Henry Coe State Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Henry Coe State Park clear sky chart from 2005-11-06 to 2025-09-04.

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	6% 4 1 3 2 3 3 2 4 6% 66%	134488
	Night	8% 5% 2 4 3 3 3 3 4 8% 59%	131677
Transparency	Day	20% 5% 8% 18% 25% 24%	129444
	Night	24% 5% 9% 18% 22% 22%	127707
Seeing	Day	9% 9% 23% 32% 21% 6%	16492
	Night	10% 5% 15% 33% 30% 8%	25859

By Month

Percentage Cloud Cover SW (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	12% 8% 3 5% 3 2 3 2 4 7% 51%	3823
	11th to 20th	Day	11% 5% 2 5% 4 4 4 2 3 4 56%	4100
	21st to end	Day	15% 8% 2 6% 4 4 3 3 4 6% 44%	4510
February	1st to 10th	Day	19% 8% 2 5% 3 2 2 3 5% 6% 45%	1800
	11th to 20th	Day	12% 8% 2 5% 4 4 5% 4 4 7% 45%	1896
	21st to end	Day	16% 7% 2 5% 4 4 4 3 4 5% 45%	1650
March	1st to 10th	Day	11% 12% 3 10% 6% 6% 6% 4 5% 6% 31%	4777
	11th to 20th	Day	12% 7% 2 6% 4 4 4 3 5% 7% 48%	5000
	21st to end	Day	12% 8% 3 7% 6% 5% 5% 5% 6% 8% 36%	5634
April	1st to 10th	Day	12% 7% 3 6% 4 4 3 3 4 6% 48%	2400
	11th to 20th	Day	9% 5% 1 4 3 3 3 3 5% 7% 56%	2369
	21st to end	Day	6% 4 1 4 3 3 4 3 5% 8% 60%	2400
May	1st to 10th	Day	4 2 1 2 2 2 2 2 3 6% 74%	5440
	11th to 20th	Day	2 2 1 3 2 3 2 2 4 7% 73%	6047
	21st to end	Day	2 1 1 2 2 2 2 2 4 6% 77%	6902
June	1st to 10th	Day	3 4 1 3 3 2 2 3 4 8% 67%	2800
	11th to 20th	Day	1 1 1 2 1 1 2 2 3 6% 80%	2800
	21st to end	Day	2 2 1 1 1 1 1 2 2 3 83%	2800
July	1st to 10th	Day	1 1 1 1 1 1 3 91%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 88%	5978
	21st to end	Day	1 1 2 2 2 3 6% 83%	6402
August	1st to 10th	Day	1 1 1 1 1 2 2 3 6% 82%	5777
	11th to 20th	Day	1 1 1 1 1 2 4 89%	5406
	21st to end	Day	1 1 1 1 1 2 2 3 4 85%	5981
September	1st to 10th	Day	1 1 1 1 1 1 1 1 2 3 86%	2317
	11th to 20th	Day	1 2 1 3 2 2 3 2 4 7% 75%	2280
	21st to end	Day	3 1 2 1 2 1 1 4 5% 80%	2040
October	1st to 10th	Day	1 3 1 3 2 2 2 2 4 6% 74%	3180
	11th to 20th	Day	3 2 1 2 2 2 2 2 3 5% 77%	3161
	21st to end	Day	3 3 1 4 3 3 4 3 5% 6% 65%	3448
November	1st to 10th	Day	7% 5% 1 3 4 4 5% 3 5% 8% 56%	1889
	11th to 20th	Day	9% 4 2 4 3 4 4 4 6% 7% 54%	1800
	21st to end	Day	11% 7% 2 3 2 3 2 3 5% 7% 55%	1710
December	1st to 10th	Day	8% 4 2 5% 4 3 4 3 5% 8% 54%	2620
	11th to 20th	Day	22% 9% 3 6% 4 3 3 3 4 6% 37%	3028
	21st to end	Day	16% 13% 4 7% 4 4 4 4 6% 6% 31%	4004
January	1st to 10th	Night	9% 6% 2 5% 3 3 4 3 5% 8% 52%	6113
	11th to 20th	Night	13% 6% 2 4 3 3 4 3 4 6% 53%	5880
	21st to end	Night	14% 8% 3 6% 4 3 3 3 4 6% 46%	6468
February	1st to 10th	Night	18% 7% 2 4 3 3 3 3 4 6% 49%	2600
	11th to 20th	Night	12% 7% 2 4 4 4 4 4 6% 7% 47%	2392
	21st to end	Night	12% 7% 2 5% 4 3 3 3 4 7% 51%	1980
March	1st to 10th	Night	14% 13% 3 9% 5% 5% 4 3 5% 6% 33%	5112
	11th to 20th	Night	13% 9% 2 6% 3 3 3 3 3 6% 49%	5000
	21st to end	Night	14% 13% 2 7% 4 3 4 3 4 7% 39%	5341
April	1st to 10th	Night	10% 7% 1 4 4 4 3 4 6% 6% 50%	2000
	11th to 20th	Night	7% 7% 2 4 4 3 3 3 4 8% 55%	1979
	21st to end	Night	4 5% 2 4 3 3 4 4 6% 7% 58%	2000
May	1st to 10th	Night	2 2 1 3 2 2 2 2 3 8% 71%	4544
	11th to 20th	Night	1 2 2 4 3 4 4 3 6% 13% 58%	3852
	21st to end	Night	1 1 1 2 2 4 3 2 5% 9% 69%	3972
June	1st to 10th	Night	2 2 1 4 4 3 3 3 5% 9% 64%	1600
	11th to 20th	Night	1 1 1 3 3 2 3 3 5% 7% 73%	1600
	21st to end	Night	1 1 1 3 3 2 2 2 4 6% 76%	1600
July	1st to 10th	Night	1 1 1 1 1 2 7% 87%	3640
	11th to 20th	Night	1 1 1 1 1 2 2 10% 80%	3980
	21st to end	Night	1 1 3 3 6% 4 3 6% 18% 56%	4472
August	1st to 10th	Night	1 1 1 2 2 1 2 2 3 11% 74%	4035
	11th to 20th	Night	1 1 2 1 1 2 9% 81%	4461
	21st to end	Night	1 1 2 1 2 2 2 3 6% 80%	5015
September	1st to 10th	Night	1 1 2 1 2 2 1 3 6% 80%	1930
	11th to 20th	Night	3 2 1 3 2 2 3 2 4 7% 71%	1900
	21st to end	Night	2 2 1 3 1 2 1 2 2 5% 79%	2134
October	1st to 10th	Night	2 2 1 2 2 2 2 2 3 6% 77%	3800
	11th to 20th	Night	2 2 1 3 2 2 2 2 3 7% 73%	3777
	21st to end	Night	4 3 2 4 3 2 3 3 4 6% 66%	4121
November	1st to 10th	Night	6% 6% 1 4 3 3 3 3 4 7% 60%	2406
	11th to 20th	Night	8% 5% 2 4 3 3 3 3 6% 9% 55%	2600
	21st to end	Night	9% 6% 1 4 3 3 4 3 4 6% 55%	2685
December	1st to 10th	Night	9% 4 1 4 3 3 4 3 5% 6% 59%	4560
	11th to 20th	Night	21% 10% 3 5% 3 3 4 3 6% 9% 33%	5176
	21st to end	Night	20% 10% 4 8% 5% 5% 5% 3 5% 5% 31%	6952

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	31% 7% 16% 25% 13% 8%	3823
	11th to 20th	Day	33% 6% 8% 11% 22% 21%	4100
	21st to end	Day	39% 7% 7% 12% 19% 16%	4510
February	1st to 10th	Day	39% 4 8% 19% 19% 11%	1800
	11th to 20th	Day	37% 5% 11% 19% 21% 7%	1906
	21st to end	Day	39% 6% 8% 21% 19% 7%	1650
March	1st to 10th	Day	45% 10% 9% 13% 20% 3	4777
	11th to 20th	Day	33% 6% 13% 29% 11% 8%	5000
	21st to end	Day	38% 13% 14% 22% 11% 2	5634
April	1st to 10th	Day	36% 6% 12% 22% 19% 5%	2400
	11th to 20th	Day	27% 7% 13% 25% 20% 8%	2372
	21st to end	Day	21% 7% 13% 30% 24% 5%	2400
May	1st to 10th	Day	12% 3 10% 25% 36% 14%	5440
	11th to 20th	Day	11% 4 9% 31% 36% 8%	6050
	21st to end	Day	9% 5% 10% 27% 41% 9%	6904
June	1st to 10th	Day	16% 6% 11% 26% 31% 11%	2800
	11th to 20th	Day	8% 3 9% 21% 36% 24%	2800
	21st to end	Day	8% 2 4 14% 30% 42%	2800
July	1st to 10th	Day	3 1 1 5% 19% 71%	6251
	11th to 20th	Day	5% 2 2 6% 12% 73%	5906
	21st to end	Day	4 1 3 15% 32% 45%	6217
August	1st to 10th	Day	9% 2 4 9% 25% 51%	4206
	11th to 20th	Day	3 1 3 9% 29% 55%	3910
	21st to end	Day	6% 2 4 11% 31% 46%	4311
September	1st to 10th	Day	6% 2 3 10% 30% 49%	2317
	11th to 20th	Day	10% 4 8% 23% 34% 20%	2280
	21st to end	Day	10% 2 4 20% 38% 26%	2040
October	1st to 10th	Day	12% 3 5% 11% 24% 45%	3180
	11th to 20th	Day	12% 3 6% 22% 34% 24%	3161
	21st to end	Day	19% 3 7% 18% 32% 20%	3448
November	1st to 10th	Day	25% 6% 9% 22% 25% 14%	1889
	11th to 20th	Day	28% 7% 10% 22% 25% 8%	1800
	21st to end	Day	28% 5% 10% 17% 29% 11%	1710
December	1st to 10th	Day	28% 5% 8% 16% 34% 10%	2620
	11th to 20th	Day	47% 5% 9% 15% 17% 8%	3028
	21st to end	Day	48% 8% 11% 19% 9% 4	4004
January	1st to 10th	Night	27% 8% 18% 23% 18% 6%	6113
	11th to 20th	Night	32% 6% 9% 11% 25% 16%	5880
	21st to end	Night	37% 7% 8% 12% 16% 20%	6468
February	1st to 10th	Night	37% 4 9% 18% 20% 13%	2600
	11th to 20th	Night	33% 5% 12% 19% 21% 9%	2393
	21st to end	Night	31% 5% 11% 27% 18% 8%	1980
March	1st to 10th	Night	46% 10% 10% 12% 18% 4	5112
	11th to 20th	Night	35% 6% 12% 23% 16% 8%	5000
	21st to end	Night	39% 10% 12% 22% 13% 4	5341
April	1st to 10th	Night	31% 7% 14% 22% 20% 6%	2000
	11th to 20th	Night	27% 6% 12% 28% 19% 8%	1979
	21st to end	Night	20% 7% 14% 29% 24% 6%	2000
May	1st to 10th	Night	12% 5% 10% 23% 33% 16%	4541
	11th to 20th	Night	11% 4 13% 36% 23% 14%	3852
	21st to end	Night	9% 4 11% 24% 35% 17%	3972
June	1st to 10th	Night	17% 7% 11% 23% 29% 14%	1600
	11th to 20th	Night	11% 4 8% 19% 28% 31%	1600
	21st to end	Night	10% 3 6% 18% 23% 41%	1600
July	1st to 10th	Night	3 1 2 7% 17% 71%	3600
	11th to 20th	Night	6% 2 2 8% 17% 65%	3929
	21st to end	Night	10% 2 11% 15% 30% 31%	4345
August	1st to 10th	Night	12% 2 4 13% 17% 52%	2929
	11th to 20th	Night	5% 2 4 11% 23% 56%	3224
	21st to end	Night	8% 2 5% 13% 27% 45%	3608
September	1st to 10th	Night	8% 2 3 11% 23% 52%	1930
	11th to 20th	Night	13% 5% 7% 21% 29% 25%	1900
	21st to end	Night	11% 2 7% 15% 36% 29%	2134
October	1st to 10th	Night	12% 2 5% 11% 17% 52%	3800
	11th to 20th	Night	13% 3 6% 22% 33% 23%	3777
	21st to end	Night	17% 4 6% 19% 30% 25%	4121
November	1st to 10th	Night	22% 5% 9% 22% 24% 18%	2406
	11th to 20th	Night	25% 6% 11% 20% 28% 10%	2600
	21st to end	Night	28% 5% 8% 19% 24% 15%	2685
December	1st to 10th	Night	26% 4 7% 19% 27% 17%	4560
	11th to 20th	Night	46% 5% 11% 12% 20% 6%	5176
	21st to end	Night	51% 9% 10% 16% 9% 4	6952

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	28% 12% 22% 23% 14% 1	280
	11th to 20th	Day	18% 9% 23% 30% 16% 4	400
	21st to end	Day	17% 9% 21% 29% 21% 3	440
February	1st to 10th	Day	25% 11% 23% 21% 16% 5%	400
	11th to 20th	Day	17% 17% 28% 26% 12% 1	468
	21st to end	Day	17% 15% 26% 29% 12% 1	414
March	1st to 10th	Day	24% 16% 28% 25% 7% 1	500
	11th to 20th	Day	23% 15% 22% 23% 13% 4	500
	21st to end	Day	19% 15% 33% 22% 11% 1	550
April	1st to 10th	Day	18% 12% 31% 24% 14% 2	500
	11th to 20th	Day	13% 17% 29% 26% 13% 3	492
	21st to end	Day	7% 16% 30% 28% 13% 7%	500
May	1st to 10th	Day	9% 13% 34% 29% 12% 4	503
	11th to 20th	Day	9% 10% 32% 29% 16% 4	528
	21st to end	Day	6% 12% 27% 32% 17% 5%	573
June	1st to 10th	Day	5% 8% 28% 32% 19% 8%	530
	11th to 20th	Day	2 9% 29% 38% 16% 5%	530
	21st to end	Day	2 5% 22% 39% 25% 8%	530
July	1st to 10th	Day	1 1 18% 42% 30% 8%	526
	11th to 20th	Day	1 10% 44% 34% 11%	524
	21st to end	Day	1 10% 46% 30% 14%	575
August	1st to 10th	Day	3 2 18% 40% 27% 10%	527
	11th to 20th	Day	2 3 19% 38% 27% 12%	517
	21st to end	Day	1 1 11% 43% 32% 11%	583
September	1st to 10th	Day	2 3 15% 39% 30% 10%	503
	11th to 20th	Day	3 6% 22% 38% 21% 9%	490
	21st to end	Day	3 7% 16% 40% 27% 8%	490
October	1st to 10th	Day	3 8% 22% 33% 29% 6%	490
	11th to 20th	Day	5% 8% 21% 31% 27% 8%	490
	21st to end	Day	8% 6% 23% 33% 24% 5%	539
November	1st to 10th	Day	8% 11% 20% 26% 29% 6%	425
	11th to 20th	Day	9% 10% 20% 29% 24% 9%	300
	21st to end	Day	13% 7% 24% 29% 24% 4	255
December	1st to 10th	Day	13% 10% 19% 33% 19% 8%	200
	11th to 20th	Day	18% 13% 15% 28% 20% 7%	200
	21st to end	Day	21% 14% 21% 23% 15% 5%	220
January	1st to 10th	Night	17% 6% 18% 32% 20% 8%	800
	11th to 20th	Night	14% 5% 15% 36% 22% 8%	800
	21st to end	Night	20% 5% 13% 33% 24% 6%	880
February	1st to 10th	Night	21% 7% 14% 30% 21% 7%	800
	11th to 20th	Night	16% 8% 19% 32% 19% 6%	784
	21st to end	Night	15% 8% 20% 37% 18% 3	660
March	1st to 10th	Night	20% 8% 18% 30% 20% 4	800
	11th to 20th	Night	19% 7% 18% 28% 23% 5%	800
	21st to end	Night	15% 9% 23% 33% 18% 3	880
April	1st to 10th	Night	13% 8% 21% 33% 22% 3	800
	11th to 20th	Night	10% 10% 21% 33% 20% 5%	791
	21st to end	Night	7% 8% 26% 30% 23% 6%	800
May	1st to 10th	Night	6% 4 25% 36% 24% 6%	797
	11th to 20th	Night	6% 3 17% 36% 32% 6%	500
	21st to end	Night	4 5% 20% 36% 29% 5%	433
June	1st to 10th	Night	4 4 16% 37% 34% 6%	400
	11th to 20th	Night	1 4 16% 43% 32% 4	400
	21st to end	Night	2 1 8% 38% 45% 6%	400
July	1st to 10th	Night	1 8% 36% 48% 7%	390
	11th to 20th	Night	1 2 26% 56% 15%	530
	21st to end	Night	1 6% 29% 50% 14%	657
August	1st to 10th	Night	1 1 9% 32% 46% 10%	597
	11th to 20th	Night	1 1 10% 37% 41% 10%	772
	21st to end	Night	1 1 5% 31% 51% 11%	880
September	1st to 10th	Night	1 1 6% 34% 47% 11%	772
	11th to 20th	Night	4 1 11% 35% 39% 10%	760
	21st to end	Night	3 2 12% 31% 40% 12%	760
October	1st to 10th	Night	5% 3 12% 36% 35% 11%	760
	11th to 20th	Night	7% 3 13% 31% 34% 11%	760
	21st to end	Night	6% 3 14% 32% 27% 17%	836
November	1st to 10th	Night	8% 5% 15% 30% 31% 11%	780
	11th to 20th	Night	11% 5% 14% 34% 29% 8%	800
	21st to end	Night	12% 5% 15% 36% 26% 7%	800
December	1st to 10th	Night	16% 5% 16% 33% 23% 6%	800
	11th to 20th	Night	22% 5% 16% 31% 21% 5%	800
	21st to end	Night	19% 8% 21% 30% 18% 4	880

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