Dark Horse Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dark Horse 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	16% 7% 2 6% 4 4 4 4 6% 7% 40%	120191
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 43%	118928
Transparency	Day	42% 5% 11% 23% 17% 3	123779
	Night	41% 4 11% 23% 17% 3	123500
Seeing	Day	23% 15% 28% 21% 10% 3	15845
	Night	20% 14% 30% 26% 9% 1	25128

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% 7% 2 6% 4 4 4 3 5% 6% 48%	3597
	11th to 20th	Day	13% 9% 3 9% 5% 5% 4 3 5% 6% 39%	3725
	21st to end	Day	17% 7% 3 7% 5% 4 4 3 4 6% 40%	4466
February	1st to 10th	Day	15% 8% 3 7% 5% 4 4 5% 5% 7% 38%	1800
	11th to 20th	Day	15% 9% 2 8% 5% 4 4 3 5% 5% 39%	1879
	21st to end	Day	17% 11% 3 7% 4 3 4 3 4 6% 39%	1650
March	1st to 10th	Day	21% 5% 2 5% 4 4 4 3 4 6% 44%	4539
	11th to 20th	Day	13% 6% 2 6% 4 5% 4 4 5% 7% 44%	4980
	21st to end	Day	18% 6% 2 6% 4 4 4 3 4 6% 43%	5758
April	1st to 10th	Day	15% 8% 3 6% 5% 3 4 4 6% 8% 39%	2400
	11th to 20th	Day	14% 8% 2 6% 4 4 4 4 5% 7% 41%	2374
	21st to end	Day	16% 6% 2 6% 4 4 5% 5% 6% 8% 40%	2400
May	1st to 10th	Day	25% 7% 2 6% 4 4 5% 4 6% 8% 29%	5847
	11th to 20th	Day	24% 8% 3 7% 4 4 4 3 5% 8% 31%	6318
	21st to end	Day	19% 6% 2 6% 4 5% 4 4 6% 8% 37%	6949
June	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 5% 7% 9% 39%	2800
	11th to 20th	Day	13% 6% 2 6% 5% 5% 5% 5% 6% 9% 37%	2800
	21st to end	Day	10% 5% 2 5% 5% 5% 5% 5% 7% 10% 42%	2800
July	1st to 10th	Day	7% 5% 2 6% 5% 6% 6% 6% 8% 12% 37%	4580
	11th to 20th	Day	9% 6% 2 6% 5% 4 5% 4 8% 10% 39%	4580
	21st to end	Day	10% 9% 3 8% 6% 6% 6% 5% 7% 8% 34%	4697
August	1st to 10th	Day	18% 6% 2 6% 5% 5% 5% 4 7% 7% 34%	4104
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 6% 9% 42%	3765
	21st to end	Day	12% 5% 2 5% 4 4 4 3 5% 8% 48%	4180
September	1st to 10th	Day	15% 6% 2 5% 3 5% 4 4 5% 7% 45%	2280
	11th to 20th	Day	13% 5% 2 5% 2 3 3 3 5% 7% 51%	2265
	21st to end	Day	19% 7% 3 6% 5% 5% 4 3 5% 6% 38%	2066
October	1st to 10th	Day	18% 5% 2 6% 4 3 4 3 5% 8% 42%	1900
	11th to 20th	Day	15% 5% 2 4 4 4 4 3 5% 7% 48%	1900
	21st to end	Day	19% 6% 2 5% 3 3 3 3 5% 6% 45%	2041
November	1st to 10th	Day	12% 6% 2 5% 2 3 4 3 4 7% 52%	1761
	11th to 20th	Day	15% 6% 2 5% 3 3 4 4 6% 6% 47%	1800
	21st to end	Day	15% 7% 2 5% 3 4 4 5% 5% 7% 44%	1644
December	1st to 10th	Day	13% 9% 2 6% 4 4 5% 4 5% 6% 43%	2600
	11th to 20th	Day	20% 9% 2 4 3 4 3 2 4 6% 42%	2986
	21st to end	Day	16% 6% 2 6% 5% 5% 5% 4 6% 7% 38%	3960
January	1st to 10th	Night	12% 6% 2 6% 4 4 4 3 4 6% 49%	6349
	11th to 20th	Night	14% 8% 2 5% 3 3 4 3 4 6% 48%	6158
	21st to end	Night	17% 8% 2 6% 4 4 4 3 4 6% 42%	6490
February	1st to 10th	Night	17% 9% 3 5% 4 4 4 4 5% 6% 40%	2600
	11th to 20th	Night	17% 8% 2 6% 4 4 3 3 5% 6% 39%	2405
	21st to end	Night	17% 8% 2 6% 3 4 4 4 5% 6% 41%	1980
March	1st to 10th	Night	17% 8% 2 5% 4 3 4 3 4 6% 43%	5285
	11th to 20th	Night	11% 7% 2 5% 4 4 4 3 4 6% 50%	4979
	21st to end	Night	15% 8% 2 7% 4 4 3 3 5% 6% 43%	5174
April	1st to 10th	Night	17% 6% 3 6% 5% 4 5% 4 6% 6% 39%	2000
	11th to 20th	Night	15% 7% 3 6% 4 4 4 4 5% 7% 41%	1975
	21st to end	Night	17% 6% 2 5% 3 3 4 3 6% 7% 44%	2000
May	1st to 10th	Night	24% 8% 3 7% 4 4 4 4 6% 7% 28%	4135
	11th to 20th	Night	23% 9% 3 7% 4 3 3 4 5% 7% 31%	3575
	21st to end	Night	21% 7% 2 5% 4 3 4 3 6% 9% 37%	3926
June	1st to 10th	Night	14% 6% 3 6% 5% 4 4 3 5% 8% 42%	1600
	11th to 20th	Night	12% 6% 2 5% 4 6% 5% 4 7% 9% 40%	1600
	21st to end	Night	11% 4 2 5% 5% 4 5% 4 7% 10% 43%	1600
July	1st to 10th	Night	7% 5% 3 8% 5% 5% 6% 5% 8% 10% 38%	2600
	11th to 20th	Night	9% 7% 3 7% 4 5% 5% 5% 9% 9% 37%	2600
	21st to end	Night	13% 7% 3 6% 5% 4 5% 4 7% 9% 36%	2853
August	1st to 10th	Night	19% 5% 2 7% 4 5% 5% 4 6% 9% 34%	2825
	11th to 20th	Night	12% 7% 3 7% 4 4 5% 3 6% 8% 40%	3029
	21st to end	Night	11% 5% 2 5% 4 4 4 4 6% 8% 48%	3476
September	1st to 10th	Night	14% 7% 3 6% 4 3 3 3 4 6% 47%	1900
	11th to 20th	Night	12% 5% 2 5% 3 3 4 3 4 6% 53%	1914
	21st to end	Night	18% 8% 2 5% 4 4 4 4 5% 6% 41%	2110
October	1st to 10th	Night	19% 7% 3 6% 4 4 3 2 4 6% 43%	2280
	11th to 20th	Night	15% 5% 2 4 3 3 3 3 5% 5% 51%	2280
	21st to end	Night	18% 7% 2 3 3 2 4 3 5% 6% 48%	2555
November	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 5% 6% 53%	2537
	11th to 20th	Night	17% 7% 2 5% 3 3 4 3 5% 5% 47%	2600
	21st to end	Night	13% 6% 2 5% 5% 5% 4 4 5% 7% 45%	2754
December	1st to 10th	Night	12% 7% 3 6% 4 4 4 3 5% 6% 46%	4580
	11th to 20th	Night	22% 7% 2 5% 3 3 4 3 4 6% 41%	5208
	21st to end	Night	19% 8% 3 6% 4 4 4 3 5% 6% 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	38% 2 9% 26% 22% 3	3598
	11th to 20th	Day	47% 3 8% 21% 15% 6%	3756
	21st to end	Day	46% 4 10% 24% 15% 1	4466
February	1st to 10th	Day	45% 4 13% 24% 12% 2	1800
	11th to 20th	Day	47% 4 9% 24% 15% 2	1888
	21st to end	Day	47% 4 9% 23% 14% 4	1650
March	1st to 10th	Day	42% 2 9% 23% 17% 7%	4542
	11th to 20th	Day	39% 4 9% 28% 15% 5%	4980
	21st to end	Day	44% 3 9% 16% 23% 5%	5758
April	1st to 10th	Day	43% 5% 10% 21% 16% 5%	2400
	11th to 20th	Day	42% 6% 11% 21% 16% 4	2374
	21st to end	Day	42% 6% 11% 19% 18% 4	2400
May	1st to 10th	Day	53% 5% 11% 18% 10% 2	5856
	11th to 20th	Day	52% 5% 8% 23% 11% 1	6322
	21st to end	Day	45% 5% 10% 24% 15% 1	6949
June	1st to 10th	Day	40% 7% 12% 24% 16% 1	2800
	11th to 20th	Day	42% 7% 14% 23% 11% 3	2800
	21st to end	Day	37% 7% 14% 25% 17% 1	2800
July	1st to 10th	Day	39% 8% 15% 21% 14% 3	4510
	11th to 20th	Day	39% 6% 16% 25% 13% 1	4505
	21st to end	Day	47% 6% 10% 19% 17% 1	4547
August	1st to 10th	Day	47% 6% 14% 23% 10%	4091
	11th to 20th	Day	38% 4 11% 31% 15% 1	3802
	21st to end	Day	35% 5% 10% 28% 21% 2	4180
September	1st to 10th	Day	40% 6% 10% 21% 21% 3	2280
	11th to 20th	Day	33% 5% 9% 26% 23% 4	2265
	21st to end	Day	48% 5% 7% 18% 20% 2	2066
October	1st to 10th	Day	38% 3 8% 18% 28% 5%	3160
	11th to 20th	Day	26% 2 11% 26% 28% 6%	3140
	21st to end	Day	25% 2 12% 27% 29% 4	3343
November	1st to 10th	Day	34% 4 13% 25% 20% 4	1761
	11th to 20th	Day	37% 5% 10% 25% 21% 2	1800
	21st to end	Day	39% 4 13% 30% 12% 2	1644
December	1st to 10th	Day	42% 3 15% 25% 12% 3	2600
	11th to 20th	Day	44% 2 12% 29% 10% 1	2986
	21st to end	Day	44% 5% 13% 21% 16% 2	3960
January	1st to 10th	Night	38% 3 8% 25% 21% 5%	6349
	11th to 20th	Night	39% 3 11% 23% 17% 7%	6202
	21st to end	Night	45% 3 9% 23% 18% 3	6490
February	1st to 10th	Night	46% 4 12% 25% 12% 2	2600
	11th to 20th	Night	46% 3 9% 21% 17% 4	2407
	21st to end	Night	44% 5% 8% 22% 17% 5%	1980
March	1st to 10th	Night	43% 3 8% 24% 12% 9%	5295
	11th to 20th	Night	36% 4 14% 27% 15% 5%	4980
	21st to end	Night	44% 3 7% 17% 25% 4	5174
April	1st to 10th	Night	44% 5% 11% 19% 17% 4	2000
	11th to 20th	Night	42% 5% 10% 19% 20% 4	1975
	21st to end	Night	40% 6% 9% 21% 19% 5%	2000
May	1st to 10th	Night	55% 6% 11% 16% 11% 2	4142
	11th to 20th	Night	52% 6% 10% 19% 12% 1	3576
	21st to end	Night	45% 5% 12% 22% 14% 2	3927
June	1st to 10th	Night	40% 6% 11% 22% 20% 2	1600
	11th to 20th	Night	38% 8% 14% 24% 14% 1	1600
	21st to end	Night	35% 8% 13% 26% 15% 2	1600
July	1st to 10th	Night	40% 9% 18% 17% 14% 3	2560
	11th to 20th	Night	41% 7% 16% 23% 11% 1	2552
	21st to end	Night	44% 6% 13% 19% 16% 1	2762
August	1st to 10th	Night	48% 6% 19% 18% 9% 1	2814
	11th to 20th	Night	42% 4 11% 26% 15% 1	3058
	21st to end	Night	34% 4 11% 30% 20% 1	3476
September	1st to 10th	Night	39% 6% 10% 20% 22% 3	1900
	11th to 20th	Night	32% 5% 8% 24% 26% 4	1914
	21st to end	Night	44% 5% 8% 20% 21% 2	2110
October	1st to 10th	Night	43% 3 9% 21% 22% 2	3800
	11th to 20th	Night	24% 3 10% 29% 28% 5%	3779
	21st to end	Night	26% 3 11% 32% 24% 4	4203
November	1st to 10th	Night	32% 4 12% 26% 21% 5%	2537
	11th to 20th	Night	39% 4 9% 22% 21% 5%	2600
	21st to end	Night	38% 4 13% 27% 16% 2	2754
December	1st to 10th	Night	40% 3 12% 30% 14% 2	4580
	11th to 20th	Night	45% 3 11% 25% 13% 4	5208
	21st to end	Night	45% 4 11% 20% 18% 2	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	35% 24% 27% 12% 1 2	200
	11th to 20th	Day	39% 25% 28% 7% 1	233
	21st to end	Day	30% 28% 26% 12% 3	330
February	1st to 10th	Day	30% 27% 31% 10% 2	300
	11th to 20th	Day	35% 26% 29% 8% 1	427
	21st to end	Day	32% 23% 30% 12% 3	414
March	1st to 10th	Day	33% 19% 29% 15% 4	500
	11th to 20th	Day	30% 25% 29% 13% 3 1	500
	21st to end	Day	30% 25% 30% 10% 4 1	550
April	1st to 10th	Day	30% 24% 30% 12% 4 1	500
	11th to 20th	Day	27% 24% 27% 15% 6% 2	492
	21st to end	Day	28% 18% 34% 15% 4 2	500
May	1st to 10th	Day	31% 20% 27% 15% 5% 1	503
	11th to 20th	Day	27% 14% 32% 19% 7% 2	527
	21st to end	Day	21% 11% 27% 23% 16% 2	573
June	1st to 10th	Day	18% 12% 29% 26% 12% 2	530
	11th to 20th	Day	19% 10% 31% 25% 14% 2	530
	21st to end	Day	13% 9% 33% 27% 14% 4	530
July	1st to 10th	Day	14% 4 24% 33% 20% 6%	525
	11th to 20th	Day	9% 5% 25% 34% 22% 5%	524
	21st to end	Day	11% 6% 31% 31% 18% 3	532
August	1st to 10th	Day	12% 6% 26% 31% 18% 7%	486
	11th to 20th	Day	14% 5% 21% 36% 18% 7%	479
	21st to end	Day	12% 7% 24% 29% 19% 9%	539
September	1st to 10th	Day	19% 11% 20% 27% 16% 8%	490
	11th to 20th	Day	16% 5% 27% 26% 16% 10%	490
	21st to end	Day	25% 13% 23% 23% 12% 3	490
October	1st to 10th	Day	23% 8% 28% 22% 13% 5%	490
	11th to 20th	Day	18% 18% 27% 23% 11% 2	490
	21st to end	Day	21% 17% 32% 17% 11% 2	512
November	1st to 10th	Day	21% 13% 32% 21% 10% 3	395
	11th to 20th	Day	22% 21% 33% 19% 5% 1	400
	21st to end	Day	27% 23% 29% 17% 4	244
December	1st to 10th	Day	33% 25% 29% 9% 5%	200
	11th to 20th	Day	36% 24% 27% 12% 3	200
	21st to end	Day	28% 22% 27% 14% 9%	220
January	1st to 10th	Night	23% 23% 35% 17% 2	800
	11th to 20th	Night	27% 19% 31% 21% 3	800
	21st to end	Night	19% 23% 38% 18% 2	880
February	1st to 10th	Night	23% 20% 36% 20% 1	800
	11th to 20th	Night	30% 24% 33% 13% 1	783
	21st to end	Night	26% 21% 36% 15% 2	660
March	1st to 10th	Night	24% 24% 30% 20% 3	800
	11th to 20th	Night	24% 22% 32% 17% 5%	800
	21st to end	Night	21% 17% 40% 17% 4	880
April	1st to 10th	Night	23% 23% 35% 17% 4	800
	11th to 20th	Night	21% 14% 37% 22% 6%	791
	21st to end	Night	21% 16% 33% 24% 7%	800
May	1st to 10th	Night	24% 13% 34% 23% 6%	643
	11th to 20th	Night	20% 9% 35% 29% 7%	394
	21st to end	Night	16% 8% 26% 37% 12% 1	433
June	1st to 10th	Night	14% 6% 31% 38% 11%	400
	11th to 20th	Night	12% 8% 28% 42% 11%	400
	21st to end	Night	10% 4 27% 45% 13% 1	400
July	1st to 10th	Night	12% 4 21% 43% 18% 1	390
	11th to 20th	Night	7% 2 22% 42% 26% 1	388
	21st to end	Night	11% 4 23% 41% 18% 2	481
August	1st to 10th	Night	12% 2 19% 41% 24% 2	567
	11th to 20th	Night	13% 5% 18% 37% 25% 2	706
	21st to end	Night	13% 5% 18% 41% 22% 2	836
September	1st to 10th	Night	16% 5% 28% 31% 18% 3	760
	11th to 20th	Night	14% 5% 21% 35% 23% 3	760
	21st to end	Night	22% 5% 24% 31% 17% 1	760
October	1st to 10th	Night	23% 8% 25% 25% 16% 3	760
	11th to 20th	Night	19% 11% 29% 29% 12%	760
	21st to end	Night	23% 13% 31% 24% 9%	836
November	1st to 10th	Night	16% 12% 33% 27% 11% 1	780
	11th to 20th	Night	22% 17% 27% 26% 7%	800
	21st to end	Night	19% 18% 35% 22% 7% 1	800
December	1st to 10th	Night	26% 19% 31% 19% 5% 1	800
	11th to 20th	Night	22% 18% 34% 22% 5%	800
	21st to end	Night	22% 19% 33% 24% 3	880

Page generated at 2025-07-18 01:10:02. Version ac07

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