Sheep Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sheep Hill Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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	17% 7% 2 6% 4 4 4 4 5% 7% 39%	120110
	Night	17% 7% 2 6% 4 4 4 3 5% 6% 42%	119004
Transparency	Day	42% 5% 12% 22% 16% 3	123669
	Night	41% 5% 11% 22% 17% 3	123593
Seeing	Day	23% 14% 30% 21% 9% 3	15843
	Night	21% 11% 31% 27% 9% 1	24997

By Month

Percentage Cloud Cover NE (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% 6% 2 5% 4 4 4 4 6% 6% 47%	3597
	11th to 20th	Day	13% 9% 3 7% 6% 5% 4 3 4 5% 40%	3570
	21st to end	Day	18% 8% 4 7% 4 4 4 3 4 5% 40%	4351
February	1st to 10th	Day	17% 8% 3 7% 4 4 4 4 5% 6% 38%	1800
	11th to 20th	Day	15% 7% 3 7% 4 4 4 4 5% 6% 41%	1933
	21st to end	Day	19% 8% 3 6% 5% 4 5% 3 4 6% 38%	1650
March	1st to 10th	Day	21% 6% 2 5% 4 3 4 3 5% 5% 42%	4476
	11th to 20th	Day	11% 6% 3 5% 4 5% 5% 4 6% 8% 43%	4832
	21st to end	Day	18% 7% 2 6% 3 3 4 3 5% 7% 42%	5893
April	1st to 10th	Day	18% 8% 3 6% 5% 4 4 4 5% 7% 37%	2400
	11th to 20th	Day	17% 7% 2 6% 4 5% 4 4 5% 7% 40%	2374
	21st to end	Day	18% 6% 2 6% 5% 4 4 4 5% 7% 40%	2405
May	1st to 10th	Day	30% 7% 2 6% 3 4 4 3 5% 8% 28%	5922
	11th to 20th	Day	24% 8% 3 7% 4 3 4 3 5% 6% 33%	6318
	21st to end	Day	20% 6% 2 6% 3 4 4 4 6% 8% 35%	6949
June	1st to 10th	Day	16% 5% 2 6% 5% 5% 5% 4 6% 9% 36%	2800
	11th to 20th	Day	17% 6% 2 6% 5% 5% 5% 5% 7% 9% 34%	2800
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 4 6% 9% 39%	2800
July	1st to 10th	Day	10% 6% 3 7% 6% 6% 6% 5% 7% 10% 35%	4580
	11th to 20th	Day	11% 6% 2 6% 4 5% 5% 5% 7% 10% 37%	4580
	21st to end	Day	15% 8% 3 8% 6% 6% 5% 5% 6% 8% 32%	4814
August	1st to 10th	Day	21% 5% 3 7% 4 5% 5% 4 6% 7% 32%	4162
	11th to 20th	Day	14% 5% 2 6% 5% 4 4 4 6% 8% 42%	3730
	21st to end	Day	12% 6% 2 5% 3 3 4 3 6% 8% 47%	4180
September	1st to 10th	Day	16% 5% 2 6% 4 4 5% 4 5% 6% 43%	2280
	11th to 20th	Day	13% 6% 2 5% 4 3 3 3 5% 7% 50%	2214
	21st to end	Day	19% 6% 3 7% 4 4 4 3 5% 6% 40%	2090
October	1st to 10th	Day	20% 6% 2 5% 4 4 4 4 5% 6% 41%	1957
	11th to 20th	Day	15% 5% 2 5% 3 4 4 2 5% 7% 47%	1900
	21st to end	Day	20% 6% 2 5% 3 3 4 3 4 6% 43%	1956
November	1st to 10th	Day	14% 6% 2 5% 3 3 3 2 4 5% 53%	1761
	11th to 20th	Day	18% 6% 2 4 3 3 5% 3 5% 7% 46%	1800
	21st to end	Day	15% 7% 2 5% 3 3 4 4 6% 7% 43%	1690
December	1st to 10th	Day	17% 6% 2 6% 4 3 3 3 6% 7% 44%	2600
	11th to 20th	Day	22% 7% 2 4 3 3 3 3 5% 6% 43%	2986
	21st to end	Day	17% 8% 3 6% 5% 4 4 4 5% 8% 36%	3960
January	1st to 10th	Night	13% 5% 2 5% 4 4 4 3 6% 6% 48%	6349
	11th to 20th	Night	13% 7% 3 7% 4 4 4 3 4 6% 45%	6315
	21st to end	Night	18% 9% 2 5% 4 4 4 3 5% 7% 40%	6600
February	1st to 10th	Night	17% 9% 2 6% 4 4 4 3 5% 5% 41%	2600
	11th to 20th	Night	16% 8% 3 6% 4 4 4 4 5% 5% 42%	2353
	21st to end	Night	18% 8% 3 6% 4 4 5% 2 5% 6% 40%	1980
March	1st to 10th	Night	19% 8% 2 5% 3 3 3 3 5% 6% 43%	5364
	11th to 20th	Night	13% 8% 3 5% 4 4 3 3 4 6% 48%	5122
	21st to end	Night	16% 9% 3 6% 4 3 3 3 5% 6% 42%	5037
April	1st to 10th	Night	18% 9% 2 5% 5% 4 5% 3 5% 7% 37%	2000
	11th to 20th	Night	18% 7% 2 5% 5% 4 5% 3 5% 7% 40%	1975
	21st to end	Night	19% 6% 2 5% 3 4 3 3 5% 6% 42%	1995
May	1st to 10th	Night	28% 9% 3 7% 4 3 3 3 5% 7% 27%	4054
	11th to 20th	Night	23% 10% 3 5% 4 4 3 3 5% 5% 34%	3581
	21st to end	Night	22% 6% 2 6% 3 4 4 3 5% 8% 36%	3926
June	1st to 10th	Night	16% 6% 2 5% 5% 4 5% 3 5% 8% 44%	1600
	11th to 20th	Night	15% 7% 2 6% 4 4 4 4 6% 9% 41%	1600
	21st to end	Night	12% 5% 2 5% 4 5% 5% 4 6% 10% 44%	1600
July	1st to 10th	Night	9% 5% 3 7% 5% 5% 5% 4 7% 10% 41%	2600
	11th to 20th	Night	13% 6% 2 7% 4 5% 5% 4 7% 10% 38%	2600
	21st to end	Night	16% 8% 3 6% 4 4 4 4 6% 9% 37%	2739
August	1st to 10th	Night	21% 6% 3 6% 5% 5% 4 4 6% 8% 33%	2718
	11th to 20th	Night	14% 6% 3 7% 4 5% 5% 3 5% 7% 39%	3097
	21st to end	Night	11% 4 2 5% 4 3 4 4 5% 7% 52%	3476
September	1st to 10th	Night	16% 5% 2 7% 5% 3 4 4 4 5% 45%	1900
	11th to 20th	Night	13% 6% 2 5% 3 3 3 2 5% 6% 52%	1966
	21st to end	Night	19% 6% 2 6% 3 4 4 3 4 7% 44%	2090
October	1st to 10th	Night	20% 7% 2 6% 4 4 3 3 4 6% 42%	2218
	11th to 20th	Night	16% 6% 1 5% 3 2 3 3 4 7% 50%	2280
	21st to end	Night	20% 7% 2 5% 3 2 3 3 4 6% 45%	2641
November	1st to 10th	Night	13% 5% 2 5% 3 3 4 3 5% 6% 52%	2537
	11th to 20th	Night	19% 7% 2 4 3 3 3 2 4 5% 48%	2600
	21st to end	Night	15% 5% 2 5% 4 4 4 4 5% 7% 45%	2707
December	1st to 10th	Night	13% 7% 2 6% 4 4 4 3 5% 6% 47%	4580
	11th to 20th	Night	24% 7% 1 5% 3 3 3 2 4 5% 42%	5208
	21st to end	Night	18% 10% 3 7% 4 4 3 3 4 5% 39%	6996

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	34% 4 11% 30% 18% 3	3598
	11th to 20th	Day	44% 5% 9% 22% 14% 6%	3600
	21st to end	Day	47% 4 10% 22% 16% 1	4351
February	1st to 10th	Day	45% 5% 13% 22% 13% 2	1800
	11th to 20th	Day	43% 5% 9% 24% 17% 3	1942
	21st to end	Day	46% 4 11% 22% 14% 3	1650
March	1st to 10th	Day	43% 4 10% 21% 14% 8%	4479
	11th to 20th	Day	38% 6% 12% 28% 11% 4	4832
	21st to end	Day	43% 4 10% 13% 24% 6%	5893
April	1st to 10th	Day	46% 4 10% 20% 15% 5%	2400
	11th to 20th	Day	43% 5% 11% 21% 17% 3	2374
	21st to end	Day	42% 5% 9% 22% 17% 4	2405
May	1st to 10th	Day	54% 6% 13% 15% 11% 1	5933
	11th to 20th	Day	51% 5% 10% 20% 14% 2	6322
	21st to end	Day	44% 6% 11% 23% 14% 1	6949
June	1st to 10th	Day	42% 7% 13% 21% 15% 1	2800
	11th to 20th	Day	45% 7% 13% 23% 11% 2	2800
	21st to end	Day	39% 6% 15% 23% 16% 1	2800
July	1st to 10th	Day	42% 9% 12% 22% 13% 2	4510
	11th to 20th	Day	39% 8% 16% 23% 13% 1	4505
	21st to end	Day	48% 7% 11% 16% 16% 2	4661
August	1st to 10th	Day	49% 7% 13% 20% 10%	4148
	11th to 20th	Day	37% 7% 14% 29% 13% 1	3766
	21st to end	Day	35% 6% 10% 29% 19% 2	4180
September	1st to 10th	Day	40% 6% 10% 22% 19% 3	2280
	11th to 20th	Day	34% 4 11% 23% 25% 3	2214
	21st to end	Day	45% 5% 9% 19% 18% 3	2090
October	1st to 10th	Day	37% 6% 9% 18% 23% 6%	3256
	11th to 20th	Day	26% 4 11% 24% 30% 5%	3140
	21st to end	Day	27% 3 14% 25% 26% 4	3194
November	1st to 10th	Day	35% 3 12% 24% 23% 3	1761
	11th to 20th	Day	38% 4 10% 25% 20% 4	1800
	21st to end	Day	40% 3 14% 29% 12% 2	1690
December	1st to 10th	Day	40% 4 15% 28% 12% 2	2600
	11th to 20th	Day	42% 3 15% 23% 15% 2	2986
	21st to end	Day	45% 5% 10% 22% 16% 2	3960
January	1st to 10th	Night	35% 4 10% 25% 21% 4	6349
	11th to 20th	Night	39% 5% 15% 20% 16% 5%	6360
	21st to end	Night	45% 4 10% 20% 16% 5%	6600
February	1st to 10th	Night	44% 4 13% 24% 13% 2	2600
	11th to 20th	Night	44% 5% 9% 21% 17% 5%	2355
	21st to end	Night	46% 3 6% 24% 17% 3	1980
March	1st to 10th	Night	43% 4 10% 21% 13% 8%	5374
	11th to 20th	Night	39% 5% 15% 25% 13% 4	5123
	21st to end	Night	44% 5% 7% 16% 23% 5%	5037
April	1st to 10th	Night	46% 5% 10% 21% 14% 4	2000
	11th to 20th	Night	44% 5% 9% 21% 19% 2	1975
	21st to end	Night	42% 5% 9% 22% 18% 4	1995
May	1st to 10th	Night	56% 7% 11% 14% 10% 1	4060
	11th to 20th	Night	51% 6% 13% 14% 15% 2	3582
	21st to end	Night	45% 7% 13% 23% 10% 2	3927
June	1st to 10th	Night	40% 5% 11% 22% 20% 2	1600
	11th to 20th	Night	40% 7% 15% 25% 12% 1	1600
	21st to end	Night	34% 9% 15% 25% 16% 2	1600
July	1st to 10th	Night	39% 9% 16% 19% 15% 2	2560
	11th to 20th	Night	41% 8% 13% 24% 12% 1	2552
	21st to end	Night	42% 8% 15% 15% 18% 2	2651
August	1st to 10th	Night	49% 7% 17% 17% 9% 1	2708
	11th to 20th	Night	42% 8% 12% 27% 10% 1	3127
	21st to end	Night	31% 6% 11% 31% 20% 1	3476
September	1st to 10th	Night	41% 5% 10% 25% 19% 1	1900
	11th to 20th	Night	33% 5% 11% 23% 25% 3	1966
	21st to end	Night	42% 4 9% 22% 22% 2	2090
October	1st to 10th	Night	38% 6% 11% 22% 20% 2	3699
	11th to 20th	Night	25% 4 9% 28% 29% 4	3779
	21st to end	Night	30% 4 12% 32% 20% 2	4340
November	1st to 10th	Night	34% 4 11% 24% 22% 4	2537
	11th to 20th	Night	40% 3 8% 24% 20% 4	2600
	21st to end	Night	37% 5% 13% 29% 14% 1	2707
December	1st to 10th	Night	39% 4 13% 31% 11% 2	4580
	11th to 20th	Night	46% 3 11% 22% 15% 3	5208
	21st to end	Night	47% 5% 10% 18% 16% 4	6996

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	27% 26% 31% 14% 3	200
	11th to 20th	Day	30% 26% 35% 8% 2	200
	21st to end	Day	24% 23% 36% 12% 4 1	305
February	1st to 10th	Day	31% 22% 37% 9% 2	300
	11th to 20th	Day	29% 21% 35% 14% 1	481
	21st to end	Day	28% 19% 35% 15% 3	414
March	1st to 10th	Day	27% 22% 30% 17% 4	500
	11th to 20th	Day	24% 22% 34% 15% 4 1	500
	21st to end	Day	29% 21% 30% 15% 5% 1	550
April	1st to 10th	Day	30% 21% 30% 13% 5% 1	500
	11th to 20th	Day	25% 21% 33% 14% 6% 1	492
	21st to end	Day	31% 16% 31% 17% 3 2	500
May	1st to 10th	Day	35% 16% 27% 15% 6% 1	503
	11th to 20th	Day	24% 16% 31% 20% 8% 1	527
	21st to end	Day	21% 10% 27% 28% 13% 2	573
June	1st to 10th	Day	18% 11% 32% 25% 12% 2	530
	11th to 20th	Day	22% 12% 30% 25% 11% 1	530
	21st to end	Day	15% 11% 32% 27% 13% 2	530
July	1st to 10th	Day	14% 5% 27% 30% 18% 5%	525
	11th to 20th	Day	11% 6% 27% 34% 19% 3	524
	21st to end	Day	14% 8% 30% 32% 15% 2	532
August	1st to 10th	Day	15% 4 27% 32% 15% 7%	486
	11th to 20th	Day	15% 5% 23% 33% 19% 5%	479
	21st to end	Day	14% 8% 29% 27% 15% 7%	539
September	1st to 10th	Day	18% 11% 24% 24% 16% 7%	490
	11th to 20th	Day	16% 6% 27% 25% 14% 12%	490
	21st to end	Day	23% 10% 30% 22% 10% 5%	490
October	1st to 10th	Day	26% 8% 28% 21% 12% 5%	490
	11th to 20th	Day	18% 17% 30% 22% 11% 1	490
	21st to end	Day	26% 13% 30% 19% 11% 1	468
November	1st to 10th	Day	20% 12% 32% 24% 9% 3	395
	11th to 20th	Day	26% 16% 32% 22% 4 1	400
	21st to end	Day	22% 21% 35% 17% 4 2	290
December	1st to 10th	Day	28% 23% 29% 15% 6%	200
	11th to 20th	Day	34% 19% 32% 12% 4	200
	21st to end	Day	29% 15% 37% 14% 5% 1	220
January	1st to 10th	Night	22% 17% 38% 20% 3	800
	11th to 20th	Night	25% 16% 33% 24% 3	800
	21st to end	Night	21% 17% 39% 21% 2	880
February	1st to 10th	Night	23% 13% 38% 24% 2	800
	11th to 20th	Night	24% 17% 40% 17% 1	783
	21st to end	Night	25% 16% 36% 19% 4	660
March	1st to 10th	Night	24% 17% 36% 20% 4	800
	11th to 20th	Night	23% 17% 36% 21% 4 1	800
	21st to end	Night	22% 17% 37% 20% 4 1	880
April	1st to 10th	Night	25% 18% 34% 19% 4	800
	11th to 20th	Night	23% 16% 34% 23% 4	791
	21st to end	Night	22% 13% 34% 25% 5%	795
May	1st to 10th	Night	27% 12% 34% 21% 7%	600
	11th to 20th	Night	22% 13% 30% 29% 7%	400
	21st to end	Night	20% 6% 23% 37% 13% 1	433
June	1st to 10th	Night	19% 5% 29% 34% 13% 1	400
	11th to 20th	Night	15% 6% 30% 37% 13%	400
	21st to end	Night	12% 6% 26% 45% 11%	400
July	1st to 10th	Night	12% 5% 24% 38% 20% 1	390
	11th to 20th	Night	14% 3 20% 43% 19% 1	388
	21st to end	Night	13% 4 22% 44% 15% 2	416
August	1st to 10th	Night	12% 3 21% 41% 22% 2	499
	11th to 20th	Night	13% 4 18% 39% 23% 2	750
	21st to end	Night	15% 5% 18% 40% 21% 2	836
September	1st to 10th	Night	17% 4 25% 34% 18% 2	760
	11th to 20th	Night	17% 5% 19% 34% 21% 3	760
	21st to end	Night	21% 5% 25% 33% 14% 2	760
October	1st to 10th	Night	24% 8% 27% 24% 15% 3	760
	11th to 20th	Night	20% 10% 28% 32% 10% 1	760
	21st to end	Night	24% 10% 34% 23% 8%	836
November	1st to 10th	Night	17% 11% 31% 28% 12% 1	780
	11th to 20th	Night	24% 12% 33% 25% 7% 1	800
	21st to end	Night	18% 17% 33% 26% 6% 1	800
December	1st to 10th	Night	25% 14% 32% 24% 5% 1	800
	11th to 20th	Night	24% 13% 34% 26% 3	800
	21st to end	Night	24% 15% 34% 25% 3	880

Page generated at 2025-07-18 01:10:03. Version ac07

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