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 2026-08-30.

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 Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(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

By Month

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 Forecast	Data Points
Cloud Cover	Day	17% 8% 3 6% 4 4 5% 4 6% 7% 36%	143889
	Night	17% 8% 3 6% 4 4 4 3 5% 7% 38%	138329
Transparency	Day	31% 34% 7% 15% 10% 2	147408
	Night	30% 34% 8% 15% 11% 2	142899
Seeing	Day	25% 20% 29% 17% 7% 2	17267
	Night	23% 19% 29% 22% 7% 1	26228

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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	10% 10% 4 10% 5% 5% 5% 4 5% 6% 35%	4661
	11th to 20th	Day	14% 10% 4 8% 4 4 4 3 5% 5% 39%	4650
	21st to end	Day	14% 10% 4 6% 3 4 4 3 6% 6% 40%	5262
February	1st to 10th	Day	15% 9% 3 7% 7% 6% 5% 4 5% 6% 34%	1929
	11th to 20th	Day	13% 7% 3 8% 6% 5% 6% 4 5% 7% 37%	2048
	21st to end	Day	17% 12% 3 7% 5% 4 3 3 4 5% 36%	1730
March	1st to 10th	Day	20% 10% 3 7% 4 4 4 3 5% 5% 36%	4576
	11th to 20th	Day	14% 7% 3 6% 5% 5% 5% 5% 6% 7% 37%	4774
	21st to end	Day	20% 10% 3 6% 4 4 4 3 5% 7% 34%	6102
April	1st to 10th	Day	18% 10% 3 6% 5% 5% 4 3 5% 6% 35%	2520
	11th to 20th	Day	17% 8% 2 6% 4 5% 5% 4 6% 6% 38%	2494
	21st to end	Day	19% 6% 3 7% 4 4 4 4 5% 7% 36%	2635
May	1st to 10th	Day	29% 8% 2 6% 4 4 4 3 5% 7% 29%	7639
	11th to 20th	Day	19% 8% 3 6% 5% 4 4 3 5% 7% 36%	8102
	21st to end	Day	22% 8% 2 7% 5% 5% 5% 4 6% 8% 28%	9060
June	1st to 10th	Day	16% 7% 2 6% 5% 5% 6% 4 6% 9% 34%	2940
	11th to 20th	Day	17% 6% 2 6% 5% 5% 5% 4 7% 8% 36%	2940
	21st to end	Day	15% 7% 3 7% 5% 4 5% 4 6% 8% 35%	2940
July	1st to 10th	Day	15% 6% 2 7% 5% 5% 5% 5% 7% 9% 35%	8280
	11th to 20th	Day	10% 5% 2 5% 4 5% 5% 5% 7% 11% 40%	8280
	21st to end	Day	17% 7% 2 6% 4 4 4 4 5% 7% 38%	8993
August	1st to 10th	Day	19% 6% 3 7% 5% 6% 6% 4 7% 8% 30%	4580
	11th to 20th	Day	14% 5% 2 7% 5% 5% 5% 4 6% 9% 38%	4002
	21st to end	Day	10% 5% 2 6% 4 5% 6% 5% 7% 9% 40%	4420
September	1st to 10th	Day	13% 6% 2 6% 5% 6% 5% 4 5% 6% 43%	2400
	11th to 20th	Day	14% 5% 3 4 4 2 4 3 5% 7% 49%	2274
	21st to end	Day	18% 6% 2 6% 3 4 4 4 6% 7% 40%	2200
October	1st to 10th	Day	21% 7% 3 5% 4 3 4 3 4 5% 41%	2146
	11th to 20th	Day	13% 8% 3 5% 5% 4 4 4 5% 8% 43%	1985
	21st to end	Day	21% 8% 2 6% 5% 4 4 4 5% 5% 37%	1980
November	1st to 10th	Day	12% 8% 3 5% 4 4 5% 3 5% 7% 45%	1851
	11th to 20th	Day	17% 6% 2 4 3 4 5% 4 5% 8% 41%	1890
	21st to end	Day	17% 10% 2 6% 5% 3 5% 3 5% 6% 37%	1812
December	1st to 10th	Day	14% 9% 3 7% 5% 3 4 4 5% 7% 40%	2680
	11th to 20th	Day	20% 8% 3 5% 3 3 4 3 5% 6% 39%	3066
	21st to end	Day	18% 11% 5% 8% 4 3 3 3 4 6% 33%	4048
January	1st to 10th	Night	13% 9% 3 9% 5% 5% 5% 4 6% 7% 33%	8234
	11th to 20th	Night	14% 11% 3 9% 5% 4 4 3 5% 6% 35%	8235
	21st to end	Night	13% 9% 3 7% 5% 5% 5% 4 5% 6% 39%	8903
February	1st to 10th	Night	14% 9% 4 6% 4 4 4 4 6% 6% 38%	2689
	11th to 20th	Night	13% 9% 4 6% 5% 5% 5% 4 7% 7% 36%	2458
	21st to end	Night	17% 9% 2 8% 5% 4 3 3 5% 6% 37%	2076
March	1st to 10th	Night	19% 10% 2 6% 4 4 3 3 5% 6% 38%	5484
	11th to 20th	Night	16% 8% 3 6% 4 4 4 3 6% 7% 39%	5402
	21st to end	Night	20% 10% 3 6% 4 4 3 3 5% 5% 36%	5072
April	1st to 10th	Night	18% 9% 3 7% 5% 5% 5% 4 5% 6% 34%	2100
	11th to 20th	Night	19% 8% 2 6% 6% 4 4 4 4 6% 37%	2075
	21st to end	Night	20% 8% 3 6% 4 4 3 3 5% 6% 40%	1983
May	1st to 10th	Night	28% 9% 2 6% 4 3 3 3 5% 7% 30%	5259
	11th to 20th	Night	18% 10% 3 7% 5% 5% 4 3 5% 7% 33%	4800
	21st to end	Night	22% 9% 3 6% 4 5% 4 4 5% 6% 31%	5114
June	1st to 10th	Night	17% 7% 2 6% 4 3 4 3 6% 7% 42%	1680
	11th to 20th	Night	17% 7% 2 6% 4 4 4 5% 7% 9% 36%	1680
	21st to end	Night	17% 6% 2 5% 4 4 3 4 6% 8% 43%	1680
July	1st to 10th	Night	16% 6% 2 5% 4 4 5% 4 6% 9% 38%	4680
	11th to 20th	Night	13% 7% 2 6% 4 4 4 4 6% 9% 41%	4680
	21st to end	Night	15% 6% 2 5% 3 4 4 3 5% 6% 46%	5084
August	1st to 10th	Night	21% 7% 3 6% 4 4 4 4 6% 6% 34%	2754
	11th to 20th	Night	15% 8% 2 6% 4 4 4 3 5% 6% 42%	3262
	21st to end	Night	11% 4 1 5% 4 3 4 3 5% 8% 52%	3677
September	1st to 10th	Night	14% 6% 2 6% 4 4 4 3 5% 6% 46%	2000
	11th to 20th	Night	12% 6% 2 5% 3 3 2 2 5% 6% 53%	2125
	21st to end	Night	18% 7% 3 5% 3 3 4 3 5% 6% 44%	2200
October	1st to 10th	Night	19% 6% 2 6% 4 4 3 3 5% 7% 42%	2250
	11th to 20th	Night	14% 7% 2 6% 3 3 4 3 5% 7% 45%	2414
	21st to end	Night	21% 8% 2 5% 4 4 4 3 5% 7% 38%	2860
November	1st to 10th	Night	13% 7% 2 5% 4 3 4 3 5% 6% 47%	2667
	11th to 20th	Night	17% 7% 3 5% 4 3 4 4 5% 6% 42%	2730
	21st to end	Night	14% 8% 3 6% 4 5% 5% 3 5% 7% 41%	2804
December	1st to 10th	Night	17% 9% 3 6% 4 3 3 3 5% 6% 40%	4720
	11th to 20th	Night	22% 9% 2 5% 3 4 4 3 5% 5% 38%	5348
	21st to end	Night	18% 11% 4 7% 5% 4 3 3 4 5% 36%	7150

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	38% 22% 7% 22% 10% 2	4662
	11th to 20th	Day	35% 21% 6% 19% 14% 6%	4680
	21st to end	Day	36% 22% 6% 19% 16% 1	5262
February	1st to 10th	Day	20% 59% 5% 9% 6% 1	1929
	11th to 20th	Day	20% 58% 4 9% 9% 1	2057
	21st to end	Day	19% 59% 5% 11% 6% 1	1730
March	1st to 10th	Day	39% 28% 6% 10% 9% 7%	4579
	11th to 20th	Day	33% 28% 10% 21% 6% 2	4774
	21st to end	Day	38% 28% 4 10% 15% 5%	6102
April	1st to 10th	Day	24% 59% 3 7% 6% 2	2520
	11th to 20th	Day	18% 60% 4 9% 9%	2494
	21st to end	Day	20% 59% 3 9% 7% 1	2635
May	1st to 10th	Day	45% 24% 10% 13% 9%	7651
	11th to 20th	Day	40% 23% 11% 16% 10%	8106
	21st to end	Day	46% 23% 6% 16% 9%	9061
June	1st to 10th	Day	17% 60% 4 11% 8%	2940
	11th to 20th	Day	17% 60% 6% 11% 5% 1	2940
	21st to end	Day	17% 59% 4 10% 9% 1	2940
July	1st to 10th	Day	36% 25% 11% 17% 11%	8210
	11th to 20th	Day	28% 24% 14% 24% 10% 1	8205
	21st to end	Day	35% 22% 9% 20% 14% 1	8795
August	1st to 10th	Day	35% 37% 9% 13% 5% 1	4566
	11th to 20th	Day	27% 38% 11% 19% 5%	4039
	21st to end	Day	22% 38% 9% 21% 10% 1	4420
September	1st to 10th	Day	16% 58% 5% 13% 8%	2400
	11th to 20th	Day	16% 57% 3 11% 11% 1	2274
	21st to end	Day	21% 57% 4 10% 7% 1	2200
October	1st to 10th	Day	23% 37% 4 14% 19% 3	3497
	11th to 20th	Day	19% 36% 5% 15% 21% 4	3213
	21st to end	Day	18% 37% 9% 20% 14% 3	3180
November	1st to 10th	Day	21% 55% 6% 11% 7% 1	1851
	11th to 20th	Day	17% 54% 4 10% 14% 2	1890
	21st to end	Day	20% 54% 7% 12% 5% 1	1812
December	1st to 10th	Day	27% 36% 10% 20% 6%	2680
	11th to 20th	Day	32% 31% 8% 14% 14% 2	3066
	21st to end	Day	41% 25% 3 12% 13% 5%	4048
January	1st to 10th	Night	37% 22% 9% 19% 10% 2	8234
	11th to 20th	Night	38% 21% 8% 15% 12% 5%	8280
	21st to end	Night	37% 22% 8% 14% 16% 3	8903
February	1st to 10th	Night	18% 59% 5% 10% 6% 1	2689
	11th to 20th	Night	19% 58% 4 7% 10% 1	2460
	21st to end	Night	19% 59% 4 9% 7% 2	2076
March	1st to 10th	Night	36% 28% 7% 12% 14% 3	5494
	11th to 20th	Night	33% 27% 12% 17% 8% 3	5403
	21st to end	Night	39% 28% 4 8% 18% 3	5072
April	1st to 10th	Night	23% 59% 3 8% 6% 1	2100
	11th to 20th	Night	19% 60% 4 8% 9% 1	2075
	21st to end	Night	19% 58% 3 8% 10% 1	1983
May	1st to 10th	Night	44% 24% 11% 13% 8%	5265
	11th to 20th	Night	40% 24% 11% 17% 7% 1	4801
	21st to end	Night	45% 24% 8% 16% 7%	5115
June	1st to 10th	Night	17% 59% 2 13% 8% 1	1680
	11th to 20th	Night	17% 59% 6% 12% 6%	1680
	21st to end	Night	15% 58% 5% 13% 7% 1	1680
July	1st to 10th	Night	34% 24% 13% 16% 13% 1	4640
	11th to 20th	Night	30% 23% 12% 21% 12% 2	4632
	21st to end	Night	30% 22% 9% 23% 15% 1	4967
August	1st to 10th	Night	36% 38% 7% 12% 6% 1	2744
	11th to 20th	Night	27% 37% 10% 21% 5%	3291
	21st to end	Night	16% 36% 9% 28% 11%	3677
September	1st to 10th	Night	15% 58% 4 14% 9% 1	2000
	11th to 20th	Night	14% 57% 4 11% 12% 1	2125
	21st to end	Night	20% 57% 4 10% 8% 1	2200
October	1st to 10th	Night	24% 37% 7% 16% 14% 1	3679
	11th to 20th	Night	15% 35% 7% 17% 21% 4	3925
	21st to end	Night	18% 36% 10% 23% 12% 1	4610
November	1st to 10th	Night	19% 54% 5% 12% 9% 1	2667
	11th to 20th	Night	18% 54% 5% 9% 12% 2	2730
	21st to end	Night	18% 55% 7% 14% 6%	2804
December	1st to 10th	Night	28% 35% 8% 23% 6%	4720
	11th to 20th	Night	35% 30% 8% 9% 15% 2	5348
	21st to end	Night	39% 25% 4 12% 14% 5%	7150

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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	32% 27% 30% 8% 2	220
	11th to 20th	Day	34% 33% 25% 7% 1	220
	21st to end	Day	28% 33% 28% 7% 2 1	267
February	1st to 10th	Day	32% 33% 28% 7% 1	369
	11th to 20th	Day	32% 32% 28% 7% 1	536
	21st to end	Day	31% 30% 26% 11% 2	446
March	1st to 10th	Day	31% 27% 29% 11% 2	540
	11th to 20th	Day	32% 26% 30% 9% 2 1	540
	21st to end	Day	35% 28% 25% 8% 3 2	594
April	1st to 10th	Day	33% 26% 26% 10% 5% 1	540
	11th to 20th	Day	33% 25% 26% 11% 4 1	532
	21st to end	Day	29% 24% 30% 12% 4 1	540
May	1st to 10th	Day	36% 24% 23% 12% 5% 1	543
	11th to 20th	Day	22% 23% 32% 18% 4 1	567
	21st to end	Day	26% 14% 30% 18% 9% 2	617
June	1st to 10th	Day	19% 16% 33% 22% 9% 1	570
	11th to 20th	Day	20% 17% 34% 21% 7% 2	570
	21st to end	Day	19% 18% 32% 21% 7% 2	570
July	1st to 10th	Day	14% 12% 33% 27% 13% 1	565
	11th to 20th	Day	13% 9% 35% 30% 11% 2	564
	21st to end	Day	16% 10% 39% 24% 9% 3	620
August	1st to 10th	Day	15% 9% 29% 31% 11% 5%	566
	11th to 20th	Day	15% 11% 32% 25% 12% 6%	559
	21st to end	Day	12% 14% 32% 25% 12% 5%	619
September	1st to 10th	Day	18% 13% 26% 23% 15% 5%	530
	11th to 20th	Day	16% 12% 26% 24% 14% 8%	530
	21st to end	Day	22% 13% 31% 19% 11% 4	530
October	1st to 10th	Day	25% 14% 25% 18% 12% 4	530
	11th to 20th	Day	22% 21% 28% 23% 5% 2	522
	21st to end	Day	30% 19% 29% 16% 7%	462
November	1st to 10th	Day	22% 25% 26% 19% 7% 1	425
	11th to 20th	Day	30% 26% 26% 14% 4 1	430
	21st to end	Day	30% 26% 26% 14% 3 1	352
December	1st to 10th	Day	28% 30% 28% 10% 4	220
	11th to 20th	Day	41% 23% 29% 5% 2	220
	21st to end	Day	27% 29% 30% 11% 2 1	242
January	1st to 10th	Night	23% 29% 31% 15% 2	840
	11th to 20th	Night	29% 24% 31% 14% 2	840
	21st to end	Night	22% 28% 31% 17% 2	924
February	1st to 10th	Night	25% 24% 31% 18% 2	840
	11th to 20th	Night	24% 30% 33% 12% 1	823
	21st to end	Night	27% 27% 28% 15% 2	692
March	1st to 10th	Night	26% 25% 33% 14% 3	840
	11th to 20th	Night	25% 28% 28% 16% 3	840
	21st to end	Night	26% 26% 29% 15% 3 1	924
April	1st to 10th	Night	27% 22% 33% 14% 4	840
	11th to 20th	Night	25% 23% 30% 16% 6%	831
	21st to end	Night	25% 24% 29% 18% 4	723
May	1st to 10th	Night	28% 18% 30% 18% 5%	630
	11th to 20th	Night	22% 15% 35% 24% 4	473
	21st to end	Night	21% 11% 29% 30% 7%	455
June	1st to 10th	Night	19% 11% 32% 28% 9% 1	420
	11th to 20th	Night	20% 12% 27% 32% 9%	420
	21st to end	Night	19% 9% 29% 33% 10% 1	420
July	1st to 10th	Night	16% 6% 26% 37% 15%	410
	11th to 20th	Night	15% 7% 25% 38% 14% 1	408
	21st to end	Night	12% 8% 28% 39% 13%	460
August	1st to 10th	Night	15% 6% 24% 41% 14% 1	467
	11th to 20th	Night	15% 6% 26% 33% 17% 2	807
	21st to end	Night	16% 9% 24% 32% 17% 2	917
September	1st to 10th	Night	16% 8% 29% 32% 14% 2	800
	11th to 20th	Night	16% 9% 24% 29% 19% 3	800
	21st to end	Night	22% 9% 26% 27% 14% 3	800
October	1st to 10th	Night	25% 11% 25% 25% 12% 2	800
	11th to 20th	Night	21% 18% 27% 25% 8%	800
	21st to end	Night	27% 18% 30% 19% 6%	880
November	1st to 10th	Night	19% 21% 30% 23% 7% 1	820
	11th to 20th	Night	26% 23% 27% 17% 6% 1	840
	21st to end	Night	24% 22% 30% 20% 4 1	840
December	1st to 10th	Night	26% 24% 30% 16% 3	840
	11th to 20th	Night	27% 25% 29% 16% 2	840
	21st to end	Night	28% 26% 30% 13% 3	924

Page generated at 2026-08-27 04:07:16. Version ac07

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