USNO
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the USNO 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
35%
23727
21%
80377
11%
23631
47%
23631
23%
49168

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%
752
24%
4263
8%
737
39%
737
13%
2330
January11th to 20thNight
24%
759
27%
3954
9%
740
41%
740
13%
2200
January21st to endNight
22%
828
16%
4449
9%
814
42%
814
13%
2687
February1st to 10thNight
26%
760
19%
2470
9%
760
41%
760
21%
1546
February11th to 20thNight
14%
742
20%
2343
4%
740
34%
740
17%
1489
February21st to endNight
18%
628
24%
1884
6%
628
39%
628
21%
1222
March1st to 10thNight
22%
760
25%
2133
8%
760
39%
760
25%
1328
March11th to 20thNight
21%
760
22%
2090
8%
760
36%
760
23%
1330
March21st to endNight
20%
801
21%
2161
7%
801
32%
801
23%
1446
April1st to 10thNight
21%
720
19%
1800
4%
720
32%
720
22%
1217
April11th to 20thNight
30%
711
21%
1775
6%
711
37%
711
30%
1153
April21st to endNight
31%
720
20%
1800
6%
720
39%
720
28%
1210
May1st to 10thNight
33%
609
18%
1689
8%
609
40%
609
24%
1121
May11th to 20thNight
38%
413
18%
1474
10%
413
47%
413
20%
939
May21st to endNight
50%
388
16%
1551
15%
388
56%
388
23%
893
June1st to 10thNight
50%
360
20%
1440
19%
360
59%
360
25%
808
June11th to 20thNight
47%
360
17%
1440
15%
360
55%
360
23%
851
June21st to endNight
52%
360
18%
1440
16%
360
60%
360
25%
823
July1st to 10thNight
59%
350
15%
1400
18%
350
63%
350
21%
857
July11th to 20thNight
67%
410
17%
1454
16%
410
70%
410
30%
822
July21st to endNight
60%
591
18%
1672
13%
558
66%
558
34%
1006
August1st to 10thNight
65%
537
16%
1600
12%
534
66%
534
29%
1000
August11th to 20thNight
58%
618
17%
1682
11%
618
60%
618
27%
1133
August21st to endNight
57%
792
20%
1980
11%
792
59%
792
33%
1310
September1st to 10thNight
52%
720
23%
1800
12%
720
56%
720
32%
1199
September11th to 20thNight
58%
720
31%
1800
21%
720
62%
720
38%
1139
September21st to endNight
44%
720
21%
2059
15%
720
51%
720
25%
1382
October1st to 10thNight
46%
720
21%
2160
17%
720
53%
720
27%
1362
October11th to 20thNight
40%
720
26%
2160
18%
720
53%
720
30%
1292
October21st to endNight
32%
792
28%
2376
15%
792
48%
792
28%
1428
November1st to 10thNight
36%
740
28%
2324
16%
740
52%
740
31%
1342
November11th to 20thNight
31%
760
26%
2474
13%
760
48%
760
27%
1435
November21st to endNight
32%
760
24%
2660
12%
760
49%
760
25%
1488
December1st to 10thNight
23%
760
20%
2660
7%
760
41%
760
18%
1642
December11th to 20thNight
25%
760
21%
3166
9%
759
41%
759
18%
1857
December21st to endNight
28%
826
15%
4794
11%
817
42%
817
11%
2881

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%6%25%5%35%5%6%8%41%
78762
Night
15%6%25%5%345%5%7%44%
81128
TransparencyDay
40%6%12%24%16%3
78472
Night
39%5%11%24%17%3
80937
SeeingDay
20%12%28%24%12%4
14102
Night
18%12%29%29%12%1
23756

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
17%424424446%48%
2581
11th to 20thDay
15%11%35%5%35%446%40%
2840
21st to endDay
18%8%36%6%35%446%36%
3124
February1st to 10thDay
15%6%25%7%35%5%6%7%39%
1710
11th to 20thDay
18%6%25%5%345%5%6%41%
1724
21st to endDay
18%8%36%5%25%446%40%
1570
March1st to 10thDay
13%5%345%3445%6%48%
2042
11th to 20thDay
16%6%35%5%35%45%7%42%
2090
21st to endDay
17%7%36%5%35%45%6%39%
2239
April1st to 10thDay
12%6%35%5%345%6%7%44%
2160
11th to 20thDay
13%5%25%5%3435%6%48%
2134
21st to endDay
19%5%25%43446%7%41%
2160
May1st to 10thDay
14%6%35%5%35%6%7%9%37%
2286
11th to 20thDay
17%6%36%5%346%6%9%34%
2419
21st to endDay
13%6%26%5%45%5%6%9%40%
2725
June1st to 10thDay
12%5%25%5%35%6%6%10%42%
2520
11th to 20thDay
11%6%26%6%46%5%7%8%40%
2520
21st to endDay
10%3245%36%6%7%12%41%
2520
July1st to 10thDay
10%425%7%46%6%7%11%39%
2520
11th to 20thDay
6%4246%46%7%8%12%41%
2454
21st to endDay
9%5%26%6%37%7%8%11%35%
2547
August1st to 10thDay
9%6%26%6%47%7%7%10%34%
2327
11th to 20thDay
11%5%36%6%46%6%7%9%37%
2201
21st to endDay
11%425%6%45%7%8%8%39%
2376
September1st to 10thDay
14%5%35%4345%6%7%43%
2160
11th to 20thDay
11%5%2443446%9%46%
2160
21st to endDay
18%6%35%5%345%6%6%39%
1897
October1st to 10thDay
16%5%34434448%45%
1800
11th to 20thDay
18%425%32346%6%48%
1800
21st to endDay
14%5%25%42445%6%48%
1980
November1st to 10thDay
12%5%2343445%8%51%
1753
11th to 20thDay
14%5%3432346%7%49%
1701
21st to endDay
15%5%2442445%6%48%
1520
December1st to 10thDay
15%6%15%435%5%6%7%42%
1520
11th to 20thDay
16%6%25%43345%7%46%
1910
21st to endDay
17%7%36%5%36%5%6%7%36%
2772
January1st to 10thNight
15%7%25%435%45%7%45%
4361
11th to 20thNight
19%7%24423446%46%
4120
21st to endNight
20%9%35%423435%41%
4532
February1st to 10thNight
15%7%245%35%5%6%6%42%
2470
11th to 20thNight
18%7%25%434346%44%
2341
21st to endNight
18%7%25%43445%7%42%
1884
March1st to 10thNight
14%5%34435%346%48%
2133
11th to 20thNight
16%6%25%42345%8%44%
2090
21st to endNight
14%7%37%5%2446%6%41%
2161
April1st to 10thNight
14%7%35%5%346%6%6%41%
1800
11th to 20thNight
14%5%25%4346%6%6%46%
1775
21st to endNight
18%5%24435%46%9%41%
1800
May1st to 10thNight
16%7%25%435%5%5%7%40%
1689
11th to 20thNight
17%7%344245%7%8%37%
1474
21st to endNight
13%5%24435%6%7%11%41%
1550
June1st to 10thNight
10%424436%5%7%9%45%
1440
11th to 20thNight
12%6%25%6%35%5%7%10%40%
1440
21st to endNight
9%425%6%45%6%6%10%43%
1440
July1st to 10thNight
8%445%7%47%5%7%9%39%
1440
11th to 20thNight
6%4245%47%6%7%9%46%
1502
21st to endNight
8%425%5%35%7%9%11%41%
1771
August1st to 10thNight
9%425%6%46%6%8%9%40%
1611
11th to 20thNight
11%5%37%6%35%5%7%8%39%
1682
21st to endNight
9%6%36%6%35%5%6%9%43%
1980
September1st to 10thNight
14%6%25%435%36%7%45%
1800
11th to 20thNight
10%6%345%3445%7%50%
1800
21st to endNight
17%6%35%5%345%6%6%39%
2059
October1st to 10thNight
19%5%244245%46%45%
2160
11th to 20thNight
15%5%24433346%51%
2160
21st to endNight
15%6%2442445%6%48%
2376
November1st to 10thNight
12%5%245%23445%54%
2324
11th to 20thNight
14%6%24423346%50%
2474
21st to endNight
13%4245%345%5%8%48%
2660
December1st to 10thNight
17%6%35%5%3445%7%42%
2660
11th to 20thNight
16%5%25%43446%6%44%
3285
21st to endNight
16%7%25%5%45%6%5%7%37%
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
37%49%31%15%4
2582
11th to 20thDay
46%410%21%15%4
2840
21st to endDay
49%411%23%11%2
3124
February1st to 10thDay
43%5%13%24%14%1
1710
11th to 20thDay
43%49%26%16%3
1733
21st to endDay
47%411%20%15%3
1570
March1st to 10thDay
37%411%26%20%3
2042
11th to 20thDay
42%310%20%18%6%
2090
21st to endDay
46%48%21%17%4
2239
April1st to 10thDay
39%5%11%21%19%5%
2160
11th to 20thDay
37%410%23%21%6%
2134
21st to endDay
41%5%11%22%18%4
2160
May1st to 10thDay
42%6%13%20%17%3
2286
11th to 20thDay
45%6%13%20%13%2
2423
21st to endDay
40%6%14%25%12%2
2725
June1st to 10thDay
37%7%13%26%16%2
2520
11th to 20thDay
39%8%13%22%15%2
2520
21st to endDay
34%8%15%24%18%1
2520
July1st to 10thDay
38%8%14%21%16%3
2450
11th to 20thDay
32%8%16%26%15%3
2379
21st to endDay
39%9%13%21%17%2
2405
August1st to 10thDay
42%7%15%23%12%1
2314
11th to 20thDay
41%8%13%21%16%2
2201
21st to endDay
38%8%13%23%14%3
2376
September1st to 10thDay
38%6%13%20%18%4
2160
11th to 20thDay
34%6%9%26%22%4
2160
21st to endDay
44%6%7%22%19%2
1897
October1st to 10thDay
38%6%9%24%19%3
1800
11th to 20thDay
37%410%25%22%3
1800
21st to endDay
37%47%28%22%2
1980
November1st to 10thDay
32%5%11%27%20%5%
1753
11th to 20thDay
34%410%25%22%5%
1701
21st to endDay
38%310%28%18%3
1520
December1st to 10thDay
41%415%25%13%2
1520
11th to 20thDay
38%310%33%13%2
1906
21st to endDay
45%6%13%24%11%1
2772
January1st to 10thNight
39%49%27%16%4
4361
11th to 20thNight
41%39%26%18%3
4120
21st to endNight
46%38%27%15%1
4532
February1st to 10thNight
42%413%26%14%2
2470
11th to 20thNight
43%39%22%18%5%
2343
21st to endNight
44%311%22%16%5%
1884
March1st to 10thNight
38%410%27%18%3
2133
11th to 20thNight
39%411%22%19%4
2090
21st to endNight
43%5%10%21%16%6%
2161
April1st to 10thNight
41%6%10%19%20%4
1800
11th to 20thNight
37%5%12%21%19%6%
1775
21st to endNight
40%5%11%24%15%6%
1800
May1st to 10thNight
43%412%23%13%4
1689
11th to 20thNight
43%6%12%18%17%3
1475
21st to endNight
36%7%20%22%13%2
1551
June1st to 10thNight
34%8%12%27%17%2
1440
11th to 20thNight
38%7%15%24%13%3
1440
21st to endNight
36%6%14%23%19%3
1440
July1st to 10thNight
41%7%18%18%12%4
1400
11th to 20thNight
31%8%16%25%17%2
1454
21st to endNight
31%9%17%26%16%2
1672
August1st to 10thNight
37%7%19%22%14%1
1600
11th to 20thNight
40%8%14%22%14%2
1682
21st to endNight
37%7%14%24%15%3
1980
September1st to 10thNight
40%5%10%23%18%5%
1800
11th to 20thNight
34%5%8%27%22%4
1800
21st to endNight
43%6%9%20%19%4
2059
October1st to 10thNight
40%5%10%24%18%3
2160
11th to 20thNight
36%49%28%21%3
2160
21st to endNight
37%49%27%21%4
2376
November1st to 10thNight
33%49%26%22%6%
2324
11th to 20thNight
36%39%24%22%6%
2474
21st to endNight
34%412%26%20%3
2660
December1st to 10thNight
43%412%25%15%2
2660
11th to 20thNight
40%411%23%19%3
3288
21st to endNight
44%6%14%23%11%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
198
11th to 20thDay
31%23%29%12%4
270
21st to endDay
24%24%34%14%4
297
February1st to 10thDay
25%24%35%12%4
270
11th to 20thDay
35%21%36%7%2
331
21st to endDay
30%18%31%18%2
382
March1st to 10thDay
27%20%31%17%31
460
11th to 20thDay
28%20%32%15%41
460
21st to endDay
25%19%33%16%6%1
470
April1st to 10thDay
22%20%35%17%5%1
420
11th to 20thDay
18%18%34%19%8%3
412
21st to endDay
29%14%32%17%7%1
420
May1st to 10thDay
24%18%28%20%7%3
423
11th to 20thDay
21%15%29%24%9%2
447
21st to endDay
19%7%23%31%18%3
485
June1st to 10thDay
14%8%27%28%16%6%
450
11th to 20thDay
17%8%27%27%16%4
450
21st to endDay
10%7%29%32%18%4
450
July1st to 10thDay
13%319%34%20%11%
445
11th to 20thDay
5%223%33%30%6%
444
21st to endDay
11%324%35%21%6%
488
August1st to 10thDay
12%5%19%33%21%10%
446
11th to 20thDay
13%417%31%25%10%
439
21st to endDay
10%222%35%18%13%
495
September1st to 10thDay
18%6%21%25%17%13%
450
11th to 20thDay
12%422%34%17%10%
450
21st to endDay
24%9%24%28%9%5%
450
October1st to 10thDay
23%7%22%26%17%5%
450
11th to 20thDay
18%15%28%23%14%2
450
21st to endDay
18%14%37%21%8%2
495
November1st to 10thDay
16%15%33%23%11%3
406
11th to 20thDay
20%15%33%21%9%1
361
21st to endDay
26%20%29%17%8%1
180
December1st to 10thDay
21%24%33%17%32
180
11th to 20thDay
25%22%29%17%7%
180
21st to endDay
27%16%32%19%6%
198
January1st to 10thNight
20%18%41%19%3
760
11th to 20thNight
23%16%31%25%4
760
21st to endNight
21%17%35%25%2
836
February1st to 10thNight
19%17%33%28%3
760
11th to 20thNight
26%21%34%17%1
743
21st to endNight
26%17%35%20%3
628
March1st to 10thNight
20%17%38%23%3
760
11th to 20thNight
24%17%36%19%41
760
21st to endNight
18%17%39%21%41
801
April1st to 10thNight
20%22%32%22%5%
720
11th to 20thNight
17%16%33%27%7%
711
21st to endNight
23%12%31%27%8%
720
May1st to 10thNight
21%10%30%27%11%
609
11th to 20thNight
20%7%29%34%10%
413
21st to endNight
17%423%39%15%1
389
June1st to 10thNight
12%429%36%20%1
360
11th to 20thNight
13%6%25%40%16%
360
21st to endNight
10%424%46%15%1
360
July1st to 10thNight
8%319%42%25%2
350
11th to 20thNight
6%317%42%30%2
410
21st to endNight
9%319%42%25%3
591
August1st to 10thNight
10%212%43%30%2
537
11th to 20thNight
13%413%40%27%3
618
21st to endNight
12%318%35%27%4
792
September1st to 10thNight
17%320%30%26%4
720
11th to 20thNight
12%417%37%26%5%
720
21st to endNight
21%424%31%18%1
720
October1st to 10thNight
21%7%20%29%19%4
720
11th to 20thNight
17%11%26%32%14%1
720
21st to endNight
19%13%31%26%11%
792
November1st to 10thNight
14%10%34%29%11%1
740
11th to 20thNight
19%14%32%28%7%1
760
21st to endNight
17%14%33%27%9%1
760
December1st to 10thNight
22%17%32%21%8%1
760
11th to 20thNight
18%13%37%27%5%
760
21st to endNight
20%15%30%31%3
836
Page generated at 2024-03-19 23:31:04. Version ac07

Copyright © 2024 Attilla Danko.


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