Potomac Astrometric Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Potomac Astrometric Observatory clear sky chart from 2005-11-05 to 2017-09-16.

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
31%
15233
21%
46884
10%
15194
44%
15194
22%
29280

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
18%
717
22%
2417
8%
716
36%
716
15%
1529
January16th to endNight
16%
768
23%
2496
6%
768
37%
768
18%
1528
February1st to 15thNight
18%
703
19%
2287
5%
701
36%
701
16%
1403
February16th to endNight
15%
631
24%
1919
5%
631
37%
631
17%
1214
March1st to 15thNight
19%
720
21%
2003
7%
720
34%
720
20%
1294
March16th to endNight
19%
768
21%
2088
7%
768
31%
768
20%
1393
April1st to 15thNight
20%
718
19%
1796
4%
718
30%
718
23%
1193
April16th to endNight
30%
720
19%
1800
5%
720
38%
720
24%
1210
May1st to 15thNight
28%
558
18%
1638
7%
558
36%
558
19%
1096
May16th to endNight
42%
374
18%
1497
11%
374
54%
374
20%
861
June1st to 15thNight
42%
360
19%
1440
16%
360
52%
360
21%
813
June16th to endNight
45%
360
18%
1440
14%
360
56%
360
23%
806
July1st to 15thNight
50%
340
12%
1360
13%
340
53%
340
17%
855
July16th to endNight
59%
550
16%
1581
11%
517
68%
517
35%
898
August1st to 15thNight
56%
539
19%
1609
14%
536
59%
536
28%
1005
August16th to endNight
59%
754
20%
1900
12%
754
62%
754
33%
1238
September1st to 15thNight
49%
720
25%
1800
13%
720
54%
720
33%
1181
September16th to endNight
47%
661
22%
1819
15%
661
53%
661
26%
1211
October1st to 15thNight
40%
660
25%
1980
20%
660
49%
660
24%
1269
October16th to endNight
32%
704
28%
2112
13%
704
51%
704
29%
1224
November1st to 15thNight
30%
700
26%
2222
14%
700
46%
700
25%
1306
November16th to endNight
24%
720
22%
2472
9%
720
42%
720
19%
1459
December1st to 15thNight
21%
720
18%
2520
6%
720
37%
720
14%
1605
December16th to endNight
16%
768
22%
2688
6%
768
35%
768
14%
1689

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%25%4345%5%7%41%
47864
Night
14%5%2442445%7%43%
47066
Cloud CoverDay
14%5%245%3445%7%41%
47874
Night
14%5%2442445%7%43%
47070
TransparencyDay
40%5%12%23%16%2
47564
Night
39%411%23%17%3
46884
SeeingDay
17%16%22%22%14%5%
5668
Night
15%24%23%24%11%1
15238

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
15%6%25%425%45%7%39%
1519
16th to endDay
14%6%345%24447%41%
1728
February1st to 15thDay
13%6%25%6%35%5%6%6%38%
1575
16th to endDay
13%8%36%444445%41%
1553
March1st to 15thDay
16%5%25%424445%44%
1945
16th to endDay
16%6%35%5%23446%40%
2134
April1st to 15thDay
14%5%245%3335%7%43%
2152
16th to endDay
17%5%24423346%44%
2160
May1st to 15thDay
15%7%25%4245%6%8%36%
2322
16th to endDay
14%5%25%5%245%5%8%38%
2618
June1st to 15thDay
11%425%5%35%47%9%40%
2520
16th to endDay
9%4245%35%5%7%9%42%
2520
July1st to 15thDay
10%425%5%36%6%6%9%39%
2520
16th to endDay
7%315%6%37%7%8%11%37%
2477
August1st to 15thDay
11%6%26%5%46%7%5%8%35%
2337
16th to endDay
12%414435%5%6%8%41%
2292
September1st to 15thDay
14%4244245%6%7%44%
2160
16th to endDay
15%6%25%5%3345%7%39%
1827
October1st to 15thDay
20%41332335%5%45%
1650
16th to endDay
14%4245%34345%48%
1760
November1st to 15thDay
14%5%1332445%6%47%
1634
16th to endDay
17%5%23324445%44%
1485
December1st to 15thDay
15%4145%25%46%7%41%
1440
16th to endDay
18%6%25%42435%7%38%
1536
January1st to 15thNight
16%6%24433447%40%
2417
16th to endNight
16%6%2332435%6%43%
2496
February1st to 15thNight
14%5%25%5%3345%5%44%
2285
16th to endNight
14%6%25%5%24446%43%
1919
March1st to 15thNight
16%4243345%47%43%
1991
16th to endNight
15%6%26%42445%7%41%
2088
April1st to 15thNight
13%5%34435%45%7%42%
1796
16th to endNight
14%425%43435%8%43%
1800
May1st to 15thNight
18%6%25%32345%7%38%
1638
16th to endNight
12%42442446%10%42%
1495
June1st to 15thNight
10%314426%5%7%8%44%
1440
16th to endNight
8%425%5%35%46%11%42%
1440
July1st to 15thNight
9%5%26%6%37%7%6%9%35%
1440
16th to endNight
6%223425%5%9%12%44%
1688
August1st to 15thNight
9%5%25%435%6%6%7%44%
1620
16th to endNight
10%325%435%5%6%8%44%
1900
September1st to 15thNight
12%5%25%423347%46%
1800
16th to endNight
13%6%35%5%34447%42%
1819
October1st to 15thNight
18%5%24324345%45%
1980
16th to endNight
14%413323446%52%
2112
November1st to 15thNight
13%5%2442323451%
2222
16th to endNight
16%41342445%7%44%
2472
December1st to 15thNight
16%5%25%5%3445%6%38%
2520
16th to endNight
16%6%345%24447%40%
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%6%24435%45%6%41%
1519
16th to endDay
14%6%35%5%34446%42%
1728
February1st to 15thDay
14%6%25%6%3445%6%39%
1575
16th to endDay
13%7%36%43435%5%41%
1553
March1st to 15thDay
15%5%25%424345%44%
1956
16th to endDay
17%6%35%5%24346%40%
2133
April1st to 15thDay
13%5%245%3335%7%44%
2152
16th to endDay
18%6%14423347%44%
2160
May1st to 15thDay
16%6%25%5%245%5%8%37%
2322
16th to endDay
14%5%245%35%5%6%8%38%
2618
June1st to 15thDay
11%4245%25%5%6%9%41%
2520
16th to endDay
9%5%24435%5%7%9%43%
2520
July1st to 15thDay
10%425%6%36%6%6%9%38%
2520
16th to endDay
7%3246%46%7%8%11%38%
2477
August1st to 15thDay
11%5%26%6%46%6%6%8%35%
2337
16th to endDay
11%4245%35%6%6%8%40%
2292
September1st to 15thDay
13%4244245%5%8%43%
2160
16th to endDay
15%6%3443445%7%40%
1827
October1st to 15thDay
20%423322346%46%
1650
16th to endDay
14%424424445%48%
1760
November1st to 15thDay
15%5%23324446%48%
1634
16th to endDay
17%5%13325%35%6%44%
1485
December1st to 15thDay
16%315%425%45%8%41%
1440
16th to endDay
19%5%25%423447%39%
1536
January1st to 15thNight
17%6%24424448%41%
2417
16th to endNight
16%6%23324447%43%
2496
February1st to 15thNight
13%5%2443445%5%44%
2278
16th to endNight
14%6%34424446%44%
1919
March1st to 15thNight
16%5%24334447%44%
2003
16th to endNight
15%5%36%5%2445%6%41%
2088
April1st to 15thNight
14%5%15%4245%5%6%43%
1796
16th to endNight
14%5%2342435%7%43%
1800
May1st to 15thNight
18%7%33424447%38%
1637
16th to endNight
12%5%243245%6%10%41%
1495
June1st to 15thNight
10%314425%5%6%9%45%
1440
16th to endNight
8%425%5%25%47%11%42%
1440
July1st to 15thNight
8%5%27%6%46%6%7%7%37%
1440
16th to endNight
6%323335%6%9%12%44%
1688
August1st to 15thNight
8%425%5%35%5%6%7%44%
1620
16th to endNight
10%325%5%345%5%8%43%
1900
September1st to 15thNight
13%5%24424446%46%
1800
16th to endNight
14%6%25%5%3345%6%41%
1819
October1st to 15thNight
19%5%24323445%44%
1980
16th to endNight
14%413323446%51%
2112
November1st to 15thNight
14%5%24423235%51%
2222
16th to endNight
16%32342345%7%44%
2472
December1st to 15thNight
16%5%26%5%3445%6%39%
2520
16th to endNight
16%6%24424447%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
43%412%22%13%3
1520
16th to endDay
42%311%25%13%3
1728
February1st to 15thDay
43%411%24%13%2
1577
16th to endDay
43%310%26%13%1
1553
March1st to 15thDay
42%311%24%15%3
1956
16th to endDay
45%311%20%17%2
2134
April1st to 15thDay
39%310%23%18%3
2152
16th to endDay
41%39%18%20%6%
2160
May1st to 15thDay
43%5%13%20%15%1
2322
16th to endDay
41%5%14%23%12%2
2622
June1st to 15thDay
37%6%13%27%14%
2520
16th to endDay
35%7%14%24%17%2
2520
July1st to 15thDay
39%7%13%21%15%2
2371
16th to endDay
34%8%15%25%14%2
2321
August1st to 15thDay
42%6%12%21%15%1
2324
16th to endDay
37%6%13%24%15%2
2292
September1st to 15thDay
37%6%11%21%20%2
2160
16th to endDay
42%47%22%20%1
1827
October1st to 15thDay
39%310%25%17%3
1650
16th to endDay
37%38%28%20%1
1760
November1st to 15thDay
36%411%22%18%6%
1634
16th to endDay
40%39%27%16%2
1485
December1st to 15thDay
40%415%24%12%2
1440
16th to endDay
45%311%24%13%1
1536
January1st to 15thNight
42%410%20%17%4
2417
16th to endNight
40%312%26%16%1
2496
February1st to 15thNight
40%412%24%16%2
2287
16th to endNight
41%38%26%17%2
1919
March1st to 15thNight
39%5%12%26%13%2
2003
16th to endNight
42%411%21%17%3
2088
April1st to 15thNight
40%410%21%20%3
1796
16th to endNight
39%5%10%20%17%7%
1800
May1st to 15thNight
44%411%22%14%2
1638
16th to endNight
36%6%18%21%14%2
1497
June1st to 15thNight
34%7%13%27%16%1
1440
16th to endNight
35%5%13%24%17%2
1440
July1st to 15thNight
43%7%15%18%11%4
1360
16th to endNight
26%9%18%26%17%1
1581
August1st to 15thNight
35%7%12%24%18%2
1609
16th to endNight
35%6%12%24%18%2
1900
September1st to 15thNight
37%410%21%21%3
1800
16th to endNight
41%48%21%20%3
1819
October1st to 15thNight
41%47%25%17%3
1980
16th to endNight
33%29%29%21%3
2112
November1st to 15thNight
37%28%23%21%7%
2222
16th to endNight
37%410%23%18%4
2472
December1st to 15thNight
44%411%23%13%2
2520
16th to endNight
43%412%24%12%1
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
28%24%30%14%11
75
16th to endDay
23%26%35%12%2
96
February1st to 15thDay
26%20%43%8%1
90
16th to endDay
16%30%33%17%2
212
March1st to 15thDay
18%29%26%20%4
270
16th to endDay
19%25%27%19%5%2
288
April1st to 15thDay
15%26%29%16%9%2
270
16th to endDay
22%18%25%16%12%4
270
May1st to 15thDay
25%25%21%13%9%4
288
16th to endDay
19%14%17%27%16%4
331
June1st to 15thDay
14%14%23%22%17%6%
315
16th to endDay
12%15%26%26%15%2
315
July1st to 15thDay
13%8%20%29%20%6%
305
16th to endDay
8%10%15%29%28%7%
326
August1st to 15thDay
14%7%15%27%24%10%
313
16th to endDay
10%5%8%34%23%16%
332
September1st to 15thDay
14%12%16%23%18%14%
315
16th to endDay
24%6%15%23%16%13%
257
October1st to 15thDay
23%12%15%22%21%5%
255
16th to endDay
14%18%26%23%15%1
272
November1st to 15thDay
16%19%24%20%15%2
244
16th to endDay
21%19%32%13%12%
105
December1st to 15thDay
18%28%26%13%10%3
60
16th to endDay
32%12%34%17%3
64
January1st to 15thNight
17%32%27%19%3
720
16th to endNight
17%30%31%18%1
768
February1st to 15thNight
16%30%29%22%1
704
16th to endNight
18%33%28%17%1
631
March1st to 15thNight
19%29%28%19%2
720
16th to endNight
16%29%31%16%41
768
April1st to 15thNight
15%33%28%18%4
718
16th to endNight
14%26%24%24%9%
720
May1st to 15thNight
21%23%22%22%10%
558
16th to endNight
15%20%16%32%14%1
375
June1st to 15thNight
10%19%21%30%17%1
360
16th to endNight
8%17%20%40%13%
360
July1st to 15thNight
8%14%15%37%23%
340
16th to endNight
6%17%12%32%28%4
550
August1st to 15thNight
12%10%10%39%24%2
539
16th to endNight
11%9%10%36%27%4
754
September1st to 15thNight
12%10%20%26%27%2
720
16th to endNight
14%12%15%28%24%3
661
October1st to 15thNight
18%16%18%24%19%2
660
16th to endNight
15%21%28%22%11%
704
November1st to 15thNight
14%25%24%23%10%2
700
16th to endNight
15%30%25%20%6%
720
December1st to 15thNight
18%30%24%19%7%
720
16th to endNight
18%32%29%17%2
768
Page generated at 2017-09-17 12:05:47. Version ac04

Copyright 2017 A. Danko


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