Bolton
Clear Sky Chart
History

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

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
28%
23015
13%
80432
8%
22915
35%
22915
15%
57228

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
12%
751
8%
4407
3%
737
19%
737
4%
3314
January11th to 20thNight
13%
759
6%
4168
2%
740
18%
740
3%
3434
January21st to endNight
16%
833
6%
4529
4%
814
24%
814
5%
3697
February1st to 10thNight
11%
760
10%
2470
2%
760
21%
760
10%
1884
February11th to 20thNight
12%
740
12%
2405
3%
740
23%
740
10%
1820
February21st to endNight
16%
628
11%
1983
3%
628
25%
628
11%
1441
March1st to 10thNight
17%
743
15%
2042
5%
743
26%
743
15%
1482
March11th to 20thNight
20%
720
16%
1980
6%
720
29%
720
15%
1425
March21st to endNight
24%
792
18%
2178
7%
792
34%
792
20%
1512
April1st to 10thNight
19%
720
13%
1846
3%
720
25%
720
17%
1369
April11th to 20thNight
27%
639
18%
1703
6%
639
33%
639
21%
1197
April21st to endNight
27%
540
16%
1620
5%
540
35%
540
20%
1116
May1st to 10thNight
36%
540
16%
1620
8%
540
43%
540
23%
1082
May11th to 20thNight
36%
382
22%
1444
14%
382
50%
382
24%
847
May21st to endNight
42%
388
17%
1551
13%
388
54%
388
27%
819
June1st to 10thNight
37%
360
14%
1440
11%
360
50%
360
21%
838
June11th to 20thNight
43%
360
16%
1395
17%
360
52%
360
24%
800
June21st to endNight
46%
360
17%
1260
17%
360
54%
360
26%
738
July1st to 10thNight
58%
350
17%
1293
19%
350
65%
350
34%
665
July11th to 20thNight
50%
348
19%
1392
23%
348
60%
348
29%
758
July21st to endNight
53%
575
18%
1657
12%
543
59%
543
34%
964
August1st to 10thNight
55%
537
17%
1600
11%
534
60%
534
35%
939
August11th to 20thNight
58%
534
23%
1598
15%
534
63%
534
36%
929
August21st to endNight
46%
656
21%
1844
10%
656
52%
656
31%
1142
September1st to 10thNight
48%
720
17%
1800
7%
720
53%
720
33%
1171
September11th to 20thNight
49%
720
26%
1919
14%
720
56%
720
36%
1155
September21st to endNight
43%
720
19%
2006
14%
720
47%
720
25%
1353
October1st to 10thNight
34%
720
16%
2160
11%
720
41%
720
20%
1497
October11th to 20thNight
24%
720
13%
2160
7%
720
32%
720
16%
1549
October21st to endNight
18%
792
14%
2376
7%
792
27%
792
12%
1791
November1st to 10thNight
27%
740
17%
2474
12%
740
36%
740
18%
1649
November11th to 20thNight
20%
760
13%
2660
6%
760
29%
760
12%
1910
November21st to endNight
19%
760
8%
2660
5%
760
24%
760
10%
2023
December1st to 10thNight
13%
760
7%
2660
3%
760
18%
760
8%
2102
December11th to 20thNight
13%
760
5%
3258
1%
758
18%
758
6%
2565
December21st to endNight
9%
828
3%
4874
0%
817
15%
817
3%
4251

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%10%48%6%45%5%5%7%31%
78815
Night
15%10%47%6%45%5%5%7%32%
80690
TransparencyDay
50%9%13%18%9%1
78531
Night
49%9%13%18%9%1
80509
SeeingDay
20%18%31%19%9%3
13770
Night
19%16%31%23%9%1
23040

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
16%15%6%8%7%35%447%24%
2517
11th to 20thDay
11%19%8%13%11%5%5%445%14%
2776
21st to endDay
21%20%7%10%7%44335%16%
3124
February1st to 10thDay
14%16%5%11%8%46%6%5%6%19%
1710
11th to 20thDay
12%12%7%11%9%445%5%7%22%
1663
21st to endDay
17%15%5%11%5%35%5%45%25%
1469
March1st to 10thDay
13%11%6%8%8%45%46%5%30%
2032
11th to 20thDay
13%11%5%8%7%46%445%32%
1980
21st to endDay
17%11%48%6%45%45%5%32%
2178
April1st to 10thDay
16%12%5%8%6%345%5%6%29%
2114
11th to 20thDay
18%9%37%5%34446%36%
2222
21st to endDay
21%8%37%5%3445%6%34%
2340
May1st to 10thDay
16%7%47%6%35%5%6%7%34%
2340
11th to 20thDay
16%6%26%6%4445%6%40%
2450
21st to endDay
12%5%26%5%46%5%6%7%43%
2725
June1st to 10thDay
14%6%36%5%35%6%7%8%38%
2520
11th to 20thDay
12%6%245%35%5%6%10%41%
2557
21st to endDay
14%5%25%5%446%6%10%39%
2700
July1st to 10thDay
9%425%5%36%6%6%9%44%
2624
11th to 20thDay
11%5%36%5%36%7%7%9%39%
2520
21st to endDay
11%6%36%6%5%6%7%7%9%35%
2560
August1st to 10thDay
10%426%7%46%7%7%10%39%
2327
11th to 20thDay
8%5%36%8%46%7%7%9%37%
2303
21st to endDay
12%5%25%7%35%7%6%9%39%
2493
September1st to 10thDay
13%6%25%6%46%5%6%7%39%
2160
11th to 20thDay
12%5%35%5%35%5%6%8%43%
2038
21st to endDay
14%7%37%8%35%5%6%7%35%
1954
October1st to 10thDay
20%9%36%6%4445%7%33%
1800
11th to 20thDay
21%9%38%7%445%5%6%27%
1800
21st to endDay
26%13%47%6%35%5%5%6%21%
1980
November1st to 10thDay
14%11%46%5%45%6%5%9%30%
1601
11th to 20thDay
14%12%5%9%5%46%6%7%8%25%
1520
21st to endDay
18%15%7%9%7%35%5%6%7%20%
1520
December1st to 10thDay
16%16%8%12%8%5%45%46%16%
1520
11th to 20thDay
17%16%5%10%7%46%5%5%6%19%
1906
21st to endDay
28%18%7%11%7%3444410%
2772
January1st to 10thNight
17%15%6%9%7%45%446%21%
4429
11th to 20thNight
14%13%7%11%11%6%7%5%5%5%14%
4180
21st to endNight
19%13%6%10%10%46%446%17%
4532
February1st to 10thNight
13%14%5%9%8%45%5%5%6%25%
2470
11th to 20thNight
13%12%6%9%9%45%5%5%6%26%
2403
21st to endNight
12%13%49%7%5%5%5%6%6%28%
1983
March1st to 10thNight
13%9%5%7%7%45%5%5%5%34%
2032
11th to 20thNight
14%11%47%6%36%446%35%
1979
21st to endNight
15%9%37%6%34446%40%
2178
April1st to 10thNight
17%10%47%6%445%5%6%32%
1846
11th to 20thNight
15%9%35%5%345%6%6%39%
1703
21st to endNight
15%9%46%5%445%6%7%35%
1620
May1st to 10thNight
16%6%35%6%35%5%5%7%40%
1620
11th to 20thNight
15%5%35%43445%8%46%
1443
21st to endNight
9%425%33445%8%53%
1550
June1st to 10thNight
12%5%246%45%5%6%8%43%
1440
11th to 20thNight
9%5%26%5%247%5%9%45%
1395
21st to endNight
11%5%25%5%25%46%7%48%
1260
July1st to 10thNight
5%3335%33448%58%
1332
11th to 20thNight
10%325%5%36%45%8%50%
1440
21st to endNight
7%4244345%7%8%52%
1755
August1st to 10thNight
7%5%25%6%3446%7%52%
1611
11th to 20thNight
7%3345%35%46%9%51%
1598
21st to endNight
11%6%24435%5%5%7%48%
1844
September1st to 10thNight
9%425%5%346%7%7%48%
1800
11th to 20thNight
12%42432445%7%53%
1919
21st to endNight
15%6%25%7%3445%7%42%
2006
October1st to 10thNight
18%7%35%5%45%5%6%7%37%
2160
11th to 20thNight
18%9%36%6%35%5%5%6%34%
2160
21st to endNight
21%10%47%7%445%6%7%27%
2376
November1st to 10thNight
16%8%36%5%35%5%6%6%37%
2474
11th to 20thNight
16%11%46%6%46%5%6%7%28%
2660
21st to endNight
18%11%5%9%6%345%5%8%25%
2660
December1st to 10thNight
20%13%6%8%6%45%5%6%7%21%
2660
11th to 20thNight
15%17%6%10%7%35%5%5%7%20%
3288
21st to endNight
28%17%6%10%7%4444412%
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
56%11%16%11%6%
2518
11th to 20thDay
70%8%11%8%3
2776
21st to endDay
70%8%10%8%41
3124
February1st to 10thDay
63%10%9%14%31
1710
11th to 20thDay
58%9%12%13%6%1
1672
21st to endDay
60%8%11%14%7%1
1469
March1st to 10thDay
54%8%13%16%9%1
2035
11th to 20thDay
54%7%10%16%11%2
1980
21st to endDay
53%6%10%16%13%2
2178
April1st to 10thDay
55%7%11%17%10%1
2114
11th to 20thDay
48%6%11%20%14%1
2222
21st to endDay
50%8%12%17%11%2
2340
May1st to 10thDay
49%7%11%19%14%1
2340
11th to 20thDay
43%8%12%23%13%1
2454
21st to endDay
37%10%16%22%14%2
2725
June1st to 10thDay
42%9%12%21%15%1
2520
11th to 20thDay
36%10%15%23%15%1
2557
21st to endDay
38%10%15%23%13%1
2700
July1st to 10thDay
34%9%14%23%18%2
2554
11th to 20thDay
37%10%16%26%11%1
2445
21st to endDay
41%10%16%19%12%1
2417
August1st to 10thDay
36%12%18%21%12%2
2314
11th to 20thDay
38%11%18%22%11%
2303
21st to endDay
40%10%15%24%11%1
2493
September1st to 10thDay
41%9%15%24%10%2
2160
11th to 20thDay
36%9%15%27%13%
2038
21st to endDay
45%9%13%25%7%
1954
October1st to 10thDay
50%9%13%17%10%1
1800
11th to 20thDay
54%9%13%17%6%1
1800
21st to endDay
62%8%12%13%5%
1980
November1st to 10thDay
48%10%14%19%8%1
1601
11th to 20thDay
54%10%14%13%8%
1520
21st to endDay
62%10%15%12%2
1520
December1st to 10thDay
67%10%12%9%2
1520
11th to 20thDay
61%12%14%10%4
1906
21st to endDay
76%8%8%7%1
2772
January1st to 10thNight
60%10%14%12%3
4429
11th to 20thNight
64%14%11%8%3
4180
21st to endNight
63%12%12%8%41
4532
February1st to 10thNight
57%9%12%17%5%
2470
11th to 20thNight
56%9%11%16%8%
2405
21st to endNight
53%8%15%16%8%
1983
March1st to 10thNight
48%9%14%19%9%1
2042
11th to 20thNight
50%7%12%19%11%2
1980
21st to endNight
47%6%10%20%15%2
2178
April1st to 10thNight
51%6%11%17%13%1
1846
11th to 20thNight
45%8%10%20%15%2
1703
21st to endNight
45%8%13%17%15%2
1620
May1st to 10thNight
42%7%11%21%16%3
1620
11th to 20thNight
37%7%10%26%17%4
1444
21st to endNight
30%7%17%24%18%4
1551
June1st to 10thNight
37%8%13%23%17%2
1440
11th to 20thNight
35%8%13%23%21%2
1395
21st to endNight
35%8%14%25%17%1
1260
July1st to 10thNight
24%9%12%26%24%4
1293
11th to 20thNight
31%7%13%31%17%2
1392
21st to endNight
29%8%15%30%15%3
1657
August1st to 10thNight
31%7%19%29%13%2
1600
11th to 20thNight
28%8%16%33%13%2
1598
21st to endNight
33%8%13%29%15%1
1844
September1st to 10thNight
33%9%16%27%14%1
1800
11th to 20thNight
29%8%13%33%17%1
1919
21st to endNight
40%9%13%27%10%
2006
October1st to 10thNight
44%9%12%19%14%1
2160
11th to 20thNight
48%9%14%21%7%
2160
21st to endNight
55%9%12%15%9%
2376
November1st to 10thNight
45%7%16%21%11%1
2474
11th to 20thNight
51%10%15%16%8%1
2660
21st to endNight
56%10%17%13%3
2660
December1st to 10thNight
61%10%14%12%3
2660
11th to 20thNight
61%10%14%12%3
3288
21st to endNight
72%11%9%6%2
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
24%24%31%17%41
180
11th to 20thDay
23%30%29%13%41
252
21st to endDay
28%25%32%13%2
297
February1st to 10thDay
30%31%27%9%3
270
11th to 20thDay
23%34%37%5%1
270
21st to endDay
35%32%26%7%
230
March1st to 10thDay
22%24%35%16%4
440
11th to 20thDay
22%20%40%14%4
420
21st to endDay
29%21%31%13%6%
462
April1st to 10thDay
22%25%33%12%7%
420
11th to 20thDay
30%21%26%13%8%3
412
21st to endDay
25%20%32%15%5%1
420
May1st to 10thDay
23%16%31%19%9%3
423
11th to 20thDay
19%19%29%20%9%4
447
21st to endDay
14%15%29%27%10%5%
485
June1st to 10thDay
13%19%34%24%8%2
450
11th to 20thDay
16%15%30%24%12%2
487
21st to endDay
16%12%37%24%7%3
630
July1st to 10thDay
9%9%32%29%16%4
549
11th to 20thDay
11%9%32%27%15%6%
444
21st to endDay
12%7%36%28%11%5%
488
August1st to 10thDay
9%9%28%31%15%9%
446
11th to 20thDay
9%7%30%26%19%9%
439
21st to endDay
14%11%30%24%15%6%
495
September1st to 10thDay
14%10%29%25%13%8%
450
11th to 20thDay
10%10%28%23%19%10%
450
21st to endDay
15%15%29%22%13%5%
450
October1st to 10thDay
24%16%23%21%12%4
450
11th to 20thDay
23%21%34%14%6%1
450
21st to endDay
32%22%29%14%4
495
November1st to 10thDay
24%24%28%19%41
251
11th to 20thDay
25%24%33%12%6%1
180
21st to endDay
28%23%38%10%1
180
December1st to 10thDay
31%28%29%11%1
180
11th to 20thDay
26%32%29%12%11
180
21st to endDay
28%30%32%7%3
198
January1st to 10thNight
23%26%32%14%4
760
11th to 20thNight
26%22%32%17%3
760
21st to endNight
19%18%36%20%6%
836
February1st to 10thNight
21%24%37%16%1
760
11th to 20thNight
22%22%38%15%3
743
21st to endNight
21%25%34%17%3
628
March1st to 10thNight
20%23%35%20%3
743
11th to 20thNight
22%23%31%18%5%
720
21st to endNight
21%17%33%21%7%
792
April1st to 10thNight
23%18%35%20%3
720
11th to 20thNight
20%18%34%19%8%1
639
21st to endNight
21%14%32%25%8%
540
May1st to 10thNight
17%10%30%30%11%2
540
11th to 20thNight
16%7%34%32%9%1
382
21st to endNight
11%8%34%32%15%
389
June1st to 10thNight
13%8%34%32%12%
360
11th to 20thNight
12%9%29%35%15%
360
21st to endNight
12%6%29%37%16%
360
July1st to 10thNight
6%5%25%43%19%2
350
11th to 20thNight
11%6%28%34%20%1
348
21st to endNight
8%430%38%18%2
575
August1st to 10thNight
10%224%39%24%1
537
11th to 20thNight
8%5%21%40%23%3
534
21st to endNight
13%5%30%29%21%2
656
September1st to 10thNight
11%8%28%30%19%5%
720
11th to 20thNight
13%7%25%29%21%5%
720
21st to endNight
18%11%22%29%16%4
720
October1st to 10thNight
20%10%31%22%15%2
720
11th to 20thNight
22%17%34%20%8%1
720
21st to endNight
28%18%31%21%3
792
November1st to 10thNight
21%18%30%23%8%1
740
11th to 20thNight
21%23%30%19%6%1
760
21st to endNight
25%19%33%20%4
760
December1st to 10thNight
29%21%33%14%3
760
11th to 20thNight
23%22%36%16%3
760
21st to endNight
32%18%32%16%2
836
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