Smithsonian NASM Public Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Smithsonian NASM Public Observatory clear sky chart from 2005-11-05 to 2017-09-21.

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%
15264
21%
46935
11%
15225
49%
15225
25%
29269

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
24%
717
23%
2422
10%
716
41%
716
20%
1493
January16th to endNight
22%
768
23%
2496
8%
768
41%
768
22%
1540
February1st to 15thNight
26%
703
20%
2287
7%
701
43%
701
21%
1395
February16th to endNight
19%
631
24%
1910
6%
631
41%
631
23%
1194
March1st to 15thNight
26%
720
22%
2010
9%
720
40%
720
25%
1278
March16th to endNight
23%
768
21%
2084
8%
768
34%
768
24%
1392
April1st to 15thNight
26%
718
20%
1796
5%
718
35%
718
27%
1180
April16th to endNight
37%
720
20%
1800
7%
720
42%
720
30%
1192
May1st to 15thNight
36%
565
18%
1645
8%
565
43%
565
23%
1106
May16th to endNight
54%
374
17%
1497
15%
374
60%
374
23%
878
June1st to 15thNight
52%
360
19%
1440
19%
360
61%
360
25%
818
June16th to endNight
51%
360
17%
1440
16%
360
59%
360
25%
824
July1st to 15thNight
56%
340
13%
1360
15%
340
59%
340
19%
858
July16th to endNight
68%
545
17%
1576
14%
512
74%
512
37%
897
August1st to 15thNight
64%
539
18%
1609
14%
536
65%
536
31%
987
August16th to endNight
58%
763
19%
1909
11%
763
60%
763
32%
1275
September1st to 15thNight
55%
720
25%
1800
14%
720
60%
720
34%
1192
September16th to endNight
54%
681
23%
1867
19%
681
57%
681
29%
1242
October1st to 15thNight
45%
660
25%
1980
21%
660
53%
660
27%
1269
October16th to endNight
36%
704
29%
2112
16%
704
52%
704
32%
1219
November1st to 15thNight
37%
700
25%
2227
15%
700
52%
700
28%
1317
November16th to endNight
33%
720
22%
2460
12%
720
47%
720
23%
1457
December1st to 15thNight
28%
720
19%
2520
8%
720
42%
720
19%
1578
December16th to endNight
23%
768
21%
2688
8%
768
40%
768
18%
1688

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%24435%45%7%41%
47916
Night
14%5%2442445%7%44%
47117
Cloud CoverDay
13%5%244345%5%7%41%
47926
Night
14%5%2442445%7%43%
47121
TransparencyDay
39%6%11%23%16%2
47616
Night
38%5%11%23%17%3
46935
SeeingDay
17%9%25%26%15%6%
5706
Night
17%10%28%29%13%1
15269

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%2442445%6%42%
1515
16th to endDay
14%7%245%2435%6%42%
1728
February1st to 15thDay
14%7%25%43445%7%40%
1575
16th to endDay
14%7%25%5%24436%42%
1562
March1st to 15thDay
15%6%15%5%24345%45%
1936
16th to endDay
16%7%35%5%24346%41%
2139
April1st to 15thDay
12%5%3442435%7%45%
2152
16th to endDay
18%4145%23345%45%
2160
May1st to 15thDay
16%5%25%5%345%6%8%36%
2313
16th to endDay
14%4245%36%6%6%7%38%
2618
June1st to 15thDay
12%324435%6%5%10%41%
2520
16th to endDay
10%3145%35%6%6%10%41%
2520
July1st to 15thDay
9%5%25%5%35%5%8%9%38%
2520
16th to endDay
7%3246%37%7%7%12%37%
2483
August1st to 15thDay
11%5%25%6%5%5%6%6%9%34%
2328
16th to endDay
11%4246%36%5%7%7%39%
2292
September1st to 15thDay
13%324325%35%8%44%
2160
16th to endDay
14%5%244345%5%7%39%
1883
October1st to 15thDay
18%42332435%6%45%
1650
16th to endDay
15%424423445%48%
1760
November1st to 15thDay
14%5%1322445%7%48%
1629
16th to endDay
16%5%2422345%6%46%
1497
December1st to 15thDay
15%424425%45%6%44%
1440
16th to endDay
19%6%25%424346%40%
1536
January1st to 15thNight
17%5%25%314347%42%
2422
16th to endNight
16%6%24424446%44%
2496
February1st to 15thNight
14%6%23424446%45%
2285
16th to endNight
14%5%24424346%45%
1910
March1st to 15thNight
16%424423348%44%
1998
16th to endNight
16%5%25%5%23446%41%
2084
April1st to 15thNight
14%5%244335%5%6%44%
1796
16th to endNight
17%41442445%7%43%
1800
May1st to 15thNight
18%6%35%42445%6%39%
1645
16th to endNight
12%5%143245%7%9%41%
1495
June1st to 15thNight
9%424326%5%7%8%45%
1440
16th to endNight
8%415%6%3447%9%43%
1440
July1st to 15thNight
7%5%26%7%37%6%6%10%36%
1440
16th to endNight
6%323326%6%8%12%44%
1683
August1st to 15thNight
9%325%5%245%6%9%43%
1620
16th to endNight
10%5%35%5%35%46%8%42%
1909
September1st to 15thNight
12%5%24425%45%6%45%
1800
16th to endNight
12%6%345%34447%42%
1867
October1st to 15thNight
18%5%23423345%45%
1980
16th to endNight
13%412323446%52%
2112
November1st to 15thNight
14%433422335%51%
2227
16th to endNight
16%4134235%5%7%45%
2460
December1st to 15thNight
16%6%25%5%2445%6%39%
2520
16th to endNight
17%5%244345%46%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
16%5%2442445%7%42%
1515
16th to endDay
14%6%345%2435%7%42%
1728
February1st to 15thDay
15%7%245%245%5%6%40%
1575
16th to endDay
14%7%25%6%23446%42%
1562
March1st to 15thDay
14%6%25%433346%44%
1947
16th to endDay
16%6%25%424436%41%
2138
April1st to 15thDay
13%5%34434447%45%
2152
16th to endDay
17%5%14424345%44%
2160
May1st to 15thDay
16%6%25%425%45%8%36%
2313
16th to endDay
15%4245%35%6%6%7%38%
2618
June1st to 15thDay
12%3245%246%7%8%42%
2520
16th to endDay
10%315%5%35%6%7%9%41%
2520
July1st to 15thDay
9%5%25%5%36%5%7%10%38%
2520
16th to endDay
7%325%447%6%8%12%37%
2483
August1st to 15thDay
11%6%35%6%46%6%7%8%34%
2328
16th to endDay
11%4245%45%5%7%9%38%
2292
September1st to 15thDay
13%415%42445%8%44%
2160
16th to endDay
15%6%24435%5%5%8%39%
1883
October1st to 15thDay
17%5%1342435%5%45%
1650
16th to endDay
15%4144225%45%49%
1760
November1st to 15thDay
14%5%1322335%7%48%
1629
16th to endDay
16%5%243235%46%46%
1497
December1st to 15thDay
15%324425%45%7%44%
1440
16th to endDay
19%6%25%424346%41%
1536
January1st to 15thNight
16%6%2432345%6%43%
2422
16th to endNight
16%6%24424446%44%
2496
February1st to 15thNight
14%6%24424446%45%
2278
16th to endNight
14%6%34323346%45%
1910
March1st to 15thNight
16%424424447%44%
2010
16th to endNight
16%5%36%5%2345%6%41%
2084
April1st to 15thNight
14%5%245%245%46%44%
1796
16th to endNight
16%5%234145%5%7%43%
1800
May1st to 15thNight
17%7%3442445%7%38%
1644
16th to endNight
12%5%143245%7%10%41%
1495
June1st to 15thNight
9%5%23425%5%7%10%44%
1440
16th to endNight
8%325%6%345%6%10%42%
1440
July1st to 15thNight
7%5%36%7%35%7%6%10%37%
1440
16th to endNight
6%213326%6%8%12%44%
1683
August1st to 15thNight
8%325%5%36%5%6%9%44%
1620
16th to endNight
9%5%25%5%35%5%5%9%42%
1909
September1st to 15thNight
11%5%25%425%5%5%7%44%
1800
16th to endNight
12%6%25%5%3445%7%42%
1867
October1st to 15thNight
17%6%24413345%45%
1980
16th to endNight
13%423323346%52%
2112
November1st to 15thNight
14%434422335%51%
2227
16th to endNight
15%42342346%6%45%
2460
December1st to 15thNight
16%5%25%42445%6%39%
2520
16th to endNight
17%5%25%424446%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%412%22%13%3
1516
16th to endDay
41%411%25%13%3
1728
February1st to 15thDay
42%410%25%14%2
1577
16th to endDay
42%411%26%14%1
1562
March1st to 15thDay
41%410%24%16%3
1947
16th to endDay
44%410%19%17%3
2139
April1st to 15thDay
38%49%24%18%4
2152
16th to endDay
39%410%18%20%6%
2160
May1st to 15thDay
43%7%10%20%15%2
2313
16th to endDay
40%7%13%23%12%2
2622
June1st to 15thDay
36%9%11%26%14%1
2520
16th to endDay
35%8%14%21%17%2
2520
July1st to 15thDay
38%8%13%20%15%3
2371
16th to endDay
35%9%14%23%15%2
2327
August1st to 15thDay
43%7%11%20%16%
2315
16th to endDay
39%8%12%21%15%2
2292
September1st to 15thDay
36%5%12%21%20%3
2160
16th to endDay
41%6%8%22%19%1
1883
October1st to 15thDay
39%5%9%25%17%3
1650
16th to endDay
36%38%27%20%2
1760
November1st to 15thDay
35%411%24%18%5%
1629
16th to endDay
38%49%26%18%2
1497
December1st to 15thDay
38%5%14%25%12%3
1440
16th to endDay
44%410%25%13%1
1536
January1st to 15thNight
41%410%20%18%4
2422
16th to endNight
40%410%26%17%1
2496
February1st to 15thNight
39%411%24%17%3
2287
16th to endNight
39%49%25%19%2
1910
March1st to 15thNight
38%411%26%14%2
2010
16th to endNight
42%5%10%21%16%3
2084
April1st to 15thNight
38%5%9%20%21%3
1796
16th to endNight
38%5%10%20%17%7%
1800
May1st to 15thNight
44%6%11%20%14%3
1645
16th to endNight
35%8%17%20%14%2
1497
June1st to 15thNight
33%8%12%27%16%1
1440
16th to endNight
34%7%15%21%17%2
1440
July1st to 15thNight
41%8%14%17%12%4
1360
16th to endNight
27%7%17%27%17%1
1576
August1st to 15thNight
34%7%14%23%18%1
1609
16th to endNight
37%7%11%24%16%3
1909
September1st to 15thNight
37%6%10%22%20%4
1800
16th to endNight
40%6%8%21%19%3
1867
October1st to 15thNight
41%47%24%18%3
1980
16th to endNight
32%49%28%22%3
2112
November1st to 15thNight
36%38%22%21%7%
2227
16th to endNight
36%5%10%23%19%4
2460
December1st to 15thNight
43%412%22%15%2
2520
16th to endNight
43%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
24%24%37%10%11
74
16th to endDay
21%28%31%14%4
96
February1st to 15thDay
27%21%37%10%3
90
16th to endDay
19%17%40%20%2
221
March1st to 15thDay
18%17%32%23%6%
270
16th to endDay
20%13%37%20%5%2
288
April1st to 15thDay
16%16%31%22%10%2
270
16th to endDay
20%10%26%25%10%5%
270
May1st to 15thDay
26%18%22%18%11%3
288
16th to endDay
19%422%30%18%4
331
June1st to 15thDay
14%8%22%28%16%9%
315
16th to endDay
11%428%33%16%4
315
July1st to 15thDay
14%120%35%21%5%
305
16th to endDay
8%115%34%30%10%
326
August1st to 15thDay
15%115%31%24%10%
313
16th to endDay
9%114%34%25%14%
332
September1st to 15thDay
13%5%20%25%17%17%
315
16th to endDay
21%216%28%16%14%
275
October1st to 15thDay
23%5%20%22%22%5%
255
16th to endDay
14%12%30%26%14%1
272
November1st to 15thDay
17%11%27%23%14%5%
244
16th to endDay
17%14%33%22%10%1
117
December1st to 15thDay
16%31%20%15%13%3
60
16th to endDay
37%14%29%15%3
64
January1st to 15thNight
20%16%34%23%5%
720
16th to endNight
19%16%37%24%1
768
February1st to 15thNight
20%15%33%28%2
704
16th to endNight
19%20%36%21%1
631
March1st to 15thNight
20%13%36%25%3
720
16th to endNight
18%14%40%20%5%1
768
April1st to 15thNight
16%16%36%24%4
718
16th to endNight
19%9%29%30%11%
720
May1st to 15thNight
21%8%28%30%11%
565
16th to endNight
13%5%20%39%19%1
375
June1st to 15thNight
10%228%36%20%1
360
16th to endNight
10%323%43%17%
360
July1st to 15thNight
9%221%40%25%
340
16th to endNight
7%115%41%31%3
545
August1st to 15thNight
11%111%42%29%1
539
16th to endNight
13%215%35%29%3
763
September1st to 15thNight
14%219%32%27%3
720
16th to endNight
15%318%33%25%4
681
October1st to 15thNight
20%6%20%27%21%3
660
16th to endNight
16%11%32%25%13%
704
November1st to 15thNight
15%10%30%28%12%2
700
16th to endNight
18%15%29%25%9%
720
December1st to 15thNight
20%11%33%25%8%
720
16th to endNight
22%13%35%25%3
768
Page generated at 2017-09-22 07:35:35. Version ac04

Copyright 2017 A. Danko


Smithsonian NASM Public Observatory Clear Sky Chart | Charts Home