Cambridge
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cambridge clear sky chart from 2005-11-05 to 2018-02-14.

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
33%
15472
14%
48608
9%
15435
39%
15435
18%
33865

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 15thNight
13%
776
9%
2629
4%
776
20%
776
8%
2081
January16th to endNight
14%
832
10%
2704
2%
832
22%
832
9%
2129
February1st to 15thNight
15%
758
8%
2468
3%
757
22%
757
10%
1924
February16th to endNight
20%
631
11%
1991
4%
631
27%
631
12%
1475
March1st to 15thNight
22%
720
13%
1990
6%
720
28%
720
16%
1515
March16th to endNight
31%
768
21%
2112
10%
768
39%
768
23%
1447
April1st to 15thNight
26%
712
15%
1838
6%
712
31%
712
21%
1332
April16th to endNight
37%
540
21%
1620
9%
540
45%
540
25%
1045
May1st to 15thNight
40%
537
17%
1617
10%
537
47%
537
24%
1069
May16th to endNight
44%
374
17%
1497
14%
374
55%
374
23%
860
June1st to 15thNight
45%
360
13%
1440
12%
360
55%
360
22%
836
June16th to endNight
47%
360
15%
1272
16%
360
55%
360
27%
748
July1st to 15thNight
61%
340
18%
1273
22%
340
66%
340
36%
637
July16th to endNight
57%
545
15%
1576
12%
512
60%
512
33%
927
August1st to 15thNight
66%
539
19%
1609
13%
536
70%
536
37%
928
August16th to endNight
60%
603
23%
1749
15%
603
65%
603
34%
1044
September1st to 15thNight
54%
720
22%
1848
12%
720
59%
720
34%
1177
September16th to endNight
52%
717
23%
1980
16%
717
56%
717
32%
1235
October1st to 15thNight
40%
720
18%
2160
15%
720
46%
720
23%
1430
October16th to endNight
26%
768
16%
2304
9%
768
35%
768
15%
1685
November1st to 15thNight
33%
760
17%
2559
15%
760
39%
760
18%
1736
November16th to endNight
22%
780
10%
2730
6%
780
28%
780
11%
2007
December1st to 15thNight
13%
780
6%
2730
2%
780
18%
780
8%
2167
December16th to endNight
11%
832
6%
2912
1%
832
16%
832
6%
2431

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%9%47%6%35%5%5%7%30%
49455
Night
15%8%46%6%3445%7%33%
48787
Cloud CoverDay
15%9%47%6%35%5%5%7%31%
49465
Night
15%8%36%5%3445%7%33%
48791
TransparencyDay
51%6%12%18%9%
49152
Night
49%5%12%20%10%1
48608
SeeingDay
18%10%27%23%13%6%
5998
Night
19%11%29%26%11%1
15479

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 15thDay
19%14%6%9%7%35%436%19%
1637
16th to endDay
14%17%7%12%9%445%35%16%
1872
February1st to 15thDay
16%16%7%9%8%45%446%16%
1695
16th to endDay
16%14%5%9%8%45%35%6%20%
1480
March1st to 15thDay
15%13%6%8%6%25%437%24%
1958
16th to endDay
14%9%6%7%6%3345%7%31%
2112
April1st to 15thDay
21%9%46%435%446%29%
2120
16th to endDay
18%9%26%433345%37%
2340
May1st to 15thDay
17%7%37%4245%47%34%
2331
16th to endDay
12%5%36%6%35%5%5%8%38%
2618
June1st to 15thDay
14%5%25%5%35%5%5%8%37%
2520
16th to endDay
12%4245%35%6%7%10%35%
2682
July1st to 15thDay
8%425%6%35%6%7%10%39%
2607
16th to endDay
9%436%6%36%5%8%9%34%
2483
August1st to 15thDay
9%5%26%6%46%6%7%8%36%
2340
16th to endDay
11%6%25%6%35%5%6%8%38%
2439
September1st to 15thDay
13%6%35%5%245%6%8%37%
2110
16th to endDay
14%6%25%4345%5%8%40%
1978
October1st to 15thDay
19%8%36%5%3435%5%32%
1800
16th to endDay
24%10%37%5%345%5%6%23%
1920
November1st to 15thDay
15%9%47%6%35%447%31%
1629
16th to endDay
19%15%48%6%3445%6%21%
1560
December1st to 15thDay
17%15%7%10%7%45%45%5%15%
1560
16th to endDay
20%15%8%10%7%45%445%12%
1664
January1st to 15thNight
21%12%5%9%7%45%445%19%
2629
16th to endNight
15%13%7%10%7%45%446%20%
2704
February1st to 15thNight
14%14%6%9%6%36%5%5%6%21%
2466
16th to endNight
15%11%5%9%7%35%447%24%
1991
March1st to 15thNight
17%9%47%6%4445%6%28%
1978
16th to endNight
15%9%35%5%24335%40%
2112
April1st to 15thNight
19%8%35%6%23347%35%
1838
16th to endNight
12%6%36%5%4345%6%42%
1620
May1st to 15thNight
15%5%45%5%2445%8%37%
1617
16th to endNight
10%5%245%245%6%7%46%
1495
June1st to 15thNight
11%325%5%45%5%7%7%41%
1440
16th to endNight
9%4145%3446%9%44%
1272
July1st to 15thNight
43133235%410%56%
1350
16th to endNight
8%3245%2346%9%48%
1683
August1st to 15thNight
5%3135%25%47%9%49%
1620
16th to endNight
7%424434446%52%
1749
September1st to 15thNight
11%424425%448%48%
1848
16th to endNight
12%4235%2445%8%46%
1980
October1st to 15thNight
17%6%25%5%2345%7%40%
2160
16th to endNight
19%9%36%6%35%45%6%30%
2304
November1st to 15thNight
15%8%35%6%335%6%7%34%
2559
16th to endNight
20%9%47%6%35%45%6%25%
2730
December1st to 15thNight
20%13%5%8%6%35%5%5%6%18%
2730
16th to endNight
21%13%8%10%7%4445%6%13%
2912

Percentage Cloud Cover SE (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 15thDay
19%13%7%8%6%45%436%19%
1637
16th to endDay
13%17%7%13%7%46%335%16%
1872
February1st to 15thDay
16%16%7%9%7%5%5%5%46%16%
1695
16th to endDay
16%14%5%9%7%45%35%6%21%
1480
March1st to 15thDay
15%13%6%8%6%25%446%24%
1969
16th to endDay
14%9%6%6%6%34447%32%
2111
April1st to 15thDay
21%9%46%5%345%5%6%29%
2120
16th to endDay
17%9%36%423345%38%
2340
May1st to 15thDay
17%7%36%5%2445%7%34%
2331
16th to endDay
12%5%25%6%35%45%7%39%
2618
June1st to 15thDay
14%5%35%5%45%5%5%8%37%
2520
16th to endDay
12%4245%35%6%6%9%36%
2682
July1st to 15thDay
9%425%5%35%6%7%9%39%
2607
16th to endDay
9%5%35%6%36%6%8%9%34%
2483
August1st to 15thDay
9%5%25%7%45%6%6%8%37%
2340
16th to endDay
12%6%26%5%35%5%6%8%38%
2439
September1st to 15thDay
13%7%246%35%46%7%38%
2110
16th to endDay
14%5%345%3445%8%39%
1978
October1st to 15thDay
20%8%36%6%3435%6%33%
1800
16th to endDay
24%9%37%5%3445%5%24%
1920
November1st to 15thDay
15%9%37%5%35%5%5%7%31%
1629
16th to endDay
19%14%48%6%3445%7%21%
1560
December1st to 15thDay
17%15%6%10%8%45%45%5%16%
1560
16th to endDay
20%15%7%10%7%45%5%45%13%
1664
January1st to 15thNight
21%12%5%9%7%345%45%19%
2629
16th to endNight
15%13%7%10%6%35%45%5%20%
2704
February1st to 15thNight
14%14%6%9%6%35%5%5%5%21%
2459
16th to endNight
15%11%5%8%7%4446%6%24%
1991
March1st to 15thNight
18%8%48%6%4445%5%28%
1990
16th to endNight
15%10%36%423335%40%
2112
April1st to 15thNight
19%8%36%5%33347%35%
1838
16th to endNight
11%6%35%6%33447%42%
1620
May1st to 15thNight
15%5%45%5%2445%8%38%
1616
16th to endNight
11%5%235%2446%8%45%
1495
June1st to 15thNight
11%4245%35%5%6%7%41%
1440
16th to endNight
10%415%5%3347%8%44%
1272
July1st to 15thNight
5%3134245%5%9%55%
1350
16th to endNight
8%3245%2446%9%48%
1683
August1st to 15thNight
6%3134245%8%9%49%
1620
16th to endNight
7%425%5%33446%52%
1749
September1st to 15thNight
11%414334448%48%
1848
16th to endNight
11%5%145%235%5%7%46%
1980
October1st to 15thNight
17%7%25%423447%40%
2160
16th to endNight
19%8%36%6%3445%6%30%
2304
November1st to 15thNight
16%7%35%6%245%5%8%34%
2559
16th to endNight
20%9%38%6%25%5%5%6%26%
2730
December1st to 15thNight
20%14%48%6%35%5%5%6%19%
2730
16th to endNight
20%13%7%10%7%45%5%5%6%13%
2912

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 15thDay
67%410%10%6%
1638
16th to endDay
70%410%9%4
1872
February1st to 15thDay
67%6%9%11%3
1697
16th to endDay
63%5%11%13%5%
1480
March1st to 15thDay
58%6%12%14%7%
1969
16th to endDay
53%411%17%11%1
2112
April1st to 15thDay
54%5%11%17%10%
2120
16th to endDay
48%49%20%14%2
2340
May1st to 15thDay
48%6%10%22%11%
2331
16th to endDay
42%6%14%21%13%1
2622
June1st to 15thDay
43%7%13%22%12%1
2520
16th to endDay
41%7%16%21%12%
2682
July1st to 15thDay
37%7%12%23%17%1
2455
16th to endDay
40%9%14%22%10%2
2327
August1st to 15thDay
40%8%17%19%12%1
2327
16th to endDay
42%6%14%24%11%1
2439
September1st to 15thDay
43%6%14%23%11%
2110
16th to endDay
41%6%14%27%10%
1978
October1st to 15thDay
52%5%11%17%11%1
1800
16th to endDay
59%6%13%13%5%
1920
November1st to 15thDay
51%7%14%18%8%
1629
16th to endDay
62%6%13%12%5%
1560
December1st to 15thDay
67%8%12%8%3
1560
16th to endDay
72%7%9%8%2
1664
January1st to 15thNight
65%410%11%6%
2629
16th to endNight
63%5%12%13%4
2704
February1st to 15thNight
62%5%12%14%4
2468
16th to endNight
57%5%13%15%7%
1991
March1st to 15thNight
56%5%11%18%6%
1990
16th to endNight
46%49%22%14%2
2112
April1st to 15thNight
50%410%20%12%1
1838
16th to endNight
42%49%20%19%3
1620
May1st to 15thNight
43%5%11%24%12%2
1617
16th to endNight
35%7%13%25%13%3
1497
June1st to 15thNight
38%5%12%26%16%1
1440
16th to endNight
34%7%14%25%17%1
1272
July1st to 15thNight
24%6%11%30%23%3
1273
16th to endNight
32%6%16%24%16%3
1576
August1st to 15thNight
28%7%16%32%12%2
1609
16th to endNight
31%6%11%30%17%1
1749
September1st to 15thNight
34%5%12%28%16%1
1848
16th to endNight
35%5%12%30%14%
1980
October1st to 15thNight
43%6%9%25%14%
2160
16th to endNight
53%5%12%19%7%
2304
November1st to 15thNight
47%6%13%19%12%1
2559
16th to endNight
57%5%16%14%5%
2730
December1st to 15thNight
65%6%13%10%4
2730
16th to endNight
70%6%11%8%3
2912

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 15thDay
20%26%31%16%4
110
16th to endDay
29%16%40%11%1
144
February1st to 15thDay
30%23%33%11%
130
16th to endDay
23%22%43%9%1
81
March1st to 15thDay
25%12%30%25%5%
259
16th to endDay
23%9%34%23%6%2
288
April1st to 15thDay
22%15%30%21%9%
270
16th to endDay
20%11%30%20%14%3
270
May1st to 15thDay
20%9%27%23%11%7%
288
16th to endDay
14%8%31%25%12%7%
331
June1st to 15thDay
18%10%22%30%13%4
315
16th to endDay
14%8%31%30%11%3
477
July1st to 15thDay
10%6%24%29%19%9%
389
16th to endDay
10%5%23%28%21%10%
326
August1st to 15thDay
9%222%25%25%14%
313
16th to endDay
15%317%21%23%18%
332
September1st to 15thDay
13%414%27%26%13%
315
16th to endDay
15%5%17%20%22%17%
314
October1st to 15thDay
20%8%24%24%14%6%
315
16th to endDay
24%15%31%22%6%
336
November1st to 15thDay
19%15%31%18%12%2
119
16th to endDay
28%15%35%12%6%1
90
December1st to 15thDay
28%28%26%11%4
90
16th to endDay
32%19%38%9%
96
January1st to 15thNight
26%18%33%17%3
780
16th to endNight
20%15%39%19%4
832
February1st to 15thNight
20%20%34%20%3
760
16th to endNight
21%15%34%23%5%
631
March1st to 15thNight
22%14%33%23%5%
720
16th to endNight
19%11%31%27%9%
768
April1st to 15thNight
23%14%30%25%5%
712
16th to endNight
13%9%33%30%10%1
540
May1st to 15thNight
15%7%29%32%13%1
537
16th to endNight
12%5%30%30%19%
375
June1st to 15thNight
13%430%38%12%
360
16th to endNight
11%328%40%15%
360
July1st to 15thNight
3225%45%20%1
340
16th to endNight
8%224%40%21%2
545
August1st to 15thNight
7%117%44%25%3
539
16th to endNight
9%220%33%30%2
603
September1st to 15thNight
13%6%20%32%23%4
720
16th to endNight
15%419%26%27%6%
717
October1st to 15thNight
20%7%26%26%15%3
720
16th to endNight
24%13%30%26%5%
768
November1st to 15thNight
19%10%31%25%11%1
760
16th to endNight
26%17%28%19%8%
780
December1st to 15thNight
27%18%31%17%4
780
16th to endNight
28%16%35%16%3
832
Page generated at 2018-02-15 10:58:52. Version ac04

Copyright 2018 A. Danko


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