USNO
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the USNO clear sky chart from 2005-11-05 to 2018-02-14.

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%
15847
21%
48836
10%
15808
45%
15808
23%
30288

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
20%
777
22%
2622
9%
776
37%
776
18%
1603
January16th to endNight
20%
832
24%
2704
8%
832
39%
832
20%
1642
February1st to 15thNight
20%
759
19%
2468
6%
757
37%
757
18%
1519
February16th to endNight
16%
631
24%
1913
5%
631
39%
631
20%
1200
March1st to 15thNight
20%
720
22%
2007
8%
720
34%
720
22%
1273
March16th to endNight
19%
768
21%
2085
7%
768
31%
768
21%
1394
April1st to 15thNight
22%
718
19%
1796
5%
718
30%
718
23%
1179
April16th to endNight
31%
720
20%
1800
5%
720
39%
720
25%
1195
May1st to 15thNight
28%
564
17%
1644
7%
564
36%
564
19%
1102
May16th to endNight
43%
374
17%
1497
12%
374
53%
374
20%
857
June1st to 15thNight
43%
360
19%
1440
16%
360
53%
360
22%
811
June16th to endNight
44%
360
17%
1440
13%
360
54%
360
23%
819
July1st to 15thNight
50%
340
13%
1360
13%
340
54%
340
18%
853
July16th to endNight
61%
547
16%
1578
12%
514
69%
514
35%
899
August1st to 15thNight
59%
539
19%
1609
14%
536
61%
536
29%
970
August16th to endNight
56%
761
19%
1907
11%
761
58%
761
31%
1266
September1st to 15thNight
51%
720
25%
1800
14%
720
56%
720
32%
1192
September16th to endNight
51%
717
24%
1974
18%
717
57%
717
27%
1295
October1st to 15thNight
42%
720
23%
2160
19%
720
51%
720
25%
1379
October16th to endNight
34%
768
31%
2304
16%
768
53%
768
31%
1324
November1st to 15thNight
32%
760
24%
2418
14%
760
47%
760
25%
1445
November16th to endNight
28%
780
24%
2668
12%
780
45%
780
22%
1531
December1st to 15thNight
24%
780
18%
2730
6%
780
39%
780
15%
1720
December16th to endNight
18%
832
22%
2912
7%
832
36%
832
16%
1820

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%5%25%435%5%5%7%41%
49233
Night
14%5%2442445%7%43%
49018
Cloud CoverDay
13%5%24435%5%5%7%41%
49243
Night
14%5%2442445%7%44%
49022
TransparencyDay
40%5%11%23%16%2
48933
Night
38%411%23%17%3
48836
SeeingDay
16%16%23%23%14%5%
6134
Night
15%21%24%25%11%1
15852

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%4244245%46%42%
1643
16th to endDay
13%7%25%5%24347%42%
1872
February1st to 15thDay
15%6%25%6%245%5%7%39%
1695
16th to endDay
13%7%25%5%25%446%41%
1560
March1st to 15thDay
15%5%25%5%24345%45%
1939
16th to endDay
16%6%35%5%25%336%41%
2139
April1st to 15thDay
13%5%25%42435%6%45%
2152
16th to endDay
17%42442335%5%44%
2160
May1st to 15thDay
16%6%25%5%345%6%8%35%
2316
16th to endDay
14%5%245%35%5%5%8%38%
2618
June1st to 15thDay
11%4245%25%6%6%9%41%
2520
16th to endDay
9%4245%35%5%6%10%40%
2520
July1st to 15thDay
9%425%5%36%6%7%10%37%
2520
16th to endDay
7%4145%47%7%8%11%37%
2481
August1st to 15thDay
11%6%26%6%36%6%7%9%33%
2331
16th to endDay
11%4145%46%6%7%8%39%
2292
September1st to 15thDay
13%5%24435%46%7%43%
2160
16th to endDay
14%6%25%4245%6%7%41%
1983
October1st to 15thDay
18%423323446%45%
1800
16th to endDay
14%42432445%5%49%
1920
November1st to 15thDay
14%6%2332345%7%46%
1770
16th to endDay
15%5%23324446%48%
1618
December1st to 15thDay
15%5%144245%5%7%43%
1560
16th to endDay
18%6%25%324446%40%
1664
January1st to 15thNight
17%5%24414347%43%
2622
16th to endNight
16%6%24324446%44%
2704
February1st to 15thNight
15%6%145%2445%6%44%
2466
16th to endNight
14%6%25%324346%45%
1913
March1st to 15thNight
16%425%32435%7%44%
1995
16th to endNight
15%6%35%5%2345%6%41%
2085
April1st to 15thNight
14%5%244245%5%6%44%
1796
16th to endNight
16%424325%45%8%42%
1800
May1st to 15thNight
18%7%34424447%39%
1644
16th to endNight
12%5%234245%7%10%41%
1495
June1st to 15thNight
10%424325%5%7%9%44%
1440
16th to endNight
8%415%6%245%6%10%42%
1440
July1st to 15thNight
7%436%7%36%6%7%8%36%
1440
16th to endNight
6%313325%7%9%12%44%
1685
August1st to 15thNight
9%325%5%2447%8%44%
1620
16th to endNight
9%5%245%35%5%5%9%42%
1907
September1st to 15thNight
12%5%24425%45%6%45%
1800
16th to endNight
12%6%245%345%5%7%42%
1974
October1st to 15thNight
18%5%24423345%44%
2160
16th to endNight
13%323323346%52%
2304
November1st to 15thNight
14%5%24423335%50%
2418
16th to endNight
15%41342345%7%47%
2668
December1st to 15thNight
16%5%25%5%3445%6%38%
2730
16th to endNight
16%5%25%5%2445%6%41%
2912

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
16%5%244245%46%42%
1643
16th to endDay
13%7%25%425%347%42%
1872
February1st to 15thDay
15%6%25%6%2445%6%39%
1695
16th to endDay
13%7%25%5%25%436%41%
1560
March1st to 15thDay
15%435%5%24345%45%
1950
16th to endDay
16%6%35%425%346%41%
2138
April1st to 15thDay
13%5%2443435%6%45%
2152
16th to endDay
17%41443325%5%44%
2160
May1st to 15thDay
16%5%35%4345%6%8%35%
2316
16th to endDay
14%5%245%35%5%5%8%38%
2618
June1st to 15thDay
11%424425%5%6%9%41%
2520
16th to endDay
9%3245%26%5%7%10%41%
2520
July1st to 15thDay
9%425%6%35%6%7%10%37%
2520
16th to endDay
7%4246%47%7%8%11%36%
2481
August1st to 15thDay
11%6%26%6%36%6%6%9%34%
2331
16th to endDay
11%4246%46%6%7%8%38%
2292
September1st to 15thDay
13%424425%5%6%7%43%
2160
16th to endDay
14%5%35%4345%6%7%41%
1983
October1st to 15thDay
18%424223446%45%
1800
16th to endDay
14%414424445%49%
1920
November1st to 15thDay
14%5%2332345%7%46%
1770
16th to endDay
15%424323446%48%
1618
December1st to 15thDay
15%5%15%42445%7%43%
1560
16th to endDay
17%6%25%424446%40%
1664
January1st to 15thNight
17%5%24314347%43%
2622
16th to endNight
16%6%24324446%44%
2704
February1st to 15thNight
14%6%145%2445%5%44%
2459
16th to endNight
14%6%24424346%45%
1913
March1st to 15thNight
16%42432435%7%44%
2007
16th to endNight
15%6%35%5%2345%6%41%
2085
April1st to 15thNight
14%5%244345%5%6%43%
1796
16th to endNight
16%42432445%8%43%
1800
May1st to 15thNight
18%6%24424447%39%
1643
16th to endNight
12%5%233245%7%9%42%
1495
June1st to 15thNight
10%423325%5%7%9%45%
1440
16th to endNight
8%4246%345%6%10%42%
1440
July1st to 15thNight
8%435%7%47%6%6%8%36%
1440
16th to endNight
6%313325%6%9%12%44%
1685
August1st to 15thNight
9%325%6%3447%8%44%
1620
16th to endNight
10%5%245%35%5%6%8%42%
1907
September1st to 15thNight
12%5%25%42445%6%45%
1800
16th to endNight
12%5%245%345%5%7%43%
1974
October1st to 15thNight
18%5%14423445%45%
2160
16th to endNight
13%32332335%6%52%
2304
November1st to 15thNight
14%5%24422335%50%
2418
16th to endNight
15%4134235%5%7%47%
2668
December1st to 15thNight
16%5%25%5%3446%6%39%
2730
16th to endNight
16%5%25%424446%41%
2912

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%413%22%13%4
1644
16th to endDay
41%312%25%14%3
1872
February1st to 15thDay
43%411%24%13%2
1697
16th to endDay
43%311%25%13%1
1560
March1st to 15thDay
41%311%24%15%3
1950
16th to endDay
44%310%20%17%3
2139
April1st to 15thDay
39%410%23%18%3
2152
16th to endDay
40%49%18%20%6%
2160
May1st to 15thDay
44%6%12%19%15%2
2316
16th to endDay
41%6%14%23%12%2
2622
June1st to 15thDay
37%8%12%26%14%1
2520
16th to endDay
35%8%13%22%17%2
2520
July1st to 15thDay
39%7%14%20%15%2
2371
16th to endDay
35%9%14%23%14%2
2325
August1st to 15thDay
43%7%12%20%15%1
2318
16th to endDay
38%7%12%22%15%2
2292
September1st to 15thDay
37%5%12%20%21%2
2160
16th to endDay
40%6%7%24%19%2
1983
October1st to 15thDay
40%49%24%17%3
1800
16th to endDay
35%38%29%22%1
1920
November1st to 15thDay
36%411%24%16%5%
1770
16th to endDay
37%39%28%17%2
1618
December1st to 15thDay
40%414%25%12%3
1560
16th to endDay
44%411%25%12%1
1664
January1st to 15thNight
40%310%20%18%5%
2622
16th to endNight
40%211%26%17%1
2704
February1st to 15thNight
40%412%24%15%2
2468
16th to endNight
40%39%25%18%2
1913
March1st to 15thNight
39%412%26%14%2
2007
16th to endNight
42%411%21%16%3
2085
April1st to 15thNight
38%5%10%20%21%3
1796
16th to endNight
38%410%21%17%7%
1800
May1st to 15thNight
44%411%21%13%3
1644
16th to endNight
35%7%18%20%14%2
1497
June1st to 15thNight
33%7%13%27%15%1
1440
16th to endNight
35%6%15%23%17%2
1440
July1st to 15thNight
43%6%15%17%12%4
1360
16th to endNight
26%9%19%25%17%1
1578
August1st to 15thNight
34%5%15%23%18%1
1609
16th to endNight
37%7%11%24%17%2
1907
September1st to 15thNight
38%5%9%22%20%4
1800
16th to endNight
39%5%7%22%20%3
1974
October1st to 15thNight
41%48%24%17%3
2160
16th to endNight
32%38%30%21%3
2304
November1st to 15thNight
37%39%23%19%6%
2418
16th to endNight
35%311%25%20%3
2668
December1st to 15thNight
44%412%22%13%2
2730
16th to endNight
42%5%12%24%12%1
2912

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
22%23%34%15%3
111
16th to endDay
18%27%30%19%3
144
February1st to 15thDay
29%20%42%6%2
130
16th to endDay
15%27%36%17%2
219
March1st to 15thDay
19%25%28%21%5%
270
16th to endDay
18%23%32%17%5%2
288
April1st to 15thDay
15%26%26%20%9%1
270
16th to endDay
19%20%22%21%10%6%
270
May1st to 15thDay
25%26%20%15%9%2
288
16th to endDay
18%13%17%29%17%2
331
June1st to 15thDay
13%15%21%25%17%6%
315
16th to endDay
10%14%28%27%14%3
315
July1st to 15thDay
11%8%20%32%20%6%
305
16th to endDay
8%10%13%31%27%8%
326
August1st to 15thDay
15%7%14%29%23%9%
313
16th to endDay
10%5%11%32%24%15%
332
September1st to 15thDay
13%10%20%23%17%15%
315
16th to endDay
20%8%14%27%15%13%
314
October1st to 15thDay
22%8%19%21%22%5%
315
16th to endDay
12%16%28%25%14%2
336
November1st to 15thDay
16%18%25%21%14%3
293
16th to endDay
12%21%35%19%8%1
148
December1st to 15thDay
18%24%24%16%12%3
90
16th to endDay
27%20%32%16%3
96
January1st to 15thNight
17%28%29%19%4
780
16th to endNight
18%27%30%21%1
832
February1st to 15thNight
17%27%29%23%1
760
16th to endNight
17%31%31%18%1
631
March1st to 15thNight
17%27%32%19%3
720
16th to endNight
15%27%33%17%41
768
April1st to 15thNight
14%30%29%20%3
718
16th to endNight
16%25%23%24%9%
720
May1st to 15thNight
19%24%23%22%9%
564
16th to endNight
13%20%16%32%15%1
375
June1st to 15thNight
9%19%22%31%16%
360
16th to endNight
7%20%20%38%12%
360
July1st to 15thNight
7%14%19%35%21%
340
16th to endNight
6%16%11%32%28%3
547
August1st to 15thNight
11%10%9%40%25%1
539
16th to endNight
12%9%12%33%27%3
761
September1st to 15thNight
11%10%19%28%26%3
720
16th to endNight
15%9%16%28%26%3
717
October1st to 15thNight
19%13%17%26%20%2
720
16th to endNight
14%18%30%22%13%
768
November1st to 15thNight
14%21%26%24%11%1
760
16th to endNight
14%27%27%22%7%
780
December1st to 15thNight
18%25%26%21%7%
780
16th to endNight
18%27%31%20%3
832
Page generated at 2018-02-15 10:58:40. Version ac04

Copyright 2018 A. Danko


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