Chesterfield
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chesterfield clear sky chart from 2005-11-06 to 2024-02-05.

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
47%
23014
7%
81611
6%
22946
48%
22946
13%
59359

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
57%
1011
14%
5576
12%
1006
59%
1006
14%
3039
January11th to 20thNight
55%
990
6%
5569
10%
976
59%
976
10%
4139
January21st to endNight
59%
1038
9%
5636
13%
1023
63%
1023
16%
3100
February1st to 10thNight
67%
923
12%
2838
10%
923
68%
923
22%
1927
February11th to 20thNight
68%
874
11%
2529
9%
874
68%
874
22%
1785
February21st to endNight
63%
740
17%
1973
13%
740
64%
740
27%
1356
March1st to 10thNight
55%
799
14%
2239
11%
799
59%
799
22%
1579
March11th to 20thNight
63%
644
16%
2084
16%
644
67%
644
23%
1383
March21st to endNight
53%
594
14%
2016
13%
594
57%
594
20%
1363
April1st to 10thNight
59%
540
8%
1698
6%
540
59%
540
20%
1196
April11th to 20thNight
58%
533
4%
1421
3%
533
59%
533
23%
1028
April21st to endNight
54%
476
0%
1313
0%
476
54%
476
24%
934
May1st to 10thNight
46%
360
0%
1080
0%
360
46%
360
21%
770
May11th to 20thNight
39%
296
0%
932
0%
296
39%
296
15%
681
May21st to endNight
35%
194
0%
776
0%
194
35%
194
9%
601
June1st to 10thNight
54%
180
0%
720
0%
180
54%
180
19%
460
June11th to 20thNight
58%
180
0%
557
0%
180
58%
180
23%
367
June21st to endNight
54%
180
0%
540
0%
180
54%
180
22%
362
July1st to 10thNight
56%
175
0%
631
0%
175
56%
175
19%
418
July11th to 20thNight
60%
174
0%
696
0%
174
60%
174
18%
441
July21st to endNight
59%
223
0%
909
0%
211
60%
211
15%
592
August1st to 10thNight
53%
358
0%
1068
0%
356
53%
356
24%
725
August11th to 20thNight
53%
489
0%
1245
0%
489
53%
489
21%
923
August21st to endNight
48%
594
2%
1584
2%
594
48%
594
21%
1154
September1st to 10thNight
40%
540
5%
1671
4%
540
40%
540
15%
1255
September11th to 20thNight
38%
540
5%
1800
5%
540
39%
540
11%
1415
September21st to endNight
27%
688
6%
2044
4%
688
29%
688
9%
1712
October1st to 10thNight
18%
720
2%
2190
1%
720
19%
720
5%
1955
October11th to 20thNight
16%
796
3%
2416
3%
796
17%
796
4%
2192
October21st to endNight
12%
990
0%
2855
0%
990
12%
990
2%
2720
November1st to 10thNight
25%
925
1%
2883
0%
925
25%
925
3%
2698
November11th to 20thNight
38%
995
5%
3135
3%
995
40%
995
9%
2602
November21st to endNight
43%
1040
5%
3353
5%
1040
44%
1040
10%
2692
December1st to 10thNight
53%
1040
10%
3420
9%
1040
55%
1040
14%
2545
December11th to 20thNight
46%
1039
11%
4130
7%
1039
49%
1039
12%
2938
December21st to endNight
45%
1136
5%
6084
7%
1116
48%
1116
8%
4312

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
11%10%5%8%8%46%6%7%9%26%
76647
Night
9%11%5%10%8%5%8%7%8%11%19%
82193
TransparencyDay
52%7%15%21%5%
76281
Night
55%7%14%18%6%
82142
SeeingDay
16%210%26%31%16%
15724
Night
14%312%30%30%11%
23076

By Month

Percentage Cloud Cover NW (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
2438%8%5%7%9%10%17%27%
1270
11th to 20thDay
313%6%13%9%7%8%8%9%11%13%
1381
21st to endDay
15%38%10%5%7%9%9%20%21%
1898
February1st to 10thDay
25%47%7%6%7%9%15%21%17%
1216
11th to 20thDay
25%36%8%5%11%9%11%20%20%
1321
21st to endDay
16%49%8%48%9%11%21%17%
1282
March1st to 10thDay
5%6%48%9%5%9%9%11%18%16%
1720
11th to 20thDay
49%36%7%5%6%11%10%19%19%
1891
21st to endDay
27%410%8%7%9%7%10%13%22%
2340
April1st to 10thDay
5%9%49%9%5%9%7%8%9%26%
2260
11th to 20thDay
8%8%47%7%5%9%8%8%9%27%
2490
21st to endDay
9%6%49%10%6%7%6%7%11%26%
2646
May1st to 10thDay
9%12%6%10%10%46%5%5%8%25%
2880
11th to 20thDay
11%11%5%9%10%5%6%5%6%8%24%
2961
21st to endDay
13%12%6%10%7%35%5%5%7%26%
3499
June1st to 10thDay
14%10%5%9%6%45%45%8%30%
3240
11th to 20thDay
10%9%36%6%35%5%6%8%39%
3402
21st to endDay
13%7%37%6%35%5%7%9%36%
3420
July1st to 10thDay
16%6%36%5%35%46%8%38%
3308
11th to 20thDay
13%5%36%6%45%6%7%10%36%
3240
21st to endDay
14%5%36%6%45%6%7%8%35%
3354
August1st to 10thDay
17%7%25%5%3446%7%40%
2865
11th to 20thDay
20%9%36%6%35%6%5%7%31%
2655
21st to endDay
17%10%46%6%35%45%6%33%
2772
September1st to 10thDay
18%13%5%7%6%45%446%27%
2289
11th to 20thDay
20%16%6%9%6%45%45%420%
2160
21st to endDay
22%18%6%8%5%24445%23%
1896
October1st to 10thDay
20%21%11%13%7%3433311%
1770
11th to 20thDay
24%19%8%17%11%422337%
1542
21st to endDay
19%22%8%16%18%5%33223
1501
November1st to 10thDay
8%22%12%13%10%6%12%10%322
1193
11th to 20thDay
7%15%6%11%6%6%11%11%6%10%10%
1045
21st to endDay
7%10%10%11%12%5%7%12%9%7%11%
825
December1st to 10thDay
48%410%11%6%11%7%12%12%15%
760
11th to 20thDay
5%9%48%8%6%8%9%10%15%18%
958
21st to endDay
412%7%11%10%6%7%7%7%10%18%
1397
January1st to 10thNight
2437%8%6%9%9%10%17%26%
5685
11th to 20thNight
415%6%11%7%5%9%8%10%13%12%
5520
21st to endNight
38%47%6%46%8%10%19%24%
5763
February1st to 10thNight
2438%10%6%9%12%14%16%16%
2838
11th to 20thNight
25%37%10%6%12%12%9%15%19%
2526
21st to endNight
15%310%9%410%8%13%16%20%
1973
March1st to 10thNight
5%7%5%8%9%5%10%10%10%18%14%
2239
11th to 20thNight
36%48%8%6%9%8%10%15%24%
2084
21st to endNight
38%5%10%10%47%6%8%11%28%
2016
April1st to 10thNight
5%10%49%7%5%10%8%7%9%27%
1698
11th to 20thNight
7%7%46%8%7%9%8%6%8%29%
1421
21st to endNight
7%7%5%9%8%5%8%6%5%8%32%
1313
May1st to 10thNight
11%13%411%7%44445%33%
1080
11th to 20thNight
11%12%5%12%9%45%5%46%28%
932
21st to endNight
14%15%9%11%6%5%6%5%4323%
776
June1st to 10thNight
16%10%5%45%445%5%6%37%
720
11th to 20thNight
13%8%37%5%347%37%40%
557
21st to endNight
12%9%46%5%37%5%6%9%34%
540
July1st to 10thNight
16%9%38%5%35%5%46%36%
651
11th to 20thNight
13%6%48%446%7%5%8%34%
720
21st to endNight
15%7%47%6%45%6%9%7%31%
962
August1st to 10thNight
16%9%46%425%46%6%37%
1074
11th to 20thNight
19%10%347%345%6%6%32%
1245
21st to endNight
19%10%47%5%3445%6%34%
1584
September1st to 10thNight
22%13%46%7%24446%28%
1671
11th to 20thNight
20%17%47%6%45%45%5%22%
1800
21st to endNight
21%22%8%8%5%3444417%
2044
October1st to 10thNight
19%25%13%12%5%332349%
2190
11th to 20thNight
23%21%8%16%9%2332310%
2416
21st to endNight
21%22%6%17%16%5%32114
2855
November1st to 10thNight
11%21%9%16%10%7%11%7%323
2883
11th to 20thNight
9%14%6%11%10%6%13%7%7%10%7%
3135
21st to endNight
7%13%7%13%10%6%9%8%8%10%10%
3353
December1st to 10thNight
48%412%11%6%10%8%12%12%13%
3420
11th to 20thNight
5%9%5%10%10%7%9%8%10%16%12%
4239
21st to endNight
6%13%6%11%9%5%7%6%7%11%18%
6270

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
36%9%26%19%9%
1270
11th to 20thDay
60%8%19%9%4
1394
21st to endDay
40%8%8%22%20%2
1898
February1st to 10thDay
40%6%21%28%5%
1216
11th to 20thDay
42%6%19%24%9%
1328
21st to endDay
42%6%15%25%12%
1282
March1st to 10thDay
46%7%15%23%9%
1720
11th to 20thDay
43%6%15%26%9%1
1893
21st to endDay
49%6%12%22%11%1
2340
April1st to 10thDay
50%6%12%23%9%
2260
11th to 20thDay
49%6%11%25%9%
2490
21st to endDay
50%8%13%20%9%
2646
May1st to 10thDay
56%5%12%20%6%
2880
11th to 20thDay
56%7%14%18%5%
2965
21st to endDay
56%6%15%19%4
3500
June1st to 10thDay
52%6%15%21%6%
3240
11th to 20thDay
43%7%14%31%5%
3402
21st to endDay
42%8%17%30%3
3420
July1st to 10thDay
42%6%16%31%5%
3219
11th to 20thDay
39%8%21%29%2
3141
21st to endDay
42%9%23%24%2
3167
August1st to 10thDay
42%7%23%23%4
2847
11th to 20thDay
50%8%17%21%3
2655
21st to endDay
52%7%17%23%2
2772
September1st to 10thDay
57%7%18%15%3
2289
11th to 20thDay
65%7%18%10%1
2160
21st to endDay
63%5%16%14%2
1896
October1st to 10thDay
78%5%10%6%
1770
11th to 20thDay
84%5%6%41
1542
21st to endDay
87%5%5%21
1501
November1st to 10thDay
78%13%7%3
1193
11th to 20thDay
60%10%16%10%2
1045
21st to endDay
59%13%13%12%21
825
December1st to 10thDay
55%7%16%19%4
760
11th to 20thDay
49%8%13%27%3
958
21st to endDay
55%8%11%12%10%4
1397
January1st to 10thNight
39%6%22%24%8%
5685
11th to 20thNight
59%8%20%10%3
5569
21st to endNight
39%6%10%23%22%1
5763
February1st to 10thNight
43%9%20%23%5%
2838
11th to 20thNight
46%9%17%22%6%
2529
21st to endNight
43%6%13%25%12%
1973
March1st to 10thNight
47%7%15%22%8%
2239
11th to 20thNight
44%7%13%27%9%
2084
21st to endNight
47%7%12%23%11%1
2016
April1st to 10thNight
48%8%13%22%8%1
1698
11th to 20thNight
48%8%13%25%7%1
1421
21st to endNight
49%5%11%24%10%
1313
May1st to 10thNight
52%410%24%9%
1080
11th to 20thNight
55%6%12%19%8%
932
21st to endNight
65%5%9%18%3
776
June1st to 10thNight
47%416%24%8%
720
11th to 20thNight
44%6%11%31%8%
557
21st to endNight
44%8%16%28%4
540
July1st to 10thNight
48%6%12%27%8%
631
11th to 20thNight
44%8%15%26%7%
696
21st to endNight
47%8%20%23%2
909
August1st to 10thNight
45%6%18%27%3
1068
11th to 20thNight
50%8%16%23%4
1245
21st to endNight
51%5%15%26%2
1584
September1st to 10thNight
57%7%19%15%3
1671
11th to 20thNight
62%7%18%13%
1800
21st to endNight
70%6%10%13%1
2044
October1st to 10thNight
80%410%5%
2190
11th to 20thNight
83%48%5%1
2416
21st to endNight
88%4421
2855
November1st to 10thNight
80%10%7%3
2883
11th to 20thNight
65%10%12%11%1
3135
21st to endNight
63%8%14%12%2
3353
December1st to 10thNight
55%8%15%18%3
3420
11th to 20thNight
54%6%15%21%3
4239
21st to endNight
56%6%11%14%12%1
6270

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
17%33%38%21%
90
11th to 20thDay
6%219%42%26%6%
90
21st to endDay
2215%33%38%11%
144
February1st to 10thDay
6%8%29%33%24%
218
11th to 20thDay
18%30%38%23%
240
21st to endDay
17%24%37%31%
198
March1st to 10thDay
5%114%27%35%18%
240
11th to 20thDay
7%19%29%35%18%
240
21st to endDay
5%419%26%28%18%
426
April1st to 10thDay
10%19%24%33%23%
494
11th to 20thDay
10%112%30%29%18%
590
21st to endDay
9%212%29%26%23%
600
May1st to 10thDay
14%110%24%30%20%
603
11th to 20thDay
15%216%29%27%11%
698
21st to endDay
19%212%27%27%13%
874
June1st to 10thDay
18%27%24%31%17%
810
11th to 20thDay
14%18%31%33%13%
972
21st to endDay
15%26%28%34%15%
990
July1st to 10thDay
19%1421%35%20%
864
11th to 20thDay
16%16%27%37%14%
792
21st to endDay
16%15%20%36%22%
732
August1st to 10thDay
20%36%21%31%19%
626
11th to 20thDay
21%15%23%31%19%
615
21st to endDay
22%29%27%26%14%
693
September1st to 10thDay
30%38%19%28%12%
529
11th to 20thDay
28%29%26%26%8%
450
21st to endDay
30%313%25%23%7%
352
October1st to 10thDay
25%6%17%27%19%6%
270
11th to 20thDay
31%312%21%24%8%
270
21st to endDay
24%5%13%29%24%5%
255
November1st to 10thDay
12%317%30%28%9%
180
11th to 20thDay
11%414%32%29%10%
180
21st to endDay
11%418%23%34%10%
120
December1st to 10thDay
4117%32%38%8%
90
11th to 20thDay
7%212%31%28%20%
90
21st to endDay
5%211%35%27%19%
99
January1st to 10thNight
4211%30%39%14%
1040
11th to 20thNight
6%216%37%29%9%
1002
21st to endNight
4216%35%30%14%
1045
February1st to 10thNight
4210%33%34%18%
923
11th to 20thNight
318%35%36%17%
879
21st to endNight
3110%32%41%14%
740
March1st to 10thNight
7%314%33%30%13%
799
11th to 20thNight
5%111%33%37%13%
644
21st to endNight
7%115%33%29%15%
594
April1st to 10thNight
9%113%26%34%16%
540
11th to 20thNight
11%211%33%33%11%
533
21st to endNight
9%213%30%30%16%
476
May1st to 10thNight
15%311%31%30%10%
360
11th to 20thNight
14%415%35%25%7%
296
21st to endNight
21%311%29%24%12%
194
June1st to 10thNight
19%18%31%29%11%
180
11th to 20thNight
14%18%37%37%4
180
21st to endNight
17%38%25%38%8%
180
July1st to 10thNight
18%15%25%38%12%
175
11th to 20thNight
15%210%30%35%9%
174
21st to endNight
17%25%21%39%15%
223
August1st to 10thNight
22%39%20%34%11%
358
11th to 20thNight
25%2423%34%12%
489
21st to endNight
25%28%29%27%9%
594
September1st to 10thNight
31%39%23%27%8%
540
11th to 20thNight
29%210%29%24%5%
540
21st to endNight
32%415%27%18%5%
688
October1st to 10thNight
28%7%16%27%18%3
720
11th to 20thNight
32%5%12%24%22%5%
796
21st to endNight
35%313%25%20%4
990
November1st to 10thNight
19%413%30%26%7%
925
11th to 20thNight
14%415%29%29%8%
995
21st to endNight
12%413%27%32%13%
1040
December1st to 10thNight
8%211%36%31%13%
1040
11th to 20thNight
10%316%28%27%16%
1040
21st to endNight
11%315%29%28%16%
1144
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