Truro
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Truro clear sky chart from 2005-11-05 to 2018-04-23.

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
20%
16054
10%
49368
4%
16009
26%
16009
12%
37504

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
11%
825
7%
2716
1%
821
19%
821
7%
2212
January16th to endNight
14%
847
10%
2769
2%
847
22%
847
9%
2138
February1st to 15thNight
14%
764
8%
2481
2%
761
21%
761
9%
1946
February16th to endNight
14%
683
8%
2187
2%
683
20%
683
8%
1756
March1st to 15thNight
17%
780
12%
2180
4%
780
24%
780
11%
1742
March16th to endNight
18%
816
11%
2272
3%
816
24%
816
13%
1774
April1st to 15thNight
16%
584
12%
1946
6%
584
23%
584
10%
1463
April16th to endNight
19%
557
13%
1705
6%
557
26%
557
14%
1258
May1st to 15thNight
20%
540
9%
1488
4%
540
23%
540
12%
1182
May16th to endNight
16%
561
9%
1496
2%
560
21%
560
16%
1149
June1st to 15thNight
15%
540
6%
1440
2%
540
18%
540
13%
1155
June16th to endNight
25%
392
7%
1292
3%
392
30%
392
14%
943
July1st to 15thNight
33%
448
7%
1298
3%
448
37%
448
24%
889
July16th to endNight
32%
565
9%
1425
6%
532
33%
532
18%
1085
August1st to 15thNight
34%
539
10%
1608
7%
535
37%
535
19%
1174
August16th to endNight
36%
573
14%
1797
9%
573
41%
573
21%
1224
September1st to 15thNight
29%
540
15%
1800
9%
540
37%
540
21%
1165
September16th to endNight
29%
672
20%
1992
10%
672
37%
672
22%
1321
October1st to 15thNight
26%
720
16%
2160
7%
720
33%
720
20%
1488
October16th to endNight
18%
773
13%
2323
5%
773
25%
773
12%
1738
November1st to 15thNight
21%
805
10%
2621
5%
805
26%
805
12%
1995
November16th to endNight
19%
825
9%
2730
4%
825
25%
825
9%
2133
December1st to 15thNight
10%
825
8%
2730
1%
825
19%
825
7%
2188
December16th to endNight
9%
880
8%
2912
1%
880
17%
880
6%
2386

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
22%11%46%5%24446%27%
50144
Night
21%10%47%5%24446%28%
49540
TransparencyDay
59%6%11%15%7%
49835
Night
57%6%12%16%6%
49368
SeeingDay
31%17%28%16%5%
8297
Night
28%15%31%19%4
16054

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
17%17%49%6%45%45%6%18%
1552
16th to endDay
18%14%5%8%7%24436%23%
1804
February1st to 15thDay
18%14%5%11%8%3344422%
1710
16th to endDay
14%15%7%9%5%34346%25%
1571
March1st to 15thDay
18%15%5%9%7%3433423%
2098
16th to endDay
17%14%8%8%6%2233426%
2304
April1st to 15thDay
25%14%45%5%23345%24%
2333
16th to endDay
24%11%45%5%23345%28%
2369
May1st to 15thDay
28%10%36%5%34345%24%
2464
16th to endDay
27%10%46%5%24345%26%
2618
June1st to 15thDay
30%9%36%43435%6%22%
2520
16th to endDay
28%9%35%5%24446%26%
2673
July1st to 15thDay
20%6%244245%5%8%34%
2565
16th to endDay
22%9%36%5%245%5%6%26%
2646
August1st to 15thDay
21%7%26%5%345%6%7%28%
2340
16th to endDay
17%6%144235%5%8%39%
2404
September1st to 15thDay
22%6%35%423445%37%
2160
16th to endDay
20%7%2443335%6%39%
1965
October1st to 15thDay
20%8%35%43335%6%34%
1800
16th to endDay
24%10%36%6%33446%26%
1900
November1st to 15thDay
24%11%47%5%34325%28%
1564
16th to endDay
20%15%5%8%5%24445%23%
1560
December1st to 15thDay
27%15%5%8%5%2323420%
1560
16th to endDay
23%16%6%9%5%2333420%
1664
January1st to 15thNight
19%13%6%8%7%45%45%5%20%
2716
16th to endNight
19%12%48%7%34445%25%
2769
February1st to 15thNight
18%13%49%7%34346%23%
2480
16th to endNight
15%14%6%11%7%2444424%
2187
March1st to 15thNight
19%12%5%8%6%34435%24%
2168
16th to endNight
19%12%5%8%5%23345%28%
2272
April1st to 15thNight
25%12%36%5%2333428%
1946
16th to endNight
24%9%35%325%445%30%
1705
May1st to 15thNight
28%10%35%5%35%33426%
1488
16th to endNight
23%9%35%42445%8%28%
1494
June1st to 15thNight
27%9%36%6%23346%25%
1440
16th to endNight
23%8%35%42346%7%29%
1292
July1st to 15thNight
16%6%25%4245%7%9%35%
1377
16th to endNight
19%6%26%6%45%5%49%29%
1521
August1st to 15thNight
20%8%25%4245%5%7%33%
1620
16th to endNight
17%5%244345%5%7%38%
1797
September1st to 15thNight
17%6%25%324448%40%
1800
16th to endNight
19%5%34423346%41%
1992
October1st to 15thNight
20%7%24424447%36%
2160
16th to endNight
26%9%25%5%23346%29%
2323
November1st to 15thNight
23%10%36%424446%28%
2621
16th to endNight
23%12%5%6%5%33446%25%
2730
December1st to 15thNight
25%13%48%5%24335%23%
2730
16th to endNight
25%15%5%7%5%34345%19%
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
65%7%11%11%4
1553
16th to endDay
63%5%10%11%8%
1804
February1st to 15thDay
65%5%9%11%6%
1713
16th to endDay
63%410%13%7%
1571
March1st to 15thDay
66%5%8%13%6%
2109
16th to endDay
63%38%14%7%1
2304
April1st to 15thDay
62%5%7%13%9%1
2333
16th to endDay
58%5%9%15%10%
2369
May1st to 15thDay
63%5%9%14%6%
2464
16th to endDay
60%6%11%15%6%
2622
June1st to 15thDay
63%6%11%12%5%
2520
16th to endDay
60%6%10%15%6%
2673
July1st to 15thDay
44%10%14%20%8%
2417
16th to endDay
58%7%12%15%5%
2479
August1st to 15thDay
54%8%13%16%7%
2327
16th to endDay
42%7%17%24%7%
2404
September1st to 15thDay
49%5%13%21%9%
2160
16th to endDay
46%5%11%24%11%
1965
October1st to 15thDay
50%5%12%22%9%
1800
16th to endDay
60%5%12%16%4
1900
November1st to 15thDay
61%5%11%14%7%
1564
16th to endDay
64%7%13%10%4
1560
December1st to 15thDay
68%5%10%13%2
1560
16th to endDay
69%5%10%8%5%
1664
January1st to 15thNight
66%6%11%10%3
2716
16th to endNight
61%5%12%15%5%
2769
February1st to 15thNight
63%5%11%13%6%
2481
16th to endNight
65%5%10%12%5%
2187
March1st to 15thNight
62%5%9%14%7%
2180
16th to endNight
59%5%9%15%7%1
2272
April1st to 15thNight
60%47%15%9%1
1946
16th to endNight
55%5%9%15%11%1
1705
May1st to 15thNight
61%6%8%17%6%
1488
16th to endNight
55%6%13%17%5%
1496
June1st to 15thNight
61%6%10%15%5%
1440
16th to endNight
53%8%13%17%6%
1292
July1st to 15thNight
42%9%19%18%9%
1298
16th to endNight
52%9%15%15%6%
1425
August1st to 15thNight
50%8%13%21%5%
1608
16th to endNight
44%9%16%23%7%
1797
September1st to 15thNight
43%5%12%27%9%
1800
16th to endNight
44%5%13%25%10%
1992
October1st to 15thNight
48%5%13%23%9%
2160
16th to endNight
57%411%18%6%
2323
November1st to 15thNight
57%8%13%14%6%
2621
16th to endNight
61%7%13%12%5%
2730
December1st to 15thNight
64%6%11%12%3
2730
16th to endNight
68%5%9%11%5%
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
31%20%30%16%
285
16th to endDay
30%22%33%11%2
304
February1st to 15thDay
30%20%30%16%1
281
16th to endDay
33%18%30%12%4
251
March1st to 15thDay
29%25%29%11%3
322
16th to endDay
31%27%25%10%4
352
April1st to 15thDay
38%25%24%9%2
330
16th to endDay
25%22%32%13%4
289
May1st to 15thDay
38%15%24%14%5%2
420
16th to endDay
33%15%31%14%4
519
June1st to 15thDay
40%13%27%15%4
495
16th to endDay
35%13%25%18%42
495
July1st to 15thDay
25%9%32%23%8%
475
16th to endDay
34%8%26%21%7%
507
August1st to 15thDay
26%9%31%21%8%1
313
16th to endDay
20%10%33%21%11%1
332
September1st to 15thDay
26%14%30%21%6%
315
16th to endDay
24%14%28%22%9%1
300
October1st to 15thDay
24%13%32%15%11%2
270
16th to endDay
30%25%22%11%9%
288
November1st to 15thDay
29%18%24%19%6%2
280
16th to endDay
30%21%28%13%5%
285
December1st to 15thDay
40%20%27%11%1
285
16th to endDay
34%26%25%11%
304
January1st to 15thNight
28%21%32%15%1
825
16th to endNight
29%20%32%14%3
847
February1st to 15thNight
28%19%32%17%1
764
16th to endNight
27%24%28%14%4
683
March1st to 15thNight
26%21%29%18%3
780
16th to endNight
29%17%28%19%4
816
April1st to 15thNight
33%16%29%17%3
584
16th to endNight
30%13%33%18%2
557
May1st to 15thNight
33%13%29%16%5%
540
16th to endNight
27%13%40%15%3
561
June1st to 15thNight
34%13%33%16%2
540
16th to endNight
26%8%34%23%5%
392
July1st to 15thNight
19%439%29%6%
448
16th to endNight
22%7%32%29%7%
565
August1st to 15thNight
24%5%30%30%9%
539
16th to endNight
21%5%32%31%7%
573
September1st to 15thNight
22%8%35%25%7%
540
16th to endNight
24%10%31%24%7%1
672
October1st to 15thNight
26%11%30%21%8%
720
16th to endNight
32%17%28%15%4
773
November1st to 15thNight
30%13%30%19%6%1
805
16th to endNight
30%19%24%20%4
825
December1st to 15thNight
36%19%29%13%2
825
16th to endNight
33%22%27%13%1
880
Page generated at 2018-04-24 11:03:06. Version ac04

Copyright 2018 A. Danko


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