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 2024-11-05.

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 6% 4 4 4 4 6% 7% 40%	101321
	Night	14% 7% 2 6% 4 4 4 3 5% 7% 42%	97664
Transparency	Day	42% 4 10% 24% 17% 3	105055
	Night	40% 4 9% 24% 19% 4	102070
Seeing	Day	16% 9% 20% 24% 20% 10%	15141
	Night	17% 7% 23% 31% 19% 3	24750

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	15% 15% 5% 9% 4 5% 5% 4 4 5% 29%	2527
	11th to 20th	Day	22% 9% 4 10% 4 4 5% 2 4 5% 31%	2776
	21st to end	Day	25% 11% 5% 6% 4 4 4 3 4 4 30%	3348
February	1st to 10th	Day	18% 13% 4 7% 4 4 3 3 4 6% 35%	1900
	11th to 20th	Day	23% 8% 3 8% 5% 3 3 3 4 6% 34%	1848
	21st to end	Day	22% 9% 3 8% 4 3 3 3 4 6% 36%	1570
March	1st to 10th	Day	24% 12% 3 8% 4 4 3 3 4 4 31%	3120
	11th to 20th	Day	15% 7% 2 6% 4 4 4 3 4 5% 47%	3370
	21st to end	Day	13% 7% 3 6% 5% 4 4 4 5% 7% 42%	3967
April	1st to 10th	Day	14% 6% 3 6% 4 4 3 3 5% 7% 45%	2280
	11th to 20th	Day	15% 7% 2 6% 4 4 4 4 6% 6% 43%	2254
	21st to end	Day	13% 7% 2 7% 4 5% 5% 4 5% 7% 40%	2354
May	1st to 10th	Day	21% 6% 2 6% 5% 4 4 4 6% 9% 34%	4082
	11th to 20th	Day	17% 9% 3 7% 4 4 4 3 5% 7% 36%	4081
	21st to end	Day	11% 7% 2 7% 5% 4 5% 4 5% 8% 40%	4476
June	1st to 10th	Day	11% 5% 2 6% 5% 5% 4 4 7% 8% 43%	2470
	11th to 20th	Day	5% 4 2 6% 5% 5% 6% 5% 8% 11% 45%	2622
	21st to end	Day	5% 5% 2 6% 5% 6% 5% 5% 9% 11% 40%	2660
July	1st to 10th	Day	8% 5% 2 7% 7% 6% 6% 5% 8% 10% 36%	4440
	11th to 20th	Day	9% 6% 2 6% 5% 5% 5% 5% 7% 12% 39%	4280
	21st to end	Day	19% 9% 3 8% 6% 6% 6% 4 7% 8% 24%	4466
August	1st to 10th	Day	5% 3 1 5% 4 5% 5% 4 7% 10% 51%	3991
	11th to 20th	Day	7% 5% 2 7% 5% 6% 6% 5% 8% 10% 39%	3729
	21st to end	Day	7% 3 1 4 4 4 5% 4 6% 8% 55%	4180
September	1st to 10th	Day	12% 6% 3 6% 5% 5% 5% 4 6% 7% 41%	2233
	11th to 20th	Day	13% 7% 2 5% 4 4 4 4 5% 7% 46%	2090
	21st to end	Day	12% 6% 2 6% 4 5% 4 3 5% 6% 48%	2090
October	1st to 10th	Day	10% 4 2 4 3 3 4 3 4 7% 56%	2090
	11th to 20th	Day	13% 5% 2 5% 3 3 3 2 4 6% 53%	2062
	21st to end	Day	17% 6% 2 5% 3 4 3 3 5% 5% 46%	1881
November	1st to 10th	Day	12% 5% 1 4 4 3 3 2 4 5% 56%	1708
	11th to 20th	Day	15% 7% 2 5% 3 3 3 3 5% 6% 49%	1710
	21st to end	Day	17% 8% 2 5% 3 4 4 3 4 5% 46%	1710
December	1st to 10th	Day	25% 9% 2 5% 4 4 3 3 5% 5% 34%	1710
	11th to 20th	Day	18% 9% 3 8% 5% 4 3 3 5% 6% 36%	2153
	21st to end	Day	19% 13% 5% 6% 4 5% 5% 3 6% 7% 27%	3093
January	1st to 10th	Night	19% 12% 3 6% 4 3 4 3 5% 7% 35%	4428
	11th to 20th	Night	20% 12% 2 8% 6% 5% 4 2 5% 4 32%	4173
	21st to end	Night	22% 13% 3 7% 4 3 3 2 4 5% 34%	4304
February	1st to 10th	Night	23% 10% 4 7% 4 4 3 3 5% 5% 33%	2280
	11th to 20th	Night	23% 9% 4 6% 4 4 3 3 4 5% 36%	2218
	21st to end	Night	25% 9% 3 8% 4 3 3 3 4 5% 31%	1884
March	1st to 10th	Night	25% 9% 3 6% 5% 4 4 3 5% 5% 31%	3733
	11th to 20th	Night	13% 6% 2 6% 4 4 4 3 5% 6% 48%	3590
	21st to end	Night	14% 7% 2 8% 4 5% 4 4 5% 6% 40%	3692
April	1st to 10th	Night	12% 6% 1 6% 5% 4 5% 4 6% 7% 46%	1900
	11th to 20th	Night	16% 6% 2 5% 3 3 4 3 5% 6% 46%	1875
	21st to end	Night	14% 8% 2 6% 4 4 3 4 5% 7% 43%	1824
May	1st to 10th	Night	14% 8% 3 8% 5% 4 4 3 6% 6% 38%	2813
	11th to 20th	Night	14% 6% 3 6% 4 4 4 4 5% 7% 41%	2812
	21st to end	Night	7% 5% 3 7% 5% 5% 5% 5% 6% 9% 43%	3079
June	1st to 10th	Night	8% 6% 3 7% 4 4 4 4 5% 9% 46%	1710
	11th to 20th	Night	4 3 1 3 4 4 4 4 8% 10% 54%	1551
	21st to end	Night	5% 5% 1 5% 5% 5% 5% 4 7% 13% 45%	1520
July	1st to 10th	Night	7% 4 3 6% 5% 5% 6% 5% 9% 11% 40%	2530
	11th to 20th	Night	7% 5% 2 5% 4 4 5% 4 7% 10% 46%	2680
	21st to end	Night	13% 9% 3 9% 7% 5% 6% 5% 7% 9% 27%	3087
August	1st to 10th	Night	5% 3 2 5% 3 3 3 3 5% 9% 59%	2920
	11th to 20th	Night	6% 3 2 5% 4 4 4 4 7% 11% 48%	3100
	21st to end	Night	5% 3 1 4 3 4 4 3 5% 9% 60%	3476
September	1st to 10th	Night	11% 5% 3 6% 4 4 4 4 5% 7% 48%	1928
	11th to 20th	Night	10% 6% 2 7% 4 5% 4 3 6% 7% 47%	2090
	21st to end	Night	12% 4 2 5% 4 3 4 3 5% 5% 52%	2090
October	1st to 10th	Night	9% 5% 1 4 3 2 2 2 4 6% 61%	2090
	11th to 20th	Night	14% 5% 2 5% 3 3 2 2 5% 5% 52%	2112
	21st to end	Night	15% 6% 2 5% 3 3 3 2 5% 5% 49%	2717
November	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 4 6% 53%	2470
	11th to 20th	Night	13% 7% 2 5% 3 3 3 3 5% 7% 50%	2470
	21st to end	Night	18% 7% 3 5% 4 3 3 3 4 6% 45%	2470
December	1st to 10th	Night	23% 10% 4 6% 4 3 4 3 5% 5% 33%	2470
	11th to 20th	Night	19% 8% 3 6% 3 3 3 3 5% 5% 42%	3042
	21st to end	Night	18% 13% 4 8% 5% 5% 5% 3 4 6% 28%	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	2280
	11th to 20th	Day	42% 3 6% 23% 22% 4	2254
	21st to end	Day	44% 4 9% 25% 16% 2	2354
May	1st to 10th	Day	48% 3 13% 22% 13% 1	4120
	11th to 20th	Day	48% 3 11% 26% 11% 1	4085
	21st to end	Day	42% 5% 11% 21% 20% 1	4489
June	1st to 10th	Day	38% 6% 11% 24% 18% 3	2470
	11th to 20th	Day	31% 6% 17% 25% 18% 3	2622
	21st to end	Day	34% 7% 17% 22% 16% 4	2660
July	1st to 10th	Day	40% 7% 17% 24% 9% 2	4371
	11th to 20th	Day	37% 6% 17% 30% 9% 2	4206
	21st to end	Day	56% 7% 12% 18% 7% 1	4324
August	1st to 10th	Day	27% 7% 14% 33% 18% 1	3978
	11th to 20th	Day	37% 5% 14% 25% 18% 2	3765
	21st to end	Day	27% 6% 10% 35% 21% 2	4179
September	1st to 10th	Day	40% 6% 10% 19% 19% 4	2233
	11th to 20th	Day	38% 6% 9% 24% 21% 2	2090
	21st to end	Day	36% 5% 9% 24% 22% 4	2090
October	1st to 10th	Day	22% 3 6% 19% 42% 8%	3480
	11th to 20th	Day	24% 2 7% 25% 30% 12%	3406
	21st to end	Day	31% 2 7% 25% 27% 8%	3081
November	1st to 10th	Day	31% 3 8% 26% 27% 6%	1708
	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%	1875
	21st to end	Night	41% 3 9% 28% 18% 1	1824
May	1st to 10th	Night	46% 4 12% 24% 12% 2	2840
	11th to 20th	Night	42% 5% 14% 22% 16% 1	2813
	21st to end	Night	35% 6% 14% 25% 18% 2	3087
June	1st to 10th	Night	35% 5% 13% 28% 15% 3	1710
	11th to 20th	Night	23% 5% 18% 31% 19% 4	1551
	21st to end	Night	30% 6% 18% 27% 14% 5%	1520
July	1st to 10th	Night	35% 8% 17% 28% 10% 2	2490
	11th to 20th	Night	31% 6% 18% 33% 10% 3	2632
	21st to end	Night	53% 7% 12% 19% 7% 1	2988
August	1st to 10th	Night	23% 5% 11% 38% 23% 1	2909
	11th to 20th	Night	28% 5% 18% 31% 15% 1	3130
	21st to end	Night	22% 6% 11% 35% 24% 2	3476
September	1st to 10th	Night	36% 5% 11% 23% 21% 5%	1928
	11th to 20th	Night	37% 6% 8% 22% 24% 3	2090
	21st to end	Night	33% 4 8% 23% 25% 7%	2090
October	1st to 10th	Night	23% 2 6% 19% 39% 11%	3480
	11th to 20th	Night	22% 2 5% 25% 33% 14%	3507
	21st to end	Night	27% 2 6% 24% 31% 10%	4467
November	1st to 10th	Night	33% 3 7% 24% 26% 7%	2470
	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	20% 16% 26% 25% 11% 2	460
	11th to 20th	Day	21% 11% 26% 29% 12% 2	452
	21st to end	Day	19% 10% 30% 26% 12% 3	460
May	1st to 10th	Day	19% 11% 25% 24% 16% 4	463
	11th to 20th	Day	17% 8% 22% 24% 20% 9%	487
	21st to end	Day	13% 6% 16% 27% 20% 17%	529
June	1st to 10th	Day	12% 6% 16% 25% 26% 16%	490
	11th to 20th	Day	5% 3 15% 28% 33% 16%	642
	21st to end	Day	5% 3 18% 32% 27% 15%	680
July	1st to 10th	Day	10% 2 11% 26% 30% 21%	671
	11th to 20th	Day	11% 3 8% 22% 32% 24%	574
	21st to end	Day	11% 3 8% 26% 32% 20%	532
August	1st to 10th	Day	9% 3 7% 25% 34% 22%	486
	11th to 20th	Day	10% 2 10% 30% 33% 14%	479
	21st to end	Day	12% 2 7% 19% 35% 26%	539
September	1st to 10th	Day	11% 3 12% 24% 30% 19%	490
	11th to 20th	Day	14% 3 11% 20% 34% 18%	490
	21st to end	Day	17% 6% 13% 24% 27% 12%	490
October	1st to 10th	Day	12% 4 14% 31% 31% 8%	490
	11th to 20th	Day	13% 10% 27% 28% 16% 6%	481
	21st to end	Day	23% 11% 22% 29% 13% 2	330
November	1st to 10th	Day	18% 6% 27% 27% 17% 5%	282
	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% 9% 30% 29% 12% 1	563
	21st to end	Night	19% 6% 30% 34% 10% 1	570
May	1st to 10th	Night	12% 5% 30% 35% 17% 1	570
	11th to 20th	Night	13% 4 21% 35% 24% 3	561
	21st to end	Night	6% 2 14% 38% 33% 6%	617
June	1st to 10th	Night	10% 2 17% 36% 30% 5%	570
	11th to 20th	Night	4 2 9% 39% 40% 6%	570
	21st to end	Night	5% 2 11% 38% 38% 6%	570
July	1st to 10th	Night	10% 2 8% 30% 42% 8%	555
	11th to 20th	Night	7% 3 7% 28% 46% 10%	552
	21st to end	Night	9% 2 6% 33% 42% 8%	624
August	1st to 10th	Night	8% 1 6% 30% 48% 7%	567
	11th to 20th	Night	9% 3 6% 40% 39% 4	564
	21st to end	Night	8% 1 6% 30% 47% 8%	627
September	1st to 10th	Night	11% 2 8% 32% 41% 7%	598
	11th to 20th	Night	11% 2 12% 32% 37% 7%	760
	21st to end	Night	13% 2 14% 33% 33% 5%	760
October	1st to 10th	Night	11% 4 16% 33% 30% 5%	760
	11th to 20th	Night	18% 6% 20% 36% 18% 2	760
	21st to end	Night	19% 6% 27% 32% 15% 2	836
November	1st to 10th	Night	15% 5% 21% 38% 19% 3	760
	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 2024-11-03 01:11:27. Version ac07

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