Chester
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chester clear sky chart from 2005-11-06 to 2024-03-23.

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
29%
23074
15%
80930
8%
22983
39%
22983
16%
54739

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%
751
16%
4362
6%
737
31%
737
8%
2738
January11th to 20thNight
18%
758
13%
4377
4%
740
31%
740
6%
3087
January21st to endNight
19%
831
9%
4869
6%
814
33%
814
7%
3624
February1st to 10thNight
13%
760
13%
2469
2%
760
27%
760
11%
1751
February11th to 20thNight
15%
743
16%
2220
3%
740
30%
740
13%
1523
February21st to endNight
18%
628
17%
1884
4%
628
34%
628
14%
1293
March1st to 10thNight
19%
760
19%
2280
8%
760
33%
760
16%
1570
March11th to 20thNight
20%
760
16%
2231
4%
760
32%
760
15%
1549
March21st to endNight
24%
801
20%
2001
6%
801
34%
801
18%
1406
April1st to 10thNight
22%
720
17%
1800
4%
720
29%
720
19%
1269
April11th to 20thNight
25%
711
15%
1775
4%
711
32%
711
20%
1245
April21st to endNight
23%
569
17%
1649
6%
569
31%
569
17%
1156
May1st to 10thNight
25%
540
13%
1620
7%
540
30%
540
15%
1168
May11th to 20thNight
28%
384
17%
1446
12%
384
38%
384
16%
951
May21st to endNight
39%
389
13%
1551
13%
388
46%
388
16%
984
June1st to 10thNight
38%
360
13%
1440
12%
360
48%
360
17%
925
June11th to 20thNight
39%
360
12%
1440
11%
360
47%
360
17%
908
June21st to endNight
43%
360
12%
1440
13%
360
51%
360
18%
906
July1st to 10thNight
51%
350
13%
1400
17%
350
58%
350
22%
846
July11th to 20thNight
55%
348
14%
1392
18%
348
58%
348
24%
804
July21st to endNight
53%
394
18%
1486
20%
372
59%
372
25%
843
August1st to 10thNight
56%
389
18%
1453
17%
387
61%
387
25%
851
August11th to 20thNight
46%
614
15%
1678
9%
614
48%
614
26%
1136
August21st to endNight
50%
792
20%
1980
10%
792
53%
792
34%
1279
September1st to 10thNight
48%
720
20%
1800
8%
720
53%
720
35%
1165
September11th to 20thNight
47%
720
24%
1927
15%
720
53%
720
33%
1193
September21st to endNight
42%
720
23%
1980
15%
720
50%
720
30%
1265
October1st to 10thNight
37%
720
16%
2008
10%
720
43%
720
20%
1410
October11th to 20thNight
35%
720
17%
2214
9%
720
45%
720
21%
1469
October21st to endNight
23%
792
13%
2574
7%
792
33%
792
13%
1860
November1st to 10thNight
31%
740
18%
2407
11%
740
41%
740
18%
1562
November11th to 20thNight
26%
760
21%
2470
11%
760
39%
760
17%
1604
November21st to endNight
24%
760
16%
2565
7%
760
37%
760
16%
1706
December1st to 10thNight
17%
760
14%
2660
5%
760
30%
760
10%
1919
December11th to 20thNight
19%
760
13%
3234
5%
759
30%
759
9%
2250
December21st to endNight
16%
830
9%
4848
3%
817
28%
817
6%
3524

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
15%7%36%6%35%5%6%7%36%
78545
Night
15%8%36%6%35%5%5%7%37%
81336
TransparencyDay
47%8%13%20%11%1
78250
Night
45%8%13%21%11%1
81165
SeeingDay
23%19%30%18%8%2
13967
Night
20%16%29%26%8%1
23096

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
11%8%38%7%45%6%6%6%35%
2517
11th to 20thDay
13%10%5%11%9%5%6%5%5%5%28%
2520
21st to endDay
15%12%6%10%7%4445%6%26%
2772
February1st to 10thDay
12%10%5%7%7%46%5%6%8%30%
1709
11th to 20thDay
9%8%5%9%7%46%6%6%8%33%
1848
21st to endDay
16%9%5%8%6%35%5%5%6%32%
1570
March1st to 10thDay
13%9%37%7%46%5%6%6%33%
1897
11th to 20thDay
15%9%5%7%6%35%5%6%6%34%
1944
21st to endDay
16%7%37%7%35%5%46%38%
2400
April1st to 10thDay
17%9%36%6%35%45%6%36%
2160
11th to 20thDay
16%6%26%6%35%5%6%8%37%
2134
21st to endDay
21%8%38%6%35%5%5%6%31%
2309
May1st to 10thDay
21%10%47%5%3446%6%32%
2340
11th to 20thDay
17%6%35%5%345%5%7%39%
2464
21st to endDay
15%6%35%5%35%5%5%8%40%
2725
June1st to 10thDay
18%6%36%6%46%5%7%8%31%
2520
11th to 20thDay
17%6%346%35%5%6%8%37%
2520
21st to endDay
17%6%36%7%45%6%6%7%32%
2520
July1st to 10thDay
13%6%36%5%36%6%6%8%39%
2520
11th to 20thDay
11%6%36%6%36%5%7%10%37%
2520
21st to endDay
13%435%6%46%6%7%8%38%
2744
August1st to 10thDay
10%436%7%46%6%6%10%38%
2458
11th to 20thDay
12%426%6%46%8%7%9%36%
2219
21st to endDay
12%425%5%45%47%9%43%
2376
September1st to 10thDay
12%5%25%435%5%6%9%44%
2160
11th to 20thDay
13%5%25%435%45%6%49%
2032
21st to endDay
15%5%2443446%8%44%
1980
October1st to 10thDay
22%7%26%5%4445%5%36%
1949
11th to 20thDay
14%7%36%5%445%6%7%39%
1746
21st to endDay
23%8%37%6%35%5%5%6%31%
1782
November1st to 10thDay
12%7%36%4345%6%8%43%
1671
11th to 20thDay
16%7%36%6%35%5%5%6%39%
1710
21st to endDay
17%7%47%5%345%7%7%35%
1611
December1st to 10thDay
17%10%5%7%6%5%5%5%5%6%29%
1520
11th to 20thDay
21%10%36%5%34435%36%
1906
21st to endDay
21%12%48%6%46%445%27%
2772
January1st to 10thNight
15%7%37%6%35%5%6%7%37%
4429
11th to 20thNight
11%10%5%10%9%45%6%5%6%28%
4440
21st to endNight
18%13%49%6%46%5%5%6%26%
4884
February1st to 10thNight
11%10%47%7%45%6%6%8%32%
2469
11th to 20thNight
9%8%37%6%46%7%7%7%35%
2218
21st to endNight
15%8%36%5%35%5%7%7%36%
1884
March1st to 10thNight
14%10%36%5%45%5%5%7%37%
2270
11th to 20thNight
14%9%35%5%36%5%6%7%36%
2230
21st to endNight
14%9%25%5%35%5%5%6%42%
2001
April1st to 10thNight
18%6%25%5%345%6%8%38%
1800
11th to 20thNight
16%7%35%5%25%6%5%7%39%
1775
21st to endNight
20%8%25%6%34448%36%
1649
May1st to 10thNight
21%8%46%6%34447%34%
1620
11th to 20thNight
16%7%36%7%35%46%7%37%
1445
21st to endNight
15%5%25%5%35%5%5%9%41%
1550
June1st to 10thNight
19%425%5%35%45%8%38%
1440
11th to 20thNight
19%5%26%4245%5%8%41%
1440
21st to endNight
13%6%25%6%35%7%6%7%39%
1440
July1st to 10thNight
12%425%5%35%5%6%9%43%
1440
11th to 20thNight
13%4234346%7%11%43%
1440
21st to endNight
12%424435%6%7%8%46%
1574
August1st to 10thNight
10%5%25%435%5%6%9%47%
1463
11th to 20thNight
12%6%25%6%45%5%5%9%41%
1678
21st to endNight
14%5%2442445%7%49%
1980
September1st to 10thNight
12%5%2442446%8%50%
1800
11th to 20thNight
13%5%24424446%53%
1927
21st to endNight
14%6%2443436%7%48%
1980
October1st to 10thNight
21%6%345%3445%6%39%
2008
11th to 20thNight
14%6%26%5%35%7%5%8%40%
2214
21st to endNight
22%7%25%5%35%5%6%5%33%
2574
November1st to 10thNight
13%6%36%6%3445%6%44%
2407
11th to 20thNight
16%6%35%6%345%5%7%41%
2470
21st to endNight
13%8%35%5%35%5%6%7%40%
2565
December1st to 10thNight
16%10%46%7%4446%6%32%
2660
11th to 20thNight
19%8%36%5%35%5%5%7%34%
3288
21st to endNight
21%11%5%8%6%3445%6%27%
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
45%9%12%27%7%
2518
11th to 20thDay
55%10%10%18%6%1
2520
21st to endDay
58%7%10%17%6%1
2772
February1st to 10thDay
52%8%13%17%9%1
1709
11th to 20thDay
48%10%11%19%11%2
1857
21st to endDay
53%6%12%17%11%2
1570
March1st to 10thDay
50%7%12%16%11%3
1900
11th to 20thDay
50%8%12%14%14%3
1944
21st to endDay
48%6%9%18%16%4
2400
April1st to 10thDay
49%6%10%20%12%3
2160
11th to 20thDay
44%9%11%19%15%2
2134
21st to endDay
53%7%10%16%12%3
2309
May1st to 10thDay
53%6%8%19%12%2
2340
11th to 20thDay
45%7%10%20%16%2
2468
21st to endDay
42%9%15%22%11%2
2725
June1st to 10thDay
50%8%14%18%10%1
2520
11th to 20thDay
44%8%13%18%15%2
2520
21st to endDay
49%11%13%17%10%1
2520
July1st to 10thDay
41%9%13%21%16%1
2450
11th to 20thDay
40%9%17%24%9%1
2445
21st to endDay
39%10%15%24%11%1
2591
August1st to 10thDay
40%11%16%22%11%1
2444
11th to 20thDay
41%12%15%22%10%1
2219
21st to endDay
36%8%17%25%13%1
2376
September1st to 10thDay
37%8%15%26%14%1
2160
11th to 20thDay
36%7%11%26%19%2
2032
21st to endDay
38%6%14%25%14%2
1980
October1st to 10thDay
50%7%9%19%13%2
1949
11th to 20thDay
42%8%15%26%8%1
1746
21st to endDay
55%8%10%17%9%2
1782
November1st to 10thDay
39%8%15%23%15%1
1671
11th to 20thDay
45%8%12%18%15%2
1710
21st to endDay
47%8%17%19%7%1
1611
December1st to 10thDay
55%8%15%14%8%1
1520
11th to 20thDay
52%7%11%18%11%1
1906
21st to endDay
58%8%11%14%9%
2772
January1st to 10thNight
44%10%14%21%10%1
4429
11th to 20thNight
53%9%11%16%8%1
4440
21st to endNight
58%9%10%15%8%1
4884
February1st to 10thNight
50%8%14%17%10%1
2469
11th to 20thNight
44%9%14%19%13%1
2220
21st to endNight
45%9%13%22%8%3
1884
March1st to 10thNight
46%7%13%21%11%2
2280
11th to 20thNight
46%8%12%19%13%2
2231
21st to endNight
44%7%11%16%18%4
2001
April1st to 10thNight
43%8%13%21%14%2
1800
11th to 20thNight
43%7%11%23%14%1
1775
21st to endNight
47%7%11%18%13%4
1649
May1st to 10thNight
52%7%8%17%14%2
1620
11th to 20thNight
45%8%13%19%13%2
1446
21st to endNight
41%10%13%22%11%2
1551
June1st to 10thNight
44%8%15%21%12%
1440
11th to 20thNight
42%9%16%20%12%1
1440
21st to endNight
41%10%16%22%11%1
1440
July1st to 10thNight
36%10%16%24%13%1
1400
11th to 20thNight
33%10%19%26%11%
1392
21st to endNight
33%9%16%29%11%2
1486
August1st to 10thNight
33%10%16%28%13%
1453
11th to 20thNight
39%10%17%23%10%
1678
21st to endNight
35%7%15%28%15%
1980
September1st to 10thNight
33%8%15%28%16%
1800
11th to 20thNight
32%7%12%28%20%1
1927
21st to endNight
36%7%12%28%16%2
1980
October1st to 10thNight
46%8%11%22%11%2
2008
11th to 20thNight
41%9%16%24%9%1
2214
21st to endNight
52%8%12%18%9%1
2574
November1st to 10thNight
41%8%13%21%14%3
2407
11th to 20thNight
42%7%13%22%14%2
2470
21st to endNight
44%8%17%21%9%1
2565
December1st to 10thNight
53%8%13%19%7%
2660
11th to 20thNight
50%9%12%15%12%1
3288
21st to endNight
57%8%12%18%6%
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
28%24%27%17%3
180
11th to 20thDay
29%29%26%14%21
180
21st to endDay
19%29%37%11%5%
198
February1st to 10thDay
28%35%27%9%1
284
11th to 20thDay
30%27%27%14%2
456
21st to endDay
28%27%28%15%3
382
March1st to 10thDay
29%20%34%15%2
460
11th to 20thDay
29%28%28%11%31
460
21st to endDay
32%21%27%11%5%3
470
April1st to 10thDay
34%21%25%12%6%1
420
11th to 20thDay
31%24%24%13%6%3
412
21st to endDay
31%20%29%14%5%1
420
May1st to 10thDay
28%26%24%12%7%2
423
11th to 20thDay
20%20%35%19%32
447
21st to endDay
19%14%33%25%8%2
485
June1st to 10thDay
21%17%36%17%9%1
450
11th to 20thDay
20%18%34%18%9%2
450
21st to endDay
18%17%35%18%8%4
450
July1st to 10thDay
16%12%32%24%13%3
445
11th to 20thDay
15%7%38%26%12%2
444
21st to endDay
13%11%40%24%10%2
488
August1st to 10thDay
10%13%31%28%14%4
446
11th to 20thDay
14%10%33%22%15%6%
439
21st to endDay
13%15%27%25%13%7%
495
September1st to 10thDay
16%12%25%24%16%6%
450
11th to 20thDay
16%14%28%18%14%9%
450
21st to endDay
16%16%30%22%11%6%
450
October1st to 10thDay
24%18%23%19%12%4
450
11th to 20thDay
20%17%28%24%10%1
423
21st to endDay
33%16%25%19%6%
396
November1st to 10thDay
23%18%24%23%11%1
365
11th to 20thDay
30%22%25%17%5%1
370
21st to endDay
29%21%28%17%4
271
December1st to 10thDay
24%30%26%15%5%
180
11th to 20thDay
37%23%30%7%3
180
21st to endDay
32%21%30%14%3
198
January1st to 10thNight
21%20%36%21%3
760
11th to 20thNight
25%19%32%21%2
760
21st to endNight
17%22%34%22%4
836
February1st to 10thNight
22%21%37%18%2
760
11th to 20thNight
20%24%36%18%2
743
21st to endNight
25%23%30%20%2
628
March1st to 10thNight
26%18%33%21%2
760
11th to 20thNight
24%23%29%20%4
760
21st to endNight
23%20%29%21%6%
801
April1st to 10thNight
25%20%29%22%4
720
11th to 20thNight
21%23%28%21%7%
711
21st to endNight
25%17%30%24%5%
569
May1st to 10thNight
26%16%27%23%7%
540
11th to 20thNight
16%16%32%29%6%
384
21st to endNight
17%9%30%31%13%1
389
June1st to 10thNight
18%9%28%33%12%
360
11th to 20thNight
18%12%25%34%12%
360
21st to endNight
14%7%29%39%10%1
360
July1st to 10thNight
12%9%21%43%14%1
350
11th to 20thNight
12%424%39%18%2
348
21st to endNight
12%5%27%38%17%1
394
August1st to 10thNight
9%422%46%18%1
389
11th to 20thNight
13%7%26%34%18%3
614
21st to endNight
16%7%21%33%21%2
792
September1st to 10thNight
13%7%27%34%18%1
720
11th to 20thNight
15%10%24%29%18%4
720
21st to endNight
17%9%26%31%14%3
720
October1st to 10thNight
24%14%19%28%13%1
720
11th to 20thNight
18%13%28%30%11%1
720
21st to endNight
26%16%29%23%6%
792
November1st to 10thNight
19%19%26%27%9%1
740
11th to 20thNight
21%19%30%23%7%1
760
21st to endNight
22%19%29%22%6%1
760
December1st to 10thNight
23%22%33%17%4
760
11th to 20thNight
26%21%29%21%3
760
21st to endNight
27%22%30%19%2
836
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