Horsethief Lake State Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Horsethief Lake 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	7% 7% 2 6% 4 4 4 4 5% 7% 50%	136420
	Night	10% 8% 3 7% 4 4 4 4 5% 7% 44%	132253
Transparency	Day	37% 4 12% 24% 16% 6%	131169
	Night	41% 5% 14% 23% 12% 4	128503
Seeing	Day	16% 15% 27% 23% 14% 4	15915
	Night	15% 10% 24% 32% 17% 2	24986

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	20% 18% 6% 15% 7% 6% 5% 4 4 5% 10%	3647
	11th to 20th	Day	16% 13% 4 7% 5% 5% 5% 4 5% 8% 29%	3640
	21st to end	Day	18% 15% 5% 10% 6% 4 3 4 5% 4 24%	4160
February	1st to 10th	Day	13% 14% 5% 11% 7% 6% 6% 4 5% 7% 22%	1800
	11th to 20th	Day	12% 11% 3 8% 5% 6% 6% 5% 7% 7% 29%	1755
	21st to end	Day	10% 10% 3 8% 7% 6% 5% 6% 8% 7% 29%	1638
March	1st to 10th	Day	7% 9% 4 8% 6% 6% 7% 6% 8% 10% 28%	4518
	11th to 20th	Day	11% 10% 4 9% 5% 4 4 4 6% 8% 35%	4995
	21st to end	Day	10% 10% 3 9% 6% 7% 7% 6% 7% 9% 26%	5749
April	1st to 10th	Day	8% 8% 3 7% 6% 6% 6% 5% 7% 9% 35%	2400
	11th to 20th	Day	7% 8% 2 6% 5% 5% 5% 5% 7% 8% 42%	2369
	21st to end	Day	5% 5% 2 5% 4 6% 5% 5% 8% 10% 47%	2546
May	1st to 10th	Day	8% 4 2 4 4 4 4 3 5% 6% 57%	6320
	11th to 20th	Day	4 4 2 4 4 4 4 3 5% 9% 57%	6277
	21st to end	Day	5% 4 2 4 4 3 5% 4 6% 8% 55%	6902
June	1st to 10th	Day	7% 5% 2 6% 4 4 5% 4 6% 8% 51%	2800
	11th to 20th	Day	4 3 1 4 4 4 4 4 5% 8% 57%	2800
	21st to end	Day	3 2 1 3 3 3 3 3 5% 7% 69%	2800
July	1st to 10th	Day	1 1 1 1 1 1 2 3 3 84%	6319
	11th to 20th	Day	1 1 1 1 1 1 2 2 2 3 86%	6320
	21st to end	Day	2 2 1 2 1 1 2 1 3 4 81%	6880
August	1st to 10th	Day	2 3 1 3 2 2 3 2 4 6% 72%	6090
	11th to 20th	Day	1 1 1 3 2 3 3 3 5% 6% 72%	5796
	21st to end	Day	4 4 1 4 3 3 3 3 4 6% 64%	5994
September	1st to 10th	Day	2 2 1 2 2 2 2 3 4 6% 74%	2400
	11th to 20th	Day	4 3 1 3 3 4 4 3 6% 6% 62%	2400
	21st to end	Day	7% 4 1 5% 3 4 4 3 5% 5% 58%	2140
October	1st to 10th	Day	5% 4 1 4 3 3 4 3 6% 7% 59%	3280
	11th to 20th	Day	6% 5% 2 7% 5% 6% 6% 4 7% 9% 42%	3261
	21st to end	Day	10% 7% 3 9% 6% 6% 6% 5% 7% 8% 34%	3335
November	1st to 10th	Day	14% 11% 3 9% 7% 7% 5% 5% 7% 8% 24%	1829
	11th to 20th	Day	14% 14% 5% 10% 6% 7% 7% 5% 7% 6% 19%	1680
	21st to end	Day	14% 12% 5% 11% 6% 6% 5% 5% 6% 7% 24%	1680
December	1st to 10th	Day	11% 10% 6% 11% 6% 7% 5% 4 5% 6% 29%	2700
	11th to 20th	Day	20% 17% 7% 15% 8% 6% 5% 3 5% 4 10%	3108
	21st to end	Day	18% 20% 6% 13% 7% 7% 4 4 5% 5% 12%	4092
January	1st to 10th	Night	22% 20% 6% 15% 7% 5% 5% 3 4 5% 9%	6320
	11th to 20th	Night	18% 12% 4 7% 5% 4 4 3 5% 9% 30%	6320
	21st to end	Night	20% 14% 5% 10% 5% 4 3 2 4 5% 29%	6801
February	1st to 10th	Night	14% 13% 4 9% 5% 5% 5% 4 6% 7% 27%	2600
	11th to 20th	Night	12% 11% 3 8% 5% 5% 4 4 5% 7% 34%	2532
	21st to end	Night	11% 12% 3 8% 5% 5% 5% 5% 7% 8% 30%	1991
March	1st to 10th	Night	7% 8% 3 8% 6% 6% 7% 6% 8% 8% 32%	5376
	11th to 20th	Night	8% 9% 3 8% 5% 5% 5% 4 7% 7% 40%	5001
	21st to end	Night	10% 7% 3 8% 7% 7% 7% 6% 8% 9% 27%	5213
April	1st to 10th	Night	9% 8% 2 7% 4 5% 5% 5% 9% 8% 39%	2000
	11th to 20th	Night	6% 6% 2 6% 5% 5% 4 4 7% 8% 47%	1979
	21st to end	Night	6% 5% 2 5% 4 4 5% 4 7% 9% 49%	1850
May	1st to 10th	Night	8% 5% 2 4 3 2 3 3 5% 5% 61%	3640
	11th to 20th	Night	4 4 2 5% 3 3 4 3 5% 7% 60%	3616
	21st to end	Night	6% 4 1 3 3 3 4 4 5% 9% 57%	3972
June	1st to 10th	Night	8% 5% 1 5% 4 5% 4 4 6% 7% 50%	1600
	11th to 20th	Night	4 4 1 3 3 3 4 2 6% 8% 61%	1600
	21st to end	Night	2 3 1 3 3 3 3 2 5% 5% 70%	1600
July	1st to 10th	Night	1 1 1 1 2 2 2 3 4 83%	3640
	11th to 20th	Night	1 1 1 2 1 2 2 3 86%	3640
	21st to end	Night	1 1 1 1 1 1 1 3 6% 82%	3970
August	1st to 10th	Night	3 2 1 3 2 3 3 2 4 7% 69%	3706
	11th to 20th	Night	3 2 1 3 2 3 3 3 4 5% 73%	4052
	21st to end	Night	2 3 1 4 3 3 4 3 4 6% 67%	5005
September	1st to 10th	Night	1 2 1 3 2 2 3 2 4 6% 75%	2000
	11th to 20th	Night	4 3 1 3 3 3 4 3 5% 6% 64%	2000
	21st to end	Night	5% 4 1 4 3 3 3 3 5% 6% 62%	2255
October	1st to 10th	Night	5% 4 1 3 3 3 4 3 5% 7% 63%	3920
	11th to 20th	Night	6% 5% 2 5% 4 5% 5% 4 8% 9% 47%	3897
	21st to end	Night	10% 6% 2 8% 4 6% 7% 5% 7% 8% 38%	4469
November	1st to 10th	Night	15% 9% 3 8% 5% 6% 5% 5% 6% 7% 30%	2686
	11th to 20th	Night	16% 13% 3 10% 7% 5% 6% 5% 6% 6% 24%	2940
	21st to end	Night	12% 11% 4 9% 7% 6% 6% 5% 6% 7% 27%	2940
December	1st to 10th	Night	10% 10% 5% 11% 6% 5% 6% 4 5% 6% 31%	4700
	11th to 20th	Night	24% 17% 6% 15% 7% 5% 5% 3 4 4 9%	5316
	21st to end	Night	20% 17% 6% 11% 6% 6% 5% 3 4 5% 16%	7106

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	79% 5% 7% 8% 1	3647
	11th to 20th	Day	55% 4 19% 17% 5%	3640
	21st to end	Day	62% 4 7% 17% 9%	4160
February	1st to 10th	Day	61% 9% 16% 12% 3	1800
	11th to 20th	Day	52% 9% 18% 17% 4	1764
	21st to end	Day	51% 9% 17% 14% 8%	1638
March	1st to 10th	Day	48% 8% 12% 19% 12%	4518
	11th to 20th	Day	47% 5% 13% 28% 6%	4995
	21st to end	Day	54% 6% 16% 20% 4	5749
April	1st to 10th	Day	44% 8% 19% 21% 8%	2400
	11th to 20th	Day	39% 7% 18% 21% 12% 2	2372
	21st to end	Day	32% 7% 21% 26% 12% 2	2546
May	1st to 10th	Day	29% 4 10% 30% 24% 3	6320
	11th to 20th	Day	27% 4 15% 35% 16% 3	6280
	21st to end	Day	28% 5% 22% 34% 10% 1	6904
June	1st to 10th	Day	32% 5% 15% 30% 15% 3	2800
	11th to 20th	Day	26% 4 15% 37% 15% 3	2800
	21st to end	Day	18% 3 16% 35% 21% 8%	2800
July	1st to 10th	Day	8% 1 6% 25% 32% 28%	6251
	11th to 20th	Day	7% 1 3 17% 33% 38%	6245
	21st to end	Day	11% 1 8% 33% 32% 15%	6681
August	1st to 10th	Day	15% 2 6% 21% 34% 22%	4438
	11th to 20th	Day	15% 2 11% 39% 24% 8%	4195
	21st to end	Day	22% 2 11% 26% 31% 8%	4321
September	1st to 10th	Day	13% 2 10% 34% 27% 13%	2400
	11th to 20th	Day	24% 4 16% 28% 23% 6%	2400
	21st to end	Day	28% 4 14% 30% 19% 5%	2140
October	1st to 10th	Day	26% 4 12% 32% 23% 3	3280
	11th to 20th	Day	40% 5% 18% 23% 13% 1	3261
	21st to end	Day	48% 6% 12% 26% 8% 1	3335
November	1st to 10th	Day	57% 8% 14% 17% 4	1829
	11th to 20th	Day	62% 9% 13% 12% 3	1680
	21st to end	Day	61% 7% 16% 11% 5%	1680
December	1st to 10th	Day	60% 5% 13% 19% 4	2700
	11th to 20th	Day	80% 4 8% 6% 1	3108
	21st to end	Day	79% 4 8% 8% 1	4092
January	1st to 10th	Night	79% 6% 7% 7% 1	6320
	11th to 20th	Night	49% 5% 19% 22% 4	6320
	21st to end	Night	59% 4 8% 19% 10%	6801
February	1st to 10th	Night	56% 10% 18% 14% 3	2600
	11th to 20th	Night	49% 8% 18% 19% 5% 1	2534
	21st to end	Night	48% 9% 16% 19% 6% 1	1991
March	1st to 10th	Night	45% 7% 15% 23% 10%	5376
	11th to 20th	Night	44% 4 16% 29% 7%	5001
	21st to end	Night	53% 7% 19% 19% 2	5213
April	1st to 10th	Night	40% 8% 21% 23% 7% 1	2000
	11th to 20th	Night	35% 6% 20% 25% 12% 2	1979
	21st to end	Night	30% 9% 19% 28% 12% 2	1850
May	1st to 10th	Night	28% 3 9% 31% 28% 2	3637
	11th to 20th	Night	24% 5% 18% 37% 14% 2	3616
	21st to end	Night	25% 5% 30% 31% 9%	3972
June	1st to 10th	Night	34% 6% 14% 34% 10% 2	1600
	11th to 20th	Night	22% 4 20% 38% 15% 1	1600
	21st to end	Night	18% 3 14% 37% 21% 6%	1600
July	1st to 10th	Night	9% 2 8% 25% 32% 26%	3600
	11th to 20th	Night	8% 1 6% 15% 38% 32%	3592
	21st to end	Night	8% 1 8% 34% 33% 16%	3857
August	1st to 10th	Night	14% 2 6% 23% 34% 21%	2688
	11th to 20th	Night	15% 2 13% 40% 22% 7%	2929
	21st to end	Night	16% 3 13% 29% 31% 8%	3598
September	1st to 10th	Night	13% 3 13% 31% 27% 13%	2000
	11th to 20th	Night	22% 5% 14% 30% 19% 10%	2000
	21st to end	Night	23% 4 15% 30% 22% 6%	2255
October	1st to 10th	Night	22% 3 12% 37% 24% 1	3920
	11th to 20th	Night	35% 5% 20% 25% 13% 1	3897
	21st to end	Night	43% 7% 13% 26% 10% 1	4469
November	1st to 10th	Night	51% 8% 15% 20% 6%	2686
	11th to 20th	Night	58% 8% 14% 15% 4	2940
	21st to end	Night	54% 9% 16% 16% 5% 1	2940
December	1st to 10th	Night	51% 6% 14% 26% 3	4700
	11th to 20th	Night	78% 6% 9% 6% 1	5316
	21st to end	Night	71% 6% 10% 11% 2	7106

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	34% 17% 17% 18% 11% 2	202
	11th to 20th	Day	33% 10% 18% 27% 11% 2	200
	21st to end	Day	23% 9% 24% 25% 18% 2	285
February	1st to 10th	Day	25% 15% 24% 22% 11% 2	400
	11th to 20th	Day	27% 15% 24% 24% 9% 1	395
	21st to end	Day	22% 18% 25% 26% 8% 2	409
March	1st to 10th	Day	22% 17% 27% 23% 9% 2	500
	11th to 20th	Day	21% 17% 23% 23% 12% 3	500
	21st to end	Day	22% 20% 28% 20% 9% 1	550
April	1st to 10th	Day	23% 21% 27% 17% 10% 2	500
	11th to 20th	Day	17% 27% 27% 17% 9% 4	492
	21st to end	Day	16% 22% 31% 21% 8% 2	500
May	1st to 10th	Day	16% 20% 30% 20% 11% 3	503
	11th to 20th	Day	19% 17% 30% 17% 13% 4	528
	21st to end	Day	13% 17% 30% 23% 12% 4	573
June	1st to 10th	Day	13% 19% 30% 19% 14% 5%	530
	11th to 20th	Day	15% 24% 34% 16% 9% 2	530
	21st to end	Day	7% 16% 33% 25% 14% 4	530
July	1st to 10th	Day	5% 11% 43% 24% 14% 3	526
	11th to 20th	Day	5% 13% 36% 27% 16% 3	524
	21st to end	Day	3 10% 35% 25% 18% 8%	575
August	1st to 10th	Day	6% 8% 30% 24% 22% 10%	527
	11th to 20th	Day	2 10% 30% 27% 20% 11%	517
	21st to end	Day	8% 13% 26% 23% 22% 9%	583
September	1st to 10th	Day	4 11% 25% 27% 23% 10%	530
	11th to 20th	Day	9% 12% 26% 25% 22% 5%	530
	21st to end	Day	12% 13% 22% 28% 19% 6%	530
October	1st to 10th	Day	13% 14% 23% 25% 16% 9%	530
	11th to 20th	Day	16% 15% 22% 23% 17% 7%	530
	21st to end	Day	19% 15% 23% 22% 15% 4	448
November	1st to 10th	Day	27% 12% 20% 26% 12% 3	316
	11th to 20th	Day	30% 10% 23% 24% 9% 3	220
	21st to end	Day	26% 8% 18% 29% 16% 2	220
December	1st to 10th	Day	25% 10% 22% 25% 13% 5%	220
	11th to 20th	Day	35% 10% 21% 25% 7% 1	220
	21st to end	Day	29% 12% 21% 26% 11% 1	242
January	1st to 10th	Night	29% 9% 21% 28% 11% 1	804
	11th to 20th	Night	27% 9% 18% 30% 14% 2	800
	21st to end	Night	23% 8% 18% 31% 18% 2	880
February	1st to 10th	Night	24% 12% 17% 33% 14% 1	800
	11th to 20th	Night	19% 12% 22% 29% 16% 1	784
	21st to end	Night	21% 14% 20% 33% 11% 1	660
March	1st to 10th	Night	18% 11% 21% 35% 14% 1	800
	11th to 20th	Night	15% 12% 24% 35% 13% 2	800
	21st to end	Night	16% 16% 27% 30% 10% 1	880
April	1st to 10th	Night	14% 16% 26% 29% 14% 1	800
	11th to 20th	Night	13% 17% 28% 26% 14% 2	791
	21st to end	Night	9% 17% 33% 27% 13% 1	650
May	1st to 10th	Night	10% 15% 30% 31% 15% 1	400
	11th to 20th	Night	10% 11% 28% 32% 16% 2	394
	21st to end	Night	7% 8% 34% 35% 14% 2	433
June	1st to 10th	Night	12% 13% 33% 28% 14% 1	400
	11th to 20th	Night	9% 18% 38% 26% 9%	400
	21st to end	Night	5% 13% 35% 30% 16% 1	400
July	1st to 10th	Night	3 9% 35% 35% 18% 1	390
	11th to 20th	Night	1 9% 40% 36% 13% 1	388
	21st to end	Night	2 9% 31% 38% 19% 1	438
August	1st to 10th	Night	2 6% 28% 40% 21% 3	446
	11th to 20th	Night	4 5% 21% 32% 31% 7%	594
	21st to end	Night	3 10% 27% 33% 23% 4	870
September	1st to 10th	Night	3 7% 27% 31% 30% 3	800
	11th to 20th	Night	6% 7% 26% 27% 30% 5%	800
	21st to end	Night	7% 8% 20% 34% 27% 3	800
October	1st to 10th	Night	11% 7% 20% 38% 20% 4	800
	11th to 20th	Night	14% 8% 21% 30% 25% 2	800
	21st to end	Night	16% 8% 25% 32% 17% 2	880
November	1st to 10th	Night	20% 9% 21% 36% 13% 1	820
	11th to 20th	Night	25% 10% 22% 30% 12% 1	840
	21st to end	Night	18% 8% 22% 34% 15% 3	840
December	1st to 10th	Night	24% 7% 20% 31% 16% 2	840
	11th to 20th	Night	27% 9% 18% 33% 14% 1	840
	21st to end	Night	27% 11% 21% 31% 9% 1	924

Page generated at 2025-12-29 22:14:10. Version ac07

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