Cumberland Plateau Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cumberland Plateau Observatory clear sky chart from 2005-11-06 to 2024-11-10.

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	14% 8% 3 7% 5% 4 5% 4 6% 7% 38%	100941
	Night	14% 9% 3 6% 4 4 4 3 5% 7% 40%	98163
Transparency	Day	43% 7% 11% 22% 15% 3	104696
	Night	41% 7% 11% 22% 17% 4	102546
Seeing	Day	19% 13% 24% 22% 15% 6%	14670
	Night	18% 11% 23% 27% 17% 3	24689

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% 14% 6% 11% 5% 5% 5% 4 5% 5% 25%	2836
	11th to 20th	Day	19% 13% 5% 13% 4 4 4 2 3 4 27%	2840
	21st to end	Day	24% 14% 5% 6% 3 3 3 3 3 5% 28%	3225
February	1st to 10th	Day	18% 13% 4 8% 4 3 4 3 5% 5% 32%	1900
	11th to 20th	Day	20% 12% 4 9% 5% 4 3 2 3 5% 31%	1848
	21st to end	Day	21% 11% 5% 8% 4 4 3 3 5% 5% 32%	1570
March	1st to 10th	Day	23% 12% 3 7% 4 4 4 3 4 5% 31%	3120
	11th to 20th	Day	14% 8% 3 6% 4 4 4 3 4 5% 46%	3448
	21st to end	Day	15% 9% 3 7% 5% 4 4 3 5% 6% 39%	3815
April	1st to 10th	Day	14% 7% 2 7% 5% 5% 3 3 6% 6% 42%	2280
	11th to 20th	Day	15% 9% 2 6% 4 3 4 3 6% 6% 42%	2254
	21st to end	Day	14% 8% 2 7% 5% 5% 5% 4 5% 6% 38%	2418
May	1st to 10th	Day	19% 8% 3 7% 4 5% 5% 4 6% 7% 33%	4082
	11th to 20th	Day	18% 9% 3 8% 5% 5% 4 4 5% 7% 33%	4081
	21st to end	Day	11% 6% 3 7% 5% 5% 5% 4 7% 8% 38%	4476
June	1st to 10th	Day	11% 7% 3 7% 4 4 6% 5% 6% 8% 39%	2470
	11th to 20th	Day	6% 5% 2 6% 5% 5% 6% 5% 7% 10% 43%	2470
	21st to end	Day	7% 5% 1 6% 5% 6% 6% 5% 10% 11% 38%	2470
July	1st to 10th	Day	8% 5% 2 7% 5% 6% 6% 5% 8% 11% 36%	4120
	11th to 20th	Day	9% 6% 2 7% 5% 5% 6% 4 7% 9% 38%	4120
	21st to end	Day	19% 10% 3 10% 7% 6% 5% 4 6% 8% 21%	4466
August	1st to 10th	Day	7% 5% 2 5% 5% 5% 6% 5% 7% 10% 43%	4104
	11th to 20th	Day	8% 4 2 7% 5% 5% 6% 6% 8% 12% 37%	3772
	21st to end	Day	6% 4 1 5% 4 5% 5% 4 6% 9% 50%	4180
September	1st to 10th	Day	14% 6% 3 6% 5% 5% 6% 4 5% 6% 40%	2180
	11th to 20th	Day	14% 7% 2 7% 3 5% 5% 4 6% 6% 42%	2090
	21st to end	Day	14% 6% 2 5% 4 3 4 4 6% 5% 48%	2090
October	1st to 10th	Day	10% 6% 2 4 3 3 4 2 5% 6% 55%	2090
	11th to 20th	Day	13% 5% 2 5% 3 3 3 2 4 6% 52%	2014
	21st to end	Day	18% 6% 2 4 4 3 3 3 5% 7% 45%	1952
November	1st to 10th	Day	12% 6% 2 4 4 3 3 3 5% 5% 54%	1753
	11th to 20th	Day	14% 8% 2 5% 3 3 4 3 5% 6% 46%	1710
	21st to end	Day	16% 9% 3 6% 3 4 2 3 5% 5% 43%	1710
December	1st to 10th	Day	23% 11% 4 7% 4 4 4 2 4 5% 31%	1710
	11th to 20th	Day	17% 13% 4 7% 5% 4 4 3 4 6% 32%	2153
	21st to end	Day	18% 15% 4 6% 4 4 4 3 6% 7% 28%	3124
January	1st to 10th	Night	19% 16% 5% 8% 4 3 3 2 4 5% 31%	4109
	11th to 20th	Night	21% 13% 4 8% 5% 5% 3 2 3 4 32%	4120
	21st to end	Night	22% 15% 4 7% 4 3 3 3 4 4 32%	4427
February	1st to 10th	Night	20% 11% 4 8% 4 4 4 3 5% 5% 31%	2280
	11th to 20th	Night	22% 10% 5% 9% 4 4 3 3 3 5% 33%	2218
	21st to end	Night	22% 13% 4 7% 4 4 3 4 4 5% 29%	1884
March	1st to 10th	Night	22% 14% 3 6% 4 4 3 4 4 5% 31%	3733
	11th to 20th	Night	14% 7% 2 5% 3 3 4 4 5% 6% 47%	3496
	21st to end	Night	14% 9% 3 8% 5% 4 5% 4 5% 6% 38%	3817
April	1st to 10th	Night	13% 7% 2 7% 4 3 5% 4 6% 7% 43%	1914
	11th to 20th	Night	16% 8% 3 4 3 3 3 3 5% 7% 46%	1875
	21st to end	Night	14% 8% 3 6% 4 4 4 3 5% 6% 42%	1762
May	1st to 10th	Night	14% 9% 3 8% 5% 4 4 4 6% 7% 36%	2813
	11th to 20th	Night	15% 9% 3 7% 5% 5% 5% 4 6% 8% 35%	2812
	21st to end	Night	7% 5% 2 7% 6% 5% 5% 4 8% 11% 42%	3079
June	1st to 10th	Night	7% 6% 2 6% 5% 6% 5% 4 7% 10% 42%	1710
	11th to 20th	Night	4 4 1 5% 5% 4 5% 5% 7% 12% 49%	1710
	21st to end	Night	5% 5% 1 5% 5% 5% 6% 5% 8% 11% 42%	1710
July	1st to 10th	Night	8% 5% 2 6% 5% 5% 6% 6% 8% 11% 39%	2840
	11th to 20th	Night	10% 4 2 5% 4 4 4 4 6% 9% 47%	2840
	21st to end	Night	13% 10% 4 10% 7% 5% 6% 5% 7% 8% 25%	3087
August	1st to 10th	Night	6% 4 2 5% 4 4 5% 3 6% 9% 52%	2825
	11th to 20th	Night	7% 5% 2 6% 4 4 5% 4 8% 11% 44%	3053
	21st to end	Night	4 3 1 4 2 3 4 4 6% 9% 61%	3476
September	1st to 10th	Night	11% 6% 2 6% 5% 4 4 3 6% 7% 47%	1981
	11th to 20th	Night	11% 7% 2 7% 4 4 5% 3 5% 7% 45%	2090
	21st to end	Night	11% 5% 2 5% 4 3 4 3 5% 7% 50%	2090
October	1st to 10th	Night	11% 5% 2 3 2 2 3 2 4 6% 60%	2090
	11th to 20th	Night	17% 4 2 4 3 4 4 2 4 5% 51%	2166
	21st to end	Night	15% 7% 2 6% 3 3 2 2 4 6% 49%	2642
November	1st to 10th	Night	12% 6% 3 6% 4 3 3 3 4 6% 50%	2530
	11th to 20th	Night	14% 8% 2 4 3 3 3 3 5% 7% 48%	2470
	21st to end	Night	16% 10% 3 5% 4 3 3 3 4 6% 44%	2470
December	1st to 10th	Night	20% 15% 4 7% 4 3 4 3 4 5% 30%	2470
	11th to 20th	Night	17% 13% 5% 8% 4 3 3 3 3 5% 36%	3042
	21st to end	Night	14% 14% 5% 10% 6% 6% 5% 4 5% 6% 24%	4532

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% 6% 7% 16% 11% 2	2837
	11th to 20th	Day	62% 4 6% 14% 13% 1	2840
	21st to end	Day	58% 5% 7% 20% 10% 1	3225
February	1st to 10th	Day	55% 5% 6% 16% 15% 2	1900
	11th to 20th	Day	57% 4 7% 17% 13% 3	1858
	21st to end	Day	54% 4 10% 14% 14% 5%	1570
March	1st to 10th	Day	54% 6% 6% 16% 13% 6%	3120
	11th to 20th	Day	40% 5% 10% 24% 16% 5%	3448
	21st to end	Day	44% 7% 9% 25% 14% 1	3815
April	1st to 10th	Day	41% 6% 11% 22% 16% 3	2280
	11th to 20th	Day	41% 7% 8% 20% 22% 3	2254
	21st to end	Day	44% 7% 10% 25% 13% 2	2418
May	1st to 10th	Day	47% 10% 11% 21% 10% 1	4120
	11th to 20th	Day	48% 8% 12% 23% 9%	4085
	21st to end	Day	40% 10% 11% 20% 17% 2	4489
June	1st to 10th	Day	40% 9% 13% 22% 14% 2	2470
	11th to 20th	Day	33% 10% 17% 22% 16% 2	2470
	21st to end	Day	35% 11% 17% 19% 13% 4	2470
July	1st to 10th	Day	37% 10% 21% 21% 8% 2	4055
	11th to 20th	Day	40% 8% 17% 26% 7% 2	4051
	21st to end	Day	59% 8% 13% 14% 5% 1	4324
August	1st to 10th	Day	32% 10% 16% 27% 14% 1	4091
	11th to 20th	Day	34% 10% 17% 25% 13% 1	3809
	21st to end	Day	28% 9% 12% 36% 13% 1	4179
September	1st to 10th	Day	42% 8% 12% 16% 17% 4	2180
	11th to 20th	Day	41% 7% 12% 21% 17% 2	2090
	21st to end	Day	36% 7% 11% 23% 20% 4	2090
October	1st to 10th	Day	24% 4 7% 20% 38% 7%	3480
	11th to 20th	Day	24% 4 10% 22% 31% 10%	3332
	21st to end	Day	30% 4 8% 27% 22% 9%	3189
November	1st to 10th	Day	33% 5% 9% 22% 26% 4	1753
	11th to 20th	Day	38% 5% 7% 22% 22% 5%	1710
	21st to end	Day	42% 5% 7% 21% 20% 4	1710
December	1st to 10th	Day	55% 5% 7% 19% 12% 2	1710
	11th to 20th	Day	53% 5% 8% 17% 15% 2	2150
	21st to end	Day	53% 6% 10% 20% 9% 2	3124
January	1st to 10th	Night	55% 5% 7% 19% 13% 1	4109
	11th to 20th	Night	57% 4 6% 13% 16% 5%	4120
	21st to end	Night	56% 4 8% 15% 16% 1	4427
February	1st to 10th	Night	54% 5% 8% 17% 14% 1	2280
	11th to 20th	Night	55% 4 7% 17% 13% 4	2219
	21st to end	Night	57% 5% 6% 13% 15% 3	1884
March	1st to 10th	Night	55% 6% 9% 13% 15% 3	3733
	11th to 20th	Night	37% 6% 9% 26% 13% 9%	3496
	21st to end	Night	45% 6% 10% 23% 15% 1	3817
April	1st to 10th	Night	39% 7% 10% 23% 18% 3	1914
	11th to 20th	Night	38% 5% 8% 20% 22% 5%	1875
	21st to end	Night	40% 7% 10% 28% 14% 1	1762
May	1st to 10th	Night	44% 9% 13% 23% 9% 1	2840
	11th to 20th	Night	44% 9% 14% 20% 13%	2813
	21st to end	Night	31% 12% 16% 24% 15% 2	3087
June	1st to 10th	Night	35% 9% 13% 27% 14% 2	1710
	11th to 20th	Night	25% 8% 21% 26% 17% 2	1710
	21st to end	Night	32% 11% 16% 24% 13% 4	1710
July	1st to 10th	Night	34% 12% 20% 24% 9% 1	2795
	11th to 20th	Night	31% 8% 18% 31% 10% 2	2786
	21st to end	Night	55% 10% 14% 14% 7% 1	2988
August	1st to 10th	Night	27% 8% 14% 34% 17% 1	2814
	11th to 20th	Night	30% 10% 20% 28% 12% 1	3082
	21st to end	Night	20% 7% 13% 39% 19% 2	3476
September	1st to 10th	Night	35% 9% 12% 21% 18% 5%	1981
	11th to 20th	Night	38% 7% 11% 22% 22% 2	2090
	21st to end	Night	32% 6% 11% 22% 23% 5%	2090
October	1st to 10th	Night	22% 5% 7% 23% 34% 9%	3480
	11th to 20th	Night	23% 3 6% 25% 27% 15%	3587
	21st to end	Night	25% 3 8% 23% 28% 12%	4355
November	1st to 10th	Night	36% 4 7% 23% 24% 6%	2530
	11th to 20th	Night	36% 5% 8% 21% 24% 7%	2470
	21st to end	Night	43% 3 7% 24% 19% 4	2470
December	1st to 10th	Night	56% 5% 7% 17% 11% 2	2470
	11th to 20th	Night	51% 5% 7% 18% 17% 3	3044
	21st to end	Night	56% 8% 10% 15% 10% 1	4532

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	29% 23% 34% 11% 2 1	270
	11th to 20th	Day	38% 30% 19% 9% 3	270
	21st to end	Day	29% 27% 34% 7% 3	297
February	1st to 10th	Day	30% 26% 29% 14% 1	270
	11th to 20th	Day	40% 26% 26% 5% 2 1	270
	21st to end	Day	42% 24% 30% 3 1	225
March	1st to 10th	Day	32% 21% 33% 11% 3	270
	11th to 20th	Day	29% 23% 27% 16% 4 1	441
	21st to end	Day	29% 20% 29% 15% 6%	506
April	1st to 10th	Day	28% 17% 27% 19% 8%	460
	11th to 20th	Day	21% 19% 31% 20% 8% 1	452
	21st to end	Day	23% 16% 33% 17% 9% 2	460
May	1st to 10th	Day	20% 17% 33% 18% 8% 4	463
	11th to 20th	Day	16% 12% 29% 25% 10% 7%	487
	21st to end	Day	14% 8% 25% 26% 17% 10%	529
June	1st to 10th	Day	12% 6% 24% 28% 19% 10%	490
	11th to 20th	Day	7% 3 20% 32% 28% 10%	490
	21st to end	Day	9% 5% 25% 29% 20% 11%	490
July	1st to 10th	Day	12% 2 17% 27% 25% 16%	485
	11th to 20th	Day	9% 7% 15% 26% 27% 15%	484
	21st to end	Day	13% 5% 14% 28% 28% 13%	532
August	1st to 10th	Day	13% 2 16% 27% 30% 12%	486
	11th to 20th	Day	11% 5% 15% 35% 28% 6%	479
	21st to end	Day	12% 4 9% 27% 32% 16%	539
September	1st to 10th	Day	13% 4 15% 26% 30% 12%	490
	11th to 20th	Day	15% 4 13% 28% 27% 13%	490
	21st to end	Day	16% 7% 19% 29% 21% 8%	490
October	1st to 10th	Day	12% 6% 22% 31% 23% 7%	490
	11th to 20th	Day	14% 13% 28% 25% 15% 4	490
	21st to end	Day	23% 12% 28% 26% 9% 2	401
November	1st to 10th	Day	17% 16% 32% 25% 9% 2	297
	11th to 20th	Day	21% 17% 38% 20% 5%	270
	21st to end	Day	22% 18% 33% 18% 7% 2	270
December	1st to 10th	Day	34% 20% 27% 14% 4	270
	11th to 20th	Day	30% 21% 26% 15% 7%	270
	21st to end	Day	31% 24% 28% 13% 3	297
January	1st to 10th	Night	26% 21% 29% 21% 4	760
	11th to 20th	Night	30% 24% 25% 18% 3	760
	21st to end	Night	24% 19% 36% 19% 2	836
February	1st to 10th	Night	26% 16% 34% 21% 3	760
	11th to 20th	Night	30% 20% 33% 16% 1	743
	21st to end	Night	34% 22% 28% 15% 1	628
March	1st to 10th	Night	24% 16% 33% 23% 3	760
	11th to 20th	Night	24% 18% 31% 20% 6% 1	760
	21st to end	Night	24% 17% 30% 22% 7%	836
April	1st to 10th	Night	16% 16% 32% 27% 8%	584
	11th to 20th	Night	20% 12% 34% 23% 11%	563
	21st to end	Night	18% 11% 32% 30% 9% 1	570
May	1st to 10th	Night	16% 9% 30% 30% 14% 1	570
	11th to 20th	Night	13% 6% 25% 34% 18% 3	561
	21st to end	Night	7% 3 21% 35% 29% 4	617
June	1st to 10th	Night	8% 4 17% 37% 29% 5%	570
	11th to 20th	Night	4 2 16% 38% 33% 7%	570
	21st to end	Night	5% 2 16% 39% 32% 6%	570
July	1st to 10th	Night	11% 3 11% 26% 42% 8%	555
	11th to 20th	Night	9% 3 11% 26% 43% 8%	552
	21st to end	Night	9% 2 7% 33% 42% 6%	624
August	1st to 10th	Night	11% 2 7% 29% 44% 7%	567
	11th to 20th	Night	8% 1 9% 41% 34% 5%	564
	21st to end	Night	8% 2 7% 32% 42% 8%	627
September	1st to 10th	Night	11% 4 11% 30% 38% 6%	651
	11th to 20th	Night	13% 3 13% 30% 35% 6%	760
	21st to end	Night	13% 4 18% 32% 29% 5%	760
October	1st to 10th	Night	11% 5% 18% 31% 29% 5%	760
	11th to 20th	Night	19% 8% 21% 34% 17% 2	760
	21st to end	Night	19% 9% 27% 29% 15% 2	836
November	1st to 10th	Night	15% 10% 25% 34% 15% 2	779
	11th to 20th	Night	17% 16% 29% 27% 10% 1	760
	21st to end	Night	22% 15% 27% 28% 9%	760
December	1st to 10th	Night	29% 16% 27% 25% 4	760
	11th to 20th	Night	24% 16% 29% 24% 6% 1	760
	21st to end	Night	27% 20% 26% 24% 4	836

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