Wilton
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Wilton clear sky chart from 2005-11-05 to 2019-07-16.

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
22%
17157
18%
54144
6%
17125
35%
17125
17%
35312

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
12%
840
21%
2926
5%
836
31%
836
13%
1866
January16th to endNight
11%
896
17%
3136
2%
896
29%
896
12%
2041
February1st to 15thNight
11%
824
17%
2586
3%
821
28%
821
12%
1749
February16th to endNight
10%
735
20%
2205
4%
735
29%
735
12%
1478
March1st to 15thNight
14%
840
17%
2520
6%
840
28%
840
12%
1744
March16th to endNight
17%
896
22%
2358
5%
896
31%
896
18%
1583
April1st to 15thNight
15%
838
18%
2096
2%
838
25%
838
17%
1485
April16th to endNight
18%
688
17%
1934
2%
688
29%
688
18%
1314
May1st to 15thNight
18%
630
14%
1890
5%
630
28%
630
14%
1326
May16th to endNight
20%
452
11%
1766
6%
451
31%
451
13%
1171
June1st to 15thNight
24%
420
13%
1680
8%
420
37%
420
14%
1116
June16th to endNight
30%
420
11%
1680
9%
420
40%
420
18%
1033
July1st to 15thNight
42%
400
14%
1600
16%
400
50%
400
21%
936
July16th to endNight
40%
410
15%
1554
14%
388
51%
388
23%
913
August1st to 15thNight
40%
511
17%
1652
11%
509
45%
509
22%
1023
August16th to endNight
46%
792
19%
2034
9%
792
50%
792
31%
1315
September1st to 15thNight
35%
780
20%
2005
8%
780
42%
780
32%
1280
September16th to endNight
40%
777
26%
2145
14%
777
50%
777
30%
1312
October1st to 15thNight
29%
780
21%
2190
9%
780
41%
780
22%
1452
October16th to endNight
21%
832
17%
2701
5%
832
35%
832
17%
1753
November1st to 15thNight
23%
820
21%
2667
9%
820
38%
820
17%
1713
November16th to endNight
18%
840
21%
2743
6%
840
35%
840
16%
1744
December1st to 15thNight
17%
840
18%
2940
5%
840
34%
840
13%
1913
December16th to endNight
12%
896
18%
3136
3%
896
29%
896
11%
2052

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
17%6%25%5%3445%6%38%
55179
Night
16%6%2442445%6%41%
54314
Cloud CoverDay
17%5%25%5%3445%6%39%
55189
Night
17%6%2442445%6%42%
54319
TransparencyDay
44%6%11%20%14%2
54867
Night
42%6%12%21%14%1
54144
SeeingDay
22%21%28%17%7%2
8124
Night
18%25%28%19%5%
17157

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
13%7%26%5%245%5%7%39%
1672
16th to endDay
14%7%245%34448%40%
1792
February1st to 15thDay
16%6%26%5%35%7%5%7%32%
1931
16th to endDay
14%8%36%5%3445%6%37%
1840
March1st to 15thDay
18%9%35%434446%34%
2090
16th to endDay
15%7%35%5%24345%41%
2566
April1st to 15thDay
17%7%35%6%24446%36%
2512
16th to endDay
20%6%15%42435%6%39%
2630
May1st to 15thDay
24%7%35%5%24435%32%
2722
16th to endDay
18%5%35%5%34447%38%
3065
June1st to 15thDay
23%5%25%5%3446%7%33%
2940
16th to endDay
16%5%26%6%35%5%6%8%33%
2940
July1st to 15thDay
12%425%6%35%5%5%8%40%
2940
16th to endDay
12%5%35%6%36%5%6%7%36%
2882
August1st to 15thDay
13%5%25%6%45%6%6%7%36%
2537
16th to endDay
11%4145%345%7%8%42%
2518
September1st to 15thDay
15%5%15%5%3345%6%42%
2285
16th to endDay
15%424423335%50%
2145
October1st to 15thDay
22%5%24434345%39%
2100
16th to endDay
18%5%25%424446%40%
1873
November1st to 15thDay
19%5%24323345%43%
1851
16th to endDay
18%6%1443444444%
1876
December1st to 15thDay
17%6%25%5%335%5%5%39%
1680
16th to endDay
21%8%345%23446%35%
1792
January1st to 15thNight
13%8%25%42445%6%42%
2926
16th to endNight
14%9%35%4345%46%38%
3136
February1st to 15thNight
14%9%35%5%35%5%5%6%37%
2585
16th to endNight
14%8%35%42445%5%40%
2205
March1st to 15thNight
17%7%25%435%445%37%
2507
16th to endNight
15%6%2442435%6%44%
2358
April1st to 15thNight
17%6%35%5%3445%6%38%
2096
16th to endNight
22%5%25%324335%41%
1934
May1st to 15thNight
20%6%35%5%3345%6%34%
1890
16th to endNight
18%5%26%5%2445%7%35%
1764
June1st to 15thNight
22%5%24424446%36%
1680
16th to endNight
16%41442446%7%41%
1680
July1st to 15thNight
12%423425%5%6%8%44%
1680
16th to endNight
13%424435%46%8%43%
1650
August1st to 15thNight
9%6%25%435%46%7%44%
1662
16th to endNight
13%31443446%8%46%
2034
September1st to 15thNight
15%41431246%6%48%
2005
16th to endNight
13%424323345%52%
2145
October1st to 15thNight
19%5%13433345%45%
2190
16th to endNight
19%6%24424447%40%
2701
November1st to 15thNight
18%5%25%424435%43%
2667
16th to endNight
17%6%23423337%43%
2743
December1st to 15thNight
17%7%35%43435%5%39%
2940
16th to endNight
17%7%35%32445%7%37%
3136

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
13%7%26%5%245%5%6%40%
1672
16th to endDay
14%7%2443435%6%42%
1792
February1st to 15thDay
16%7%36%5%35%6%6%6%33%
1931
16th to endDay
15%8%35%5%35%45%5%38%
1840
March1st to 15thDay
19%9%345%34446%34%
2100
16th to endDay
16%6%35%5%24445%42%
2566
April1st to 15thDay
17%6%35%5%2445%6%37%
2512
16th to endDay
20%5%25%42445%6%39%
2630
May1st to 15thDay
24%7%35%5%24445%33%
2722
16th to endDay
18%5%25%5%34446%38%
3065
June1st to 15thDay
23%42443445%7%33%
2940
16th to endDay
17%425%5%45%5%6%8%35%
2940
July1st to 15thDay
12%425%5%35%5%5%8%40%
2940
16th to endDay
13%436%6%35%6%6%7%37%
2882
August1st to 15thDay
13%425%6%35%5%7%7%36%
2537
16th to endDay
12%3245%345%6%8%43%
2518
September1st to 15thDay
15%4245%35%347%44%
2285
16th to endDay
15%5%23423335%50%
2145
October1st to 15thDay
22%5%14433445%40%
2100
16th to endDay
19%415%425%446%41%
1873
November1st to 15thDay
19%6%24323345%43%
1851
16th to endDay
18%5%245%2434444%
1876
December1st to 15thDay
17%5%25%43445%5%40%
1680
16th to endDay
22%8%235%23346%36%
1792
January1st to 15thNight
14%7%3442435%6%42%
2926
16th to endNight
15%8%35%42445%5%39%
3136
February1st to 15thNight
14%8%25%5%35%5%5%6%38%
2578
16th to endNight
15%7%35%42345%6%40%
2205
March1st to 15thNight
19%7%25%5%2445%5%38%
2520
16th to endNight
15%6%24324346%45%
2357
April1st to 15thNight
18%6%345%2445%6%38%
2096
16th to endNight
22%5%24424245%41%
1934
May1st to 15thNight
20%6%25%5%3445%5%35%
1890
16th to endNight
19%5%26%425%45%7%36%
1764
June1st to 15thNight
22%5%34423446%37%
1680
16th to endNight
17%42343446%7%42%
1680
July1st to 15thNight
13%324435%45%8%44%
1680
16th to endNight
13%425%335%46%8%43%
1650
August1st to 15thNight
10%5%245%35%5%6%7%44%
1662
16th to endNight
13%41333435%8%46%
2034
September1st to 15thNight
15%32332335%6%49%
2005
16th to endNight
14%414323346%52%
2145
October1st to 15thNight
20%413423345%45%
2190
16th to endNight
19%6%24324446%41%
2701
November1st to 15thNight
18%5%15%425%345%43%
2667
16th to endNight
18%5%24333346%43%
2743
December1st to 15thNight
18%7%35%425%45%5%39%
2940
16th to endNight
18%7%2442345%6%38%
3136

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%6%10%24%12%2
1673
16th to endDay
41%5%14%23%14%1
1792
February1st to 15thDay
48%7%12%19%11%1
1934
16th to endDay
46%5%10%19%15%3
1840
March1st to 15thDay
50%5%11%17%11%2
2100
16th to endDay
44%5%9%19%15%5%
2566
April1st to 15thDay
47%5%9%18%14%3
2512
16th to endDay
45%48%18%18%3
2630
May1st to 15thDay
52%5%8%18%12%2
2722
16th to endDay
44%7%12%20%12%2
3069
June1st to 15thDay
48%6%13%19%11%
2940
16th to endDay
45%8%12%19%12%1
2940
July1st to 15thDay
38%8%12%22%16%1
2791
16th to endDay
41%7%14%20%13%2
2715
August1st to 15thDay
42%8%14%21%11%1
2523
16th to endDay
35%8%12%25%15%1
2518
September1st to 15thDay
39%6%11%21%18%2
2285
16th to endDay
37%49%23%23%2
2145
October1st to 15thDay
45%6%8%23%13%2
2100
16th to endDay
42%7%10%23%13%2
1873
November1st to 15thDay
43%412%22%14%1
1851
16th to endDay
43%5%13%19%16%2
1876
December1st to 15thDay
44%6%13%17%16%
1680
16th to endDay
49%5%11%19%12%1
1792
January1st to 15thNight
42%5%11%23%13%2
2926
16th to endNight
45%5%11%20%14%2
3136
February1st to 15thNight
45%6%13%19%12%2
2586
16th to endNight
44%6%10%22%13%3
2205
March1st to 15thNight
47%6%10%20%12%2
2520
16th to endNight
41%5%9%17%20%4
2358
April1st to 15thNight
46%5%10%18%17%2
2096
16th to endNight
45%410%18%15%4
1934
May1st to 15thNight
49%5%11%17%13%2
1890
16th to endNight
46%8%13%18%11%1
1766
June1st to 15thNight
48%6%11%21%11%1
1680
16th to endNight
40%8%13%22%14%1
1680
July1st to 15thNight
36%7%14%25%15%
1600
16th to endNight
37%7%16%25%11%1
1554
August1st to 15thNight
38%7%14%22%14%1
1652
16th to endNight
35%7%12%26%18%
2034
September1st to 15thNight
34%6%13%25%18%
2005
16th to endNight
34%48%27%22%1
2145
October1st to 15thNight
41%5%11%22%14%3
2190
16th to endNight
43%6%13%24%12%
2701
November1st to 15thNight
42%5%11%20%16%2
2667
16th to endNight
42%5%12%23%14%1
2743
December1st to 15thNight
45%6%13%22%12%
2940
16th to endNight
45%6%12%21%13%
3136

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
23%30%29%13%2
120
16th to endDay
30%28%28%11%
128
February1st to 15thDay
28%33%26%10%1
302
16th to endDay
22%35%27%12%1
343
March1st to 15thDay
29%31%26%10%2
390
16th to endDay
22%32%30%8%5%1
416
April1st to 15thDay
26%27%28%12%4
390
16th to endDay
27%22%29%12%6%2
379
May1st to 15thDay
32%24%24%11%5%1
408
16th to endDay
18%22%35%19%3
459
June1st to 15thDay
21%19%34%18%5%1
435
16th to endDay
16%20%36%18%5%1
435
July1st to 15thDay
13%11%29%29%14%2
425
16th to endDay
13%13%29%29%10%3
391
August1st to 15thDay
15%12%27%25%16%2
358
16th to endDay
12%10%27%27%16%5%
396
September1st to 15thDay
18%11%27%20%16%5%
375
16th to endDay
19%12%26%17%12%10%
374
October1st to 15thDay
25%14%24%21%11%2
375
16th to endDay
25%21%29%18%5%
336
November1st to 15thDay
24%24%25%17%6%1
325
16th to endDay
25%27%23%15%6%1
316
December1st to 15thDay
24%29%28%12%5%
120
16th to endDay
33%33%25%6%
128
January1st to 15thNight
17%35%32%12%2
840
16th to endNight
19%32%32%12%2
896
February1st to 15thNight
19%32%32%14%
824
16th to endNight
20%36%28%12%1
735
March1st to 15thNight
23%30%29%14%2
840
16th to endNight
19%33%26%15%4
896
April1st to 15thNight
21%30%29%15%2
838
16th to endNight
19%31%28%16%4
688
May1st to 15thNight
20%26%30%17%4
630
16th to endNight
19%22%33%20%4
452
June1st to 15thNight
20%22%30%22%4
420
16th to endNight
15%17%32%30%4
420
July1st to 15thNight
12%14%27%37%8%1
400
16th to endNight
11%16%28%32%10%
410
August1st to 15thNight
12%11%27%36%10%
511
16th to endNight
12%10%24%34%16%1
792
September1st to 15thNight
15%13%31%25%13%1
780
16th to endNight
16%13%24%24%17%2
777
October1st to 15thNight
22%17%27%23%9%1
780
16th to endNight
22%23%29%17%5%
832
November1st to 15thNight
18%28%26%18%6%1
820
16th to endNight
19%31%28%14%6%
840
December1st to 15thNight
21%30%27%15%4
840
16th to endNight
22%34%28%13%1
896
Page generated at 2019-07-17 08:53:09. Version ac04

Copyright 2019 A. Danko


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