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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 2017-05-24.

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%
14977
21%
45893
11%
14940
48%
14940
25%
28804

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%
717
23%
2399
9%
716
40%
716
19%
1477
January16th to endNight
20%
768
23%
2496
8%
768
39%
768
20%
1565
February1st to 15thNight
21%
701
20%
2287
7%
701
38%
701
21%
1383
February16th to endNight
19%
631
25%
1909
6%
631
41%
631
22%
1174
March1st to 15thNight
23%
720
22%
2001
8%
720
38%
720
24%
1291
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%
360
17%
1441
16%
360
60%
360
23%
862
June1st to 15thNight
56%
330
21%
1320
20%
330
64%
330
24%
778
June16th to endNight
60%
330
16%
1320
18%
330
63%
330
24%
767
July1st to 15thNight
55%
316
15%
1246
17%
316
59%
316
21%
777
July16th to endNight
70%
522
15%
1469
10%
489
74%
489
33%
887
August1st to 15thNight
66%
496
18%
1476
13%
493
67%
493
30%
938
August16th to endNight
63%
700
17%
1750
10%
700
64%
700
31%
1184
September1st to 15thNight
58%
660
26%
1650
15%
660
63%
660
33%
1095
September16th to endNight
52%
657
19%
1809
15%
657
56%
657
27%
1215
October1st to 15thNight
43%
660
24%
1980
20%
660
51%
660
26%
1259
October16th to endNight
37%
704
29%
2112
17%
704
52%
704
30%
1248
November1st to 15thNight
36%
700
25%
2212
13%
700
51%
700
27%
1317
November16th to endNight
32%
720
23%
2453
12%
720
47%
720
22%
1444
December1st to 15thNight
26%
720
20%
2520
8%
720
41%
720
18%
1573
December16th to endNight
22%
768
22%
2688
9%
768
40%
768
19%
1676

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%
46371
Night
14%5%2442445%7%44%
46076
Cloud CoverDay
13%5%2442445%7%43%
46381
Night
14%5%2442445%7%44%
46080
TransparencyDay
38%6%12%22%16%3
46072
Night
39%5%11%22%17%3
45893
SeeingDay
17%8%25%25%16%6%
5251
Night
17%10%28%27%13%1
14984

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
16%7%245%2235%6%43%
1537
16th to endDay
17%5%25%424346%42%
1728
February1st to 15thDay
14%6%25%435%446%41%
1575
16th to endDay
14%7%245%35%446%41%
1563
March1st to 15thDay
15%5%24423445%46%
1946
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%5%9%42%
2517
June1st to 15thDay
11%3145%25%5%6%10%43%
2310
16th to endDay
7%324437%6%7%9%43%
2310
July1st to 15thDay
9%5%25%435%6%6%11%40%
2301
16th to endDay
5%225%5%37%7%8%10%40%
2283
August1st to 15thDay
10%5%26%6%45%5%6%9%37%
2131
16th to endDay
10%425%5%36%46%9%40%
2100
September1st to 15thDay
12%41443446%8%46%
1980
16th to endDay
15%7%25%43445%6%40%
1818
October1st to 15thDay
17%5%23323346%47%
1650
16th to endDay
14%5%24423335%50%
1760
November1st to 15thDay
15%423423346%49%
1646
16th to endDay
16%424314345%49%
1506
December1st to 15thDay
16%5%24424437%44%
1440
16th to endDay
20%6%25%32335%6%40%
1536
January1st to 15thNight
17%6%24424346%43%
2399
16th to endNight
16%7%25%32344445%
2496
February1st to 15thNight
15%6%24424345%46%
2285
16th to endNight
14%7%24324345%47%
1909
March1st to 15thNight
16%5%13424446%44%
1989
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%
1439
June1st to 15thNight
10%5%15%4245%8%10%41%
1320
16th to endNight
9%426%4345%6%9%42%
1320
July1st to 15thNight
7%6%26%5%45%6%7%9%38%
1326
16th to endNight
5%2245%46%6%8%11%42%
1577
August1st to 15thNight
9%325%5%35%5%6%9%43%
1487
16th to endNight
10%425%6%3446%9%40%
1750
September1st to 15thNight
11%5%1343445%7%46%
1650
16th to endNight
15%6%245%245%5%6%40%
1809
October1st to 15thNight
16%5%25%423345%45%
1980
16th to endNight
14%31432345%6%50%
2112
November1st to 15thNight
14%5%23413335%51%
2212
16th to endNight
14%424324446%46%
2453
December1st to 15thNight
17%6%25%42445%6%41%
2520
16th to endNight
18%6%24423347%41%
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
16%7%245%23347%43%
1537
16th to endDay
17%6%24423446%42%
1728
February1st to 15thDay
14%7%25%5%34446%41%
1575
16th to endDay
15%6%24435%45%7%41%
1563
March1st to 15thDay
15%5%2442345%5%46%
1957
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%3144245%6%9%43%
2517
June1st to 15thDay
11%324425%6%6%9%44%
2310
16th to endDay
6%3245%36%6%6%10%43%
2310
July1st to 15thDay
8%425%5%35%5%6%10%40%
2301
16th to endDay
5%3246%36%6%8%11%40%
2283
August1st to 15thDay
9%5%16%6%45%5%7%10%37%
2131
16th to endDay
10%415%5%345%6%10%41%
2100
September1st to 15thDay
12%32342436%7%48%
1980
16th to endDay
15%7%2442445%7%40%
1818
October1st to 15thDay
17%5%24313345%48%
1650
16th to endDay
14%415%42434451%
1760
November1st to 15thDay
15%413423346%49%
1646
16th to endDay
16%413323345%49%
1506
December1st to 15thDay
16%5%24424447%44%
1440
16th to endDay
20%6%24323345%41%
1536
January1st to 15thNight
17%6%24323346%43%
2399
16th to endNight
16%6%25%42434445%
2496
February1st to 15thNight
15%6%24324345%47%
2278
16th to endNight
15%6%24424345%47%
1909
March1st to 15thNight
16%5%24323446%44%
2001
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
12%414345%5%5%10%41%
1439
June1st to 15thNight
11%4244345%7%10%41%
1320
16th to endNight
9%425%4345%5%10%43%
1320
July1st to 15thNight
7%5%25%6%35%6%6%10%38%
1326
16th to endNight
5%223436%8%8%10%42%
1577
August1st to 15thNight
8%4245%36%6%7%7%43%
1487
16th to endNight
11%5%25%5%35%46%9%40%
1750
September1st to 15thNight
12%5%1442345%7%47%
1650
16th to endNight
15%6%244445%5%7%40%
1809
October1st to 15thNight
17%4245%24445%44%
1980
16th to endNight
14%314423447%50%
2112
November1st to 15thNight
14%5%24313335%51%
2212
16th to endNight
14%42342445%6%47%
2453
December1st to 15thNight
16%6%25%43445%6%41%
2520
16th to endNight
18%5%34414346%42%
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
42%312%22%14%3
1538
16th to endDay
42%411%22%15%3
1728
February1st to 15thDay
42%39%25%15%2
1577
16th to endDay
42%412%22%16%1
1563
March1st to 15thDay
39%5%10%23%16%4
1957
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
35%8%14%25%13%1
2521
June1st to 15thDay
34%8%15%27%13%1
2310
16th to endDay
33%9%15%22%16%2
2310
July1st to 15thDay
36%9%13%20%15%3
2152
16th to endDay
32%10%16%23%14%1
2128
August1st to 15thDay
39%8%13%21%15%
2118
16th to endDay
36%7%14%22%15%2
2100
September1st to 15thDay
34%6%14%18%21%3
1980
16th to endDay
42%5%9%21%19%1
1818
October1st to 15thDay
38%48%27%17%3
1650
16th to endDay
36%38%25%22%3
1760
November1st to 15thDay
35%411%23%18%6%
1646
16th to endDay
36%410%24%20%3
1506
December1st to 15thDay
39%412%24%13%4
1440
16th to endDay
44%310%24%15%1
1536
January1st to 15thNight
41%311%19%18%5%
2399
16th to endNight
42%49%24%17%2
2496
February1st to 15thNight
39%39%24%19%3
2287
16th to endNight
39%39%25%19%2
1909
March1st to 15thNight
39%5%10%26%15%2
2001
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
36%8%16%21%15%2
1441
June1st to 15thNight
35%8%12%25%16%1
1320
16th to endNight
35%8%15%18%20%2
1320
July1st to 15thNight
39%9%15%18%13%4
1246
16th to endNight
29%10%16%26%15%1
1469
August1st to 15thNight
36%8%13%22%17%1
1476
16th to endNight
39%6%13%22%15%2
1750
September1st to 15thNight
35%8%11%19%20%4
1650
16th to endNight
41%6%10%17%19%3
1809
October1st to 15thNight
41%48%22%18%4
1980
16th to endNight
33%48%26%22%4
2112
November1st to 15thNight
36%39%21%21%7%
2212
16th to endNight
36%5%9%24%19%4
2453
December1st to 15thNight
43%411%21%15%3
2520
16th to endNight
43%412%22%14%2
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
24%24%35%12%11
77
16th to endDay
18%30%32%16%2
96
February1st to 15thDay
27%20%37%12%2
90
16th to endDay
18%20%39%20%1
222
March1st to 15thDay
18%18%34%24%4
270
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%121%26%22%8%
302
June1st to 15thDay
13%326%25%21%9%
255
16th to endDay
8%322%36%23%5%
255
July1st to 15thDay
12%119%33%24%8%
245
16th to endDay
4114%32%33%14%
270
August1st to 15thDay
14%115%26%33%8%
253
16th to endDay
8%112%31%25%20%
268
September1st to 15thDay
11%318%26%19%20%
255
16th to endDay
25%419%21%16%12%
254
October1st to 15thDay
21%5%24%19%22%5%
255
16th to endDay
13%11%29%29%13%2
272
November1st to 15thDay
19%8%29%23%15%4
246
16th to endDay
19%15%28%23%9%3
126
December1st to 15thDay
20%15%33%15%13%3
60
16th to endDay
40%9%28%20%1
64
January1st to 15thNight
21%19%33%20%5%
720
16th to endNight
22%17%37%21%1
768
February1st to 15thNight
20%16%37%23%2
704
16th to endNight
20%19%37%20%1
631
March1st to 15thNight
21%14%36%23%3
720
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
11%6%20%36%23%1
361
June1st to 15thNight
10%223%37%24%1
330
16th to endNight
7%317%45%24%
330
July1st to 15thNight
9%222%34%29%1
316
16th to endNight
5%114%37%33%6%
522
August1st to 15thNight
11%114%41%27%3
496
16th to endNight
12%213%37%30%4
700
September1st to 15thNight
13%218%31%31%2
660
16th to endNight
17%319%30%26%2
657
October1st to 15thNight
19%8%21%27%21%3
660
16th to endNight
16%12%30%25%14%
704
November1st to 15thNight
17%12%28%25%13%1
700
16th to endNight
18%16%29%25%10%1
720
December1st to 15thNight
20%15%33%20%10%
720
16th to endNight
21%13%38%23%2
768
Page generated at 2017-05-25 13:27:36. Version ac04

Copyright 2017 A. Danko


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