JGS Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the JGS Observatory clear sky chart from 2005-11-06 to 2024-01-16.

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%
22999
17%
74720
8%
22955
38%
22955
18%
49654

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
14%
760
18%
2649
5%
757
31%
757
16%
1690
January11th to 20thNight
15%
744
14%
2602
4%
744
29%
744
12%
1811
January21st to endNight
15%
792
18%
2700
6%
792
32%
792
15%
1718
February1st to 10thNight
14%
720
13%
2326
3%
720
27%
720
13%
1620
February11th to 20thNight
11%
703
16%
2100
3%
700
26%
700
14%
1465
February21st to endNight
14%
592
20%
1776
6%
592
31%
592
14%
1221
March1st to 10thNight
17%
720
18%
2160
7%
720
31%
720
16%
1449
March11th to 20thNight
17%
720
17%
2094
4%
720
30%
720
16%
1442
March21st to endNight
18%
792
20%
1980
4%
792
30%
792
18%
1398
April1st to 10thNight
18%
720
15%
1800
2%
720
26%
720
17%
1331
April11th to 20thNight
22%
711
15%
1775
3%
711
29%
711
20%
1276
April21st to endNight
21%
645
17%
1725
4%
645
30%
645
20%
1215
May1st to 10thNight
21%
540
13%
1620
5%
540
28%
540
14%
1181
May11th to 20thNight
24%
382
18%
1444
11%
382
36%
382
16%
949
May21st to endNight
40%
388
13%
1551
12%
388
47%
388
18%
986
June1st to 10thNight
36%
360
15%
1440
13%
360
47%
360
17%
908
June11th to 20thNight
37%
360
12%
1440
12%
360
45%
360
18%
908
June21st to endNight
47%
360
14%
1440
15%
360
55%
360
23%
854
July1st to 10thNight
52%
350
14%
1400
18%
350
57%
350
22%
834
July11th to 20thNight
51%
348
13%
1392
16%
348
54%
348
22%
843
July21st to endNight
52%
394
16%
1486
18%
372
58%
372
23%
888
August1st to 10thNight
53%
394
17%
1458
14%
392
59%
392
25%
867
August11th to 20thNight
51%
710
13%
1774
8%
710
52%
710
26%
1275
August21st to endNight
49%
792
18%
1980
9%
792
53%
792
31%
1332
September1st to 10thNight
41%
720
19%
1800
8%
720
46%
720
29%
1213
September11th to 20thNight
45%
720
25%
1894
14%
720
51%
720
35%
1154
September21st to endNight
40%
720
23%
1980
15%
720
48%
720
29%
1298
October1st to 10thNight
35%
720
16%
2052
10%
720
41%
720
22%
1398
October11th to 20thNight
32%
720
20%
2160
10%
720
44%
720
24%
1376
October21st to endNight
23%
792
15%
2562
8%
792
34%
792
17%
1756
November1st to 10thNight
29%
740
22%
2407
10%
740
45%
740
22%
1483
November11th to 20thNight
23%
760
21%
2470
10%
760
38%
760
19%
1605
November21st to endNight
24%
760
18%
2559
8%
760
37%
760
18%
1601
December1st to 10thNight
18%
760
15%
2660
5%
760
31%
760
13%
1823
December11th to 20thNight
19%
760
13%
3227
3%
759
30%
759
10%
2163
December21st to endNight
13%
830
11%
4837
3%
817
30%
817
8%
3323

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
16%7%36%5%35%5%5%7%37%
74803
Night
16%7%36%5%345%5%6%38%
74999
TransparencyDay
44%8%13%21%13%2
74508
Night
44%7%13%21%13%2
74828
SeeingDay
23%18%30%19%8%2
13812
Night
21%16%31%23%7%1
23006

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%9%36%5%36%6%7%6%37%
1517
11th to 20thDay
17%9%36%7%36%5%5%7%31%
1486
21st to endDay
12%7%46%5%46%5%6%7%38%
1653
February1st to 10thDay
18%9%38%7%25%5%5%6%31%
1632
11th to 20thDay
15%7%37%6%45%45%6%37%
1748
21st to endDay
19%10%48%5%35%445%34%
1480
March1st to 10thDay
14%8%47%7%445%5%6%38%
1797
11th to 20thDay
16%8%47%6%35%5%5%6%34%
1862
21st to endDay
20%9%36%5%3445%5%36%
2376
April1st to 10thDay
19%8%37%5%35%5%46%35%
2160
11th to 20thDay
17%7%35%6%44447%40%
2134
21st to endDay
21%8%25%5%345%5%6%36%
2233
May1st to 10thDay
24%8%36%5%35%45%6%32%
2340
11th to 20thDay
19%7%2442446%7%39%
2465
21st to endDay
14%6%35%5%35%6%5%8%40%
2725
June1st to 10thDay
17%5%36%5%45%6%6%9%33%
2520
11th to 20thDay
17%7%35%6%35%5%7%9%34%
2520
21st to endDay
14%7%36%7%36%5%6%8%36%
2520
July1st to 10thDay
12%6%36%5%45%5%6%8%40%
2520
11th to 20thDay
12%5%25%6%46%5%6%10%39%
2520
21st to endDay
12%6%36%5%46%5%7%9%37%
2744
August1st to 10thDay
12%6%26%7%45%6%7%9%37%
2450
11th to 20thDay
13%7%25%6%36%5%7%9%37%
2126
21st to endDay
13%6%246%35%5%6%10%41%
2376
September1st to 10thDay
14%7%35%5%35%6%6%7%40%
2160
11th to 20thDay
13%6%2442345%7%49%
2065
21st to endDay
17%6%244345%6%7%41%
1980
October1st to 10thDay
22%8%36%434445%40%
1904
11th to 20thDay
15%7%25%5%3445%7%42%
1800
21st to endDay
23%8%35%5%35%5%5%6%33%
1792
November1st to 10thDay
14%7%3442345%7%47%
1671
11th to 20thDay
17%8%34425%5%5%7%40%
1710
21st to endDay
17%7%25%5%35%35%7%40%
1619
December1st to 10thDay
19%8%48%6%35%45%6%32%
1520
11th to 20thDay
20%9%35%5%335%5%5%38%
1906
21st to endDay
20%11%47%7%45%45%6%27%
2772
January1st to 10thNight
15%8%36%5%25%5%6%7%38%
2649
11th to 20thNight
18%8%37%6%46%45%7%32%
2602
21st to endNight
12%9%36%6%35%5%6%6%38%
2700
February1st to 10thNight
17%10%47%436%5%6%6%34%
2326
11th to 20thNight
16%8%46%7%35%6%6%6%33%
2098
21st to endNight
18%8%46%6%3445%5%36%
1776
March1st to 10thNight
16%7%35%5%45%6%6%6%38%
2150
11th to 20thNight
15%8%37%7%45%46%6%36%
2093
21st to endNight
17%7%36%5%3445%6%39%
1980
April1st to 10thNight
17%9%37%7%45%5%6%6%32%
1800
11th to 20thNight
18%7%36%6%3335%7%39%
1775
21st to endNight
19%9%35%5%2345%6%38%
1725
May1st to 10thNight
21%10%37%6%34446%32%
1620
11th to 20thNight
17%7%36%6%3445%8%37%
1443
21st to endNight
17%7%246%3447%7%40%
1550
June1st to 10thNight
17%6%26%5%246%5%7%40%
1440
11th to 20thNight
14%6%36%5%45%5%6%8%37%
1440
21st to endNight
13%4245%345%6%9%44%
1440
July1st to 10thNight
13%5%245%25%5%7%8%43%
1440
11th to 20thNight
13%435%5%35%6%8%10%37%
1440
21st to endNight
10%6%35%5%36%47%9%43%
1574
August1st to 10thNight
9%6%26%435%7%7%8%44%
1468
11th to 20thNight
12%6%36%6%45%6%7%8%37%
1774
21st to endNight
15%6%25%4345%5%7%45%
1980
September1st to 10thNight
15%6%35%5%34446%46%
1800
11th to 20thNight
14%5%23323447%54%
1894
21st to endNight
16%7%345%24446%45%
1980
October1st to 10thNight
20%7%35%6%24345%42%
2052
11th to 20thNight
14%6%35%5%345%6%6%43%
2160
21st to endNight
22%8%36%43445%5%36%
2562
November1st to 10thNight
15%6%35%42446%6%45%
2407
11th to 20thNight
17%8%36%5%24446%40%
2470
21st to endNight
13%7%36%5%35%5%5%6%41%
2559
December1st to 10thNight
20%8%38%5%345%5%6%33%
2660
11th to 20thNight
20%8%36%6%35%6%5%5%33%
3288
21st to endNight
21%11%47%5%345%46%30%
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
43%6%13%25%10%2
1518
11th to 20thDay
49%8%14%18%9%2
1486
21st to endDay
42%8%14%21%13%2
1653
February1st to 10thDay
52%7%12%17%11%1
1632
11th to 20thDay
47%6%10%21%13%2
1757
21st to endDay
52%5%11%19%11%2
1480
March1st to 10thDay
45%7%13%19%13%3
1800
11th to 20thDay
47%6%11%21%13%2
1862
21st to endDay
49%5%9%16%16%5%
2376
April1st to 10thDay
48%6%11%20%12%3
2160
11th to 20thDay
43%7%11%21%16%2
2134
21st to endDay
46%6%11%19%14%3
2233
May1st to 10thDay
52%7%10%17%12%2
2340
11th to 20thDay
42%8%12%21%16%2
2469
21st to endDay
41%10%15%20%12%2
2725
June1st to 10thDay
44%9%15%20%10%1
2520
11th to 20thDay
44%9%14%22%10%1
2520
21st to endDay
43%9%14%22%12%1
2520
July1st to 10thDay
41%10%12%20%16%1
2450
11th to 20thDay
37%8%17%25%12%1
2445
21st to endDay
41%10%14%19%15%2
2591
August1st to 10thDay
40%11%15%23%11%1
2436
11th to 20thDay
38%11%18%22%10%1
2126
21st to endDay
37%8%14%24%15%1
2376
September1st to 10thDay
40%8%12%23%15%2
2160
11th to 20thDay
34%6%11%23%23%2
2065
21st to endDay
39%7%13%21%17%2
1980
October1st to 10thDay
46%6%10%22%13%3
1904
11th to 20thDay
40%7%15%24%12%2
1800
21st to endDay
50%7%11%18%12%2
1792
November1st to 10thDay
37%6%15%25%15%3
1671
11th to 20thDay
41%7%12%23%15%2
1710
21st to endDay
44%5%14%26%9%2
1619
December1st to 10thDay
53%6%14%16%10%1
1520
11th to 20thDay
46%6%17%20%9%2
1906
21st to endDay
55%8%11%18%7%
2772
January1st to 10thNight
43%6%15%21%13%2
2649
11th to 20thNight
50%7%15%18%8%2
2602
21st to endNight
43%6%13%21%16%2
2700
February1st to 10thNight
49%7%14%18%10%2
2326
11th to 20thNight
48%6%12%16%14%3
2100
21st to endNight
48%6%11%21%11%3
1776
March1st to 10thNight
42%9%13%21%13%3
2160
11th to 20thNight
46%7%12%20%13%2
2094
21st to endNight
44%7%10%18%17%4
1980
April1st to 10thNight
49%7%11%19%12%2
1800
11th to 20thNight
45%8%9%20%17%1
1775
21st to endNight
45%7%10%20%13%4
1725
May1st to 10thNight
53%7%10%16%12%2
1620
11th to 20thNight
46%8%9%20%15%3
1444
21st to endNight
42%10%16%19%12%1
1551
June1st to 10thNight
43%8%12%22%13%2
1440
11th to 20thNight
42%8%17%22%9%1
1440
21st to endNight
34%11%15%26%14%1
1440
July1st to 10thNight
37%9%15%23%14%2
1400
11th to 20thNight
37%10%16%28%9%1
1392
21st to endNight
35%11%17%21%14%3
1486
August1st to 10thNight
33%11%17%24%13%1
1458
11th to 20thNight
41%12%16%21%10%
1774
21st to endNight
37%8%12%25%17%1
1980
September1st to 10thNight
40%7%12%25%15%1
1800
11th to 20thNight
31%7%12%24%25%1
1894
21st to endNight
39%6%11%22%21%1
1980
October1st to 10thNight
45%6%13%20%14%2
2052
11th to 20thNight
38%8%15%25%13%1
2160
21st to endNight
48%6%14%19%11%1
2562
November1st to 10thNight
39%6%12%21%18%3
2407
11th to 20thNight
44%5%12%21%15%3
2470
21st to endNight
41%6%18%26%8%1
2559
December1st to 10thNight
49%7%14%20%10%1
2660
11th to 20thNight
48%8%16%15%12%2
3288
21st to endNight
52%8%13%17%9%
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%28%25%16%2
180
11th to 20thDay
34%26%29%9%2
172
21st to endDay
22%35%31%9%21
207
February1st to 10thDay
37%26%30%6%2
252
11th to 20thDay
27%29%33%10%
416
21st to endDay
33%25%31%9%2
346
March1st to 10thDay
27%24%35%13%2
420
11th to 20thDay
28%26%31%12%2
420
21st to endDay
32%21%32%9%5%1
462
April1st to 10thDay
32%25%27%10%6%1
420
11th to 20thDay
26%27%26%13%6%1
412
21st to endDay
28%21%32%14%5%1
420
May1st to 10thDay
33%20%27%13%5%2
423
11th to 20thDay
23%19%36%17%5%1
447
21st to endDay
19%14%27%26%11%2
485
June1st to 10thDay
20%13%32%22%13%1
450
11th to 20thDay
16%16%31%25%10%2
450
21st to endDay
15%16%30%26%12%2
450
July1st to 10thDay
13%10%31%29%14%3
445
11th to 20thDay
14%9%27%32%16%3
444
21st to endDay
11%11%34%30%11%2
488
August1st to 10thDay
14%7%31%31%11%6%
446
11th to 20thDay
14%10%30%24%15%7%
439
21st to endDay
13%11%30%25%15%6%
495
September1st to 10thDay
20%11%23%23%16%6%
450
11th to 20thDay
14%12%31%20%14%8%
450
21st to endDay
20%12%30%22%9%5%
450
October1st to 10thDay
30%11%24%18%14%3
450
11th to 20thDay
20%19%32%20%8%1
450
21st to endDay
30%15%26%21%7%1
401
November1st to 10thDay
21%17%29%21%10%2
365
11th to 20thDay
29%22%27%17%31
370
21st to endDay
24%25%28%20%3
279
December1st to 10thDay
24%31%29%11%5%
180
11th to 20thDay
33%28%25%13%1
180
21st to endDay
26%28%27%16%3
198
January1st to 10thNight
20%23%40%16%2
760
11th to 20thNight
26%22%33%17%2
744
21st to endNight
19%24%37%17%3
792
February1st to 10thNight
24%23%34%18%1
720
11th to 20thNight
24%24%37%14%1
703
21st to endNight
27%22%34%15%2
592
March1st to 10thNight
23%25%33%16%3
720
11th to 20thNight
23%26%32%16%3
720
21st to endNight
23%24%32%16%5%
792
April1st to 10thNight
25%24%30%18%3
720
11th to 20thNight
23%20%32%20%5%
711
21st to endNight
24%20%34%19%4
645
May1st to 10thNight
26%18%31%20%6%
540
11th to 20thNight
19%15%38%23%4
382
21st to endNight
20%8%26%34%11%1
389
June1st to 10thNight
17%8%33%32%11%1
360
11th to 20thNight
16%9%32%29%14%
360
21st to endNight
14%5%30%39%12%
360
July1st to 10thNight
15%5%21%41%18%1
350
11th to 20thNight
15%325%41%15%1
348
21st to endNight
13%5%25%41%16%1
394
August1st to 10thNight
8%5%27%44%13%3
394
11th to 20thNight
14%422%38%19%2
710
21st to endNight
16%7%23%34%20%2
792
September1st to 10thNight
17%6%30%31%14%2
720
11th to 20thNight
15%8%28%27%18%4
720
21st to endNight
20%7%28%30%12%3
720
October1st to 10thNight
24%10%27%26%11%1
720
11th to 20thNight
18%15%31%28%8%
720
21st to endNight
27%16%31%21%5%
792
November1st to 10thNight
20%16%32%23%9%1
740
11th to 20thNight
22%20%32%20%6%1
760
21st to endNight
18%22%33%21%5%1
760
December1st to 10thNight
26%20%31%18%4
760
11th to 20thNight
25%21%31%21%3
760
21st to endNight
27%19%38%15%2
836
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