Montgomery County Astrophysical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Montgomery County Astrophysical Observatory clear sky chart from 2005-11-05 to 2017-12-20.

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
37%
15601
21%
48086
11%
15562
48%
15562
25%
29964

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
21%
717
22%
2422
8%
716
39%
716
18%
1530
January16th to endNight
19%
768
23%
2496
7%
768
39%
768
21%
1528
February1st to 15thNight
20%
703
19%
2287
5%
701
38%
701
20%
1403
February16th to endNight
18%
631
24%
1919
6%
631
39%
631
20%
1214
March1st to 15thNight
23%
720
21%
2006
7%
720
37%
720
23%
1295
March16th to endNight
21%
768
21%
2088
8%
768
33%
768
23%
1393
April1st to 15thNight
24%
718
19%
1796
4%
718
33%
718
26%
1193
April16th to endNight
37%
720
19%
1800
7%
720
42%
720
29%
1210
May1st to 15thNight
36%
552
18%
1632
8%
552
43%
552
24%
1090
May16th to endNight
51%
374
18%
1497
14%
374
60%
374
24%
861
June1st to 15thNight
52%
360
19%
1440
17%
360
60%
360
25%
813
June16th to endNight
53%
360
18%
1440
16%
360
61%
360
26%
806
July1st to 15thNight
56%
340
12%
1360
15%
340
58%
340
19%
855
July16th to endNight
68%
543
17%
1574
12%
510
73%
510
37%
891
August1st to 15thNight
65%
539
19%
1609
15%
536
66%
536
31%
1005
August16th to endNight
62%
755
20%
1901
13%
755
64%
755
35%
1239
September1st to 15thNight
52%
720
25%
1800
14%
720
57%
720
34%
1181
September16th to endNight
54%
717
25%
1974
20%
717
58%
717
30%
1290
October1st to 15thNight
45%
720
24%
2160
20%
720
53%
720
26%
1384
October16th to endNight
36%
768
30%
2304
17%
768
53%
768
32%
1331
November1st to 15thNight
36%
760
25%
2418
14%
760
51%
760
27%
1435
November16th to endNight
29%
780
24%
2678
12%
780
46%
780
23%
1546
December1st to 15thNight
26%
780
18%
2730
7%
780
41%
780
17%
1745
December16th to endNight
22%
788
22%
2755
8%
788
41%
788
18%
1726

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%245%345%5%7%41%
48747
Night
14%5%25%42445%7%43%
48268
Cloud CoverDay
14%5%2443445%7%41%
48757
Night
14%5%2442445%7%43%
48272
TransparencyDay
40%5%12%23%16%2
48447
Night
39%411%24%17%3
48086
SeeingDay
18%9%24%24%16%6%
5969
Night
17%11%27%28%12%1
15606

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%4245%47%40%
1513
16th to endDay
14%6%34424347%41%
1728
February1st to 15thDay
13%6%35%5%345%6%6%38%
1575
16th to endDay
13%8%35%435%445%41%
1554
March1st to 15thDay
16%5%25%5%24345%44%
1942
16th to endDay
16%6%35%6%23446%40%
2136
April1st to 15thDay
13%5%245%2445%7%43%
2152
16th to endDay
17%5%34423346%44%
2160
May1st to 15thDay
15%6%35%4245%6%8%36%
2326
16th to endDay
14%4245%35%5%5%8%38%
2618
June1st to 15thDay
11%4245%35%5%6%9%41%
2520
16th to endDay
9%423435%5%6%9%43%
2520
July1st to 15thDay
9%5%245%35%6%6%8%39%
2520
16th to endDay
7%3246%46%7%8%11%37%
2484
August1st to 15thDay
12%5%26%6%36%6%6%8%35%
2337
16th to endDay
12%424435%6%6%8%42%
2292
September1st to 15thDay
14%3244335%5%8%44%
2160
16th to endDay
14%6%25%5%2445%7%41%
1983
October1st to 15thDay
19%41432335%6%45%
1800
16th to endDay
13%424424345%49%
1920
November1st to 15thDay
14%5%2442345%6%45%
1770
16th to endDay
15%43332445%5%47%
1609
December1st to 15thDay
16%425%5%25%45%7%41%
1560
16th to endDay
17%6%25%5%2435%7%39%
1568
January1st to 15thNight
16%6%25%423448%40%
2422
16th to endNight
16%6%2442445%6%43%
2496
February1st to 15thNight
14%6%245%34445%44%
2285
16th to endNight
14%6%26%424446%44%
1919
March1st to 15thNight
16%425%32445%6%44%
1994
16th to endNight
15%5%36%424447%41%
2088
April1st to 15thNight
13%5%34425%45%8%42%
1796
16th to endNight
14%5%25%42445%7%43%
1800
May1st to 15thNight
18%7%15%41445%7%38%
1632
16th to endNight
12%5%234335%6%10%42%
1495
June1st to 15thNight
10%315%326%6%6%9%44%
1440
16th to endNight
7%426%5%245%5%11%43%
1440
July1st to 15thNight
8%5%26%7%37%6%6%9%35%
1440
16th to endNight
6%223425%6%8%12%45%
1681
August1st to 15thNight
9%5%15%4345%7%7%44%
1620
16th to endNight
10%325%4345%6%8%44%
1901
September1st to 15thNight
12%5%25%424446%47%
1800
16th to endNight
12%6%25%5%34446%44%
1974
October1st to 15thNight
18%5%24334445%44%
2160
16th to endNight
14%41232335%6%52%
2304
November1st to 15thNight
13%5%34422345%50%
2418
16th to endNight
15%31342445%7%46%
2678
December1st to 15thNight
16%6%26%6%3445%6%38%
2730
16th to endNight
16%5%35%5%24447%41%
2755

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
15%7%25%4245%5%7%40%
1513
16th to endDay
15%6%3442435%7%41%
1728
February1st to 15thDay
13%6%25%5%346%6%6%38%
1575
16th to endDay
13%8%35%434445%41%
1554
March1st to 15thDay
15%5%25%424345%44%
1953
16th to endDay
16%7%25%5%1445%5%41%
2135
April1st to 15thDay
13%5%245%2435%7%43%
2152
16th to endDay
17%6%34323346%44%
2160
May1st to 15thDay
15%6%35%4345%6%8%36%
2326
16th to endDay
15%4245%35%5%5%8%38%
2618
June1st to 15thDay
11%4245%35%5%7%9%41%
2520
16th to endDay
10%4245%25%5%7%9%43%
2520
July1st to 15thDay
10%5%25%5%45%5%6%8%40%
2520
16th to endDay
8%315%5%36%7%8%11%38%
2484
August1st to 15thDay
12%5%26%6%35%6%7%7%36%
2337
16th to endDay
12%424435%6%5%8%42%
2292
September1st to 15thDay
14%4244345%5%7%44%
2160
16th to endDay
14%6%25%5%2445%6%41%
1983
October1st to 15thDay
20%41332335%6%45%
1800
16th to endDay
13%424424445%49%
1920
November1st to 15thDay
15%5%2432445%6%45%
1770
16th to endDay
15%433324445%47%
1609
December1st to 15thDay
16%315%5%2445%8%41%
1560
16th to endDay
17%6%25%5%2435%7%39%
1568
January1st to 15thNight
16%6%25%423448%40%
2422
16th to endNight
16%6%2432445%6%43%
2496
February1st to 15thNight
13%6%245%33446%44%
2278
16th to endNight
14%6%26%424446%44%
1919
March1st to 15thNight
16%42422445%6%44%
2006
16th to endNight
15%6%36%324347%41%
2088
April1st to 15thNight
13%5%344245%48%42%
1796
16th to endNight
15%5%25%42445%7%43%
1800
May1st to 15thNight
19%7%25%41345%7%38%
1631
16th to endNight
12%5%234235%5%10%42%
1495
June1st to 15thNight
10%315%326%5%6%9%44%
1440
16th to endNight
8%426%5%245%5%11%42%
1440
July1st to 15thNight
8%5%26%6%47%5%7%8%36%
1440
16th to endNight
6%223425%5%8%12%45%
1681
August1st to 15thNight
9%5%25%435%5%7%7%44%
1620
16th to endNight
10%325%435%5%6%8%44%
1901
September1st to 15thNight
12%5%34424446%47%
1800
16th to endNight
13%5%25%5%34437%44%
1974
October1st to 15thNight
18%423424435%45%
2160
16th to endNight
14%41232335%6%52%
2304
November1st to 15thNight
13%5%24422335%50%
2418
16th to endNight
15%31342445%7%46%
2678
December1st to 15thNight
16%6%26%5%3445%6%37%
2730
16th to endNight
16%6%35%424347%41%
2755

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
1514
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
1554
March1st to 15thDay
42%311%24%15%3
1953
16th to endDay
45%311%20%17%2
2136
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
2326
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
2328
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
41%47%24%20%2
1983
October1st to 15thDay
39%310%24%17%3
1800
16th to endDay
36%38%28%21%1
1920
November1st to 15thDay
38%411%22%17%5%
1770
16th to endDay
38%39%28%17%2
1609
December1st to 15thDay
41%414%24%12%2
1560
16th to endDay
44%312%24%13%1
1568
January1st to 15thNight
42%410%20%17%4
2422
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
2006
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
1632
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
1574
August1st to 15thNight
35%7%12%24%18%2
1609
16th to endNight
35%6%12%24%18%2
1901
September1st to 15thNight
37%410%21%21%3
1800
16th to endNight
39%47%23%20%3
1974
October1st to 15thNight
41%48%25%17%3
2160
16th to endNight
33%38%30%21%3
2304
November1st to 15thNight
38%28%24%20%6%
2418
16th to endNight
36%410%24%20%4
2678
December1st to 15thNight
45%411%23%13%2
2730
16th to endNight
42%412%24%12%1
2755

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
27%23%31%15%11
73
16th to endDay
23%26%35%12%2
96
February1st to 15thDay
26%20%43%8%1
90
16th to endDay
16%24%36%19%2
213
March1st to 15thDay
21%19%30%22%4
270
16th to endDay
20%16%31%22%5%3
288
April1st to 15thDay
16%17%32%19%11%2
270
16th to endDay
23%9%29%19%13%5%
270
May1st to 15thDay
26%15%24%17%11%5%
288
16th to endDay
19%5%21%29%19%4
331
June1st to 15thDay
16%5%26%25%20%7%
315
16th to endDay
13%5%28%29%18%4
315
July1st to 15thDay
14%322%32%21%6%
305
16th to endDay
8%117%32%30%9%
326
August1st to 15thDay
14%117%28%25%12%
313
16th to endDay
11%9%35%25%16%
332
September1st to 15thDay
15%8%16%23%20%15%
315
16th to endDay
21%315%27%18%14%
314
October1st to 15thDay
25%5%14%24%23%6%
315
16th to endDay
14%13%28%24%16%2
336
November1st to 15thDay
19%9%27%25%16%2
293
16th to endDay
20%13%37%14%12%2
139
December1st to 15thDay
16%26%27%15%10%3
90
16th to endDay
29%15%31%20%2
72
January1st to 15thNight
20%19%35%21%3
720
16th to endNight
21%19%36%21%1
768
February1st to 15thNight
19%17%36%25%1
704
16th to endNight
21%21%34%21%1
631
March1st to 15thNight
21%16%35%23%3
720
16th to endNight
19%16%37%19%5%1
768
April1st to 15thNight
17%19%35%22%4
718
16th to endNight
17%11%29%30%10%
720
May1st to 15thNight
22%7%27%28%13%
552
16th to endNight
15%5%20%39%17%1
375
June1st to 15thNight
11%325%36%22%1
360
16th to endNight
9%222%47%16%1
360
July1st to 15thNight
9%317%42%26%
340
16th to endNight
6%214%41%30%4
543
August1st to 15thNight
12%111%44%27%2
539
16th to endNight
12%212%39%28%4
755
September1st to 15thNight
15%320%29%28%2
720
16th to endNight
14%417%32%27%3
717
October1st to 15thNight
20%8%19%26%21%3
720
16th to endNight
17%13%31%24%12%
768
November1st to 15thNight
17%12%27%29%10%1
760
16th to endNight
18%16%30%26%7%
780
December1st to 15thNight
20%15%31%24%8%
780
16th to endNight
20%17%35%23%3
788
Page generated at 2017-12-21 07:04:54. Version ac04

Copyright 2017 A. Danko


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