19%7%
515
August1st to 15thDay
9%324%30%21%9%
493
16th to endDay
8%5%25%32%22%5%
523
September1st to 15thDay
12%5%29%33%17%2
288
16th to endDay
20%10%34%20%12%2
135
October1st to 15thDay
14%7%42%28%5%
135
16th to endDay
24%436%22%11%1
144
November1st to 15thDay
20%9%23%34%10%
135
16th to endDay
29%14%27%19%8%
135
December1st to 15thDay
22%5%25%35%8%1
135
16th to endDay
15%9%27%29%14%2
144
January1st to 15thNight
14%9%29%33%12%1
975
16th to endNight
16%10%33%31%8%
1040
February1st to 15thNight
11%6%27%36%14%1
941
16th to endNight
13%5%31%32%14%2
732
March1st to 15thNight
15%6%30%30%14%2
639
16th to endNight
18%5%26%31%14%3
576
April1st to 15thNight
11%7%33%35%11%
539
16th to endNight
19%6%26%35%11%
540
May1st to 15thNight
20%8%29%30%10%
540
16th to endNight
17%6%28%35%12%
563
June1st to 15thNight
17%424%37%15%
540
16th to endNight
10%426%37%18%
540
July1st to 15thNight
7%321%42%23%
510
16th to endNight
5%323%42%23%2
564
August1st to 15thNight
7%121%43%23%3
539
16th to endNight
8%326%36%22%1
573
September1st to 15thNight
10%327%37%18%2
540
16th to endNight
16%426%36%15%
537
October1st to 15thNight
19%7%32%30%9%
672
16th to endNight
21%7%30%29%10%
924
November1st to 15thNight
20%8%31%28%10%1
950
16th to endNight
18%9%37%25%7%2
975
December1st to 15thNight
17%7%27%33%12%
975
16th to endNight
20%7%31%26%12%1
1040
Page generated at 2018-02-13 19:46:05. Version ac04

Copyright 2018 A. Danko


Gretna Clear Sky Chart |Charts Home