Williamsburg
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Williamsburg clear sky chart from 2005-11-05 to 2017-10-06.

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
39%
16060
19%
47094
11%
16016
46%
16016
27%
29972

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
27%
900
15%
2506
5%
895
35%
895
23%
1716
January16th to endNight
22%
960
17%
2499
5%
960
31%
960
21%
1793
February1st to 15thNight
24%
877
15%
2288
4%
876
31%
876
22%
1618
February16th to endNight
30%
681
20%
1943
8%
681
41%
681
26%
1254
March1st to 15thNight
33%
603
20%
2043
12%
603
42%
603
24%
1242
March16th to endNight
26%
576
11%
2074
7%
576
34%
576
15%
1456
April1st to 15thNight
28%
539
18%
1797
12%
539
38%
539
23%
1134
April16th to endNight
33%
540
18%
1785
12%
540
46%
540
22%
1179
May1st to 15thNight
35%
540
18%
1521
8%
540
45%
540
27%
1002
May16th to endNight
46%
563
20%
1497
9%
561
55%
561
32%
965
June1st to 15thNight
54%
540
16%
1440
8%
540
57%
540
37%
891
June16th to endNight
54%
540
18%
1440
9%
540
57%
540
40%
854
July1st to 15thNight
67%
510
18%
1360
11%
510
69%
510
45%
768
July16th to endNight
70%
564
24%
1477
18%
531
72%
531
46%
816
August1st to 15thNight
70%
539
22%
1610
24%
536
72%
536
41%
878
August16th to endNight
67%
573
23%
1869
26%
573
71%
573
35%
1035
September1st to 15thNight
62%
540
28%
1800
27%
540
68%
540
36%
946
September16th to endNight
55%
615
28%
1962
20%
615
65%
615
39%
1057
October1st to 15thNight
44%
684
26%
2052
18%
684
50%
684
32%
1229
October16th to endNight
34%
704
20%
2115
11%
704
43%
704
28%
1309
November1st to 15thNight
35%
712
26%
2288
15%
712
47%
712
30%
1311
November16th to endNight
27%
900
17%
2520
8%
900
36%
900
24%
1669
December1st to 15thNight
21%
900
14%
2520
5%
900
29%
900
17%
1831
December16th to endNight
22%
960
15%
2688
5%
960
31%
960
16%
2019

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%7%35%5%3445%7%38%
48102
Night
12%7%35%424446%43%
47269
Cloud CoverDay
13%7%35%5%3445%7%38%
48111
Night
12%7%35%424446%43%
47273
TransparencyDay
44%313%23%12%1
47794
Night
41%312%25%14%2
47095
SeeingDay
14%6%26%28%18%4
6074
Night
16%7%30%31%13%1
16063

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
10%10%5%7%7%45%448%30%
1432
16th to endDay
12%13%6%7%8%445%47%25%
1722
February1st to 15thDay
12%11%5%7%6%3435%8%30%
1575
16th to endDay
13%8%47%6%24446%35%
1529
March1st to 15thDay
14%9%46%5%33447%36%
1915
16th to endDay
19%11%46%424446%30%
2149
April1st to 15thDay
17%9%36%5%245%46%33%
2151
16th to endDay
24%8%25%424446%31%
2173
May1st to 15thDay
19%8%35%5%35%5%46%31%
2429
16th to endDay
13%5%36%6%45%5%5%8%36%
2617
June1st to 15thDay
12%6%25%5%26%46%7%41%
2520
16th to endDay
9%425%5%345%6%10%42%
2520
July1st to 15thDay
7%425%4346%6%9%46%
2520
16th to endDay
6%4145%35%6%7%9%44%
2588
August1st to 15thDay
5%425%5%46%6%7%9%42%
2340
16th to endDay
11%425%6%345%5%7%43%
2331
September1st to 15thDay
12%31432435%7%51%
2160
16th to endDay
10%424435%5%47%47%
1983
October1st to 15thDay
14%5%26%423245%46%
1706
16th to endDay
16%7%25%5%33445%42%
1757
November1st to 15thDay
11%7%3443445%6%44%
1569
16th to endDay
19%8%34333348%38%
1440
December1st to 15thDay
19%12%5%8%5%35%336%26%
1440
16th to endDay
17%16%5%8%6%24335%24%
1536
January1st to 15thNight
10%9%5%7%5%45%35%6%36%
2506
16th to endNight
11%13%6%7%5%24345%35%
2499
February1st to 15thNight
13%11%47%5%24346%35%
2285
16th to endNight
13%6%36%423446%43%
1943
March1st to 15thNight
14%7%3443335%6%43%
2031
16th to endNight
16%9%48%6%35%43432%
2074
April1st to 15thNight
15%8%26%424346%40%
1797
16th to endNight
19%8%2443345%5%37%
1785
May1st to 15thNight
16%424425%45%7%40%
1521
16th to endNight
11%425%425%5%5%7%44%
1496
June1st to 15thNight
8%5%345%2346%8%47%
1440
16th to endNight
7%424425%47%9%47%
1440
July1st to 15thNight
43143245%6%9%54%
1440
16th to endNight
43233245%7%10%52%
1578
August1st to 15thNight
34245%235%48%54%
1620
16th to endNight
8%424424446%53%
1869
September1st to 15thNight
9%414423347%55%
1800
16th to endNight
7%41432335%8%54%
1962
October1st to 15thNight
13%5%24424335%49%
2052
16th to endNight
14%6%25%424336%45%
2115
November1st to 15thNight
12%5%24324336%51%
2288
16th to endNight
17%7%35%423346%41%
2520
December1st to 15thNight
15%13%5%7%5%2443433%
2520
16th to endNight
20%12%5%6%5%3434430%
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
10%9%5%8%7%5%5%5%48%30%
1432
16th to endDay
13%13%6%6%8%44446%25%
1722
February1st to 15thDay
12%12%5%7%6%34448%30%
1575
16th to endDay
13%8%47%5%345%47%35%
1529
March1st to 15thDay
15%10%35%5%24447%35%
1926
16th to endDay
19%10%46%42445%6%31%
2147
April1st to 15thDay
17%9%36%6%235%46%33%
2151
16th to endDay
24%9%25%424446%31%
2173
May1st to 15thDay
19%8%36%5%35%45%6%31%
2429
16th to endDay
13%6%26%5%36%5%5%8%35%
2617
June1st to 15thDay
13%5%245%25%5%6%6%41%
2520
16th to endDay
9%4245%345%6%9%42%
2520
July1st to 15thDay
7%4245%35%6%6%9%46%
2520
16th to endDay
7%4245%35%6%7%9%43%
2588
August1st to 15thDay
6%3346%45%5%7%9%42%
2340
16th to endDay
11%425%6%35%45%7%43%
2331
September1st to 15thDay
12%314323448%51%
2160
16th to endDay
10%424425%45%7%47%
1983
October1st to 15thDay
15%5%25%423245%46%
1706
16th to endDay
16%6%25%433445%42%
1757
November1st to 15thDay
11%7%345%245%5%6%44%
1569
16th to endDay
18%8%34333348%38%
1440
December1st to 15thDay
18%13%49%5%25%436%26%
1440
16th to endDay
18%15%5%8%6%34335%24%
1536
January1st to 15thNight
10%8%5%7%5%35%447%36%
2506
16th to endNight
10%12%6%8%6%24336%34%
2499
February1st to 15thNight
13%11%5%7%5%23445%35%
2278
16th to endNight
13%6%36%5%23446%43%
1943
March1st to 15thNight
14%8%34323345%44%
2043
16th to endNight
16%9%5%8%6%45%44432%
2074
April1st to 15thNight
16%8%35%42445%6%40%
1797
16th to endNight
19%7%344335%46%37%
1785
May1st to 15thNight
17%425%4345%5%7%39%
1520
16th to endNight
10%425%5%36%45%7%43%
1496
June1st to 15thNight
8%6%35%42245%9%46%
1440
16th to endNight
7%423335%47%8%48%
1440
July1st to 15thNight
4243245%6%9%55%
1440
16th to endNight
43133246%6%10%51%
1578
August1st to 15thNight
432443445%8%54%
1620
16th to endNight
9%32442345%6%53%
1869
September1st to 15thNight
9%3135%1345%6%55%
1800
16th to endNight
7%413423448%54%
1962
October1st to 15thNight
13%424324345%49%
2052
16th to endNight
14%6%25%424346%46%
2115
November1st to 15thNight
12%5%24323336%51%
2288
16th to endNight
16%8%25%424336%41%
2520
December1st to 15thNight
16%12%5%7%5%2343433%
2520
16th to endNight
20%12%46%5%3343429%
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
53%212%19%10%
1433
16th to endDay
58%313%17%7%
1722
February1st to 15thDay
52%215%23%5%
1576
16th to endDay
47%416%21%8%1
1529
March1st to 15thDay
48%315%22%9%
1926
16th to endDay
53%314%19%6%1
2149
April1st to 15thDay
51%39%25%9%
2151
16th to endDay
52%311%19%10%1
2173
May1st to 15thDay
53%411%22%8%
2429
16th to endDay
45%5%12%22%12%2
2621
June1st to 15thDay
40%413%25%14%1
2520
16th to endDay
36%5%13%24%18%2
2520
July1st to 15thDay
34%5%13%29%13%3
2371
16th to endDay
33%5%16%26%18%1
2426
August1st to 15thDay
34%5%15%28%15%1
2326
16th to endDay
39%310%27%16%2
2331
September1st to 15thDay
31%310%29%22%2
2160
16th to endDay
36%311%30%16%1
1983
October1st to 15thDay
40%210%24%21%1
1706
16th to endDay
44%311%24%15%1
1757
November1st to 15thDay
39%312%26%16%2
1569
16th to endDay
46%217%24%8%
1440
December1st to 15thDay
60%113%18%5%
1440
16th to endDay
63%212%12%7%
1536
January1st to 15thNight
47%315%18%14%
2506
16th to endNight
51%215%19%10%
2499
February1st to 15thNight
49%212%24%9%
2288
16th to endNight
41%313%27%12%1
1943
March1st to 15thNight
41%213%27%13%1
2043
16th to endNight
55%313%17%8%
2074
April1st to 15thNight
44%110%27%13%2
1797
16th to endNight
47%310%22%13%2
1785
May1st to 15thNight
43%315%24%13%
1521
16th to endNight
38%412%24%17%3
1498
June1st to 15thNight
35%314%28%14%3
1440
16th to endNight
30%414%25%20%4
1440
July1st to 15thNight
26%416%30%17%3
1360
16th to endNight
24%416%33%19%1
1477
August1st to 15thNight
27%414%33%16%3
1610
16th to endNight
31%411%27%20%5%
1869
September1st to 15thNight
29%37%30%26%3
1800
16th to endNight
28%311%34%19%2
1962
October1st to 15thNight
36%28%24%22%4
2052
16th to endNight
40%213%24%15%2
2115
November1st to 15thNight
35%211%26%19%4
2288
16th to endNight
45%212%28%10%
2520
December1st to 15thNight
55%212%21%8%
2520
16th to endNight
58%311%17%9%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
25%7%37%21%6%1
90
16th to endDay
19%8%32%33%42
96
February1st to 15thDay
8%24%38%18%8%
90
16th to endDay
24%6%38%14%16%
81
March1st to 15thDay
22%18%43%12%3
90
16th to endDay
39%8%25%18%8%
133
April1st to 15thDay
17%12%32%24%11%
270
16th to endDay
30%6%29%21%9%2
283
May1st to 15thDay
19%12%31%26%7%2
468
16th to endDay
16%7%28%29%14%3
517
June1st to 15thDay
13%326%31%19%4
495
16th to endDay
8%6%29%31%18%4
495
July1st to 15thDay
6%323%32%25%8%
475
16th to endDay
7%219%37%26%5%
515
August1st to 15thDay
8%220%33%29%5%
493
16th to endDay
12%314%33%28%7%
371
September1st to 15thDay
9%218%32%28%7%
315
16th to endDay
9%427%30%23%4
228
October1st to 15thDay
11%12%28%26%15%6%
107
16th to endDay
18%6%40%25%9%
96
November1st to 15thDay
5%6%34%24%20%8%
90
16th to endDay
45%13%27%11%2
90
December1st to 15thDay
26%16%30%15%11%
90
16th to endDay
32%15%37%13%1
96
January1st to 15thNight
15%12%37%26%8%
900
16th to endNight
17%11%38%26%6%
960
February1st to 15thNight
19%10%35%27%5%
880
16th to endNight
16%9%39%27%5%1
681
March1st to 15thNight
19%8%35%24%11%
603
16th to endNight
21%9%34%27%7%
576
April1st to 15thNight
21%8%36%27%5%
539
16th to endNight
22%7%32%29%8%
540
May1st to 15thNight
18%6%34%32%7%1
540
16th to endNight
12%231%35%16%
563
June1st to 15thNight
11%421%44%17%
540
16th to endNight
8%226%44%17%
540
July1st to 15thNight
6%115%41%32%1
510
16th to endNight
6%113%45%29%3
564
August1st to 15thNight
6%211%45%31%2
539
16th to endNight
12%112%40%29%4
573
September1st to 15thNight
10%316%36%28%4
540
16th to endNight
8%423%35%24%2
615
October1st to 15thNight
17%5%27%28%17%1
684
16th to endNight
18%8%32%29%10%
704
November1st to 15thNight
15%10%35%26%10%
712
16th to endNight
21%10%35%24%7%
900
December1st to 15thNight
23%11%34%24%5%
900
16th to endNight
27%10%30%24%7%
960
Page generated at 2017-10-07 13:07:52. Version ac04

Copyright 2017 A. Danko


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