Grayson Lake State Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grayson Lake State Park clear sky chart from 2005-11-06 to 2025-01-07.

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% 9% 3 7% 5% 4 5% 4 5% 7% 36%	105222
	Night	14% 9% 3 7% 5% 4 4 3 5% 7% 38%	103772
Transparency	Day	44% 6% 11% 23% 14% 2	108965
	Night	44% 6% 10% 22% 16% 3	108168
Seeing	Day	21% 12% 26% 22% 14% 6%	15078
	Night	19% 10% 24% 28% 16% 2	25189

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% 17% 6% 10% 5% 4 3 4 5% 7% 23%	2890
	11th to 20th	Day	16% 15% 8% 13% 7% 4 4 2 3 4 23%	2840
	21st to end	Day	24% 18% 5% 7% 4 3 3 3 4 6% 23%	3124
February	1st to 10th	Day	18% 13% 6% 10% 5% 3 4 3 4 5% 30%	1746
	11th to 20th	Day	20% 11% 5% 8% 4 4 4 4 5% 5% 30%	1848
	21st to end	Day	19% 13% 4 8% 4 4 4 3 3 6% 31%	1570
March	1st to 10th	Day	21% 14% 4 8% 4 4 4 3 4 6% 28%	3276
	11th to 20th	Day	14% 7% 3 6% 4 4 4 3 5% 7% 43%	3480
	21st to end	Day	17% 8% 2 6% 4 4 4 4 6% 8% 37%	3815
April	1st to 10th	Day	13% 9% 3 8% 5% 5% 4 4 5% 7% 37%	2110
	11th to 20th	Day	15% 9% 3 6% 5% 4 3 3 5% 6% 41%	2366
	21st to end	Day	16% 8% 2 7% 6% 4 4 4 5% 6% 38%	2470
May	1st to 10th	Day	18% 10% 3 8% 5% 5% 5% 4 6% 7% 28%	4082
	11th to 20th	Day	17% 9% 3 8% 5% 5% 5% 3 5% 6% 34%	4081
	21st to end	Day	9% 6% 2 7% 6% 6% 6% 6% 7% 9% 36%	4476
June	1st to 10th	Day	10% 7% 2 6% 5% 4 5% 6% 7% 8% 41%	2470
	11th to 20th	Day	8% 5% 2 7% 5% 6% 6% 4 7% 9% 42%	2470
	21st to end	Day	8% 6% 1 7% 5% 4 5% 5% 7% 9% 40%	2470
July	1st to 10th	Day	9% 5% 2 8% 5% 5% 5% 4 7% 9% 40%	4120
	11th to 20th	Day	8% 5% 2 6% 5% 5% 6% 4 7% 11% 41%	4120
	21st to end	Day	15% 10% 3 10% 6% 6% 6% 4 6% 8% 27%	4466
August	1st to 10th	Day	8% 6% 2 7% 5% 5% 6% 4 7% 11% 39%	4104
	11th to 20th	Day	9% 7% 2 6% 5% 5% 6% 5% 7% 10% 39%	4044
	21st to end	Day	5% 4 1 5% 4 4 4 4 6% 9% 54%	4350
September	1st to 10th	Day	13% 6% 2 5% 4 4 3 3 5% 7% 47%	2090
	11th to 20th	Day	11% 6% 2 6% 4 3 5% 4 5% 7% 48%	2090
	21st to end	Day	18% 7% 2 6% 4 3 4 3 4 7% 43%	2090
October	1st to 10th	Day	12% 5% 2 5% 3 5% 3 3 5% 7% 51%	2057
	11th to 20th	Day	13% 6% 2 5% 4 3 4 3 5% 6% 48%	1900
	21st to end	Day	19% 7% 3 7% 3 3 3 2 4 6% 43%	2076
November	1st to 10th	Day	11% 6% 2 5% 4 5% 3 3 4 5% 51%	1762
	11th to 20th	Day	13% 10% 3 8% 4 3 4 4 4 7% 40%	1800
	21st to end	Day	16% 11% 4 7% 4 4 3 3 4 5% 39%	1800
December	1st to 10th	Day	17% 11% 3 7% 4 4 5% 3 5% 4 37%	2930
	11th to 20th	Day	22% 13% 5% 8% 6% 4 4 3 4 5% 27%	3373
	21st to end	Day	16% 15% 5% 12% 6% 6% 6% 3 4 5% 22%	4466
January	1st to 10th	Night	14% 16% 6% 12% 6% 5% 4 3 5% 5% 25%	4188
	11th to 20th	Night	20% 14% 5% 11% 5% 3 3 2 4 4 28%	4120
	21st to end	Night	22% 13% 4 9% 5% 4 4 3 4 5% 28%	4532
February	1st to 10th	Night	18% 12% 4 10% 5% 4 4 4 5% 5% 30%	2432
	11th to 20th	Night	21% 11% 4 9% 4 4 4 3 5% 5% 30%	2218
	21st to end	Night	22% 13% 4 9% 5% 4 4 2 4 5% 29%	1884
March	1st to 10th	Night	21% 13% 3 7% 4 4 3 3 4 6% 32%	3575
	11th to 20th	Night	13% 9% 2 6% 4 4 4 3 5% 6% 44%	3480
	21st to end	Night	12% 7% 2 7% 5% 5% 5% 4 6% 7% 39%	3817
April	1st to 10th	Night	13% 8% 2 6% 5% 4 3 4 7% 7% 40%	2085
	11th to 20th	Night	14% 7% 3 5% 4 4 4 4 5% 8% 42%	1763
	21st to end	Night	16% 8% 2 5% 5% 5% 4 4 5% 7% 39%	1710
May	1st to 10th	Night	18% 11% 3 9% 5% 5% 5% 4 5% 7% 27%	2813
	11th to 20th	Night	18% 8% 3 6% 4 4 4 3 5% 9% 37%	2812
	21st to end	Night	8% 6% 2 6% 5% 5% 5% 4 6% 10% 43%	3079
June	1st to 10th	Night	8% 5% 2 5% 4 5% 5% 5% 8% 9% 43%	1710
	11th to 20th	Night	6% 4 2 5% 5% 4 5% 4 8% 11% 45%	1710
	21st to end	Night	8% 6% 1 5% 5% 3 5% 5% 8% 11% 44%	1710
July	1st to 10th	Night	7% 6% 2 5% 5% 5% 5% 5% 7% 9% 44%	2840
	11th to 20th	Night	6% 4 2 6% 5% 4 5% 4 6% 9% 48%	2840
	21st to end	Night	12% 10% 4 9% 5% 5% 5% 4 6% 8% 33%	3087
August	1st to 10th	Night	7% 6% 3 7% 4 5% 5% 5% 7% 10% 41%	2825
	11th to 20th	Night	8% 5% 3 7% 5% 5% 5% 4 7% 9% 43%	2785
	21st to end	Night	5% 3 1 4 3 4 4 4 6% 8% 59%	3280
September	1st to 10th	Night	10% 6% 2 5% 4 4 5% 4 5% 7% 48%	2080
	11th to 20th	Night	10% 6% 2 6% 3 3 3 4 5% 7% 51%	2090
	21st to end	Night	12% 6% 3 6% 4 4 4 4 5% 6% 46%	2090
October	1st to 10th	Night	11% 5% 2 5% 3 4 3 3 5% 7% 53%	2122
	11th to 20th	Night	13% 5% 3 6% 4 3 4 3 4 7% 48%	2280
	21st to end	Night	17% 8% 3 6% 4 3 4 3 4 5% 44%	2517
November	1st to 10th	Night	12% 8% 2 5% 4 3 4 3 4 5% 50%	2536
	11th to 20th	Night	14% 8% 3 7% 4 4 4 3 5% 6% 42%	2600
	21st to end	Night	17% 11% 3 6% 5% 3 3 3 5% 6% 39%	2600
December	1st to 10th	Night	14% 12% 5% 9% 5% 3 4 3 4 4 36%	4250
	11th to 20th	Night	18% 12% 7% 9% 5% 4 3 3 4 5% 30%	4822
	21st to end	Night	16% 16% 5% 10% 6% 5% 6% 4 5% 6% 21%	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	59% 6% 11% 15% 7% 1	2891
	11th to 20th	Day	66% 5% 6% 18% 5% 1	2840
	21st to end	Day	62% 4 8% 20% 5% 1	3124
February	1st to 10th	Day	58% 5% 8% 18% 11% 1	1746
	11th to 20th	Day	56% 5% 7% 19% 11% 2	1858
	21st to end	Day	55% 4 8% 19% 11% 2	1570
March	1st to 10th	Day	57% 5% 9% 18% 9% 2	3276
	11th to 20th	Day	40% 6% 11% 26% 17% 1	3480
	21st to end	Day	43% 7% 10% 26% 12% 1	3815
April	1st to 10th	Day	46% 5% 10% 22% 13% 3	2110
	11th to 20th	Day	43% 5% 9% 21% 18% 4	2366
	21st to end	Day	46% 6% 13% 22% 11% 3	2470
May	1st to 10th	Day	52% 8% 10% 20% 9% 1	4120
	11th to 20th	Day	49% 7% 10% 21% 12% 1	4085
	21st to end	Day	38% 11% 14% 19% 15% 3	4489
June	1st to 10th	Day	37% 10% 12% 25% 16% 1	2470
	11th to 20th	Day	36% 9% 13% 21% 17% 2	2470
	21st to end	Day	37% 10% 14% 21% 15% 4	2470
July	1st to 10th	Day	38% 7% 16% 23% 14% 1	4055
	11th to 20th	Day	34% 7% 12% 33% 12% 1	4051
	21st to end	Day	54% 6% 10% 17% 10% 2	4324
August	1st to 10th	Day	36% 8% 16% 28% 11% 1	4091
	11th to 20th	Day	37% 8% 12% 29% 14% 1	4083
	21st to end	Day	25% 6% 14% 32% 20% 2	4350
September	1st to 10th	Day	35% 6% 12% 22% 19% 6%	2090
	11th to 20th	Day	35% 7% 10% 25% 21% 1	2090
	21st to end	Day	42% 5% 9% 22% 20% 2	2090
October	1st to 10th	Day	26% 4 9% 25% 35% 2	3421
	11th to 20th	Day	26% 4 10% 30% 27% 3	3140
	21st to end	Day	31% 3 10% 30% 23% 3	3402
November	1st to 10th	Day	36% 4 8% 26% 22% 4	1762
	11th to 20th	Day	45% 4 8% 22% 19% 3	1800
	21st to end	Day	48% 3 10% 20% 17% 1	1800
December	1st to 10th	Day	51% 4 7% 23% 15%	2930
	11th to 20th	Day	60% 4 9% 16% 10% 1	3370
	21st to end	Day	62% 6% 10% 16% 5%	4466
January	1st to 10th	Night	59% 8% 8% 16% 8% 1	4188
	11th to 20th	Night	60% 4 7% 20% 8% 2	4120
	21st to end	Night	59% 5% 8% 20% 7% 1	4532
February	1st to 10th	Night	55% 5% 9% 19% 11% 1	2432
	11th to 20th	Night	57% 4 8% 18% 12% 1	2219
	21st to end	Night	59% 4 9% 15% 10% 3	1884
March	1st to 10th	Night	53% 5% 8% 21% 13% 2	3575
	11th to 20th	Night	37% 7% 11% 27% 16% 1	3480
	21st to end	Night	41% 6% 11% 24% 17% 1	3817
April	1st to 10th	Night	40% 7% 11% 21% 17% 3	2085
	11th to 20th	Night	39% 6% 10% 21% 17% 6%	1763
	21st to end	Night	44% 6% 12% 21% 15% 2	1710
May	1st to 10th	Night	53% 8% 11% 17% 8% 2	2840
	11th to 20th	Night	44% 8% 13% 23% 11% 1	2813
	21st to end	Night	33% 9% 14% 23% 17% 4	3087
June	1st to 10th	Night	33% 9% 15% 23% 17% 3	1710
	11th to 20th	Night	30% 9% 17% 20% 20% 4	1710
	21st to end	Night	32% 10% 15% 24% 15% 4	1710
July	1st to 10th	Night	34% 9% 14% 22% 17% 4	2795
	11th to 20th	Night	32% 8% 12% 30% 16% 2	2786
	21st to end	Night	48% 7% 10% 20% 12% 3	2988
August	1st to 10th	Night	36% 8% 14% 29% 12% 1	2814
	11th to 20th	Night	37% 7% 10% 27% 18% 1	2812
	21st to end	Night	21% 7% 14% 31% 23% 4	3280
September	1st to 10th	Night	33% 8% 12% 20% 19% 8%	2080
	11th to 20th	Night	32% 6% 12% 22% 24% 4	2090
	21st to end	Night	37% 6% 10% 21% 23% 2	2090
October	1st to 10th	Night	26% 4 7% 25% 35% 3	3538
	11th to 20th	Night	26% 4 8% 26% 29% 8%	3779
	21st to end	Night	30% 3 7% 25% 27% 8%	4141
November	1st to 10th	Night	37% 4 8% 22% 23% 5%	2536
	11th to 20th	Night	43% 4 9% 19% 21% 5%	2600
	21st to end	Night	47% 4 9% 24% 15% 2	2600
December	1st to 10th	Night	51% 4 10% 21% 12% 1	4250
	11th to 20th	Night	57% 4 10% 20% 9% 1	4824
	21st to end	Night	61% 6% 10% 16% 7%	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	29% 20% 33% 13% 4 1	288
	11th to 20th	Day	34% 22% 31% 11%	270
	21st to end	Day	30% 28% 28% 12% 2	297
February	1st to 10th	Day	32% 20% 32% 12% 4	270
	11th to 20th	Day	41% 24% 27% 6% 1 1	270
	21st to end	Day	37% 20% 36% 6% 1	225
March	1st to 10th	Day	34% 19% 30% 11% 5% 1	361
	11th to 20th	Day	26% 20% 33% 16% 4 1	460
	21st to end	Day	31% 18% 30% 15% 6%	506
April	1st to 10th	Day	24% 19% 29% 20% 8% 1	460
	11th to 20th	Day	21% 18% 32% 17% 8% 3	452
	21st to end	Day	25% 15% 34% 18% 5% 2	460
May	1st to 10th	Day	23% 18% 31% 15% 10% 3	463
	11th to 20th	Day	20% 12% 31% 21% 12% 5%	487
	21st to end	Day	14% 9% 23% 27% 16% 10%	529
June	1st to 10th	Day	13% 7% 24% 27% 20% 9%	490
	11th to 20th	Day	12% 7% 25% 29% 19% 9%	490
	21st to end	Day	13% 7% 28% 24% 19% 10%	490
July	1st to 10th	Day	15% 5% 16% 25% 20% 20%	485
	11th to 20th	Day	8% 5% 23% 30% 20% 14%	484
	21st to end	Day	16% 5% 18% 29% 20% 12%	532
August	1st to 10th	Day	14% 3 19% 26% 24% 14%	486
	11th to 20th	Day	14% 4 17% 33% 22% 9%	479
	21st to end	Day	9% 4 14% 31% 28% 15%	539
September	1st to 10th	Day	13% 4 16% 24% 30% 12%	490
	11th to 20th	Day	13% 3 13% 31% 26% 13%	490
	21st to end	Day	21% 9% 20% 26% 19% 6%	490
October	1st to 10th	Day	13% 6% 26% 27% 21% 7%	490
	11th to 20th	Day	16% 13% 29% 23% 14% 5%	490
	21st to end	Day	23% 11% 31% 23% 10% 1	525
November	1st to 10th	Day	18% 12% 35% 25% 9% 2	300
	11th to 20th	Day	25% 17% 29% 19% 7% 2	300
	21st to end	Day	28% 18% 31% 16% 6% 1	300
December	1st to 10th	Day	30% 20% 30% 15% 5% 1	300
	11th to 20th	Day	31% 17% 30% 15% 6% 1	300
	21st to end	Day	32% 19% 32% 15% 2	330
January	1st to 10th	Night	27% 19% 30% 19% 5%	785
	11th to 20th	Night	33% 17% 27% 20% 3	760
	21st to end	Night	24% 19% 33% 20% 3	836
February	1st to 10th	Night	28% 14% 33% 24% 2	760
	11th to 20th	Night	31% 18% 30% 19% 2	743
	21st to end	Night	31% 18% 32% 18% 2	628
March	1st to 10th	Night	25% 17% 30% 23% 5%	760
	11th to 20th	Night	26% 15% 30% 22% 7%	760
	21st to end	Night	20% 16% 32% 25% 7%	836
April	1st to 10th	Night	19% 15% 32% 25% 8%	755
	11th to 20th	Night	17% 12% 31% 27% 12% 1	563
	21st to end	Night	19% 9% 38% 28% 6%	570
May	1st to 10th	Night	20% 10% 27% 26% 14% 2	570
	11th to 20th	Night	17% 7% 22% 35% 18% 1	561
	21st to end	Night	9% 4 21% 35% 28% 4	617
June	1st to 10th	Night	9% 5% 22% 40% 21% 3	570
	11th to 20th	Night	8% 3 17% 40% 28% 5%	570
	21st to end	Night	9% 4 20% 37% 28% 3	570
July	1st to 10th	Night	12% 4 13% 32% 30% 9%	555
	11th to 20th	Night	7% 2 14% 37% 35% 5%	552
	21st to end	Night	13% 3 12% 34% 33% 5%	624
August	1st to 10th	Night	10% 2 11% 35% 37% 6%	567
	11th to 20th	Night	9% 3 10% 38% 32% 7%	564
	21st to end	Night	9% 2 9% 35% 35% 9%	627
September	1st to 10th	Night	11% 2 13% 34% 34% 6%	750
	11th to 20th	Night	13% 3 15% 28% 36% 6%	760
	21st to end	Night	16% 5% 20% 30% 27% 2	760
October	1st to 10th	Night	14% 5% 17% 33% 27% 4	760
	11th to 20th	Night	17% 8% 27% 28% 18% 2	760
	21st to end	Night	22% 9% 25% 28% 15% 1	836
November	1st to 10th	Night	17% 8% 26% 31% 17% 1	780
	11th to 20th	Night	20% 14% 28% 27% 10% 1	800
	21st to end	Night	23% 12% 25% 29% 10% 1	800
December	1st to 10th	Night	28% 15% 27% 25% 5%	800
	11th to 20th	Night	25% 13% 30% 25% 6% 1	800
	21st to end	Night	29% 15% 27% 25% 4	880

Page generated at 2025-01-04 03:24:04. Version ac07

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