Whitetail Hollow Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Whitetail Hollow Observatory 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	11% 5% 2 6% 4 5% 5% 4 6% 9% 42%	126174
	Night	12% 6% 2 6% 4 4 4 4 6% 8% 44%	122887
Transparency	Day	36% 7% 13% 19% 18% 7%	129885
	Night	36% 7% 11% 17% 20% 8%	127245
Seeing	Day	13% 7% 22% 24% 21% 13%	16402
	Night	13% 7% 23% 33% 21% 3	26419

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	13% 4 1 5% 3 3 3 3 4 6% 55%	4056
	11th to 20th	Day	13% 5% 2 5% 3 3 2 2 4 7% 54%	4060
	21st to end	Day	14% 7% 2 6% 3 4 4 3 5% 7% 46%	4466
February	1st to 10th	Day	14% 7% 3 6% 4 4 5% 3 7% 7% 41%	1945
	11th to 20th	Day	18% 7% 2 4 4 3 4 3 5% 7% 44%	1948
	21st to end	Day	15% 5% 2 7% 3 3 5% 4 4 6% 46%	1655
March	1st to 10th	Day	19% 8% 3 7% 4 3 4 3 5% 6% 39%	4935
	11th to 20th	Day	8% 4 1 4 4 4 3 4 5% 8% 55%	4980
	21st to end	Day	14% 7% 3 7% 4 4 4 3 4 5% 47%	5465
April	1st to 10th	Day	9% 5% 2 5% 4 4 4 4 5% 7% 49%	2200
	11th to 20th	Day	11% 5% 1 5% 4 3 5% 4 5% 8% 49%	2257
	21st to end	Day	6% 4 1 6% 4 4 5% 3 6% 7% 54%	2597
May	1st to 10th	Day	10% 6% 3 6% 5% 4 5% 5% 6% 9% 42%	5862
	11th to 20th	Day	15% 6% 2 7% 4 5% 5% 5% 7% 9% 34%	5861
	21st to end	Day	8% 5% 2 6% 5% 4 5% 4 6% 9% 45%	6433
June	1st to 10th	Day	9% 6% 2 7% 5% 6% 6% 6% 8% 10% 34%	2600
	11th to 20th	Day	5% 4 2 6% 5% 7% 6% 5% 9% 11% 41%	2600
	21st to end	Day	4 5% 2 7% 5% 7% 7% 6% 9% 11% 36%	2600
July	1st to 10th	Day	10% 7% 3 9% 7% 7% 8% 6% 10% 11% 23%	5900
	11th to 20th	Day	3 3 1 5% 5% 6% 7% 7% 11% 16% 36%	5900
	21st to end	Day	5% 5% 3 8% 6% 7% 8% 7% 10% 13% 28%	6424
August	1st to 10th	Day	15% 6% 3 9% 6% 8% 7% 6% 9% 11% 23%	4234
	11th to 20th	Day	7% 4 2 6% 5% 5% 6% 5% 9% 11% 38%	4174
	21st to end	Day	7% 5% 2 6% 4 5% 6% 5% 8% 11% 41%	4135
September	1st to 10th	Day	7% 5% 2 8% 6% 7% 6% 5% 7% 9% 38%	2200
	11th to 20th	Day	12% 6% 2 6% 4 5% 5% 5% 7% 9% 39%	2178
	21st to end	Day	15% 6% 2 6% 5% 5% 4 3 5% 6% 43%	2090
October	1st to 10th	Day	12% 5% 2 6% 4 4 3 3 4 6% 51%	2059
	11th to 20th	Day	10% 3 2 3 3 3 3 3 5% 6% 59%	1900
	21st to end	Day	9% 3 2 5% 3 4 3 3 4 5% 58%	2090
November	1st to 10th	Day	13% 6% 2 5% 4 4 3 3 4 6% 51%	1957
	11th to 20th	Day	14% 6% 2 4 3 2 3 2 4 6% 54%	1844
	21st to end	Day	14% 6% 2 6% 4 4 3 4 5% 6% 48%	1800
December	1st to 10th	Day	9% 5% 2 7% 5% 3 4 4 5% 6% 50%	2930
	11th to 20th	Day	14% 6% 2 5% 3 3 4 3 5% 7% 48%	3373
	21st to end	Day	19% 8% 3 5% 3 3 4 3 4 5% 43%	4466
January	1st to 10th	Night	12% 4 1 5% 3 3 3 3 4 5% 56%	5889
	11th to 20th	Night	13% 5% 2 6% 4 4 4 4 5% 6% 46%	5900
	21st to end	Night	15% 8% 2 6% 3 4 4 3 4 7% 44%	6490
February	1st to 10th	Night	14% 6% 2 6% 4 4 4 4 5% 6% 46%	2450
	11th to 20th	Night	17% 7% 3 5% 4 3 3 3 4 6% 46%	2338
	21st to end	Night	13% 5% 3 6% 4 4 5% 4 5% 6% 46%	1975
March	1st to 10th	Night	18% 8% 2 7% 4 4 4 3 4 6% 40%	4918
	11th to 20th	Night	9% 4 1 4 3 3 4 4 6% 8% 54%	4980
	21st to end	Night	14% 5% 2 4 3 3 4 4 5% 6% 49%	5467
April	1st to 10th	Night	10% 4 2 6% 5% 4 4 4 5% 8% 49%	2200
	11th to 20th	Night	10% 5% 2 5% 4 3 3 4 4 8% 52%	2090
	21st to end	Night	7% 3 2 4 4 4 4 3 6% 7% 54%	1818
May	1st to 10th	Night	8% 7% 2 7% 4 4 5% 4 8% 10% 40%	4033
	11th to 20th	Night	15% 5% 2 5% 4 4 5% 5% 8% 10% 37%	4032
	21st to end	Night	9% 5% 2 5% 4 4 4 4 7% 9% 47%	4421
June	1st to 10th	Night	10% 6% 3 7% 5% 6% 6% 5% 8% 11% 34%	1800
	11th to 20th	Night	6% 6% 2 6% 6% 5% 6% 4 7% 10% 41%	1800
	21st to end	Night	6% 5% 2 7% 5% 6% 7% 7% 8% 13% 35%	1800
July	1st to 10th	Night	12% 9% 3 9% 7% 7% 7% 5% 8% 10% 23%	4058
	11th to 20th	Night	4 5% 2 7% 6% 7% 7% 6% 11% 15% 31%	4057
	21st to end	Night	8% 7% 3 9% 7% 7% 7% 6% 8% 12% 26%	4427
August	1st to 10th	Night	17% 7% 3 7% 6% 6% 6% 6% 9% 10% 23%	2915
	11th to 20th	Night	8% 6% 3 7% 5% 6% 6% 5% 9% 10% 36%	2875
	21st to end	Night	6% 6% 2 5% 4 4 5% 4 7% 11% 46%	3723
September	1st to 10th	Night	10% 5% 2 6% 5% 5% 6% 5% 7% 11% 38%	2200
	11th to 20th	Night	10% 6% 3 7% 5% 5% 5% 4 6% 8% 40%	2179
	21st to end	Night	13% 6% 2 7% 4 5% 5% 4 5% 7% 42%	2090
October	1st to 10th	Night	12% 6% 2 6% 4 3 3 3 5% 7% 50%	2118
	11th to 20th	Night	12% 4 1 4 3 3 2 2 4 5% 59%	2280
	21st to end	Night	8% 4 2 5% 4 4 3 3 4 5% 57%	2508
November	1st to 10th	Night	12% 5% 2 5% 3 3 3 3 4 5% 53%	2341
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 4 6% 51%	2553
	21st to end	Night	13% 5% 2 4 4 4 4 3 4 7% 51%	2600
December	1st to 10th	Night	11% 5% 2 4 3 3 3 3 4 5% 56%	4250
	11th to 20th	Night	15% 8% 3 6% 4 3 3 3 4 5% 46%	4822
	21st to end	Night	19% 8% 3 5% 3 3 3 3 4 5% 43%	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	32% 4 5% 16% 34% 8%	4057
	11th to 20th	Day	31% 3 5% 16% 32% 12%	4060
	21st to end	Day	40% 4 5% 19% 21% 11%	4466
February	1st to 10th	Day	40% 5% 9% 19% 20% 7%	1945
	11th to 20th	Day	40% 5% 6% 17% 25% 7%	1958
	21st to end	Day	39% 5% 7% 17% 21% 11%	1655
March	1st to 10th	Day	46% 5% 7% 10% 23% 9%	4935
	11th to 20th	Day	27% 5% 9% 16% 22% 22%	4980
	21st to end	Day	42% 4 6% 18% 20% 11%	5465
April	1st to 10th	Day	33% 6% 10% 20% 21% 9%	2200
	11th to 20th	Day	33% 5% 9% 20% 21% 12%	2257
	21st to end	Day	28% 6% 11% 25% 25% 5%	2597
May	1st to 10th	Day	37% 7% 16% 24% 14% 2	5897
	11th to 20th	Day	44% 8% 17% 19% 11% 1	5865
	21st to end	Day	34% 7% 16% 19% 22% 2	6447
June	1st to 10th	Day	40% 12% 17% 18% 12% 1	2600
	11th to 20th	Day	32% 11% 20% 24% 14% 1	2600
	21st to end	Day	36% 12% 19% 18% 12% 2	2600
July	1st to 10th	Day	47% 12% 20% 16% 3 1	5835
	11th to 20th	Day	29% 16% 34% 18% 3 1	5831
	21st to end	Day	40% 15% 23% 18% 3	6240
August	1st to 10th	Day	51% 12% 19% 16% 3	4221
	11th to 20th	Day	34% 11% 17% 26% 12% 1	4213
	21st to end	Day	33% 9% 15% 31% 11% 1	4135
September	1st to 10th	Day	40% 10% 16% 15% 14% 5%	2200
	11th to 20th	Day	39% 9% 11% 19% 18% 5%	2178
	21st to end	Day	42% 6% 10% 18% 18% 6%	2090
October	1st to 10th	Day	34% 4 7% 14% 30% 12%	3423
	11th to 20th	Day	16% 3 5% 15% 29% 32%	3140
	21st to end	Day	22% 3 6% 19% 35% 16%	3428
November	1st to 10th	Day	36% 4 8% 16% 23% 14%	1957
	11th to 20th	Day	33% 3 7% 19% 25% 13%	1844
	21st to end	Day	37% 4 9% 19% 22% 10%	1800
December	1st to 10th	Day	34% 5% 8% 17% 27% 8%	2930
	11th to 20th	Day	37% 4 7% 24% 22% 6%	3370
	21st to end	Day	43% 4 6% 21% 22% 3	4466
January	1st to 10th	Night	29% 5% 6% 18% 33% 10%	5889
	11th to 20th	Night	36% 5% 6% 16% 28% 10%	5900
	21st to end	Night	41% 4 6% 12% 27% 10%	6490
February	1st to 10th	Night	38% 5% 8% 18% 21% 10%	2450
	11th to 20th	Night	41% 4 8% 15% 22% 10%	2339
	21st to end	Night	38% 6% 9% 15% 18% 15%	1975
March	1st to 10th	Night	45% 5% 9% 13% 18% 10%	4918
	11th to 20th	Night	28% 4 9% 13% 27% 20%	4980
	21st to end	Night	36% 4 8% 17% 21% 14%	5467
April	1st to 10th	Night	33% 5% 11% 20% 20% 11%	2200
	11th to 20th	Night	31% 5% 9% 20% 23% 12%	2090
	21st to end	Night	27% 7% 13% 25% 21% 7%	1818
May	1st to 10th	Night	35% 9% 15% 25% 13% 3	4060
	11th to 20th	Night	39% 10% 18% 21% 11% 1	4033
	21st to end	Night	32% 8% 14% 23% 21% 3	4429
June	1st to 10th	Night	40% 11% 18% 20% 11% 1	1800
	11th to 20th	Night	35% 10% 19% 23% 11% 1	1800
	21st to end	Night	34% 15% 22% 16% 11% 3	1800
July	1st to 10th	Night	52% 13% 18% 13% 3 1	4015
	11th to 20th	Night	36% 15% 31% 15% 2	4006
	21st to end	Night	47% 15% 18% 15% 4	4297
August	1st to 10th	Night	51% 13% 19% 14% 3	2904
	11th to 20th	Night	38% 12% 20% 23% 7% 1	2902
	21st to end	Night	30% 8% 18% 32% 10% 1	3723
September	1st to 10th	Night	37% 12% 17% 17% 12% 5%	2200
	11th to 20th	Night	40% 9% 14% 16% 16% 4	2179
	21st to end	Night	40% 7% 13% 17% 17% 5%	2090
October	1st to 10th	Night	35% 4 10% 13% 25% 11%	3534
	11th to 20th	Night	18% 2 5% 12% 25% 38%	3779
	21st to end	Night	21% 3 6% 17% 33% 20%	4120
November	1st to 10th	Night	33% 4 9% 14% 24% 15%	2341
	11th to 20th	Night	35% 4 7% 15% 24% 14%	2553
	21st to end	Night	34% 4 8% 19% 23% 11%	2600
December	1st to 10th	Night	32% 4 7% 17% 33% 8%	4250
	11th to 20th	Night	40% 4 11% 18% 19% 8%	4824
	21st to end	Night	43% 4 5% 20% 25% 3	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	15% 16% 48% 17% 4	300
	11th to 20th	Day	15% 20% 33% 23% 7% 2	300
	21st to end	Day	21% 20% 42% 13% 4	330
February	1st to 10th	Day	13% 17% 42% 20% 7%	300
	11th to 20th	Day	28% 15% 36% 18% 4	300
	21st to end	Day	17% 18% 39% 18% 7% 1	254
March	1st to 10th	Day	18% 15% 31% 24% 12% 1	500
	11th to 20th	Day	14% 13% 37% 27% 8% 1	500
	21st to end	Day	18% 11% 38% 23% 9% 1	550
April	1st to 10th	Day	15% 14% 29% 28% 12% 2	500
	11th to 20th	Day	12% 12% 28% 24% 18% 6%	492
	21st to end	Day	8% 8% 26% 27% 23% 8%	500
May	1st to 10th	Day	8% 11% 26% 29% 17% 9%	503
	11th to 20th	Day	12% 4 18% 29% 21% 15%	527
	21st to end	Day	13% 3 16% 25% 24% 18%	573
June	1st to 10th	Day	10% 3 15% 26% 28% 17%	530
	11th to 20th	Day	6% 4 12% 21% 29% 28%	530
	21st to end	Day	4 1 10% 26% 32% 26%	530
July	1st to 10th	Day	7% 1 7% 19% 34% 32%	525
	11th to 20th	Day	6% 1 4 18% 36% 35%	524
	21st to end	Day	5% 1 6% 24% 34% 29%	576
August	1st to 10th	Day	10% 3 10% 27% 28% 22%	526
	11th to 20th	Day	10% 2 7% 18% 34% 29%	519
	21st to end	Day	14% 1 8% 24% 28% 25%	583
September	1st to 10th	Day	8% 4 10% 26% 33% 19%	530
	11th to 20th	Day	14% 3 13% 26% 30% 14%	522
	21st to end	Day	18% 2 13% 24% 29% 14%	490
October	1st to 10th	Day	15% 3 20% 30% 21% 11%	490
	11th to 20th	Day	10% 4 22% 32% 21% 10%	490
	21st to end	Day	13% 5% 22% 28% 27% 5%	539
November	1st to 10th	Day	17% 5% 26% 25% 21% 6%	495
	11th to 20th	Day	19% 8% 35% 26% 10% 2	344
	21st to end	Day	11% 17% 27% 29% 14% 2	300
December	1st to 10th	Day	16% 15% 36% 25% 8% 1	300
	11th to 20th	Day	19% 15% 34% 20% 11% 1	300
	21st to end	Day	25% 12% 32% 21% 8% 2	330
January	1st to 10th	Night	16% 13% 37% 28% 5%	800
	11th to 20th	Night	18% 12% 33% 29% 9%	800
	21st to end	Night	19% 11% 39% 27% 4	880
February	1st to 10th	Night	16% 11% 32% 35% 7%	800
	11th to 20th	Night	20% 12% 35% 29% 4	783
	21st to end	Night	15% 12% 38% 32% 4	660
March	1st to 10th	Night	15% 10% 34% 36% 6%	800
	11th to 20th	Night	14% 10% 35% 32% 8% 1	800
	21st to end	Night	13% 11% 37% 29% 10%	880
April	1st to 10th	Night	11% 10% 34% 34% 11% 1	800
	11th to 20th	Night	12% 11% 29% 33% 15%	791
	21st to end	Night	7% 5% 27% 44% 16%	618
May	1st to 10th	Night	7% 5% 29% 42% 16% 1	600
	11th to 20th	Night	9% 4 24% 40% 20% 3	591
	21st to end	Night	10% 4 12% 39% 31% 4	650
June	1st to 10th	Night	11% 2 14% 39% 30% 4	600
	11th to 20th	Night	8% 2 11% 35% 35% 9%	600
	21st to end	Night	7% 1 10% 34% 40% 8%	600
July	1st to 10th	Night	9% 2 8% 28% 45% 9%	585
	11th to 20th	Night	6% 2 4 28% 49% 11%	582
	21st to end	Night	7% 1 7% 32% 45% 8%	657
August	1st to 10th	Night	10% 1 8% 27% 44% 10%	597
	11th to 20th	Night	13% 2 6% 28% 42% 9%	594
	21st to end	Night	10% 3 9% 28% 40% 9%	782
September	1st to 10th	Night	11% 3 8% 32% 40% 7%	800
	11th to 20th	Night	12% 2 11% 31% 41% 3	793
	21st to end	Night	15% 2 13% 34% 31% 4	760
October	1st to 10th	Night	13% 4 16% 38% 24% 5%	760
	11th to 20th	Night	14% 3 21% 35% 24% 3	760
	21st to end	Night	10% 6% 23% 35% 24% 2	836
November	1st to 10th	Night	15% 5% 21% 34% 22% 3	780
	11th to 20th	Night	16% 9% 26% 32% 16% 1	800
	21st to end	Night	15% 9% 26% 32% 17% 1	800
December	1st to 10th	Night	16% 8% 30% 33% 11% 1	800
	11th to 20th	Night	18% 9% 28% 31% 12% 1	800
	21st to end	Night	22% 11% 27% 31% 9%	880

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