JMS Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the JMS Observatory clear sky chart from 2005-11-05 to 2017-08-12.

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
37%
16024
15%
46364
8%
15978
43%
15978
23%
31353

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
28%
900
13%
2506
6%
895
34%
895
20%
1823
January16th to endNight
22%
960
11%
2547
4%
960
28%
960
17%
1928
February1st to 15thNight
29%
879
13%
2288
4%
875
35%
875
22%
1650
February16th to endNight
29%
756
15%
2018
4%
756
36%
756
22%
1423
March1st to 15thNight
31%
579
13%
2019
9%
579
38%
579
19%
1344
March16th to endNight
33%
576
15%
2099
12%
576
40%
576
17%
1417
April1st to 15thNight
33%
539
17%
1797
13%
539
42%
539
21%
1186
April16th to endNight
30%
540
16%
1665
11%
540
38%
540
17%
1205
May1st to 15thNight
31%
540
15%
1494
6%
540
37%
540
22%
1044
May16th to endNight
42%
562
15%
1498
5%
561
47%
561
29%
1000
June1st to 15thNight
47%
540
13%
1440
5%
540
51%
540
32%
920
June16th to endNight
58%
540
17%
1316
7%
540
61%
540
42%
812
July1st to 15thNight
60%
510
15%
1321
7%
510
62%
510
43%
786
July16th to endNight
62%
564
19%
1431
10%
531
65%
531
45%
811
August1st to 15thNight
65%
527
21%
1575
18%
524
68%
524
37%
895
August16th to endNight
56%
525
21%
1638
19%
525
63%
525
34%
944
September1st to 15thNight
56%
495
23%
1650
20%
495
65%
495
33%
917
September16th to endNight
44%
532
18%
1816
12%
532
53%
532
28%
1066
October1st to 15thNight
40%
660
23%
1980
15%
660
47%
660
30%
1200
October16th to endNight
34%
704
15%
2126
9%
704
40%
704
21%
1451
November1st to 15thNight
34%
836
19%
2412
10%
836
41%
836
24%
1607
November16th to endNight
28%
900
14%
2520
8%
900
33%
900
18%
1855
December1st to 15thNight
24%
900
10%
2520
3%
900
29%
900
15%
1922
December16th to endNight
19%
960
10%
2688
3%
960
26%
960
12%
2147

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%8%36%5%3445%7%36%
47596
Night
12%8%36%5%24446%40%
46534
Cloud CoverDay
26%7%25%423446%31%
47607
Night
25%7%25%423335%35%
46538
Cloud CoverDay
13%8%36%5%3445%7%36%
47605
Night
12%8%36%5%24446%40%
46538
TransparencyDay
46%6%14%22%9%
47284
Night
44%5%13%23%11%1
46364
SeeingDay
16%8%29%27%14%3
5980
Night
16%8%31%31%11%
16026

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
10%10%7%8%9%45%445%28%
1432
16th to endDay
9%15%6%9%6%45%46%7%22%
1674
February1st to 15thDay
8%11%8%9%7%45%445%31%
1575
16th to endDay
10%13%5%9%5%345%5%6%29%
1453
March1st to 15thDay
13%11%5%7%424446%32%
1939
16th to endDay
17%11%46%5%245%46%31%
2124
April1st to 15thDay
17%7%36%5%33345%37%
2151
16th to endDay
22%12%35%5%24446%28%
2293
May1st to 15thDay
20%7%36%6%3446%5%32%
2457
16th to endDay
15%6%26%5%35%45%7%36%
2617
June1st to 15thDay
16%5%25%4345%5%9%37%
2520
16th to endDay
9%425%5%35%6%7%10%39%
2636
July1st to 15thDay
7%4244245%6%9%48%
2550
16th to endDay
7%323436%5%7%9%46%
2637
August1st to 15thDay
8%425%6%36%5%6%8%40%
2289
16th to endDay
11%5%2433445%8%47%
2212
September1st to 15thDay
10%5%145%3445%8%45%
1980
16th to endDay
15%6%245%3345%8%41%
1799
October1st to 15thDay
16%8%25%423345%43%
1650
16th to endDay
18%10%35%432445%37%
1746
November1st to 15thDay
10%11%48%424435%38%
1446
16th to endDay
15%10%38%5%3445%8%30%
1440
December1st to 15thDay
15%12%5%9%8%45%44424%
1440
16th to endDay
18%17%8%10%7%2322421%
1536
January1st to 15thNight
9%9%6%8%6%45%5%5%6%32%
2506
16th to endNight
11%12%6%11%5%3435%6%28%
2547
February1st to 15thNight
7%11%5%8%6%35%445%36%
2285
16th to endNight
9%9%47%7%45%5%47%36%
2018
March1st to 15thNight
12%10%46%5%24346%38%
2007
16th to endNight
15%10%47%5%23346%35%
2099
April1st to 15thNight
16%9%35%323445%39%
1797
16th to endNight
23%9%2443345%433%
1665
May1st to 15thNight
17%8%35%422436%40%
1494
16th to endNight
10%5%25%5%35%5%6%7%42%
1496
June1st to 15thNight
12%424435%45%8%45%
1440
16th to endNight
7%31433435%7%53%
1316
July1st to 15thNight
5%3134245%5%8%55%
1401
16th to endNight
5%21332446%8%55%
1528
August1st to 15thNight
6%32442345%7%54%
1585
16th to endNight
9%314423446%54%
1638
September1st to 15thNight
7%425%433446%53%
1650
16th to endNight
12%424423446%49%
1816
October1st to 15thNight
14%7%23312346%49%
1980
16th to endNight
19%9%34423345%39%
2126
November1st to 15thNight
13%7%36%424445%42%
2412
16th to endNight
14%10%47%6%35%447%31%
2520
December1st to 15thNight
13%12%48%7%34445%29%
2520
16th to endNight
17%15%7%9%6%34345%22%
2688

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
23%8%47%8%4444425%
1432
16th to endDay
24%14%48%5%34446%18%
1674
February1st to 15thDay
23%10%5%8%6%34245%26%
1575
16th to endDay
23%11%48%5%24445%25%
1453
March1st to 15thDay
26%9%47%422345%29%
1950
16th to endDay
30%8%26%423435%26%
2124
April1st to 15thDay
31%6%25%42233431%
2151
16th to endDay
37%10%24423335%22%
2293
May1st to 15thDay
32%6%25%6%3434426%
2457
16th to endDay
30%5%15%433446%29%
2617
June1st to 15thDay
31%414423447%30%
2520
16th to endDay
25%3145%245%5%8%32%
2636
July1st to 15thDay
23%31332245%7%40%
2550
16th to endDay
23%313425%45%7%37%
2637
August1st to 15thDay
16%4245%35%5%6%8%37%
2289
16th to endDay
18%423423448%44%
2212
September1st to 15thDay
18%5%1342335%7%42%
1980
16th to endDay
22%5%14423446%40%
1799
October1st to 15thDay
24%6%24323345%39%
1650
16th to endDay
25%8%25%422345%35%
1746
November1st to 15thDay
23%10%27%42433335%
1446
16th to endDay
29%9%26%5%23436%27%
1440
December1st to 15thDay
29%10%48%6%4444220%
1440
16th to endDay
29%16%6%9%6%3212318%
1536
January1st to 15thNight
23%8%47%5%3444428%
2506
16th to endNight
25%11%49%423335%24%
2547
February1st to 15thNight
22%10%46%42323432%
2278
16th to endNight
22%7%36%6%44336%31%
2018
March1st to 15thNight
25%9%35%414245%33%
2019
16th to endNight
30%8%35%422235%31%
2099
April1st to 15thNight
30%7%2432233434%
1797
16th to endNight
37%7%1432334326%
1665
May1st to 15thNight
32%6%24412325%33%
1493
16th to endNight
25%5%15%42345%6%35%
1496
June1st to 15thNight
27%314324345%38%
1440
16th to endNight
22%213323236%46%
1316
July1st to 15thNight
21%212423446%46%
1401
16th to endNight
21%2322335%6%46%
1528
August1st to 15thNight
15%314323446%50%
1585
16th to endNight
17%213322335%52%
1638
September1st to 15thNight
16%315%333446%48%
1650
16th to endNight
19%313323445%47%
1816
October1st to 15thNight
22%6%12312335%45%
1980
16th to endNight
27%8%2331233437%
2126
November1st to 15thNight
25%6%25%42333438%
2412
16th to endNight
27%9%26%5%33335%27%
2520
December1st to 15thNight
28%10%37%6%3323425%
2520
16th to endNight
29%13%5%8%5%2333419%
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
10%10%7%8%8%5%5%5%45%28%
1432
16th to endDay
9%15%6%9%7%36%5%5%7%22%
1674
February1st to 15thDay
7%12%7%9%7%35%35%5%31%
1575
16th to endDay
10%13%5%8%6%25%45%6%29%
1453
March1st to 15thDay
14%10%5%8%5%34446%32%
1950
16th to endDay
17%10%46%5%245%46%31%
2122
April1st to 15thDay
17%7%36%5%33346%37%
2151
16th to endDay
22%11%35%5%24446%28%
2293
May1st to 15thDay
19%7%36%6%3445%6%31%
2457
16th to endDay
15%6%26%5%35%5%5%7%36%
2617
June1st to 15thDay
16%5%25%4245%6%9%37%
2520
16th to endDay
9%425%5%35%5%8%9%39%
2636
July1st to 15thDay
7%3244245%7%8%48%
2550
16th to endDay
6%423436%5%7%9%45%
2637
August1st to 15thDay
8%5%25%5%45%6%6%8%40%
2289
16th to endDay
11%5%2343445%8%46%
2212
September1st to 15thDay
9%5%235%3445%8%45%
1980
16th to endDay
15%6%245%24448%41%
1799
October1st to 15thDay
16%8%24423445%42%
1650
16th to endDay
18%11%35%423345%37%
1746
November1st to 15thDay
10%11%5%8%434445%38%
1446
16th to endDay
15%10%37%5%35%448%30%
1440
December1st to 15thDay
15%12%6%8%8%45%44424%
1440
16th to endDay
17%18%7%10%7%3322421%
1536
January1st to 15thNight
9%10%5%7%7%35%5%5%6%32%
2506
16th to endNight
11%12%6%10%6%34346%29%
2547
February1st to 15thNight
7%11%6%7%6%34446%36%
2278
16th to endNight
9%8%37%7%45%5%46%36%
2018
March1st to 15thNight
11%11%46%5%24346%38%
2019
16th to endNight
16%9%46%5%23437%35%
2099
April1st to 15thNight
17%9%34424446%39%
1797
16th to endNight
24%8%2443445%5%32%
1665
May1st to 15thNight
18%7%25%5%23346%40%
1493
16th to endNight
10%5%25%5%35%5%6%7%42%
1496
June1st to 15thNight
12%42443446%7%45%
1440
16th to endNight
6%31442445%7%54%
1316
July1st to 15thNight
5%3134235%6%8%55%
1401
16th to endNight
5%32331446%9%55%
1528
August1st to 15thNight
6%3144245%48%54%
1585
16th to endNight
9%323423345%54%
1638
September1st to 15thNight
7%425%423446%53%
1650
16th to endNight
12%424424446%49%
1816
October1st to 15thNight
14%6%23322346%49%
1980
16th to endNight
19%10%33423345%39%
2126
November1st to 15thNight
13%8%36%423445%42%
2412
16th to endNight
14%10%47%6%35%447%31%
2520
December1st to 15thNight
13%12%5%8%7%35%345%29%
2520
16th to endNight
16%15%7%10%6%34345%22%
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
57%5%11%18%7%
1433
16th to endDay
60%6%13%15%3
1674
February1st to 15thDay
54%5%14%18%6%1
1576
16th to endDay
55%5%12%21%5%
1453
March1st to 15thDay
52%6%15%19%6%
1950
16th to endDay
53%6%13%23%3
2124
April1st to 15thDay
47%5%12%21%11%1
2151
16th to endDay
56%5%12%15%8%
2293
May1st to 15thDay
52%6%12%20%7%
2457
16th to endDay
45%7%12%21%11%1
2621
June1st to 15thDay
41%7%15%26%7%
2520
16th to endDay
37%8%16%25%10%1
2636
July1st to 15thDay
30%6%17%27%15%1
2401
16th to endDay
32%9%17%24%15%1
2471
August1st to 15thDay
38%8%14%27%9%2
2275
16th to endDay
33%6%15%28%13%1
2212
September1st to 15thDay
35%7%15%30%11%
1980
16th to endDay
40%7%14%30%6%
1799
October1st to 15thDay
42%412%26%12%
1650
16th to endDay
49%415%21%7%
1746
November1st to 15thDay
47%412%20%12%1
1446
16th to endDay
52%6%18%15%6%1
1440
December1st to 15thDay
61%6%15%11%4
1440
16th to endDay
68%310%12%4
1536
January1st to 15thNight
51%5%14%20%7%
2506
16th to endNight
56%415%16%5%
2547
February1st to 15thNight
48%5%14%22%8%
2288
16th to endNight
46%6%13%23%9%
2018
March1st to 15thNight
47%5%15%21%10%
2019
16th to endNight
50%413%24%6%
2099
April1st to 15thNight
45%5%8%24%14%1
1797
16th to endNight
52%6%9%17%11%2
1665
May1st to 15thNight
45%6%12%20%12%1
1494
16th to endNight
38%7%13%22%15%1
1498
June1st to 15thNight
35%6%15%28%12%1
1440
16th to endNight
27%6%14%30%16%3
1316
July1st to 15thNight
26%6%15%31%17%2
1321
16th to endNight
23%7%16%29%17%4
1431
August1st to 15thNight
28%6%15%29%16%4
1575
16th to endNight
30%7%12%28%17%3
1638
September1st to 15thNight
31%5%13%30%18%
1650
16th to endNight
34%6%14%29%13%
1816
October1st to 15thNight
36%412%28%15%2
1980
16th to endNight
46%5%13%21%11%
2126
November1st to 15thNight
43%5%12%23%14%
2412
16th to endNight
51%6%15%17%7%
2520
December1st to 15thNight
57%6%13%18%4
2520
16th to endNight
64%313%13%4
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
16%12%32%26%11%1
90
16th to endDay
30%7%35%21%41
96
February1st to 15thDay
14%18%40%22%4
90
16th to endDay
20%12%35%25%31
81
March1st to 15thDay
15%11%35%31%6%
90
16th to endDay
28%11%31%22%6%
108
April1st to 15thDay
20%11%31%26%9%
270
16th to endDay
31%9%31%16%9%
403
May1st to 15thDay
22%14%29%22%9%1
468
16th to endDay
18%9%30%24%12%4
517
June1st to 15thDay
16%7%31%27%14%2
495
16th to endDay
11%7%32%28%15%3
495
July1st to 15thDay
8%6%26%30%19%8%
475
16th to endDay
7%426%35%21%5%
515
August1st to 15thDay
11%421%34%24%4
474
16th to endDay
12%5%23%33%17%6%
380
September1st to 15thDay
12%5%27%29%19%5%
255
16th to endDay
19%427%28%14%5%
126
October1st to 15thDay
8%11%28%24%22%4
90
16th to endDay
28%5%33%19%13%
96
November1st to 15thDay
7%5%30%40%13%3
90
16th to endDay
36%11%35%10%6%
90
December1st to 15thDay
23%8%30%31%6%
90
16th to endDay
35%10%35%17%1
96
January1st to 15thNight
13%11%35%29%8%1
900
16th to endNight
17%11%39%27%4
960
February1st to 15thNight
14%9%37%29%8%1
880
16th to endNight
14%11%37%26%10%
756
March1st to 15thNight
15%8%36%29%8%
579
16th to endNight
20%10%30%24%12%1
576
April1st to 15thNight
21%8%32%29%7%
539
16th to endNight
27%7%30%26%7%
540
May1st to 15thNight
21%8%34%28%5%
540
16th to endNight
12%6%31%32%15%
563
June1st to 15thNight
14%5%26%43%10%
540
16th to endNight
9%325%46%14%
540
July1st to 15thNight
6%125%39%24%1
510
16th to endNight
6%224%44%19%1
564
August1st to 15thNight
7%218%45%23%2
527
16th to endNight
9%425%33%26%1
525
September1st to 15thNight
8%5%23%38%20%4
495
16th to endNight
13%7%31%27%18%1
532
October1st to 15thNight
19%8%29%28%13%1
660
16th to endNight
25%7%26%30%8%
704
November1st to 15thNight
18%10%31%30%9%1
836
16th to endNight
20%14%31%25%8%
900
December1st to 15thNight
18%10%38%25%6%
900
16th to endNight
24%10%32%25%6%
960
Page generated at 2017-08-13 13:16:19. Version ac04

Copyright 2017 A. Danko


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