Chelsea
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chelsea clear sky chart from 2005-11-06 to 2024-01-16.

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
28%
22561
15%
74774
8%
22520
36%
22520
16%
52605

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
14%
760
9%
2649
2%
757
22%
757
7%
2128
January11th to 20thNight
17%
744
9%
2602
4%
744
24%
744
8%
2025
January21st to endNight
20%
792
14%
2704
7%
792
30%
792
13%
1944
February1st to 10thNight
16%
720
11%
2340
2%
720
26%
720
10%
1782
February11th to 20thNight
15%
703
14%
2275
5%
700
28%
700
12%
1660
February21st to endNight
21%
592
16%
1776
7%
592
33%
592
14%
1300
March1st to 10thNight
19%
720
15%
2157
7%
720
29%
720
14%
1607
March11th to 20thNight
20%
720
16%
1980
6%
720
30%
720
15%
1458
March21st to endNight
24%
792
22%
2063
8%
792
34%
792
19%
1435
April1st to 10thNight
21%
720
16%
1800
4%
720
28%
720
18%
1333
April11th to 20thNight
24%
711
18%
1775
6%
711
31%
711
20%
1285
April21st to endNight
26%
600
17%
1680
7%
600
33%
600
18%
1192
May1st to 10thNight
33%
360
19%
1440
15%
360
44%
360
17%
904
May11th to 20thNight
32%
354
22%
1416
18%
354
48%
354
18%
880
May21st to endNight
34%
389
13%
1551
10%
388
47%
388
21%
913
June1st to 10thNight
36%
360
11%
1440
11%
360
46%
360
17%
918
June11th to 20thNight
39%
360
13%
1440
15%
360
48%
360
20%
845
June21st to endNight
42%
360
11%
1440
13%
360
50%
360
18%
911
July1st to 10thNight
51%
350
14%
1400
18%
350
60%
350
27%
780
July11th to 20thNight
51%
348
16%
1392
23%
348
56%
348
25%
780
July21st to endNight
54%
394
15%
1486
17%
372
59%
372
25%
840
August1st to 10thNight
53%
388
16%
1452
17%
386
59%
386
25%
848
August11th to 20thNight
56%
534
19%
1598
13%
534
60%
534
33%
969
August21st to endNight
45%
792
18%
1980
8%
792
49%
792
32%
1307
September1st to 10thNight
44%
720
19%
1800
7%
720
49%
720
34%
1170
September11th to 20thNight
48%
720
23%
1800
12%
720
52%
720
33%
1167
September21st to endNight
40%
720
22%
2036
15%
720
47%
720
27%
1334
October1st to 10thNight
32%
720
19%
2160
11%
720
41%
720
21%
1455
October11th to 20thNight
26%
720
15%
2160
8%
720
35%
720
16%
1564
October21st to endNight
23%
792
13%
2520
6%
792
33%
792
14%
1860
November1st to 10thNight
29%
740
16%
2407
9%
740
38%
740
17%
1654
November11th to 20thNight
20%
760
15%
2660
9%
760
30%
760
12%
1928
November21st to endNight
19%
760
11%
2660
5%
760
28%
760
9%
2023
December1st to 10thNight
16%
760
9%
2660
4%
760
23%
760
8%
2106
December11th to 20thNight
16%
759
13%
3242
6%
759
26%
759
8%
2419
December21st to endNight
11%
827
7%
4833
1%
817
19%
817
4%
3881

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%
74761
Night
18%10%47%6%34446%33%
75042
TransparencyDay
53%311%21%10%1
74466
Night
54%310%21%10%1
74871
SeeingDay
24%15%31%19%8%2
13350
Night
24%13%28%25%9%1
22571

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
17%17%8%10%7%34445%21%
1517
11th to 20thDay
20%15%5%9%7%45%445%23%
1480
21st to endDay
15%14%5%9%7%45%5%5%6%25%
1650
February1st to 10thDay
14%15%6%12%7%445%45%23%
1620
11th to 20thDay
14%15%6%9%6%3445%5%28%
1573
21st to endDay
19%12%5%9%8%445%5%5%24%
1480
March1st to 10thDay
16%12%49%7%45%45%5%28%
1797
11th to 20thDay
16%12%5%7%7%45%445%31%
1980
21st to endDay
20%8%37%6%335%45%36%
2292
April1st to 10thDay
23%11%46%5%34337%31%
2160
11th to 20thDay
22%9%36%42444437%
2134
21st to endDay
23%10%47%6%34445%31%
2294
May1st to 10thDay
22%7%36%5%35%45%5%34%
2520
11th to 20thDay
17%6%37%5%45%446%39%
2478
21st to endDay
17%6%35%6%45%5%6%7%36%
2725
June1st to 10thDay
22%8%36%6%45%5%5%6%30%
2520
11th to 20thDay
15%5%26%6%36%5%6%8%38%
2520
21st to endDay
20%6%35%6%46%6%7%6%32%
2520
July1st to 10thDay
12%5%26%5%36%6%7%9%39%
2520
11th to 20thDay
13%5%26%5%45%5%7%8%38%
2520
21st to endDay
13%5%35%6%36%5%6%10%39%
2744
August1st to 10thDay
12%5%36%5%45%6%6%9%39%
2473
11th to 20thDay
13%5%26%6%35%6%6%9%39%
2284
21st to endDay
14%6%25%6%35%5%6%8%41%
2376
September1st to 10thDay
15%425%5%346%5%8%42%
2160
11th to 20thDay
13%5%25%5%3445%7%45%
2160
21st to endDay
19%5%26%5%335%5%8%39%
1923
October1st to 10thDay
22%8%35%5%3445%6%35%
1800
11th to 20thDay
25%8%38%5%35%5%5%7%27%
1800
21st to endDay
28%8%37%5%44445%27%
1835
November1st to 10thDay
16%9%36%6%45%46%7%33%
1668
11th to 20thDay
21%12%46%6%45%5%6%6%26%
1520
21st to endDay
22%14%5%7%8%35%446%21%
1520
December1st to 10thDay
22%17%7%10%6%3446%6%17%
1520
11th to 20thDay
21%12%5%9%7%45%44425%
1906
21st to endDay
28%18%6%8%6%3333419%
2772
January1st to 10thNight
18%18%6%11%8%44445%19%
2649
11th to 20thNight
18%18%5%8%7%45%43423%
2602
21st to endNight
12%15%6%9%7%35%446%30%
2704
February1st to 10thNight
15%13%6%11%7%45%5%5%6%23%
2340
11th to 20thNight
14%13%6%9%8%34446%29%
2273
21st to endNight
17%13%5%7%6%35%5%45%30%
1776
March1st to 10thNight
17%12%5%7%8%46%44430%
2147
11th to 20thNight
16%12%5%7%6%45%345%33%
1979
21st to endNight
17%9%46%6%24446%39%
2063
April1st to 10thNight
21%10%47%434445%34%
1800
11th to 20thNight
19%8%46%6%34446%36%
1775
21st to endNight
21%9%45%5%4435%6%33%
1680
May1st to 10thNight
21%7%25%43445%7%37%
1440
11th to 20thNight
18%8%35%5%24446%40%
1415
21st to endNight
13%5%36%5%345%6%6%44%
1550
June1st to 10thNight
18%5%36%6%45%5%6%8%35%
1440
11th to 20thNight
15%5%25%5%3446%7%43%
1440
21st to endNight
17%7%25%5%46%45%8%36%
1440
July1st to 10thNight
11%425%445%5%5%8%46%
1440
11th to 20thNight
13%6%245%245%5%8%44%
1440
21st to endNight
10%426%5%46%6%6%8%44%
1574
August1st to 10thNight
10%6%36%5%345%6%9%43%
1462
11th to 20thNight
11%5%25%5%345%6%8%45%
1598
21st to endNight
12%5%26%6%445%5%7%45%
1980
September1st to 10thNight
12%5%25%5%345%5%7%46%
1800
11th to 20thNight
16%424424446%48%
1800
21st to endNight
18%6%26%5%24346%44%
2036
October1st to 10thNight
20%6%25%5%44446%39%
2160
11th to 20thNight
23%7%26%7%45%5%5%5%31%
2160
21st to endNight
24%9%37%6%45%446%28%
2520
November1st to 10thNight
20%10%36%5%345%47%33%
2407
11th to 20thNight
20%10%47%7%35%5%5%6%29%
2660
21st to endNight
22%12%5%8%6%4445%6%25%
2660
December1st to 10thNight
22%16%5%9%6%35%5%5%6%18%
2660
11th to 20thNight
23%13%6%8%7%33335%27%
3288
21st to endNight
29%18%48%6%2433419%
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%28%15%5%
1518
11th to 20thDay
66%38%12%11%1
1480
21st to endDay
61%48%16%10%1
1650
February1st to 10thDay
64%29%18%6%2
1620
11th to 20thDay
59%212%16%10%1
1582
21st to endDay
63%29%14%10%1
1480
March1st to 10thDay
58%39%18%11%1
1800
11th to 20thDay
57%311%15%12%3
1980
21st to endDay
52%28%19%14%5%
2292
April1st to 10thDay
58%26%19%12%3
2160
11th to 20thDay
52%26%22%16%2
2134
21st to endDay
58%27%15%13%4
2294
May1st to 10thDay
53%36%21%14%3
2520
11th to 20thDay
49%38%22%16%3
2482
21st to endDay
48%411%21%14%2
2725
June1st to 10thDay
55%410%19%12%1
2520
11th to 20thDay
44%411%26%13%2
2520
21st to endDay
50%413%22%10%1
2520
July1st to 10thDay
41%413%29%13%1
2450
11th to 20thDay
42%415%27%12%
2445
21st to endDay
41%5%16%27%10%
2591
August1st to 10thDay
43%418%26%9%
2459
11th to 20thDay
43%416%28%9%1
2284
21st to endDay
42%314%29%12%1
2376
September1st to 10thDay
41%414%30%12%
2160
11th to 20thDay
40%413%32%11%1
2160
21st to endDay
45%316%28%8%
1923
October1st to 10thDay
51%312%23%10%1
1800
11th to 20thDay
59%414%17%6%1
1800
21st to endDay
62%311%16%7%
1835
November1st to 10thDay
49%315%24%7%1
1668
11th to 20thDay
61%313%15%8%1
1520
21st to endDay
66%412%10%7%1
1520
December1st to 10thDay
70%311%12%3
1520
11th to 20thDay
64%312%15%6%
1906
21st to endDay
72%26%16%4
2772
January1st to 10thNight
70%38%14%5%
2649
11th to 20thNight
66%310%12%8%1
2602
21st to endNight
57%38%19%12%1
2704
February1st to 10thNight
63%39%17%8%1
2340
11th to 20thNight
59%39%20%9%1
2275
21st to endNight
56%48%20%11%1
1776
March1st to 10thNight
59%38%19%10%2
2157
11th to 20thNight
56%39%17%12%3
1980
21st to endNight
49%37%21%15%5%
2063
April1st to 10thNight
54%28%19%15%2
1800
11th to 20thNight
51%36%22%17%2
1775
21st to endNight
53%28%17%16%4
1680
May1st to 10thNight
47%37%22%16%4
1440
11th to 20thNight
46%36%23%19%3
1416
21st to endNight
40%414%21%17%3
1551
June1st to 10thNight
47%412%26%9%1
1440
11th to 20thNight
41%411%27%14%2
1440
21st to endNight
46%5%13%22%14%1
1440
July1st to 10thNight
35%5%14%29%17%1
1400
11th to 20thNight
38%313%32%14%
1392
21st to endNight
37%5%16%28%14%
1486
August1st to 10thNight
38%5%19%25%12%1
1452
11th to 20thNight
36%417%31%11%
1598
21st to endNight
39%416%30%11%
1980
September1st to 10thNight
38%412%32%14%1
1800
11th to 20thNight
38%412%35%10%1
1800
21st to endNight
44%211%33%10%
2036
October1st to 10thNight
48%49%26%13%
2160
11th to 20thNight
55%311%22%8%1
2160
21st to endNight
58%313%17%8%
2520
November1st to 10thNight
51%414%22%9%
2407
11th to 20thNight
58%311%18%9%1
2660
21st to endNight
62%412%17%4
2660
December1st to 10thNight
68%410%15%4
2660
11th to 20thNight
63%28%18%8%1
3288
21st to endNight
73%26%14%5%
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
29%16%33%18%4
180
11th to 20thDay
29%19%35%13%31
170
21st to endDay
24%21%35%14%5%
206
February1st to 10thDay
38%25%29%8%1
240
11th to 20thDay
32%26%35%7%1
240
21st to endDay
32%23%29%14%2
346
March1st to 10thDay
28%20%33%15%5%
420
11th to 20thDay
29%21%31%15%31
420
21st to endDay
32%18%30%14%6%1
462
April1st to 10thDay
31%22%27%14%5%1
420
11th to 20thDay
34%18%24%15%8%1
412
21st to endDay
31%20%30%12%6%1
420
May1st to 10thDay
27%17%30%17%6%3
423
11th to 20thDay
20%18%34%17%7%2
447
21st to endDay
22%15%33%19%9%2
485
June1st to 10thDay
26%11%37%18%6%1
450
11th to 20thDay
19%15%30%22%11%3
450
21st to endDay
23%11%31%22%10%3
450
July1st to 10thDay
14%8%30%33%11%3
445
11th to 20thDay
14%9%35%27%11%3
444
21st to endDay
14%7%36%28%12%3
488
August1st to 10thDay
15%9%27%31%14%4
446
11th to 20thDay
17%428%30%13%8%
439
21st to endDay
17%11%29%24%13%6%
495
September1st to 10thDay
18%10%30%22%13%7%
450
11th to 20thDay
15%11%28%20%17%9%
450
21st to endDay
21%13%27%21%14%4
450
October1st to 10thDay
25%16%23%22%10%4
450
11th to 20thDay
28%18%31%16%6%1
450
21st to endDay
34%15%27%20%4
422
November1st to 10thDay
27%16%30%19%7%1
362
11th to 20thDay
29%26%28%13%3
180
21st to endDay
27%24%30%13%32
180
December1st to 10thDay
36%13%39%11%2
180
11th to 20thDay
28%22%37%11%2
180
21st to endDay
35%20%32%11%21
198
January1st to 10thNight
28%17%32%19%4
760
11th to 20thNight
30%17%26%21%5%
744
21st to endNight
20%15%34%26%5%1
792
February1st to 10thNight
22%20%32%23%3
720
11th to 20thNight
22%20%36%17%4
703
21st to endNight
26%19%28%23%4
592
March1st to 10thNight
26%16%32%22%3
720
11th to 20thNight
25%19%32%20%5%
720
21st to endNight
25%17%28%22%7%
792
April1st to 10thNight
29%16%26%24%4
720
11th to 20thNight
24%19%28%20%7%1
711
21st to endNight
27%15%26%23%8%
600
May1st to 10thNight
25%10%26%28%8%2
360
11th to 20thNight
24%9%28%29%10%
354
21st to endNight
18%8%35%27%11%1
389
June1st to 10thNight
20%9%28%34%9%
360
11th to 20thNight
16%9%26%32%15%1
360
21st to endNight
18%7%24%36%15%
360
July1st to 10thNight
12%328%37%19%
350
11th to 20thNight
13%322%43%17%1
348
21st to endNight
11%5%25%41%17%2
394
August1st to 10thNight
12%5%18%41%23%1
388
11th to 20thNight
13%5%17%37%25%3
534
21st to endNight
15%6%25%34%18%2
792
September1st to 10thNight
15%6%29%29%19%1
720
11th to 20thNight
18%7%21%32%19%3
720
21st to endNight
21%8%25%25%18%2
720
October1st to 10thNight
24%12%25%23%15%1
720
11th to 20thNight
27%12%27%25%8%1
720
21st to endNight
29%15%27%24%5%
792
November1st to 10thNight
27%14%23%26%9%1
740
11th to 20thNight
26%17%31%16%10%
760
21st to endNight
29%19%24%22%6%
760
December1st to 10thNight
30%16%29%21%3
760
11th to 20thNight
28%19%27%20%5%1
760
21st to endNight
34%16%31%16%4
836
Page generated at 2024-01-13 09:37:57. Version ac07

Copyright 2024 Attilla Danko


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