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 2026-06-01.

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	16% 13% 5% 11% 6% 5% 5% 4 5% 6% 24%	131929
	Night	15% 14% 5% 13% 7% 6% 5% 3 4 5% 23%	131232
Transparency	Day	59% 6% 10% 16% 8% 1	135410
	Night	64% 5% 9% 14% 7% 1	135838
Seeing	Day	30% 19% 27% 16% 7% 2	16699
	Night	27% 17% 25% 22% 9% 1	25617

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% 15% 11% 20% 12% 10% 5% 3 3 4 10%	4661
	11th to 20th	Day	6% 16% 8% 25% 16% 7% 3 2 4 3 10%	4650
	21st to end	Day	7% 13% 7% 16% 10% 12% 8% 3 4 4 16%	5124
February	1st to 10th	Day	11% 17% 6% 15% 10% 7% 6% 2 4 3 18%	1785
	11th to 20th	Day	9% 15% 9% 18% 10% 7% 5% 3 3 4 16%	2048
	21st to end	Day	11% 17% 6% 16% 8% 7% 5% 3 4 4 20%	1730
March	1st to 10th	Day	16% 16% 6% 16% 7% 6% 4 3 3 4 19%	4576
	11th to 20th	Day	13% 17% 6% 10% 5% 4 4 3 5% 5% 27%	4847
	21st to end	Day	18% 15% 7% 13% 5% 4 3 2 3 5% 24%	5953
April	1st to 10th	Day	18% 20% 5% 10% 5% 3 3 3 3 4 26%	2520
	11th to 20th	Day	17% 16% 5% 8% 5% 5% 3 3 4 4 31%	2550
	21st to end	Day	21% 15% 4 9% 4 5% 4 3 4 6% 26%	2730
May	1st to 10th	Day	28% 15% 4 9% 5% 5% 4 3 4 4 20%	6016
	11th to 20th	Day	19% 10% 3 10% 6% 5% 5% 4 5% 7% 26%	6429
	21st to end	Day	20% 9% 3 7% 5% 5% 6% 5% 7% 8% 27%	7103
June	1st to 10th	Day	26% 10% 3 7% 5% 5% 5% 4 6% 7% 23%	2800
	11th to 20th	Day	19% 8% 3 6% 6% 4 5% 4 6% 7% 32%	2800
	21st to end	Day	22% 9% 3 8% 5% 5% 5% 5% 6% 6% 25%	2800
July	1st to 10th	Day	15% 7% 3 8% 6% 6% 7% 6% 9% 9% 24%	6360
	11th to 20th	Day	13% 7% 2 8% 5% 6% 7% 5% 8% 9% 31%	6360
	21st to end	Day	15% 8% 2 7% 4 5% 5% 4 6% 9% 36%	6926
August	1st to 10th	Day	17% 9% 3 9% 6% 6% 6% 4 7% 7% 26%	4448
	11th to 20th	Day	17% 9% 3 9% 5% 7% 6% 5% 7% 9% 24%	4175
	21st to end	Day	17% 8% 3 6% 4 5% 4 3 6% 8% 35%	4359
September	1st to 10th	Day	16% 8% 2 6% 5% 4 4 4 5% 6% 39%	2400
	11th to 20th	Day	17% 7% 2 6% 4 4 3 3 5% 6% 42%	2220
	21st to end	Day	17% 9% 3 7% 5% 3 5% 4 6% 7% 34%	2200
October	1st to 10th	Day	21% 12% 4 7% 4 4 4 3 5% 6% 30%	2200
	11th to 20th	Day	16% 17% 4 9% 4 5% 3 3 6% 6% 27%	1875
	21st to end	Day	21% 22% 5% 8% 4 4 3 2 3 4 22%	1980
November	1st to 10th	Day	15% 21% 5% 8% 5% 5% 4 3 4 5% 26%	1851
	11th to 20th	Day	15% 21% 7% 12% 6% 4 4 2 4 5% 21%	1890
	21st to end	Day	15% 18% 7% 15% 6% 4 4 3 5% 5% 17%	1769
December	1st to 10th	Day	10% 19% 7% 24% 11% 4 4 3 4 4 11%	2680
	11th to 20th	Day	13% 18% 9% 15% 6% 4 4 2 5% 5% 20%	3066
	21st to end	Day	15% 18% 6% 15% 7% 5% 6% 3 3 4 18%	4048
January	1st to 10th	Night	8% 18% 10% 21% 13% 8% 6% 3 3 3 10%	8234
	11th to 20th	Night	8% 17% 10% 21% 15% 7% 5% 3 3 3 8%	8235
	21st to end	Night	6% 14% 6% 16% 10% 13% 8% 4 4 4 15%	9057
February	1st to 10th	Night	10% 15% 6% 17% 11% 7% 6% 3 4 5% 17%	2833
	11th to 20th	Night	8% 16% 7% 18% 10% 8% 6% 4 3 3 17%	2458
	21st to end	Night	10% 18% 5% 13% 8% 7% 5% 2 4 5% 23%	2076
March	1st to 10th	Night	14% 17% 6% 17% 7% 6% 6% 2 3 4 18%	5484
	11th to 20th	Night	13% 18% 6% 11% 6% 5% 4 3 5% 6% 23%	5330
	21st to end	Night	16% 17% 6% 13% 7% 5% 4 3 4 4 23%	5218
April	1st to 10th	Night	19% 15% 5% 10% 6% 4 4 3 4 5% 25%	2100
	11th to 20th	Night	18% 16% 4 8% 5% 4 4 3 4 5% 29%	2019
	21st to end	Night	21% 13% 4 8% 5% 4 4 3 5% 5% 29%	1890
May	1st to 10th	Night	27% 14% 3 9% 5% 4 4 3 4 5% 22%	4144
	11th to 20th	Night	21% 11% 3 8% 6% 5% 5% 4 5% 7% 27%	3715
	21st to end	Night	19% 10% 3 7% 4 4 4 3 5% 7% 33%	4014
June	1st to 10th	Night	27% 9% 2 7% 4 4 5% 3 4 6% 29%	1600
	11th to 20th	Night	18% 7% 2 6% 5% 5% 5% 4 6% 7% 35%	1600
	21st to end	Night	20% 8% 3 7% 5% 5% 5% 5% 6% 6% 30%	1600
July	1st to 10th	Night	15% 8% 2 7% 6% 6% 6% 5% 7% 9% 29%	3600
	11th to 20th	Night	17% 8% 3 6% 5% 5% 5% 4 6% 6% 35%	3600
	21st to end	Night	12% 8% 2 7% 4 5% 5% 4 7% 8% 38%	3926
August	1st to 10th	Night	22% 9% 3 8% 4 4 5% 3 5% 6% 31%	2668
	11th to 20th	Night	16% 8% 3 9% 5% 6% 6% 5% 7% 7% 28%	2872
	21st to end	Night	17% 9% 3 6% 4 4 4 3 4 7% 39%	3536
September	1st to 10th	Night	16% 7% 2 6% 4 4 5% 3 5% 6% 42%	2000
	11th to 20th	Night	16% 7% 2 6% 3 3 4 3 4 6% 46%	2180
	21st to end	Night	18% 9% 2 5% 4 4 4 3 5% 7% 39%	2200
October	1st to 10th	Night	18% 11% 2 6% 4 5% 5% 4 5% 6% 34%	2200
	11th to 20th	Night	18% 16% 3 8% 5% 4 4 3 4 6% 28%	2520
	21st to end	Night	22% 18% 5% 7% 4 4 5% 3 5% 6% 21%	2860
November	1st to 10th	Night	17% 18% 5% 10% 5% 4 3 3 4 5% 25%	2667
	11th to 20th	Night	14% 18% 7% 14% 6% 4 2 3 4 5% 24%	2730
	21st to end	Night	15% 19% 8% 15% 8% 4 4 3 4 4 18%	2848
December	1st to 10th	Night	12% 17% 9% 27% 11% 5% 4 3 3 3 8%	4720
	11th to 20th	Night	14% 20% 6% 19% 9% 5% 4 2 3 3 14%	5348
	21st to end	Night	15% 17% 7% 16% 7% 5% 5% 3 3 4 18%	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	79% 3 8% 8% 2	4662
	11th to 20th	Day	79% 4 5% 8% 3 1	4680
	21st to end	Day	71% 4 7% 9% 8% 1	5124
February	1st to 10th	Day	72% 3 5% 12% 7% 1	1785
	11th to 20th	Day	73% 3 7% 10% 6% 1	2057
	21st to end	Day	67% 5% 6% 13% 7% 2	1730
March	1st to 10th	Day	71% 4 6% 7% 8% 4	4579
	11th to 20th	Day	59% 5% 11% 14% 9% 2	4847
	21st to end	Day	65% 3 4 13% 13% 1	5953
April	1st to 10th	Day	63% 3 8% 15% 9% 2	2520
	11th to 20th	Day	58% 4 8% 18% 11% 1	2550
	21st to end	Day	61% 4 8% 16% 8% 3	2730
May	1st to 10th	Day	68% 5% 7% 15% 5%	6027
	11th to 20th	Day	57% 7% 13% 16% 6% 1	6433
	21st to end	Day	52% 8% 11% 20% 8%	7103
June	1st to 10th	Day	60% 6% 10% 18% 5% 1	2800
	11th to 20th	Day	50% 6% 11% 22% 10% 1	2800
	21st to end	Day	57% 7% 10% 19% 7%	2800
July	1st to 10th	Day	50% 9% 17% 17% 7%	6290
	11th to 20th	Day	44% 10% 14% 25% 6%	6285
	21st to end	Day	44% 7% 12% 29% 7%	6728
August	1st to 10th	Day	54% 8% 14% 18% 6%	4434
	11th to 20th	Day	54% 9% 15% 18% 4	4214
	21st to end	Day	47% 7% 12% 20% 12% 2	4359
September	1st to 10th	Day	43% 7% 13% 26% 11%	2400
	11th to 20th	Day	42% 4 12% 27% 15% 1	2220
	21st to end	Day	48% 5% 13% 22% 11% 1	2200
October	1st to 10th	Day	51% 6% 11% 22% 10%	3590
	11th to 20th	Day	56% 4 12% 21% 8%	3026
	21st to end	Day	55% 4 13% 19% 9% 1	3180
November	1st to 10th	Day	62% 4 9% 16% 8% 1	1851
	11th to 20th	Day	67% 5% 7% 10% 8% 2	1890
	21st to end	Day	69% 5% 11% 12% 3	1769
December	1st to 10th	Day	79% 4 7% 7% 3	2680
	11th to 20th	Day	68% 5% 5% 12% 9% 1	3066
	21st to end	Day	71% 4 4 9% 9% 3	4048
January	1st to 10th	Night	82% 4 7% 6% 2	8234
	11th to 20th	Night	82% 5% 5% 5% 3 1	8280
	21st to end	Night	72% 4 6% 9% 8% 1	9057
February	1st to 10th	Night	71% 4 7% 11% 6%	2833
	11th to 20th	Night	73% 4 6% 11% 5% 1	2460
	21st to end	Night	66% 4 6% 13% 9% 1	2076
March	1st to 10th	Night	73% 2 7% 10% 6% 2	5494
	11th to 20th	Night	62% 6% 13% 12% 7% 1	5331
	21st to end	Night	66% 3 4 13% 13% 1	5218
April	1st to 10th	Night	63% 5% 7% 15% 10% 1	2100
	11th to 20th	Night	58% 5% 7% 19% 12%	2019
	21st to end	Night	57% 6% 9% 15% 11% 2	1890
May	1st to 10th	Night	64% 6% 11% 13% 5%	4150
	11th to 20th	Night	55% 9% 11% 19% 6% 1	3716
	21st to end	Night	50% 8% 13% 19% 9%	4015
June	1st to 10th	Night	57% 5% 9% 20% 8% 1	1600
	11th to 20th	Night	48% 7% 12% 21% 10% 2	1600
	21st to end	Night	53% 8% 12% 19% 6% 1	1600
July	1st to 10th	Night	49% 9% 16% 20% 5%	3560
	11th to 20th	Night	47% 8% 12% 27% 6%	3552
	21st to end	Night	44% 8% 15% 26% 7%	3809
August	1st to 10th	Night	54% 6% 13% 22% 5%	2658
	11th to 20th	Night	51% 10% 17% 15% 6%	2899
	21st to end	Night	45% 7% 13% 21% 13% 1	3536
September	1st to 10th	Night	43% 6% 14% 28% 9%	2000
	11th to 20th	Night	41% 5% 13% 28% 12% 1	2180
	21st to end	Night	45% 5% 12% 26% 11% 2	2200
October	1st to 10th	Night	49% 8% 11% 22% 10%	3590
	11th to 20th	Night	57% 4 9% 19% 10%	4108
	21st to end	Night	55% 6% 13% 17% 8% 1	4610
November	1st to 10th	Night	64% 4 10% 16% 7%	2667
	11th to 20th	Night	64% 4 8% 14% 8% 2	2730
	21st to end	Night	71% 5% 9% 11% 4	2848
December	1st to 10th	Night	83% 4 5% 6% 2	4720
	11th to 20th	Night	78% 3 4 9% 7%	5348
	21st to end	Night	71% 4 6% 10% 8% 2	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	28% 24% 34% 13% 1	220
	11th to 20th	Day	34% 24% 31% 9% 2	220
	21st to end	Day	27% 27% 29% 14% 3	242
February	1st to 10th	Day	34% 28% 26% 9% 2	297
	11th to 20th	Day	31% 28% 29% 9% 2	536
	21st to end	Day	32% 25% 28% 12% 2	446
March	1st to 10th	Day	33% 26% 27% 10% 4	540
	11th to 20th	Day	35% 26% 24% 11% 3 1	540
	21st to end	Day	37% 22% 23% 11% 5% 1	594
April	1st to 10th	Day	42% 20% 23% 10% 4 1	540
	11th to 20th	Day	38% 22% 24% 9% 5% 2	532
	21st to end	Day	36% 21% 26% 10% 5% 2	540
May	1st to 10th	Day	44% 17% 24% 11% 4 1	543
	11th to 20th	Day	30% 18% 30% 16% 5% 1	567
	21st to end	Day	27% 22% 28% 17% 5% 1	617
June	1st to 10th	Day	33% 15% 31% 14% 7% 1	530
	11th to 20th	Day	25% 17% 29% 19% 8% 2	530
	21st to end	Day	26% 18% 32% 15% 7% 2	530
July	1st to 10th	Day	20% 15% 32% 24% 7% 2	525
	11th to 20th	Day	17% 13% 36% 23% 9% 2	524
	21st to end	Day	21% 14% 33% 25% 6% 2	576
August	1st to 10th	Day	19% 12% 30% 22% 14% 3	526
	11th to 20th	Day	18% 10% 31% 21% 14% 5%	519
	21st to end	Day	25% 13% 23% 22% 12% 5%	583
September	1st to 10th	Day	22% 14% 25% 21% 12% 5%	530
	11th to 20th	Day	22% 10% 22% 21% 17% 8%	530
	21st to end	Day	23% 14% 25% 22% 12% 5%	530
October	1st to 10th	Day	27% 17% 23% 17% 12% 3	530
	11th to 20th	Day	31% 16% 27% 14% 11% 1	454
	21st to end	Day	36% 20% 24% 15% 5%	462
November	1st to 10th	Day	37% 17% 25% 15% 4	425
	11th to 20th	Day	30% 25% 25% 14% 5% 1	430
	21st to end	Day	38% 22% 24% 12% 3 1	309
December	1st to 10th	Day	32% 23% 28% 15% 3	220
	11th to 20th	Day	37% 25% 29% 7% 3	220
	21st to end	Day	37% 19% 29% 12% 3	242
January	1st to 10th	Night	26% 25% 32% 15% 2	840
	11th to 20th	Night	27% 25% 28% 16% 4	840
	21st to end	Night	22% 24% 32% 17% 4 1	924
February	1st to 10th	Night	24% 25% 31% 18% 2	840
	11th to 20th	Night	24% 27% 30% 16% 2	823
	21st to end	Night	25% 22% 32% 19% 2	692
March	1st to 10th	Night	29% 21% 30% 17% 3	840
	11th to 20th	Night	32% 20% 25% 17% 5%	840
	21st to end	Night	30% 20% 25% 17% 7% 1	924
April	1st to 10th	Night	36% 20% 23% 16% 4	840
	11th to 20th	Night	33% 18% 24% 20% 6%	775
	21st to end	Night	31% 18% 26% 19% 5%	630
May	1st to 10th	Night	30% 15% 28% 19% 7% 1	630
	11th to 20th	Night	23% 15% 25% 28% 8% 1	448
	21st to end	Night	25% 10% 26% 30% 9% 1	455
June	1st to 10th	Night	31% 9% 21% 28% 11% 1	400
	11th to 20th	Night	24% 9% 20% 31% 16%	400
	21st to end	Night	24% 9% 20% 36% 11% 1	400
July	1st to 10th	Night	17% 6% 24% 38% 15% 1	390
	11th to 20th	Night	18% 6% 21% 37% 17% 1	388
	21st to end	Night	17% 5% 20% 42% 16% 1	438
August	1st to 10th	Night	19% 5% 15% 39% 21%	441
	11th to 20th	Night	18% 7% 21% 32% 21% 1	592
	21st to end	Night	21% 7% 19% 32% 18% 2	843
September	1st to 10th	Night	21% 8% 21% 33% 16% 2	800
	11th to 20th	Night	21% 8% 17% 26% 25% 2	800
	21st to end	Night	25% 9% 24% 25% 16% 2	800
October	1st to 10th	Night	27% 11% 20% 27% 15% 1	800
	11th to 20th	Night	31% 17% 19% 21% 11% 2	800
	21st to end	Night	34% 18% 23% 20% 5%	880
November	1st to 10th	Night	33% 18% 22% 19% 7%	820
	11th to 20th	Night	28% 22% 24% 17% 8% 2	840
	21st to end	Night	31% 22% 25% 16% 4 1	840
December	1st to 10th	Night	32% 23% 26% 14% 5%	840
	11th to 20th	Night	33% 25% 24% 14% 4	840
	21st to end	Night	35% 20% 26% 15% 3	924

Page generated at 2026-05-29 01:53:25. Version ac07

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