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 2026-05-02.

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	13% 9% 3 8% 5% 5% 5% 4 6% 7% 35%	131911
	Night	13% 9% 3 8% 5% 4 4 4 5% 7% 37%	130598
Transparency	Day	45% 6% 11% 22% 14% 2	135610
	Night	45% 6% 10% 22% 15% 2	134969
Seeing	Day	21% 13% 26% 21% 13% 6%	16814
	Night	19% 10% 25% 28% 16% 2	26944

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% 13% 6% 4 4 3 4 6% 23%	5258
	11th to 20th	Day	12% 13% 7% 14% 6% 3 4 2 3 4 32%	5280
	21st to end	Day	18% 13% 4 9% 7% 4 4 3 5% 6% 26%	5780
February	1st to 10th	Day	18% 13% 6% 10% 5% 3 4 3 4 5% 30%	1930
	11th to 20th	Day	21% 11% 5% 9% 4 4 4 4 5% 5% 29%	2048
	21st to end	Day	18% 13% 5% 8% 4 4 4 3 4 6% 31%	1730
March	1st to 10th	Day	17% 14% 3 9% 4 4 4 3 4 6% 32%	4811
	11th to 20th	Day	11% 8% 3 6% 4 4 5% 4 7% 8% 41%	5090
	21st to end	Day	16% 8% 2 7% 5% 4 5% 4 6% 8% 36%	5586
April	1st to 10th	Day	13% 9% 3 7% 5% 5% 4 3 5% 7% 38%	2332
	11th to 20th	Day	14% 8% 3 6% 5% 4 4 3 5% 6% 41%	2618
	21st to end	Day	15% 8% 3 7% 6% 4 5% 4 5% 6% 37%	2730
May	1st to 10th	Day	16% 9% 3 8% 6% 5% 6% 5% 7% 8% 26%	5875
	11th to 20th	Day	16% 9% 3 9% 5% 5% 5% 4 6% 7% 30%	5861
	21st to end	Day	12% 6% 3 7% 5% 6% 6% 6% 7% 9% 32%	6433
June	1st to 10th	Day	10% 6% 2 6% 5% 4 5% 6% 7% 8% 41%	2600
	11th to 20th	Day	9% 5% 2 7% 5% 6% 6% 4 7% 9% 40%	2600
	21st to end	Day	8% 6% 1 7% 5% 4 6% 5% 8% 10% 40%	2600
July	1st to 10th	Day	8% 6% 2 8% 5% 5% 5% 4 7% 9% 42%	5900
	11th to 20th	Day	9% 6% 3 8% 6% 6% 7% 5% 8% 11% 33%	5900
	21st to end	Day	12% 9% 3 9% 6% 6% 6% 4 7% 9% 30%	6424
August	1st to 10th	Day	8% 6% 2 7% 5% 5% 6% 5% 7% 11% 39%	4234
	11th to 20th	Day	8% 6% 2 6% 5% 5% 6% 5% 7% 10% 39%	4174
	21st to end	Day	5% 4 1 5% 4 4 4 4 6% 9% 54%	4487
September	1st to 10th	Day	13% 6% 2 5% 4 4 3 3 6% 7% 48%	2200
	11th to 20th	Day	11% 6% 2 6% 4 3 5% 4 5% 8% 48%	2200
	21st to end	Day	18% 7% 2 7% 4 3 4 3 4 7% 42%	2200
October	1st to 10th	Day	12% 5% 2 5% 3 4 3 3 5% 7% 52%	2165
	11th to 20th	Day	13% 6% 2 5% 4 3 4 3 5% 6% 49%	2000
	21st to end	Day	19% 7% 3 7% 3 3 3 2 4 6% 43%	2185
November	1st to 10th	Day	11% 7% 2 5% 4 4 4 3 4 6% 50%	1852
	11th to 20th	Day	13% 10% 3 8% 4 3 4 3 4 7% 40%	1890
	21st to end	Day	16% 12% 4 7% 5% 4 3 3 4 5% 38%	1890
December	1st to 10th	Day	17% 12% 3 8% 4 4 5% 3 5% 4 36%	3020
	11th to 20th	Day	21% 13% 5% 9% 6% 4 4 3 4 5% 26%	3463
	21st to end	Day	16% 15% 5% 12% 6% 6% 6% 3 4 6% 22%	4565
January	1st to 10th	Night	14% 14% 6% 14% 5% 5% 4 3 5% 6% 25%	7635
	11th to 20th	Night	16% 12% 5% 11% 5% 4 4 3 4 5% 30%	7680
	21st to end	Night	17% 10% 4 10% 5% 4 4 3 4 5% 33%	8401
February	1st to 10th	Night	18% 11% 4 9% 6% 4 4 4 5% 5% 30%	2688
	11th to 20th	Night	21% 12% 4 10% 5% 4 4 3 4 5% 29%	2458
	21st to end	Night	20% 13% 4 10% 4 4 4 3 4 5% 30%	2076
March	1st to 10th	Night	17% 12% 3 7% 4 4 3 3 5% 6% 36%	5260
	11th to 20th	Night	11% 7% 2 6% 3 3 4 3 5% 8% 46%	5090
	21st to end	Night	13% 9% 3 9% 5% 5% 5% 4 6% 7% 34%	5588
April	1st to 10th	Night	13% 8% 2 6% 5% 4 4 4 7% 7% 39%	2305
	11th to 20th	Night	13% 7% 2 5% 4 4 5% 4 6% 8% 41%	1951
	21st to end	Night	15% 8% 2 5% 5% 5% 5% 4 5% 7% 39%	1890
May	1st to 10th	Night	16% 11% 3 9% 4 5% 5% 5% 6% 8% 28%	4042
	11th to 20th	Night	14% 7% 3 7% 5% 6% 5% 4 7% 9% 32%	4032
	21st to end	Night	12% 7% 3 7% 5% 5% 5% 5% 6% 9% 36%	4421
June	1st to 10th	Night	8% 6% 2 5% 4 5% 5% 5% 7% 9% 44%	1800
	11th to 20th	Night	6% 5% 2 6% 5% 4 4 4 8% 11% 45%	1800
	21st to end	Night	7% 6% 1 5% 4 3 5% 5% 8% 12% 44%	1800
July	1st to 10th	Night	6% 6% 2 6% 5% 5% 5% 5% 7% 9% 44%	4058
	11th to 20th	Night	8% 6% 3 8% 5% 5% 5% 5% 7% 11% 38%	4057
	21st to end	Night	10% 9% 3 9% 5% 5% 5% 4 7% 9% 33%	4427
August	1st to 10th	Night	7% 6% 3 7% 4 5% 5% 5% 7% 10% 41%	2915
	11th to 20th	Night	8% 5% 3 7% 5% 5% 5% 4 7% 9% 43%	2875
	21st to end	Night	4 3 1 4 3 4 4 4 6% 9% 59%	3384
September	1st to 10th	Night	10% 5% 2 5% 4 4 5% 3 5% 7% 49%	2190
	11th to 20th	Night	10% 6% 2 6% 3 4 3 4 6% 7% 51%	2200
	21st to end	Night	13% 7% 3 6% 4 4 4 4 5% 6% 46%	2200
October	1st to 10th	Night	11% 5% 2 5% 3 4 3 3 5% 7% 54%	2234
	11th to 20th	Night	13% 5% 3 6% 4 3 4 3 4 7% 48%	2400
	21st to end	Night	18% 8% 2 6% 4 3 4 3 4 5% 44%	2650
November	1st to 10th	Night	12% 8% 2 5% 3 3 4 3 4 5% 50%	2666
	11th to 20th	Night	14% 9% 3 6% 4 4 4 3 5% 6% 43%	2730
	21st to end	Night	18% 11% 3 7% 5% 3 3 3 4 5% 38%	2730
December	1st to 10th	Night	14% 12% 5% 9% 5% 3 4 3 4 4 36%	4380
	11th to 20th	Night	18% 13% 7% 9% 5% 4 3 3 4 5% 30%	4952
	21st to end	Night	16% 15% 5% 10% 6% 5% 6% 4 5% 6% 21%	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	63% 5% 10% 16% 6%	5259
	11th to 20th	Day	57% 4 6% 18% 13% 2	5280
	21st to end	Day	59% 4 8% 22% 6% 1	5780
February	1st to 10th	Day	57% 5% 8% 18% 11% 1	1930
	11th to 20th	Day	57% 5% 7% 19% 10% 2	2058
	21st to end	Day	54% 4 8% 20% 11% 2	1730
March	1st to 10th	Day	53% 4 7% 17% 14% 5%	4811
	11th to 20th	Day	39% 5% 9% 24% 20% 3	5090
	21st to end	Day	45% 6% 9% 25% 14% 1	5586
April	1st to 10th	Day	45% 5% 11% 23% 13% 3	2332
	11th to 20th	Day	43% 5% 10% 21% 17% 4	2618
	21st to end	Day	46% 6% 12% 22% 12% 3	2730
May	1st to 10th	Day	52% 8% 11% 19% 10% 1	5910
	11th to 20th	Day	51% 7% 10% 21% 10% 1	5865
	21st to end	Day	42% 9% 13% 21% 12% 2	6447
June	1st to 10th	Day	36% 10% 12% 24% 16% 1	2600
	11th to 20th	Day	38% 9% 13% 21% 17% 2	2600
	21st to end	Day	36% 10% 14% 21% 15% 4	2600
July	1st to 10th	Day	38% 7% 14% 25% 16% 1	5835
	11th to 20th	Day	40% 10% 15% 26% 8% 1	5831
	21st to end	Day	49% 7% 11% 20% 11% 1	6240
August	1st to 10th	Day	36% 8% 16% 28% 10% 1	4221
	11th to 20th	Day	37% 8% 12% 29% 14%	4213
	21st to end	Day	25% 6% 14% 32% 21% 2	4487
September	1st to 10th	Day	35% 6% 12% 22% 19% 5%	2200
	11th to 20th	Day	34% 7% 10% 26% 22% 1	2200
	21st to end	Day	43% 5% 9% 22% 20% 2	2200
October	1st to 10th	Day	26% 4 8% 25% 36% 2	3529
	11th to 20th	Day	26% 3 10% 30% 27% 4	3240
	21st to end	Day	32% 3 10% 29% 23% 3	3511
November	1st to 10th	Day	37% 4 8% 26% 22% 3	1852
	11th to 20th	Day	44% 4 8% 23% 18% 3	1890
	21st to end	Day	49% 3 10% 20% 16% 1	1890
December	1st to 10th	Day	51% 4 7% 23% 14%	3020
	11th to 20th	Day	60% 4 9% 16% 10% 1	3460
	21st to end	Day	62% 6% 10% 16% 5%	4565
January	1st to 10th	Night	60% 6% 10% 18% 7%	7636
	11th to 20th	Night	55% 4 5% 20% 13% 3	7680
	21st to end	Night	54% 4 8% 21% 11% 2	8401
February	1st to 10th	Night	55% 5% 9% 19% 11% 1	2688
	11th to 20th	Night	58% 4 7% 19% 11% 1	2459
	21st to end	Night	58% 4 9% 16% 10% 3	2076
March	1st to 10th	Night	48% 4 6% 23% 15% 3	5260
	11th to 20th	Night	35% 6% 9% 28% 22% 1	5090
	21st to end	Night	48% 6% 9% 22% 15% 1	5588
April	1st to 10th	Night	41% 7% 11% 21% 17% 2	2305
	11th to 20th	Night	39% 6% 11% 22% 16% 6%	1951
	21st to end	Night	44% 6% 11% 22% 15% 2	1890
May	1st to 10th	Night	52% 8% 12% 18% 9% 2	4069
	11th to 20th	Night	47% 8% 14% 21% 9% 1	4033
	21st to end	Night	41% 9% 15% 20% 13% 3	4429
June	1st to 10th	Night	33% 8% 15% 23% 18% 3	1800
	11th to 20th	Night	32% 9% 16% 20% 20% 3	1800
	21st to end	Night	31% 10% 15% 24% 17% 4	1800
July	1st to 10th	Night	35% 9% 12% 23% 19% 3	4015
	11th to 20th	Night	38% 10% 16% 23% 11% 2	4006
	21st to end	Night	45% 8% 11% 22% 12% 2	4297
August	1st to 10th	Night	37% 8% 14% 28% 12% 1	2904
	11th to 20th	Night	36% 7% 11% 28% 18% 1	2902
	21st to end	Night	22% 6% 14% 31% 23% 3	3384
September	1st to 10th	Night	32% 7% 13% 20% 20% 8%	2190
	11th to 20th	Night	32% 6% 12% 22% 25% 3	2200
	21st to end	Night	38% 6% 10% 20% 23% 2	2200
October	1st to 10th	Night	26% 4 7% 25% 35% 3	3650
	11th to 20th	Night	26% 4 8% 26% 29% 8%	3899
	21st to end	Night	31% 3 7% 24% 27% 8%	4274
November	1st to 10th	Night	37% 4 9% 23% 23% 5%	2666
	11th to 20th	Night	43% 4 9% 20% 20% 5%	2730
	21st to end	Night	48% 4 9% 24% 14% 2	2730
December	1st to 10th	Night	52% 4 10% 21% 12% 1	4380
	11th to 20th	Night	57% 4 10% 20% 9% 1	4954
	21st to end	Night	61% 6% 10% 16% 7%	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	28% 20% 35% 12% 4 1	330
	11th to 20th	Day	37% 23% 30% 10%	330
	21st to end	Day	30% 30% 28% 10% 2	363
February	1st to 10th	Day	30% 21% 34% 12% 3	330
	11th to 20th	Day	42% 23% 26% 7% 1 1	330
	21st to end	Day	35% 20% 37% 7% 2	273
March	1st to 10th	Day	33% 21% 29% 12% 5% 1	431
	11th to 20th	Day	27% 19% 34% 15% 4 1	540
	21st to end	Day	29% 19% 32% 14% 5%	594
April	1st to 10th	Day	25% 19% 31% 18% 7% 1	540
	11th to 20th	Day	21% 19% 33% 17% 7% 3	532
	21st to end	Day	24% 16% 34% 19% 5% 2	540
May	1st to 10th	Day	23% 19% 30% 15% 9% 3	507
	11th to 20th	Day	20% 13% 31% 21% 11% 5%	527
	21st to end	Day	15% 10% 24% 26% 15% 9%	573
June	1st to 10th	Day	13% 8% 24% 28% 19% 8%	530
	11th to 20th	Day	13% 7% 24% 29% 19% 8%	530
	21st to end	Day	12% 6% 26% 25% 20% 11%	530
July	1st to 10th	Day	14% 5% 15% 26% 20% 19%	525
	11th to 20th	Day	9% 5% 23% 30% 20% 13%	524
	21st to end	Day	15% 4 17% 30% 21% 13%	576
August	1st to 10th	Day	13% 3 19% 27% 25% 13%	526
	11th to 20th	Day	13% 4 16% 34% 22% 10%	519
	21st to end	Day	8% 4 15% 31% 27% 14%	583
September	1st to 10th	Day	13% 4 17% 25% 29% 12%	530
	11th to 20th	Day	12% 3 13% 30% 27% 15%	530
	21st to end	Day	20% 9% 21% 25% 19% 6%	530
October	1st to 10th	Day	13% 6% 24% 27% 22% 7%	530
	11th to 20th	Day	15% 12% 29% 25% 14% 5%	530
	21st to end	Day	25% 12% 31% 21% 10% 1	568
November	1st to 10th	Day	18% 14% 35% 23% 8% 2	330
	11th to 20th	Day	27% 17% 30% 18% 6% 2	330
	21st to end	Day	29% 18% 31% 15% 6% 1	330
December	1st to 10th	Day	31% 19% 31% 14% 5% 1	330
	11th to 20th	Day	30% 18% 32% 14% 5% 1	330
	21st to end	Day	32% 20% 31% 15% 2	363
January	1st to 10th	Night	26% 18% 30% 20% 5%	840
	11th to 20th	Night	32% 18% 28% 19% 3	840
	21st to end	Night	24% 19% 34% 20% 3	924
February	1st to 10th	Night	27% 14% 33% 24% 2	840
	11th to 20th	Night	32% 18% 30% 18% 2	823
	21st to end	Night	30% 18% 31% 19% 2	692
March	1st to 10th	Night	24% 17% 30% 24% 5%	840
	11th to 20th	Night	26% 15% 31% 21% 7%	840
	21st to end	Night	19% 17% 33% 24% 6%	924
April	1st to 10th	Night	20% 15% 33% 24% 8%	835
	11th to 20th	Night	16% 12% 33% 27% 11% 1	623
	21st to end	Night	18% 9% 37% 28% 8%	630
May	1st to 10th	Night	19% 11% 27% 28% 13% 2	603
	11th to 20th	Night	17% 8% 22% 35% 17% 1	591
	21st to end	Night	10% 4 21% 35% 27% 3	650
June	1st to 10th	Night	9% 5% 22% 40% 22% 3	600
	11th to 20th	Night	8% 3 17% 40% 27% 5%	600
	21st to end	Night	9% 3 19% 36% 30% 3	600
July	1st to 10th	Night	12% 4 13% 32% 31% 8%	585
	11th to 20th	Night	7% 2 14% 36% 36% 5%	582
	21st to end	Night	13% 3 12% 33% 33% 6%	657
August	1st to 10th	Night	10% 2 11% 35% 37% 5%	597
	11th to 20th	Night	9% 3 10% 37% 33% 7%	594
	21st to end	Night	9% 2 10% 35% 35% 9%	660
September	1st to 10th	Night	11% 2 13% 34% 34% 6%	790
	11th to 20th	Night	12% 2 15% 28% 36% 7%	800
	21st to end	Night	17% 5% 20% 30% 27% 2	800
October	1st to 10th	Night	13% 5% 16% 33% 29% 4	800
	11th to 20th	Night	17% 8% 27% 29% 17% 2	800
	21st to end	Night	23% 10% 25% 28% 14% 1	880
November	1st to 10th	Night	17% 8% 26% 31% 16% 1	820
	11th to 20th	Night	20% 15% 27% 27% 10% 1	840
	21st to end	Night	24% 12% 25% 28% 10% 1	840
December	1st to 10th	Night	28% 15% 28% 25% 5%	840
	11th to 20th	Night	25% 13% 30% 25% 6% 1	840
	21st to end	Night	29% 16% 28% 24% 3	924

Page generated at 2026-04-29 02:21:46. Version ac07

Copyright © 2026 Attilla Danko.