Winchester
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Winchester clear sky chart from 2005-11-06 to 2024-01-12.

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
30%
22588
14%
74716
9%
22549
37%
22549
16%
53131

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 10thNight
17%
760
8%
2649
4%
757
23%
757
8%
2119
January11th to 20thNight
19%
725
9%
2535
5%
725
24%
725
8%
2017
January21st to endNight
24%
792
13%
2714
8%
792
31%
792
13%
2001
February1st to 10thNight
16%
720
9%
2340
3%
720
24%
720
10%
1842
February11th to 20thNight
20%
701
13%
2244
7%
700
29%
700
13%
1661
February21st to endNight
22%
592
16%
1776
7%
592
34%
592
15%
1327
March1st to 10thNight
23%
720
14%
2160
9%
720
30%
720
15%
1629
March11th to 20thNight
22%
720
16%
2002
6%
720
32%
720
18%
1453
March21st to endNight
30%
792
21%
2044
8%
792
37%
792
22%
1467
April1st to 10thNight
25%
720
16%
1800
5%
720
30%
720
20%
1338
April11th to 20thNight
27%
711
17%
1775
5%
711
33%
711
22%
1301
April21st to endNight
29%
632
16%
1712
7%
632
35%
632
21%
1232
May1st to 10thNight
32%
360
16%
1440
14%
360
41%
360
17%
934
May11th to 20thNight
34%
354
20%
1416
16%
354
49%
354
19%
894
May21st to endNight
35%
389
13%
1551
11%
388
48%
388
24%
902
June1st to 10thNight
40%
360
11%
1440
11%
360
48%
360
16%
968
June11th to 20thNight
40%
360
13%
1440
15%
360
48%
360
20%
856
June21st to endNight
40%
360
12%
1440
15%
360
48%
360
19%
914
July1st to 10thNight
51%
350
15%
1400
19%
350
60%
350
26%
788
July11th to 20thNight
52%
348
16%
1392
23%
348
57%
348
24%
797
July21st to endNight
51%
394
15%
1486
17%
372
55%
372
24%
859
August1st to 10thNight
52%
373
16%
1437
17%
371
58%
371
25%
851
August11th to 20thNight
54%
565
17%
1629
12%
565
57%
565
29%
1058
August21st to endNight
47%
792
18%
1980
9%
792
50%
792
33%
1297
September1st to 10thNight
48%
720
16%
1800
7%
720
50%
720
31%
1218
September11th to 20thNight
52%
720
23%
1816
14%
720
55%
720
36%
1161
September21st to endNight
44%
720
22%
2009
17%
720
48%
720
28%
1323
October1st to 10thNight
37%
720
17%
2160
12%
720
45%
720
22%
1479
October11th to 20thNight
29%
720
16%
2160
8%
720
39%
720
18%
1526
October21st to endNight
25%
792
13%
2535
7%
792
35%
792
14%
1872
November1st to 10thNight
27%
740
16%
2407
10%
740
37%
740
18%
1655
November11th to 20thNight
25%
760
17%
2616
10%
760
35%
760
15%
1851
November21st to endNight
20%
760
11%
2660
5%
760
28%
760
10%
2027
December1st to 10thNight
18%
760
8%
2660
3%
760
24%
760
8%
2150
December11th to 20thNight
17%
759
12%
3245
6%
759
25%
759
8%
2431
December21st to endNight
16%
827
5%
4846
2%
817
22%
817
4%
3933

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
18%9%37%6%35%5%5%6%33%
74732
Night
19%10%47%6%3445%6%32%
74969
TransparencyDay
51%5%11%21%10%1
74437
Night
53%5%11%20%10%1
74798
SeeingDay
22%13%30%22%10%3
13404
Night
23%10%29%28%10%1
22600

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 10thDay
20%17%7%10%7%45%44420%
1517
11th to 20thDay
19%16%6%9%7%45%5%45%21%
1448
21st to endDay
14%13%5%10%9%5%5%5%5%6%23%
1638
February1st to 10thDay
17%14%6%10%8%45%35%5%23%
1620
11th to 20thDay
15%14%5%10%7%5%4436%27%
1603
21st to endDay
20%15%5%9%6%35%445%24%
1480
March1st to 10thDay
15%12%6%11%7%35%44429%
1797
11th to 20thDay
18%11%47%6%36%5%46%30%
1953
21st to endDay
19%11%36%5%33446%36%
2311
April1st to 10thDay
22%12%36%5%3435%6%32%
2160
11th to 20thDay
21%8%47%5%33436%36%
2134
21st to endDay
24%9%26%5%3445%6%31%
2264
May1st to 10thDay
20%8%36%6%3445%6%35%
2520
11th to 20thDay
17%7%36%6%3445%6%39%
2478
21st to endDay
16%6%35%5%35%5%7%7%38%
2725
June1st to 10thDay
23%7%36%5%35%6%5%7%30%
2520
11th to 20thDay
16%5%25%6%35%5%6%8%39%
2520
21st to endDay
20%7%26%6%35%5%6%7%32%
2520
July1st to 10thDay
12%5%25%5%35%5%6%8%43%
2520
11th to 20thDay
12%5%35%6%36%7%7%10%37%
2520
21st to endDay
12%5%25%6%46%5%7%10%38%
2744
August1st to 10thDay
12%6%35%6%45%6%6%7%39%
2489
11th to 20thDay
13%5%25%5%45%6%6%7%41%
2252
21st to endDay
15%6%245%46%5%6%7%41%
2376
September1st to 10thDay
14%5%26%5%35%5%6%7%42%
2160
11th to 20thDay
15%5%245%4445%7%46%
2143
21st to endDay
19%5%25%4345%6%8%39%
1949
October1st to 10thDay
21%6%37%5%35%5%46%35%
1800
11th to 20thDay
21%9%38%7%45%5%5%6%27%
1800
21st to endDay
27%10%36%6%3445%5%26%
1819
November1st to 10thDay
16%9%46%6%346%5%8%32%
1671
11th to 20thDay
19%12%47%7%35%45%5%29%
1563
21st to endDay
21%14%5%9%7%45%5%46%21%
1520
December1st to 10thDay
24%15%5%10%6%45%5%45%16%
1520
11th to 20thDay
23%16%5%7%5%34445%23%
1906
21st to endDay
26%17%5%10%5%3433320%
2772
January1st to 10thNight
19%16%7%11%7%44445%19%
2649
11th to 20thNight
19%18%6%10%7%35%44421%
2535
21st to endNight
13%15%6%9%7%4445%5%28%
2714
February1st to 10thNight
16%15%6%11%7%45%45%5%21%
2340
11th to 20thNight
15%13%5%10%7%4445%6%28%
2242
21st to endNight
17%14%5%9%6%35%445%29%
1776
March1st to 10thNight
17%14%5%9%6%3444429%
2150
11th to 20thNight
18%12%47%6%24345%34%
2002
21st to endNight
19%8%36%5%3444439%
2044
April1st to 10thNight
21%9%47%5%44445%34%
1800
11th to 20thNight
19%9%36%5%345%5%6%35%
1775
21st to endNight
23%9%36%5%34446%34%
1712
May1st to 10thNight
22%7%35%6%25%45%7%35%
1440
11th to 20thNight
17%9%36%5%335%5%5%40%
1415
21st to endNight
12%6%25%5%35%5%5%8%44%
1550
June1st to 10thNight
19%6%37%5%46%5%7%9%30%
1440
11th to 20thNight
16%6%35%425%45%9%40%
1440
21st to endNight
18%7%36%435%46%8%36%
1440
July1st to 10thNight
10%435%5%35%5%5%10%44%
1440
11th to 20thNight
12%5%25%4345%7%10%43%
1440
21st to endNight
10%5%25%5%45%6%7%8%44%
1574
August1st to 10thNight
11%6%35%6%345%5%9%42%
1447
11th to 20thNight
11%7%35%6%446%5%8%43%
1629
21st to endNight
13%6%345%345%5%7%45%
1980
September1st to 10thNight
13%5%35%6%3445%6%44%
1800
11th to 20thNight
14%5%2343345%7%49%
1816
21st to endNight
15%6%36%5%3445%6%43%
2009
October1st to 10thNight
19%7%35%5%35%45%7%36%
2160
11th to 20thNight
21%8%37%5%345%5%6%32%
2160
21st to endNight
25%9%37%6%35%45%6%27%
2535
November1st to 10thNight
20%9%46%6%34437%35%
2407
11th to 20thNight
18%11%37%6%35%446%32%
2616
21st to endNight
22%12%6%8%6%34447%24%
2660
December1st to 10thNight
22%16%6%9%6%45%45%5%18%
2660
11th to 20thNight
23%16%48%5%3433427%
3288
21st to endNight
32%15%5%9%5%3333318%
4884

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 10thDay
69%410%11%6%
1518
11th to 20thDay
66%48%11%9%1
1448
21st to endDay
62%5%11%15%7%1
1638
February1st to 10thDay
64%49%19%41
1620
11th to 20thDay
60%410%17%8%1
1612
21st to endDay
63%410%13%9%1
1480
March1st to 10thDay
59%311%17%10%1
1800
11th to 20thDay
55%412%15%11%3
1953
21st to endDay
50%49%19%14%5%
2311
April1st to 10thDay
55%46%20%13%3
2160
11th to 20thDay
51%47%21%15%2
2134
21st to endDay
54%48%16%14%3
2264
May1st to 10thDay
49%5%7%20%15%3
2520
11th to 20thDay
45%5%9%22%16%2
2482
21st to endDay
43%6%14%21%13%2
2725
June1st to 10thDay
53%6%11%18%12%1
2520
11th to 20thDay
43%5%12%25%13%2
2520
21st to endDay
50%6%13%20%10%1
2520
July1st to 10thDay
38%6%12%28%15%1
2450
11th to 20thDay
39%6%16%29%10%
2445
21st to endDay
41%7%16%25%11%
2591
August1st to 10thDay
42%7%16%26%9%
2475
11th to 20thDay
40%6%17%27%10%1
2252
21st to endDay
41%6%13%28%12%
2376
September1st to 10thDay
39%7%13%29%13%
2160
11th to 20thDay
38%5%11%33%13%
2143
21st to endDay
43%6%14%28%9%
1949
October1st to 10thDay
50%5%11%22%11%
1800
11th to 20thDay
57%6%14%17%6%1
1800
21st to endDay
60%411%16%8%1
1819
November1st to 10thDay
50%5%15%23%7%1
1671
11th to 20thDay
55%5%13%17%9%1
1563
21st to endDay
63%6%13%13%5%
1520
December1st to 10thDay
69%7%11%10%2
1520
11th to 20thDay
64%5%13%14%4
1906
21st to endDay
72%37%15%3
2772
January1st to 10thNight
68%410%12%5%
2649
11th to 20thNight
67%49%11%8%1
2535
21st to endNight
59%49%17%10%1
2714
February1st to 10thNight
65%410%15%6%1
2340
11th to 20thNight
58%5%9%20%8%
2244
21st to endNight
58%48%18%11%1
1776
March1st to 10thNight
60%310%16%11%1
2160
11th to 20thNight
54%410%19%11%2
2003
21st to endNight
50%47%20%15%4
2044
April1st to 10thNight
54%39%19%14%2
1800
11th to 20thNight
49%6%7%20%16%2
1775
21st to endNight
52%49%16%16%3
1712
May1st to 10thNight
49%5%8%19%16%5%
1440
11th to 20thNight
45%6%7%21%18%3
1416
21st to endNight
39%5%17%20%16%3
1551
June1st to 10thNight
49%7%13%19%11%1
1440
11th to 20thNight
41%6%12%22%17%2
1440
21st to endNight
45%6%14%21%13%1
1440
July1st to 10thNight
34%7%13%27%16%2
1400
11th to 20thNight
36%6%14%31%13%
1392
21st to endNight
36%8%16%26%14%1
1486
August1st to 10thNight
38%6%19%24%12%1
1437
11th to 20thNight
41%5%15%26%13%1
1629
21st to endNight
37%6%16%28%13%1
1980
September1st to 10thNight
40%6%12%27%15%1
1800
11th to 20thNight
36%413%34%12%1
1816
21st to endNight
41%5%11%30%11%
2009
October1st to 10thNight
47%5%11%24%12%1
2160
11th to 20thNight
52%412%22%9%
2160
21st to endNight
58%413%15%8%1
2535
November1st to 10thNight
51%414%21%10%1
2407
11th to 20thNight
54%5%12%18%10%1
2616
21st to endNight
61%5%12%16%5%
2660
December1st to 10thNight
68%5%10%14%3
2660
11th to 20thNight
63%310%16%7%1
3288
21st to endNight
72%48%13%3
4884

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 10thDay
23%16%32%21%8%
180
11th to 20thDay
29%20%31%14%6%1
162
21st to endDay
24%15%35%20%8%1
200
February1st to 10thDay
31%21%36%11%1
240
11th to 20thDay
24%20%41%13%2
270
21st to endDay
29%14%36%18%2
346
March1st to 10thDay
22%13%36%20%7%
420
11th to 20thDay
24%14%37%19%5%1
420
21st to endDay
29%14%29%19%8%2
462
April1st to 10thDay
26%16%34%17%6%1
420
11th to 20thDay
27%19%25%19%7%3
412
21st to endDay
29%17%31%13%9%1
420
May1st to 10thDay
25%13%30%21%8%2
423
11th to 20thDay
17%15%37%19%9%2
447
21st to endDay
18%15%31%21%13%2
485
June1st to 10thDay
27%10%27%25%9%2
450
11th to 20thDay
16%13%29%27%10%4
450
21st to endDay
22%10%30%22%11%4
450
July1st to 10thDay
13%7%24%36%16%4
445
11th to 20thDay
14%8%27%33%15%3
444
21st to endDay
14%7%29%33%14%2
488
August1st to 10thDay
16%7%24%33%17%4
446
11th to 20thDay
14%5%23%29%20%8%
439
21st to endDay
14%11%27%24%15%8%
495
September1st to 10thDay
14%8%28%26%14%10%
450
11th to 20thDay
14%8%25%24%18%12%
450
21st to endDay
19%9%30%19%17%5%
450
October1st to 10thDay
22%12%27%18%14%6%
450
11th to 20thDay
25%18%26%24%6%1
450
21st to endDay
32%15%25%22%6%
414
November1st to 10thDay
25%19%27%20%7%1
365
11th to 20thDay
25%18%35%14%8%
223
21st to endDay
26%22%31%13%7%1
180
December1st to 10thDay
31%14%32%19%4
180
11th to 20thDay
27%25%33%11%41
180
21st to endDay
34%18%29%15%32
198
January1st to 10thNight
27%14%31%24%4
760
11th to 20thNight
30%12%30%22%5%1
725
21st to endNight
21%10%33%29%6%
792
February1st to 10thNight
22%14%37%23%4
720
11th to 20thNight
21%14%37%23%4
703
21st to endNight
24%15%32%26%4
592
March1st to 10thNight
24%13%32%26%5%
720
11th to 20thNight
24%16%32%23%5%
720
21st to endNight
23%11%29%27%9%
792
April1st to 10thNight
27%12%28%29%5%
720
11th to 20thNight
24%14%31%23%8%
711
21st to endNight
27%11%28%26%8%1
632
May1st to 10thNight
26%7%28%27%10%2
360
11th to 20thNight
19%7%34%31%8%
354
21st to endNight
14%6%38%28%13%1
389
June1st to 10thNight
19%7%29%37%9%
360
11th to 20thNight
18%6%29%33%13%1
360
21st to endNight
17%6%30%32%14%1
360
July1st to 10thNight
11%5%28%42%14%
350
11th to 20thNight
16%322%40%19%1
348
21st to endNight
11%326%48%11%1
394
August1st to 10thNight
15%323%40%19%1
373
11th to 20thNight
15%218%38%25%2
565
21st to endNight
15%5%26%33%19%3
792
September1st to 10thNight
15%7%25%35%18%1
720
11th to 20thNight
17%6%20%33%21%4
720
21st to endNight
17%7%26%28%21%2
720
October1st to 10thNight
21%8%29%26%15%2
720
11th to 20thNight
24%11%30%26%9%1
720
21st to endNight
29%12%27%26%7%
792
November1st to 10thNight
26%12%27%26%8%1
740
11th to 20thNight
25%15%31%21%9%
760
21st to endNight
27%15%28%23%7%
760
December1st to 10thNight
30%13%28%24%4
760
11th to 20thNight
29%13%31%21%5%1
760
21st to endNight
33%12%29%22%3
836
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