United States Naval Academy
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the United States Naval Academy clear sky chart from 2005-11-06 to 2024-03-23.

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%
23691
21%
80420
11%
23596
49%
23596
23%
49511

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
18%
750
24%
4344
7%
737
40%
737
12%
2451
January11th to 20thNight
25%
760
26%
3980
10%
740
41%
740
13%
2230
January21st to endNight
23%
829
16%
4454
11%
814
42%
814
12%
2701
February1st to 10thNight
28%
760
19%
2470
10%
760
40%
760
21%
1570
February11th to 20thNight
16%
742
20%
2309
5%
740
36%
740
19%
1466
February21st to endNight
19%
628
24%
1884
7%
628
39%
628
22%
1203
March1st to 10thNight
25%
760
24%
2176
9%
760
42%
760
27%
1354
March11th to 20thNight
26%
760
22%
2090
9%
760
38%
760
24%
1349
March21st to endNight
25%
801
21%
2134
7%
801
36%
801
25%
1422
April1st to 10thNight
25%
720
20%
1800
4%
720
35%
720
25%
1207
April11th to 20thNight
36%
711
21%
1775
7%
711
42%
711
32%
1160
April21st to endNight
35%
720
20%
1800
6%
720
41%
720
30%
1188
May1st to 10thNight
38%
651
16%
1731
7%
651
43%
651
26%
1181
May11th to 20thNight
39%
364
18%
1425
13%
364
48%
364
19%
895
May21st to endNight
55%
388
16%
1551
15%
388
60%
388
22%
923
June1st to 10thNight
54%
360
20%
1440
20%
360
61%
360
25%
828
June11th to 20thNight
53%
360
17%
1440
18%
360
58%
360
22%
853
June21st to endNight
57%
360
17%
1440
17%
360
64%
360
25%
818
July1st to 10thNight
62%
350
15%
1400
18%
350
65%
350
21%
847
July11th to 20thNight
70%
352
17%
1396
22%
352
70%
352
26%
773
July21st to endNight
64%
591
19%
1672
15%
558
69%
558
32%
1028
August1st to 10thNight
65%
537
16%
1600
12%
534
67%
534
29%
1026
August11th to 20thNight
59%
648
16%
1712
9%
648
60%
648
27%
1166
August21st to endNight
61%
792
18%
1980
10%
792
63%
792
32%
1339
September1st to 10thNight
56%
720
22%
1800
12%
720
60%
720
32%
1216
September11th to 20thNight
62%
720
29%
1800
20%
720
66%
720
38%
1133
September21st to endNight
44%
720
21%
2036
15%
720
51%
720
24%
1375
October1st to 10thNight
45%
720
19%
2160
15%
720
52%
720
26%
1389
October11th to 20thNight
41%
720
26%
2160
18%
720
53%
720
30%
1295
October21st to endNight
36%
792
27%
2376
16%
792
51%
792
27%
1443
November1st to 10thNight
39%
740
26%
2365
15%
740
54%
740
30%
1364
November11th to 20thNight
32%
760
25%
2470
15%
760
46%
760
25%
1476
November21st to endNight
33%
760
24%
2626
13%
760
49%
760
25%
1490
December1st to 10thNight
27%
760
20%
2660
6%
760
44%
760
18%
1661
December11th to 20thNight
25%
760
21%
3176
8%
759
42%
759
18%
1872
December21st to endNight
29%
825
15%
4788
11%
817
43%
817
13%
2819

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
14%5%244345%6%8%44%
78740
Night
15%6%25%5%3445%7%44%
81141
TransparencyDay
37%6%13%25%16%3
78450
Night
40%5%12%24%17%3
80950
SeeingDay
16%9%25%26%17%6%
14130
Night
17%10%28%29%14%2
23721

By Month

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 10thDay
16%424423348%49%
2517
11th to 20thDay
14%10%46%434446%40%
2817
21st to endDay
18%7%35%5%3445%6%40%
3124
February1st to 10thDay
15%7%25%436%5%6%8%40%
1710
11th to 20thDay
19%6%2443345%7%43%
1757
21st to endDay
18%8%26%5%34346%39%
1570
March1st to 10thDay
13%6%25%425%45%7%48%
2000
11th to 20thDay
16%5%245%35%45%6%44%
2090
21st to endDay
18%6%36%5%345%46%40%
2266
April1st to 10thDay
13%5%35%5%345%6%7%45%
2160
11th to 20thDay
13%425%5%245%5%7%48%
2134
21st to endDay
17%6%2443446%8%41%
2160
May1st to 10thDay
16%5%25%4345%6%9%41%
2243
11th to 20thDay
17%5%34435%5%7%9%38%
2464
21st to endDay
14%4245%35%5%6%10%42%
2725
June1st to 10thDay
11%3245%35%6%7%10%44%
2520
11th to 20thDay
10%5%24436%5%7%10%44%
2520
21st to endDay
9%324435%6%7%11%46%
2520
July1st to 10thDay
8%425%5%46%6%8%10%43%
2520
11th to 20thDay
5%2145%36%7%8%11%48%
2512
21st to endDay
9%425%6%46%8%8%11%38%
2547
August1st to 10thDay
8%5%25%6%35%7%8%10%40%
2327
11th to 20thDay
10%5%25%5%35%6%7%10%43%
2170
21st to endDay
11%425%5%345%7%10%44%
2376
September1st to 10thDay
15%42443446%8%46%
2160
11th to 20thDay
12%5%345%24448%50%
2160
21st to endDay
18%7%25%425%5%5%7%40%
1918
October1st to 10thDay
17%42432345%7%48%
1800
11th to 20thDay
18%42332345%7%50%
1800
21st to endDay
15%6%2433445%5%49%
1980
November1st to 10thDay
13%6%13424347%53%
1712
11th to 20thDay
15%5%234345%5%6%49%
1710
21st to endDay
15%5%244345%45%49%
1549
December1st to 10thDay
17%6%25%325%46%7%43%
1520
11th to 20thDay
16%7%3442445%6%46%
1910
21st to endDay
16%7%25%6%35%5%5%7%40%
2772
January1st to 10thNight
15%7%25%5%35%446%43%
4429
11th to 20thNight
19%7%34323446%45%
4138
21st to endNight
19%10%35%433345%42%
4532
February1st to 10thNight
16%8%25%4345%6%7%41%
2470
11th to 20thNight
19%7%34434445%44%
2307
21st to endNight
18%8%345%2345%6%43%
1884
March1st to 10thNight
14%6%25%424446%50%
2176
11th to 20thNight
16%6%35%434448%44%
2090
21st to endNight
16%7%36%5%34447%42%
2134
April1st to 10thNight
16%7%25%5%3445%7%42%
1800
11th to 20thNight
13%5%26%425%45%7%47%
1775
21st to endNight
19%5%2443436%7%44%
1800
May1st to 10thNight
16%7%25%445%5%6%8%39%
1731
11th to 20thNight
18%5%25%5%35%5%5%9%37%
1425
21st to endNight
13%6%25%4345%6%10%41%
1550
June1st to 10thNight
11%435%5%25%5%6%10%45%
1440
11th to 20thNight
10%5%36%5%35%5%5%10%41%
1440
21st to endNight
10%425%6%446%6%10%43%
1440
July1st to 10thNight
9%5%26%6%5%6%6%7%10%39%
1440
11th to 20thNight
6%325%5%46%6%8%11%44%
1444
21st to endNight
8%4245%46%6%9%11%41%
1771
August1st to 10thNight
9%426%6%46%6%7%10%40%
1611
11th to 20thNight
9%6%27%6%47%6%6%8%38%
1712
21st to endNight
12%5%35%5%35%5%5%9%42%
1980
September1st to 10thNight
14%6%25%5%35%45%7%44%
1800
11th to 20thNight
9%6%345%3445%7%50%
1800
21st to endNight
18%6%25%5%3445%7%39%
2036
October1st to 10thNight
19%6%2443345%6%44%
2160
11th to 20thNight
17%424423446%50%
2160
21st to endNight
17%42442445%7%48%
2376
November1st to 10thNight
12%5%2442345%5%53%
2365
11th to 20thNight
16%6%24424346%49%
2470
21st to endNight
13%5%245%3446%7%48%
2626
December1st to 10thNight
18%6%35%5%345%6%6%41%
2660
11th to 20thNight
17%435%5%3445%7%44%
3285
21st to endNight
15%6%36%435%6%6%8%38%
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
35%410%32%15%4
2518
11th to 20thDay
45%410%22%14%4
2817
21st to endDay
44%5%14%24%10%2
3124
February1st to 10thDay
42%5%14%25%13%2
1710
11th to 20thDay
40%410%26%17%3
1766
21st to endDay
46%412%20%15%4
1570
March1st to 10thDay
36%6%12%26%18%3
2000
11th to 20thDay
40%5%11%20%19%5%
2090
21st to endDay
45%5%9%21%17%3
2266
April1st to 10thDay
37%6%13%22%17%6%
2160
11th to 20thDay
34%6%11%23%19%7%
2134
21st to endDay
39%5%13%21%18%4
2160
May1st to 10thDay
38%6%14%22%16%3
2243
11th to 20thDay
40%8%15%21%14%2
2468
21st to endDay
36%8%17%26%12%2
2725
June1st to 10thDay
33%8%17%25%15%1
2520
11th to 20thDay
33%10%15%24%15%2
2520
21st to endDay
29%8%17%27%17%1
2520
July1st to 10thDay
34%9%15%23%16%3
2450
11th to 20thDay
26%9%18%30%14%3
2437
21st to endDay
34%10%15%23%16%1
2405
August1st to 10thDay
34%9%17%27%12%1
2314
11th to 20thDay
34%8%17%26%14%2
2170
21st to endDay
34%7%16%24%14%4
2376
September1st to 10thDay
35%6%15%23%16%4
2160
11th to 20thDay
33%5%10%26%21%4
2160
21st to endDay
43%5%8%22%18%3
1918
October1st to 10thDay
36%5%12%26%18%3
1800
11th to 20thDay
35%410%27%21%3
1800
21st to endDay
36%49%28%21%2
1980
November1st to 10thDay
33%312%29%19%3
1712
11th to 20thDay
35%410%24%22%4
1710
21st to endDay
37%410%29%18%3
1549
December1st to 10thDay
39%5%17%24%12%2
1520
11th to 20thDay
39%410%32%12%3
1906
21st to endDay
43%6%15%23%12%1
2772
January1st to 10thNight
40%6%10%26%15%3
4429
11th to 20thNight
41%410%27%16%3
4138
21st to endNight
46%48%27%14%1
4532
February1st to 10thNight
42%5%13%24%14%2
2470
11th to 20thNight
43%48%23%18%5%
2309
21st to endNight
42%311%22%18%5%
1884
March1st to 10thNight
37%411%26%19%3
2176
11th to 20thNight
39%5%11%22%19%4
2090
21st to endNight
42%5%10%20%17%6%
2134
April1st to 10thNight
41%5%10%20%20%5%
1800
11th to 20thNight
37%5%11%21%20%6%
1775
21st to endNight
40%5%11%23%16%5%
1800
May1st to 10thNight
43%6%12%21%14%4
1731
11th to 20thNight
44%6%13%17%18%2
1426
21st to endNight
38%8%18%21%13%3
1551
June1st to 10thNight
36%7%14%24%18%2
1440
11th to 20thNight
38%7%14%24%14%2
1440
21st to endNight
35%7%16%23%18%2
1440
July1st to 10thNight
38%9%18%19%12%3
1400
11th to 20thNight
31%9%17%26%14%2
1396
21st to endNight
33%9%14%28%15%1
1672
August1st to 10thNight
37%9%17%22%15%1
1600
11th to 20thNight
41%8%15%21%13%1
1712
21st to endNight
38%8%14%22%15%3
1980
September1st to 10thNight
40%6%11%22%16%5%
1800
11th to 20thNight
32%6%10%25%23%4
1800
21st to endNight
44%6%9%21%18%3
2036
October1st to 10thNight
42%410%23%18%3
2160
11th to 20thNight
36%49%28%20%3
2160
21st to endNight
37%410%25%21%3
2376
November1st to 10thNight
34%5%9%27%20%6%
2365
11th to 20thNight
39%310%22%21%6%
2470
21st to endNight
35%412%26%20%3
2626
December1st to 10thNight
43%5%12%24%14%2
2660
11th to 20thNight
40%5%11%22%18%4
3288
21st to endNight
43%6%16%24%10%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
18%20%35%18%7%1
180
11th to 20thDay
18%24%32%22%3
265
21st to endDay
18%20%38%17%6%1
297
February1st to 10thDay
20%12%43%16%8%
270
11th to 20thDay
26%17%40%15%2
364
21st to endDay
24%14%37%19%5%
382
March1st to 10thDay
17%20%34%20%7%1
460
11th to 20thDay
17%18%34%22%7%2
460
21st to endDay
21%12%36%24%6%2
470
April1st to 10thDay
18%15%35%25%7%1
420
11th to 20thDay
14%14%33%23%12%5%
412
21st to endDay
21%12%30%27%10%1
420
May1st to 10thDay
21%9%31%26%10%2
423
11th to 20thDay
19%6%30%27%15%5%
447
21st to endDay
20%415%26%26%9%
485
June1st to 10thDay
11%6%21%28%23%10%
450
11th to 20thDay
13%6%20%31%22%9%
450
21st to endDay
11%218%35%26%8%
450
July1st to 10thDay
10%310%32%30%15%
445
11th to 20thDay
318%36%37%14%
444
21st to endDay
9%213%35%31%9%
488
August1st to 10thDay
10%211%30%34%13%
446
11th to 20thDay
11%312%31%31%12%
439
21st to endDay
9%216%31%27%15%
495
September1st to 10thDay
18%316%22%27%14%
450
11th to 20thDay
15%216%31%25%11%
450
21st to endDay
23%6%22%29%16%4
450
October1st to 10thDay
21%5%16%30%19%8%
450
11th to 20thDay
17%9%26%27%17%4
450
21st to endDay
17%10%31%27%11%4
495
November1st to 10thDay
16%11%33%23%12%6%
386
11th to 20thDay
14%16%34%24%9%2
370
21st to endDay
13%18%31%21%14%2
209
December1st to 10thDay
19%16%35%22%6%2
180
11th to 20thDay
19%18%31%23%8%
180
21st to endDay
23%12%29%27%9%1
198
January1st to 10thNight
19%18%40%20%4
760
11th to 20thNight
21%17%31%25%6%
760
21st to endNight
19%17%36%25%2
836
February1st to 10thNight
19%13%36%28%4
760
11th to 20thNight
24%18%37%20%1
743
21st to endNight
22%14%39%21%3
628
March1st to 10thNight
17%16%38%24%5%
760
11th to 20thNight
20%15%34%23%6%1
760
21st to endNight
18%12%37%26%5%1
801
April1st to 10thNight
19%17%36%22%6%
720
11th to 20thNight
15%11%33%31%10%
711
21st to endNight
21%8%33%28%9%1
720
May1st to 10thNight
18%8%28%32%13%1
651
11th to 20thNight
19%8%25%33%14%1
364
21st to endNight
13%319%36%25%3
389
June1st to 10thNight
12%322%37%25%1
360
11th to 20thNight
10%423%41%21%1
360
21st to endNight
11%118%45%22%3
360
July1st to 10thNight
11%315%36%31%4
350
11th to 20thNight
6%210%42%37%4
352
21st to endNight
8%216%39%31%4
591
August1st to 10thNight
10%212%42%31%3
537
11th to 20thNight
11%413%34%33%5%
648
21st to endNight
13%312%34%31%8%
792
September1st to 10thNight
17%314%32%28%6%
720
11th to 20thNight
11%314%35%30%6%
720
21st to endNight
20%6%20%32%20%3
720
October1st to 10thNight
22%6%21%26%21%5%
720
11th to 20thNight
18%10%26%28%17%1
720
21st to endNight
19%13%28%26%13%1
792
November1st to 10thNight
15%9%31%30%14%2
740
11th to 20thNight
18%16%29%28%9%
760
21st to endNight
16%14%32%27%9%1
760
December1st to 10thNight
21%17%29%22%10%1
760
11th to 20thNight
17%14%39%24%6%
760
21st to endNight
19%12%35%31%4
836
Page generated at 2024-03-19 23:31:09. Version ac07

Copyright © 2024 Attilla Danko.


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