Natural Tunnel State Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Natural Tunnel State Park clear sky chart from 2005-11-06 to 2026-01-02.

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% 5% 5% 4 6% 8% 36%	127215
	Night	15% 8% 3 7% 5% 4 4 4 5% 7% 38%	124161
Transparency	Day	45% 5% 11% 22% 15% 3	130926
	Night	44% 4 10% 23% 16% 3	128519
Seeing	Day	20% 9% 20% 24% 18% 8%	16497
	Night	18% 7% 19% 34% 19% 2	26577

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% 14% 4 9% 5% 4 4 2 4 6% 31%	4065
	11th to 20th	Day	16% 13% 8% 10% 6% 3 3 2 3 4 32%	4060
	21st to end	Day	16% 11% 3 7% 5% 4 3 3 4 5% 39%	4466
February	1st to 10th	Day	19% 11% 3 7% 5% 4 4 3 5% 6% 33%	1865
	11th to 20th	Day	22% 9% 4 8% 5% 4 3 3 4 6% 30%	1948
	21st to end	Day	23% 8% 3 7% 4 4 3 3 4 6% 34%	1650
March	1st to 10th	Day	19% 12% 4 7% 4 4 4 3 4 5% 35%	4788
	11th to 20th	Day	12% 7% 2 7% 4 3 4 3 5% 7% 46%	4980
	21st to end	Day	14% 8% 2 7% 4 4 4 3 5% 7% 41%	5465
April	1st to 10th	Day	13% 7% 2 7% 4 4 4 4 7% 8% 38%	2200
	11th to 20th	Day	15% 7% 2 6% 4 4 4 3 5% 8% 42%	2446
	21st to end	Day	15% 7% 2 7% 5% 5% 4 3 6% 7% 39%	2600
May	1st to 10th	Day	17% 9% 4 9% 5% 6% 6% 5% 7% 8% 25%	5862
	11th to 20th	Day	17% 8% 3 9% 6% 7% 6% 5% 6% 7% 26%	5861
	21st to end	Day	16% 6% 3 7% 5% 5% 6% 5% 7% 9% 33%	6433
June	1st to 10th	Day	11% 7% 2 7% 5% 5% 5% 5% 7% 9% 37%	2600
	11th to 20th	Day	7% 6% 2 7% 6% 5% 6% 6% 8% 10% 36%	2600
	21st to end	Day	9% 6% 3 6% 5% 4 6% 5% 8% 11% 36%	2600
July	1st to 10th	Day	6% 5% 2 6% 6% 6% 7% 5% 9% 10% 39%	5900
	11th to 20th	Day	10% 7% 3 8% 6% 6% 7% 6% 8% 11% 29%	5900
	21st to end	Day	14% 10% 3 9% 7% 6% 6% 5% 7% 9% 25%	6424
August	1st to 10th	Day	11% 7% 3 7% 6% 6% 6% 5% 8% 11% 31%	4234
	11th to 20th	Day	9% 5% 2 7% 5% 6% 6% 5% 8% 11% 36%	4174
	21st to end	Day	5% 3 1 4 4 4 4 4 6% 10% 55%	4394
September	1st to 10th	Day	10% 6% 2 6% 5% 5% 5% 4 5% 8% 43%	2200
	11th to 20th	Day	12% 6% 3 5% 5% 6% 5% 4 6% 7% 42%	2200
	21st to end	Day	16% 6% 2 6% 5% 4 4 4 6% 6% 41%	2200
October	1st to 10th	Day	13% 5% 2 5% 4 4 4 3 4 7% 51%	2165
	11th to 20th	Day	13% 6% 1 6% 3 3 4 3 5% 6% 51%	2000
	21st to end	Day	16% 8% 2 6% 4 3 4 2 4 7% 45%	2200
November	1st to 10th	Day	9% 5% 2 6% 4 3 4 3 6% 6% 50%	1907
	11th to 20th	Day	13% 9% 2 5% 3 4 4 3 4 8% 45%	1890
	21st to end	Day	15% 9% 3 7% 4 3 3 3 5% 5% 42%	1890
December	1st to 10th	Day	18% 9% 3 7% 4 3 3 3 4 6% 39%	3020
	11th to 20th	Day	22% 13% 4 8% 4 4 3 3 5% 6% 28%	3463
	21st to end	Day	16% 12% 6% 7% 5% 5% 5% 4 7% 7% 25%	4565
January	1st to 10th	Night	15% 12% 5% 10% 5% 4 4 3 4 5% 33%	5903
	11th to 20th	Night	19% 9% 3 10% 6% 4 3 2 3 4 37%	5900
	21st to end	Night	16% 9% 3 6% 4 4 4 3 4 6% 41%	6490
February	1st to 10th	Night	17% 9% 3 7% 5% 5% 4 3 5% 6% 35%	2532
	11th to 20th	Night	23% 9% 3 7% 4 4 4 4 5% 5% 31%	2338
	21st to end	Night	21% 9% 3 8% 5% 4 4 3 5% 6% 31%	1980
March	1st to 10th	Night	18% 9% 3 6% 5% 4 4 3 4 6% 37%	5064
	11th to 20th	Night	11% 5% 3 4 4 3 3 3 5% 8% 51%	4980
	21st to end	Night	12% 7% 2 6% 5% 5% 5% 4 5% 7% 40%	5467
April	1st to 10th	Night	12% 7% 3 6% 4 4 4 4 6% 8% 41%	2200
	11th to 20th	Night	12% 7% 3 7% 4 5% 4 4 5% 7% 43%	1917
	21st to end	Night	13% 6% 2 6% 4 5% 6% 4 6% 7% 41%	1800
May	1st to 10th	Night	19% 9% 4 8% 5% 4 5% 4 6% 9% 28%	4033
	11th to 20th	Night	17% 10% 3 8% 5% 5% 5% 4 5% 7% 30%	4032
	21st to end	Night	16% 9% 3 8% 4 4 4 4 6% 8% 34%	4421
June	1st to 10th	Night	10% 7% 2 7% 5% 4 5% 4 7% 11% 39%	1800
	11th to 20th	Night	8% 5% 3 6% 5% 5% 5% 5% 8% 10% 42%	1800
	21st to end	Night	10% 6% 2 7% 6% 5% 5% 4 7% 10% 38%	1800
July	1st to 10th	Night	9% 5% 2 6% 6% 5% 6% 6% 9% 12% 35%	4058
	11th to 20th	Night	11% 8% 3 7% 5% 5% 6% 5% 7% 10% 34%	4057
	21st to end	Night	14% 10% 4 9% 5% 6% 7% 5% 7% 9% 24%	4427
August	1st to 10th	Night	9% 7% 3 8% 5% 5% 5% 5% 8% 9% 36%	2915
	11th to 20th	Night	11% 6% 2 7% 4 5% 5% 4 7% 10% 40%	2875
	21st to end	Night	6% 3 1 4 3 4 4 4 6% 9% 56%	3464
September	1st to 10th	Night	12% 6% 3 7% 5% 4 4 4 4 7% 45%	2200
	11th to 20th	Night	12% 6% 2 7% 4 4 4 3 6% 8% 44%	2200
	21st to end	Night	15% 5% 2 5% 4 4 4 3 4 6% 47%	2200
October	1st to 10th	Night	12% 5% 2 5% 3 3 4 3 4 6% 52%	2235
	11th to 20th	Night	14% 6% 2 4 3 3 3 3 4 6% 52%	2400
	21st to end	Night	14% 6% 2 6% 4 3 3 3 5% 6% 48%	2640
November	1st to 10th	Night	12% 6% 2 6% 4 4 3 3 5% 6% 51%	2608
	11th to 20th	Night	13% 7% 2 7% 4 4 3 3 4 6% 46%	2730
	21st to end	Night	16% 8% 2 6% 3 3 4 3 4 6% 44%	2730
December	1st to 10th	Night	18% 9% 3 8% 5% 4 4 3 4 5% 37%	4380
	11th to 20th	Night	21% 12% 5% 10% 4 3 3 3 4 5% 30%	4952
	21st to end	Night	17% 11% 4 8% 4 5% 5% 4 6% 9% 28%	6633

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	54% 3 4 23% 14% 1	4066
	11th to 20th	Day	58% 2 5% 15% 18% 2	4060
	21st to end	Day	49% 2 8% 26% 14% 1	4466
February	1st to 10th	Day	52% 3 7% 19% 15% 2	1865
	11th to 20th	Day	57% 2 6% 19% 13% 2	1958
	21st to end	Day	53% 2 7% 17% 17% 4	1650
March	1st to 10th	Day	52% 2 6% 16% 17% 7%	4788
	11th to 20th	Day	39% 2 5% 21% 27% 6%	4980
	21st to end	Day	44% 3 7% 29% 15% 2	5465
April	1st to 10th	Day	42% 4 11% 24% 16% 3	2200
	11th to 20th	Day	42% 3 9% 21% 21% 5%	2446
	21st to end	Day	45% 3 11% 26% 13% 2	2600
May	1st to 10th	Day	55% 5% 11% 17% 10% 1	5897
	11th to 20th	Day	57% 5% 11% 20% 7% 1	5865
	21st to end	Day	47% 5% 12% 20% 13% 3	6447
June	1st to 10th	Day	42% 5% 15% 23% 14% 1	2600
	11th to 20th	Day	40% 7% 17% 20% 14% 2	2600
	21st to end	Day	39% 7% 15% 21% 16% 2	2600
July	1st to 10th	Day	35% 11% 18% 21% 14% 1	5835
	11th to 20th	Day	45% 11% 18% 20% 5% 1	5831
	21st to end	Day	53% 8% 13% 18% 8%	6240
August	1st to 10th	Day	43% 8% 16% 24% 8%	4221
	11th to 20th	Day	37% 8% 18% 24% 11% 2	4213
	21st to end	Day	26% 5% 18% 37% 13% 2	4394
September	1st to 10th	Day	39% 5% 13% 23% 16% 6%	2200
	11th to 20th	Day	41% 5% 9% 25% 18% 2	2200
	21st to end	Day	42% 4 10% 22% 18% 3	2200
October	1st to 10th	Day	28% 4 8% 21% 33% 6%	3529
	11th to 20th	Day	26% 3 9% 24% 24% 14%	3240
	21st to end	Day	31% 3 9% 29% 18% 10%	3538
November	1st to 10th	Day	33% 3 8% 23% 28% 5%	1907
	11th to 20th	Day	39% 2 6% 25% 22% 5%	1890
	21st to end	Day	44% 3 8% 22% 21% 3	1890
December	1st to 10th	Day	47% 3 6% 21% 21% 2	3020
	11th to 20th	Day	58% 2 9% 16% 13% 2	3460
	21st to end	Day	56% 5% 10% 20% 9%	4565
January	1st to 10th	Night	55% 2 5% 20% 16% 1	5903
	11th to 20th	Night	52% 3 5% 18% 18% 4	5900
	21st to end	Night	46% 3 7% 23% 18% 2	6490
February	1st to 10th	Night	50% 2 9% 22% 15% 2	2532
	11th to 20th	Night	55% 2 7% 21% 12% 4	2339
	21st to end	Night	54% 3 7% 20% 13% 4	1980
March	1st to 10th	Night	50% 2 5% 22% 16% 5%	5064
	11th to 20th	Night	33% 2 7% 26% 24% 8%	4980
	21st to end	Night	42% 4 10% 27% 15% 1	5467
April	1st to 10th	Night	41% 3 11% 24% 17% 3	2200
	11th to 20th	Night	40% 3 9% 21% 19% 6%	1917
	21st to end	Night	42% 4 12% 26% 14% 2	1800
May	1st to 10th	Night	53% 6% 13% 19% 8% 1	4060
	11th to 20th	Night	54% 5% 12% 22% 6%	4033
	21st to end	Night	48% 5% 12% 23% 11% 2	4429
June	1st to 10th	Night	39% 7% 17% 21% 15% 2	1800
	11th to 20th	Night	36% 8% 17% 23% 13% 2	1800
	21st to end	Night	40% 8% 14% 21% 14% 4	1800
July	1st to 10th	Night	37% 11% 20% 22% 10% 1	4015
	11th to 20th	Night	42% 10% 20% 21% 6% 2	4006
	21st to end	Night	53% 11% 13% 15% 8%	4297
August	1st to 10th	Night	41% 7% 19% 26% 7% 1	2904
	11th to 20th	Night	38% 6% 17% 26% 12%	2902
	21st to end	Night	23% 6% 17% 37% 15% 1	3464
September	1st to 10th	Night	39% 6% 12% 19% 19% 5%	2200
	11th to 20th	Night	38% 5% 11% 24% 20% 2	2200
	21st to end	Night	39% 4 10% 25% 19% 3	2200
October	1st to 10th	Night	29% 6% 7% 23% 29% 7%	3651
	11th to 20th	Night	24% 3 8% 25% 27% 12%	3899
	21st to end	Night	27% 2 5% 29% 26% 10%	4252
November	1st to 10th	Night	35% 3 8% 24% 24% 6%	2608
	11th to 20th	Night	40% 2 6% 22% 23% 6%	2730
	21st to end	Night	43% 3 7% 25% 20% 2	2730
December	1st to 10th	Night	49% 4 6% 22% 17% 1	4380
	11th to 20th	Night	58% 3 7% 20% 11% 1	4954
	21st to end	Night	53% 4 12% 19% 12%	6633

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	30% 21% 28% 18% 2 1	303
	11th to 20th	Day	36% 19% 24% 15% 6%	300
	21st to end	Day	28% 22% 30% 16% 4	330
February	1st to 10th	Day	28% 18% 31% 16% 6%	300
	11th to 20th	Day	42% 20% 27% 8% 3	300
	21st to end	Day	37% 16% 32% 13% 2	249
March	1st to 10th	Day	31% 11% 27% 23% 7% 1	426
	11th to 20th	Day	27% 14% 29% 25% 4 1	500
	21st to end	Day	25% 16% 28% 23% 9%	550
April	1st to 10th	Day	27% 11% 24% 26% 10% 2	500
	11th to 20th	Day	22% 15% 25% 23% 13% 2	492
	21st to end	Day	22% 9% 27% 25% 15% 2	500
May	1st to 10th	Day	22% 11% 28% 22% 12% 5%	503
	11th to 20th	Day	19% 9% 20% 27% 18% 7%	527
	21st to end	Day	18% 7% 17% 23% 21% 14%	573
June	1st to 10th	Day	15% 4 19% 26% 25% 11%	530
	11th to 20th	Day	11% 4 17% 27% 27% 13%	530
	21st to end	Day	12% 5% 19% 23% 30% 12%	530
July	1st to 10th	Day	13% 2 14% 27% 25% 18%	525
	11th to 20th	Day	12% 2 10% 25% 31% 19%	524
	21st to end	Day	14% 3 10% 27% 30% 16%	576
August	1st to 10th	Day	16% 4 13% 26% 25% 17%	526
	11th to 20th	Day	14% 3 11% 29% 28% 15%	519
	21st to end	Day	12% 2 10% 25% 30% 21%	583
September	1st to 10th	Day	12% 2 12% 28% 31% 14%	530
	11th to 20th	Day	12% 4 11% 25% 32% 16%	530
	21st to end	Day	21% 8% 14% 29% 22% 7%	530
October	1st to 10th	Day	15% 5% 15% 30% 26% 9%	530
	11th to 20th	Day	14% 10% 21% 30% 21% 4	530
	21st to end	Day	22% 8% 21% 26% 18% 4	583
November	1st to 10th	Day	16% 11% 25% 30% 14% 4	385
	11th to 20th	Day	23% 11% 27% 26% 11% 2	330
	21st to end	Day	23% 12% 29% 21% 14% 1	330
December	1st to 10th	Day	30% 15% 29% 22% 4	330
	11th to 20th	Day	30% 15% 24% 23% 6% 2	330
	21st to end	Day	27% 17% 26% 23% 7% 1	363
January	1st to 10th	Night	24% 14% 26% 33% 3	805
	11th to 20th	Night	29% 12% 25% 31% 4	800
	21st to end	Night	20% 12% 35% 30% 2	880
February	1st to 10th	Night	23% 10% 28% 35% 3	800
	11th to 20th	Night	31% 13% 30% 26% 1	783
	21st to end	Night	28% 12% 32% 26% 3	660
March	1st to 10th	Night	21% 11% 29% 35% 5%	800
	11th to 20th	Night	22% 11% 28% 32% 7% 1	800
	21st to end	Night	19% 10% 29% 33% 9%	880
April	1st to 10th	Night	18% 13% 24% 34% 10%	800
	11th to 20th	Night	17% 9% 24% 36% 12% 1	644
	21st to end	Night	17% 4 23% 44% 11% 1	600
May	1st to 10th	Night	17% 7% 21% 36% 17% 2	600
	11th to 20th	Night	19% 7% 13% 38% 22% 2	591
	21st to end	Night	14% 3 10% 34% 35% 4	650
June	1st to 10th	Night	13% 4 9% 39% 31% 5%	600
	11th to 20th	Night	8% 3 9% 35% 38% 6%	600
	21st to end	Night	11% 4 8% 36% 37% 5%	600
July	1st to 10th	Night	13% 2 5% 28% 46% 6%	585
	11th to 20th	Night	12% 3 5% 26% 46% 8%	582
	21st to end	Night	11% 4 4 33% 40% 8%	657
August	1st to 10th	Night	12% 3 5% 30% 45% 4	597
	11th to 20th	Night	15% 1 3 35% 40% 6%	594
	21st to end	Night	10% 2 7% 30% 42% 10%	685
September	1st to 10th	Night	14% 3 8% 31% 40% 5%	800
	11th to 20th	Night	15% 3 8% 31% 38% 6%	800
	21st to end	Night	18% 4 14% 37% 27% 2	800
October	1st to 10th	Night	15% 4 13% 34% 29% 4	800
	11th to 20th	Night	17% 6% 19% 39% 18% 1	800
	21st to end	Night	19% 6% 19% 38% 16% 1	880
November	1st to 10th	Night	15% 5% 20% 42% 16% 2	820
	11th to 20th	Night	18% 10% 21% 40% 10% 1	840
	21st to end	Night	20% 10% 21% 39% 9% 1	840
December	1st to 10th	Night	27% 8% 25% 34% 6%	840
	11th to 20th	Night	23% 8% 26% 36% 6%	840
	21st to end	Night	25% 11% 24% 34% 6%	924

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