Old Tunnel State Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Old Tunnel State Park clear sky chart from 2005-11-06 to 2025-09-19.

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	12% 6% 2 5% 4 4 4 3 5% 7% 48%	126251
	Night	13% 6% 2 4 3 3 3 3 5% 6% 51%	123115
Transparency	Day	35% 5% 9% 20% 19% 11%	130056
	Night	33% 5% 10% 19% 19% 13%	127373
Seeing	Day	14% 8% 22% 29% 21% 6%	16012
	Night	15% 7% 25% 36% 16% 1	25874

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	14% 7% 3 5% 3 2 3 2 3 4 52%	4077
	11th to 20th	Day	14% 6% 2 4 2 2 3 3 4 6% 55%	4080
	21st to end	Day	24% 7% 2 4 2 2 2 2 3 6% 46%	4963
February	1st to 10th	Day	16% 8% 2 6% 3 3 3 3 5% 5% 48%	2000
	11th to 20th	Day	16% 7% 2 5% 3 3 4 3 5% 6% 47%	1949
	21st to end	Day	17% 6% 2 4 2 2 2 3 4 5% 52%	1650
March	1st to 10th	Day	12% 6% 2 5% 3 3 3 2 4 6% 54%	4778
	11th to 20th	Day	14% 8% 2 6% 3 2 3 3 3 4 52%	4980
	21st to end	Day	14% 7% 2 6% 3 3 3 3 4 6% 48%	5462
April	1st to 10th	Day	12% 9% 3 5% 4 3 3 3 4 7% 46%	2200
	11th to 20th	Day	17% 7% 2 5% 3 4 3 3 5% 5% 44%	2259
	21st to end	Day	13% 7% 2 6% 4 4 4 3 4 6% 46%	2499
May	1st to 10th	Day	18% 8% 2 6% 4 4 4 4 6% 7% 37%	5862
	11th to 20th	Day	11% 5% 2 5% 4 4 4 4 6% 9% 46%	5862
	21st to end	Day	14% 7% 2 6% 5% 5% 5% 5% 7% 9% 36%	6434
June	1st to 10th	Day	6% 5% 2 4 4 4 5% 4 7% 9% 49%	2600
	11th to 20th	Day	7% 5% 2 5% 5% 4 5% 3 7% 10% 47%	2600
	21st to end	Day	6% 5% 2 7% 5% 5% 7% 5% 7% 9% 41%	2600
July	1st to 10th	Day	14% 7% 3 7% 5% 5% 5% 4 6% 8% 37%	5900
	11th to 20th	Day	6% 4 2 5% 4 5% 6% 4 7% 10% 47%	5900
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 4 7% 9% 45%	6422
August	1st to 10th	Day	1 2 1 2 3 3 4 3 6% 9% 66%	4076
	11th to 20th	Day	4 3 1 4 3 3 3 3 6% 10% 59%	3846
	21st to end	Day	4 3 2 5% 5% 6% 6% 5% 7% 11% 47%	4213
September	1st to 10th	Day	11% 6% 3 6% 5% 4 4 4 5% 7% 44%	2200
	11th to 20th	Day	10% 5% 3 6% 4 5% 5% 4 6% 9% 42%	2178
	21st to end	Day	8% 5% 1 6% 4 5% 5% 4 6% 7% 49%	2090
October	1st to 10th	Day	6% 5% 2 5% 4 5% 4 3 5% 6% 56%	2090
	11th to 20th	Day	14% 5% 1 4 4 3 3 3 5% 7% 52%	2009
	21st to end	Day	11% 5% 2 5% 3 3 2 2 4 4 59%	2090
November	1st to 10th	Day	14% 8% 2 6% 4 3 4 3 5% 6% 45%	1958
	11th to 20th	Day	10% 7% 2 6% 3 3 3 3 5% 7% 51%	1805
	21st to end	Day	16% 7% 3 6% 3 3 3 3 4 6% 48%	1800
December	1st to 10th	Day	25% 9% 3 5% 3 2 3 2 3 4 41%	2940
	11th to 20th	Day	15% 8% 3 6% 3 4 4 3 5% 6% 44%	3391
	21st to end	Day	13% 6% 2 4 2 2 2 2 3 4 60%	4488
January	1st to 10th	Night	17% 5% 2 5% 2 2 2 2 4 5% 53%	5888
	11th to 20th	Night	11% 3 1 2 2 2 2 3 4 5% 65%	5900
	21st to end	Night	27% 6% 2 5% 3 3 2 2 4 5% 42%	6058
February	1st to 10th	Night	15% 8% 3 4 3 3 2 3 4 4 51%	2400
	11th to 20th	Night	17% 7% 2 4 3 2 3 2 5% 5% 51%	2338
	21st to end	Night	17% 7% 2 6% 3 3 3 3 5% 5% 48%	1980
March	1st to 10th	Night	13% 7% 2 5% 3 3 3 3 6% 7% 47%	5081
	11th to 20th	Night	13% 8% 2 5% 3 3 3 3 4 5% 52%	4980
	21st to end	Night	16% 10% 2 4 3 3 3 2 5% 5% 47%	5467
April	1st to 10th	Night	11% 8% 2 6% 3 3 3 3 5% 6% 49%	2200
	11th to 20th	Night	15% 7% 3 6% 3 3 3 3 4 5% 48%	2086
	21st to end	Night	17% 8% 2 5% 4 3 3 2 3 4 48%	1900
May	1st to 10th	Night	22% 9% 2 5% 3 3 4 3 4 5% 39%	4053
	11th to 20th	Night	7% 6% 2 5% 4 4 5% 4 6% 7% 50%	4053
	21st to end	Night	15% 7% 2 5% 4 3 4 4 6% 9% 41%	4443
June	1st to 10th	Night	5% 3 1 4 2 3 3 3 7% 9% 60%	1800
	11th to 20th	Night	5% 4 2 4 4 3 3 3 4 9% 60%	1800
	21st to end	Night	7% 5% 2 4 3 3 4 4 6% 10% 54%	1800
July	1st to 10th	Night	12% 6% 2 6% 4 4 5% 4 6% 9% 43%	4078
	11th to 20th	Night	6% 3 2 5% 3 4 4 4 6% 10% 53%	4077
	21st to end	Night	8% 6% 2 5% 3 4 4 4 7% 9% 48%	4449
August	1st to 10th	Night	1 1 2 2 2 3 3 4 6% 75%	3079
	11th to 20th	Night	3 2 1 3 3 3 3 3 6% 12% 61%	3213
	21st to end	Night	4 5% 2 4 4 3 4 4 6% 9% 54%	3690
September	1st to 10th	Night	8% 5% 2 5% 5% 4 4 3 5% 7% 50%	2200
	11th to 20th	Night	8% 7% 2 6% 4 4 4 4 6% 9% 46%	2178
	21st to end	Night	7% 6% 2 4 5% 4 4 4 6% 6% 52%	2090
October	1st to 10th	Night	6% 4 1 4 3 3 3 3 4 7% 63%	2090
	11th to 20th	Night	13% 4 1 4 3 3 3 3 5% 5% 55%	2149
	21st to end	Night	14% 5% 2 4 2 2 2 1 3 5% 61%	2508
November	1st to 10th	Night	13% 7% 2 5% 3 3 3 2 4 6% 53%	2340
	11th to 20th	Night	10% 7% 2 3 2 3 3 2 4 6% 57%	2591
	21st to end	Night	16% 7% 2 4 3 3 3 3 4 6% 49%	2600
December	1st to 10th	Night	24% 7% 2 5% 3 3 3 3 4 5% 41%	4250
	11th to 20th	Night	20% 9% 2 5% 2 2 2 2 3 4 48%	4816
	21st to end	Night	16% 6% 2 3 2 2 2 2 3 4 59%	6490

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	38% 2 3 19% 24% 14%	4078
	11th to 20th	Day	33% 2 4 16% 26% 19%	4080
	21st to end	Day	43% 3 4 10% 26% 14%	4963
February	1st to 10th	Day	40% 4 5% 12% 22% 18%	2000
	11th to 20th	Day	39% 4 6% 19% 19% 14%	1959
	21st to end	Day	36% 4 5% 17% 21% 17%	1650
March	1st to 10th	Day	32% 3 5% 17% 17% 25%	4778
	11th to 20th	Day	36% 5% 5% 8% 14% 32%	4980
	21st to end	Day	37% 5% 8% 17% 20% 12%	5464
April	1st to 10th	Day	40% 4 8% 17% 17% 14%	2200
	11th to 20th	Day	42% 5% 7% 16% 19% 12%	2259
	21st to end	Day	39% 5% 8% 19% 18% 10%	2499
May	1st to 10th	Day	45% 6% 8% 18% 17% 5%	5900
	11th to 20th	Day	35% 6% 12% 16% 22% 10%	5866
	21st to end	Day	42% 8% 10% 25% 13% 2	6447
June	1st to 10th	Day	29% 8% 14% 27% 19% 3	2600
	11th to 20th	Day	32% 6% 12% 25% 20% 5%	2600
	21st to end	Day	36% 8% 13% 24% 15% 4	2600
July	1st to 10th	Day	43% 7% 17% 28% 5%	5836
	11th to 20th	Day	29% 8% 16% 31% 13% 2	5831
	21st to end	Day	32% 8% 16% 26% 18% 1	6240
August	1st to 10th	Day	15% 5% 12% 40% 28% 1	4062
	11th to 20th	Day	20% 5% 23% 38% 12% 2	3882
	21st to end	Day	29% 11% 17% 24% 17% 2	4212
September	1st to 10th	Day	40% 6% 10% 18% 18% 8%	2200
	11th to 20th	Day	38% 9% 12% 19% 18% 5%	2178
	21st to end	Day	33% 7% 8% 20% 18% 13%	2090
October	1st to 10th	Day	18% 4 6% 11% 26% 36%	3480
	11th to 20th	Day	25% 4 7% 19% 24% 21%	3314
	21st to end	Day	28% 3 5% 16% 35% 13%	3428
November	1st to 10th	Day	39% 4 8% 18% 18% 11%	1958
	11th to 20th	Day	33% 5% 7% 16% 21% 18%	1805
	21st to end	Day	39% 4 5% 17% 21% 14%	1800
December	1st to 10th	Day	49% 3 5% 17% 16% 10%	2940
	11th to 20th	Day	40% 5% 8% 11% 21% 15%	3389
	21st to end	Day	31% 3 4 15% 28% 20%	4488
January	1st to 10th	Night	36% 2 4 17% 26% 15%	5888
	11th to 20th	Night	24% 2 3 18% 28% 24%	5900
	21st to end	Night	47% 3 3 10% 21% 17%	6058
February	1st to 10th	Night	38% 4 6% 11% 22% 19%	2400
	11th to 20th	Night	37% 3 7% 17% 20% 16%	2338
	21st to end	Night	39% 4 6% 12% 20% 18%	1980
March	1st to 10th	Night	36% 4 6% 18% 19% 17%	5081
	11th to 20th	Night	36% 5% 6% 10% 15% 28%	4980
	21st to end	Night	39% 5% 8% 17% 18% 13%	5467
April	1st to 10th	Night	35% 6% 9% 18% 16% 16%	2200
	11th to 20th	Night	39% 5% 8% 15% 17% 16%	2086
	21st to end	Night	41% 5% 8% 18% 18% 10%	1900
May	1st to 10th	Night	48% 5% 7% 17% 14% 9%	4080
	11th to 20th	Night	32% 7% 11% 19% 18% 13%	4054
	21st to end	Night	39% 7% 14% 26% 10% 3	4452
June	1st to 10th	Night	21% 8% 16% 33% 19% 3	1800
	11th to 20th	Night	23% 6% 15% 29% 21% 5%	1800
	21st to end	Night	27% 8% 17% 28% 17% 4	1800
July	1st to 10th	Night	37% 9% 23% 26% 4	4035
	11th to 20th	Night	26% 8% 21% 31% 13% 1	4026
	21st to end	Night	31% 8% 21% 25% 13% 2	4320
August	1st to 10th	Night	12% 4 14% 46% 24% 1	3069
	11th to 20th	Night	18% 6% 30% 35% 9% 2	3243
	21st to end	Night	25% 11% 20% 25% 18% 2	3690
September	1st to 10th	Night	34% 6% 14% 20% 18% 8%	2200
	11th to 20th	Night	33% 8% 15% 23% 15% 6%	2178
	21st to end	Night	31% 6% 12% 20% 18% 13%	2090
October	1st to 10th	Night	14% 4 8% 15% 22% 37%	3480
	11th to 20th	Night	22% 4 9% 16% 30% 19%	3570
	21st to end	Night	27% 2 5% 18% 33% 15%	4120
November	1st to 10th	Night	33% 5% 7% 19% 22% 14%	2340
	11th to 20th	Night	29% 5% 7% 21% 20% 18%	2591
	21st to end	Night	37% 4 8% 16% 20% 16%	2600
December	1st to 10th	Night	47% 3 6% 16% 16% 12%	4250
	11th to 20th	Night	42% 3 5% 11% 21% 19%	4817
	21st to end	Night	33% 3 3 15% 28% 18%	6490

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	22% 12% 36% 23% 6% 1	300
	11th to 20th	Day	21% 15% 31% 24% 8% 1	300
	21st to end	Day	20% 15% 35% 22% 8%	330
February	1st to 10th	Day	25% 14% 28% 25% 7%	300
	11th to 20th	Day	25% 19% 34% 19% 2 1	300
	21st to end	Day	28% 14% 41% 16% 2	249
March	1st to 10th	Day	22% 16% 37% 22% 3	300
	11th to 20th	Day	19% 19% 36% 19% 7%	300
	21st to end	Day	17% 18% 41% 19% 5% 1	330
April	1st to 10th	Day	20% 15% 40% 20% 5%	300
	11th to 20th	Day	19% 15% 38% 22% 6%	377
	21st to end	Day	17% 16% 39% 22% 6%	599
May	1st to 10th	Day	18% 11% 37% 27% 7%	703
	11th to 20th	Day	17% 9% 31% 31% 9% 2	725
	21st to end	Day	17% 7% 28% 31% 15% 2	789
June	1st to 10th	Day	8% 3 18% 42% 26% 4	730
	11th to 20th	Day	8% 2 16% 46% 22% 4	730
	21st to end	Day	8% 2 14% 35% 33% 8%	730
July	1st to 10th	Day	12% 2 10% 33% 33% 10%	721
	11th to 20th	Day	5% 1 6% 33% 46% 9%	718
	21st to end	Day	6% 2 5% 33% 43% 12%	794
August	1st to 10th	Day	2 1 3 33% 46% 16%	633
	11th to 20th	Day	7% 1 6% 32% 39% 16%	517
	21st to end	Day	5% 1 6% 25% 41% 21%	523
September	1st to 10th	Day	11% 2 7% 25% 32% 22%	330
	11th to 20th	Day	5% 5% 30% 42% 18%	324
	21st to end	Day	10% 2 7% 31% 36% 14%	300
October	1st to 10th	Day	6% 1 16% 31% 36% 11%	300
	11th to 20th	Day	11% 3 21% 41% 20% 4	300
	21st to end	Day	19% 11% 25% 27% 14% 3	330
November	1st to 10th	Day	17% 7% 27% 35% 9% 5%	300
	11th to 20th	Day	21% 10% 26% 26% 14% 4	300
	21st to end	Day	21% 16% 31% 20% 9% 3	300
December	1st to 10th	Day	19% 11% 30% 27% 10% 3	300
	11th to 20th	Day	17% 19% 32% 23% 9% 1	300
	21st to end	Day	18% 11% 34% 26% 9% 2	330
January	1st to 10th	Night	18% 10% 30% 35% 7% 1	1000
	11th to 20th	Night	19% 10% 32% 32% 7%	1000
	21st to end	Night	22% 11% 29% 30% 8%	915
February	1st to 10th	Night	22% 12% 31% 28% 7%	800
	11th to 20th	Night	22% 14% 33% 28% 3	784
	21st to end	Night	23% 11% 42% 23% 1	660
March	1st to 10th	Night	21% 12% 34% 30% 3	676
	11th to 20th	Night	19% 12% 34% 30% 5%	600
	21st to end	Night	16% 11% 39% 29% 4	660
April	1st to 10th	Night	15% 11% 42% 29% 3	600
	11th to 20th	Night	18% 12% 40% 27% 3	593
	21st to end	Night	19% 11% 39% 25% 6%	600
May	1st to 10th	Night	17% 11% 39% 29% 4	600
	11th to 20th	Night	12% 8% 36% 36% 8% 1	591
	21st to end	Night	15% 7% 30% 36% 12%	650
June	1st to 10th	Night	5% 2 23% 47% 22% 1	600
	11th to 20th	Night	7% 2 26% 51% 15%	600
	21st to end	Night	7% 2 18% 46% 26% 2	600
July	1st to 10th	Night	10% 3 9% 44% 31% 4	585
	11th to 20th	Night	5% 1 10% 47% 36% 2	582
	21st to end	Night	5% 2 8% 47% 36% 3	657
August	1st to 10th	Night	2 1 4 48% 42% 4	597
	11th to 20th	Night	5% 1 6% 46% 38% 4	594
	21st to end	Night	7% 1 6% 38% 42% 6%	660
September	1st to 10th	Night	10% 1 8% 39% 38% 5%	600
	11th to 20th	Night	10% 1 8% 40% 37% 4	594
	21st to end	Night	9% 2 11% 39% 34% 4	570
October	1st to 10th	Night	8% 2 10% 49% 28% 3	570
	11th to 20th	Night	13% 4 19% 42% 20% 2	629
	21st to end	Night	17% 7% 23% 29% 22% 2	836
November	1st to 10th	Night	15% 7% 20% 39% 18% 2	780
	11th to 20th	Night	14% 7% 25% 37% 16% 1	991
	21st to end	Night	20% 8% 28% 32% 10% 1	1000
December	1st to 10th	Night	19% 8% 26% 38% 9% 1	1000
	11th to 20th	Night	23% 9% 28% 33% 8%	1000
	21st to end	Night	19% 9% 30% 34% 8%	1100

Page generated at 2025-09-16 00:49:16. Version ac07

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