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 2024-04-10.

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	15% 7% 3 6% 5% 3 5% 5% 5% 7% 40%	83683
	Night	15% 7% 3 5% 5% 3 4 4 5% 7% 42%	85081
Transparency	Day	44% 4 10% 23% 16% 3	83397
	Night	43% 4 9% 23% 18% 4	84888
Seeing	Day	16% 9% 21% 24% 20% 10%	14289
	Night	17% 7% 23% 31% 19% 3	24065

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% 15% 5% 7% 6% 3 6% 5% 4 5% 29%	2527
	11th to 20th	Day	22% 8% 4 9% 6% 3 5% 3 4 5% 31%	2776
	21st to end	Day	25% 10% 6% 5% 4 3 4 4 4 5% 30%	3348
February	1st to 10th	Day	18% 12% 4 6% 5% 3 3 4 4 6% 35%	1900
	11th to 20th	Day	23% 8% 4 7% 5% 3 4 3 4 6% 34%	1848
	21st to end	Day	22% 8% 3 7% 5% 3 3 4 4 6% 36%	1570
March	1st to 10th	Day	24% 12% 3 7% 5% 3 4 3 4 5% 31%	3120
	11th to 20th	Day	15% 6% 3 5% 4 3 4 4 4 5% 47%	3370
	21st to end	Day	13% 6% 3 5% 6% 3 5% 5% 5% 7% 42%	3967
April	1st to 10th	Day	14% 5% 3 5% 5% 3 4 4 5% 7% 46%	2269
	11th to 20th	Day	16% 7% 2 5% 4 3 4 5% 5% 6% 43%	2134
	21st to end	Day	14% 7% 3 6% 5% 4 5% 5% 5% 7% 39%	2230
May	1st to 10th	Day	17% 5% 2 4 5% 3 4 4 6% 8% 41%	2340
	11th to 20th	Day	15% 7% 3 7% 5% 3 5% 4 5% 8% 38%	2301
	21st to end	Day	9% 5% 2 7% 6% 4 6% 7% 5% 10% 39%	2531
June	1st to 10th	Day	10% 5% 2 6% 6% 3 5% 5% 6% 8% 44%	2340
	11th to 20th	Day	5% 3 2 5% 6% 4 6% 7% 7% 11% 45%	2484
	21st to end	Day	5% 4 2 5% 6% 4 6% 7% 9% 12% 40%	2520
July	1st to 10th	Day	9% 6% 2 7% 8% 5% 7% 7% 7% 9% 32%	2520
	11th to 20th	Day	10% 5% 3 6% 6% 4 6% 6% 8% 10% 37%	2430
	21st to end	Day	9% 5% 3 5% 6% 4 6% 7% 8% 11% 37%	2547
August	1st to 10th	Day	8% 4 2 6% 6% 5% 6% 7% 7% 12% 36%	2270
	11th to 20th	Day	10% 5% 2 6% 7% 4 6% 7% 7% 9% 35%	2125
	21st to end	Day	10% 5% 2 4 6% 4 6% 7% 7% 9% 41%	2376
September	1st to 10th	Day	12% 5% 3 5% 6% 4 6% 5% 6% 8% 40%	2116
	11th to 20th	Day	13% 6% 2 4 5% 3 4 6% 4 7% 46%	1980
	21st to end	Day	11% 6% 2 4 4 3 4 4 5% 6% 50%	1980
October	1st to 10th	Day	11% 4 2 4 3 3 3 4 4 7% 55%	1980
	11th to 20th	Day	14% 5% 2 4 4 3 3 3 4 7% 52%	1954
	21st to end	Day	18% 6% 2 5% 4 3 4 3 5% 5% 45%	1782
November	1st to 10th	Day	12% 5% 2 4 4 2 3 3 4 5% 57%	1672
	11th to 20th	Day	15% 6% 3 4 3 3 3 3 5% 6% 49%	1710
	21st to end	Day	17% 7% 3 4 4 2 4 4 4 5% 46%	1710
December	1st to 10th	Day	25% 9% 2 4 5% 3 4 3 5% 6% 34%	1710
	11th to 20th	Day	18% 9% 3 7% 6% 3 4 4 4 6% 36%	2153
	21st to end	Day	19% 13% 5% 5% 5% 3 5% 4 5% 7% 28%	3093
January	1st to 10th	Night	19% 12% 3 5% 4 2 4 4 4 7% 35%	4428
	11th to 20th	Night	20% 12% 3 7% 7% 4 4 3 4 4 32%	4173
	21st to end	Night	22% 12% 3 6% 4 2 3 3 4 5% 34%	4304
February	1st to 10th	Night	23% 9% 4 6% 5% 3 4 3 5% 5% 33%	2280
	11th to 20th	Night	23% 8% 4 6% 4 3 3 4 4 5% 36%	2218
	21st to end	Night	25% 8% 4 7% 5% 2 3 4 4 5% 31%	1884
March	1st to 10th	Night	25% 9% 3 6% 6% 3 4 4 4 5% 31%	3733
	11th to 20th	Night	13% 6% 2 5% 4 3 4 4 5% 6% 48%	3590
	21st to end	Night	15% 7% 3 7% 5% 4 4 5% 5% 7% 41%	3692
April	1st to 10th	Night	12% 5% 2 5% 5% 3 5% 5% 5% 7% 46%	1900
	11th to 20th	Night	17% 5% 2 4 4 2 4 4 5% 7% 46%	1775
	21st to end	Night	14% 6% 3 5% 5% 3 4 4 5% 8% 43%	1728
May	1st to 10th	Night	10% 6% 3 6% 5% 3 4 4 6% 6% 45%	1620
	11th to 20th	Night	10% 5% 2 5% 5% 3 4 5% 5% 9% 45%	1592
	21st to end	Night	5% 4 2 5% 7% 4 6% 7% 7% 11% 42%	1744
June	1st to 10th	Night	8% 5% 3 6% 5% 2 5% 5% 5% 9% 47%	1620
	11th to 20th	Night	4 3 1 3 4 3 5% 5% 8% 10% 55%	1470
	21st to end	Night	4 4 1 5% 5% 4 6% 5% 7% 14% 45%	1440
July	1st to 10th	Night	8% 4 3 5% 6% 3 6% 6% 8% 10% 41%	1440
	11th to 20th	Night	8% 4 2 4 5% 3 5% 6% 8% 11% 45%	1525
	21st to end	Night	7% 4 2 5% 6% 3 6% 6% 7% 12% 43%	1771
August	1st to 10th	Night	7% 5% 2 5% 5% 3 4 5% 7% 11% 47%	1664
	11th to 20th	Night	9% 4 2 4 6% 3 5% 5% 8% 10% 45%	1776
	21st to end	Night	8% 4 2 5% 4 3 5% 5% 6% 10% 47%	1980
September	1st to 10th	Night	11% 5% 3 5% 4 3 4 5% 5% 7% 48%	1826
	11th to 20th	Night	10% 6% 3 6% 5% 4 4 5% 6% 7% 47%	1980
	21st to end	Night	10% 4 2 5% 4 2 3 5% 5% 6% 54%	1980
October	1st to 10th	Night	9% 5% 1 3 3 2 2 3 4 6% 61%	1980
	11th to 20th	Night	15% 6% 2 4 4 3 3 3 5% 6% 50%	2000
	21st to end	Night	16% 6% 2 5% 4 2 4 3 5% 5% 48%	2574
November	1st to 10th	Night	11% 5% 3 5% 4 2 3 4 4 6% 53%	2406
	11th to 20th	Night	13% 6% 2 4 3 2 3 4 5% 7% 50%	2470
	21st to end	Night	18% 6% 3 4 4 2 3 4 4 6% 45%	2470
December	1st to 10th	Night	23% 10% 4 5% 5% 3 3 4 4 5% 33%	2470
	11th to 20th	Night	20% 7% 3 5% 4 2 4 4 4 5% 42%	3042
	21st to end	Night	18% 13% 4 7% 7% 4 5% 5% 4 6% 29%	4536

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 6% 19% 13% 2	2528
	11th to 20th	Day	58% 2 7% 20% 12% 1	2776
	21st to end	Day	59% 2 7% 22% 9% 1	3348
February	1st to 10th	Day	53% 3 7% 21% 13% 2	1900
	11th to 20th	Day	54% 3 6% 19% 14% 3	1858
	21st to end	Day	51% 3 8% 19% 15% 5%	1570
March	1st to 10th	Day	58% 2 6% 16% 12% 6%	3120
	11th to 20th	Day	40% 3 8% 27% 17% 4	3370
	21st to end	Day	43% 3 10% 29% 15% 1	3967
April	1st to 10th	Day	39% 3 11% 25% 19% 3	2269
	11th to 20th	Day	42% 3 6% 23% 22% 4	2134
	21st to end	Day	44% 4 8% 25% 16% 2	2230
May	1st to 10th	Day	42% 4 13% 26% 15% 2	2340
	11th to 20th	Day	46% 4 11% 24% 14% 1	2305
	21st to end	Day	40% 6% 15% 24% 13% 2	2531
June	1st to 10th	Day	38% 7% 11% 24% 18% 2	2340
	11th to 20th	Day	31% 7% 17% 26% 17% 3	2484
	21st to end	Day	34% 7% 16% 22% 15% 5%	2520
July	1st to 10th	Day	44% 7% 16% 18% 12% 4	2451
	11th to 20th	Day	40% 7% 18% 20% 12% 4	2356
	21st to end	Day	36% 7% 17% 26% 11% 2	2405
August	1st to 10th	Day	38% 9% 16% 23% 12% 1	2257
	11th to 20th	Day	43% 6% 13% 20% 16% 2	2125
	21st to end	Day	38% 8% 12% 24% 15% 4	2376
September	1st to 10th	Day	41% 7% 11% 20% 18% 3	2116
	11th to 20th	Day	38% 6% 9% 24% 21% 2	1980
	21st to end	Day	34% 4 9% 25% 23% 4	1980
October	1st to 10th	Day	30% 4 9% 23% 28% 6%	1980
	11th to 20th	Day	35% 3 7% 22% 25% 8%	1954
	21st to end	Day	42% 3 7% 21% 23% 4	1782
November	1st to 10th	Day	31% 3 8% 26% 26% 6%	1672
	11th to 20th	Day	38% 3 7% 24% 23% 6%	1710
	21st to end	Day	42% 3 6% 24% 20% 4	1710
December	1st to 10th	Day	53% 3 7% 22% 14% 2	1710
	11th to 20th	Day	50% 3 8% 22% 15% 2	2150
	21st to end	Day	57% 3 10% 20% 9% 1	3093
January	1st to 10th	Night	50% 3 7% 25% 14% 2	4428
	11th to 20th	Night	56% 1 6% 17% 16% 3	4173
	21st to end	Night	55% 2 7% 22% 13% 1	4304
February	1st to 10th	Night	54% 3 8% 19% 15% 1	2280
	11th to 20th	Night	52% 3 6% 20% 15% 4	2219
	21st to end	Night	56% 3 7% 14% 15% 4	1884
March	1st to 10th	Night	56% 3 7% 13% 16% 4	3733
	11th to 20th	Night	38% 2 8% 25% 15% 11%	3590
	21st to end	Night	45% 3 8% 26% 17% 1	3692
April	1st to 10th	Night	38% 4 9% 25% 21% 3	1900
	11th to 20th	Night	39% 3 8% 20% 23% 7%	1775
	21st to end	Night	41% 3 9% 28% 18% 1	1728
May	1st to 10th	Night	40% 4 11% 25% 18% 3	1620
	11th to 20th	Night	37% 6% 17% 24% 16% 1	1593
	21st to end	Night	33% 9% 16% 24% 15% 3	1745
June	1st to 10th	Night	35% 5% 13% 29% 16% 3	1620
	11th to 20th	Night	23% 6% 18% 31% 18% 4	1470
	21st to end	Night	30% 7% 18% 28% 13% 5%	1440
July	1st to 10th	Night	36% 7% 18% 23% 14% 3	1400
	11th to 20th	Night	30% 8% 20% 23% 15% 5%	1477
	21st to end	Night	32% 7% 17% 30% 12% 2	1672
August	1st to 10th	Night	32% 6% 15% 30% 16% 1	1653
	11th to 20th	Night	33% 7% 19% 25% 15% 2	1776
	21st to end	Night	31% 8% 13% 27% 17% 4	1980
September	1st to 10th	Night	36% 5% 11% 24% 20% 5%	1826
	11th to 20th	Night	37% 6% 8% 22% 24% 3	1980
	21st to end	Night	31% 4 8% 23% 26% 7%	1980
October	1st to 10th	Night	27% 3 7% 24% 31% 8%	1980
	11th to 20th	Night	37% 3 8% 23% 23% 7%	2000
	21st to end	Night	39% 2 5% 24% 25% 6%	2574
November	1st to 10th	Night	33% 3 6% 24% 26% 7%	2406
	11th to 20th	Night	34% 2 7% 23% 27% 7%	2470
	21st to end	Night	42% 3 6% 26% 19% 4	2470
December	1st to 10th	Night	53% 3 8% 20% 14% 2	2470
	11th to 20th	Night	45% 3 7% 24% 19% 3	3044
	21st to end	Night	57% 3 8% 19% 12% 1	4536

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% 17% 33% 17% 8% 1	270
	11th to 20th	Day	29% 21% 30% 13% 6% 1	270
	21st to end	Day	22% 16% 42% 16% 4	297
February	1st to 10th	Day	26% 18% 30% 21% 5% 1	270
	11th to 20th	Day	33% 21% 30% 11% 4 1	270
	21st to end	Day	36% 16% 36% 10% 2	225
March	1st to 10th	Day	28% 17% 32% 16% 6% 1	270
	11th to 20th	Day	22% 19% 34% 17% 8% 1	389
	21st to end	Day	22% 17% 31% 18% 10% 1	506
April	1st to 10th	Day	19% 16% 26% 25% 11% 2	457
	11th to 20th	Day	21% 10% 25% 29% 13% 2	412
	21st to end	Day	20% 10% 29% 25% 12% 4	420
May	1st to 10th	Day	19% 11% 24% 25% 15% 4	423
	11th to 20th	Day	17% 8% 22% 25% 18% 10%	447
	21st to end	Day	13% 5% 15% 26% 22% 18%	485
June	1st to 10th	Day	12% 5% 15% 24% 26% 18%	450
	11th to 20th	Day	5% 3 16% 28% 32% 16%	594
	21st to end	Day	5% 3 18% 32% 27% 15%	630
July	1st to 10th	Day	10% 1 11% 25% 30% 22%	621
	11th to 20th	Day	10% 3 9% 22% 31% 25%	529
	21st to end	Day	9% 3 9% 26% 32% 22%	488
August	1st to 10th	Day	10% 3 7% 24% 33% 22%	446
	11th to 20th	Day	11% 3 8% 29% 34% 15%	439
	21st to end	Day	13% 2 8% 20% 35% 24%	495
September	1st to 10th	Day	11% 3 11% 24% 31% 20%	450
	11th to 20th	Day	14% 2 12% 19% 35% 18%	450
	21st to end	Day	15% 7% 13% 25% 28% 13%	450
October	1st to 10th	Day	12% 5% 15% 31% 30% 8%	450
	11th to 20th	Day	14% 11% 27% 29% 14% 5%	442
	21st to end	Day	25% 11% 22% 27% 12% 2	297
November	1st to 10th	Day	19% 7% 26% 27% 16% 5%	270
	11th to 20th	Day	14% 14% 35% 26% 10%	270
	21st to end	Day	20% 11% 30% 24% 13% 2	270
December	1st to 10th	Day	27% 19% 23% 24% 7% 1	270
	11th to 20th	Day	23% 15% 27% 21% 14% 1	270
	21st to end	Day	23% 18% 29% 23% 6%	297
January	1st to 10th	Night	24% 13% 31% 26% 5%	950
	11th to 20th	Night	29% 15% 30% 21% 4 1	787
	21st to end	Night	21% 13% 40% 24% 2	836
February	1st to 10th	Night	27% 12% 32% 25% 4	760
	11th to 20th	Night	29% 13% 34% 22% 3	743
	21st to end	Night	29% 13% 32% 24% 1	628
March	1st to 10th	Night	23% 11% 32% 29% 5%	760
	11th to 20th	Night	23% 12% 30% 28% 7% 1	760
	21st to end	Night	19% 12% 31% 30% 7% 1	762
April	1st to 10th	Night	14% 11% 32% 33% 10% 1	570
	11th to 20th	Night	20% 8% 29% 30% 13% 1	533
	21st to end	Night	19% 7% 29% 34% 10% 1	540
May	1st to 10th	Night	12% 5% 31% 35% 16% 1	540
	11th to 20th	Night	12% 3 22% 35% 24% 4	531
	21st to end	Night	7% 1 14% 37% 35% 7%	584
June	1st to 10th	Night	10% 1 17% 36% 31% 5%	540
	11th to 20th	Night	4 1 10% 39% 39% 7%	540
	21st to end	Night	5% 2 11% 38% 39% 6%	540
July	1st to 10th	Night	10% 2 7% 30% 43% 8%	525
	11th to 20th	Night	7% 3 7% 28% 46% 10%	522
	21st to end	Night	8% 1 6% 33% 43% 8%	591
August	1st to 10th	Night	8% 1 6% 30% 48% 6%	537
	11th to 20th	Night	9% 3 6% 38% 40% 4	534
	21st to end	Night	8% 2 6% 30% 46% 8%	594
September	1st to 10th	Night	11% 2 8% 31% 42% 6%	566
	11th to 20th	Night	11% 2 12% 31% 38% 7%	720
	21st to end	Night	12% 2 15% 33% 33% 5%	720
October	1st to 10th	Night	11% 4 17% 32% 31% 5%	720
	11th to 20th	Night	19% 6% 20% 36% 17% 2	720
	21st to end	Night	20% 6% 27% 31% 15% 2	792
November	1st to 10th	Night	15% 5% 21% 38% 19% 3	740
	11th to 20th	Night	17% 8% 31% 34% 10%	760
	21st to end	Night	21% 10% 24% 34% 11% 1	760
December	1st to 10th	Night	29% 10% 27% 28% 7%	760
	11th to 20th	Night	25% 7% 29% 31% 7%	760
	21st to end	Night	27% 10% 28% 28% 6%	840

Page generated at 2024-04-07 08:28:10. Version ac07

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