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Nanjemoy Creek Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Nanjemoy Creek Observatory clear sky chart from 2005-11-05 to 2018-03-11.

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
38%
16001
21%
49130
11%
15964
48%
15964
25%
30770

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
22%
777
23%
2600
8%
776
40%
776
20%
1575
January16th to endNight
22%
832
23%
2704
10%
832
40%
832
20%
1686
February1st to 15thNight
21%
761
20%
2482
7%
761
37%
761
20%
1529
February16th to endNight
18%
683
24%
2066
6%
683
39%
683
22%
1286
March1st to 15thNight
23%
764
23%
2124
8%
764
38%
764
25%
1361
March16th to endNight
24%
768
21%
2088
9%
768
35%
768
25%
1393
April1st to 15thNight
26%
718
20%
1796
5%
718
35%
718
29%
1170
April16th to endNight
39%
720
20%
1800
8%
720
44%
720
31%
1187
May1st to 15thNight
35%
591
16%
1671
8%
591
41%
591
23%
1144
May16th to endNight
54%
374
17%
1497
16%
374
59%
374
22%
904
June1st to 15thNight
57%
360
20%
1440
19%
360
65%
360
26%
825
June16th to endNight
58%
360
16%
1440
18%
360
63%
360
24%
835
July1st to 15thNight
54%
347
14%
1367
16%
347
58%
347
21%
852
July16th to endNight
69%
564
16%
1595
11%
531
74%
531
33%
958
August1st to 15thNight
66%
541
17%
1611
13%
538
66%
538
29%
1030
August16th to endNight
63%
764
18%
1910
11%
764
64%
764
31%
1291
September1st to 15thNight
56%
720
24%
1800
15%
720
61%
720
33%
1197
September16th to endNight
54%
717
22%
1974
17%
717
58%
717
29%
1299
October1st to 15thNight
44%
720
23%
2160
19%
720
52%
720
26%
1382
October16th to endNight
38%
768
30%
2304
19%
768
53%
768
31%
1350
November1st to 15thNight
36%
760
24%
2401
13%
760
50%
760
27%
1445
November16th to endNight
33%
780
25%
2658
13%
780
49%
780
24%
1535
December1st to 15thNight
26%
780
19%
2730
8%
780
41%
780
17%
1719
December16th to endNight
22%
832
22%
2912
9%
832
40%
832
19%
1817

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
13%5%2442445%7%43%
49488
Night
14%5%2442445%7%44%
49313
Cloud CoverDay
13%5%2442445%7%43%
49498
Night
14%5%2442445%7%44%
49317
TransparencyDay
38%6%12%23%16%3
49189
Night
38%5%11%22%17%3
49130
SeeingDay
17%9%26%25%15%6%
6256
Night
17%10%28%27%14%1
16008

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%245%2246%6%43%
1666
16th to endDay
17%5%25%424346%43%
1872
February1st to 15thDay
15%6%25%435%346%40%
1710
16th to endDay
16%7%245%34446%40%
1692
March1st to 15thDay
15%5%24423345%46%
2060
16th to endDay
16%6%245%23436%41%
2135
April1st to 15thDay
13%5%1442435%7%46%
2152
16th to endDay
16%5%24424345%45%
2160
May1st to 15thDay
16%5%234245%6%8%39%
2287
16th to endDay
12%315%5%25%5%6%9%41%
2618
June1st to 15thDay
11%315%5%25%5%6%10%44%
2520
16th to endDay
7%324437%6%7%10%42%
2520
July1st to 15thDay
9%425%435%6%6%10%40%
2510
16th to endDay
5%325%5%37%7%8%10%40%
2465
August1st to 15thDay
10%5%26%6%45%5%6%9%37%
2325
16th to endDay
10%4245%35%46%9%41%
2292
September1st to 15thDay
12%41443446%8%46%
2160
16th to endDay
14%7%25%42445%7%42%
1983
October1st to 15thDay
18%6%23323346%46%
1800
16th to endDay
13%424423335%52%
1920
November1st to 15thDay
16%423423346%48%
1786
16th to endDay
15%324323345%51%
1631
December1st to 15thDay
16%5%24324447%43%
1560
16th to endDay
19%7%2432335%6%40%
1664
January1st to 15thNight
16%6%23323346%45%
2600
16th to endNight
16%6%25%32344445%
2704
February1st to 15thNight
15%6%24424445%45%
2480
16th to endNight
15%7%24324345%46%
2066
March1st to 15thNight
16%5%23424447%45%
2112
16th to endNight
17%5%35%323346%42%
2088
April1st to 15thNight
12%5%2442435%7%45%
1796
16th to endNight
16%42332345%6%45%
1800
May1st to 15thNight
16%6%25%4245%5%8%38%
1671
16th to endNight
12%423445%5%6%9%41%
1495
June1st to 15thNight
9%415%4245%8%10%43%
1440
16th to endNight
9%325%4345%6%10%42%
1440
July1st to 15thNight
6%6%26%6%45%6%7%9%38%
1447
16th to endNight
5%324446%6%7%11%43%
1703
August1st to 15thNight
9%325%5%35%5%6%9%42%
1622
16th to endNight
10%425%7%3446%10%39%
1910
September1st to 15thNight
11%6%245%2445%7%45%
1800
16th to endNight
14%6%245%245%5%7%41%
1974
October1st to 15thNight
17%5%25%424345%44%
2160
16th to endNight
13%314323446%51%
2304
November1st to 15thNight
15%5%23413335%50%
2401
16th to endNight
14%423324446%48%
2658
December1st to 15thNight
17%6%25%42445%6%40%
2730
16th to endNight
17%6%25%423347%41%
2912

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%245%2335%7%43%
1666
16th to endDay
16%6%24423446%43%
1872
February1st to 15thDay
15%7%25%5%34446%41%
1710
16th to endDay
16%7%24435%45%6%40%
1692
March1st to 15thDay
15%5%2442345%5%46%
2071
16th to endDay
16%6%245%34346%42%
2134
April1st to 15thDay
13%5%2342445%6%47%
2152
16th to endDay
16%5%25%423445%46%
2160
May1st to 15thDay
16%5%244245%6%8%40%
2287
16th to endDay
12%3145%245%6%9%42%
2618
June1st to 15thDay
11%3244245%6%9%45%
2520
16th to endDay
6%3245%36%6%6%10%43%
2520
July1st to 15thDay
8%425%5%35%5%6%10%41%
2510
16th to endDay
5%3246%36%6%8%11%40%
2465
August1st to 15thDay
10%5%16%6%35%5%7%10%38%
2325
16th to endDay
10%415%5%345%6%10%41%
2292
September1st to 15thDay
12%32342436%8%47%
2160
16th to endDay
14%7%24424447%42%
1983
October1st to 15thDay
18%5%23313346%48%
1800
16th to endDay
14%41442324452%
1920
November1st to 15thDay
16%423423346%48%
1786
16th to endDay
15%413323335%51%
1631
December1st to 15thDay
16%5%2432445%7%43%
1560
16th to endDay
19%7%24413345%41%
1664
January1st to 15thNight
16%6%23323346%44%
2600
16th to endNight
16%6%25%42434445%
2704
February1st to 15thNight
15%6%24424445%45%
2473
16th to endNight
15%6%24424345%46%
2066
March1st to 15thNight
16%5%24313446%45%
2124
16th to endNight
17%6%25%323446%43%
2088
April1st to 15thNight
12%5%245%24347%46%
1796
16th to endNight
16%424323447%45%
1800
May1st to 15thNight
16%6%25%5%25%449%37%
1670
16th to endNight
13%414345%5%5%10%41%
1495
June1st to 15thNight
10%3144345%7%10%43%
1440
16th to endNight
9%425%4345%6%10%43%
1440
July1st to 15thNight
7%5%26%6%36%6%6%10%38%
1447
16th to endNight
6%323436%7%8%10%42%
1703
August1st to 15thNight
8%425%5%36%5%7%7%42%
1622
16th to endNight
11%425%5%35%46%9%39%
1910
September1st to 15thNight
12%6%1442345%6%46%
1800
16th to endNight
13%6%244445%5%7%41%
1974
October1st to 15thNight
17%5%245%24445%43%
2160
16th to endNight
13%314323446%51%
2304
November1st to 15thNight
15%5%24313335%50%
2401
16th to endNight
14%42342445%6%48%
2658
December1st to 15thNight
16%6%25%43445%6%40%
2730
16th to endNight
17%5%35%414346%42%
2912

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
41%412%22%14%4
1667
16th to endDay
42%411%22%16%3
1872
February1st to 15thDay
43%49%24%14%2
1712
16th to endDay
44%411%21%16%1
1692
March1st to 15thDay
39%5%10%22%17%4
2071
16th to endDay
44%411%18%17%3
2135
April1st to 15thDay
36%411%23%19%5%
2152
16th to endDay
40%49%19%18%7%
2160
May1st to 15thDay
41%7%13%21%15%1
2287
16th to endDay
36%8%14%24%13%1
2622
June1st to 15thDay
33%8%15%27%14%1
2520
16th to endDay
33%9%15%22%17%2
2520
July1st to 15thDay
36%9%14%20%14%3
2361
16th to endDay
32%9%16%24%15%1
2310
August1st to 15thDay
40%8%13%21%14%1
2312
16th to endDay
35%7%14%22%15%2
2292
September1st to 15thDay
34%6%14%20%20%3
2160
16th to endDay
40%5%9%22%19%1
1983
October1st to 15thDay
38%49%27%17%3
1800
16th to endDay
35%38%26%23%3
1920
November1st to 15thDay
37%411%23%17%5%
1786
16th to endDay
35%49%25%21%3
1631
December1st to 15thDay
39%5%12%24%13%4
1560
16th to endDay
44%310%25%14%1
1664
January1st to 15thNight
39%411%20%18%6%
2600
16th to endNight
41%39%24%18%2
2704
February1st to 15thNight
40%49%24%17%3
2482
16th to endNight
40%39%25%18%3
2066
March1st to 15thNight
39%5%10%26%16%2
2124
16th to endNight
41%5%10%21%17%3
2088
April1st to 15thNight
38%49%21%22%3
1796
16th to endNight
38%5%11%20%17%7%
1800
May1st to 15thNight
43%7%12%19%14%2
1671
16th to endNight
37%8%16%20%14%1
1497
June1st to 15thNight
33%7%13%26%16%1
1440
16th to endNight
35%8%15%18%20%2
1440
July1st to 15thNight
39%9%15%17%13%4
1367
16th to endNight
29%9%16%26%15%1
1595
August1st to 15thNight
36%8%14%21%17%1
1611
16th to endNight
39%6%13%22%15%2
1910
September1st to 15thNight
35%8%11%20%19%4
1800
16th to endNight
39%6%10%19%20%3
1974
October1st to 15thNight
41%48%22%18%4
2160
16th to endNight
33%47%26%22%4
2304
November1st to 15thNight
37%49%21%20%6%
2401
16th to endNight
35%49%24%21%4
2658
December1st to 15thNight
43%411%22%14%3
2730
16th to endNight
43%412%23%14%2
2912

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
20%26%31%16%4
116
16th to endDay
18%29%28%18%4
144
February1st to 15thDay
29%21%38%8%1
135
16th to endDay
22%18%35%20%2
273
March1st to 15thDay
19%20%32%22%4
312
16th to endDay
18%15%38%18%5%2
288
April1st to 15thDay
16%14%30%26%10%1
270
16th to endDay
19%7%28%26%13%4
270
May1st to 15thDay
25%12%27%17%13%2
288
16th to endDay
19%224%25%20%7%
331
June1st to 15thDay
12%5%26%24%20%10%
315
16th to endDay
9%327%35%19%4
315
July1st to 15thDay
12%120%37%21%7%
305
16th to endDay
6%113%33%31%12%
326
August1st to 15thDay
16%115%27%30%8%
313
16th to endDay
9%115%32%24%17%
332
September1st to 15thDay
13%225%24%16%17%
315
16th to endDay
21%6%18%24%18%12%
314
October1st to 15thDay
22%5%21%21%21%7%
315
16th to endDay
12%11%28%28%14%4
336
November1st to 15thDay
20%8%27%26%14%3
296
16th to endDay
14%17%34%21%9%3
161
December1st to 15thDay
22%14%32%14%13%3
90
16th to endDay
34%14%29%19%2
96
January1st to 15thNight
21%19%33%20%5%
780
16th to endNight
21%17%36%22%2
832
February1st to 15thNight
20%16%37%22%2
764
16th to endNight
21%18%38%19%2
683
March1st to 15thNight
21%14%36%23%3
764
16th to endNight
19%13%38%21%5%1
768
April1st to 15thNight
15%15%39%24%5%
718
16th to endNight
19%9%27%31%11%
720
May1st to 15thNight
18%8%32%25%13%1
591
16th to endNight
12%6%20%36%22%1
375
June1st to 15thNight
9%223%38%25%1
360
16th to endNight
8%320%44%22%
360
July1st to 15thNight
9%222%34%29%1
347
16th to endNight
6%114%37%33%6%
564
August1st to 15thNight
11%115%41%27%4
541
16th to endNight
12%213%37%30%4
764
September1st to 15thNight
14%219%31%29%2
720
16th to endNight
16%319%30%26%3
717
October1st to 15thNight
19%7%20%26%21%3
720
16th to endNight
16%12%30%24%15%1
768
November1st to 15thNight
18%11%28%26%13%1
760
16th to endNight
17%16%29%26%9%1
780
December1st to 15thNight
20%15%33%20%10%
780
16th to endNight
20%13%38%24%2
832
Page generated at 2018-03-12 13:28:07. Version ac04

Copyright 2018 A. Danko


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