Cornwall
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cornwall clear sky chart from 2005-11-05 to 2018-03-21.

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
26%
15416
12%
49241
7%
15385
32%
15385
14%
36448

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
12%
780
6%
2716
2%
776
16%
776
6%
2262
January16th to endNight
16%
832
8%
2892
4%
832
21%
832
8%
2292
February1st to 15thNight
11%
764
7%
2482
2%
761
18%
761
7%
2055
February16th to endNight
18%
683
10%
2071
4%
683
26%
683
11%
1619
March1st to 15thNight
17%
780
11%
2316
6%
780
23%
780
11%
1827
March16th to endNight
27%
792
19%
2073
9%
792
34%
792
20%
1499
April1st to 15thNight
23%
718
15%
1796
5%
718
28%
718
18%
1367
April16th to endNight
26%
660
16%
1740
5%
660
33%
660
20%
1250
May1st to 15thNight
29%
366
16%
1446
13%
366
40%
366
14%
988
May16th to endNight
31%
374
12%
1497
9%
374
41%
374
15%
975
June1st to 15thNight
31%
360
9%
1440
8%
360
38%
360
12%
1017
June16th to endNight
37%
360
9%
1440
11%
360
43%
360
19%
899
July1st to 15thNight
45%
340
13%
1360
17%
340
54%
340
26%
770
July16th to endNight
47%
376
13%
1418
13%
354
52%
354
21%
858
August1st to 15thNight
45%
407
14%
1478
12%
405
50%
405
21%
952
August16th to endNight
46%
747
19%
1893
9%
747
51%
747
33%
1234
September1st to 15thNight
44%
720
15%
1800
8%
720
46%
720
28%
1255
September16th to endNight
46%
717
22%
1947
16%
717
49%
717
29%
1268
October1st to 15thNight
31%
720
16%
2155
10%
720
38%
720
19%
1517
October16th to endNight
21%
768
9%
2427
5%
768
27%
768
11%
1886
November1st to 15thNight
25%
760
14%
2482
10%
760
32%
760
14%
1821
November16th to endNight
20%
780
10%
2730
7%
780
25%
780
9%
2119
December1st to 15thNight
13%
780
6%
2730
1%
780
18%
780
5%
2277
December16th to endNight
10%
832
7%
2912
3%
832
15%
832
5%
2441

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%36%5%34446%31%
49623
Night
19%10%47%5%34446%30%
49410
Cloud CoverDay
19%9%36%5%34446%31%
49632
Night
19%10%47%5%34445%30%
49416
TransparencyDay
53%6%11%18%9%1
49310
Night
55%6%11%17%8%
49241
SeeingDay
25%14%28%19%9%2
6095
Night
26%13%28%22%8%
15417

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
21%16%6%9%7%35%42417%
1552
16th to endDay
15%16%5%10%6%345%46%20%
1684
February1st to 15thDay
16%14%6%11%7%46%5%5%419%
1710
16th to endDay
19%15%5%7%7%3433424%
1688
March1st to 15thDay
20%14%49%6%25%33325%
1963
16th to endDay
19%8%36%42333439%
2279
April1st to 15thDay
23%10%46%5%23335%32%
2152
16th to endDay
20%9%34433345%36%
2223
May1st to 15thDay
20%6%26%434446%35%
2511
16th to endDay
17%6%35%5%3445%7%36%
2618
June1st to 15thDay
23%6%36%5%23447%30%
2520
16th to endDay
20%6%245%45%5%7%8%31%
2520
July1st to 15thDay
13%5%345%345%5%8%38%
2520
16th to endDay
12%5%35%5%36%6%7%8%36%
2654
August1st to 15thDay
13%6%26%5%36%6%7%6%35%
2471
16th to endDay
11%4145%345%7%7%43%
2297
September1st to 15thDay
18%5%24423446%42%
2160
16th to endDay
15%5%13423446%47%
2013
October1st to 15thDay
21%7%246%24446%33%
1803
16th to endDay
26%10%37%5%2442425%
1795
November1st to 15thDay
17%9%46%5%35%45%7%30%
1706
16th to endDay
23%12%5%7%5%35%345%24%
1560
December1st to 15thDay
23%17%5%9%6%3444416%
1560
16th to endDay
26%17%5%8%6%3424418%
1664
January1st to 15thNight
20%18%6%11%6%3433417%
2716
16th to endNight
14%17%6%8%8%45%345%22%
2892
February1st to 15thNight
14%15%7%14%7%35%33420%
2480
16th to endNight
18%13%5%8%7%35%44326%
2071
March1st to 15thNight
21%14%47%6%3422426%
2303
16th to endNight
19%11%35%42333438%
2073
April1st to 15thNight
25%9%36%423335%32%
1796
16th to endNight
20%8%25%433436%37%
1740
May1st to 15thNight
21%7%35%5%34445%34%
1446
16th to endNight
19%5%25%5%335%5%8%35%
1495
June1st to 15thNight
23%5%26%5%345%5%8%29%
1440
16th to endNight
15%6%245%245%6%10%35%
1440
July1st to 15thNight
12%213435%5%7%10%42%
1440
16th to endNight
10%424336%5%6%9%41%
1514
August1st to 15thNight
12%6%35%4245%5%9%39%
1488
16th to endNight
13%3245%3335%7%47%
1893
September1st to 15thNight
16%5%35%43345%6%41%
1800
16th to endNight
16%323423446%47%
1947
October1st to 15thNight
19%7%35%434446%36%
2155
16th to endNight
26%9%36%434346%25%
2427
November1st to 15thNight
21%9%36%424436%32%
2482
16th to endNight
22%12%47%5%34445%24%
2730
December1st to 15thNight
23%16%5%8%6%4443417%
2730
16th to endNight
25%17%5%8%6%3333417%
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
22%15%6%10%7%45%33317%
1552
16th to endDay
15%17%5%10%8%35%5%45%19%
1684
February1st to 15thDay
16%13%6%11%8%46%45%418%
1710
16th to endDay
19%16%5%7%7%34335%23%
1688
March1st to 15thDay
20%16%5%8%5%3433324%
1973
16th to endDay
20%9%35%323335%38%
2278
April1st to 15thDay
23%11%36%5%3324431%
2152
16th to endDay
21%8%35%43435%6%35%
2223
May1st to 15thDay
20%6%25%5%3445%6%34%
2511
16th to endDay
18%6%25%435%448%35%
2618
June1st to 15thDay
23%6%36%5%3445%7%30%
2520
16th to endDay
20%435%5%35%5%6%8%31%
2520
July1st to 15thDay
13%5%35%5%445%5%8%39%
2520
16th to endDay
12%4246%36%6%7%8%35%
2654
August1st to 15thDay
13%6%25%6%46%5%6%7%34%
2471
16th to endDay
11%4245%35%5%5%9%42%
2297
September1st to 15thDay
18%5%145%24446%42%
2160
16th to endDay
15%42432345%6%46%
2013
October1st to 15thDay
21%7%25%5%25%446%33%
1803
16th to endDay
27%11%36%5%235%3425%
1795
November1st to 15thDay
17%10%46%444447%30%
1706
16th to endDay
22%14%48%5%2443424%
1560
December1st to 15thDay
24%17%6%8%6%3434415%
1560
16th to endDay
26%18%5%8%5%3424416%
1664
January1st to 15thNight
19%17%7%11%6%24335%16%
2716
16th to endNight
15%17%6%8%7%4434421%
2892
February1st to 15thNight
15%15%7%14%8%3434318%
2473
16th to endNight
18%13%5%9%7%35%34425%
2071
March1st to 15thNight
22%15%47%6%3322426%
2316
16th to endNight
19%10%45%42323437%
2073
April1st to 15thNight
25%9%36%424335%31%
1796
16th to endNight
20%7%35%5%33345%37%
1740
May1st to 15thNight
22%6%25%43445%5%34%
1446
16th to endNight
19%5%25%5%3445%7%35%
1495
June1st to 15thNight
23%6%25%5%4446%8%28%
1440
16th to endNight
16%5%24425%46%10%36%
1440
July1st to 15thNight
12%323425%5%7%10%43%
1440
16th to endNight
10%5%334346%7%9%41%
1514
August1st to 15thNight
12%6%36%4345%6%9%39%
1488
16th to endNight
14%3245%3345%7%46%
1893
September1st to 15thNight
16%5%25%5%33346%42%
1800
16th to endNight
17%41342445%5%46%
1947
October1st to 15thNight
20%7%35%42435%6%35%
2155
16th to endNight
27%9%36%5%34346%25%
2427
November1st to 15thNight
21%10%35%6%24445%31%
2482
16th to endNight
23%12%47%6%33445%24%
2730
December1st to 15thNight
22%17%6%8%5%3443417%
2730
16th to endNight
25%16%5%9%5%3343417%
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
72%48%9%4
1553
16th to endDay
64%6%11%11%4
1684
February1st to 15thDay
68%6%7%14%2
1712
16th to endDay
63%5%10%11%8%
1688
March1st to 15thDay
64%410%11%7%1
1973
16th to endDay
49%49%16%15%4
2279
April1st to 15thDay
56%38%18%11%2
2152
16th to endDay
49%5%8%17%14%4
2223
May1st to 15thDay
49%6%9%18%14%1
2511
16th to endDay
46%7%12%18%12%1
2622
June1st to 15thDay
53%6%11%19%9%
2520
16th to endDay
48%8%12%20%9%
2520
July1st to 15thDay
40%7%12%28%10%
2371
16th to endDay
42%11%16%22%8%
2487
August1st to 15thDay
45%10%15%20%8%
2457
16th to endDay
37%8%17%25%10%
2297
September1st to 15thDay
42%6%11%26%12%
2160
16th to endDay
38%6%11%31%10%
2013
October1st to 15thDay
51%6%12%20%9%
1803
16th to endDay
62%6%11%13%5%
1795
November1st to 15thDay
53%7%12%17%7%
1706
16th to endDay
63%5%10%13%6%
1560
December1st to 15thDay
71%6%9%9%2
1560
16th to endDay
72%5%9%8%4
1664
January1st to 15thNight
70%5%10%8%4
2716
16th to endNight
65%5%9%13%5%
2892
February1st to 15thNight
70%5%8%12%3
2482
16th to endNight
62%5%10%12%8%
2071
March1st to 15thNight
63%48%15%6%
2316
16th to endNight
51%39%17%13%4
2073
April1st to 15thNight
55%48%18%12%
1796
16th to endNight
48%5%8%17%16%3
1740
May1st to 15thNight
50%6%7%15%16%3
1446
16th to endNight
46%7%12%20%12%1
1497
June1st to 15thNight
52%7%12%16%9%
1440
16th to endNight
41%8%14%22%12%
1440
July1st to 15thNight
32%8%15%28%14%
1360
16th to endNight
37%9%14%23%14%
1418
August1st to 15thNight
40%9%17%20%11%
1478
16th to endNight
36%6%14%29%12%
1893
September1st to 15thNight
42%7%10%26%12%
1800
16th to endNight
37%7%10%31%12%
1947
October1st to 15thNight
48%6%11%23%9%
2155
16th to endNight
60%6%13%14%4
2427
November1st to 15thNight
54%7%12%15%8%2
2482
16th to endNight
62%6%10%14%5%
2730
December1st to 15thNight
70%6%9%10%2
2730
16th to endNight
71%6%9%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
32%15%28%16%6%
90
16th to endDay
33%16%32%15%
101
February1st to 15thDay
42%24%28%2
135
16th to endDay
35%18%28%15%2
269
March1st to 15thDay
35%19%27%11%5%
330
16th to endDay
27%20%26%14%8%1
311
April1st to 15thDay
34%16%30%11%6%
270
16th to endDay
28%14%27%16%11%2
270
May1st to 15thDay
31%14%24%16%10%3
288
16th to endDay
22%17%32%16%10%
331
June1st to 15thDay
29%13%29%19%7%1
315
16th to endDay
21%12%33%22%8%1
315
July1st to 15thDay
16%7%31%27%13%3
305
16th to endDay
15%6%31%33%10%2
326
August1st to 15thDay
17%7%29%28%12%5%
313
16th to endDay
12%7%28%27%13%9%
332
September1st to 15thDay
19%7%24%23%14%10%
315
16th to endDay
14%10%25%22%19%7%
314
October1st to 15thDay
22%15%26%17%14%3
315
16th to endDay
36%17%23%18%3
304
November1st to 15thDay
26%15%30%15%11%1
270
16th to endDay
36%21%22%16%3
90
December1st to 15thDay
33%23%23%15%31
90
16th to endDay
50%25%15%8%1
96
January1st to 15thNight
31%19%30%15%3
780
16th to endNight
26%20%29%19%4
832
February1st to 15thNight
26%20%33%18%1
764
16th to endNight
26%17%31%21%2
683
March1st to 15thNight
33%16%28%17%3
780
16th to endNight
25%15%26%24%7%
792
April1st to 15thNight
30%13%27%23%4
718
16th to endNight
27%13%29%21%7%
660
May1st to 15thNight
26%9%29%24%8%1
366
16th to endNight
20%10%32%27%9%
375
June1st to 15thNight
25%8%27%31%7%
360
16th to endNight
18%5%33%29%11%1
360
July1st to 15thNight
10%5%31%37%14%
340
16th to endNight
12%5%31%37%12%1
376
August1st to 15thNight
15%325%35%17%1
407
16th to endNight
15%5%26%32%18%1
747
September1st to 15thNight
20%5%25%32%13%2
720
16th to endNight
18%9%23%24%22%2
717
October1st to 15thNight
24%11%27%23%12%
720
16th to endNight
34%14%25%20%5%
768
November1st to 15thNight
27%15%26%20%9%
760
16th to endNight
30%18%22%18%8%
780
December1st to 15thNight
34%20%26%16%2
780
16th to endNight
37%14%30%13%3
832
Page generated at 2018-03-22 11:34:23. Version ac04

Copyright 2018 A. Danko


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