Colebrook
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Colebrook clear sky chart from 2005-11-06 to 2024-02-20.

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
25%
23112
13%
80390
5%
23016
34%
23016
9%
60733

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
12%
756
18%
4329
3%
737
25%
737
4%
2778
January11th to 20thNight
13%
758
18%
4331
2%
740
23%
740
4%
2989
January21st to endNight
13%
833
9%
4785
4%
814
27%
814
4%
3694
February1st to 10thNight
13%
760
12%
2433
2%
760
25%
760
8%
1848
February11th to 20thNight
10%
742
13%
2215
2%
739
23%
739
7%
1730
February21st to endNight
13%
592
15%
1776
5%
592
27%
592
7%
1412
March1st to 10thNight
13%
720
13%
2160
3%
720
26%
720
9%
1669
March11th to 20thNight
16%
720
12%
2104
4%
720
24%
720
9%
1656
March21st to endNight
17%
792
16%
1980
4%
792
26%
792
11%
1544
April1st to 10thNight
16%
720
12%
1800
3%
720
22%
720
11%
1451
April11th to 20thNight
20%
711
11%
1775
1%
711
26%
711
10%
1452
April21st to endNight
19%
621
13%
1701
2%
621
27%
621
11%
1340
May1st to 10thNight
20%
540
10%
1620
3%
540
26%
540
8%
1313
May11th to 20thNight
21%
405
16%
1467
8%
405
32%
405
9%
1107
May21st to endNight
37%
388
11%
1551
9%
388
43%
388
11%
1131
June1st to 10thNight
31%
360
11%
1440
8%
360
40%
360
9%
1065
June11th to 20thNight
37%
360
9%
1440
9%
360
43%
360
10%
1089
June21st to endNight
41%
360
10%
1440
9%
360
48%
360
11%
1055
July1st to 10thNight
48%
350
9%
1400
10%
350
53%
350
11%
1031
July11th to 20thNight
48%
348
9%
1392
9%
348
52%
348
11%
1035
July21st to endNight
47%
394
12%
1486
10%
372
53%
372
12%
1086
August1st to 10thNight
51%
399
11%
1463
7%
397
57%
397
13%
1062
August11th to 20thNight
47%
690
12%
1754
5%
690
49%
690
16%
1382
August21st to endNight
46%
792
11%
1980
4%
792
49%
792
16%
1567
September1st to 10thNight
40%
720
13%
1800
4%
720
45%
720
17%
1406
September11th to 20thNight
42%
720
18%
1912
9%
720
49%
720
18%
1415
September21st to endNight
39%
720
16%
1980
9%
720
46%
720
16%
1503
October1st to 10thNight
33%
720
13%
2024
6%
720
40%
720
13%
1559
October11th to 20thNight
29%
720
12%
2172
5%
720
38%
720
11%
1671
October21st to endNight
21%
792
11%
2574
4%
792
30%
792
9%
2007
November1st to 10thNight
27%
740
14%
2407
6%
740
37%
740
12%
1779
November11th to 20thNight
21%
760
17%
2470
7%
760
34%
760
9%
1901
November21st to endNight
21%
760
12%
2536
4%
760
31%
760
8%
1968
December1st to 10thNight
15%
760
11%
2660
2%
760
25%
760
6%
2064
December11th to 20thNight
16%
760
12%
3222
1%
759
27%
759
6%
2399
December21st to endNight
11%
829
9%
4811
2%
817
23%
817
3%
3575

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
17%7%35%5%35%5%5%7%37%
78241
Night
17%8%35%5%3445%7%39%
80938
TransparencyDay
33%34%7%16%9%1
77946
Night
33%33%8%16%9%1
80767
SeeingDay
24%20%29%18%7%2
13930
Night
22%19%29%22%7%1
23129

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%8%25%5%35%5%5%6%42%
2517
11th to 20thDay
16%9%48%7%35%5%5%5%32%
2520
21st to endDay
21%13%7%7%5%3444428%
2840
February1st to 10thDay
16%9%45%8%46%5%6%6%33%
1745
11th to 20thDay
13%7%36%6%46%5%47%38%
1838
21st to endDay
19%11%36%5%44436%35%
1480
March1st to 10thDay
13%8%37%6%3445%8%38%
1797
11th to 20thDay
16%8%37%5%36%7%6%6%34%
1853
21st to endDay
19%9%25%6%34446%37%
2376
April1st to 10thDay
17%9%45%5%45%5%5%6%36%
2160
11th to 20thDay
16%7%25%5%35%45%7%40%
2134
21st to endDay
21%6%46%5%345%5%7%34%
2258
May1st to 10thDay
24%8%35%5%34446%31%
2340
11th to 20thDay
19%6%345%345%5%7%39%
2442
21st to endDay
16%5%35%5%3446%8%40%
2725
June1st to 10thDay
18%6%35%6%36%6%6%8%33%
2520
11th to 20thDay
18%5%25%6%45%5%6%8%35%
2520
21st to endDay
15%7%36%6%35%5%6%9%34%
2520
July1st to 10thDay
13%6%36%6%36%5%6%8%39%
2520
11th to 20thDay
12%6%25%5%45%6%7%8%40%
2520
21st to endDay
13%6%36%6%46%6%6%8%37%
2744
August1st to 10thDay
13%5%25%5%5%7%6%7%9%36%
2445
11th to 20thDay
13%5%26%6%46%6%7%9%36%
2143
21st to endDay
13%6%26%5%35%6%6%8%40%
2376
September1st to 10thDay
13%6%35%6%46%5%5%6%41%
2160
11th to 20thDay
15%5%344235%46%48%
2048
21st to endDay
18%6%25%4345%6%7%42%
1980
October1st to 10thDay
23%7%35%434445%38%
1932
11th to 20thDay
13%7%35%6%3445%7%43%
1787
21st to endDay
22%8%25%5%345%5%5%35%
1782
November1st to 10thDay
13%8%34434347%46%
1671
11th to 20thDay
19%6%24435%45%8%40%
1710
21st to endDay
17%8%35%5%3445%6%39%
1640
December1st to 10thDay
18%8%37%7%35%5%5%6%33%
1520
11th to 20thDay
22%6%2443436%7%39%
1906
21st to endDay
19%11%6%8%6%3445%7%27%
2772
January1st to 10thNight
16%7%345%3446%7%42%
4429
11th to 20thNight
15%9%47%6%3445%6%38%
4440
21st to endNight
21%13%46%5%35%445%29%
4814
February1st to 10thNight
15%9%45%5%345%6%6%38%
2433
11th to 20thNight
13%8%46%6%35%5%6%7%37%
2213
21st to endNight
18%9%27%5%3445%6%38%
1776
March1st to 10thNight
15%8%26%5%445%5%7%39%
2150
11th to 20thNight
15%8%35%5%4446%8%37%
2103
21st to endNight
17%7%35%5%3445%6%40%
1980
April1st to 10thNight
17%8%36%6%45%5%5%6%35%
1800
11th to 20thNight
18%7%246%445%47%38%
1775
21st to endNight
21%7%45%42335%6%39%
1701
May1st to 10thNight
22%9%36%6%25%45%7%30%
1620
11th to 20thNight
19%8%25%5%345%6%7%37%
1466
21st to endNight
17%7%35%5%3446%7%40%
1550
June1st to 10thNight
18%6%26%42436%7%41%
1440
11th to 20thNight
17%5%36%5%345%7%9%35%
1440
21st to endNight
16%5%3443446%8%43%
1440
July1st to 10thNight
14%5%335%35%5%5%8%45%
1440
11th to 20thNight
15%5%25%5%35%5%6%9%41%
1440
21st to endNight
11%335%5%45%6%5%8%44%
1574
August1st to 10thNight
10%6%35%4345%6%9%46%
1473
11th to 20thNight
13%7%25%6%45%5%7%7%40%
1754
21st to endNight
15%5%25%5%3445%8%45%
1980
September1st to 10thNight
14%6%35%5%3445%7%44%
1800
11th to 20thNight
14%6%2432325%5%53%
1912
21st to endNight
17%6%3442345%7%45%
1980
October1st to 10thNight
21%6%35%43345%6%41%
2024
11th to 20thNight
15%7%25%5%3445%8%44%
2172
21st to endNight
22%9%245%34446%37%
2574
November1st to 10thNight
14%7%345%2445%6%47%
2407
11th to 20thNight
18%6%35%5%2445%6%42%
2470
21st to endNight
15%7%35%445%447%42%
2536
December1st to 10thNight
19%7%36%6%3446%8%34%
2660
11th to 20thNight
18%7%245%35%5%5%7%37%
3288
21st to endNight
21%10%47%5%34446%32%
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
34%23%7%30%5%
2518
11th to 20thDay
44%25%7%18%6%
2520
21st to endDay
51%21%5%17%5%1
2840
February1st to 10thDay
35%34%7%14%9%1
1745
11th to 20thDay
32%36%6%16%7%2
1847
21st to endDay
38%34%6%11%10%1
1480
March1st to 10thDay
30%35%10%16%9%2
1800
11th to 20thDay
34%36%6%15%8%1
1853
21st to endDay
32%35%415%12%3
2376
April1st to 10thDay
31%35%6%16%10%2
2160
11th to 20thDay
32%35%7%15%10%2
2134
21st to endDay
32%35%7%13%10%3
2258
May1st to 10thDay
35%35%5%14%9%2
2340
11th to 20thDay
30%36%6%15%11%1
2446
21st to endDay
26%37%11%16%9%2
2725
June1st to 10thDay
33%37%9%14%7%
2520
11th to 20thDay
32%36%9%15%8%1
2520
21st to endDay
33%37%9%15%6%
2520
July1st to 10thDay
32%37%8%15%8%1
2450
11th to 20thDay
27%36%11%18%7%1
2445
21st to endDay
29%37%9%14%9%1
2591
August1st to 10thDay
31%36%9%15%9%
2431
11th to 20thDay
30%35%9%16%9%1
2143
21st to endDay
31%36%6%16%10%1
2376
September1st to 10thDay
30%36%6%17%10%1
2160
11th to 20thDay
24%36%7%18%15%1
2048
21st to endDay
30%35%6%16%12%2
1980
October1st to 10thDay
32%36%5%16%11%1
1932
11th to 20thDay
28%36%8%19%7%2
1787
21st to endDay
33%36%7%13%9%2
1782
November1st to 10thDay
23%39%8%17%12%2
1671
11th to 20thDay
28%39%6%16%9%1
1710
21st to endDay
31%39%7%15%7%1
1640
December1st to 10thDay
33%38%8%13%7%1
1520
11th to 20thDay
33%31%11%15%10%1
1906
21st to endDay
47%24%8%11%10%
2772
January1st to 10thNight
34%23%9%25%7%2
4429
11th to 20thNight
39%23%7%18%9%3
4440
21st to endNight
50%21%6%15%7%1
4814
February1st to 10thNight
32%33%9%17%9%1
2433
11th to 20thNight
31%35%7%15%10%1
2215
21st to endNight
35%34%5%17%7%2
1776
March1st to 10thNight
31%35%8%17%8%1
2160
11th to 20thNight
33%35%8%15%8%1
2104
21st to endNight
29%35%6%15%12%3
1980
April1st to 10thNight
32%36%7%14%9%2
1800
11th to 20thNight
34%35%5%14%12%
1775
21st to endNight
32%35%6%16%8%3
1701
May1st to 10thNight
37%35%5%11%10%3
1620
11th to 20thNight
31%35%5%15%11%3
1467
21st to endNight
26%38%12%14%10%1
1551
June1st to 10thNight
30%36%10%17%7%
1440
11th to 20thNight
32%35%10%16%6%
1440
21st to endNight
28%36%11%17%8%
1440
July1st to 10thNight
28%37%10%18%7%1
1400
11th to 20thNight
28%37%10%19%6%
1392
21st to endNight
25%38%10%16%9%2
1486
August1st to 10thNight
27%35%11%18%8%
1463
11th to 20thNight
30%35%9%17%8%1
1754
21st to endNight
30%36%8%16%10%1
1980
September1st to 10thNight
29%35%8%16%11%1
1800
11th to 20thNight
24%35%8%18%14%1
1912
21st to endNight
27%35%6%17%14%1
1980
October1st to 10thNight
30%34%8%16%11%1
2024
11th to 20thNight
30%35%9%18%7%1
2172
21st to endNight
32%35%8%15%8%1
2574
November1st to 10thNight
23%40%8%14%13%2
2407
11th to 20thNight
28%39%7%15%11%1
2470
21st to endNight
29%39%9%17%6%
2536
December1st to 10thNight
30%39%9%14%8%
2660
11th to 20thNight
33%32%10%12%11%2
3288
21st to endNight
45%23%5%16%10%
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
29%26%33%9%2
180
11th to 20thDay
30%34%26%8%11
180
21st to endDay
21%34%33%9%31
218
February1st to 10thDay
31%35%27%6%1
305
11th to 20thDay
29%34%28%8%1
452
21st to endDay
33%28%26%10%2
346
March1st to 10thDay
28%28%31%11%2
420
11th to 20thDay
30%28%30%10%21
420
21st to endDay
31%27%26%9%42
462
April1st to 10thDay
29%27%26%11%5%1
420
11th to 20thDay
28%25%28%13%42
412
21st to endDay
31%23%27%13%41
420
May1st to 10thDay
35%23%23%12%6%1
423
11th to 20thDay
22%21%34%19%21
447
21st to endDay
23%13%32%19%11%2
485
June1st to 10thDay
20%16%33%20%10%2
450
11th to 20thDay
20%15%35%21%7%2
450
21st to endDay
18%18%31%23%8%3
450
July1st to 10thDay
13%11%33%27%14%1
445
11th to 20thDay
14%7%34%31%12%3
444
21st to endDay
14%10%40%25%9%2
488
August1st to 10thDay
15%9%28%33%11%5%
446
11th to 20thDay
16%9%31%25%12%6%
439
21st to endDay
13%12%31%25%14%5%
495
September1st to 10thDay
19%12%23%24%16%5%
450
11th to 20thDay
18%14%26%20%13%8%
450
21st to endDay
21%14%30%18%12%4
450
October1st to 10thDay
28%14%26%16%12%4
450
11th to 20thDay
19%20%30%25%6%1
444
21st to endDay
31%18%28%15%8%1
396
November1st to 10thDay
22%22%25%21%8%1
365
11th to 20thDay
31%25%26%15%41
370
21st to endDay
26%25%27%17%41
300
December1st to 10thDay
25%32%27%11%5%
180
11th to 20thDay
37%26%31%42
180
21st to endDay
24%33%31%11%11
198
January1st to 10thNight
21%29%31%15%3
760
11th to 20thNight
27%24%31%15%2
760
21st to endNight
20%29%31%18%2
836
February1st to 10thNight
26%23%31%18%2
760
11th to 20thNight
22%32%33%12%1
742
21st to endNight
28%28%27%16%2
592
March1st to 10thNight
23%26%33%14%3
720
11th to 20thNight
25%29%28%15%3
720
21st to endNight
25%25%29%17%41
792
April1st to 10thNight
24%22%34%15%4
720
11th to 20thNight
22%23%32%17%7%
711
21st to endNight
25%22%30%18%4
621
May1st to 10thNight
28%18%30%18%6%
540
11th to 20thNight
23%15%35%23%4
405
21st to endNight
20%11%28%32%9%
389
June1st to 10thNight
20%10%31%29%9%1
360
11th to 20thNight
20%11%26%33%11%
360
21st to endNight
19%6%30%34%10%1
360
July1st to 10thNight
15%5%25%37%17%1
350
11th to 20thNight
16%5%23%39%16%1
348
21st to endNight
11%8%27%41%13%
394
August1st to 10thNight
13%5%26%42%14%1
399
11th to 20thNight
16%5%25%34%18%2
690
21st to endNight
16%8%23%32%18%2
792
September1st to 10thNight
16%7%30%32%13%2
720
11th to 20thNight
18%10%25%28%17%3
720
21st to endNight
21%9%26%27%14%3
720
October1st to 10thNight
27%11%25%25%11%2
720
11th to 20thNight
20%18%28%26%7%
720
21st to endNight
28%19%29%19%6%
792
November1st to 10thNight
18%20%30%23%7%1
740
11th to 20thNight
26%23%27%17%6%1
760
21st to endNight
23%21%30%20%41
760
December1st to 10thNight
26%24%30%15%4
760
11th to 20thNight
25%26%29%17%2
760
21st to endNight
27%27%31%13%2
836
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