Cambridge
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cambridge clear sky chart from 2005-11-06 to 2024-03-18.

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
31%
23160
13%
80496
9%
23057
38%
23057
16%
57422

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
13%
752
8%
4368
4%
737
21%
737
5%
3457
January11th to 20thNight
12%
758
5%
4117
3%
740
17%
740
4%
3393
January21st to endNight
18%
833
6%
4528
4%
814
26%
814
6%
3645
February1st to 10thNight
15%
760
8%
2470
4%
760
21%
760
10%
1931
February11th to 20thNight
18%
741
11%
2405
3%
740
27%
740
11%
1787
February21st to endNight
16%
628
10%
1946
3%
628
24%
628
12%
1463
March1st to 10thNight
22%
760
17%
2105
7%
760
30%
760
19%
1523
March11th to 20thNight
22%
752
17%
2068
7%
752
32%
752
19%
1490
March21st to endNight
28%
792
18%
2178
9%
792
35%
792
20%
1556
April1st to 10thNight
25%
720
14%
1872
5%
720
31%
720
20%
1374
April11th to 20thNight
29%
615
19%
1679
8%
615
37%
615
22%
1167
April21st to endNight
32%
540
18%
1620
7%
540
39%
540
20%
1101
May1st to 10thNight
36%
540
16%
1620
9%
540
43%
540
23%
1089
May11th to 20thNight
40%
442
19%
1504
11%
442
51%
442
23%
932
May21st to endNight
51%
388
18%
1551
18%
388
59%
388
27%
845
June1st to 10thNight
46%
360
15%
1440
14%
360
58%
360
24%
829
June11th to 20thNight
42%
360
17%
1369
17%
360
51%
360
25%
786
June21st to endNight
47%
360
17%
1260
16%
360
56%
360
27%
739
July1st to 10thNight
60%
350
19%
1266
23%
350
65%
350
36%
650
July11th to 20thNight
57%
407
19%
1451
19%
407
64%
407
32%
784
July21st to endNight
58%
591
17%
1672
13%
558
62%
558
34%
982
August1st to 10thNight
60%
537
18%
1600
13%
534
64%
534
33%
960
August11th to 20thNight
66%
534
23%
1598
16%
534
70%
534
38%
912
August21st to endNight
54%
639
21%
1827
12%
639
59%
639
32%
1138
September1st to 10thNight
54%
720
19%
1800
10%
720
57%
720
32%
1187
September11th to 20thNight
56%
720
26%
1962
17%
720
61%
720
37%
1178
September21st to endNight
43%
720
20%
1980
14%
720
48%
720
26%
1320
October1st to 10thNight
39%
720
18%
2160
14%
720
44%
720
23%
1438
October11th to 20thNight
31%
720
16%
2160
9%
720
41%
720
19%
1510
October21st to endNight
24%
792
16%
2376
10%
792
33%
792
14%
1776
November1st to 10thNight
32%
740
18%
2437
14%
740
39%
740
19%
1612
November11th to 20thNight
22%
760
14%
2660
7%
760
32%
760
12%
1914
November21st to endNight
19%
760
9%
2660
5%
760
24%
760
11%
1981
December1st to 10thNight
14%
760
6%
2660
3%
760
18%
760
7%
2131
December11th to 20thNight
14%
760
7%
3249
2%
758
20%
758
6%
2524
December21st to endNight
10%
829
3%
4878
1%
817
16%
817
3%
4318

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%30%
79025
Night
16%10%5%8%6%35%5%5%7%31%
80744
TransparencyDay
52%7%12%18%10%1
78740
Night
52%7%12%19%10%1
80564
SeeingDay
19%14%29%22%12%5%
13888
Night
19%12%30%27%11%1
23183

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%18%8%11%7%34335%23%
2558
11th to 20thDay
12%20%8%15%10%44436%15%
2840
21st to endDay
23%19%7%11%9%3444313%
3124
February1st to 10thDay
16%17%5%10%10%5%5%5%5%6%17%
1710
11th to 20thDay
14%12%9%12%8%45%35%7%21%
1663
21st to endDay
17%15%5%9%7%35%35%6%23%
1506
March1st to 10thDay
14%13%6%8%6%35%5%48%27%
2071
11th to 20thDay
14%13%6%8%6%3445%7%30%
2058
21st to endDay
18%11%5%7%7%3445%6%30%
2178
April1st to 10thDay
18%11%6%8%5%35%45%7%28%
2088
11th to 20thDay
19%9%36%5%44447%35%
2245
21st to endDay
20%10%37%5%44446%32%
2340
May1st to 10thDay
17%8%48%6%345%5%7%34%
2340
11th to 20thDay
16%7%37%6%35%5%5%7%36%
2388
21st to endDay
11%5%26%6%35%5%6%9%42%
2725
June1st to 10thDay
14%6%35%5%46%6%6%8%38%
2520
11th to 20thDay
11%6%25%6%346%6%9%40%
2582
21st to endDay
13%5%35%5%35%6%7%10%40%
2700
July1st to 10thDay
8%435%6%35%6%7%9%45%
2651
11th to 20thDay
11%6%36%6%36%6%8%10%36%
2458
21st to endDay
9%5%36%6%46%6%8%9%38%
2547
August1st to 10thDay
9%5%26%6%46%6%8%10%38%
2327
11th to 20thDay
10%5%37%7%46%6%7%9%37%
2303
21st to endDay
11%7%36%6%35%5%6%8%39%
2510
September1st to 10thDay
12%7%35%6%35%6%6%8%38%
2160
11th to 20thDay
11%6%35%5%245%6%9%43%
1996
21st to endDay
14%7%36%5%45%5%7%8%35%
1979
October1st to 10thDay
18%9%37%6%4445%7%33%
1800
11th to 20thDay
22%9%47%6%45%5%6%6%28%
1800
21st to endDay
24%12%38%6%345%6%6%22%
1980
November1st to 10thDay
15%9%47%8%35%5%5%7%31%
1640
11th to 20thDay
15%15%38%6%45%5%5%8%24%
1520
21st to endDay
19%16%6%9%7%3346%6%20%
1520
December1st to 10thDay
16%16%7%11%8%46%45%6%16%
1520
11th to 20thDay
19%15%7%10%6%46%6%5%6%16%
1906
21st to endDay
28%19%7%12%7%343349%
2772
January1st to 10thNight
17%16%7%12%8%45%5%45%17%
4384
11th to 20thNight
15%14%7%13%10%5%7%6%5%5%13%
4120
21st to endNight
22%14%9%10%6%44345%18%
4532
February1st to 10thNight
15%15%6%9%8%46%5%5%6%21%
2470
11th to 20thNight
14%13%5%10%7%36%5%6%7%24%
2403
21st to endNight
15%12%6%10%7%46%5%5%7%24%
1946
March1st to 10thNight
14%9%47%7%45%5%6%7%33%
2095
11th to 20thNight
15%11%47%6%345%5%6%34%
2067
21st to endNight
17%11%46%7%3444437%
2178
April1st to 10thNight
19%9%36%6%35%45%6%34%
1872
11th to 20thNight
15%10%36%6%3445%7%37%
1679
21st to endNight
15%8%47%5%4445%7%36%
1620
May1st to 10thNight
16%7%45%5%3445%8%38%
1620
11th to 20thNight
14%6%35%6%246%5%8%41%
1503
21st to endNight
9%5%24425%5%6%7%51%
1550
June1st to 10thNight
10%4245%46%5%7%8%44%
1440
11th to 20thNight
10%5%25%5%45%5%7%8%44%
1369
21st to endNight
10%5%25%6%345%6%8%45%
1260
July1st to 10thNight
5%415%5%3445%8%58%
1303
11th to 20thNight
9%5%234245%5%10%50%
1499
21st to endNight
8%425%5%346%6%8%49%
1771
August1st to 10thNight
6%5%25%5%35%5%7%8%48%
1611
11th to 20thNight
6%4345%245%7%10%51%
1598
21st to endNight
9%5%25%5%446%6%8%47%
1827
September1st to 10thNight
11%5%245%36%5%5%8%46%
1800
11th to 20thNight
11%5%24424448%52%
1962
21st to endNight
15%7%25%6%345%47%42%
1980
October1st to 10thNight
16%8%26%6%3345%6%41%
2160
11th to 20thNight
18%9%46%5%35%5%5%8%34%
2160
21st to endNight
21%9%47%7%45%45%6%28%
2376
November1st to 10thNight
14%8%36%5%345%6%7%37%
2437
11th to 20thNight
16%11%48%7%45%5%6%7%28%
2660
21st to endNight
20%11%5%8%7%45%5%6%6%25%
2660
December1st to 10thNight
19%14%6%9%7%35%5%5%7%18%
2660
11th to 20thNight
16%16%6%11%7%35%5%5%6%21%
3288
21st to endNight
28%20%7%10%7%344449%
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
64%7%11%10%8%
2559
11th to 20thDay
73%410%9%4
2840
21st to endDay
75%6%8%7%4
3124
February1st to 10thDay
67%7%10%12%41
1710
11th to 20thDay
63%5%10%14%7%1
1672
21st to endDay
62%5%11%14%7%1
1506
March1st to 10thDay
55%6%13%16%8%1
2074
11th to 20thDay
54%5%12%16%10%2
2058
21st to endDay
56%410%17%11%2
2178
April1st to 10thDay
55%6%12%16%11%
2088
11th to 20thDay
50%5%10%21%12%1
2245
21st to endDay
53%6%9%18%11%3
2340
May1st to 10thDay
49%5%11%22%12%1
2340
11th to 20thDay
46%7%11%23%11%2
2392
21st to endDay
39%7%16%22%14%2
2725
June1st to 10thDay
42%8%12%23%15%2
2520
11th to 20thDay
39%7%14%23%15%1
2582
21st to endDay
39%8%14%24%15%
2700
July1st to 10thDay
34%8%13%24%18%3
2579
11th to 20thDay
38%9%16%25%11%1
2383
21st to endDay
38%9%13%23%14%2
2405
August1st to 10thDay
38%9%17%24%11%2
2314
11th to 20thDay
41%8%17%23%11%1
2303
21st to endDay
40%7%13%26%12%1
2510
September1st to 10thDay
42%8%14%24%11%1
2160
11th to 20thDay
36%6%16%29%14%
1996
21st to endDay
44%8%14%24%9%1
1979
October1st to 10thDay
50%6%13%17%12%1
1800
11th to 20thDay
55%7%13%17%8%1
1800
21st to endDay
60%6%12%14%6%
1980
November1st to 10thDay
51%7%13%21%8%1
1640
11th to 20thDay
57%7%14%13%8%1
1520
21st to endDay
63%6%14%12%4
1520
December1st to 10thDay
68%8%11%9%4
1520
11th to 20thDay
67%9%11%8%5%
1906
21st to endDay
79%6%7%7%1
2772
January1st to 10thNight
65%8%10%11%41
4384
11th to 20thNight
69%9%11%8%3
4120
21st to endNight
68%7%10%11%41
4532
February1st to 10thNight
62%6%11%16%4
2470
11th to 20thNight
57%7%12%15%9%1
2405
21st to endNight
58%5%13%16%8%
1946
March1st to 10thNight
50%6%13%21%8%2
2105
11th to 20thNight
50%5%10%21%11%2
2068
21st to endNight
51%5%7%20%14%3
2178
April1st to 10thNight
51%6%10%19%14%1
1872
11th to 20thNight
46%6%9%20%17%2
1679
21st to endNight
47%5%10%20%15%3
1620
May1st to 10thNight
44%6%10%22%15%3
1620
11th to 20thNight
40%6%12%25%13%4
1504
21st to endNight
31%8%14%26%18%3
1551
June1st to 10thNight
36%7%12%26%18%2
1440
11th to 20thNight
35%7%13%25%18%2
1369
21st to endNight
35%7%15%25%18%1
1260
July1st to 10thNight
26%7%11%27%26%4
1266
11th to 20thNight
30%6%14%30%17%2
1451
21st to endNight
31%8%16%27%16%3
1672
August1st to 10thNight
32%8%15%31%11%2
1600
11th to 20thNight
28%7%17%32%14%2
1598
21st to endNight
33%8%14%28%15%2
1827
September1st to 10thNight
37%7%13%27%15%2
1800
11th to 20thNight
31%6%13%31%18%1
1962
21st to endNight
41%7%12%27%13%
1980
October1st to 10thNight
44%6%10%24%15%1
2160
11th to 20thNight
48%7%13%23%9%
2160
21st to endNight
56%6%10%18%10%1
2376
November1st to 10thNight
45%7%13%21%13%1
2437
11th to 20thNight
54%7%14%18%7%1
2660
21st to endNight
58%6%17%13%5%
2660
December1st to 10thNight
65%8%12%11%4
2660
11th to 20thNight
63%6%13%13%4
3288
21st to endNight
78%7%8%5%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%23%29%17%7%1
193
11th to 20thDay
25%23%36%11%41
270
21st to endDay
28%19%34%16%4
297
February1st to 10thDay
27%24%34%11%4
270
11th to 20thDay
25%24%38%12%1
270
21st to endDay
35%24%32%10%
225
March1st to 10thDay
25%18%33%19%6%
444
11th to 20thDay
23%17%35%21%41
449
21st to endDay
28%16%31%18%6%1
462
April1st to 10thDay
20%21%32%19%7%1
420
11th to 20thDay
26%18%25%18%10%2
412
21st to endDay
25%11%34%19%9%2
420
May1st to 10thDay
22%15%28%21%9%5%
423
11th to 20thDay
18%11%28%26%13%5%
447
21st to endDay
13%7%29%28%15%8%
485
June1st to 10thDay
15%14%27%29%11%4
450
11th to 20thDay
14%10%29%27%16%5%
512
21st to endDay
15%8%29%33%11%3
630
July1st to 10thDay
8%6%25%31%22%8%
574
11th to 20thDay
15%7%23%27%19%10%
444
21st to endDay
9%5%29%33%17%7%
488
August1st to 10thDay
9%6%22%28%24%11%
446
11th to 20thDay
12%5%21%26%23%13%
439
21st to endDay
14%6%25%25%20%10%
495
September1st to 10thDay
14%9%22%28%16%12%
450
11th to 20thDay
9%6%20%27%24%13%
450
21st to endDay
18%11%25%24%15%8%
450
October1st to 10thDay
22%12%24%22%12%7%
450
11th to 20thDay
21%19%32%19%7%2
450
21st to endDay
28%17%31%18%6%
495
November1st to 10thDay
24%22%30%15%8%1
260
11th to 20thDay
27%19%31%16%5%2
180
21st to endDay
28%19%35%14%4
180
December1st to 10thDay
29%26%31%9%41
180
11th to 20thDay
29%26%31%11%3
180
21st to endDay
30%25%35%10%1
198
January1st to 10thNight
23%21%34%16%5%
760
11th to 20thNight
27%18%33%19%3
760
21st to endNight
21%13%38%23%5%
836
February1st to 10thNight
23%19%35%21%2
760
11th to 20thNight
22%17%35%21%4
743
21st to endNight
22%21%34%20%3
628
March1st to 10thNight
18%15%37%24%6%
760
11th to 20thNight
22%16%35%21%6%
752
21st to endNight
21%13%31%26%8%1
792
April1st to 10thNight
23%15%32%26%4
720
11th to 20thNight
21%15%30%23%9%1
615
21st to endNight
16%11%35%29%9%
540
May1st to 10thNight
17%9%31%28%13%1
540
11th to 20thNight
16%5%32%35%11%1
442
21st to endNight
11%5%28%36%20%
389
June1st to 10thNight
13%6%31%39%12%1
360
11th to 20thNight
13%6%33%36%12%1
360
21st to endNight
11%6%28%38%16%
360
July1st to 10thNight
4325%43%21%2
350
11th to 20thNight
10%325%40%20%2
407
21st to endNight
9%323%45%19%2
591
August1st to 10thNight
8%320%42%24%2
537
11th to 20thNight
8%217%40%30%3
534
21st to endNight
11%424%32%26%3
639
September1st to 10thNight
13%6%21%35%21%4
720
11th to 20thNight
12%5%20%32%27%4
720
21st to endNight
18%8%22%27%20%5%
720
October1st to 10thNight
18%10%28%26%17%3
720
11th to 20thNight
22%13%29%28%8%1
720
21st to endNight
26%14%30%26%4
792
November1st to 10thNight
19%13%30%26%10%2
740
11th to 20thNight
22%18%32%20%7%1
760
21st to endNight
26%16%33%19%6%
760
December1st to 10thNight
27%21%30%17%5%
760
11th to 20thNight
24%20%35%18%4
760
21st to endNight
32%15%34%17%3
836
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