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 2025-02-06.

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% 3 5% 6% 25%	108350
	Night	15% 14% 5% 13% 7% 5% 5% 3 4 5% 23%	109910
Transparency	Day	59% 6% 10% 17% 8% 1	111879
	Night	63% 5% 9% 14% 7% 1	114545
Seeing	Day	30% 18% 27% 16% 7% 2	14976
	Night	27% 17% 24% 22% 9% 1	23977

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% 16% 11% 21% 11% 9% 4 2 3 3 9%	3597
	11th to 20th	Day	6% 15% 6% 25% 16% 6% 4 2 4 3 11%	3570
	21st to end	Day	8% 15% 9% 17% 7% 8% 8% 2 4 4 18%	3960
February	1st to 10th	Day	12% 17% 7% 15% 11% 7% 6% 2 3 3 18%	1654
	11th to 20th	Day	9% 16% 10% 17% 10% 7% 5% 3 3 4 16%	1848
	21st to end	Day	11% 18% 5% 15% 8% 7% 4 3 4 4 20%	1570
March	1st to 10th	Day	20% 18% 5% 12% 6% 5% 3 3 3 4 21%	3116
	11th to 20th	Day	14% 22% 8% 12% 6% 3 4 3 3 4 22%	3312
	21st to end	Day	17% 15% 5% 14% 5% 4 3 2 3 5% 27%	4062
April	1st to 10th	Day	18% 19% 5% 11% 5% 3 3 3 3 4 26%	2280
	11th to 20th	Day	16% 15% 4 8% 5% 5% 4 3 4 5% 32%	2304
	21st to end	Day	21% 16% 4 9% 4 5% 4 3 4 6% 25%	2470
May	1st to 10th	Day	26% 13% 3 7% 5% 5% 4 3 5% 5% 25%	4106
	11th to 20th	Day	20% 10% 3 9% 5% 6% 5% 4 5% 7% 26%	4384
	21st to end	Day	15% 7% 3 7% 5% 6% 6% 5% 8% 9% 30%	4837
June	1st to 10th	Day	27% 10% 3 7% 5% 5% 5% 4 5% 7% 23%	2660
	11th to 20th	Day	20% 7% 3 6% 6% 4 5% 4 6% 7% 32%	2660
	21st to end	Day	23% 9% 3 8% 5% 6% 5% 4 6% 6% 25%	2660
July	1st to 10th	Day	15% 6% 3 7% 5% 5% 6% 6% 8% 10% 29%	4440
	11th to 20th	Day	13% 8% 2 8% 5% 6% 7% 6% 8% 9% 28%	4440
	21st to end	Day	16% 8% 3 7% 5% 5% 6% 4 5% 7% 34%	4814
August	1st to 10th	Day	17% 9% 3 10% 6% 6% 6% 5% 7% 7% 25%	4311
	11th to 20th	Day	17% 9% 3 9% 5% 7% 6% 5% 7% 9% 23%	4045
	21st to end	Day	17% 8% 3 6% 4 5% 4 3 5% 8% 35%	4226
September	1st to 10th	Day	17% 8% 2 7% 5% 4 4 3 5% 6% 39%	2280
	11th to 20th	Day	18% 8% 2 6% 4 4 4 3 5% 6% 41%	2109
	21st to end	Day	18% 9% 3 7% 5% 3 5% 4 6% 7% 33%	2090
October	1st to 10th	Day	21% 13% 4 7% 4 4 4 3 6% 6% 28%	2090
	11th to 20th	Day	16% 17% 4 9% 4 5% 3 3 6% 6% 27%	1782
	21st to end	Day	21% 22% 5% 8% 4 4 3 2 3 4 23%	1881
November	1st to 10th	Day	15% 20% 5% 8% 5% 5% 4 3 4 5% 27%	1761
	11th to 20th	Day	15% 19% 6% 12% 6% 4 4 3 4 5% 22%	1800
	21st to end	Day	16% 18% 7% 14% 6% 4 5% 3 5% 5% 18%	1685
December	1st to 10th	Day	10% 20% 7% 24% 11% 4 4 3 3 4 10%	2600
	11th to 20th	Day	13% 18% 9% 15% 5% 4 4 2 5% 5% 20%	2986
	21st to end	Day	16% 18% 6% 15% 7% 5% 7% 3 3 4 18%	3960
January	1st to 10th	Night	9% 18% 11% 23% 12% 7% 5% 2 2 3 9%	6349
	11th to 20th	Night	8% 17% 9% 21% 16% 7% 5% 3 3 3 8%	6315
	21st to end	Night	8% 16% 7% 16% 8% 9% 7% 4 4 4 17%	6996
February	1st to 10th	Night	11% 16% 6% 16% 11% 7% 5% 3 3 5% 16%	2636
	11th to 20th	Night	8% 17% 8% 18% 10% 8% 6% 4 3 3 17%	2218
	21st to end	Night	11% 18% 5% 12% 8% 7% 5% 2 4 5% 23%	1884
March	1st to 10th	Night	19% 19% 5% 9% 5% 5% 4 2 4 4 23%	3724
	11th to 20th	Night	14% 23% 8% 12% 5% 5% 4 2 3 3 21%	3645
	21st to end	Night	18% 19% 6% 12% 7% 3 3 2 3 4 25%	3567
April	1st to 10th	Night	19% 16% 4 9% 6% 5% 4 3 4 5% 25%	1900
	11th to 20th	Night	17% 16% 4 8% 5% 4 4 3 4 5% 30%	1825
	21st to end	Night	22% 14% 4 7% 5% 4 4 3 5% 5% 28%	1710
May	1st to 10th	Night	25% 11% 3 8% 5% 4 4 3 5% 6% 26%	2834
	11th to 20th	Night	20% 8% 3 8% 6% 5% 5% 4 5% 7% 28%	2539
	21st to end	Night	18% 7% 2 6% 4 4 4 4 6% 8% 36%	2738
June	1st to 10th	Night	27% 9% 2 7% 4 4 5% 3 4 6% 29%	1520
	11th to 20th	Night	19% 7% 2 6% 5% 4 5% 4 6% 7% 35%	1520
	21st to end	Night	21% 8% 3 7% 5% 5% 5% 4 6% 6% 29%	1520
July	1st to 10th	Night	14% 7% 2 7% 7% 5% 6% 5% 6% 8% 31%	2520
	11th to 20th	Night	19% 8% 3 6% 5% 4 4 4 5% 6% 34%	2520
	21st to end	Night	14% 8% 2 8% 5% 5% 5% 4 6% 6% 36%	2739
August	1st to 10th	Night	23% 9% 3 8% 5% 4 5% 3 5% 6% 30%	2586
	11th to 20th	Night	16% 8% 4 9% 5% 6% 6% 5% 7% 7% 27%	2782
	21st to end	Night	17% 9% 3 6% 4 4 4 3 4 7% 39%	3428
September	1st to 10th	Night	16% 7% 2 6% 4 4 5% 3 5% 7% 41%	1900
	11th to 20th	Night	17% 7% 2 6% 3 3 3 3 4 6% 45%	2071
	21st to end	Night	18% 9% 2 5% 4 3 4 3 5% 7% 39%	2090
October	1st to 10th	Night	19% 12% 2 7% 4 5% 5% 4 5% 6% 32%	2090
	11th to 20th	Night	19% 16% 3 8% 5% 5% 4 3 4 5% 28%	2394
	21st to end	Night	22% 17% 5% 7% 4 4 5% 3 5% 6% 22%	2717
November	1st to 10th	Night	17% 18% 5% 10% 5% 4 3 3 5% 5% 25%	2537
	11th to 20th	Night	14% 17% 6% 13% 6% 4 2 3 4 5% 25%	2600
	21st to end	Night	16% 19% 8% 14% 8% 4 4 2 4 4 19%	2712
December	1st to 10th	Night	12% 17% 9% 27% 10% 5% 4 3 3 3 8%	4580
	11th to 20th	Night	15% 20% 6% 19% 9% 5% 4 2 3 3 14%	5208
	21st to end	Night	16% 17% 7% 15% 7% 5% 5% 3 3 4 18%	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	81% 3 6% 8% 1	3598
	11th to 20th	Day	77% 4 5% 9% 3 1	3600
	21st to end	Day	70% 4 8% 9% 7% 1	3960
February	1st to 10th	Day	73% 4 5% 11% 7% 2	1654
	11th to 20th	Day	74% 3 7% 10% 5% 1	1857
	21st to end	Day	67% 5% 7% 12% 7% 2	1570
March	1st to 10th	Day	70% 4 6% 9% 9% 2	3119
	11th to 20th	Day	68% 4 6% 11% 8% 2	3312
	21st to end	Day	62% 3 4 15% 14% 1	4062
April	1st to 10th	Day	63% 3 8% 15% 9% 2	2280
	11th to 20th	Day	56% 4 9% 19% 11% 1	2304
	21st to end	Day	62% 3 8% 15% 8% 3	2470
May	1st to 10th	Day	61% 5% 8% 18% 6% 1	4120
	11th to 20th	Day	56% 7% 15% 15% 6% 1	4388
	21st to end	Day	47% 9% 12% 22% 9%	4837
June	1st to 10th	Day	60% 6% 10% 18% 5% 1	2660
	11th to 20th	Day	50% 6% 11% 22% 10% 1	2660
	21st to end	Day	58% 6% 10% 19% 6%	2660
July	1st to 10th	Day	46% 9% 17% 21% 8%	4370
	11th to 20th	Day	47% 10% 15% 25% 3	4365
	21st to end	Day	48% 8% 10% 27% 7%	4661
August	1st to 10th	Day	55% 8% 14% 17% 5%	4297
	11th to 20th	Day	54% 9% 15% 17% 4	4084
	21st to end	Day	46% 7% 12% 20% 12% 2	4226
September	1st to 10th	Day	45% 7% 13% 25% 11%	2280
	11th to 20th	Day	43% 4 11% 27% 14% 1	2109
	21st to end	Day	49% 5% 13% 21% 10% 1	2090
October	1st to 10th	Day	52% 6% 11% 21% 9%	3480
	11th to 20th	Day	56% 4 12% 21% 8%	2933
	21st to end	Day	54% 4 14% 19% 9% 1	3081
November	1st to 10th	Day	61% 4 9% 17% 8% 1	1761
	11th to 20th	Day	66% 5% 7% 11% 9% 3	1800
	21st to end	Day	68% 5% 11% 12% 3	1685
December	1st to 10th	Day	80% 4 7% 7% 3	2600
	11th to 20th	Day	68% 5% 5% 12% 9% 1	2986
	21st to end	Day	71% 4 4 9% 9% 3	3960
January	1st to 10th	Night	83% 4 6% 5% 2	6349
	11th to 20th	Night	82% 5% 5% 6% 1 1	6360
	21st to end	Night	70% 5% 7% 9% 8% 1	6996
February	1st to 10th	Night	72% 4 7% 10% 6% 1	2636
	11th to 20th	Night	74% 4 5% 11% 5% 1	2220
	21st to end	Night	66% 4 6% 13% 9% 1	1884
March	1st to 10th	Night	67% 3 8% 13% 7% 2	3734
	11th to 20th	Night	69% 6% 6% 11% 7% 1	3646
	21st to end	Night	67% 2 4 14% 12% 1	3567
April	1st to 10th	Night	63% 5% 7% 15% 10% 1	1900
	11th to 20th	Night	56% 5% 8% 19% 12%	1825
	21st to end	Night	58% 6% 9% 15% 11% 2	1710
May	1st to 10th	Night	59% 6% 13% 16% 6% 1	2840
	11th to 20th	Night	53% 9% 12% 18% 7% 1	2540
	21st to end	Night	44% 9% 15% 19% 12%	2739
June	1st to 10th	Night	57% 5% 10% 20% 7%	1520
	11th to 20th	Night	48% 7% 12% 22% 9% 2	1520
	21st to end	Night	54% 8% 12% 19% 6% 1	1520
July	1st to 10th	Night	48% 9% 14% 21% 8%	2480
	11th to 20th	Night	50% 7% 13% 26% 5%	2472
	21st to end	Night	47% 8% 13% 24% 8%	2651
August	1st to 10th	Night	56% 6% 13% 21% 4	2576
	11th to 20th	Night	52% 10% 18% 15% 6%	2809
	21st to end	Night	45% 7% 13% 21% 13% 1	3428
September	1st to 10th	Night	43% 6% 14% 27% 9%	1900
	11th to 20th	Night	42% 5% 13% 28% 12% 1	2071
	21st to end	Night	44% 5% 12% 26% 10% 2	2090
October	1st to 10th	Night	50% 8% 11% 22% 9%	3480
	11th to 20th	Night	57% 4 9% 19% 10%	3982
	21st to end	Night	54% 6% 13% 17% 8% 1	4467
November	1st to 10th	Night	63% 4 10% 16% 7% 1	2537
	11th to 20th	Night	62% 4 8% 15% 9% 2	2600
	21st to end	Night	71% 5% 9% 11% 4	2712
December	1st to 10th	Night	84% 4 5% 6% 2	4580
	11th to 20th	Night	78% 3 4 9% 7%	5208
	21st to end	Night	70% 5% 6% 10% 8% 2	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	28% 24% 35% 13% 2	200
	11th to 20th	Day	33% 23% 33% 10% 2 1	200
	21st to end	Day	27% 27% 29% 14% 4	220
February	1st to 10th	Day	39% 28% 23% 8% 2	256
	11th to 20th	Day	32% 29% 28% 8% 2	456
	21st to end	Day	34% 24% 27% 13% 2	382
March	1st to 10th	Day	34% 25% 26% 11% 4	460
	11th to 20th	Day	34% 25% 25% 12% 3 1	460
	21st to end	Day	35% 22% 24% 12% 6% 2	506
April	1st to 10th	Day	42% 19% 22% 11% 5% 1	460
	11th to 20th	Day	36% 23% 24% 10% 6% 2	452
	21st to end	Day	36% 21% 27% 10% 5% 2	460
May	1st to 10th	Day	42% 16% 24% 12% 5% 1	463
	11th to 20th	Day	28% 17% 31% 17% 5% 1	487
	21st to end	Day	25% 21% 29% 18% 6% 1	529
June	1st to 10th	Day	34% 14% 31% 13% 7% 1	490
	11th to 20th	Day	25% 16% 29% 20% 9% 2	490
	21st to end	Day	27% 17% 33% 14% 7% 2	490
July	1st to 10th	Day	20% 13% 32% 25% 7% 2	485
	11th to 20th	Day	18% 12% 37% 22% 9% 2	484
	21st to end	Day	21% 12% 34% 25% 6% 2	532
August	1st to 10th	Day	21% 12% 30% 22% 13% 2	486
	11th to 20th	Day	19% 11% 31% 22% 13% 4	479
	21st to end	Day	25% 14% 22% 22% 12% 5%	539
September	1st to 10th	Day	23% 14% 25% 21% 12% 5%	490
	11th to 20th	Day	24% 10% 21% 21% 17% 8%	490
	21st to end	Day	23% 13% 24% 23% 12% 5%	490
October	1st to 10th	Day	28% 17% 24% 16% 12% 3	490
	11th to 20th	Day	31% 16% 28% 13% 11% 1	421
	21st to end	Day	37% 19% 23% 15% 6%	429
November	1st to 10th	Day	36% 18% 25% 16% 5%	395
	11th to 20th	Day	32% 23% 25% 14% 5% 1	400
	21st to end	Day	38% 21% 25% 12% 3 1	285
December	1st to 10th	Day	33% 24% 26% 15% 4	200
	11th to 20th	Day	37% 24% 29% 8% 3	200
	21st to end	Day	37% 18% 30% 11% 3	220
January	1st to 10th	Night	27% 24% 32% 15% 2	800
	11th to 20th	Night	26% 25% 28% 16% 4	800
	21st to end	Night	21% 25% 33% 18% 4 1	880
February	1st to 10th	Night	26% 25% 30% 16% 2	781
	11th to 20th	Night	25% 28% 29% 15% 3	743
	21st to end	Night	26% 22% 30% 19% 2	628
March	1st to 10th	Night	29% 20% 30% 19% 3	760
	11th to 20th	Night	31% 20% 25% 18% 6% 1	760
	21st to end	Night	31% 20% 23% 17% 8% 1	836
April	1st to 10th	Night	35% 21% 23% 16% 5%	760
	11th to 20th	Night	31% 17% 24% 21% 7%	701
	21st to end	Night	32% 18% 25% 19% 5%	570
May	1st to 10th	Night	29% 15% 28% 19% 8% 1	570
	11th to 20th	Night	22% 16% 25% 29% 8% 1	405
	21st to end	Night	24% 9% 26% 31% 10% 1	411
June	1st to 10th	Night	31% 8% 21% 28% 11% 1	380
	11th to 20th	Night	23% 9% 19% 31% 17%	380
	21st to end	Night	25% 8% 19% 36% 11% 1	380
July	1st to 10th	Night	18% 6% 24% 37% 15% 1	370
	11th to 20th	Night	18% 6% 21% 37% 17% 1	368
	21st to end	Night	18% 4 19% 41% 17% 1	416
August	1st to 10th	Night	20% 5% 16% 38% 21%	419
	11th to 20th	Night	18% 7% 22% 32% 20% 1	562
	21st to end	Night	22% 6% 19% 32% 18% 2	801
September	1st to 10th	Night	21% 8% 21% 32% 16% 2	760
	11th to 20th	Night	22% 8% 18% 26% 24% 3	760
	21st to end	Night	25% 8% 23% 25% 17% 2	760
October	1st to 10th	Night	28% 11% 20% 26% 14% 1	760
	11th to 20th	Night	31% 17% 19% 20% 11% 2	760
	21st to end	Night	35% 18% 23% 19% 5%	836
November	1st to 10th	Night	32% 18% 23% 19% 8%	780
	11th to 20th	Night	28% 22% 24% 17% 8% 2	800
	21st to end	Night	32% 22% 24% 17% 4 1	800
December	1st to 10th	Night	33% 23% 26% 14% 5%	800
	11th to 20th	Night	33% 24% 23% 15% 4	800
	21st to end	Night	35% 20% 26% 15% 3	880

Page generated at 2025-02-03 03:05:56. Version ac07

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