Ashburn Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ashburn Astronomical Observatory clear sky chart from 2005-11-05 to 2017-09-10.

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%
14763
14%
46820
8%
14729
40%
14729
18%
32504

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
14%
719
9%
2501
3%
716
22%
716
9%
1960
January16th to endNight
19%
768
10%
2496
4%
768
27%
768
11%
1926
February1st to 15thNight
14%
701
10%
2287
2%
701
24%
701
11%
1766
February16th to endNight
20%
631
12%
2021
4%
631
30%
631
14%
1470
March1st to 15thNight
21%
720
13%
1980
5%
720
28%
720
16%
1489
March16th to endNight
29%
768
20%
2112
9%
768
38%
768
25%
1413
April1st to 15thNight
25%
718
15%
1805
5%
718
31%
718
22%
1320
April16th to endNight
34%
579
19%
1659
8%
579
42%
579
26%
1089
May1st to 15thNight
45%
465
16%
1545
12%
465
51%
465
22%
1004
May16th to endNight
49%
374
17%
1497
17%
374
57%
374
26%
826
June1st to 15thNight
43%
360
11%
1440
12%
360
51%
360
19%
874
June16th to endNight
51%
360
11%
1440
15%
360
56%
360
24%
837
July1st to 15thNight
62%
340
15%
1360
21%
340
67%
340
33%
701
July16th to endNight
62%
456
16%
1492
17%
428
66%
428
30%
859
August1st to 15thNight
65%
539
16%
1609
12%
536
69%
536
36%
926
August16th to endNight
61%
636
22%
1782
13%
636
65%
636
36%
1067
September1st to 15thNight
50%
700
19%
1750
9%
700
54%
700
33%
1149
September16th to endNight
52%
657
20%
1838
14%
657
55%
657
30%
1170
October1st to 15thNight
39%
660
17%
1980
14%
660
45%
660
21%
1375
October16th to endNight
23%
704
11%
2112
5%
704
31%
704
15%
1582
November1st to 15thNight
32%
700
18%
2386
15%
700
39%
700
19%
1616
November16th to endNight
25%
720
10%
2520
6%
720
31%
720
11%
1889
December1st to 15thNight
13%
720
6%
2520
2%
720
17%
720
8%
2010
December16th to endNight
13%
768
6%
2688
1%
768
19%
768
7%
2186

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
15%8%36%6%35%45%7%32%
47806
Night
15%8%36%5%3445%6%35%
46997
Cloud CoverDay
15%8%36%5%35%5%5%7%33%
47816
Night
15%8%36%5%3445%6%35%
47002
TransparencyDay
48%7%13%18%9%1
47502
Night
48%7%13%19%10%1
46820
SeeingDay
17%8%28%26%13%5%
5337
Night
19%10%30%27%11%1
14768

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
19%13%5%8%5%34445%25%
1435
16th to endDay
14%14%7%10%7%5%5%5%45%20%
1728
February1st to 15thDay
14%14%6%10%8%45%44422%
1575
16th to endDay
18%12%5%8%7%2434427%
1448
March1st to 15thDay
16%10%38%7%35%446%28%
1969
16th to endDay
16%8%45%43444438%
2112
April1st to 15thDay
22%9%36%5%3345%6%29%
2143
16th to endDay
18%7%25%42435%5%39%
2311
May1st to 15thDay
15%7%36%6%3445%7%34%
2406
16th to endDay
13%6%25%5%35%5%5%7%40%
2618
June1st to 15thDay
16%6%25%5%35%5%6%7%34%
2520
16th to endDay
14%435%5%36%5%6%10%35%
2520
July1st to 15thDay
9%5%25%5%35%5%7%9%41%
2520
16th to endDay
9%5%35%6%36%6%7%9%35%
2571
August1st to 15thDay
9%5%25%6%45%6%6%8%38%
2340
16th to endDay
12%5%245%35%5%6%8%39%
2408
September1st to 15thDay
15%5%245%35%5%6%6%38%
2098
16th to endDay
14%6%35%5%34447%40%
1788
October1st to 15thDay
20%7%36%7%345%45%32%
1650
16th to endDay
25%11%37%6%34445%24%
1760
November1st to 15thDay
16%9%36%6%3435%7%31%
1470
16th to endDay
19%12%5%8%6%25%5%5%6%23%
1440
December1st to 15thDay
19%14%5%9%7%25%5%5%7%18%
1440
16th to endDay
20%14%6%8%7%34446%17%
1536
January1st to 15thNight
20%13%5%7%6%35%336%22%
2501
16th to endNight
15%12%6%8%7%36%445%23%
2496
February1st to 15thNight
14%14%6%8%7%35%45%5%25%
2285
16th to endNight
15%11%48%6%35%446%30%
2021
March1st to 15thNight
17%10%47%7%3433432%
1968
16th to endNight
14%7%35%434335%43%
2112
April1st to 15thNight
19%7%346%24446%35%
1805
16th to endNight
12%8%35%43445%6%43%
1659
May1st to 15thNight
16%426%5%346%46%39%
1545
16th to endNight
12%414324448%48%
1495
June1st to 15thNight
12%425%5%345%5%8%40%
1440
16th to endNight
10%5%25%5%2446%8%43%
1440
July1st to 15thNight
5%41332445%9%54%
1440
16th to endNight
8%314335%46%9%48%
1594
August1st to 15thNight
7%42342446%9%49%
1620
16th to endNight
6%42443445%6%52%
1782
September1st to 15thNight
14%42442445%7%46%
1750
16th to endNight
13%5%245%2335%6%46%
1838
October1st to 15thNight
18%6%35%5%24346%38%
1980
16th to endNight
21%9%45%5%35%45%6%28%
2112
November1st to 15thNight
17%7%245%3445%8%35%
2386
16th to endNight
20%10%46%5%35%5%5%6%25%
2520
December1st to 15thNight
19%14%5%7%6%35%5%5%6%20%
2520
16th to endNight
21%13%5%9%6%36%45%6%18%
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
19%13%5%9%6%33346%25%
1435
16th to endDay
14%14%6%10%7%46%5%5%5%20%
1728
February1st to 15thDay
14%14%6%9%8%45%446%21%
1575
16th to endDay
17%11%5%8%6%35%34428%
1448
March1st to 15thDay
16%10%38%7%44446%28%
1980
16th to endDay
16%8%36%434445%38%
2111
April1st to 15thDay
21%9%35%5%34446%30%
2143
16th to endDay
18%7%24424346%39%
2311
May1st to 15thDay
15%6%35%6%35%45%7%34%
2406
16th to endDay
12%6%25%5%35%5%5%7%39%
2618
June1st to 15thDay
16%6%25%435%5%6%8%34%
2520
16th to endDay
13%5%25%5%36%5%7%9%36%
2520
July1st to 15thDay
9%3245%35%5%6%9%42%
2520
16th to endDay
9%5%25%6%37%6%6%9%36%
2571
August1st to 15thDay
9%5%25%6%35%6%6%9%39%
2340
16th to endDay
11%5%244346%6%9%40%
2408
September1st to 15thDay
15%425%445%5%5%7%39%
2098
16th to endDay
14%5%35%5%25%5%5%7%40%
1788
October1st to 15thDay
20%6%36%6%35%5%45%33%
1650
16th to endDay
24%10%36%6%45%345%24%
1760
November1st to 15thDay
16%8%47%6%3445%8%31%
1470
16th to endDay
18%12%48%6%345%5%6%24%
1440
December1st to 15thDay
19%13%49%6%35%5%5%7%18%
1440
16th to endDay
20%13%6%9%6%35%447%18%
1536
January1st to 15thNight
21%13%5%8%5%25%446%22%
2501
16th to endNight
15%12%6%9%6%36%45%5%23%
2496
February1st to 15thNight
13%14%6%8%7%35%445%26%
2278
16th to endNight
15%10%48%6%34445%31%
2021
March1st to 15thNight
19%9%47%6%35%33432%
1980
16th to endNight
15%6%35%424336%43%
2112
April1st to 15thNight
20%6%35%5%24446%35%
1805
16th to endNight
12%7%344245%46%43%
1659
May1st to 15thNight
16%425%6%345%5%6%39%
1545
16th to endNight
12%41342346%9%47%
1495
June1st to 15thNight
12%426%5%35%5%6%8%40%
1440
16th to endNight
10%5%35%4345%5%9%43%
1440
July1st to 15thNight
5%32332445%8%54%
1440
16th to endNight
8%3134345%6%9%48%
1594
August1st to 15thNight
7%4133345%6%9%50%
1620
16th to endNight
7%32443445%6%52%
1782
September1st to 15thNight
13%424334447%46%
1750
16th to endNight
13%5%3442336%5%46%
1838
October1st to 15thNight
18%7%25%433446%38%
1980
16th to endNight
22%7%35%5%35%45%6%28%
2112
November1st to 15thNight
17%7%25%5%34448%35%
2386
16th to endNight
21%9%36%5%35%5%6%6%25%
2520
December1st to 15thNight
20%14%5%6%5%35%5%5%7%20%
2520
16th to endNight
21%13%48%6%45%45%6%18%
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
60%6%13%12%5%
1436
16th to endDay
64%7%10%11%5%
1728
February1st to 15thDay
64%6%10%13%3
1577
16th to endDay
58%6%12%14%7%
1448
March1st to 15thDay
55%5%13%16%7%
1980
16th to endDay
47%5%10%17%16%1
2112
April1st to 15thDay
53%5%11%17%10%
2143
16th to endDay
45%411%21%14%2
2311
May1st to 15thDay
46%6%12%19%13%1
2406
16th to endDay
41%7%14%21%12%2
2622
June1st to 15thDay
44%8%13%21%12%
2520
16th to endDay
40%9%15%20%12%
2520
July1st to 15thDay
35%7%15%24%15%1
2371
16th to endDay
40%9%15%22%10%1
2411
August1st to 15thDay
38%10%16%21%12%1
2327
16th to endDay
38%7%16%25%10%1
2408
September1st to 15thDay
40%8%15%24%9%
2098
16th to endDay
41%7%12%27%9%
1788
October1st to 15thDay
51%6%12%19%8%1
1650
16th to endDay
59%7%15%11%4
1760
November1st to 15thDay
50%7%15%16%7%1
1470
16th to endDay
59%7%14%12%5%
1440
December1st to 15thDay
62%10%15%9%1
1440
16th to endDay
66%7%12%11%2
1536
January1st to 15thNight
62%6%11%11%6%
2501
16th to endNight
61%5%12%14%4
2496
February1st to 15thNight
59%7%10%16%5%
2287
16th to endNight
53%6%14%17%6%
2021
March1st to 15thNight
56%412%18%6%
1980
16th to endNight
44%411%22%16%2
2112
April1st to 15thNight
49%5%10%18%15%
1805
16th to endNight
40%6%12%19%18%2
1659
May1st to 15thNight
42%6%11%20%15%2
1545
16th to endNight
33%7%14%25%16%2
1497
June1st to 15thNight
40%8%11%23%15%1
1440
16th to endNight
37%5%15%23%16%1
1440
July1st to 15thNight
25%8%14%27%23%1
1360
16th to endNight
31%8%12%28%16%2
1492
August1st to 15thNight
28%7%20%27%13%1
1609
16th to endNight
31%6%14%30%15%1
1782
September1st to 15thNight
36%7%15%26%13%
1750
16th to endNight
36%6%12%30%13%
1838
October1st to 15thNight
45%7%11%21%12%
1980
16th to endNight
54%7%15%14%7%
2112
November1st to 15thNight
47%6%14%18%12%1
2386
16th to endNight
56%8%15%13%5%
2520
December1st to 15thNight
62%8%14%11%3
2520
16th to endNight
65%8%12%9%3
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
18%27%31%14%8%
61
16th to endDay
34%17%33%12%2
96
February1st to 15thDay
22%27%38%11%
90
16th to endDay
23%20%38%15%1
104
March1st to 15thDay
24%14%34%21%4
270
16th to endDay
23%9%37%21%6%2
288
April1st to 15thDay
21%11%37%20%8%1
270
16th to endDay
20%11%33%17%14%2
270
May1st to 15thDay
20%7%25%28%11%6%
288
16th to endDay
14%8%29%29%14%3
331
June1st to 15thDay
18%7%29%31%11%1
315
16th to endDay
14%425%36%14%3
315
July1st to 15thDay
12%316%36%21%8%
305
16th to endDay
8%426%34%17%8%
326
August1st to 15thDay
10%320%31%21%12%
313
16th to endDay
15%316%26%24%13%
332
September1st to 15thDay
12%421%27%22%12%
294
16th to endDay
17%419%30%18%9%
254
October1st to 15thDay
17%7%32%22%16%3
255
16th to endDay
25%15%30%24%4
272
November1st to 15thDay
17%7%30%32%9%1
104
16th to endDay
21%20%28%23%6%
60
December1st to 15thDay
20%20%35%18%5%1
60
16th to endDay
28%23%39%9%
64
January1st to 15thNight
26%20%32%16%4
720
16th to endNight
20%12%37%23%5%
768
February1st to 15thNight
21%15%40%18%3
704
16th to endNight
19%13%36%23%6%
631
March1st to 15thNight
24%14%32%24%3
720
16th to endNight
18%10%37%23%8%1
768
April1st to 15thNight
21%14%33%25%4
718
16th to endNight
14%10%35%28%9%1
579
May1st to 15thNight
18%6%22%36%14%1
465
16th to endNight
13%327%34%19%1
375
June1st to 15thNight
12%9%25%36%15%
360
16th to endNight
11%324%41%18%
360
July1st to 15thNight
7%221%45%22%
340
16th to endNight
8%122%42%22%2
456
August1st to 15thNight
8%116%42%27%4
539
16th to endNight
8%221%37%25%3
636
September1st to 15thNight
16%5%23%29%21%4
700
16th to endNight
15%6%20%31%21%4
657
October1st to 15thNight
22%7%26%27%14%1
660
16th to endNight
25%11%31%24%5%
704
November1st to 15thNight
20%10%31%25%10%1
700
16th to endNight
24%13%28%23%8%
720
December1st to 15thNight
30%14%36%15%2
720
16th to endNight
26%14%36%19%2
768
Page generated at 2017-09-11 19:35:21. Version ac04

Copyright 2017 A. Danko


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