JAT Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the JAT Observatory clear sky chart from 2005-11-06 to 2024-01-12.

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
33%
23213
21%
74573
10%
23170
45%
23170
23%
46604

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
20%
760
24%
2649
9%
757
41%
757
19%
1578
January11th to 20thNight
24%
725
20%
2521
8%
725
41%
725
18%
1613
January21st to endNight
22%
792
24%
2574
8%
792
41%
792
21%
1570
February1st to 10thNight
21%
720
18%
2340
6%
720
36%
720
19%
1495
February11th to 20thNight
14%
703
21%
2119
5%
700
35%
700
17%
1400
February21st to endNight
17%
592
26%
1776
9%
592
38%
592
20%
1146
March1st to 10thNight
22%
720
24%
2160
9%
720
40%
720
23%
1311
March11th to 20thNight
21%
720
22%
2006
9%
720
34%
720
21%
1331
March21st to endNight
22%
792
22%
2034
7%
792
34%
792
25%
1370
April1st to 10thNight
19%
720
20%
1800
4%
720
29%
720
23%
1212
April11th to 20thNight
26%
711
20%
1775
6%
711
33%
711
26%
1204
April21st to endNight
28%
720
21%
1800
6%
720
36%
720
27%
1204
May1st to 10thNight
26%
581
16%
1661
7%
581
33%
581
19%
1152
May11th to 20thNight
29%
359
18%
1421
11%
359
41%
359
17%
888
May21st to endNight
46%
388
15%
1551
14%
388
52%
388
19%
935
June1st to 10thNight
43%
360
18%
1440
16%
360
54%
360
22%
863
June11th to 20thNight
46%
360
16%
1440
16%
360
53%
360
21%
856
June21st to endNight
53%
360
16%
1440
17%
360
60%
360
25%
809
July1st to 10thNight
55%
350
14%
1400
14%
350
60%
350
20%
860
July11th to 20thNight
59%
348
18%
1392
20%
348
63%
348
26%
784
July21st to endNight
53%
397
20%
1489
18%
375
62%
375
25%
854
August1st to 10thNight
58%
537
17%
1600
12%
534
60%
534
30%
997
August11th to 20thNight
56%
706
16%
1770
9%
706
58%
706
29%
1225
August21st to endNight
55%
792
17%
1980
8%
792
57%
792
32%
1318
September1st to 10thNight
46%
720
22%
1800
10%
720
52%
720
31%
1207
September11th to 20thNight
53%
720
29%
1844
19%
720
58%
720
38%
1126
September21st to endNight
45%
720
25%
1980
17%
720
52%
720
28%
1297
October1st to 10thNight
41%
720
18%
2127
13%
720
48%
720
25%
1382
October11th to 20thNight
42%
720
25%
2160
16%
720
54%
720
31%
1280
October21st to endNight
29%
792
25%
2462
14%
792
44%
792
25%
1533
November1st to 10thNight
37%
740
26%
2407
14%
740
53%
740
31%
1354
November11th to 20thNight
30%
760
26%
2470
14%
760
46%
760
23%
1488
November21st to endNight
30%
760
23%
2567
10%
760
47%
760
24%
1516
December1st to 10thNight
21%
760
20%
2660
7%
760
37%
760
17%
1668
December11th to 20thNight
23%
760
20%
3185
6%
759
41%
759
16%
1916
December21st to endNight
24%
828
15%
4773
9%
817
41%
817
12%
2862

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
15%6%35%5%35%5%6%8%41%
74738
Night
16%6%25%5%3445%7%43%
74960
TransparencyDay
42%411%24%16%3
74444
Night
42%411%24%17%3
74788
SeeingDay
22%13%28%23%11%4
13872
Night
20%11%29%29%10%1
23222

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
12%7%35%5%25%5%6%6%43%
1517
11th to 20thDay
15%8%36%5%35%5%5%7%39%
1459
21st to endDay
16%6%245%35%5%5%8%41%
1782
February1st to 10thDay
17%6%35%6%35%5%5%7%38%
1620
11th to 20thDay
17%7%35%6%345%5%6%41%
1726
21st to endDay
19%8%35%5%3445%6%37%
1480
March1st to 10thDay
13%5%35%5%35%446%47%
1797
11th to 20thDay
17%5%35%6%35%5%5%7%39%
1952
21st to endDay
21%6%34435%5%46%40%
2322
April1st to 10thDay
16%7%35%5%2445%8%40%
2160
11th to 20thDay
15%6%245%35%46%8%42%
2134
21st to endDay
16%5%26%6%345%6%8%39%
2160
May1st to 10thDay
20%7%35%6%35%46%7%35%
2305
11th to 20thDay
19%6%35%5%35%5%6%8%36%
2478
21st to endDay
15%5%35%5%35%5%5%8%41%
2725
June1st to 10thDay
13%5%36%5%35%5%8%10%38%
2520
11th to 20thDay
13%6%35%6%45%6%6%9%38%
2520
21st to endDay
12%5%25%5%35%5%7%10%41%
2520
July1st to 10thDay
11%5%25%7%36%6%7%9%38%
2520
11th to 20thDay
9%426%5%36%6%7%11%42%
2520
21st to endDay
11%5%36%6%36%5%6%9%39%
2738
August1st to 10thDay
9%6%26%6%46%5%7%8%40%
2312
11th to 20thDay
12%5%35%5%45%6%7%9%39%
2126
21st to endDay
14%6%25%5%345%5%9%41%
2376
September1st to 10thDay
14%6%25%5%3446%7%43%
2160
11th to 20thDay
13%5%2343346%7%51%
2116
21st to endDay
17%7%36%5%4445%8%39%
1980
October1st to 10thDay
18%6%3443445%6%44%
1831
11th to 20thDay
16%5%244245%47%47%
1800
21st to endDay
19%6%25%42345%5%44%
1892
November1st to 10thDay
13%6%23423346%52%
1671
11th to 20thDay
17%6%2342445%5%48%
1710
21st to endDay
15%425%4235%48%47%
1611
December1st to 10thDay
19%7%35%42345%7%39%
1520
11th to 20thDay
20%6%34424446%43%
1906
21st to endDay
17%6%35%5%46%6%6%7%36%
2772
January1st to 10thNight
15%7%25%425%446%46%
2649
11th to 20thNight
19%6%36%5%3445%6%40%
2521
21st to endNight
14%8%35%5%3445%7%44%
2574
February1st to 10thNight
17%9%35%43445%6%40%
2340
11th to 20thNight
18%8%25%5%3445%5%41%
2118
21st to endNight
19%7%35%5%2334444%
1776
March1st to 10thNight
15%5%25%42445%7%46%
2150
11th to 20thNight
17%7%36%435%435%44%
2005
21st to endNight
17%7%35%5%34446%43%
2034
April1st to 10thNight
18%5%25%5%345%5%7%42%
1800
11th to 20thNight
15%6%26%5%35%446%42%
1775
21st to endNight
19%6%25%43445%7%44%
1800
May1st to 10thNight
19%7%36%5%35%5%5%7%35%
1661
11th to 20thNight
18%6%36%5%346%5%6%39%
1420
21st to endNight
15%6%35%435%46%9%40%
1550
June1st to 10thNight
16%5%25%5%3446%7%43%
1440
11th to 20thNight
13%5%36%5%45%6%5%8%41%
1440
21st to endNight
10%425%5%36%6%7%10%43%
1440
July1st to 10thNight
10%6%36%447%5%8%10%38%
1440
11th to 20thNight
8%435%5%36%6%7%9%43%
1440
21st to endNight
9%5%25%5%36%7%7%9%43%
1577
August1st to 10thNight
9%6%26%5%36%6%7%9%42%
1611
11th to 20thNight
11%6%26%7%46%7%5%8%38%
1770
21st to endNight
14%6%246%35%46%9%41%
1980
September1st to 10thNight
14%7%346%245%5%6%45%
1800
11th to 20thNight
11%6%2432445%6%53%
1844
21st to endNight
18%6%245%3345%7%42%
1980
October1st to 10thNight
19%7%2343345%5%44%
2127
11th to 20thNight
15%424424347%50%
2160
21st to endNight
19%7%24423345%46%
2462
November1st to 10thNight
11%5%25%5%3335%6%53%
2407
11th to 20thNight
18%5%25%423445%48%
2470
21st to endNight
14%5%25%5%3445%7%46%
2567
December1st to 10thNight
20%6%35%43345%7%40%
2660
11th to 20thNight
18%6%25%4345%6%7%41%
3288
21st to endNight
18%7%36%5%3445%6%39%
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
41%413%23%14%5%
1518
11th to 20thDay
45%5%12%23%13%3
1459
21st to endDay
42%410%25%16%3
1782
February1st to 10thDay
46%412%25%11%2
1620
11th to 20thDay
45%48%24%15%3
1735
21st to endDay
49%48%22%13%4
1480
March1st to 10thDay
41%411%27%14%3
1800
11th to 20thDay
45%410%22%16%4
1952
21st to endDay
48%37%21%17%4
2322
April1st to 10thDay
43%49%22%16%5%
2160
11th to 20thDay
40%411%24%18%4
2134
21st to endDay
43%39%21%19%5%
2160
May1st to 10thDay
49%49%19%17%3
2305
11th to 20thDay
46%311%22%15%2
2482
21st to endDay
43%414%23%14%2
2725
June1st to 10thDay
41%6%13%25%15%1
2520
11th to 20thDay
42%6%13%24%12%3
2520
21st to endDay
38%6%14%25%17%1
2520
July1st to 10thDay
41%413%24%15%3
2450
11th to 20thDay
35%414%28%17%2
2445
21st to endDay
41%6%10%22%18%2
2585
August1st to 10thDay
40%5%13%26%16%1
2299
11th to 20thDay
40%5%14%23%15%2
2126
21st to endDay
40%5%11%22%19%3
2376
September1st to 10thDay
41%411%21%21%3
2160
11th to 20thDay
33%39%27%24%4
2116
21st to endDay
46%38%19%22%2
1980
October1st to 10thDay
42%311%25%17%2
1831
11th to 20thDay
38%310%25%22%2
1800
21st to endDay
42%49%24%19%2
1892
November1st to 10thDay
35%212%26%22%3
1671
11th to 20thDay
39%39%25%22%3
1710
21st to endDay
37%313%31%13%3
1611
December1st to 10thDay
45%215%23%14%1
1520
11th to 20thDay
43%412%27%12%2
1906
21st to endDay
47%411%25%12%1
2772
January1st to 10thNight
41%39%26%17%4
2649
11th to 20thNight
46%313%24%11%2
2521
21st to endNight
40%310%26%18%3
2574
February1st to 10thNight
45%313%25%12%2
2340
11th to 20thNight
46%38%21%17%5%
2120
21st to endNight
45%26%24%19%4
1776
March1st to 10thNight
39%211%27%17%4
2160
11th to 20thNight
44%39%22%18%4
2006
21st to endNight
43%39%19%19%6%
2034
April1st to 10thNight
44%311%21%17%5%
1800
11th to 20thNight
43%48%21%21%3
1775
21st to endNight
41%49%21%20%5%
1800
May1st to 10thNight
49%49%18%18%2
1661
11th to 20thNight
46%5%12%17%18%3
1421
21st to endNight
42%6%16%20%14%2
1551
June1st to 10thNight
41%5%11%22%19%2
1440
11th to 20thNight
42%414%26%13%1
1440
21st to endNight
35%6%14%28%15%2
1440
July1st to 10thNight
42%7%15%21%14%2
1400
11th to 20thNight
35%6%15%27%15%2
1392
21st to endNight
34%8%15%25%15%3
1489
August1st to 10thNight
37%6%17%23%15%2
1600
11th to 20thNight
44%5%13%22%15%2
1770
21st to endNight
40%414%19%20%3
1980
September1st to 10thNight
41%5%10%22%20%3
1800
11th to 20thNight
34%39%23%26%4
1844
21st to endNight
43%37%23%23%2
1980
October1st to 10thNight
44%310%25%16%2
2127
11th to 20thNight
36%312%29%18%3
2160
21st to endNight
42%28%27%19%2
2462
November1st to 10thNight
34%212%25%22%5%
2407
11th to 20thNight
40%28%24%20%5%
2470
21st to endNight
38%312%28%16%2
2567
December1st to 10thNight
46%311%24%14%2
2660
11th to 20thNight
42%412%20%17%4
3288
21st to endNight
47%211%27%12%1
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
29%20%33%13%41
180
11th to 20thDay
35%23%29%11%2
168
21st to endDay
26%24%32%14%41
264
February1st to 10thDay
33%25%30%10%2
240
11th to 20thDay
30%21%35%13%2
394
21st to endDay
30%21%31%15%3
346
March1st to 10thDay
27%20%31%18%3
420
11th to 20thDay
26%21%34%14%42
420
21st to endDay
29%22%28%15%5%1
462
April1st to 10thDay
28%21%28%16%5%1
420
11th to 20thDay
24%20%30%16%7%3
412
21st to endDay
28%16%35%15%42
420
May1st to 10thDay
31%14%29%16%7%2
423
11th to 20thDay
23%12%33%21%9%2
447
21st to endDay
21%6%25%29%16%4
485
June1st to 10thDay
16%10%30%25%15%3
450
11th to 20thDay
18%12%27%29%11%3
450
21st to endDay
17%8%29%31%12%4
450
July1st to 10thDay
13%422%32%20%8%
445
11th to 20thDay
9%323%35%24%5%
444
21st to endDay
13%5%29%32%15%6%
488
August1st to 10thDay
11%424%35%19%7%
446
11th to 20thDay
13%418%36%23%6%
439
21st to endDay
16%425%29%17%9%
495
September1st to 10thDay
18%8%23%25%19%8%
450
11th to 20thDay
16%5%26%28%16%10%
450
21st to endDay
24%10%23%24%13%6%
450
October1st to 10thDay
26%5%26%22%13%7%
450
11th to 20thDay
18%13%31%24%12%2
450
21st to endDay
23%13%32%20%10%2
450
November1st to 10thDay
21%11%31%22%12%3
365
11th to 20thDay
25%16%32%21%6%1
370
21st to endDay
21%16%38%17%7%
271
December1st to 10thDay
26%24%31%12%7%
180
11th to 20thDay
35%19%29%12%4
180
21st to endDay
31%15%32%16%6%1
198
January1st to 10thNight
20%19%34%22%4
760
11th to 20thNight
23%14%34%24%4
725
21st to endNight
18%18%36%26%2
792
February1st to 10thNight
22%16%34%26%2
720
11th to 20thNight
25%21%34%18%1
703
21st to endNight
25%18%35%20%2
592
March1st to 10thNight
23%17%34%21%4
720
11th to 20thNight
24%17%34%19%5%1
720
21st to endNight
23%12%38%21%6%
792
April1st to 10thNight
22%18%37%17%5%
720
11th to 20thNight
19%13%37%24%6%
711
21st to endNight
22%12%33%27%6%
720
May1st to 10thNight
23%12%31%26%7%
581
11th to 20thNight
21%10%34%27%7%1
359
21st to endNight
17%6%21%40%15%1
389
June1st to 10thNight
18%5%28%35%14%
360
11th to 20thNight
14%426%41%14%
360
21st to endNight
10%325%49%13%1
360
July1st to 10thNight
11%322%42%21%1
350
11th to 20thNight
9%222%45%22%1
348
21st to endNight
10%225%44%19%1
397
August1st to 10thNight
13%114%45%25%2
537
11th to 20thNight
15%215%38%27%3
706
21st to endNight
17%215%38%25%3
792
September1st to 10thNight
17%323%34%20%3
720
11th to 20thNight
16%420%35%20%5%
720
21st to endNight
22%421%32%20%1
720
October1st to 10thNight
22%7%23%26%18%4
720
11th to 20thNight
18%8%26%34%13%
720
21st to endNight
23%10%32%26%9%1
792
November1st to 10thNight
15%9%34%30%11%1
740
11th to 20thNight
21%16%27%27%8%1
760
21st to endNight
17%15%32%28%8%1
760
December1st to 10thNight
26%14%33%20%7%1
760
11th to 20thNight
21%14%34%25%6%
760
21st to endNight
23%14%31%29%3
836
Page generated at 2024-01-08 22:31:25. Version ac07

Copyright 2024 Attilla Danko


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