Colby Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Colby Observing Site clear sky chart from 2005-11-06 to 2025-10-04.

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	13% 5% 2 6% 4 4 5% 4 6% 8% 42%	126475
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 44%	122919
Transparency	Day	38% 5% 12% 21% 18% 6%	130147
	Night	38% 5% 10% 21% 20% 6%	127316
Seeing	Day	18% 6% 20% 26% 20% 10%	16407
	Night	17% 6% 20% 33% 20% 3	26442

By Month

Percentage Cloud Cover SE (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	13% 5% 2 4 3 2 2 2 3 6% 58%	4056
	11th to 20th	Day	11% 5% 1 4 3 2 3 2 3 5% 60%	4060
	21st to end	Day	19% 5% 2 5% 3 3 4 3 4 5% 49%	4466
February	1st to 10th	Day	15% 6% 2 5% 4 3 3 4 6% 8% 44%	1800
	11th to 20th	Day	18% 7% 3 5% 3 4 4 3 5% 6% 43%	1943
	21st to end	Day	18% 6% 2 5% 4 3 4 4 5% 6% 43%	1660
March	1st to 10th	Day	19% 7% 2 6% 3 4 4 3 4 6% 42%	4935
	11th to 20th	Day	10% 5% 2 5% 4 3 3 2 4 6% 55%	4980
	21st to end	Day	17% 6% 2 5% 4 3 4 3 4 5% 47%	5465
April	1st to 10th	Day	12% 4 1 4 3 4 5% 4 7% 8% 49%	2200
	11th to 20th	Day	15% 4 1 5% 3 4 4 3 5% 6% 48%	2368
	21st to end	Day	14% 5% 1 5% 5% 4 5% 3 5% 7% 46%	2600
May	1st to 10th	Day	11% 6% 3 7% 5% 6% 6% 5% 7% 9% 35%	5862
	11th to 20th	Day	19% 7% 3 8% 5% 5% 6% 5% 6% 8% 29%	5861
	21st to end	Day	13% 5% 2 6% 4 4 5% 4 6% 9% 40%	6433
June	1st to 10th	Day	11% 5% 2 6% 5% 6% 6% 5% 7% 9% 37%	2600
	11th to 20th	Day	7% 4 2 7% 6% 6% 6% 5% 7% 11% 38%	2600
	21st to end	Day	7% 4 2 5% 6% 5% 7% 6% 8% 11% 39%	2600
July	1st to 10th	Day	7% 5% 3 8% 6% 6% 7% 6% 8% 10% 36%	5900
	11th to 20th	Day	7% 5% 3 8% 6% 7% 7% 6% 9% 12% 29%	5900
	21st to end	Day	10% 8% 3 9% 6% 7% 7% 6% 9% 10% 25%	6424
August	1st to 10th	Day	16% 8% 3 7% 5% 6% 6% 5% 8% 11% 25%	4234
	11th to 20th	Day	10% 5% 2 6% 5% 6% 6% 6% 7% 11% 35%	4174
	21st to end	Day	7% 4 2 5% 4 4 4 4 7% 10% 49%	4424
September	1st to 10th	Day	11% 5% 2 5% 5% 4 5% 4 7% 8% 43%	2200
	11th to 20th	Day	17% 6% 2 6% 5% 4 5% 4 5% 7% 40%	2200
	21st to end	Day	21% 6% 2 6% 4 4 4 4 5% 6% 38%	2200
October	1st to 10th	Day	15% 5% 3 4 2 3 3 2 4 6% 54%	2030
	11th to 20th	Day	14% 4 2 3 3 2 3 3 4 6% 56%	1900
	21st to end	Day	15% 4 2 4 2 2 2 2 4 4 57%	2090
November	1st to 10th	Day	13% 6% 1 4 3 3 3 2 3 5% 55%	1941
	11th to 20th	Day	14% 4 2 4 3 2 3 2 4 7% 54%	1800
	21st to end	Day	14% 4 1 4 3 3 4 3 5% 6% 53%	1800
December	1st to 10th	Day	15% 5% 2 4 3 3 4 3 4 4 53%	2930
	11th to 20th	Day	17% 5% 3 5% 4 3 3 4 5% 6% 45%	3373
	21st to end	Day	21% 6% 1 4 3 3 3 3 4 8% 44%	4466
January	1st to 10th	Night	13% 4 1 4 2 3 3 2 3 5% 60%	5889
	11th to 20th	Night	11% 6% 2 4 3 3 3 3 4 5% 57%	5900
	21st to end	Night	18% 6% 2 4 3 3 3 3 5% 6% 48%	6490
February	1st to 10th	Night	15% 5% 2 5% 5% 4 5% 4 5% 7% 45%	2600
	11th to 20th	Night	19% 7% 2 4 4 3 4 2 5% 4 46%	2338
	21st to end	Night	16% 6% 2 5% 4 4 4 3 5% 6% 45%	1970
March	1st to 10th	Night	19% 6% 2 5% 4 3 4 3 4 5% 45%	4918
	11th to 20th	Night	9% 4 1 4 3 3 4 3 5% 8% 56%	4980
	21st to end	Night	15% 5% 2 5% 4 3 4 3 5% 7% 46%	5467
April	1st to 10th	Night	12% 5% 2 5% 4 4 4 4 5% 7% 49%	2200
	11th to 20th	Night	14% 5% 2 4 4 3 3 3 5% 8% 49%	1995
	21st to end	Night	14% 5% 2 5% 4 3 4 3 5% 7% 49%	1800
May	1st to 10th	Night	14% 7% 2 6% 5% 5% 4 4 6% 9% 36%	4033
	11th to 20th	Night	21% 7% 3 8% 4 5% 5% 4 5% 9% 30%	4032
	21st to end	Night	15% 6% 2 7% 4 3 4 4 4 8% 42%	4421
June	1st to 10th	Night	10% 5% 2 5% 4 5% 7% 6% 8% 11% 37%	1800
	11th to 20th	Night	10% 6% 2 6% 5% 5% 5% 5% 7% 9% 40%	1800
	21st to end	Night	6% 5% 2 6% 6% 5% 6% 5% 8% 11% 39%	1800
July	1st to 10th	Night	9% 7% 3 8% 6% 6% 6% 5% 8% 10% 32%	4058
	11th to 20th	Night	9% 6% 3 9% 7% 5% 6% 6% 8% 11% 29%	4057
	21st to end	Night	10% 10% 4 10% 6% 6% 7% 5% 8% 10% 24%	4427
August	1st to 10th	Night	16% 8% 3 8% 6% 6% 6% 5% 7% 9% 26%	2915
	11th to 20th	Night	12% 7% 3 10% 6% 6% 6% 5% 7% 9% 29%	2875
	21st to end	Night	7% 5% 2 6% 5% 5% 5% 4 7% 10% 45%	3435
September	1st to 10th	Night	12% 5% 2 7% 4 4 5% 3 5% 7% 44%	2200
	11th to 20th	Night	16% 6% 2 8% 6% 5% 4 4 6% 6% 38%	2200
	21st to end	Night	21% 6% 2 6% 4 5% 5% 3 4 6% 39%	2200
October	1st to 10th	Night	17% 5% 1 5% 4 3 3 4 4 5% 49%	2215
	11th to 20th	Night	15% 4 2 4 3 3 4 3 4 6% 53%	2280
	21st to end	Night	14% 4 1 4 3 3 3 2 4 5% 58%	2508
November	1st to 10th	Night	15% 5% 2 5% 4 3 4 3 3 4 53%	2354
	11th to 20th	Night	14% 5% 1 4 3 3 3 3 5% 6% 53%	2600
	21st to end	Night	13% 4 2 4 3 4 3 4 5% 7% 51%	2600
December	1st to 10th	Night	13% 6% 2 4 3 3 3 3 5% 5% 53%	4250
	11th to 20th	Night	20% 9% 2 6% 4 3 4 3 4 4 42%	4822
	21st to end	Night	21% 5% 2 4 3 3 3 3 5% 6% 46%	6490

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	31% 2 5% 20% 35% 7%	4057
	11th to 20th	Day	29% 2 5% 17% 37% 9%	4060
	21st to end	Day	38% 2 4 23% 26% 6%	4466
February	1st to 10th	Day	38% 3 11% 23% 20% 5%	1800
	11th to 20th	Day	42% 3 9% 21% 20% 5%	1953
	21st to end	Day	42% 5% 8% 17% 21% 7%	1660
March	1st to 10th	Day	43% 4 5% 16% 19% 13%	4935
	11th to 20th	Day	32% 3 6% 17% 23% 18%	4980
	21st to end	Day	40% 3 6% 25% 19% 5%	5465
April	1st to 10th	Day	32% 4 11% 24% 23% 5%	2200
	11th to 20th	Day	35% 4 9% 21% 23% 7%	2368
	21st to end	Day	38% 4 12% 26% 16% 4	2600
May	1st to 10th	Day	44% 6% 16% 20% 12% 2	5897
	11th to 20th	Day	52% 6% 12% 19% 10% 1	5865
	21st to end	Day	39% 5% 14% 20% 17% 4	6447
June	1st to 10th	Day	40% 7% 17% 23% 11% 1	2600
	11th to 20th	Day	38% 8% 18% 21% 14% 2	2600
	21st to end	Day	35% 8% 19% 21% 16% 1	2600
July	1st to 10th	Day	40% 9% 18% 21% 11% 2	5835
	11th to 20th	Day	43% 10% 26% 16% 4	5831
	21st to end	Day	49% 9% 16% 19% 6% 1	6240
August	1st to 10th	Day	50% 8% 19% 19% 5%	4221
	11th to 20th	Day	41% 7% 23% 18% 10% 2	4213
	21st to end	Day	29% 6% 16% 36% 13% 2	4424
September	1st to 10th	Day	38% 7% 13% 21% 18% 3	2200
	11th to 20th	Day	43% 6% 11% 20% 17% 3	2200
	21st to end	Day	45% 5% 10% 20% 16% 4	2200
October	1st to 10th	Day	28% 3 7% 22% 31% 10%	3355
	11th to 20th	Day	21% 2 6% 21% 26% 24%	3140
	21st to end	Day	27% 2 7% 25% 22% 16%	3428
November	1st to 10th	Day	33% 3 8% 21% 28% 7%	1941
	11th to 20th	Day	33% 3 7% 23% 27% 8%	1800
	21st to end	Day	33% 3 8% 24% 26% 6%	1800
December	1st to 10th	Day	35% 3 6% 20% 28% 8%	2930
	11th to 20th	Day	40% 4 10% 18% 21% 7%	3370
	21st to end	Day	40% 4 10% 29% 16% 2	4466
January	1st to 10th	Night	29% 3 5% 20% 33% 10%	5889
	11th to 20th	Night	30% 3 5% 19% 34% 9%	5900
	21st to end	Night	38% 2 6% 21% 24% 9%	6490
February	1st to 10th	Night	38% 5% 11% 22% 20% 5%	2600
	11th to 20th	Night	42% 3 8% 18% 21% 6%	2339
	21st to end	Night	41% 3 9% 19% 22% 7%	1970
March	1st to 10th	Night	41% 4 5% 16% 25% 9%	4918
	11th to 20th	Night	27% 4 6% 19% 26% 18%	4980
	21st to end	Night	39% 3 7% 26% 18% 7%	5467
April	1st to 10th	Night	34% 4 11% 22% 23% 5%	2200
	11th to 20th	Night	34% 4 9% 24% 22% 8%	1995
	21st to end	Night	36% 4 12% 25% 19% 4	1800
May	1st to 10th	Night	44% 6% 15% 23% 12% 1	4060
	11th to 20th	Night	51% 6% 13% 20% 10%	4033
	21st to end	Night	39% 5% 12% 26% 15% 2	4429
June	1st to 10th	Night	37% 9% 18% 22% 12% 1	1800
	11th to 20th	Night	39% 8% 19% 23% 10% 1	1800
	21st to end	Night	36% 9% 18% 20% 14% 4	1800
July	1st to 10th	Night	44% 10% 16% 20% 9% 1	4015
	11th to 20th	Night	47% 10% 25% 15% 3 1	4006
	21st to end	Night	52% 9% 17% 16% 5%	4297
August	1st to 10th	Night	53% 7% 20% 16% 5%	2904
	11th to 20th	Night	49% 8% 19% 14% 9% 1	2902
	21st to end	Night	33% 7% 14% 33% 10% 2	3435
September	1st to 10th	Night	39% 7% 14% 20% 15% 4	2200
	11th to 20th	Night	45% 7% 10% 20% 17% 1	2200
	21st to end	Night	47% 6% 10% 18% 16% 3	2200
October	1st to 10th	Night	35% 5% 7% 19% 25% 10%	3670
	11th to 20th	Night	22% 3 6% 24% 28% 18%	3779
	21st to end	Night	28% 2 4 24% 28% 14%	4120
November	1st to 10th	Night	35% 4 7% 19% 25% 9%	2354
	11th to 20th	Night	33% 3 7% 20% 27% 10%	2600
	21st to end	Night	33% 3 7% 24% 26% 6%	2600
December	1st to 10th	Night	34% 3 8% 21% 28% 6%	4250
	11th to 20th	Night	46% 3 7% 17% 19% 8%	4824
	21st to end	Night	39% 4 12% 22% 21% 2	6490

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	24% 15% 32% 22% 6%	300
	11th to 20th	Day	25% 14% 32% 23% 6%	300
	21st to end	Day	23% 15% 34% 22% 5%	330
February	1st to 10th	Day	21% 13% 37% 22% 7%	300
	11th to 20th	Day	34% 14% 32% 17% 3	300
	21st to end	Day	29% 13% 35% 19% 4	259
March	1st to 10th	Day	23% 10% 31% 26% 7% 2	500
	11th to 20th	Day	23% 11% 35% 25% 5% 1	500
	21st to end	Day	25% 12% 30% 28% 6%	550
April	1st to 10th	Day	22% 12% 32% 21% 12% 2	500
	11th to 20th	Day	18% 12% 25% 27% 14% 3	492
	21st to end	Day	18% 6% 30% 23% 16% 7%	500
May	1st to 10th	Day	13% 11% 27% 30% 14% 5%	503
	11th to 20th	Day	21% 4 17% 27% 21% 10%	527
	21st to end	Day	19% 4 14% 25% 24% 15%	573
June	1st to 10th	Day	13% 3 13% 33% 26% 11%	530
	11th to 20th	Day	8% 3 15% 29% 25% 19%	530
	21st to end	Day	9% 1 11% 32% 32% 15%	530
July	1st to 10th	Day	10% 3 6% 22% 33% 27%	525
	11th to 20th	Day	7% 2 5% 24% 33% 29%	524
	21st to end	Day	10% 2 6% 27% 32% 24%	576
August	1st to 10th	Day	16% 3 12% 25% 25% 19%	526
	11th to 20th	Day	14% 2 7% 24% 30% 21%	519
	21st to end	Day	12% 2 8% 23% 31% 25%	583
September	1st to 10th	Day	14% 2 10% 25% 31% 19%	530
	11th to 20th	Day	19% 2 11% 18% 32% 18%	530
	21st to end	Day	23% 3 17% 23% 26% 8%	530
October	1st to 10th	Day	20% 3 13% 28% 26% 11%	502
	11th to 20th	Day	16% 6% 20% 29% 21% 8%	490
	21st to end	Day	19% 6% 16% 33% 22% 5%	539
November	1st to 10th	Day	22% 4 21% 32% 18% 4	479
	11th to 20th	Day	17% 10% 33% 26% 11% 3	300
	21st to end	Day	15% 11% 30% 27% 14% 3	300
December	1st to 10th	Day	29% 9% 29% 27% 6% 1	300
	11th to 20th	Day	18% 8% 30% 31% 11% 1	300
	21st to end	Day	26% 8% 28% 26% 9% 2	330
January	1st to 10th	Night	18% 10% 34% 31% 7%	800
	11th to 20th	Night	20% 9% 33% 33% 5%	800
	21st to end	Night	18% 7% 38% 33% 3	880
February	1st to 10th	Night	18% 7% 33% 38% 4	800
	11th to 20th	Night	24% 9% 37% 27% 3	783
	21st to end	Night	19% 13% 33% 31% 3	660
March	1st to 10th	Night	18% 10% 30% 37% 6%	800
	11th to 20th	Night	18% 11% 33% 30% 9% 1	800
	21st to end	Night	19% 8% 32% 33% 8%	880
April	1st to 10th	Night	17% 10% 33% 30% 10%	800
	11th to 20th	Night	17% 10% 26% 33% 13% 1	783
	21st to end	Night	16% 5% 22% 41% 17%	600
May	1st to 10th	Night	16% 6% 25% 35% 18% 1	600
	11th to 20th	Night	18% 5% 17% 36% 23% 2	591
	21st to end	Night	17% 3 9% 32% 32% 6%	650
June	1st to 10th	Night	11% 2 9% 39% 33% 6%	600
	11th to 20th	Night	14% 2 10% 30% 34% 10%	600
	21st to end	Night	8% 2 7% 32% 44% 7%	600
July	1st to 10th	Night	12% 2 3 25% 49% 8%	585
	11th to 20th	Night	11% 2 2 24% 49% 12%	582
	21st to end	Night	7% 4 2 30% 47% 10%	657
August	1st to 10th	Night	18% 2 4 26% 43% 8%	597
	11th to 20th	Night	14% 2 3 32% 40% 9%	594
	21st to end	Night	13% 2 5% 25% 45% 10%	771
September	1st to 10th	Night	14% 2 6% 32% 38% 9%	800
	11th to 20th	Night	18% 2 7% 27% 39% 7%	800
	21st to end	Night	23% 2 11% 34% 27% 3	800
October	1st to 10th	Night	19% 3 14% 30% 28% 6%	773
	11th to 20th	Night	18% 4 21% 36% 19% 2	760
	21st to end	Night	16% 5% 19% 39% 19% 2	836
November	1st to 10th	Night	18% 5% 18% 41% 16% 3	780
	11th to 20th	Night	16% 6% 27% 38% 12% 1	800
	21st to end	Night	15% 6% 27% 37% 14% 1	800
December	1st to 10th	Night	21% 7% 25% 39% 7%	800
	11th to 20th	Night	20% 5% 27% 39% 9% 1	800
	21st to end	Night	24% 8% 24% 38% 7%	880

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