Mount Vernon
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mount Vernon clear sky chart from 2005-11-05 to 2017-09-23.

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
44%
15820
22%
46919
15%
15773
51%
15773
28%
29536

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
26%
900
19%
2506
7%
895
36%
895
22%
1700
January16th to endNight
25%
960
19%
2536
8%
960
32%
960
24%
1747
February1st to 15thNight
24%
760
17%
2168
6%
757
32%
757
20%
1515
February16th to endNight
25%
631
19%
1893
7%
631
35%
631
22%
1258
March1st to 15thNight
25%
720
17%
2160
7%
720
33%
720
20%
1464
March16th to endNight
35%
582
16%
1995
12%
582
42%
582
20%
1317
April1st to 15thNight
36%
539
22%
1797
13%
539
50%
539
26%
1074
April16th to endNight
43%
540
21%
1800
17%
540
51%
540
25%
1140
May1st to 15thNight
45%
540
18%
1643
13%
539
51%
539
24%
1104
May16th to endNight
57%
563
20%
1544
15%
561
60%
561
30%
1017
June1st to 15thNight
68%
540
22%
1440
18%
540
70%
540
42%
849
June16th to endNight
69%
540
23%
1440
17%
540
72%
540
42%
834
July1st to 15thNight
70%
510
20%
1360
16%
510
71%
510
40%
804
July16th to endNight
75%
564
24%
1426
21%
531
77%
531
42%
830
August1st to 15thNight
78%
539
23%
1748
27%
536
78%
536
40%
919
August16th to endNight
76%
573
27%
1910
32%
573
78%
573
37%
1006
September1st to 15thNight
65%
540
28%
1800
29%
540
69%
540
33%
1011
September16th to endNight
62%
680
32%
1894
25%
680
66%
680
38%
1099
October1st to 15thNight
55%
660
30%
1906
22%
660
62%
660
37%
1111
October16th to endNight
38%
704
27%
2174
18%
704
48%
704
29%
1316
November1st to 15thNight
43%
700
30%
2276
21%
700
55%
700
33%
1261
November16th to endNight
34%
720
23%
2340
16%
720
44%
720
25%
1463
December1st to 15thNight
29%
855
15%
2475
9%
855
36%
855
20%
1754
December16th to endNight
24%
960
17%
2688
6%
960
34%
960
19%
1943

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
12%6%25%5%3445%7%42%
47996
Night
13%6%25%42345%7%45%
47095
TransparencyDay
40%5%12%24%14%2
47676
Night
39%5%10%24%17%3
46921
SeeingDay
13%5%19%25%24%11%
5850
Night
15%6%26%31%17%2
15820

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 15thDay
18%8%28%4245%45%36%
1432
16th to endDay
12%8%47%6%34446%39%
1686
February1st to 15thDay
15%8%47%5%23346%37%
1693
16th to endDay
12%10%37%436%5%46%36%
1580
March1st to 15thDay
17%7%35%424445%38%
1803
16th to endDay
16%8%36%6%35%45%6%32%
2233
April1st to 15thDay
13%6%2442445%7%45%
2151
16th to endDay
16%7%36%6%34446%37%
2160
May1st to 15thDay
18%5%25%6%245%5%7%36%
2304
16th to endDay
10%6%25%5%36%5%5%8%38%
2567
June1st to 15thDay
6%425%5%345%6%8%47%
2520
16th to endDay
6%3145%45%46%10%45%
2520
July1st to 15thDay
9%3245%36%5%6%10%42%
2520
16th to endDay
42235%46%6%8%10%44%
2643
August1st to 15thDay
6%4245%45%6%5%10%43%
2202
16th to endDay
6%415%5%36%5%6%9%44%
2292
September1st to 15thDay
10%5%25%5%3445%6%44%
2157
16th to endDay
9%4245%2445%7%48%
1894
October1st to 15thDay
13%424323336%52%
1722
16th to endDay
13%424323446%51%
1697
November1st to 15thDay
11%5%25%423445%50%
1582
16th to endDay
19%8%24422335%42%
1620
December1st to 15thDay
18%10%45%424446%35%
1482
16th to endDay
18%11%36%413445%36%
1536
January1st to 15thNight
17%6%25%5%3435%6%39%
2506
16th to endNight
13%8%46%5%2445%7%38%
2536
February1st to 15thNight
16%9%36%5%25%346%36%
2159
16th to endNight
13%7%37%434446%39%
1893
March1st to 15thNight
16%7%36%5%33335%39%
2160
16th to endNight
17%6%35%5%2445%6%37%
1995
April1st to 15thNight
12%5%25%5%23445%47%
1797
16th to endNight
17%5%25%4135%5%8%40%
1800
May1st to 15thNight
17%5%245%34446%41%
1643
16th to endNight
6%436%7%345%7%9%42%
1543
June1st to 15thNight
7%323325%46%8%51%
1440
16th to endNight
43143245%7%10%51%
1440
July1st to 15thNight
7%32442435%9%51%
1440
16th to endNight
5%2144445%5%9%51%
1522
August1st to 15thNight
5%22333345%9%55%
1758
16th to endNight
432443446%8%54%
1910
September1st to 15thNight
9%42432245%7%50%
1800
16th to endNight
9%424423347%53%
1894
October1st to 15thNight
10%414423347%52%
1906
16th to endNight
16%5%1432333451%
2174
November1st to 15thNight
11%424322446%53%
2276
16th to endNight
17%7%24413235%46%
2340
December1st to 15thNight
21%9%35%423445%34%
2475
16th to endNight
19%10%45%5%33345%34%
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
49%5%10%22%11%
1433
16th to endDay
46%412%27%9%
1686
February1st to 15thDay
48%410%24%10%1
1695
16th to endDay
47%5%11%21%12%1
1580
March1st to 15thDay
47%410%23%12%1
1803
16th to endDay
51%5%15%20%7%
2235
April1st to 15thDay
37%5%11%27%17%1
2151
16th to endDay
46%410%24%10%1
2160
May1st to 15thDay
45%7%12%23%10%
2304
16th to endDay
40%7%14%19%12%3
2571
June1st to 15thDay
32%6%14%26%18%2
2520
16th to endDay
32%7%15%22%19%3
2520
July1st to 15thDay
34%6%13%25%14%5%
2371
16th to endDay
28%8%16%28%15%2
2477
August1st to 15thDay
34%7%12%26%17%1
2188
16th to endDay
33%6%14%23%16%5%
2292
September1st to 15thDay
38%5%9%23%19%4
2157
16th to endDay
34%5%8%27%21%3
1894
October1st to 15thDay
33%39%27%20%4
1722
16th to endDay
33%48%30%19%3
1697
November1st to 15thDay
35%311%21%19%7%
1582
16th to endDay
44%29%27%13%2
1620
December1st to 15thDay
50%411%20%11%
1482
16th to endDay
50%411%23%10%1
1536
January1st to 15thNight
46%49%23%15%1
2506
16th to endNight
44%5%12%23%13%1
2536
February1st to 15thNight
49%48%24%11%1
2168
16th to endNight
45%413%23%12%
1893
March1st to 15thNight
47%410%20%14%3
2160
16th to endNight
46%5%14%21%10%1
1995
April1st to 15thNight
37%410%25%18%2
1797
16th to endNight
41%5%9%25%14%3
1800
May1st to 15thNight
44%5%10%23%14%1
1644
16th to endNight
36%8%13%20%16%4
1545
June1st to 15thNight
28%5%13%27%22%2
1440
16th to endNight
25%7%15%21%22%6%
1440
July1st to 15thNight
29%5%12%28%16%6%
1360
16th to endNight
26%6%14%31%17%2
1426
August1st to 15thNight
24%6%16%25%23%3
1748
16th to endNight
26%5%12%26%22%6%
1910
September1st to 15thNight
32%5%6%27%21%7%
1800
16th to endNight
31%37%26%24%7%
1894
October1st to 15thNight
32%39%21%26%6%
1906
16th to endNight
37%36%27%18%5%
2174
November1st to 15thNight
31%48%24%24%6%
2276
16th to endNight
41%38%25%18%1
2340
December1st to 15thNight
51%411%20%11%
2475
16th to endNight
51%5%10%20%10%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
36%8%33%14%5%1
90
16th to endDay
11%20%47%15%4
96
February1st to 15thDay
22%15%42%17%2
90
16th to endDay
19%18%34%22%4
81
March1st to 15thDay
23%12%38%15%8%1
90
16th to endDay
21%10%31%23%11%1
219
April1st to 15thDay
15%12%29%24%16%1
270
16th to endDay
23%6%25%24%14%5%
270
May1st to 15thDay
18%10%23%27%15%4
288
16th to endDay
14%425%27%19%9%
467
June1st to 15thDay
9%215%27%31%13%
495
16th to endDay
7%422%30%26%8%
495
July1st to 15thDay
11%211%33%26%14%
475
16th to endDay
47%26%35%24%
515
August1st to 15thDay
12%16%17%37%24%
355
16th to endDay
7%6%24%43%17%
332
September1st to 15thDay
12%38%19%37%18%
315
16th to endDay
12%39%23%28%22%
283
October1st to 15thDay
11%20%33%25%9%
162
16th to endDay
13%430%31%20%
96
November1st to 15thDay
47%23%25%24%14%
90
16th to endDay
37%7%22%22%8%1
90
December1st to 15thDay
20%7%37%24%5%4
90
16th to endDay
27%12%46%12%1
96
January1st to 15thNight
20%14%35%24%5%
900
16th to endNight
18%11%38%28%3
960
February1st to 15thNight
20%12%36%27%2
760
16th to endNight
16%12%39%29%2
631
March1st to 15thNight
19%11%39%25%4
720
16th to endNight
19%6%32%30%10%
582
April1st to 15thNight
15%9%34%29%10%
539
16th to endNight
19%427%35%12%1
540
May1st to 15thNight
18%5%22%37%14%1
540
16th to endNight
7%323%36%25%3
563
June1st to 15thNight
9%115%41%30%2
540
16th to endNight
4115%45%30%2
540
July1st to 15thNight
9%110%37%37%4
510
16th to endNight
7%16%33%41%9%
564
August1st to 15thNight
6%15%34%42%9%
539
16th to endNight
6%7%35%43%6%
573
September1st to 15thNight
12%111%30%36%7%
540
16th to endNight
12%213%29%33%7%
680
October1st to 15thNight
12%321%31%25%5%
660
16th to endNight
19%7%31%30%11%
704
November1st to 15thNight
11%8%30%31%16%
700
16th to endNight
20%9%30%27%10%
720
December1st to 15thNight
27%9%28%26%6%1
855
16th to endNight
25%9%35%26%4
960
Page generated at 2017-09-24 08:15:48. Version ac04

Copyright 2017 A. Danko


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