Mammoth Cave National Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mammoth Cave National Park clear sky chart from 2005-11-06 to 2025-05-11.

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% 4 4 4 4 5% 7% 39%	114080
	Night	14% 9% 3 7% 4 4 4 3 5% 6% 41%	114932
Transparency	Day	42% 6% 10% 23% 17% 2	117790
	Night	42% 5% 9% 22% 18% 3	119363
Seeing	Day	17% 10% 24% 23% 17% 8%	15863
	Night	18% 8% 24% 30% 18% 2	25664

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% 12% 9% 9% 5% 4 4 4 5% 5% 28%	3617
	11th to 20th	Day	15% 10% 4 12% 5% 3 4 2 3 4 38%	3812
	21st to end	Day	20% 10% 6% 5% 3 4 3 3 4 6% 36%	4678
February	1st to 10th	Day	18% 12% 4 8% 4 3 3 3 4 5% 34%	2000
	11th to 20th	Day	21% 9% 4 9% 5% 3 3 2 4 6% 33%	1948
	21st to end	Day	19% 10% 3 7% 4 3 5% 4 5% 5% 36%	1650
March	1st to 10th	Day	19% 12% 4 7% 4 4 3 3 4 5% 35%	4480
	11th to 20th	Day	15% 9% 3 6% 3 3 3 3 4 7% 45%	4837
	21st to end	Day	12% 7% 3 9% 5% 4 5% 3 5% 7% 40%	5645
April	1st to 10th	Day	13% 7% 2 7% 5% 4 4 4 5% 7% 42%	2400
	11th to 20th	Day	15% 8% 2 7% 5% 4 4 3 4 6% 41%	2374
	21st to end	Day	15% 7% 3 6% 5% 4 5% 4 6% 7% 37%	2513
May	1st to 10th	Day	20% 9% 2 7% 5% 5% 5% 4 5% 8% 31%	4212
	11th to 20th	Day	17% 8% 3 7% 4 4 4 3 5% 7% 37%	4081
	21st to end	Day	12% 7% 3 7% 5% 5% 6% 4 6% 8% 39%	4476
June	1st to 10th	Day	11% 6% 2 6% 5% 5% 6% 5% 6% 7% 41%	2486
	11th to 20th	Day	6% 4 2 4 5% 5% 6% 5% 8% 11% 44%	2660
	21st to end	Day	7% 4 2 6% 5% 4 6% 5% 8% 11% 41%	2660
July	1st to 10th	Day	9% 5% 2 7% 6% 6% 6% 5% 7% 10% 37%	4440
	11th to 20th	Day	7% 5% 2 6% 5% 5% 5% 4 8% 11% 42%	4347
	21st to end	Day	17% 10% 3 9% 7% 7% 6% 4 6% 9% 23%	4466
August	1st to 10th	Day	5% 4 2 4 4 4 4 5% 7% 11% 51%	4084
	11th to 20th	Day	8% 6% 2 6% 5% 5% 6% 5% 8% 11% 39%	3729
	21st to end	Day	5% 3 1 4 4 4 5% 5% 7% 8% 53%	4180
September	1st to 10th	Day	12% 7% 2 6% 5% 4 5% 4 6% 8% 41%	2227
	11th to 20th	Day	12% 6% 2 5% 4 5% 4 3 6% 7% 46%	2090
	21st to end	Day	13% 5% 2 5% 5% 4 3 3 4 7% 48%	2090
October	1st to 10th	Day	10% 5% 2 4 3 4 3 2 5% 7% 56%	2090
	11th to 20th	Day	13% 6% 2 5% 4 3 3 3 5% 6% 51%	2038
	21st to end	Day	19% 6% 3 5% 4 3 4 3 5% 5% 44%	1881
November	1st to 10th	Day	12% 6% 2 4 4 3 4 2 4 7% 52%	1762
	11th to 20th	Day	16% 6% 3 5% 3 4 3 2 4 5% 47%	1800
	21st to end	Day	17% 9% 3 6% 3 4 3 3 4 5% 43%	1800
December	1st to 10th	Day	17% 9% 3 6% 4 3 4 3 4 5% 42%	2930
	11th to 20th	Day	20% 10% 5% 8% 5% 3 4 3 4 5% 31%	3373
	21st to end	Day	21% 15% 5% 8% 5% 5% 5% 3 4 5% 26%	4224
January	1st to 10th	Night	16% 14% 5% 10% 5% 4 3 3 5% 6% 29%	6348
	11th to 20th	Night	18% 12% 4 9% 5% 5% 4 2 3 3 35%	6152
	21st to end	Night	20% 11% 3 6% 4 3 3 2 3 5% 40%	6263
February	1st to 10th	Night	21% 11% 4 10% 4 3 4 3 5% 5% 30%	2400
	11th to 20th	Night	22% 10% 4 7% 4 4 3 3 4 4 34%	2338
	21st to end	Night	22% 10% 3 8% 4 4 5% 3 3 5% 33%	1980
March	1st to 10th	Night	20% 14% 3 7% 3 3 3 3 4 5% 35%	5373
	11th to 20th	Night	12% 6% 2 5% 4 3 4 4 5% 6% 48%	5123
	21st to end	Night	14% 8% 3 7% 5% 4 5% 4 6% 7% 37%	5313
April	1st to 10th	Night	13% 7% 2 6% 5% 3 4 4 6% 7% 43%	2000
	11th to 20th	Night	15% 8% 2 6% 4 4 3 3 5% 6% 44%	1975
	21st to end	Night	13% 8% 3 8% 4 5% 4 5% 7% 6% 38%	1885
May	1st to 10th	Night	18% 9% 3 7% 5% 4 4 4 6% 6% 33%	2903
	11th to 20th	Night	15% 7% 3 6% 4 4 4 4 6% 8% 40%	2818
	21st to end	Night	7% 6% 2 7% 5% 4 5% 5% 7% 10% 42%	3079
June	1st to 10th	Night	10% 4 2 6% 4 4 4 4 6% 9% 46%	1687
	11th to 20th	Night	4 3 2 4 4 4 4 5% 7% 11% 52%	1520
	21st to end	Night	4 3 1 5% 4 5% 6% 5% 7% 11% 48%	1520
July	1st to 10th	Night	6% 5% 2 5% 6% 4 4 5% 8% 11% 44%	2530
	11th to 20th	Night	7% 4 2 5% 4 4 4 4 7% 10% 50%	2615
	21st to end	Night	11% 10% 4 10% 7% 5% 7% 4 7% 7% 27%	3087
August	1st to 10th	Night	6% 4 2 4 3 3 3 3 5% 9% 58%	2842
	11th to 20th	Night	5% 4 2 5% 4 4 5% 5% 7% 10% 47%	3100
	21st to end	Night	4 3 1 4 3 3 3 4 6% 9% 61%	3476
September	1st to 10th	Night	11% 5% 2 5% 3 5% 4 4 5% 7% 50%	1933
	11th to 20th	Night	10% 6% 2 6% 4 3 4 3 5% 8% 48%	2090
	21st to end	Night	12% 6% 2 5% 3 4 3 3 4 6% 53%	2090
October	1st to 10th	Night	9% 5% 2 4 3 3 2 2 4 5% 61%	2090
	11th to 20th	Night	14% 6% 2 5% 3 3 3 2 4 6% 52%	2139
	21st to end	Night	16% 8% 2 5% 3 3 3 3 4 5% 48%	2717
November	1st to 10th	Night	12% 7% 2 5% 4 4 4 3 3 5% 52%	2536
	11th to 20th	Night	15% 7% 2 5% 4 4 4 3 5% 6% 47%	2600
	21st to end	Night	17% 10% 3 5% 4 3 3 2 5% 6% 42%	2600
December	1st to 10th	Night	19% 11% 3 8% 4 3 3 2 4 4 39%	4250
	11th to 20th	Night	23% 9% 3 7% 3 3 3 3 4 6% 36%	4822
	21st to end	Night	20% 15% 6% 10% 6% 5% 4 3 4 4 24%	6738

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	57% 4 10% 18% 10% 1	3618
	11th to 20th	Day	51% 3 7% 21% 17% 2	3812
	21st to end	Day	49% 3 9% 21% 15% 3	4678
February	1st to 10th	Day	52% 3 10% 19% 14% 2	2000
	11th to 20th	Day	54% 4 8% 20% 12% 3	1958
	21st to end	Day	49% 5% 10% 16% 15% 5%	1650
March	1st to 10th	Day	52% 4 5% 17% 18% 5%	4480
	11th to 20th	Day	40% 4 8% 25% 20% 3	4837
	21st to end	Day	43% 5% 10% 28% 13% 1	5647
April	1st to 10th	Day	41% 5% 11% 24% 17% 2	2400
	11th to 20th	Day	43% 5% 8% 23% 18% 3	2374
	21st to end	Day	44% 7% 9% 24% 13% 2	2513
May	1st to 10th	Day	50% 7% 12% 20% 10%	4250
	11th to 20th	Day	46% 7% 10% 23% 14%	4085
	21st to end	Day	40% 8% 11% 19% 20% 2	4489
June	1st to 10th	Day	39% 8% 11% 22% 18% 2	2486
	11th to 20th	Day	31% 8% 17% 24% 18% 2	2660
	21st to end	Day	34% 9% 16% 20% 18% 4	2660
July	1st to 10th	Day	39% 8% 16% 23% 12% 2	4371
	11th to 20th	Day	34% 8% 15% 31% 10% 2	4272
	21st to end	Day	56% 8% 12% 17% 6% 1	4324
August	1st to 10th	Day	25% 9% 15% 33% 17% 1	4071
	11th to 20th	Day	36% 8% 14% 25% 17% 1	3765
	21st to end	Day	26% 8% 11% 36% 18% 2	4179
September	1st to 10th	Day	40% 7% 12% 18% 18% 5%	2227
	11th to 20th	Day	38% 5% 9% 23% 22% 3	2090
	21st to end	Day	35% 6% 9% 24% 22% 4	2090
October	1st to 10th	Day	20% 3 6% 19% 47% 4	3480
	11th to 20th	Day	24% 4 8% 27% 32% 6%	3357
	21st to end	Day	30% 3 7% 29% 24% 8%	3081
November	1st to 10th	Day	34% 4 8% 26% 23% 5%	1762
	11th to 20th	Day	39% 3 8% 24% 21% 5%	1800
	21st to end	Day	45% 4 7% 23% 19% 2	1800
December	1st to 10th	Day	45% 3 7% 23% 20% 1	2930
	11th to 20th	Day	55% 3 9% 18% 13% 2	3370
	21st to end	Day	60% 5% 10% 18% 7%	4224
January	1st to 10th	Night	54% 4 10% 20% 10% 1	6348
	11th to 20th	Night	55% 4 5% 18% 14% 4	6152
	21st to end	Night	48% 3 7% 19% 20% 3	6263
February	1st to 10th	Night	55% 4 10% 18% 13% 1	2400
	11th to 20th	Night	54% 4 7% 18% 15% 3	2339
	21st to end	Night	54% 4 8% 16% 15% 3	1980
March	1st to 10th	Night	52% 3 6% 18% 16% 4	5373
	11th to 20th	Night	35% 5% 8% 25% 20% 8%	5123
	21st to end	Night	45% 5% 10% 25% 13% 1	5313
April	1st to 10th	Night	38% 7% 10% 23% 20% 3	2000
	11th to 20th	Night	42% 4 8% 19% 22% 6%	1975
	21st to end	Night	43% 6% 8% 24% 16% 2	1885
May	1st to 10th	Night	48% 8% 11% 22% 9% 2	2930
	11th to 20th	Night	42% 8% 13% 23% 15%	2819
	21st to end	Night	33% 11% 14% 22% 18% 3	3087
June	1st to 10th	Night	33% 8% 11% 27% 16% 4	1687
	11th to 20th	Night	25% 6% 16% 27% 23% 3	1520
	21st to end	Night	27% 9% 18% 22% 18% 6%	1520
July	1st to 10th	Night	32% 8% 16% 29% 13% 2	2490
	11th to 20th	Night	29% 7% 14% 35% 13% 3	2567
	21st to end	Night	53% 9% 11% 17% 9% 1	2988
August	1st to 10th	Night	23% 6% 12% 37% 21% 1	2831
	11th to 20th	Night	29% 8% 17% 29% 16% 1	3130
	21st to end	Night	19% 7% 12% 36% 23% 3	3476
September	1st to 10th	Night	34% 6% 9% 22% 23% 5%	1933
	11th to 20th	Night	34% 6% 10% 24% 22% 4	2090
	21st to end	Night	33% 5% 8% 24% 25% 6%	2090
October	1st to 10th	Night	19% 4 6% 18% 45% 7%	3480
	11th to 20th	Night	22% 3 6% 25% 36% 9%	3559
	21st to end	Night	28% 3 8% 21% 30% 10%	4467
November	1st to 10th	Night	36% 3 7% 24% 24% 5%	2536
	11th to 20th	Night	38% 4 6% 22% 21% 8%	2600
	21st to end	Night	43% 4 7% 24% 19% 3	2600
December	1st to 10th	Night	50% 3 7% 20% 19% 1	4250
	11th to 20th	Night	50% 4 8% 21% 16% 2	4824
	21st to end	Night	63% 4 8% 17% 7% 1	6738

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	26% 18% 31% 19% 5% 1	300
	11th to 20th	Day	30% 21% 31% 13% 4	300
	21st to end	Day	21% 23% 40% 12% 3	330
February	1st to 10th	Day	28% 21% 35% 13% 3 1	300
	11th to 20th	Day	34% 20% 33% 9% 4	300
	21st to end	Day	33% 16% 36% 12% 3	249
March	1st to 10th	Day	25% 19% 36% 16% 2 1	300
	11th to 20th	Day	25% 18% 37% 14% 6% 1	435
	21st to end	Day	27% 17% 33% 17% 6% 1	550
April	1st to 10th	Day	20% 17% 29% 24% 9% 1	500
	11th to 20th	Day	20% 17% 26% 22% 13% 2	492
	21st to end	Day	21% 13% 32% 21% 9% 4	500
May	1st to 10th	Day	23% 12% 30% 21% 10% 4	503
	11th to 20th	Day	17% 9% 22% 29% 15% 7%	487
	21st to end	Day	15% 5% 19% 28% 19% 15%	529
June	1st to 10th	Day	13% 5% 17% 25% 27% 13%	506
	11th to 20th	Day	6% 2 19% 31% 26% 14%	680
	21st to end	Day	8% 3 21% 31% 24% 12%	680
July	1st to 10th	Day	10% 3 15% 25% 27% 20%	671
	11th to 20th	Day	8% 4 16% 26% 28% 19%	614
	21st to end	Day	13% 3 13% 27% 26% 18%	532
August	1st to 10th	Day	9% 4 13% 24% 31% 19%	486
	11th to 20th	Day	11% 4 16% 32% 27% 11%	479
	21st to end	Day	10% 2 8% 25% 32% 23%	539
September	1st to 10th	Day	14% 2 11% 27% 30% 15%	490
	11th to 20th	Day	13% 3 13% 23% 35% 14%	490
	21st to end	Day	17% 6% 18% 25% 23% 12%	490
October	1st to 10th	Day	11% 5% 17% 30% 28% 9%	490
	11th to 20th	Day	16% 12% 26% 26% 16% 5%	481
	21st to end	Day	23% 12% 28% 27% 9% 1	330
November	1st to 10th	Day	17% 10% 30% 27% 13% 3	300
	11th to 20th	Day	18% 16% 34% 24% 8%	300
	21st to end	Day	24% 13% 30% 20% 11% 1	300
December	1st to 10th	Day	26% 15% 32% 18% 8% 1	300
	11th to 20th	Day	27% 14% 29% 23% 7%	300
	21st to end	Day	28% 16% 34% 17% 5%	330
January	1st to 10th	Night	23% 14% 33% 25% 4	1000
	11th to 20th	Night	29% 15% 32% 21% 4	909
	21st to end	Night	22% 14% 39% 22% 2	880
February	1st to 10th	Night	28% 11% 37% 21% 3	800
	11th to 20th	Night	30% 14% 36% 19% 2	783
	21st to end	Night	27% 13% 36% 22% 2	660
March	1st to 10th	Night	23% 14% 33% 26% 5%	800
	11th to 20th	Night	21% 14% 33% 26% 6%	800
	21st to end	Night	22% 13% 29% 30% 6%	816
April	1st to 10th	Night	16% 14% 31% 30% 10% 1	600
	11th to 20th	Night	20% 10% 31% 27% 12% 1	593
	21st to end	Night	17% 8% 33% 32% 10% 1	600
May	1st to 10th	Night	18% 7% 28% 31% 14% 2	600
	11th to 20th	Night	12% 6% 21% 37% 20% 3	563
	21st to end	Night	6% 2 17% 37% 32% 6%	617
June	1st to 10th	Night	11% 4 15% 37% 30% 4	570
	11th to 20th	Night	6% 2 13% 41% 35% 4	570
	21st to end	Night	5% 2 13% 42% 33% 5%	570
July	1st to 10th	Night	10% 2 8% 29% 43% 8%	555
	11th to 20th	Night	9% 1 8% 30% 45% 7%	552
	21st to end	Night	10% 3 9% 36% 35% 7%	624
August	1st to 10th	Night	10% 2 7% 31% 44% 6%	567
	11th to 20th	Night	8% 1 12% 38% 36% 5%	564
	21st to end	Night	7% 1 5% 32% 47% 8%	627
September	1st to 10th	Night	14% 1 11% 32% 38% 5%	603
	11th to 20th	Night	12% 3 12% 31% 38% 5%	760
	21st to end	Night	14% 4 17% 31% 30% 4	760
October	1st to 10th	Night	11% 4 16% 37% 28% 4	760
	11th to 20th	Night	18% 6% 22% 33% 18% 2	760
	21st to end	Night	18% 7% 31% 28% 14% 1	836
November	1st to 10th	Night	15% 8% 25% 34% 16% 2	780
	11th to 20th	Night	17% 12% 30% 31% 10% 1	800
	21st to end	Night	22% 11% 25% 31% 10% 1	800
December	1st to 10th	Night	27% 11% 29% 27% 7%	800
	11th to 20th	Night	24% 10% 33% 26% 7% 1	800
	21st to end	Night	28% 13% 28% 26% 4	985

Page generated at 2025-05-08 08:33:59. Version ac07

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