Gemma Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Gemma Observatory clear sky chartfrom 2005-11-06 to 2024-01-15.

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
27%
22795
17%
74684
8%
22751
38%
22751
17%
50062

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
17%
760
19%
2649
6%
757
34%
757
12%
1742
January11th to 20thNight
18%
740
14%
2588
3%
740
30%
740
10%
1824
January21st to endNight
16%
792
18%
2772
5%
792
32%
792
12%
1841
February1st to 10thNight
14%
720
15%
2350
3%
720
28%
720
11%
1654
February11th to 20thNight
15%
703
19%
2100
3%
700
33%
700
14%
1442
February21st to endNight
18%
592
20%
1776
4%
592
36%
592
15%
1206
March1st to 10thNight
18%
720
20%
2160
7%
720
35%
720
15%
1475
March11th to 20thNight
21%
720
17%
2089
5%
720
32%
720
15%
1451
March21st to endNight
22%
792
22%
2017
6%
792
33%
792
18%
1392
April1st to 10thNight
20%
720
17%
1800
4%
720
28%
720
17%
1298
April11th to 20thNight
24%
704
16%
1768
4%
704
31%
704
19%
1264
April21st to endNight
21%
540
17%
1620
6%
540
31%
540
17%
1119
May1st to 10thNight
22%
540
13%
1620
7%
540
28%
540
15%
1179
May11th to 20thNight
27%
418
18%
1480
12%
418
38%
418
16%
979
May21st to endNight
37%
389
13%
1551
13%
388
45%
388
17%
993
June1st to 10thNight
33%
360
12%
1440
10%
360
44%
360
15%
966
June11th to 20thNight
36%
360
11%
1440
10%
360
45%
360
16%
926
June21st to endNight
44%
360
11%
1440
12%
360
50%
360
17%
939
July1st to 10thNight
45%
350
13%
1400
16%
350
53%
350
22%
830
July11th to 20thNight
51%
348
14%
1392
18%
348
56%
348
24%
815
July21st to endNight
49%
394
18%
1486
19%
372
57%
372
25%
853
August1st to 10thNight
52%
417
18%
1481
13%
415
58%
415
25%
868
August11th to 20thNight
43%
563
17%
1627
12%
563
47%
563
24%
1066
August21st to endNight
49%
792
20%
1980
9%
792
52%
792
32%
1305
September1st to 10thNight
45%
720
20%
1800
8%
720
50%
720
35%
1139
September11th to 20thNight
44%
720
24%
1952
14%
720
51%
720
32%
1207
September21st to endNight
40%
720
25%
1980
14%
720
50%
720
31%
1248
October1st to 10thNight
35%
720
16%
1980
9%
720
42%
720
21%
1392
October11th to 20thNight
31%
720
17%
2237
8%
720
41%
720
19%
1491
October21st to endNight
24%
792
14%
2574
7%
792
34%
792
13%
1830
November1st to 10thNight
30%
740
21%
2407
11%
740
42%
740
19%
1523
November11th to 20thNight
24%
760
23%
2470
9%
760
40%
760
17%
1590
November21st to endNight
22%
760
18%
2532
7%
760
36%
760
15%
1680
December1st to 10thNight
15%
760
16%
2660
4%
760
30%
760
10%
1897
December11th to 20thNight
15%
760
15%
3230
4%
759
30%
759
9%
2204
December21st to endNight
14%
829
11%
4836
2%
817
29%
817
5%
3434

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
17%7%36%6%35%5%5%7%35%
74812
Night
17%8%36%5%35%5%5%7%37%
74962
TransparencyDay
47%7%11%21%13%2
74517
Night
46%6%12%22%13%2
74791
SeeingDay
24%19%28%18%8%2
13768
Night
23%17%27%24%8%1
22802

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 10thDay
13%11%46%6%36%6%6%8%32%
1517
11th to 20thDay
17%9%36%6%46%46%8%30%
1478
21st to endDay
12%9%45%8%45%5%6%10%33%
1584
February1st to 10thDay
16%11%5%6%6%45%6%6%6%30%
1606
11th to 20thDay
11%9%47%8%45%46%7%34%
1748
21st to endDay
16%12%46%5%45%5%5%6%31%
1480
March1st to 10thDay
13%9%37%7%46%5%5%7%33%
1797
11th to 20thDay
16%9%45%5%45%5%6%7%34%
1869
21st to endDay
17%9%37%5%45%45%6%36%
2335
April1st to 10thDay
18%9%46%6%45%447%34%
2160
11th to 20thDay
17%6%36%445%6%6%8%36%
2139
21st to endDay
21%8%47%7%34446%30%
2340
May1st to 10thDay
22%10%47%6%345%36%31%
2340
11th to 20thDay
19%7%36%5%3445%7%37%
2428
21st to endDay
15%5%36%5%36%5%5%8%38%
2725
June1st to 10thDay
20%6%36%7%36%5%6%8%30%
2520
11th to 20thDay
17%6%35%6%45%5%6%9%35%
2520
21st to endDay
19%7%37%7%45%6%6%7%29%
2520
July1st to 10thDay
16%5%36%5%46%6%5%8%37%
2520
11th to 20thDay
13%5%36%6%46%6%6%9%36%
2520
21st to endDay
14%5%35%6%46%6%6%8%36%
2744
August1st to 10thDay
12%7%26%6%46%6%7%9%35%
2427
11th to 20thDay
13%6%26%7%46%6%7%8%35%
2270
21st to endDay
13%325%5%35%6%7%9%42%
2376
September1st to 10thDay
12%5%36%4346%6%8%44%
2160
11th to 20thDay
15%5%24333446%50%
2007
21st to endDay
15%5%24435%5%5%7%45%
1980
October1st to 10thDay
23%8%36%434446%35%
1980
11th to 20thDay
16%7%26%5%35%5%5%9%39%
1717
21st to endDay
24%10%35%6%345%5%5%30%
1782
November1st to 10thDay
14%6%36%5%3445%8%42%
1671
11th to 20thDay
18%5%35%5%26%5%5%5%40%
1710
21st to endDay
19%8%36%435%5%6%7%34%
1644
December1st to 10thDay
18%10%47%6%45%6%6%8%27%
1520
11th to 20thDay
20%9%36%5%34446%36%
1906
21st to endDay
23%12%47%5%35%5%46%25%
2772
January1st to 10thNight
14%8%35%5%45%5%6%7%36%
2649
11th to 20thNight
15%10%47%6%36%5%6%8%30%
2588
21st to endNight
11%8%5%8%7%35%6%5%7%36%
2772
February1st to 10thNight
16%10%46%6%36%5%7%7%31%
2350
11th to 20thNight
12%8%46%6%36%6%6%8%34%
2098
21st to endNight
14%10%36%436%5%6%9%35%
1776
March1st to 10thNight
14%9%37%5%45%45%7%37%
2150
11th to 20thNight
16%8%36%5%35%6%7%8%33%
2089
21st to endNight
16%9%36%5%2445%6%41%
2017
April1st to 10thNight
18%8%46%6%35%45%6%35%
1800
11th to 20thNight
18%8%35%6%44447%38%
1768
21st to endNight
20%8%36%5%345%5%7%35%
1620
May1st to 10thNight
22%9%37%6%4445%6%31%
1620
11th to 20thNight
19%6%36%5%35%5%6%6%36%
1479
21st to endNight
15%6%46%7%35%5%5%7%38%
1550
June1st to 10thNight
21%6%36%6%5%445%8%33%
1440
11th to 20thNight
20%5%25%5%36%5%6%6%38%
1440
21st to endNight
17%5%35%5%45%5%7%8%35%
1440
July1st to 10thNight
16%5%24435%5%6%9%42%
1440
11th to 20thNight
13%5%25%5%35%6%6%9%42%
1440
21st to endNight
13%5%245%35%5%6%8%45%
1574
August1st to 10thNight
13%6%244345%5%10%45%
1491
11th to 20thNight
13%6%26%6%46%46%7%40%
1627
21st to endNight
14%6%244335%5%8%45%
1980
September1st to 10thNight
13%5%2432445%7%51%
1800
11th to 20thNight
14%5%24424446%50%
1952
21st to endNight
14%5%25%43345%7%48%
1980
October1st to 10thNight
22%6%35%5%34445%38%
1980
11th to 20thNight
16%7%25%6%45%5%6%7%37%
2237
21st to endNight
24%8%25%6%345%6%6%32%
2574
November1st to 10thNight
16%7%25%5%3445%6%42%
2407
11th to 20thNight
16%7%25%5%345%5%7%40%
2470
21st to endNight
15%7%35%5%35%5%6%7%37%
2532
December1st to 10thNight
18%8%49%6%345%6%7%30%
2660
11th to 20thNight
20%9%36%5%26%46%6%34%
3288
21st to endNight
22%11%47%6%3445%6%28%
4884

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
47%8%9%25%11%1
1518
11th to 20thDay
50%7%12%18%10%3
1478
21st to endDay
46%6%11%21%13%3
1584
February1st to 10thDay
52%7%12%18%10%2
1606
11th to 20thDay
49%7%10%19%12%3
1757
21st to endDay
52%6%9%18%13%1
1480
March1st to 10thDay
50%6%12%18%12%4
1800
11th to 20thDay
48%5%12%15%16%4
1869
21st to endDay
49%6%7%18%16%4
2335
April1st to 10thDay
51%5%8%21%13%3
2160
11th to 20thDay
45%6%10%20%16%2
2139
21st to endDay
55%5%8%16%13%3
2340
May1st to 10thDay
55%46%21%11%3
2340
11th to 20thDay
46%6%9%21%15%3
2432
21st to endDay
44%7%13%22%12%2
2725
June1st to 10thDay
51%7%12%20%10%1
2520
11th to 20thDay
45%7%10%20%15%2
2520
21st to endDay
51%8%11%18%11%1
2520
July1st to 10thDay
43%7%10%23%16%1
2450
11th to 20thDay
43%7%15%25%9%1
2445
21st to endDay
41%8%14%23%12%1
2591
August1st to 10thDay
42%8%14%23%11%
2413
11th to 20thDay
44%8%14%23%11%1
2270
21st to endDay
37%7%13%26%16%1
2376
September1st to 10thDay
36%7%13%27%15%2
2160
11th to 20thDay
35%5%8%27%22%2
2007
21st to endDay
37%6%12%27%16%2
1980
October1st to 10thDay
51%5%9%20%14%2
1980
11th to 20thDay
42%7%12%28%9%1
1717
21st to endDay
55%6%8%19%10%2
1782
November1st to 10thDay
41%5%14%24%15%1
1671
11th to 20thDay
43%8%10%19%17%2
1710
21st to endDay
48%7%16%19%9%1
1644
December1st to 10thDay
53%9%13%16%8%1
1520
11th to 20thDay
50%6%10%18%15%1
1906
21st to endDay
59%7%8%16%10%
2772
January1st to 10thNight
45%7%10%22%14%2
2649
11th to 20thNight
52%7%13%18%9%1
2588
21st to endNight
46%7%9%20%16%2
2772
February1st to 10thNight
51%6%11%19%11%2
2350
11th to 20thNight
45%7%11%21%14%2
2100
21st to endNight
45%7%12%21%11%3
1776
March1st to 10thNight
46%6%10%22%13%3
2160
11th to 20thNight
47%6%10%20%13%3
2090
21st to endNight
45%5%9%17%19%5%
2017
April1st to 10thNight
49%5%9%19%15%2
1800
11th to 20thNight
46%6%9%23%16%1
1768
21st to endNight
48%6%11%18%14%3
1620
May1st to 10thNight
54%5%7%19%13%2
1620
11th to 20thNight
46%7%9%20%15%3
1480
21st to endNight
45%6%14%22%11%2
1551
June1st to 10thNight
51%5%12%20%11%1
1440
11th to 20thNight
45%7%14%19%12%2
1440
21st to endNight
46%7%14%21%12%1
1440
July1st to 10thNight
38%8%16%24%13%1
1400
11th to 20thNight
39%5%18%25%13%
1392
21st to endNight
36%7%14%29%12%2
1486
August1st to 10thNight
37%7%15%28%13%1
1481
11th to 20thNight
42%6%16%24%12%
1627
21st to endNight
37%6%12%29%16%
1980
September1st to 10thNight
33%6%14%29%17%1
1800
11th to 20thNight
34%6%11%28%20%1
1952
21st to endNight
36%5%11%30%16%2
1980
October1st to 10thNight
47%6%12%22%11%2
1980
11th to 20thNight
44%7%14%25%10%1
2237
21st to endNight
52%6%11%20%10%2
2574
November1st to 10thNight
42%5%12%23%15%2
2407
11th to 20thNight
43%6%10%23%16%2
2470
21st to endNight
42%9%15%23%10%1
2532
December1st to 10thNight
52%8%10%21%8%1
2660
11th to 20thNight
49%7%11%16%15%1
3288
21st to endNight
57%7%8%20%8%
4884

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
28%24%29%13%4
180
11th to 20thDay
29%27%31%11%21
170
21st to endDay
21%32%30%14%4
176
February1st to 10thDay
33%30%26%10%2
266
11th to 20thDay
28%31%27%12%2
416
21st to endDay
29%30%25%15%1
346
March1st to 10thDay
30%21%32%12%3
420
11th to 20thDay
31%27%25%11%41
420
21st to endDay
28%25%25%15%6%3
462
April1st to 10thDay
34%19%28%10%6%2
420
11th to 20thDay
28%23%26%15%6%1
412
21st to endDay
32%23%26%12%6%2
420
May1st to 10thDay
32%22%26%14%42
423
11th to 20thDay
21%21%34%16%5%2
447
21st to endDay
20%16%30%24%8%2
485
June1st to 10thDay
22%16%38%14%10%1
450
11th to 20thDay
20%18%31%19%9%3
450
21st to endDay
21%16%31%22%9%2
450
July1st to 10thDay
19%12%30%24%12%3
445
11th to 20thDay
17%8%32%30%11%3
444
21st to endDay
15%10%38%26%8%2
488
August1st to 10thDay
13%12%30%27%15%2
446
11th to 20thDay
19%10%29%23%14%6%
439
21st to endDay
13%14%28%27%13%5%
495
September1st to 10thDay
16%10%29%22%16%5%
450
11th to 20thDay
19%14%26%19%14%8%
450
21st to endDay
17%16%28%22%11%6%
450
October1st to 10thDay
28%16%22%19%11%4
450
11th to 20thDay
21%21%25%23%8%2
405
21st to endDay
35%16%26%17%7%
396
November1st to 10thDay
23%19%24%24%8%1
365
11th to 20thDay
29%24%23%17%7%1
370
21st to endDay
33%21%25%16%5%1
304
December1st to 10thDay
27%29%25%15%4
180
11th to 20thDay
36%22%30%10%2
180
21st to endDay
34%25%23%13%41
198
January1st to 10thNight
22%24%30%22%2
760
11th to 20thNight
28%21%28%21%2
740
21st to endNight
20%25%32%19%4
792
February1st to 10thNight
24%25%30%18%3
720
11th to 20thNight
22%26%31%18%2
703
21st to endNight
25%22%29%20%3
592
March1st to 10thNight
28%19%30%21%2
720
11th to 20thNight
25%22%26%22%5%
720
21st to endNight
26%21%25%21%6%1
792
April1st to 10thNight
27%18%29%22%4
720
11th to 20thNight
23%22%27%21%6%
704
21st to endNight
27%16%30%21%5%
540
May1st to 10thNight
28%14%30%21%6%
540
11th to 20thNight
21%14%31%27%7%
418
21st to endNight
19%8%27%35%10%1
389
June1st to 10thNight
22%9%28%29%12%1
360
11th to 20thNight
22%11%24%31%13%
360
21st to endNight
19%8%23%41%9%
360
July1st to 10thNight
17%5%24%37%16%1
350
11th to 20thNight
15%324%37%19%2
348
21st to endNight
13%5%27%37%16%1
394
August1st to 10thNight
15%5%21%40%19%1
417
11th to 20thNight
16%6%27%33%16%3
563
21st to endNight
17%7%21%32%21%2
792
September1st to 10thNight
14%9%27%30%19%1
720
11th to 20thNight
16%12%24%27%17%4
720
21st to endNight
18%10%27%26%18%2
720
October1st to 10thNight
25%13%20%26%14%2
720
11th to 20thNight
21%15%27%25%12%1
720
21st to endNight
28%18%25%22%7%
792
November1st to 10thNight
20%18%26%26%9%1
740
11th to 20thNight
23%21%27%21%7%1
760
21st to endNight
23%21%28%22%6%1
760
December1st to 10thNight
26%20%32%18%4
760
11th to 20thNight
29%21%28%18%3
760
21st to endNight
28%25%25%19%2
836
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