C&O Canal National Historical Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the C&O Canal National Historical Park clear sky chart from 2005-11-05 to 2017-10-31.

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
34%
15369
20%
47424
10%
15331
45%
15331
21%
30179

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
19%
717
18%
2398
8%
716
33%
716
13%
1598
January16th to endNight
17%
768
19%
2496
5%
768
34%
768
15%
1629
February1st to 15thNight
21%
702
19%
2287
6%
701
37%
701
16%
1432
February16th to endNight
16%
631
20%
1949
5%
631
33%
631
14%
1288
March1st to 15thNight
22%
720
20%
1980
8%
720
35%
720
18%
1323
March16th to endNight
20%
768
20%
2111
7%
768
31%
768
18%
1420
April1st to 15thNight
22%
718
17%
1796
5%
718
29%
718
21%
1251
April16th to endNight
35%
703
19%
1783
6%
703
43%
703
24%
1190
May1st to 15thNight
31%
540
18%
1620
9%
540
39%
540
18%
1111
May16th to endNight
43%
379
18%
1502
15%
379
53%
379
17%
910
June1st to 15thNight
45%
360
19%
1440
17%
360
55%
360
22%
817
June16th to endNight
52%
360
19%
1440
18%
360
61%
360
24%
794
July1st to 15thNight
53%
349
12%
1369
13%
349
56%
349
18%
854
July16th to endNight
60%
564
16%
1595
10%
531
69%
531
33%
931
August1st to 15thNight
55%
539
19%
1609
12%
536
60%
536
28%
992
August16th to endNight
63%
718
22%
1864
14%
718
65%
718
34%
1175
September1st to 15thNight
54%
720
24%
1800
14%
720
58%
720
33%
1168
September16th to endNight
51%
717
25%
1995
18%
717
56%
717
28%
1268
October1st to 15thNight
42%
720
22%
2160
17%
720
51%
720
25%
1380
October16th to endNight
36%
768
29%
2303
15%
768
53%
768
29%
1354
November1st to 15thNight
35%
700
25%
2199
16%
700
47%
700
24%
1314
November16th to endNight
29%
720
20%
2520
10%
720
44%
720
15%
1566
December1st to 15thNight
23%
720
16%
2520
6%
720
37%
720
12%
1663
December16th to endNight
20%
768
19%
2688
7%
768
35%
768
13%
1751

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%5%7%39%
48319
Night
14%5%2442445%7%42%
47607
Cloud CoverDay
14%5%25%5%35%5%5%7%39%
48329
Night
14%5%2442445%7%42%
47611
TransparencyDay
41%6%12%22%15%2
48021
Night
39%6%12%23%15%2
47424
SeeingDay
17%17%19%20%17%6%
5772
Night
15%23%20%26%12%1
15375

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
14%5%46%6%36%5%5%8%34%
1537
16th to endDay
14%9%26%6%2445%6%36%
1728
February1st to 15thDay
14%7%47%6%35%246%36%
1575
16th to endDay
12%7%48%6%44346%38%
1524
March1st to 15thDay
15%7%35%5%25%445%40%
1969
16th to endDay
17%7%26%4245%5%7%35%
2112
April1st to 15thDay
15%426%5%3446%7%38%
2152
16th to endDay
18%5%24424445%42%
2187
May1st to 15thDay
19%5%24435%5%6%8%34%
2331
16th to endDay
15%6%25%5%35%45%7%37%
2613
June1st to 15thDay
12%314435%6%6%11%40%
2520
16th to endDay
9%425%5%35%5%6%10%41%
2520
July1st to 15thDay
9%425%5%46%6%7%9%38%
2508
16th to endDay
8%325%6%36%7%7%10%38%
2465
August1st to 15thDay
11%4146%36%7%7%10%35%
2340
16th to endDay
10%424335%5%6%8%44%
2324
September1st to 15thDay
15%5%14424446%45%
2160
16th to endDay
14%5%25%42445%6%42%
1963
October1st to 15thDay
20%41341345%6%44%
1800
16th to endDay
14%415%5%2345%6%46%
1920
November1st to 15thDay
14%5%23424445%48%
1655
16th to endDay
20%6%25%324446%38%
1440
December1st to 15thDay
17%6%35%5%345%6%7%33%
1440
16th to endDay
17%7%26%6%25%5%6%5%34%
1536
January1st to 15thNight
16%7%36%42445%6%36%
2398
16th to endNight
15%8%36%42445%8%36%
2496
February1st to 15thNight
14%6%245%2445%7%41%
2285
16th to endNight
11%8%35%6%35%46%6%37%
1949
March1st to 15thNight
17%5%235%3445%6%40%
1968
16th to endNight
16%6%35%434346%41%
2111
April1st to 15thNight
15%6%35%5%245%6%6%38%
1796
16th to endNight
16%4234245%47%44%
1783
May1st to 15thNight
19%5%35%424448%36%
1620
16th to endNight
14%5%135%25%45%7%44%
1500
June1st to 15thNight
10%32442446%10%45%
1440
16th to endNight
9%3145%335%7%10%45%
1440
July1st to 15thNight
10%425%5%35%47%9%40%
1449
16th to endNight
5%3145%35%6%7%11%45%
1703
August1st to 15thNight
8%425%6%345%6%9%44%
1620
16th to endNight
11%313425%45%7%49%
1864
September1st to 15thNight
13%32442445%7%47%
1800
16th to endNight
14%5%24334446%47%
1995
October1st to 15thNight
19%42442345%5%44%
2160
16th to endNight
14%313323447%50%
2303
November1st to 15thNight
13%6%34413346%48%
2199
16th to endNight
16%5%24425%45%7%40%
2520
December1st to 15thNight
17%6%35%5%25%5%5%6%35%
2520
16th to endNight
17%6%35%5%245%5%8%36%
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
14%5%35%6%35%5%5%8%34%
1537
16th to endDay
14%8%36%5%34447%36%
1728
February1st to 15thDay
13%7%37%6%45%346%36%
1575
16th to endDay
12%7%48%5%34346%38%
1524
March1st to 15thDay
15%7%25%5%25%446%40%
1980
16th to endDay
17%7%245%25%5%5%6%36%
2111
April1st to 15thDay
15%5%25%5%345%6%7%39%
2152
16th to endDay
18%5%25%324445%42%
2187
May1st to 15thDay
19%5%24435%5%6%8%34%
2331
16th to endDay
15%5%26%5%35%5%5%7%38%
2613
June1st to 15thDay
12%324425%6%6%10%41%
2520
16th to endDay
9%335%5%35%6%6%10%41%
2520
July1st to 15thDay
9%5%25%5%45%5%7%9%38%
2508
16th to endDay
7%425%6%36%7%7%10%38%
2465
August1st to 15thDay
10%5%25%6%36%7%7%9%36%
2340
16th to endDay
10%325%335%46%9%44%
2324
September1st to 15thDay
14%5%1443445%6%45%
2160
16th to endDay
14%5%25%43446%7%41%
1963
October1st to 15thDay
20%42342335%6%43%
1800
16th to endDay
14%4245%2445%6%46%
1920
November1st to 15thDay
13%5%23424346%48%
1655
16th to endDay
19%5%25%325%346%39%
1440
December1st to 15thDay
18%6%345%246%5%7%33%
1440
16th to endDay
16%7%35%6%25%45%6%34%
1536
January1st to 15thNight
16%7%36%43445%6%37%
2398
16th to endNight
15%7%45%5%2445%7%37%
2496
February1st to 15thNight
14%7%25%42445%7%42%
2278
16th to endNight
11%8%35%6%35%45%6%38%
1949
March1st to 15thNight
17%5%234345%5%6%40%
1980
16th to endNight
15%6%35%424347%41%
2111
April1st to 15thNight
14%6%35%5%245%6%6%39%
1796
16th to endNight
16%423425%5%47%44%
1783
May1st to 15thNight
18%5%35%5%2445%8%36%
1619
16th to endNight
13%6%145%35%45%7%43%
1500
June1st to 15thNight
10%4235%345%6%9%45%
1440
16th to endNight
9%3245%2347%11%44%
1440
July1st to 15thNight
10%426%5%35%6%7%8%40%
1449
16th to endNight
5%2145%35%6%8%10%45%
1703
August1st to 15thNight
8%425%5%345%5%9%44%
1620
16th to endNight
11%3135%2445%7%49%
1864
September1st to 15thNight
12%423425%46%6%46%
1800
16th to endNight
13%6%24334446%47%
1995
October1st to 15thNight
19%324423445%43%
2160
16th to endNight
14%313323446%50%
2303
November1st to 15thNight
13%6%24423336%48%
2199
16th to endNight
16%5%34425%5%5%6%40%
2520
December1st to 15thNight
16%6%35%5%35%5%5%6%36%
2520
16th to endNight
16%5%35%4245%5%7%37%
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
45%6%12%21%12%1
1538
16th to endDay
46%412%22%11%2
1728
February1st to 15thDay
49%411%21%11%2
1577
16th to endDay
47%412%24%11%1
1524
March1st to 15thDay
44%5%10%22%14%1
1980
16th to endDay
47%5%11%18%14%2
2112
April1st to 15thDay
42%6%11%18%17%2
2152
16th to endDay
43%48%20%18%5%
2187
May1st to 15thDay
46%6%12%19%14%1
2331
16th to endDay
44%5%13%21%12%2
2617
June1st to 15thDay
36%8%14%24%15%1
2520
16th to endDay
37%8%15%21%15%1
2520
July1st to 15thDay
40%9%12%20%14%3
2360
16th to endDay
36%9%14%21%16%2
2310
August1st to 15thDay
41%8%13%21%14%
2327
16th to endDay
35%7%13%25%14%2
2324
September1st to 15thDay
37%6%11%20%21%2
2160
16th to endDay
41%5%7%23%19%2
1963
October1st to 15thDay
41%49%25%15%2
1800
16th to endDay
36%5%11%26%18%1
1920
November1st to 15thDay
36%412%21%20%5%
1655
16th to endDay
44%5%10%24%14%1
1440
December1st to 15thDay
45%7%15%20%9%1
1440
16th to endDay
49%6%11%21%10%
1536
January1st to 15thNight
45%6%11%18%14%3
2398
16th to endNight
45%413%23%11%1
2496
February1st to 15thNight
40%5%13%23%13%2
2287
16th to endNight
44%5%11%23%13%1
1949
March1st to 15thNight
42%5%12%26%11%1
1980
16th to endNight
43%411%22%15%1
2111
April1st to 15thNight
43%5%10%21%15%3
1796
16th to endNight
39%5%10%21%16%6%
1783
May1st to 15thNight
45%7%8%22%13%2
1620
16th to endNight
38%7%13%22%15%1
1502
June1st to 15thNight
34%7%12%28%16%1
1440
16th to endNight
31%7%15%26%16%1
1440
July1st to 15thNight
38%10%14%18%13%4
1369
16th to endNight
27%9%19%23%17%2
1595
August1st to 15thNight
34%7%13%24%18%1
1609
16th to endNight
33%6%12%25%20%3
1864
September1st to 15thNight
34%6%13%20%22%2
1800
16th to endNight
38%5%8%24%20%2
1995
October1st to 15thNight
41%49%24%18%2
2160
16th to endNight
32%49%31%20%1
2303
November1st to 15thNight
36%48%24%19%6%
2199
16th to endNight
41%5%10%22%17%2
2520
December1st to 15thNight
46%6%12%22%11%
2520
16th to endNight
43%7%14%22%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
27%33%22%10%6%
77
16th to endDay
33%28%25%10%3
96
February1st to 15thDay
30%28%30%10%1
90
16th to endDay
21%28%25%22%2
177
March1st to 15thDay
20%28%19%26%4
270
16th to endDay
22%27%24%16%5%2
288
April1st to 15thDay
16%25%25%19%10%2
270
16th to endDay
23%18%20%15%14%7%
270
May1st to 15thDay
28%25%17%13%10%4
288
16th to endDay
19%16%15%18%22%6%
331
June1st to 15thDay
14%18%21%17%20%6%
315
16th to endDay
11%17%26%26%13%4
315
July1st to 15thDay
14%9%24%23%19%7%
305
16th to endDay
7%11%13%30%25%11%
326
August1st to 15thDay
15%8%16%20%25%14%
313
16th to endDay
8%7%12%25%28%17%
332
September1st to 15thDay
15%12%15%17%22%16%
315
16th to endDay
17%8%13%23%25%12%
314
October1st to 15thDay
23%8%13%24%23%6%
315
16th to endDay
13%21%24%22%16%2
336
November1st to 15thDay
16%19%20%20%19%3
245
16th to endDay
28%15%20%26%6%3
60
December1st to 15thDay
23%25%21%13%13%3
60
16th to endDay
26%15%32%21%3
64
January1st to 15thNight
18%31%24%21%3
720
16th to endNight
17%31%27%21%1
768
February1st to 15thNight
16%31%25%24%1
704
16th to endNight
16%35%28%17%2
631
March1st to 15thNight
19%29%25%21%4
720
16th to endNight
16%29%28%19%5%
768
April1st to 15thNight
16%30%29%20%4
718
16th to endNight
16%25%18%27%11%
703
May1st to 15thNight
20%22%19%24%12%
540
16th to endNight
15%21%14%30%17%1
380
June1st to 15thNight
9%19%20%26%23%1
360
16th to endNight
6%17%15%40%18%
360
July1st to 15thNight
11%13%12%32%27%1
349
16th to endNight
5%17%10%32%30%3
564
August1st to 15thNight
11%10%12%36%26%2
539
16th to endNight
11%9%9%35%28%5%
718
September1st to 15thNight
11%10%15%30%28%3
720
16th to endNight
16%11%13%24%29%4
717
October1st to 15thNight
20%12%16%28%18%2
720
16th to endNight
14%18%26%28%11%
768
November1st to 15thNight
13%23%22%27%12%1
700
16th to endNight
16%32%19%24%7%
720
December1st to 15thNight
18%29%23%22%6%
720
16th to endNight
18%30%25%23%2
768
Page generated at 2017-11-01 17:23:41. Version ac04

Copyright 2017 A. Danko


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