Grafton
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grafton clear sky chart from 2005-11-06 to 2024-01-15.

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%
22830
16%
74694
9%
22785
40%
22785
18%
50180

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
20%
760
17%
2649
5%
757
35%
757
13%
1767
January11th to 20thNight
19%
737
13%
2577
5%
737
31%
737
11%
1865
January21st to endNight
20%
792
17%
2772
7%
792
34%
792
15%
1817
February1st to 10thNight
15%
720
13%
2332
3%
720
27%
720
12%
1646
February11th to 20thNight
16%
703
17%
2100
3%
700
33%
700
15%
1442
February21st to endNight
17%
592
20%
1776
5%
592
35%
592
14%
1226
March1st to 10thNight
20%
720
20%
2160
9%
720
34%
720
17%
1463
March11th to 20thNight
21%
720
16%
2114
6%
720
32%
720
15%
1466
March21st to endNight
25%
792
21%
1980
6%
792
35%
792
20%
1378
April1st to 10thNight
23%
720
18%
1800
4%
720
31%
720
21%
1266
April11th to 20thNight
27%
711
16%
1775
5%
711
33%
711
21%
1247
April21st to endNight
23%
572
17%
1652
5%
572
31%
572
17%
1159
May1st to 10thNight
25%
540
14%
1620
8%
540
30%
540
15%
1172
May11th to 20thNight
30%
387
17%
1449
13%
387
40%
387
17%
957
May21st to endNight
41%
389
13%
1551
14%
388
49%
388
17%
975
June1st to 10thNight
39%
360
14%
1440
13%
360
49%
360
17%
914
June11th to 20thNight
37%
360
12%
1440
10%
360
46%
360
17%
915
June21st to endNight
46%
360
12%
1440
15%
360
53%
360
19%
919
July1st to 10thNight
51%
350
14%
1400
17%
350
59%
350
23%
850
July11th to 20thNight
56%
348
14%
1392
18%
348
59%
348
23%
822
July21st to endNight
55%
394
18%
1486
20%
372
62%
372
26%
856
August1st to 10thNight
59%
391
18%
1455
16%
389
63%
389
26%
846
August11th to 20thNight
49%
618
15%
1682
10%
618
50%
618
25%
1147
August21st to endNight
52%
792
21%
1980
10%
792
55%
792
34%
1291
September1st to 10thNight
49%
720
20%
1800
9%
720
54%
720
34%
1164
September11th to 20thNight
50%
720
25%
1927
16%
720
55%
720
33%
1199
September21st to endNight
43%
720
24%
1980
15%
720
52%
720
29%
1267
October1st to 10thNight
39%
720
16%
2007
10%
720
45%
720
20%
1399
October11th to 20thNight
36%
720
18%
2213
10%
720
46%
720
21%
1479
October21st to endNight
24%
792
13%
2574
7%
792
34%
792
13%
1859
November1st to 10thNight
31%
740
20%
2407
11%
740
44%
740
20%
1544
November11th to 20thNight
29%
760
22%
2470
11%
760
42%
760
18%
1606
November21st to endNight
26%
760
17%
2560
8%
760
38%
760
16%
1682
December1st to 10thNight
19%
760
15%
2660
6%
760
32%
760
12%
1890
December11th to 20thNight
21%
760
14%
3229
5%
759
32%
759
10%
2231
December21st to endNight
16%
830
10%
4845
3%
817
30%
817
7%
3454

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%7%36%6%35%5%5%7%36%
74797
Night
15%7%36%5%35%5%6%7%39%
74963
TransparencyDay
46%7%12%20%13%2
74502
Night
45%8%12%22%13%2
74792
SeeingDay
23%18%29%19%9%3
13747
Night
21%14%28%27%9%1
22836

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
12%9%36%7%46%6%7%7%34%
1517
11th to 20thDay
14%9%48%6%36%47%7%32%
1472
21st to endDay
10%8%47%8%4445%9%36%
1584
February1st to 10thDay
13%11%49%7%46%5%6%8%28%
1625
11th to 20thDay
9%8%48%7%5%6%6%6%8%33%
1748
21st to endDay
17%10%49%7%3445%5%31%
1480
March1st to 10thDay
12%7%47%7%46%5%6%7%34%
1797
11th to 20thDay
15%9%5%8%5%45%5%5%6%35%
1840
21st to endDay
16%8%46%6%34446%39%
2376
April1st to 10thDay
17%9%36%6%35%446%36%
2160
11th to 20thDay
16%6%446%45%5%6%7%37%
2134
21st to endDay
20%8%46%7%36%5%5%5%32%
2308
May1st to 10thDay
21%9%36%5%345%5%6%32%
2340
11th to 20thDay
18%6%35%5%35%5%5%7%38%
2460
21st to endDay
15%6%26%6%35%5%5%8%41%
2725
June1st to 10thDay
17%5%37%7%46%5%7%8%32%
2520
11th to 20thDay
17%6%35%5%45%6%6%8%36%
2520
21st to endDay
17%7%35%6%5%6%7%6%7%32%
2520
July1st to 10thDay
13%5%35%6%35%6%6%9%39%
2520
11th to 20thDay
11%5%36%6%46%6%6%9%38%
2520
21st to endDay
14%5%26%6%46%6%7%8%38%
2744
August1st to 10thDay
10%5%27%6%47%6%7%9%38%
2454
11th to 20thDay
12%5%26%7%46%7%6%9%36%
2214
21st to endDay
12%425%5%35%5%6%9%43%
2376
September1st to 10thDay
12%5%25%5%35%5%6%9%44%
2160
11th to 20thDay
13%6%24434447%48%
2032
21st to endDay
16%5%245%3446%8%43%
1980
October1st to 10thDay
22%7%35%5%35%45%6%36%
1949
11th to 20thDay
15%6%36%5%35%5%6%7%39%
1746
21st to endDay
23%8%36%7%45%45%6%31%
1782
November1st to 10thDay
12%7%25%5%35%45%7%44%
1671
11th to 20thDay
16%6%36%6%35%45%6%39%
1710
21st to endDay
17%8%27%426%5%6%7%36%
1615
December1st to 10thDay
17%10%48%7%45%46%5%30%
1520
11th to 20thDay
20%9%38%5%34435%36%
1906
21st to endDay
20%11%48%7%46%5%45%28%
2772
January1st to 10thNight
13%8%36%5%35%5%6%7%37%
2649
11th to 20thNight
15%10%47%6%46%6%5%7%32%
2577
21st to endNight
10%7%46%6%36%6%6%8%38%
2772
February1st to 10thNight
13%9%48%6%46%5%6%6%33%
2332
11th to 20thNight
10%8%46%6%5%6%6%7%7%35%
2098
21st to endNight
15%8%26%5%45%47%7%36%
1776
March1st to 10thNight
13%9%36%6%45%45%7%38%
2150
11th to 20thNight
14%8%36%6%36%5%6%6%37%
2113
21st to endNight
14%9%35%5%345%5%6%41%
1980
April1st to 10thNight
17%7%36%5%445%6%7%38%
1800
11th to 20thNight
16%7%345%35%47%7%39%
1775
21st to endNight
21%8%25%5%3445%7%37%
1652
May1st to 10thNight
22%7%45%7%24447%33%
1620
11th to 20thNight
16%6%37%6%45%47%7%36%
1448
21st to endNight
15%435%5%345%5%9%41%
1550
June1st to 10thNight
19%6%25%445%447%40%
1440
11th to 20thNight
19%5%245%235%6%7%40%
1440
21st to endNight
14%5%25%5%36%6%6%7%40%
1440
July1st to 10thNight
13%425%6%35%6%6%9%42%
1440
11th to 20thNight
12%5%244346%7%10%43%
1440
21st to endNight
12%5%24335%5%6%8%46%
1574
August1st to 10thNight
10%435%4245%6%9%48%
1465
11th to 20thNight
12%6%25%6%45%5%6%8%40%
1682
21st to endNight
14%5%2442445%7%49%
1980
September1st to 10thNight
12%42433445%8%50%
1800
11th to 20thNight
13%5%24334446%52%
1927
21st to endNight
15%5%2443445%7%48%
1980
October1st to 10thNight
21%6%25%5%2435%6%40%
2007
11th to 20thNight
15%5%26%5%35%6%5%7%41%
2213
21st to endNight
22%8%35%5%35%5%5%6%33%
2574
November1st to 10thNight
14%6%35%445%45%6%44%
2407
11th to 20thNight
16%6%25%5%35%5%6%6%41%
2470
21st to endNight
13%7%35%5%35%5%6%7%41%
2560
December1st to 10thNight
16%10%37%7%5%45%5%7%32%
2660
11th to 20thNight
19%9%35%5%45%45%6%35%
3288
21st to endNight
21%11%48%6%345%5%6%27%
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
46%8%11%25%10%1
1518
11th to 20thDay
50%9%13%17%10%2
1472
21st to endDay
45%7%13%22%11%3
1584
February1st to 10thDay
54%7%13%16%9%1
1625
11th to 20thDay
49%7%11%18%11%3
1757
21st to endDay
55%5%9%16%13%2
1480
March1st to 10thDay
49%6%12%18%11%4
1800
11th to 20thDay
50%6%11%15%15%3
1840
21st to endDay
49%5%8%18%17%4
2376
April1st to 10thDay
50%5%9%21%12%3
2160
11th to 20thDay
45%8%11%19%16%2
2134
21st to endDay
53%6%9%16%13%3
2308
May1st to 10thDay
53%6%8%19%13%2
2340
11th to 20thDay
45%6%9%20%17%3
2464
21st to endDay
42%7%15%23%11%2
2725
June1st to 10thDay
50%7%13%19%10%1
2520
11th to 20thDay
45%8%12%19%15%2
2520
21st to endDay
48%9%13%19%10%1
2520
July1st to 10thDay
41%9%12%21%16%1
2450
11th to 20thDay
40%10%14%26%10%1
2445
21st to endDay
41%9%13%24%13%1
2591
August1st to 10thDay
40%9%15%23%11%1
2440
11th to 20thDay
41%10%14%23%11%1
2214
21st to endDay
36%9%15%25%14%1
2376
September1st to 10thDay
37%8%12%26%15%1
2160
11th to 20thDay
36%6%10%25%21%2
2032
21st to endDay
40%6%12%24%15%3
1980
October1st to 10thDay
51%7%7%20%13%3
1949
11th to 20thDay
42%8%13%27%8%1
1746
21st to endDay
56%5%10%18%9%2
1782
November1st to 10thDay
40%7%14%22%16%1
1671
11th to 20thDay
46%7%11%18%15%2
1710
21st to endDay
47%6%16%21%8%1
1615
December1st to 10thDay
55%7%12%15%9%1
1520
11th to 20thDay
53%5%11%18%13%1
1906
21st to endDay
59%7%10%15%9%
2772
January1st to 10thNight
45%8%11%20%14%1
2649
11th to 20thNight
53%8%12%17%8%2
2577
21st to endNight
44%8%11%20%15%2
2772
February1st to 10thNight
50%7%13%17%10%1
2332
11th to 20thNight
46%7%12%21%12%2
2100
21st to endNight
47%7%11%23%9%3
1776
March1st to 10thNight
46%5%10%23%12%3
2160
11th to 20thNight
47%7%11%18%14%3
2114
21st to endNight
44%6%11%17%19%4
1980
April1st to 10thNight
45%6%11%21%16%2
1800
11th to 20thNight
44%7%10%23%16%1
1775
21st to endNight
47%8%10%18%14%4
1652
May1st to 10thNight
53%5%7%18%14%2
1620
11th to 20thNight
46%8%12%18%14%3
1449
21st to endNight
42%9%13%23%12%2
1551
June1st to 10thNight
45%7%14%23%12%1
1440
11th to 20thNight
43%9%14%20%13%1
1440
21st to endNight
42%10%14%23%12%1
1440
July1st to 10thNight
37%10%14%25%13%1
1400
11th to 20thNight
34%11%16%27%11%
1392
21st to endNight
34%8%15%29%12%2
1486
August1st to 10thNight
33%10%15%29%13%1
1455
11th to 20thNight
40%10%14%24%11%
1682
21st to endNight
35%7%12%30%16%
1980
September1st to 10thNight
33%8%13%28%17%1
1800
11th to 20thNight
33%7%10%28%21%1
1927
21st to endNight
36%8%10%29%16%2
1980
October1st to 10thNight
46%7%11%22%11%3
2007
11th to 20thNight
41%10%14%25%10%1
2213
21st to endNight
52%7%11%19%9%1
2574
November1st to 10thNight
41%7%13%21%15%2
2407
11th to 20thNight
43%7%11%23%15%3
2470
21st to endNight
43%8%16%23%10%1
2560
December1st to 10thNight
52%7%13%19%8%1
2660
11th to 20thNight
51%7%12%15%14%1
3288
21st to endNight
58%6%11%19%6%
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%26%27%18%6%
180
11th to 20thDay
31%25%26%15%21
168
21st to endDay
20%31%32%11%5%
176
February1st to 10thDay
31%34%22%11%1
258
11th to 20thDay
28%23%32%14%2
416
21st to endDay
29%24%28%16%1
346
March1st to 10thDay
27%20%34%16%4
420
11th to 20thDay
29%25%29%13%31
420
21st to endDay
29%23%27%11%7%3
462
April1st to 10thDay
34%21%25%11%8%1
420
11th to 20thDay
31%22%24%13%7%2
412
21st to endDay
30%19%30%14%5%2
420
May1st to 10thDay
30%21%27%13%8%2
423
11th to 20thDay
23%18%32%20%42
447
21st to endDay
18%15%32%24%9%2
485
June1st to 10thDay
19%18%34%19%10%1
450
11th to 20thDay
20%16%33%20%10%2
450
21st to endDay
18%15%33%21%9%4
450
July1st to 10thDay
16%10%29%27%14%4
445
11th to 20thDay
15%6%34%29%13%3
444
21st to endDay
13%10%36%27%11%2
488
August1st to 10thDay
11%13%28%28%16%4
446
11th to 20thDay
14%10%28%26%13%8%
439
21st to endDay
14%13%28%25%15%7%
495
September1st to 10thDay
17%12%24%24%16%8%
450
11th to 20thDay
16%14%27%19%15%9%
450
21st to endDay
16%14%28%24%11%6%
450
October1st to 10thDay
27%15%22%20%11%5%
450
11th to 20thDay
21%17%26%24%10%2
423
21st to endDay
33%13%28%17%8%1
396
November1st to 10thDay
22%18%24%24%11%1
365
11th to 20thDay
30%19%26%18%5%1
370
21st to endDay
31%18%26%19%6%1
275
December1st to 10thDay
25%32%22%16%5%
180
11th to 20thDay
37%23%26%11%2
180
21st to endDay
32%20%29%15%4
198
January1st to 10thNight
22%19%33%23%3
760
11th to 20thNight
26%19%31%22%2
737
21st to endNight
19%18%35%24%4
792
February1st to 10thNight
23%18%37%20%2
720
11th to 20thNight
20%22%37%19%2
703
21st to endNight
25%22%31%20%2
592
March1st to 10thNight
26%18%32%23%2
720
11th to 20thNight
24%21%29%21%4
720
21st to endNight
24%18%29%21%7%
792
April1st to 10thNight
25%18%30%23%5%
720
11th to 20thNight
23%20%28%22%8%
711
21st to endNight
27%15%30%23%5%
572
May1st to 10thNight
27%13%27%25%6%
540
11th to 20thNight
21%12%31%29%7%
387
21st to endNight
18%8%26%33%14%1
389
June1st to 10thNight
21%7%26%34%12%
360
11th to 20thNight
19%9%29%31%13%
360
21st to endNight
13%6%27%42%11%1
360
July1st to 10thNight
14%6%23%42%14%1
350
11th to 20thNight
13%421%42%19%2
348
21st to endNight
14%324%39%19%2
394
August1st to 10thNight
11%419%46%19%1
391
11th to 20thNight
14%7%23%34%19%3
618
21st to endNight
16%6%20%36%20%2
792
September1st to 10thNight
13%5%28%34%19%1
720
11th to 20thNight
17%8%20%33%18%4
720
21st to endNight
18%9%24%31%16%2
720
October1st to 10thNight
25%12%18%30%13%2
720
11th to 20thNight
20%11%29%28%12%1
720
21st to endNight
27%16%26%24%6%
792
November1st to 10thNight
20%16%26%28%9%
740
11th to 20thNight
21%17%29%26%7%1
760
21st to endNight
21%17%31%24%6%1
760
December1st to 10thNight
23%21%33%20%4
760
11th to 20thNight
25%18%31%23%3
760
21st to endNight
28%17%34%19%2
836
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