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 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
33%
15103
21%
46534
10%
15064
45%
15064
22%
28904

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%
717
22%
2419
9%
716
37%
716
17%
1501
January16th to endNight
19%
768
23%
2496
7%
768
39%
768
19%
1521
February1st to 15thNight
20%
703
19%
2287
6%
701
38%
701
18%
1389
February16th to endNight
16%
631
24%
1913
5%
631
39%
631
20%
1200
March1st to 15thNight
20%
720
22%
2007
8%
720
35%
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
37%
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
54%
360
22%
811
June16th to endNight
45%
360
17%
1440
13%
360
55%
360
23%
819
July1st to 15thNight
50%
340
13%
1360
13%
340
55%
340
18%
853
July16th to endNight
61%
547
16%
1578
12%
514
69%
514
35%
899
August1st to 15thNight
60%
527
19%
1575
14%
524
62%
524
29%
945
August16th to endNight
55%
697
18%
1747
10%
697
57%
697
31%
1160
September1st to 15thNight
53%
660
26%
1650
15%
660
57%
660
33%
1088
September16th to endNight
48%
657
22%
1810
15%
657
54%
657
25%
1212
October1st to 15thNight
41%
660
25%
1980
20%
660
50%
660
25%
1261
October16th to endNight
33%
704
29%
2112
14%
704
52%
704
29%
1224
November1st to 15thNight
32%
700
25%
2227
14%
700
47%
700
25%
1319
November16th to endNight
26%
720
22%
2463
10%
720
43%
720
20%
1443
December1st to 15thNight
23%
720
18%
2520
7%
720
38%
720
15%
1573
December16th to endNight
17%
768
21%
2688
6%
768
36%
768
15%
1686

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%
47433
Night
14%5%2442445%7%43%
46716
Cloud CoverDay
13%5%24435%5%5%7%41%
47443
Night
14%5%2442445%7%43%
46720
TransparencyDay
40%5%11%23%16%2
47133
Night
38%411%23%17%3
46534
SeeingDay
16%16%22%23%14%5%
5540
Night
15%22%24%24%11%1
15108

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
17%5%344145%46%42%
1516
16th to endDay
14%7%24424347%41%
1728
February1st to 15thDay
14%6%25%6%245%5%7%40%
1575
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
10%6%26%6%36%6%7%9%34%
2280
16th to endDay
11%415%5%46%5%7%8%39%
2100
September1st to 15thDay
12%5%2432446%7%44%
1980
16th to endDay
15%6%25%4245%6%7%39%
1818
October1st to 15thDay
18%424323446%45%
1650
16th to endDay
14%425%314445%48%
1760
November1st to 15thDay
14%5%2222345%7%47%
1631
16th to endDay
16%5%24324446%45%
1494
December1st to 15thDay
15%4144245%5%7%43%
1440
16th to endDay
18%6%25%324446%40%
1536
January1st to 15thNight
17%5%24414348%42%
2419
16th to endNight
16%6%24324446%43%
2496
February1st to 15thNight
14%6%1442445%5%45%
2285
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
8%325%5%2447%8%44%
1586
16th to endNight
10%5%245%3445%9%42%
1747
September1st to 15thNight
11%5%24435%46%7%45%
1650
16th to endNight
13%6%345%345%5%7%40%
1810
October1st to 15thNight
18%5%24423345%45%
1980
16th to endNight
14%323323346%52%
2112
November1st to 15thNight
14%424422335%51%
2227
16th to endNight
15%4144235%5%7%45%
2463
December1st to 15thNight
16%5%25%5%2445%6%39%
2520
16th to endNight
17%5%245%2445%6%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%244145%46%42%
1516
16th to endDay
14%7%34425%347%42%
1728
February1st to 15thDay
14%6%25%5%245%5%7%40%
1575
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
10%5%26%6%37%6%6%9%34%
2280
16th to endDay
12%4246%46%5%7%8%39%
2100
September1st to 15thDay
12%424325%5%6%7%43%
1980
16th to endDay
15%5%35%4345%6%7%39%
1818
October1st to 15thDay
18%424223446%45%
1650
16th to endDay
15%415%413445%48%
1760
November1st to 15thDay
14%5%2232345%7%47%
1631
16th to endDay
16%5%24323446%45%
1494
December1st to 15thDay
15%415%32445%7%43%
1440
16th to endDay
18%6%25%424446%40%
1536
January1st to 15thNight
17%5%24424347%42%
2419
16th to endNight
16%6%24324446%43%
2496
February1st to 15thNight
14%6%145%2445%5%45%
2278
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
8%325%6%3447%8%44%
1586
16th to endNight
10%5%245%3446%8%42%
1747
September1st to 15thNight
11%5%24435%46%6%45%
1650
16th to endNight
13%6%345%345%5%7%41%
1810
October1st to 15thNight
18%5%14413445%45%
1980
16th to endNight
14%32232335%6%52%
2112
November1st to 15thNight
14%5%24423335%51%
2227
16th to endNight
16%4144235%5%7%45%
2463
December1st to 15thNight
16%5%25%5%2446%6%39%
2520
16th to endNight
17%5%24424446%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%413%22%13%3
1517
16th to endDay
41%312%25%13%3
1728
February1st to 15thDay
42%411%25%13%2
1577
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
42%7%12%20%16%1
2267
16th to endDay
39%7%11%22%16%2
2100
September1st to 15thDay
36%5%13%19%21%2
1980
16th to endDay
41%6%7%22%19%1
1818
October1st to 15thDay
39%49%25%17%3
1650
16th to endDay
37%38%28%20%1
1760
November1st to 15thDay
35%411%24%18%5%
1631
16th to endDay
39%310%27%16%2
1494
December1st to 15thDay
39%415%25%11%3
1440
16th to endDay
44%410%25%13%1
1536
January1st to 15thNight
41%411%20%17%4
2419
16th to endNight
40%211%26%16%1
2496
February1st to 15thNight
39%312%24%16%2
2287
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
1575
16th to endNight
37%6%11%23%17%3
1747
September1st to 15thNight
37%5%10%21%20%4
1650
16th to endNight
41%5%7%20%19%4
1810
October1st to 15thNight
41%47%25%17%3
1980
16th to endNight
32%39%29%21%3
2112
November1st to 15thNight
37%38%22%20%7%
2227
16th to endNight
37%311%24%18%4
2463
December1st to 15thNight
43%412%22%14%2
2520
16th to endNight
42%5%12%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
25%24%36%10%11
74
16th to endDay
21%28%31%14%4
96
February1st to 15thDay
26%22%40%7%3
90
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
13%7%14%29%24%9%
297
16th to endDay
9%6%9%32%24%18%
268
September1st to 15thDay
10%10%16%24%19%18%
255
16th to endDay
23%8%15%25%13%13%
254
October1st to 15thDay
21%10%20%21%21%3
255
16th to endDay
13%18%27%25%13%1
272
November1st to 15thDay
15%20%25%19%15%4
244
16th to endDay
16%21%30%21%9%
114
December1st to 15thDay
20%25%25%13%13%3
60
16th to endDay
34%18%28%15%3
64
January1st to 15thNight
17%30%28%19%4
720
16th to endNight
17%27%31%20%1
768
February1st to 15thNight
17%29%28%23%2
704
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
10%10%9%40%25%1
527
16th to endNight
12%10%12%32%27%4
697
September1st to 15thNight
10%11%18%27%28%3
660
16th to endNight
16%10%16%28%25%2
657
October1st to 15thNight
19%14%18%25%20%2
660
16th to endNight
14%19%29%23%12%
704
November1st to 15thNight
13%22%26%24%11%2
700
16th to endNight
14%29%26%21%7%
720
December1st to 15thNight
18%26%26%20%7%
720
16th to endNight
19%28%30%18%3
768
Page generated at 2017-08-13 14:16:24. Version ac04

Copyright 2017 A. Danko


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