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 2005-11-05 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
34%
15079
21%
46514
10%
15040
45%
15040
22%
29165

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
19%
717
22%
2449
7%
716
37%
716
16%
1513
January16th to endNight
18%
768
23%
2496
7%
768
38%
768
18%
1547
February1st to 15thNight
20%
703
19%
2286
6%
701
37%
701
16%
1411
February16th to endNight
17%
631
25%
1893
6%
631
40%
631
19%
1168
March1st to 15thNight
25%
720
21%
2034
9%
720
38%
720
22%
1298
March16th to endNight
24%
768
21%
2067
7%
768
36%
768
22%
1380
April1st to 15thNight
24%
718
20%
1796
5%
718
33%
718
25%
1175
April16th to endNight
34%
720
20%
1800
5%
720
41%
720
26%
1179
May1st to 15thNight
31%
560
17%
1640
7%
560
37%
560
19%
1112
May16th to endNight
45%
374
17%
1497
12%
374
54%
374
18%
888
June1st to 15thNight
46%
360
19%
1440
19%
360
55%
360
22%
833
June16th to endNight
48%
360
16%
1440
13%
360
57%
360
21%
829
July1st to 15thNight
52%
340
13%
1360
15%
340
55%
340
18%
851
July16th to endNight
59%
508
17%
1539
14%
475
66%
475
30%
898
August1st to 15thNight
57%
543
18%
1591
12%
540
60%
540
28%
994
August16th to endNight
58%
700
17%
1750
9%
700
60%
700
30%
1172
September1st to 15thNight
56%
660
25%
1650
15%
660
59%
660
31%
1105
September16th to endNight
48%
657
21%
1795
13%
657
54%
657
25%
1189
October1st to 15thNight
41%
660
24%
1980
19%
660
49%
660
24%
1258
October16th to endNight
34%
704
27%
2112
14%
704
51%
704
28%
1246
November1st to 15thNight
30%
700
24%
2252
11%
700
46%
700
24%
1354
November16th to endNight
27%
720
21%
2439
10%
720
42%
720
18%
1475
December1st to 15thNight
23%
720
18%
2520
6%
720
38%
720
14%
1603
December16th to endNight
18%
768
22%
2688
7%
768
37%
768
15%
1687

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%42442446%8%43%
47448
Night
14%5%2442445%7%43%
46696
Cloud CoverDay
13%42442445%8%44%
47458
Night
14%5%2442445%7%43%
46700
TransparencyDay
36%6%13%24%16%2
47148
Night
39%5%11%23%17%3
46514
SeeingDay
16%14%22%24%17%5%
5575
Night
14%22%23%24%12%1
15084

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
16%5%245%2335%8%42%
1486
16th to endDay
15%6%25%415%446%43%
1728
February1st to 15thDay
15%6%24225%46%6%42%
1575
16th to endDay
13%8%25%424446%42%
1580
March1st to 15thDay
14%6%34424346%45%
1912
16th to endDay
18%5%34423346%42%
2157
April1st to 15thDay
14%425%42445%7%45%
2152
16th to endDay
16%5%2432435%6%45%
2160
May1st to 15thDay
16%42442446%9%40%
2318
16th to endDay
13%413425%5%7%9%40%
2618
June1st to 15thDay
10%3144345%7%10%43%
2520
16th to endDay
9%314435%5%6%10%45%
2520
July1st to 15thDay
7%425%436%5%8%9%42%
2520
16th to endDay
6%2145%26%7%9%12%41%
2519
August1st to 15thDay
9%425%5%35%6%7%9%40%
2261
16th to endDay
10%4144245%6%8%45%
2100
September1st to 15thDay
11%4244345%5%8%46%
1980
16th to endDay
15%6%25%5%245%46%42%
1832
October1st to 15thDay
19%41321336%6%47%
1650
16th to endDay
15%5%1332435%5%49%
1760
November1st to 15thDay
16%5%13324446%47%
1606
16th to endDay
15%5%24325%445%45%
1518
December1st to 15thDay
16%42432445%7%44%
1440
16th to endDay
19%5%24434446%40%
1536
January1st to 15thNight
16%5%25%424437%42%
2449
16th to endNight
16%6%2442345%5%44%
2496
February1st to 15thNight
14%6%2443345%6%43%
2284
16th to endNight
15%7%24324346%46%
1893
March1st to 15thNight
15%6%24313348%45%
2022
16th to endNight
16%5%25%424446%42%
2067
April1st to 15thNight
14%6%2442345%7%44%
1796
16th to endNight
17%41432435%7%45%
1800
May1st to 15thNight
18%7%245%25%35%7%38%
1640
16th to endNight
13%5%15%325%46%10%42%
1495
June1st to 15thNight
10%425%43445%9%45%
1440
16th to endNight
9%435%5%345%5%10%42%
1440
July1st to 15thNight
8%5%26%7%47%47%8%37%
1440
16th to endNight
7%213336%6%8%11%43%
1646
August1st to 15thNight
8%415%5%35%5%5%9%44%
1602
16th to endNight
11%5%25%5%346%5%7%43%
1750
September1st to 15thNight
11%5%245%25%45%7%44%
1650
16th to endNight
14%5%25%5%2345%8%41%
1795
October1st to 15thNight
18%5%24333335%46%
1980
16th to endNight
14%314423346%51%
2112
November1st to 15thNight
15%5%24313345%50%
2252
16th to endNight
15%5%2442445%6%43%
2439
December1st to 15thNight
17%5%25%5%3445%6%39%
2520
16th to endNight
17%5%25%4235%47%41%
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
17%5%24413347%43%
1486
16th to endDay
15%6%24425%437%43%
1728
February1st to 15thDay
15%7%1432445%7%41%
1575
16th to endDay
13%8%24434445%42%
1580
March1st to 15thDay
14%5%24424446%45%
1923
16th to endDay
18%5%25%423346%42%
2156
April1st to 15thDay
14%325%5%2445%6%45%
2152
16th to endDay
16%5%2332345%7%45%
2160
May1st to 15thDay
16%42443446%9%40%
2318
16th to endDay
13%3234345%6%9%41%
2618
June1st to 15thDay
10%214435%5%7%9%44%
2520
16th to endDay
9%314435%5%6%10%45%
2520
July1st to 15thDay
7%3145%36%6%8%9%43%
2520
16th to endDay
6%2145%25%7%9%12%42%
2519
August1st to 15thDay
9%425%5%35%6%7%9%41%
2261
16th to endDay
10%3244245%6%9%46%
2100
September1st to 15thDay
11%32343346%8%46%
1980
16th to endDay
15%6%25%424446%42%
1832
October1st to 15thDay
19%413313347%48%
1650
16th to endDay
15%5%1332335%5%49%
1760
November1st to 15thDay
16%6%12324347%47%
1606
16th to endDay
16%5%243245%45%45%
1518
December1st to 15thDay
16%42432445%6%44%
1440
16th to endDay
19%5%24424446%40%
1536
January1st to 15thNight
16%424434447%42%
2449
16th to endNight
17%6%2432345%5%44%
2496
February1st to 15thNight
14%6%2443345%6%44%
2277
16th to endNight
15%7%23423446%46%
1893
March1st to 15thNight
15%6%24314347%45%
2034
16th to endNight
16%5%25%424447%42%
2067
April1st to 15thNight
14%6%1442445%8%44%
1796
16th to endNight
17%41332435%7%45%
1800
May1st to 15thNight
18%6%24434348%38%
1639
16th to endNight
13%5%243245%5%10%42%
1495
June1st to 15thNight
11%435%42445%8%45%
1440
16th to endNight
9%435%5%345%5%10%42%
1440
July1st to 15thNight
8%426%6%5%6%47%9%37%
1440
16th to endNight
7%213336%6%8%11%43%
1646
August1st to 15thNight
8%325%5%35%6%5%9%44%
1602
16th to endNight
11%4245%25%5%5%7%43%
1750
September1st to 15thNight
11%5%145%35%447%44%
1650
16th to endNight
14%6%25%5%235%5%8%41%
1795
October1st to 15thNight
18%5%24323345%45%
1980
16th to endNight
15%314323346%50%
2112
November1st to 15thNight
15%5%2431334450%
2252
16th to endNight
15%5%2342445%6%44%
2439
December1st to 15thNight
17%5%25%5%3445%6%40%
2520
16th to endNight
17%5%25%32445%6%41%
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
41%414%22%12%4
1487
16th to endDay
40%5%11%25%13%3
1728
February1st to 15thDay
40%411%26%14%1
1577
16th to endDay
42%412%25%14%1
1580
March1st to 15thDay
39%413%24%14%3
1923
16th to endDay
42%410%21%17%2
2157
April1st to 15thDay
38%5%11%24%16%4
2152
16th to endDay
37%5%11%18%20%6%
2160
May1st to 15thDay
38%7%14%22%15%2
2318
16th to endDay
35%8%17%24%12%1
2622
June1st to 15thDay
32%9%15%27%14%1
2520
16th to endDay
30%9%16%23%18%1
2520
July1st to 15thDay
32%10%15%23%14%3
2371
16th to endDay
29%10%17%25%15%1
2363
August1st to 15thDay
34%8%15%24%15%1
2248
16th to endDay
32%7%15%24%16%4
2100
September1st to 15thDay
33%6%15%21%18%3
1980
16th to endDay
41%49%21%19%2
1832
October1st to 15thDay
35%411%27%16%3
1650
16th to endDay
35%39%28%20%2
1760
November1st to 15thDay
37%312%24%17%4
1606
16th to endDay
39%49%25%17%2
1518
December1st to 15thDay
38%6%16%24%11%3
1440
16th to endDay
44%5%11%24%14%1
1536
January1st to 15thNight
41%411%21%17%3
2449
16th to endNight
41%310%26%16%1
2496
February1st to 15thNight
39%5%11%24%16%3
2286
16th to endNight
39%39%25%19%2
1893
March1st to 15thNight
38%412%26%14%2
2034
16th to endNight
41%5%11%20%17%3
2067
April1st to 15thNight
38%411%21%20%3
1796
16th to endNight
37%5%10%20%18%6%
1800
May1st to 15thNight
44%5%11%20%14%2
1640
16th to endNight
36%8%17%20%14%2
1497
June1st to 15thNight
36%6%14%24%17%1
1440
16th to endNight
37%6%14%22%17%2
1440
July1st to 15thNight
42%8%16%18%12%3
1360
16th to endNight
30%9%15%27%16%1
1539
August1st to 15thNight
34%7%15%22%18%
1591
16th to endNight
37%7%12%22%16%3
1750
September1st to 15thNight
36%7%10%21%18%4
1650
16th to endNight
41%5%8%20%19%4
1795
October1st to 15thNight
41%48%24%17%3
1980
16th to endNight
34%39%27%21%3
2112
November1st to 15thNight
37%48%23%18%7%
2252
16th to endNight
39%411%23%17%4
2439
December1st to 15thNight
43%5%11%23%13%2
2520
16th to endNight
42%5%13%23%13%2
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
24%17%40%14%11
69
16th to endDay
19%22%36%16%4
96
February1st to 15thDay
24%18%38%14%3
90
16th to endDay
16%20%39%21%1
239
March1st to 15thDay
15%24%29%24%5%
270
16th to endDay
20%20%32%19%42
288
April1st to 15thDay
16%20%29%22%9%1
270
16th to endDay
20%15%22%24%13%3
270
May1st to 15thDay
25%21%26%12%11%3
288
16th to endDay
18%11%18%23%20%7%
331
June1st to 15thDay
11%13%20%24%22%7%
315
16th to endDay
10%11%20%32%20%3
315
July1st to 15thDay
9%8%10%36%28%5%
305
16th to endDay
5%9%10%33%30%10%
326
August1st to 15thDay
12%7%12%27%31%9%
297
16th to endDay
9%6%7%27%32%17%
268
September1st to 15thDay
10%9%16%20%26%16%
255
16th to endDay
26%7%14%24%16%10%
254
October1st to 15thDay
21%10%19%25%17%5%
255
16th to endDay
14%13%28%25%15%2
272
November1st to 15thDay
17%18%26%18%13%5%
240
16th to endDay
15%23%28%17%10%2
138
December1st to 15thDay
16%23%25%18%11%5%
60
16th to endDay
35%9%29%20%4
64
January1st to 15thNight
16%30%27%19%5%
720
16th to endNight
19%27%31%20%1
768
February1st to 15thNight
15%30%30%21%2
704
16th to endNight
15%31%31%20%1
631
March1st to 15thNight
16%27%28%22%4
720
16th to endNight
16%23%31%20%5%1
768
April1st to 15thNight
14%26%30%23%4
718
16th to endNight
16%21%24%25%10%
720
May1st to 15thNight
17%25%20%24%11%1
560
16th to endNight
13%19%16%26%22%1
375
June1st to 15thNight
9%18%18%30%21%1
360
16th to endNight
5%18%17%40%16%
360
July1st to 15thNight
8%12%17%33%27%1
340
16th to endNight
6%16%11%31%29%4
508
August1st to 15thNight
10%10%13%36%26%2
543
16th to endNight
11%10%9%32%30%5%
700
September1st to 15thNight
11%11%14%28%29%4
660
16th to endNight
16%12%14%29%23%3
657
October1st to 15thNight
20%15%18%24%20%1
660
16th to endNight
15%18%28%23%12%
704
November1st to 15thNight
15%23%25%21%13%1
700
16th to endNight
15%28%25%22%6%
720
December1st to 15thNight
17%29%24%17%9%1
720
16th to endNight
17%30%30%18%2
768
Page generated at 2017-08-13 14:16:20. Version ac04

Copyright 2017 A. Danko


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