Cumberland Astronomical Society Dark Sky Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cumberland Astronomical Society Dark Sky Site clear sky chart from 2005-11-06 to 2025-10-19.

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	14% 7% 3 7% 4 4 4 4 6% 8% 39%	126781
	Night	15% 8% 3 6% 4 4 4 3 5% 7% 41%	122927
Transparency	Day	43% 4 10% 23% 16% 3	130473
	Night	42% 4 9% 23% 18% 4	127307
Seeing	Day	16% 9% 20% 24% 20% 10%	16436
	Night	17% 7% 23% 31% 19% 3	25986

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	16% 13% 7% 10% 4 4 4 4 4 5% 29%	3617
	11th to 20th	Day	17% 8% 5% 10% 4 4 4 2 4 5% 38%	3970
	21st to end	Day	21% 8% 4 5% 3 4 4 3 4 6% 37%	4788
February	1st to 10th	Day	18% 13% 4 7% 4 4 3 3 4 6% 35%	2000
	11th to 20th	Day	23% 9% 3 8% 5% 4 3 3 4 6% 33%	1948
	21st to end	Day	20% 8% 3 7% 4 3 3 3 4 6% 37%	1650
March	1st to 10th	Day	18% 12% 4 8% 5% 4 4 3 4 5% 35%	4480
	11th to 20th	Day	15% 7% 2 6% 4 4 3 3 4 5% 47%	4814
	21st to end	Day	13% 7% 3 6% 4 4 4 3 5% 7% 43%	5685
April	1st to 10th	Day	15% 6% 2 6% 4 4 3 3 4 7% 45%	2400
	11th to 20th	Day	15% 7% 2 6% 4 4 4 4 6% 6% 44%	2374
	21st to end	Day	13% 7% 3 7% 4 5% 5% 4 5% 7% 39%	2478
May	1st to 10th	Day	21% 8% 3 7% 5% 5% 5% 4 6% 9% 27%	5862
	11th to 20th	Day	17% 9% 3 9% 5% 4 5% 4 6% 7% 30%	5861
	21st to end	Day	15% 7% 3 8% 5% 4 4 4 5% 7% 38%	6433
June	1st to 10th	Day	10% 5% 3 6% 5% 5% 4 4 7% 8% 42%	2600
	11th to 20th	Day	6% 4 2 6% 5% 5% 6% 5% 7% 11% 43%	2760
	21st to end	Day	5% 5% 2 6% 5% 6% 5% 5% 9% 12% 40%	2800
July	1st to 10th	Day	8% 5% 2 7% 6% 6% 6% 5% 7% 10% 38%	6360
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 6% 8% 13% 33%	6130
	21st to end	Day	14% 7% 3 7% 6% 6% 6% 5% 8% 11% 29%	6424
August	1st to 10th	Day	5% 3 1 5% 4 5% 5% 4 7% 10% 50%	4118
	11th to 20th	Day	8% 5% 2 7% 5% 6% 6% 6% 8% 10% 39%	3849
	21st to end	Day	7% 3 1 4 4 4 5% 4 6% 8% 55%	4312
September	1st to 10th	Day	11% 6% 3 6% 5% 5% 5% 4 6% 7% 41%	2350
	11th to 20th	Day	12% 6% 2 5% 4 4 4 4 5% 7% 47%	2200
	21st to end	Day	12% 6% 2 6% 4 5% 4 3 5% 6% 47%	2200
October	1st to 10th	Day	10% 4 2 4 3 3 3 3 4 7% 56%	2200
	11th to 20th	Day	13% 5% 2 5% 3 3 3 2 4 7% 54%	2150
	21st to end	Day	17% 6% 2 5% 3 4 3 3 5% 5% 46%	1881
November	1st to 10th	Day	13% 6% 2 4 4 3 3 2 4 5% 55%	1762
	11th to 20th	Day	15% 8% 2 5% 3 3 3 3 5% 6% 48%	1800
	21st to end	Day	16% 8% 3 5% 3 4 4 3 4 5% 44%	1800
December	1st to 10th	Day	18% 8% 2 6% 4 4 4 3 5% 5% 43%	2930
	11th to 20th	Day	23% 9% 4 7% 4 4 3 3 4 6% 32%	3373
	21st to end	Day	22% 12% 4 7% 4 5% 4 3 6% 7% 26%	4422
January	1st to 10th	Night	17% 12% 4 9% 4 3 4 3 5% 6% 34%	6348
	11th to 20th	Night	19% 11% 2 7% 5% 4 3 2 4 4 37%	5967
	21st to end	Night	19% 10% 3 7% 4 3 3 2 4 4 40%	6150
February	1st to 10th	Night	23% 10% 4 7% 4 4 3 3 5% 5% 33%	2400
	11th to 20th	Night	24% 9% 4 6% 4 4 3 3 4 5% 34%	2338
	21st to end	Night	24% 9% 3 8% 4 3 3 3 4 5% 33%	1980
March	1st to 10th	Night	21% 10% 3 6% 5% 3 4 3 5% 5% 34%	5373
	11th to 20th	Night	12% 5% 2 5% 3 4 4 4 5% 6% 49%	5132
	21st to end	Night	15% 7% 2 7% 4 5% 4 3 5% 7% 40%	5286
April	1st to 10th	Night	13% 5% 2 6% 5% 4 5% 4 6% 7% 45%	2000
	11th to 20th	Night	16% 6% 2 5% 3 3 4 3 5% 7% 46%	1975
	21st to end	Night	14% 7% 2 6% 4 4 4 4 5% 7% 42%	1920
May	1st to 10th	Night	16% 9% 3 9% 5% 5% 5% 4 7% 7% 30%	4033
	11th to 20th	Night	17% 8% 4 8% 5% 5% 5% 4 5% 7% 31%	4032
	21st to end	Night	13% 6% 3 7% 5% 5% 5% 4 6% 8% 37%	4421
June	1st to 10th	Night	8% 6% 3 7% 4 4 4 4 5% 9% 46%	1800
	11th to 20th	Night	5% 3 1 4 4 4 4 4 8% 10% 53%	1632
	21st to end	Night	5% 4 1 5% 5% 5% 5% 4 7% 13% 45%	1600
July	1st to 10th	Night	9% 5% 2 6% 5% 4 5% 4 8% 9% 43%	3618
	11th to 20th	Night	7% 5% 2 5% 5% 5% 6% 5% 8% 13% 39%	3832
	21st to end	Night	10% 7% 3 8% 6% 5% 6% 5% 8% 11% 30%	4427
August	1st to 10th	Night	5% 3 2 5% 3 3 3 3 5% 9% 58%	3013
	11th to 20th	Night	6% 3 2 5% 4 4 4 4 7% 11% 48%	3200
	21st to end	Night	5% 3 1 4 3 4 4 3 6% 9% 59%	3586
September	1st to 10th	Night	11% 5% 3 6% 4 4 4 4 5% 7% 48%	2030
	11th to 20th	Night	9% 6% 2 6% 4 5% 4 3 6% 7% 48%	2200
	21st to end	Night	12% 5% 2 5% 4 3 4 3 5% 5% 52%	2200
October	1st to 10th	Night	9% 5% 1 4 3 2 2 2 4 5% 62%	2200
	11th to 20th	Night	14% 5% 2 5% 3 3 2 2 5% 5% 54%	2201
	21st to end	Night	15% 6% 2 5% 3 3 3 2 5% 5% 49%	2717
November	1st to 10th	Night	13% 5% 2 5% 3 3 3 3 4 6% 52%	2536
	11th to 20th	Night	14% 7% 2 5% 3 3 3 3 5% 7% 49%	2600
	21st to end	Night	18% 7% 3 5% 4 3 3 3 4 6% 45%	2600
December	1st to 10th	Night	20% 10% 3 7% 4 3 3 3 4 4 39%	4250
	11th to 20th	Night	26% 7% 3 5% 3 3 3 3 4 5% 38%	4822
	21st to end	Night	19% 13% 4 8% 5% 5% 5% 3 4 5% 28%	6508

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	58% 3 9% 17% 12% 1	3618
	11th to 20th	Day	52% 2 5% 22% 18% 1	3970
	21st to end	Day	50% 2 7% 23% 16% 3	4788
February	1st to 10th	Day	53% 3 8% 21% 13% 2	2000
	11th to 20th	Day	55% 3 6% 18% 14% 3	1958
	21st to end	Day	49% 2 7% 20% 16% 6%	1650
March	1st to 10th	Day	54% 2 5% 15% 17% 7%	4480
	11th to 20th	Day	42% 3 6% 23% 20% 6%	4814
	21st to end	Day	42% 3 9% 30% 15% 1	5687
April	1st to 10th	Day	39% 3 11% 25% 18% 3	2400
	11th to 20th	Day	41% 3 6% 23% 22% 4	2374
	21st to end	Day	44% 4 9% 25% 15% 2	2478
May	1st to 10th	Day	53% 4 14% 19% 10% 1	5897
	11th to 20th	Day	53% 4 13% 21% 9%	5865
	21st to end	Day	46% 5% 9% 22% 17% 2	6447
June	1st to 10th	Day	39% 7% 11% 24% 17% 3	2600
	11th to 20th	Day	33% 6% 16% 24% 18% 2	2760
	21st to end	Day	33% 8% 17% 22% 16% 4	2800
July	1st to 10th	Day	38% 7% 15% 24% 13% 2	6291
	11th to 20th	Day	39% 9% 19% 26% 6% 1	6056
	21st to end	Day	46% 8% 18% 22% 6% 1	6240
August	1st to 10th	Day	27% 7% 14% 33% 18% 1	4105
	11th to 20th	Day	37% 6% 14% 25% 17% 1	3885
	21st to end	Day	27% 6% 9% 34% 22% 2	4311
September	1st to 10th	Day	41% 6% 10% 19% 19% 4	2350
	11th to 20th	Day	37% 6% 9% 25% 22% 2	2200
	21st to end	Day	37% 5% 9% 23% 23% 4	2200
October	1st to 10th	Day	22% 3 6% 19% 42% 8%	3590
	11th to 20th	Day	23% 2 7% 26% 29% 12%	3494
	21st to end	Day	31% 2 7% 25% 27% 8%	3081
November	1st to 10th	Day	33% 3 8% 25% 26% 5%	1762
	11th to 20th	Day	38% 3 7% 24% 23% 6%	1800
	21st to end	Day	43% 3 6% 25% 19% 4	1800
December	1st to 10th	Day	45% 2 6% 26% 20% 1	2930
	11th to 20th	Day	56% 3 7% 21% 11% 2	3370
	21st to end	Day	58% 3 11% 21% 7% 1	4422
January	1st to 10th	Night	52% 2 9% 22% 14% 1	6348
	11th to 20th	Night	52% 1 5% 19% 19% 4	5967
	21st to end	Night	50% 2 6% 22% 16% 5%	6150
February	1st to 10th	Night	54% 3 9% 19% 15% 1	2400
	11th to 20th	Night	54% 3 6% 19% 14% 4	2339
	21st to end	Night	54% 3 6% 16% 16% 4	1980
March	1st to 10th	Night	52% 2 6% 16% 19% 5%	5373
	11th to 20th	Night	36% 2 8% 22% 21% 12%	5132
	21st to end	Night	45% 3 8% 28% 15% 2	5286
April	1st to 10th	Night	39% 4 9% 25% 20% 3	2000
	11th to 20th	Night	39% 3 8% 21% 23% 7%	1975
	21st to end	Night	42% 3 10% 27% 17% 1	1920
May	1st to 10th	Night	53% 4 12% 20% 9% 1	4060
	11th to 20th	Night	52% 6% 13% 17% 11%	4033
	21st to end	Night	44% 5% 13% 23% 14% 1	4429
June	1st to 10th	Night	36% 5% 12% 28% 15% 3	1800
	11th to 20th	Night	25% 5% 17% 30% 19% 4	1632
	21st to end	Night	30% 7% 17% 28% 14% 4	1600
July	1st to 10th	Night	34% 8% 16% 26% 14% 1	3580
	11th to 20th	Night	34% 9% 23% 25% 7% 2	3787
	21st to end	Night	46% 8% 18% 21% 5% 1	4297
August	1st to 10th	Night	24% 5% 11% 38% 22% 1	3002
	11th to 20th	Night	28% 5% 18% 32% 15% 1	3230
	21st to end	Night	22% 6% 10% 35% 24% 2	3586
September	1st to 10th	Night	36% 5% 11% 22% 21% 5%	2030
	11th to 20th	Night	36% 6% 8% 23% 25% 3	2200
	21st to end	Night	34% 4 9% 22% 25% 7%	2200
October	1st to 10th	Night	23% 2 5% 20% 39% 11%	3590
	11th to 20th	Night	21% 2 5% 25% 33% 14%	3596
	21st to end	Night	27% 2 6% 24% 31% 10%	4467
November	1st to 10th	Night	35% 3 6% 23% 26% 7%	2536
	11th to 20th	Night	35% 2 7% 23% 26% 7%	2600
	21st to end	Night	42% 3 6% 26% 19% 4	2600
December	1st to 10th	Night	50% 2 6% 19% 22% 1	4250
	11th to 20th	Night	50% 2 6% 24% 16% 2	4824
	21st to end	Night	59% 3 8% 20% 9% 1	6508

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% 18% 32% 18% 8% 1	300
	11th to 20th	Day	27% 21% 32% 14% 5% 1	300
	21st to end	Day	22% 16% 42% 16% 4	330
February	1st to 10th	Day	25% 18% 31% 21% 4 1	300
	11th to 20th	Day	33% 22% 31% 10% 3 1	300
	21st to end	Day	33% 15% 36% 13% 2	249
March	1st to 10th	Day	27% 17% 33% 17% 6% 1	300
	11th to 20th	Day	22% 20% 33% 16% 8% 1	425
	21st to end	Day	22% 17% 32% 18% 9% 1	550
April	1st to 10th	Day	21% 16% 27% 24% 10% 2	500
	11th to 20th	Day	19% 12% 26% 29% 12% 2	492
	21st to end	Day	19% 9% 29% 27% 13% 3	500
May	1st to 10th	Day	20% 13% 24% 25% 15% 4	503
	11th to 20th	Day	17% 8% 23% 24% 19% 9%	527
	21st to end	Day	14% 7% 17% 27% 19% 16%	573
June	1st to 10th	Day	12% 5% 16% 26% 25% 15%	530
	11th to 20th	Day	7% 3 16% 27% 32% 16%	690
	21st to end	Day	5% 3 17% 30% 28% 17%	730
July	1st to 10th	Day	9% 2 11% 26% 32% 21%	721
	11th to 20th	Day	11% 3 8% 22% 33% 24%	619
	21st to end	Day	10% 3 8% 24% 31% 24%	576
August	1st to 10th	Day	10% 3 6% 25% 34% 22%	526
	11th to 20th	Day	10% 3 10% 29% 33% 16%	519
	21st to end	Day	11% 2 8% 20% 35% 24%	583
September	1st to 10th	Day	11% 3 12% 26% 30% 18%	530
	11th to 20th	Day	14% 3 10% 19% 36% 19%	530
	21st to end	Day	16% 6% 14% 24% 28% 12%	530
October	1st to 10th	Day	12% 5% 15% 29% 32% 8%	530
	11th to 20th	Day	12% 10% 26% 28% 18% 6%	513
	21st to end	Day	23% 11% 22% 29% 13% 2	330
November	1st to 10th	Day	19% 7% 27% 26% 16% 5%	300
	11th to 20th	Day	15% 15% 33% 27% 10%	300
	21st to end	Day	21% 12% 29% 24% 13% 2	300
December	1st to 10th	Day	25% 19% 24% 24% 7% 1	300
	11th to 20th	Day	24% 15% 26% 21% 13% 1	300
	21st to end	Day	24% 16% 29% 23% 7%	330
January	1st to 10th	Night	24% 13% 31% 27% 5%	1000
	11th to 20th	Night	29% 15% 30% 22% 4	828
	21st to end	Night	21% 12% 40% 25% 2	880
February	1st to 10th	Night	28% 12% 33% 24% 4	800
	11th to 20th	Night	30% 13% 34% 21% 2	783
	21st to end	Night	28% 13% 32% 26% 1	660
March	1st to 10th	Night	22% 11% 33% 29% 5%	800
	11th to 20th	Night	23% 12% 30% 29% 7% 1	800
	21st to end	Night	19% 12% 31% 30% 7% 1	802
April	1st to 10th	Night	15% 11% 32% 32% 10% 1	600
	11th to 20th	Night	19% 8% 31% 30% 12% 1	593
	21st to end	Night	19% 6% 29% 34% 10% 2	600
May	1st to 10th	Night	12% 5% 30% 35% 16% 1	600
	11th to 20th	Night	13% 4 22% 35% 23% 3	591
	21st to end	Night	7% 2 16% 38% 32% 6%	650
June	1st to 10th	Night	10% 2 16% 38% 29% 5%	600
	11th to 20th	Night	5% 2 10% 38% 39% 6%	600
	21st to end	Night	5% 2 10% 37% 40% 7%	600
July	1st to 10th	Night	10% 2 7% 30% 43% 8%	585
	11th to 20th	Night	8% 2 6% 27% 46% 10%	582
	21st to end	Night	9% 2 6% 32% 44% 8%	657
August	1st to 10th	Night	8% 1 6% 30% 47% 7%	597
	11th to 20th	Night	9% 3 6% 39% 40% 5%	594
	21st to end	Night	8% 1 6% 31% 46% 8%	660
September	1st to 10th	Night	10% 2 8% 33% 40% 6%	630
	11th to 20th	Night	10% 2 11% 31% 39% 7%	800
	21st to end	Night	13% 2 14% 33% 33% 5%	800
October	1st to 10th	Night	11% 4 16% 33% 31% 5%	800
	11th to 20th	Night	17% 6% 19% 36% 19% 2	793
	21st to end	Night	19% 6% 27% 32% 15% 2	836
November	1st to 10th	Night	16% 5% 21% 38% 18% 2	780
	11th to 20th	Night	17% 8% 31% 33% 10%	800
	21st to end	Night	21% 10% 25% 33% 10% 1	800
December	1st to 10th	Night	28% 10% 28% 27% 6%	800
	11th to 20th	Night	26% 7% 29% 30% 7%	800
	21st to end	Night	27% 10% 28% 28% 6%	885

Page generated at 2025-10-16 00:50:25. Version ac07

Copyright © 2025 Attilla Danko.