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 2005-11-06 to 2024-01-12.

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
35%
23443
21%
74622
11%
23391
47%
23391
24%
46389

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 10thNight
16%
757
24%
2603
7%
757
38%
757
20%
1570
January11th to 20thNight
24%
725
22%
2354
9%
725
39%
725
18%
1517
January21st to endNight
20%
792
25%
2574
9%
792
41%
792
22%
1545
February1st to 10thNight
23%
720
18%
2340
7%
720
37%
720
19%
1497
February11th to 20thNight
13%
702
20%
2225
3%
700
33%
700
17%
1416
February21st to endNight
16%
592
25%
1776
7%
592
37%
592
19%
1169
March1st to 10thNight
21%
720
24%
2014
7%
720
39%
720
24%
1244
March11th to 20thNight
21%
720
21%
1980
7%
720
35%
720
21%
1296
March21st to endNight
20%
792
21%
2143
7%
792
32%
792
23%
1435
April1st to 10thNight
22%
720
19%
1800
4%
720
32%
720
22%
1227
April11th to 20thNight
29%
711
21%
1775
6%
711
37%
711
30%
1163
April21st to endNight
30%
720
20%
1800
6%
720
38%
720
27%
1215
May1st to 10thNight
36%
594
18%
1674
8%
594
43%
594
25%
1103
May11th to 20thNight
39%
418
18%
1479
9%
418
48%
418
20%
951
May21st to endNight
52%
388
15%
1551
14%
388
59%
388
24%
900
June1st to 10thNight
51%
360
20%
1440
19%
360
60%
360
25%
800
June11th to 20thNight
49%
360
16%
1440
15%
360
55%
360
22%
869
June21st to endNight
51%
360
18%
1440
16%
360
59%
360
26%
797
July1st to 10thNight
60%
350
15%
1400
18%
350
63%
350
21%
862
July11th to 20thNight
68%
414
17%
1458
17%
414
70%
414
29%
826
July21st to endNight
63%
591
18%
1672
13%
558
68%
558
34%
1007
August1st to 10thNight
64%
537
16%
1600
13%
534
65%
534
28%
1046
August11th to 20thNight
60%
606
17%
1670
11%
606
61%
606
28%
1118
August21st to endNight
61%
792
20%
1980
11%
792
63%
792
34%
1299
September1st to 10thNight
51%
720
23%
1800
12%
720
56%
720
32%
1200
September11th to 20thNight
59%
720
31%
1800
21%
720
63%
720
38%
1132
September21st to endNight
44%
720
22%
2060
15%
720
50%
720
26%
1371
October1st to 10thNight
47%
720
20%
2160
18%
720
53%
720
27%
1384
October11th to 20thNight
39%
720
26%
2160
18%
720
51%
720
30%
1291
October21st to endNight
33%
792
27%
2376
15%
792
48%
792
28%
1446
November1st to 10thNight
37%
740
28%
2317
17%
740
53%
740
32%
1305
November11th to 20thNight
30%
760
27%
2489
13%
760
47%
760
25%
1454
November21st to endNight
30%
760
24%
2660
12%
760
48%
760
23%
1513
December1st to 10thNight
23%
760
20%
2660
6%
760
40%
760
18%
1668
December11th to 20thNight
23%
760
21%
3168
8%
759
39%
759
17%
1911
December21st to endNight
26%
830
15%
4784
10%
817
42%
817
11%
2842

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%5%35%5%6%8%42%
74668
Night
15%5%25%5%345%5%7%44%
75034
TransparencyDay
40%5%12%24%16%3
74378
Night
39%5%11%24%17%3
74843
SeeingDay
19%11%27%24%14%5%
13836
Night
18%12%29%29%12%1
23454

By Month

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 10thDay
15%6%25%435%5%5%6%43%
1559
11th to 20thDay
15%8%345%34447%42%
1629
21st to endDay
15%7%36%6%44446%41%
1782
February1st to 10thDay
15%6%26%6%45%5%6%7%37%
1620
11th to 20thDay
18%6%35%5%345%6%5%40%
1622
21st to endDay
18%7%36%5%34445%40%
1480
March1st to 10thDay
13%6%25%43445%6%49%
1944
11th to 20thDay
17%6%35%426%45%6%42%
1980
21st to endDay
18%6%36%5%4445%7%38%
2211
April1st to 10thDay
13%6%25%5%35%46%8%44%
2160
11th to 20thDay
13%5%35%5%3445%6%48%
2134
21st to endDay
20%6%25%5%2445%8%40%
2160
May1st to 10thDay
15%7%26%5%345%6%8%39%
2300
11th to 20thDay
18%7%35%6%35%5%6%8%35%
2413
21st to endDay
14%6%35%5%35%5%6%8%41%
2725
June1st to 10thDay
12%425%5%345%7%9%43%
2520
11th to 20thDay
12%6%35%6%36%6%6%8%40%
2520
21st to endDay
10%4245%36%5%8%11%44%
2520
July1st to 10thDay
10%425%7%35%6%6%10%41%
2520
11th to 20thDay
7%3245%46%7%9%10%44%
2449
21st to endDay
10%5%25%7%46%6%7%10%37%
2547
August1st to 10thDay
11%5%26%7%46%7%7%9%37%
2327
11th to 20thDay
12%5%26%6%46%6%7%8%38%
2210
21st to endDay
11%5%35%5%35%6%6%10%41%
2376
September1st to 10thDay
14%5%25%5%345%6%7%44%
2160
11th to 20thDay
13%5%34435%5%5%9%47%
2160
21st to endDay
18%7%35%43446%5%40%
1893
October1st to 10thDay
18%5%235%2445%6%47%
1800
11th to 20thDay
19%32442445%6%48%
1800
21st to endDay
15%5%245%3445%5%48%
1980
November1st to 10thDay
13%5%2432445%7%51%
1758
11th to 20thDay
16%42432446%6%50%
1687
21st to endDay
15%6%14435%45%6%47%
1520
December1st to 10thDay
16%6%35%435%5%6%8%40%
1520
11th to 20thDay
16%5%2442436%7%45%
1910
21st to endDay
18%7%36%5%35%5%6%8%36%
2772
January1st to 10thNight
16%6%25%42445%7%45%
2603
11th to 20thNight
18%6%25%5%345%47%40%
2354
21st to endNight
15%7%34325%45%7%45%
2574
February1st to 10thNight
15%6%35%5%45%5%5%6%40%
2340
11th to 20thNight
19%7%25%433446%43%
2223
21st to endNight
18%7%345%34446%42%
1776
March1st to 10thNight
14%434335%45%7%49%
2014
11th to 20thNight
17%6%26%42445%6%43%
1980
21st to endNight
15%6%36%6%245%5%6%41%
2143
April1st to 10thNight
15%7%35%5%35%5%6%6%41%
1800
11th to 20thNight
14%426%5%25%45%7%46%
1775
21st to endNight
17%5%2343446%7%43%
1800
May1st to 10thNight
17%6%245%345%5%8%40%
1674
11th to 20thNight
18%7%345%345%6%8%37%
1479
21st to endNight
12%5%34435%5%7%11%41%
1550
June1st to 10thNight
11%3245%35%5%7%10%45%
1440
11th to 20thNight
12%6%26%5%35%6%6%10%40%
1440
21st to endNight
9%435%6%36%5%7%11%43%
1440
July1st to 10thNight
10%5%26%7%5%5%6%7%9%39%
1440
11th to 20thNight
6%425%5%46%6%8%9%46%
1506
21st to endNight
9%424435%6%8%12%42%
1771
August1st to 10thNight
9%5%36%7%47%7%7%8%40%
1611
11th to 20thNight
10%5%37%6%36%6%5%8%41%
1670
21st to endNight
10%5%36%5%345%6%9%44%
1980
September1st to 10thNight
16%6%35%42445%7%46%
1800
11th to 20thNight
11%5%345%245%5%6%51%
1800
21st to endNight
18%6%36%5%3445%6%41%
2060
October1st to 10thNight
20%5%2432345%6%45%
2160
11th to 20thNight
16%424433345%51%
2160
21st to endNight
16%6%1442445%6%48%
2376
November1st to 10thNight
12%5%245%23345%54%
2317
11th to 20thNight
15%6%24423346%51%
2489
21st to endNight
14%3245%345%6%7%47%
2660
December1st to 10thNight
18%6%26%4335%5%7%41%
2660
11th to 20thNight
17%5%25%5%35%45%6%43%
3285
21st to endNight
17%6%25%5%45%5%6%7%38%
4884

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 10thDay
42%5%9%24%15%4
1560
11th to 20thDay
43%313%24%14%2
1629
21st to endDay
46%48%23%15%4
1782
February1st to 10thDay
45%6%13%23%12%2
1620
11th to 20thDay
44%49%25%15%3
1631
21st to endDay
47%410%21%15%4
1480
March1st to 10thDay
37%312%25%20%2
1944
11th to 20thDay
44%410%19%18%6%
1980
21st to endDay
47%49%20%17%3
2211
April1st to 10thDay
39%5%12%21%18%5%
2160
11th to 20thDay
37%5%10%22%20%6%
2134
21st to endDay
43%410%22%18%4
2160
May1st to 10thDay
41%6%14%20%17%2
2300
11th to 20thDay
46%6%12%20%14%2
2417
21st to endDay
40%5%16%25%11%2
2725
June1st to 10thDay
37%6%14%26%15%2
2520
11th to 20thDay
40%8%13%24%14%2
2520
21st to endDay
33%7%15%26%17%2
2520
July1st to 10thDay
37%7%14%22%16%3
2450
11th to 20thDay
31%8%16%27%16%3
2374
21st to endDay
39%8%13%22%16%2
2405
August1st to 10thDay
41%7%16%24%11%1
2314
11th to 20thDay
41%7%14%22%16%2
2210
21st to endDay
37%7%15%25%14%3
2376
September1st to 10thDay
39%6%13%21%18%4
2160
11th to 20thDay
35%49%28%21%4
2160
21st to endDay
44%5%8%21%20%2
1893
October1st to 10thDay
39%411%25%18%3
1800
11th to 20thDay
37%5%10%25%22%3
1800
21st to endDay
38%48%27%21%2
1980
November1st to 10thDay
33%411%27%20%4
1758
11th to 20thDay
35%410%24%22%4
1687
21st to endDay
39%5%10%28%17%2
1520
December1st to 10thDay
43%5%13%25%13%2
1520
11th to 20thDay
39%5%10%32%12%3
1906
21st to endDay
45%5%13%24%11%1
2772
January1st to 10thNight
40%48%25%18%5%
2603
11th to 20thNight
44%5%12%22%15%1
2354
21st to endNight
39%39%28%19%2
2574
February1st to 10thNight
44%412%24%14%2
2340
11th to 20thNight
43%49%22%18%4
2225
21st to endNight
45%39%22%16%5%
1776
March1st to 10thNight
36%5%11%27%19%3
2014
11th to 20thNight
41%5%11%21%18%4
1980
21st to endNight
43%410%21%15%6%
2143
April1st to 10thNight
42%5%9%20%19%4
1800
11th to 20thNight
38%412%22%19%6%
1775
21st to endNight
39%6%10%25%14%5%
1800
May1st to 10thNight
42%5%12%23%13%4
1674
11th to 20thNight
44%6%11%19%17%2
1480
21st to endNight
37%6%20%22%13%2
1551
June1st to 10thNight
34%7%13%27%17%2
1440
11th to 20thNight
40%7%13%24%14%3
1440
21st to endNight
35%6%14%24%19%2
1440
July1st to 10thNight
40%8%17%18%13%4
1400
11th to 20thNight
31%8%15%26%17%2
1458
21st to endNight
31%9%17%26%16%1
1672
August1st to 10thNight
38%8%16%22%14%1
1600
11th to 20thNight
40%7%13%23%14%2
1670
21st to endNight
36%7%14%24%16%3
1980
September1st to 10thNight
39%5%11%22%18%5%
1800
11th to 20thNight
34%5%9%27%22%4
1800
21st to endNight
44%49%21%18%4
2060
October1st to 10thNight
41%5%9%24%18%3
2160
11th to 20thNight
36%39%29%21%3
2160
21st to endNight
37%48%26%21%4
2376
November1st to 10thNight
33%39%28%22%5%
2317
11th to 20thNight
37%38%24%22%5%
2489
21st to endNight
35%411%26%20%3
2660
December1st to 10thNight
43%412%24%14%2
2660
11th to 20thNight
41%5%9%24%18%3
3288
21st to endNight
44%5%15%24%11%1
4884

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 10thDay
32%16%35%13%31
200
11th to 20thDay
28%23%32%14%3
243
21st to endDay
26%23%35%10%41
264
February1st to 10thDay
26%22%38%10%5%
240
11th to 20thDay
32%20%40%7%1
289
21st to endDay
28%20%34%16%2
346
March1st to 10thDay
24%24%30%19%3
420
11th to 20thDay
27%20%30%18%32
420
21st to endDay
21%21%32%19%6%2
462
April1st to 10thDay
21%20%33%18%6%1
420
11th to 20thDay
18%17%33%17%12%3
412
21st to endDay
28%11%34%18%7%2
420
May1st to 10thDay
24%16%28%20%8%4
423
11th to 20thDay
20%11%32%24%11%2
447
21st to endDay
17%6%23%29%20%5%
485
June1st to 10thDay
15%7%27%26%19%6%
450
11th to 20thDay
17%6%25%30%17%5%
450
21st to endDay
10%7%30%30%18%6%
450
July1st to 10thDay
13%318%31%22%12%
445
11th to 20thDay
5%220%34%30%9%
444
21st to endDay
13%323%31%24%6%
488
August1st to 10thDay
13%419%28%25%12%
446
11th to 20thDay
12%416%32%25%11%
439
21st to endDay
11%318%34%22%13%
495
September1st to 10thDay
18%7%17%24%22%13%
450
11th to 20thDay
14%5%18%32%19%13%
450
21st to endDay
24%8%23%28%11%5%
450
October1st to 10thDay
24%6%20%26%20%6%
450
11th to 20thDay
18%13%26%26%14%3
450
21st to endDay
19%13%34%23%9%2
495
November1st to 10thDay
17%9%35%24%13%2
408
11th to 20thDay
21%14%34%20%10%1
347
21st to endDay
23%18%33%14%11%1
180
December1st to 10thDay
23%18%34%19%32
180
11th to 20thDay
26%21%27%19%7%
180
21st to endDay
25%16%35%17%7%1
198
January1st to 10thNight
20%18%41%17%3
760
11th to 20thNight
23%16%31%26%3
725
21st to endNight
20%19%37%22%2
792
February1st to 10thNight
20%17%36%26%2
720
11th to 20thNight
26%21%35%16%1
703
21st to endNight
25%18%36%19%2
592
March1st to 10thNight
20%17%38%22%2
720
11th to 20thNight
23%19%33%19%41
720
21st to endNight
19%17%39%20%41
792
April1st to 10thNight
20%22%32%22%4
720
11th to 20thNight
16%14%36%26%7%1
711
21st to endNight
20%12%34%27%7%
720
May1st to 10thNight
21%8%31%27%12%1
594
11th to 20thNight
22%6%26%35%11%
418
21st to endNight
15%5%22%39%17%1
389
June1st to 10thNight
12%327%35%22%1
360
11th to 20thNight
14%324%41%16%2
360
21st to endNight
9%324%45%19%1
360
July1st to 10thNight
10%315%42%27%3
350
11th to 20thNight
5%314%41%34%3
414
21st to endNight
9%216%43%26%4
591
August1st to 10thNight
11%310%44%30%2
537
11th to 20thNight
13%311%44%25%4
606
21st to endNight
11%315%39%27%4
792
September1st to 10thNight
18%320%29%27%4
720
11th to 20thNight
10%5%15%36%30%4
720
21st to endNight
20%5%23%33%19%1
720
October1st to 10thNight
21%7%19%29%20%4
720
11th to 20thNight
18%11%27%29%15%1
720
21st to endNight
20%11%32%25%12%
792
November1st to 10thNight
15%10%31%32%10%1
740
11th to 20thNight
18%17%31%27%7%1
760
21st to endNight
16%16%33%28%7%1
760
December1st to 10thNight
23%17%32%21%7%1
760
11th to 20thNight
19%14%38%24%5%
760
21st to endNight
19%16%34%28%3
836
Page generated at 2024-01-08 22:31:27. Version ac07

Copyright 2024 Attilla Danko


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