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 2025-04-11.

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% 8% 3 7% 5% 4 5% 4 6% 7% 36%	113195
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 39%	115076
Transparency	Day	27% 42% 6% 13% 9% 2	116760
	Night	27% 40% 7% 14% 10% 2	119675
Seeing	Day	24% 20% 29% 17% 7% 2	15419
	Night	23% 19% 29% 22% 7% 1	24597

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	11% 9% 4 10% 5% 5% 5% 4 5% 6% 37%	3597
	11th to 20th	Day	14% 9% 3 9% 5% 4 5% 4 5% 5% 36%	3570
	21st to end	Day	17% 10% 5% 7% 3 4 4 3 5% 5% 37%	4070
February	1st to 10th	Day	15% 9% 3 7% 7% 6% 5% 4 6% 6% 32%	1837
	11th to 20th	Day	13% 7% 3 8% 6% 5% 6% 4 5% 7% 37%	1948
	21st to end	Day	18% 12% 2 7% 5% 4 3 3 4 5% 36%	1650
March	1st to 10th	Day	20% 10% 3 7% 4 4 4 3 5% 5% 36%	4476
	11th to 20th	Day	14% 7% 3 6% 5% 5% 5% 5% 5% 7% 37%	4671
	21st to end	Day	20% 10% 3 6% 5% 4 4 3 5% 7% 35%	5970
April	1st to 10th	Day	18% 10% 3 6% 5% 5% 4 3 5% 6% 35%	2400
	11th to 20th	Day	17% 8% 2 7% 4 5% 5% 3 5% 6% 39%	2254
	21st to end	Day	20% 6% 3 7% 5% 4 4 4 5% 7% 35%	2384
May	1st to 10th	Day	27% 9% 3 6% 4 4 4 3 5% 7% 28%	4106
	11th to 20th	Day	21% 9% 3 7% 5% 4 4 3 5% 7% 33%	4334
	21st to end	Day	14% 5% 2 6% 4 4 4 4 7% 9% 41%	4837
June	1st to 10th	Day	17% 7% 2 6% 5% 5% 6% 5% 6% 8% 33%	2660
	11th to 20th	Day	17% 6% 2 6% 5% 5% 5% 4 7% 8% 35%	2660
	21st to end	Day	15% 7% 3 7% 5% 4 5% 4 6% 9% 34%	2660
July	1st to 10th	Day	11% 6% 2 7% 6% 6% 6% 5% 7% 9% 35%	4440
	11th to 20th	Day	12% 5% 2 6% 5% 5% 5% 5% 7% 10% 38%	4440
	21st to end	Day	16% 9% 3 8% 5% 6% 5% 5% 6% 7% 30%	4814
August	1st to 10th	Day	20% 6% 3 7% 5% 6% 6% 4 7% 8% 29%	4307
	11th to 20th	Day	14% 5% 2 7% 5% 5% 5% 4 7% 9% 37%	3760
	21st to end	Day	11% 5% 2 6% 5% 5% 6% 5% 7% 9% 40%	4180
September	1st to 10th	Day	13% 6% 2 6% 5% 6% 5% 4 5% 6% 43%	2280
	11th to 20th	Day	14% 5% 3 4 3 2 4 4 4 7% 48%	2161
	21st to end	Day	18% 6% 2 6% 3 4 4 4 5% 7% 40%	2090
October	1st to 10th	Day	22% 7% 3 5% 4 4 4 3 5% 5% 38%	2039
	11th to 20th	Day	13% 7% 3 5% 5% 3 4 4 5% 8% 44%	1886
	21st to end	Day	21% 8% 2 6% 5% 4 4 4 5% 5% 36%	1881
November	1st to 10th	Day	12% 8% 3 5% 4 4 4 2 5% 7% 46%	1761
	11th to 20th	Day	18% 6% 2 4 3 4 4 3 5% 8% 42%	1800
	21st to end	Day	17% 9% 2 5% 5% 3 5% 3 5% 6% 38%	1726
December	1st to 10th	Day	14% 9% 3 7% 5% 3 4 4 5% 7% 40%	2600
	11th to 20th	Day	20% 8% 3 5% 3 3 4 3 5% 6% 39%	2986
	21st to end	Day	18% 11% 5% 8% 4 3 3 3 4 6% 33%	3960
January	1st to 10th	Night	13% 7% 3 9% 5% 5% 5% 4 7% 7% 36%	6349
	11th to 20th	Night	12% 9% 3 9% 5% 4 5% 3 6% 6% 37%	6315
	21st to end	Night	15% 10% 3 6% 5% 5% 5% 4 5% 6% 37%	6884
February	1st to 10th	Night	15% 10% 4 6% 4 4 4 4 6% 6% 37%	2561
	11th to 20th	Night	13% 9% 3 6% 5% 5% 5% 4 7% 7% 37%	2338
	21st to end	Night	17% 9% 2 8% 5% 4 3 3 5% 6% 37%	1980
March	1st to 10th	Night	19% 10% 2 6% 4 4 4 3 5% 6% 38%	5364
	11th to 20th	Night	16% 8% 3 6% 4 4 4 3 6% 7% 38%	5285
	21st to end	Night	20% 10% 3 6% 4 4 3 3 5% 5% 36%	4962
April	1st to 10th	Night	18% 9% 3 7% 5% 5% 6% 4 5% 6% 33%	2000
	11th to 20th	Night	19% 7% 2 6% 5% 4 4 4 4 6% 38%	1882
	21st to end	Night	20% 8% 3 6% 4 3 3 3 5% 6% 40%	1795
May	1st to 10th	Night	26% 9% 3 7% 4 4 4 3 5% 7% 28%	2834
	11th to 20th	Night	20% 9% 2 7% 6% 5% 4 3 5% 6% 30%	2574
	21st to end	Night	16% 7% 2 5% 4 4 4 4 6% 8% 41%	2738
June	1st to 10th	Night	18% 7% 2 6% 4 3 4 3 6% 7% 42%	1520
	11th to 20th	Night	16% 6% 2 6% 4 5% 4 4 7% 9% 37%	1520
	21st to end	Night	17% 6% 2 5% 4 4 3 4 6% 8% 42%	1520
July	1st to 10th	Night	12% 6% 2 5% 5% 4 5% 5% 6% 9% 41%	2520
	11th to 20th	Night	16% 6% 2 6% 5% 3 4 3 5% 9% 40%	2520
	21st to end	Night	17% 8% 3 7% 4 5% 4 4 4 6% 38%	2739
August	1st to 10th	Night	22% 7% 3 6% 4 4 4 4 6% 7% 33%	2589
	11th to 20th	Night	16% 8% 2 6% 4 4 4 3 5% 6% 42%	3064
	21st to end	Night	11% 4 1 5% 4 3 4 3 5% 8% 52%	3476
September	1st to 10th	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	1900
	11th to 20th	Night	13% 6% 2 4 3 3 2 2 5% 6% 53%	2019
	21st to end	Night	18% 7% 3 5% 3 3 4 3 5% 7% 44%	2090
October	1st to 10th	Night	20% 6% 3 6% 4 4 3 3 5% 6% 40%	2137
	11th to 20th	Night	14% 7% 2 6% 3 3 4 3 5% 7% 45%	2293
	21st to end	Night	21% 9% 2 5% 4 4 4 3 5% 7% 38%	2717
November	1st to 10th	Night	13% 7% 3 5% 4 3 4 3 5% 6% 47%	2537
	11th to 20th	Night	18% 7% 3 5% 4 3 4 4 5% 6% 43%	2600
	21st to end	Night	15% 8% 3 5% 4 5% 5% 3 5% 7% 42%	2670
December	1st to 10th	Night	17% 9% 3 6% 4 3 3 3 5% 6% 40%	4580
	11th to 20th	Night	23% 9% 2 5% 3 4 4 3 5% 5% 38%	5208
	21st to end	Night	18% 10% 4 7% 5% 4 3 3 4 5% 36%	6996

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	33% 27% 5% 23% 10% 2	3598
	11th to 20th	Day	34% 27% 5% 18% 10% 6%	3600
	21st to end	Day	38% 28% 4 16% 13% 1	4070
February	1st to 10th	Day	20% 62% 6% 8% 4	1837
	11th to 20th	Day	19% 60% 3 8% 9% 1	1957
	21st to end	Day	17% 62% 4 11% 5% 1	1650
March	1st to 10th	Day	39% 29% 6% 10% 9% 7%	4479
	11th to 20th	Day	33% 28% 10% 21% 6% 2	4671
	21st to end	Day	37% 29% 3 10% 16% 5%	5970
April	1st to 10th	Day	23% 62% 2 7% 5% 2	2400
	11th to 20th	Day	16% 65% 2 8% 8%	2254
	21st to end	Day	18% 65% 2 8% 6% 1	2384
May	1st to 10th	Day	37% 40% 8% 8% 6%	4120
	11th to 20th	Day	36% 39% 10% 12% 3	4338
	21st to end	Day	25% 39% 4 19% 11%	4837
June	1st to 10th	Day	16% 65% 4 10% 5%	2660
	11th to 20th	Day	15% 66% 5% 9% 4 1	2660
	21st to end	Day	15% 65% 4 8% 8%	2660
July	1st to 10th	Day	28% 39% 7% 14% 11% 1	4370
	11th to 20th	Day	27% 39% 10% 15% 9% 1	4365
	21st to end	Day	35% 37% 8% 10% 9% 1	4661
August	1st to 10th	Day	34% 39% 9% 13% 4	4293
	11th to 20th	Day	27% 40% 11% 18% 5%	3797
	21st to end	Day	22% 39% 9% 21% 10% 1	4180
September	1st to 10th	Day	16% 60% 5% 11% 8%	2280
	11th to 20th	Day	16% 59% 3 10% 10% 1	2161
	21st to end	Day	19% 60% 4 9% 7% 1	2090
October	1st to 10th	Day	24% 38% 4 13% 18% 3	3390
	11th to 20th	Day	18% 37% 5% 15% 22% 4	3114
	21st to end	Day	18% 38% 9% 20% 13% 3	3081
November	1st to 10th	Day	19% 57% 6% 11% 6% 1	1761
	11th to 20th	Day	16% 56% 3 9% 14% 2	1800
	21st to end	Day	18% 57% 7% 12% 5% 2	1726
December	1st to 10th	Day	27% 37% 10% 20% 6%	2600
	11th to 20th	Day	32% 32% 8% 13% 13% 2	2986
	21st to end	Day	41% 26% 3 12% 13% 5%	3960
January	1st to 10th	Night	33% 26% 8% 20% 11% 2	6349
	11th to 20th	Night	33% 27% 10% 15% 11% 4	6360
	21st to end	Night	38% 28% 8% 10% 13% 3	6884
February	1st to 10th	Night	18% 62% 5% 9% 5% 1	2561
	11th to 20th	Night	18% 61% 3 6% 11% 2	2340
	21st to end	Night	17% 62% 4 8% 7% 2	1980
March	1st to 10th	Night	36% 29% 7% 12% 14% 3	5374
	11th to 20th	Night	33% 28% 12% 17% 8% 3	5286
	21st to end	Night	38% 29% 3 8% 19% 3	4962
April	1st to 10th	Night	22% 62% 3 8% 5% 1	2000
	11th to 20th	Night	16% 65% 4 7% 8% 1	1882
	21st to end	Night	17% 64% 3 7% 7% 1	1795
May	1st to 10th	Night	37% 40% 8% 10% 4	2840
	11th to 20th	Night	37% 41% 10% 10% 3	2575
	21st to end	Night	27% 40% 9% 16% 9%	2739
June	1st to 10th	Night	15% 65% 2 11% 7%	1520
	11th to 20th	Night	14% 65% 6% 10% 5%	1520
	21st to end	Night	13% 64% 4 11% 7%	1520
July	1st to 10th	Night	25% 39% 11% 12% 12% 1	2480
	11th to 20th	Night	27% 38% 9% 17% 9%	2472
	21st to end	Night	33% 37% 7% 10% 13% 1	2651
August	1st to 10th	Night	36% 40% 7% 11% 5% 1	2579
	11th to 20th	Night	27% 39% 9% 20% 4	3093
	21st to end	Night	15% 38% 9% 28% 11%	3476
September	1st to 10th	Night	14% 61% 4 13% 8% 1	1900
	11th to 20th	Night	14% 60% 4 10% 11% 1	2019
	21st to end	Night	19% 60% 3 10% 8%	2090
October	1st to 10th	Night	24% 39% 7% 15% 13% 1	3566
	11th to 20th	Night	14% 36% 6% 17% 22% 5%	3804
	21st to end	Night	18% 37% 10% 22% 11% 1	4467
November	1st to 10th	Night	18% 56% 5% 10% 9% 1	2537
	11th to 20th	Night	17% 56% 5% 8% 12% 2	2600
	21st to end	Night	16% 58% 7% 13% 6%	2670
December	1st to 10th	Night	27% 36% 8% 23% 6%	4580
	11th to 20th	Night	35% 31% 8% 9% 14% 2	5208
	21st to end	Night	39% 26% 4 12% 14% 5%	6996

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% 31% 8% 2	200
	11th to 20th	Day	31% 34% 26% 8% 1 1	200
	21st to end	Day	25% 34% 30% 8% 2 1	243
February	1st to 10th	Day	33% 34% 26% 6% 1	337
	11th to 20th	Day	32% 33% 26% 8% 1	496
	21st to end	Day	31% 29% 26% 11% 3	414
March	1st to 10th	Day	30% 27% 30% 10% 2	500
	11th to 20th	Day	31% 26% 31% 10% 2 1	500
	21st to end	Day	34% 27% 26% 8% 3 2	550
April	1st to 10th	Day	32% 27% 26% 10% 5% 1	500
	11th to 20th	Day	31% 25% 27% 12% 4 2	452
	21st to end	Day	29% 25% 28% 13% 5% 1	460
May	1st to 10th	Day	34% 23% 23% 13% 6% 1	463
	11th to 20th	Day	22% 20% 33% 20% 4 1	487
	21st to end	Day	22% 14% 31% 20% 10% 2	529
June	1st to 10th	Day	19% 16% 34% 20% 9% 2	490
	11th to 20th	Day	18% 16% 34% 22% 8% 2	490
	21st to end	Day	18% 19% 31% 22% 8% 3	490
July	1st to 10th	Day	13% 11% 34% 27% 14% 1	485
	11th to 20th	Day	13% 8% 36% 30% 11% 2	484
	21st to end	Day	15% 11% 40% 24% 9% 2	532
August	1st to 10th	Day	16% 9% 29% 31% 10% 5%	486
	11th to 20th	Day	16% 10% 32% 25% 11% 6%	479
	21st to end	Day	12% 13% 31% 25% 14% 5%	539
September	1st to 10th	Day	19% 12% 24% 24% 16% 5%	490
	11th to 20th	Day	17% 13% 25% 23% 14% 9%	490
	21st to end	Day	22% 13% 31% 19% 11% 4	490
October	1st to 10th	Day	27% 14% 26% 17% 12% 4	490
	11th to 20th	Day	20% 20% 28% 24% 6% 2	483
	21st to end	Day	30% 17% 29% 16% 7%	429
November	1st to 10th	Day	22% 23% 26% 20% 8% 1	395
	11th to 20th	Day	30% 25% 26% 14% 4 1	400
	21st to end	Day	29% 25% 26% 15% 4 1	326
December	1st to 10th	Day	26% 31% 28% 11% 5%	200
	11th to 20th	Day	41% 23% 29% 6% 2	200
	21st to end	Day	24% 31% 31% 11% 2 1	220
January	1st to 10th	Night	23% 30% 31% 15% 2	800
	11th to 20th	Night	27% 24% 31% 15% 2	800
	21st to end	Night	20% 28% 32% 18% 2	880
February	1st to 10th	Night	27% 23% 31% 18% 2	800
	11th to 20th	Night	25% 31% 32% 12% 1	783
	21st to end	Night	28% 27% 27% 15% 2	660
March	1st to 10th	Night	26% 26% 32% 14% 3	800
	11th to 20th	Night	24% 29% 29% 16% 3	800
	21st to end	Night	26% 26% 29% 15% 3 1	880
April	1st to 10th	Night	26% 22% 34% 14% 4	800
	11th to 20th	Night	24% 22% 31% 16% 6%	754
	21st to end	Night	25% 23% 29% 18% 4	655
May	1st to 10th	Night	28% 18% 30% 19% 6%	570
	11th to 20th	Night	23% 14% 36% 23% 4	427
	21st to end	Night	19% 11% 29% 32% 8%	411
June	1st to 10th	Night	19% 11% 32% 28% 9% 1	380
	11th to 20th	Night	19% 11% 27% 33% 10%	380
	21st to end	Night	19% 8% 29% 33% 9% 1	380
July	1st to 10th	Night	16% 5% 25% 38% 16% 1	370
	11th to 20th	Night	15% 5% 24% 39% 15% 1	368
	21st to end	Night	12% 8% 27% 40% 13%	416
August	1st to 10th	Night	15% 5% 25% 41% 14% 1	422
	11th to 20th	Night	16% 6% 25% 33% 17% 2	729
	21st to end	Night	16% 8% 23% 32% 19% 2	836
September	1st to 10th	Night	16% 8% 30% 31% 14% 2	760
	11th to 20th	Night	17% 10% 24% 28% 19% 3	760
	21st to end	Night	21% 9% 26% 27% 14% 3	760
October	1st to 10th	Night	26% 11% 25% 25% 11% 2	760
	11th to 20th	Night	20% 18% 28% 26% 8%	760
	21st to end	Night	27% 18% 29% 19% 6%	836
November	1st to 10th	Night	18% 20% 30% 23% 7% 1	780
	11th to 20th	Night	26% 23% 27% 17% 6% 1	800
	21st to end	Night	24% 22% 30% 20% 4 1	800
December	1st to 10th	Night	26% 24% 30% 16% 4	800
	11th to 20th	Night	27% 26% 29% 17% 2	800
	21st to end	Night	27% 26% 30% 14% 3	880

Page generated at 2025-04-08 09:17:23. Version ac07

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