Granby
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Granby clear sky chart from 2005-11-06 to 2024-01-22.

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
30%
22697
12%
74839
8%
22655
35%
22655
13%
55595

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
16%
760
6%
2649
4%
757
19%
757
6%
2197
January11th to 20thNight
17%
760
7%
2660
4%
760
21%
760
7%
2155
January21st to endNight
20%
800
10%
2798
6%
800
26%
800
9%
2144
February1st to 10thNight
15%
720
8%
2412
4%
720
21%
720
7%
1970
February11th to 20thNight
18%
703
9%
2100
4%
700
25%
700
10%
1685
February21st to endNight
21%
592
13%
1776
7%
592
31%
592
12%
1364
March1st to 10thNight
22%
720
13%
2160
10%
720
29%
720
11%
1702
March11th to 20thNight
25%
720
13%
2086
7%
720
31%
720
14%
1576
March21st to endNight
26%
792
17%
2026
7%
792
33%
792
16%
1553
April1st to 10thNight
25%
720
14%
1800
5%
720
30%
720
16%
1400
April11th to 20thNight
27%
680
13%
1744
5%
680
32%
680
18%
1279
April21st to endNight
24%
540
14%
1620
6%
540
30%
540
15%
1175
May1st to 10thNight
26%
540
13%
1620
8%
540
31%
540
15%
1189
May11th to 20thNight
34%
382
17%
1444
15%
382
42%
382
18%
942
May21st to endNight
44%
389
12%
1551
14%
388
51%
388
17%
1006
June1st to 10thNight
35%
360
11%
1440
12%
360
43%
360
14%
1001
June11th to 20thNight
46%
360
11%
1440
14%
360
52%
360
17%
936
June21st to endNight
43%
360
8%
1440
11%
360
47%
360
14%
995
July1st to 10thNight
53%
350
12%
1400
18%
350
56%
350
18%
900
July11th to 20thNight
54%
348
13%
1392
18%
348
57%
348
22%
847
July21st to endNight
55%
394
13%
1486
19%
372
55%
372
19%
942
August1st to 10thNight
59%
394
15%
1458
17%
392
61%
392
23%
889
August11th to 20thNight
50%
532
16%
1596
12%
532
51%
532
23%
1059
August21st to endNight
50%
782
17%
1970
9%
782
52%
782
29%
1328
September1st to 10thNight
48%
720
13%
1800
6%
720
50%
720
30%
1217
September11th to 20thNight
48%
720
20%
1952
13%
720
52%
720
30%
1262
September21st to endNight
43%
720
20%
1980
15%
720
47%
720
28%
1302
October1st to 10thNight
36%
720
14%
1980
9%
720
41%
720
18%
1451
October11th to 20thNight
35%
720
14%
2263
10%
720
40%
720
16%
1621
October21st to endNight
25%
792
10%
2574
6%
792
30%
792
11%
1996
November1st to 10thNight
29%
740
14%
2407
10%
740
34%
740
14%
1745
November11th to 20thNight
27%
760
16%
2470
13%
760
33%
760
13%
1845
November21st to endNight
22%
760
9%
2576
7%
760
26%
760
9%
2028
December1st to 10thNight
16%
760
5%
2660
2%
760
19%
760
5%
2256
December11th to 20thNight
17%
760
7%
3260
5%
759
21%
759
5%
2676
December21st to endNight
16%
827
5%
4849
3%
817
20%
817
3%
3962

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%11%5%8%7%35%5%5%6%27%
74862
Night
17%12%5%9%7%45%45%6%27%
75073
TransparencyDay
57%5%11%18%9%1
74567
Night
58%411%18%8%1
74902
SeeingDay
28%16%26%19%9%3
13690
Night
25%13%23%27%10%1
22706

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
14%14%10%15%8%5%5%445%15%
1517
11th to 20thDay
17%15%6%10%11%5%4445%19%
1520
21st to endDay
10%13%9%11%9%45%6%45%23%
1592
February1st to 10thDay
14%15%7%13%11%5%6%44418%
1548
11th to 20thDay
12%12%9%17%10%36%44419%
1748
21st to endDay
14%15%6%13%10%46%346%20%
1480
March1st to 10thDay
14%15%7%11%9%45%44424%
1797
11th to 20thDay
15%14%6%8%8%3433431%
1873
21st to endDay
17%14%6%11%7%3433428%
2329
April1st to 10thDay
22%15%5%7%6%3433428%
2160
11th to 20thDay
18%12%46%6%25%45%5%32%
2164
21st to endDay
23%13%48%6%3445%5%26%
2340
May1st to 10thDay
24%11%46%5%44445%29%
2340
11th to 20thDay
20%10%37%6%3445%5%33%
2465
21st to endDay
17%9%36%5%35%6%6%8%31%
2725
June1st to 10thDay
26%9%37%6%3446%7%25%
2520
11th to 20thDay
18%7%37%6%35%5%6%7%33%
2520
21st to endDay
22%8%37%7%46%6%6%6%25%
2520
July1st to 10thDay
17%7%36%7%46%5%6%9%31%
2520
11th to 20thDay
14%7%37%7%47%6%8%9%28%
2520
21st to endDay
18%7%36%6%47%5%7%8%28%
2744
August1st to 10thDay
16%8%36%7%46%6%6%8%31%
2453
11th to 20thDay
15%8%37%5%36%6%8%9%29%
2304
21st to endDay
15%7%35%6%46%6%6%9%34%
2382
September1st to 10thDay
16%7%36%5%345%5%7%39%
2160
11th to 20thDay
17%7%345%23446%43%
2007
21st to endDay
17%7%35%6%35%5%6%8%36%
1980
October1st to 10thDay
25%11%47%5%3445%6%27%
1980
11th to 20thDay
17%13%46%6%345%7%6%30%
1692
21st to endDay
24%19%38%7%3433323%
1782
November1st to 10thDay
15%15%5%8%5%35%5%6%6%29%
1671
11th to 20thDay
18%14%6%10%8%445%5%5%22%
1710
21st to endDay
19%15%5%11%6%346%5%5%22%
1601
December1st to 10thDay
19%21%6%15%5%4433417%
1520
11th to 20thDay
20%16%7%8%6%43345%23%
1906
21st to endDay
23%18%7%14%6%45%43315%
2772
January1st to 10thNight
13%16%7%16%10%46%5%5%414%
2649
11th to 20thNight
15%17%6%11%9%5%5%5%45%17%
2660
21st to endNight
10%14%7%13%10%5%6%5%5%5%20%
2798
February1st to 10thNight
13%13%8%12%11%6%6%445%17%
2412
11th to 20thNight
11%13%6%12%11%5%7%6%45%20%
2098
21st to endNight
13%13%6%10%10%5%7%44426%
1776
March1st to 10thNight
13%14%9%11%8%46%5%4424%
2150
11th to 20thNight
16%13%6%9%6%45%445%29%
2085
21st to endNight
16%14%410%6%34435%31%
2026
April1st to 10thNight
22%12%5%7%6%45%45%427%
1800
11th to 20thNight
18%10%5%7%5%35%45%6%32%
1744
21st to endNight
23%9%47%5%3445%6%30%
1620
May1st to 10thNight
21%12%47%7%34445%29%
1620
11th to 20thNight
18%8%45%6%3346%7%36%
1443
21st to endNight
19%7%36%5%36%45%7%36%
1550
June1st to 10thNight
24%9%46%5%3445%6%30%
1440
11th to 20thNight
19%5%25%6%46%6%6%8%34%
1440
21st to endNight
18%9%37%5%45%5%7%7%30%
1440
July1st to 10thNight
16%7%36%6%36%6%6%8%34%
1440
11th to 20thNight
15%6%26%4345%7%11%36%
1440
21st to endNight
14%8%25%6%36%5%7%7%36%
1574
August1st to 10thNight
15%6%26%5%346%7%8%39%
1468
11th to 20thNight
15%8%27%5%35%46%9%36%
1596
21st to endNight
16%6%35%435%45%8%40%
1970
September1st to 10thNight
15%6%25%435%5%5%7%42%
1800
11th to 20thNight
16%7%25%434347%45%
1952
21st to endNight
18%7%345%3445%7%41%
1980
October1st to 10thNight
21%9%36%6%35%45%7%31%
1980
11th to 20thNight
17%11%47%6%46%5%6%6%28%
2263
21st to endNight
23%15%47%5%445%46%22%
2574
November1st to 10thNight
18%13%48%6%345%6%5%29%
2407
11th to 20thNight
17%12%7%10%7%34446%27%
2470
21st to endNight
18%14%8%11%8%34435%21%
2576
December1st to 10thNight
20%18%9%12%9%4444413%
2660
11th to 20thNight
20%17%7%12%9%45%335%16%
3288
21st to endNight
22%16%6%15%7%3433415%
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
71%38%14%4
1518
11th to 20thDay
67%48%12%7%1
1520
21st to endDay
61%5%9%14%8%1
1592
February1st to 10thDay
69%47%12%6%1
1548
11th to 20thDay
69%38%13%5%1
1757
21st to endDay
67%37%12%9%2
1480
March1st to 10thDay
64%49%14%7%3
1800
11th to 20thDay
59%310%14%12%2
1873
21st to endDay
60%46%17%10%3
2329
April1st to 10thDay
62%37%16%9%2
2160
11th to 20thDay
53%49%20%12%1
2164
21st to endDay
61%49%16%8%3
2340
May1st to 10thDay
57%38%20%10%2
2340
11th to 20thDay
52%49%20%13%2
2469
21st to endDay
50%6%14%20%10%1
2725
June1st to 10thDay
58%412%18%7%
2520
11th to 20thDay
49%5%10%23%11%2
2520
21st to endDay
57%6%12%18%7%1
2520
July1st to 10thDay
48%5%14%22%11%
2450
11th to 20thDay
50%6%13%23%7%
2445
21st to endDay
51%6%14%21%8%
2591
August1st to 10thDay
48%6%17%21%7%
2439
11th to 20thDay
48%6%17%21%9%
2304
21st to endDay
44%7%13%23%12%1
2382
September1st to 10thDay
43%5%13%26%13%1
2160
11th to 20thDay
41%410%29%15%1
2007
21st to endDay
46%5%14%22%12%1
1980
October1st to 10thDay
57%410%20%9%1
1980
11th to 20thDay
52%5%15%22%6%
1692
21st to endDay
66%49%12%7%2
1782
November1st to 10thDay
54%6%13%17%10%1
1671
11th to 20thDay
63%5%9%12%9%2
1710
21st to endDay
63%5%13%15%3
1601
December1st to 10thDay
73%39%11%4
1520
11th to 20thDay
64%47%16%8%1
1906
21st to endDay
74%46%10%6%
2772
January1st to 10thNight
72%5%10%9%4
2649
11th to 20thNight
69%410%13%41
2660
21st to endNight
63%410%15%6%1
2798
February1st to 10thNight
70%48%11%6%1
2412
11th to 20thNight
67%48%14%7%1
2100
21st to endNight
62%39%16%9%1
1776
March1st to 10thNight
64%46%16%7%2
2160
11th to 20thNight
59%410%16%10%1
2086
21st to endNight
58%48%19%10%2
2026
April1st to 10thNight
60%38%19%10%1
1800
11th to 20thNight
52%5%9%23%11%
1744
21st to endNight
54%5%11%16%12%2
1620
May1st to 10thNight
57%47%18%13%2
1620
11th to 20thNight
47%6%10%21%14%2
1444
21st to endNight
46%6%15%22%9%1
1551
June1st to 10thNight
55%411%21%9%1
1440
11th to 20thNight
46%7%14%21%10%2
1440
21st to endNight
50%8%13%20%8%
1440
July1st to 10thNight
46%7%15%20%12%1
1400
11th to 20thNight
40%7%19%26%8%
1392
21st to endNight
44%6%16%24%9%
1486
August1st to 10thNight
41%5%18%29%7%
1458
11th to 20thNight
44%6%17%22%10%
1596
21st to endNight
42%5%16%26%11%
1970
September1st to 10thNight
40%7%15%27%11%
1800
11th to 20thNight
40%412%31%13%1
1952
21st to endNight
43%415%26%12%2
1980
October1st to 10thNight
53%5%12%19%10%1
1980
11th to 20thNight
54%5%13%22%6%
2263
21st to endNight
63%5%10%14%7%1
2574
November1st to 10thNight
56%413%19%7%1
2407
11th to 20thNight
60%410%16%8%2
2470
21st to endNight
65%412%14%5%
2576
December1st to 10thNight
76%38%10%2
2660
11th to 20thNight
72%37%11%6%
3288
21st to endNight
74%37%12%3
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
27%19%29%21%4
180
11th to 20thDay
36%22%23%13%5%1
180
21st to endDay
25%20%33%15%7%1
178
February1st to 10thDay
40%21%25%11%4
228
11th to 20thDay
30%22%30%15%4
416
21st to endDay
28%22%27%18%5%
346
March1st to 10thDay
32%17%29%18%3
420
11th to 20thDay
30%22%28%14%5%1
420
21st to endDay
32%18%24%16%8%3
462
April1st to 10thDay
40%14%22%17%5%2
420
11th to 20thDay
32%22%21%15%8%2
412
21st to endDay
36%18%24%13%7%1
420
May1st to 10thDay
39%16%23%14%6%2
423
11th to 20thDay
27%17%31%18%5%3
447
21st to endDay
22%17%31%19%9%2
485
June1st to 10thDay
33%15%29%14%8%1
450
11th to 20thDay
23%15%30%20%10%2
450
21st to endDay
25%17%28%19%9%2
450
July1st to 10thDay
21%10%29%27%10%2
445
11th to 20thDay
17%10%34%24%11%4
444
21st to endDay
21%10%31%24%12%2
488
August1st to 10thDay
17%11%27%22%19%3
446
11th to 20thDay
20%9%28%23%15%6%
439
21st to endDay
17%16%25%22%14%6%
495
September1st to 10thDay
17%12%26%20%18%6%
450
11th to 20thDay
22%10%25%18%17%9%
450
21st to endDay
19%13%26%22%13%7%
450
October1st to 10thDay
28%14%22%18%13%5%
450
11th to 20thDay
27%14%25%21%12%2
396
21st to endDay
36%15%22%19%8%
396
November1st to 10thDay
31%16%19%26%9%
365
11th to 20thDay
31%18%24%19%6%2
370
21st to endDay
33%18%25%18%5%2
261
December1st to 10thDay
36%18%23%16%7%
180
11th to 20thDay
41%16%28%11%4
180
21st to endDay
39%16%26%15%5%
198
January1st to 10thNight
26%20%26%23%4
760
11th to 20thNight
29%18%25%23%5%
760
21st to endNight
20%20%28%27%5%1
800
February1st to 10thNight
21%20%31%23%4
720
11th to 20thNight
23%20%31%22%5%
703
21st to endNight
24%17%28%27%4
592
March1st to 10thNight
26%16%29%25%4
720
11th to 20thNight
29%16%25%23%8%
720
21st to endNight
29%15%23%24%7%1
792
April1st to 10thNight
31%14%24%24%7%
720
11th to 20thNight
26%17%25%24%7%1
680
21st to endNight
30%12%28%23%7%
540
May1st to 10thNight
28%14%25%23%9%1
540
11th to 20thNight
21%11%27%30%9%1
382
21st to endNight
22%10%17%38%13%
389
June1st to 10thNight
29%7%20%34%11%
360
11th to 20thNight
20%8%18%36%18%
360
21st to endNight
20%7%21%37%15%1
360
July1st to 10thNight
15%7%14%44%19%1
350
11th to 20thNight
15%5%18%40%19%2
348
21st to endNight
16%316%45%20%1
394
August1st to 10thNight
17%311%41%27%1
394
11th to 20thNight
18%6%17%37%20%2
532
21st to endNight
20%6%16%32%23%4
782
September1st to 10thNight
18%6%20%35%19%2
720
11th to 20thNight
20%8%18%28%23%3
720
21st to endNight
22%9%21%26%20%3
720
October1st to 10thNight
28%11%16%27%15%2
720
11th to 20thNight
23%12%22%28%13%2
720
21st to endNight
33%12%23%26%6%
792
November1st to 10thNight
26%15%23%27%9%1
740
11th to 20thNight
27%15%24%24%9%1
760
21st to endNight
27%19%22%24%6%2
760
December1st to 10thNight
32%19%24%20%5%
760
11th to 20thNight
30%17%25%23%5%
760
21st to endNight
32%16%24%24%3
836
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