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 2018-02-13.

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
59%
15295
17%
48771
17%
15268
62%
15268
26%
30962

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
43%
776
6%
2716
5%
776
46%
776
11%
2136
January16th to endNight
48%
832
8%
2912
9%
832
50%
832
13%
2188
February1st to 15thNight
48%
747
8%
2447
7%
744
52%
744
16%
1790
February16th to endNight
47%
632
12%
1895
10%
632
50%
632
17%
1398
March1st to 15thNight
47%
720
9%
2160
7%
720
49%
720
17%
1608
March16th to endNight
49%
768
12%
2028
8%
768
51%
768
22%
1496
April1st to 15thNight
54%
719
16%
1799
9%
719
56%
719
28%
1240
April16th to endNight
56%
573
21%
1653
14%
573
60%
573
31%
1028
May1st to 15thNight
61%
498
21%
1578
18%
498
66%
498
40%
825
May16th to endNight
67%
374
16%
1496
24%
374
70%
374
25%
843
June1st to 15thNight
63%
360
12%
1440
17%
360
67%
360
26%
818
June16th to endNight
75%
360
22%
1440
35%
360
79%
360
41%
640
July1st to 15thNight
88%
340
23%
1360
37%
340
92%
340
59%
491
July16th to endNight
86%
376
34%
1416
49%
354
90%
354
63%
486
August1st to 15thNight
82%
447
32%
1519
33%
445
85%
445
56%
648
August16th to endNight
83%
697
36%
1843
26%
697
84%
697
66%
870
September1st to 15thNight
81%
720
35%
1834
23%
720
83%
720
62%
935
September16th to endNight
73%
716
34%
1980
28%
716
76%
716
44%
1099
October1st to 15thNight
67%
720
24%
2040
21%
720
70%
720
38%
1193
October16th to endNight
63%
768
22%
2496
23%
768
66%
768
29%
1513
November1st to 15thNight
54%
760
17%
2470
18%
760
56%
760
22%
1652
November16th to endNight
50%
780
12%
2607
11%
780
53%
780
18%
1818
December1st to 15thNight
50%
780
9%
2730
7%
780
53%
780
13%
2032
December16th to endNight
38%
832
10%
2912
8%
832
42%
832
12%
2215

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
8%5%25%5%3445%6%48%
49283
Night
10%7%25%5%24446%45%
48941
Cloud CoverDay
8%5%25%42445%7%48%
49284
Night
10%6%25%42445%6%45%
48944
TransparencyDay
36%7%13%19%15%7%
48955
Night
39%6%13%20%13%6%
48772
SeeingDay
11%8%25%32%18%3
5769
Night
14%315%37%25%4
15306

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 15thDay
14%10%410%8%36%45%6%24%
1552
16th to endDay
15%11%47%9%5%5%5%5%7%23%
1664
February1st to 15thDay
12%9%5%8%5%25%45%7%32%
1677
16th to endDay
10%8%47%7%35%6%5%7%31%
1579
March1st to 15thDay
14%10%46%6%46%6%7%6%26%
1800
16th to endDay
13%10%36%6%35%6%7%6%29%
2192
April1st to 15thDay
8%7%38%7%45%5%6%8%35%
2149
16th to endDay
8%6%245%35%45%7%47%
2306
May1st to 15thDay
6%325%42445%8%52%
2370
16th to endDay
7%6%26%6%35%5%6%8%41%
2615
June1st to 15thDay
8%5%345%36%5%6%8%41%
2520
16th to endDay
4213323435%64%
2520
July1st to 15thDay
22221235%6%69%
2520
16th to endDay
11212245%76%
2657
August1st to 15thDay
212323345%70%
2431
16th to endDay
111212236%74%
2357
September1st to 15thDay
121312236%73%
2124
16th to endDay
7%313423445%60%
1980
October1st to 15thDay
7%41442445%6%53%
1917
16th to endDay
9%426%6%35%5%47%43%
1728
November1st to 15thDay
14%8%25%5%2445%8%38%
1719
16th to endDay
14%7%26%5%36%5%46%36%
1682
December1st to 15thDay
15%11%45%6%45%5%7%6%27%
1560
16th to endDay
19%11%47%5%3445%5%27%
1664
January1st to 15thNight
15%12%5%8%7%35%6%45%23%
2716
16th to endNight
14%10%38%6%4445%7%29%
2912
February1st to 15thNight
12%12%46%5%25%45%7%33%
2436
16th to endNight
12%10%46%5%35%5%5%6%32%
1895
March1st to 15thNight
13%9%47%6%35%5%5%7%30%
2160
16th to endNight
13%8%36%5%36%5%5%8%34%
2025
April1st to 15thNight
10%6%26%6%345%5%7%41%
1799
16th to endNight
8%5%34435%347%48%
1653
May1st to 15thNight
4313423447%59%
1578
16th to endNight
5%435%5%245%8%9%45%
1496
June1st to 15thNight
8%424436%45%7%48%
1440
16th to endNight
4212213447%63%
1440
July1st to 15thNight
1113348%73%
1440
16th to endNight
111112225%78%
1512
August1st to 15thNight
1212212348%68%
1527
16th to endNight
112212335%75%
1843
September1st to 15thNight
2212211246%72%
1834
16th to endNight
7%322323335%61%
1980
October1st to 15thNight
6%42432435%7%54%
2040
16th to endNight
9%5%245%3345%8%46%
2496
November1st to 15thNight
16%7%2442445%7%39%
2470
16th to endNight
14%9%35%435%445%37%
2607
December1st to 15thNight
13%9%36%6%36%6%5%6%31%
2730
16th to endNight
18%13%37%6%35%44428%
2912

Percentage Cloud Cover SW (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
15%10%49%8%45%6%47%24%
1552
16th to endDay
16%10%48%8%46%445%24%
1664
February1st to 15thDay
13%9%37%5%35%448%33%
1677
16th to endDay
11%8%47%6%36%6%5%7%32%
1579
March1st to 15thDay
14%9%46%6%46%6%6%7%27%
1800
16th to endDay
13%9%36%6%45%5%6%9%29%
2193
April1st to 15thDay
8%6%47%7%35%46%8%36%
2149
16th to endDay
8%5%343345%5%7%47%
2306
May1st to 15thDay
4425%42446%8%52%
2370
16th to endDay
7%6%26%6%345%5%8%42%
2615
June1st to 15thDay
8%5%35%5%35%5%7%8%42%
2520
16th to endDay
4213214345%65%
2520
July1st to 15thDay
222212338%69%
2520
16th to endDay
11212335%78%
2657
August1st to 15thDay
212222346%70%
2431
16th to endDay
111212236%75%
2357
September1st to 15thDay
112222235%74%
2124
16th to endDay
6%313423346%60%
1980
October1st to 15thDay
7%414424446%54%
1917
16th to endDay
9%5%16%5%35%5%46%44%
1728
November1st to 15thDay
15%7%25%43445%8%38%
1719
16th to endDay
15%7%35%5%36%5%45%36%
1682
December1st to 15thDay
16%9%48%5%345%6%7%27%
1560
16th to endDay
21%10%37%5%3445%5%28%
1664
January1st to 15thNight
16%13%47%7%5%6%6%45%23%
2716
16th to endNight
14%12%38%6%35%45%6%29%
2912
February1st to 15thNight
14%12%35%5%245%5%7%32%
2436
16th to endNight
13%10%37%5%35%5%5%7%31%
1895
March1st to 15thNight
12%10%47%6%35%5%5%6%31%
2160
16th to endNight
14%7%36%435%6%5%8%33%
2028
April1st to 15thNight
9%6%35%6%345%6%6%43%
1799
16th to endNight
8%435%325%45%6%49%
1653
May1st to 15thNight
421342345%7%59%
1578
16th to endNight
4426%4245%8%9%46%
1496
June1st to 15thNight
8%323435%45%8%48%
1440
16th to endNight
4212313446%64%
1440
July1st to 15thNight
111236%6%74%
1440
16th to endNight
11111223479%
1512
August1st to 15thNight
1112312337%70%
1527
16th to endNight
112112246%75%
1843
September1st to 15thNight
212212335%73%
1834
16th to endNight
7%213223335%62%
1980
October1st to 15thNight
6%32432345%7%55%
2040
16th to endNight
9%5%25%5%34448%47%
2496
November1st to 15thNight
16%7%25%42445%7%39%
2470
16th to endNight
16%9%245%35%446%37%
2607
December1st to 15thNight
14%8%38%6%36%45%6%31%
2730
16th to endNight
19%12%48%6%2444428%
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
61%9%16%11%1
1552
16th to endDay
62%9%15%10%2
1664
February1st to 15thDay
52%12%19%13%3
1677
16th to endDay
51%11%18%15%2
1579
March1st to 15thDay
56%9%15%13%5%
1800
16th to endDay
51%10%15%17%4
2193
April1st to 15thDay
45%10%17%20%5%
2149
16th to endDay
36%7%15%26%12%1
2306
May1st to 15thDay
29%6%13%32%15%2
2370
16th to endDay
39%8%14%22%12%1
2619
June1st to 15thDay
38%9%12%20%16%3
2520
16th to endDay
21%5%8%22%28%12%
2520
July1st to 15thDay
15%47%15%29%27%
2371
16th to endDay
9%25%12%29%40%
2490
August1st to 15thDay
15%37%17%32%24%
2414
16th to endDay
13%46%20%33%21%
2357
September1st to 15thDay
14%36%22%34%18%
2124
16th to endDay
26%5%9%30%22%6%
1980
October1st to 15thDay
31%6%14%25%18%2
1917
16th to endDay
40%7%16%21%13%
1728
November1st to 15thDay
45%9%18%17%7%
1719
16th to endDay
50%6%19%16%5%
1682
December1st to 15thDay
55%11%18%12%2
1560
16th to endDay
58%9%14%13%3
1664
January1st to 15thNight
61%8%14%12%2
2716
16th to endNight
56%8%16%15%3
2912
February1st to 15thNight
51%10%18%15%4
2447
16th to endNight
53%7%16%16%41
1895
March1st to 15thNight
54%7%13%18%5%1
2160
16th to endNight
48%8%17%20%4
2028
April1st to 15thNight
41%7%14%26%9%
1799
16th to endNight
34%7%12%26%17%1
1653
May1st to 15thNight
24%6%13%33%20%1
1579
16th to endNight
32%9%13%30%11%2
1496
June1st to 15thNight
34%7%15%23%14%3
1440
16th to endNight
20%5%9%25%27%11%
1440
July1st to 15thNight
10%38%20%29%26%
1360
16th to endNight
9%2312%30%39%
1416
August1st to 15thNight
14%46%18%30%25%
1519
16th to endNight
12%36%19%32%25%
1843
September1st to 15thNight
14%45%20%33%21%
1834
16th to endNight
25%5%6%26%23%12%
1980
October1st to 15thNight
28%7%12%27%20%3
2040
16th to endNight
36%6%15%22%18%
2496
November1st to 15thNight
44%7%16%22%9%
2470
16th to endNight
49%7%16%17%8%1
2607
December1st to 15thNight
53%9%15%15%5%
2730
16th to endNight
60%6%14%14%4
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
23%111%27%22%14%
90
16th to endDay
30%320%31%10%4
96
February1st to 15thDay
22%423%32%15%1
198
16th to endDay
16%15%30%26%9%1
238
March1st to 15thDay
25%10%30%25%7%
270
16th to endDay
20%15%32%22%7%
288
April1st to 15thDay
10%15%31%23%12%6%
269
16th to endDay
12%17%31%27%9%1
270
May1st to 15thDay
8%15%31%30%12%1
288
16th to endDay
12%8%22%31%20%5%
330
June1st to 15thDay
8%13%29%28%16%3
315
16th to endDay
6%9%28%32%20%3
315
July1st to 15thDay
5%225%39%24%2
305
16th to endDay
1329%41%20%3
327
August1st to 15thDay
2323%41%26%3
315
16th to endDay
2418%36%31%7%
332
September1st to 15thDay
4421%34%27%7%
315
16th to endDay
8%420%40%24%2
315
October1st to 15thDay
10%8%26%33%18%2
310
16th to endDay
20%421%29%19%5%
144
November1st to 15thDay
22%319%34%20%
135
16th to endDay
27%330%25%11%1
118
December1st to 15thDay
27%5%20%26%10%10%
90
16th to endDay
22%325%29%16%3
96
January1st to 15thNight
25%312%28%25%4
780
16th to endNight
21%212%33%23%7%
832
February1st to 15thNight
23%216%34%19%3
752
16th to endNight
19%318%33%19%4
632
March1st to 15thNight
19%5%18%36%16%4
720
16th to endNight
17%6%20%38%14%1
768
April1st to 15thNight
11%6%19%36%22%4
719
16th to endNight
12%5%21%34%22%3
573
May1st to 15thNight
5%425%39%23%2
499
16th to endNight
6%214%36%35%4
375
June1st to 15thNight
10%217%39%28%2
360
16th to endNight
5%212%40%36%1
360
July1st to 15thNight
17%52%33%2
340
16th to endNight
28%52%34%2
376
August1st to 15thNight
228%40%43%4
447
16th to endNight
2110%41%38%5%
697
September1st to 15thNight
2110%35%42%7%
720
16th to endNight
10%110%33%34%9%
716
October1st to 15thNight
9%215%42%23%6%
720
16th to endNight
12%214%38%24%6%
768
November1st to 15thNight
21%214%35%23%2
760
16th to endNight
24%213%33%19%5%
780
December1st to 15thNight
20%213%38%19%4
780
16th to endNight
27%317%34%14%2
832
Page generated at 2018-02-13 18:46:07. Version ac04

Copyright 2018 A. Danko


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