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 2024-11-04.

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	15% 7% 2 6% 4 4 4 4 6% 7% 40%	101348
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 41%	97683
Transparency	Day	42% 5% 11% 23% 16% 3	104908
	Night	41% 5% 11% 24% 17% 3	102211
Seeing	Day	22% 15% 28% 21% 10% 3	14966
	Night	20% 14% 30% 26% 10% 1	24245

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	15% 8% 2 6% 4 4 4 3 5% 5% 45%	2517
	11th to 20th	Day	14% 10% 3 10% 5% 4 4 3 5% 6% 35%	2620
	21st to end	Day	20% 8% 3 6% 5% 5% 4 3 4 5% 35%	3124
February	1st to 10th	Day	16% 7% 3 6% 5% 4 4 5% 5% 7% 39%	1710
	11th to 20th	Day	15% 9% 2 8% 5% 4 4 3 5% 5% 40%	1782
	21st to end	Day	18% 11% 2 7% 4 3 4 3 4 6% 38%	1570
March	1st to 10th	Day	26% 6% 2 6% 4 4 3 3 4 5% 36%	3165
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 5% 6% 8% 44%	3480
	21st to end	Day	22% 7% 2 6% 3 3 3 2 3 5% 44%	4024
April	1st to 10th	Day	14% 8% 2 6% 5% 4 4 3 6% 8% 39%	2280
	11th to 20th	Day	14% 8% 2 5% 4 4 4 3 5% 7% 42%	2254
	21st to end	Day	16% 6% 2 6% 4 4 5% 5% 6% 7% 39%	2280
May	1st to 10th	Day	27% 8% 3 6% 4 4 4 4 5% 7% 29%	4081
	11th to 20th	Day	24% 9% 3 8% 5% 4 4 3 5% 7% 28%	4398
	21st to end	Day	12% 6% 2 5% 4 4 4 4 6% 9% 44%	4837
June	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 5% 7% 9% 39%	2660
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 5% 7% 9% 37%	2660
	21st to end	Day	10% 5% 2 5% 5% 5% 6% 5% 7% 10% 42%	2660
July	1st to 10th	Day	7% 5% 2 6% 5% 6% 6% 6% 8% 12% 37%	4440
	11th to 20th	Day	9% 6% 2 6% 5% 4 5% 5% 8% 10% 40%	4440
	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	13% 7% 2 5% 3 3 4 3 4 7% 52%	1698
	11th to 20th	Day	16% 6% 2 5% 3 4 4 4 6% 5% 47%	1710
	21st to end	Day	15% 6% 2 4 4 4 4 5% 5% 7% 44%	1562
December	1st to 10th	Day	17% 9% 2 6% 5% 4 4 4 5% 6% 38%	1520
	11th to 20th	Day	19% 8% 3 5% 3 4 3 3 5% 7% 41%	1906
	21st to end	Day	16% 7% 3 7% 5% 5% 6% 4 6% 6% 35%	2772
January	1st to 10th	Night	15% 6% 2 6% 4 4 4 3 4 6% 44%	4429
	11th to 20th	Night	18% 9% 2 6% 3 4 4 3 4 6% 42%	4338
	21st to end	Night	23% 9% 2 7% 4 4 3 3 4 5% 36%	4532
February	1st to 10th	Night	16% 9% 3 5% 4 4 4 4 5% 6% 41%	2470
	11th to 20th	Night	17% 9% 2 7% 4 4 3 3 5% 6% 39%	2282
	21st to end	Night	18% 8% 2 5% 3 3 4 4 5% 6% 41%	1884
March	1st to 10th	Night	22% 9% 3 6% 4 4 4 2 4 6% 37%	3673
	11th to 20th	Night	11% 6% 2 5% 4 4 4 3 5% 6% 49%	3479
	21st to end	Night	16% 10% 3 7% 4 3 2 2 4 5% 46%	3608
April	1st to 10th	Night	16% 6% 3 6% 5% 4 4 4 6% 6% 40%	1900
	11th to 20th	Night	15% 7% 3 6% 4 4 4 3 5% 7% 41%	1875
	21st to end	Night	18% 6% 2 5% 3 3 3 3 6% 7% 43%	1900
May	1st to 10th	Night	24% 8% 3 7% 4 4 4 4 6% 7% 29%	2887
	11th to 20th	Night	24% 9% 3 7% 5% 4 4 4 5% 6% 28%	2495
	21st to end	Night	12% 6% 2 6% 4 4 4 3 6% 11% 42%	2738
June	1st to 10th	Night	14% 6% 3 6% 5% 4 4 3 5% 8% 43%	1520
	11th to 20th	Night	11% 6% 2 5% 4 6% 5% 4 7% 9% 40%	1520
	21st to end	Night	10% 4 2 5% 5% 4 5% 4 8% 9% 43%	1520
July	1st to 10th	Night	7% 5% 3 8% 5% 5% 7% 5% 8% 10% 38%	2520
	11th to 20th	Night	9% 7% 3 7% 4 5% 5% 5% 8% 9% 38%	2520
	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% 6% 2 5% 3 3 3 3 5% 6% 53%	2453
	11th to 20th	Night	17% 7% 2 5% 3 3 4 3 4 5% 47%	2470
	21st to end	Night	13% 5% 2 5% 5% 5% 4 4 5% 7% 46%	2616
December	1st to 10th	Night	19% 7% 3 6% 4 4 4 4 5% 6% 38%	2660
	11th to 20th	Night	17% 6% 2 6% 4 4 4 3 5% 7% 40%	3288
	21st to end	Night	18% 8% 3 7% 4 4 4 3 6% 7% 37%	4884

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	40% 3 13% 27% 13% 4	2518
	11th to 20th	Day	49% 5% 10% 20% 13% 3	2620
	21st to end	Day	50% 6% 10% 22% 10% 2	3124
February	1st to 10th	Day	45% 5% 14% 24% 12% 2	1710
	11th to 20th	Day	46% 4 9% 24% 15% 2	1791
	21st to end	Day	48% 4 9% 22% 13% 4	1570
March	1st to 10th	Day	49% 3 9% 26% 10% 2	3168
	11th to 20th	Day	36% 5% 10% 29% 17% 3	3480
	21st to end	Day	45% 4 8% 19% 21% 4	4024
April	1st to 10th	Day	42% 5% 10% 21% 16% 5%	2280
	11th to 20th	Day	41% 6% 11% 22% 16% 4	2254
	21st to end	Day	43% 6% 11% 18% 18% 4	2280
May	1st to 10th	Day	54% 7% 9% 18% 10% 2	4093
	11th to 20th	Day	55% 7% 11% 18% 8% 1	4402
	21st to end	Day	35% 7% 12% 27% 17% 2	4837
June	1st to 10th	Day	40% 7% 12% 24% 15% 1	2660
	11th to 20th	Day	41% 8% 14% 24% 11% 3	2660
	21st to end	Day	36% 7% 14% 25% 16% 1	2660
July	1st to 10th	Day	39% 9% 15% 21% 13% 3	4370
	11th to 20th	Day	38% 6% 16% 25% 13% 1	4365
	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% 26% 20% 4	1698
	11th to 20th	Day	37% 5% 10% 24% 21% 2	1710
	21st to end	Day	39% 4 12% 31% 12% 2	1562
December	1st to 10th	Day	46% 4 15% 20% 13% 1	1520
	11th to 20th	Day	43% 4 14% 27% 11% 1	1906
	21st to end	Day	46% 6% 14% 21% 12% 1	2772
January	1st to 10th	Night	42% 4 10% 28% 13% 3	4429
	11th to 20th	Night	44% 4 11% 25% 13% 3	4338
	21st to end	Night	50% 4 7% 26% 11% 2	4532
February	1st to 10th	Night	45% 4 12% 25% 12% 2	2470
	11th to 20th	Night	46% 3 9% 20% 18% 4	2284
	21st to end	Night	44% 5% 8% 23% 16% 5%	1884
March	1st to 10th	Night	49% 4 9% 24% 12% 2	3683
	11th to 20th	Night	35% 5% 12% 29% 16% 3	3480
	21st to end	Night	43% 4 7% 18% 23% 5%	3608
April	1st to 10th	Night	43% 5% 11% 20% 17% 4	1900
	11th to 20th	Night	42% 5% 10% 20% 20% 4	1875
	21st to end	Night	40% 6% 9% 21% 18% 5%	1900
May	1st to 10th	Night	53% 7% 9% 18% 11% 2	2894
	11th to 20th	Night	54% 9% 11% 14% 10% 2	2496
	21st to end	Night	36% 8% 15% 22% 16% 3	2739
June	1st to 10th	Night	40% 6% 11% 22% 20% 2	1520
	11th to 20th	Night	37% 9% 14% 25% 15% 1	1520
	21st to end	Night	35% 9% 13% 27% 15% 2	1520
July	1st to 10th	Night	40% 9% 18% 17% 13% 3	2480
	11th to 20th	Night	41% 7% 16% 23% 12% 1	2472
	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% 20% 6%	2453
	11th to 20th	Night	39% 5% 9% 22% 20% 5%	2470
	21st to end	Night	37% 4 13% 28% 16% 2	2616
December	1st to 10th	Night	46% 4 12% 22% 14% 1	2660
	11th to 20th	Night	42% 4 12% 21% 16% 4	3288
	21st to end	Night	45% 6% 14% 23% 11% 1	4884

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	32% 23% 29% 13% 1 2	180
	11th to 20th	Day	36% 26% 29% 8% 1	209
	21st to end	Day	30% 28% 26% 12% 3	297
February	1st to 10th	Day	30% 27% 31% 10% 2	270
	11th to 20th	Day	32% 27% 32% 9% 1	390
	21st to end	Day	34% 21% 29% 12% 3	382
March	1st to 10th	Day	32% 18% 29% 16% 4	460
	11th to 20th	Day	30% 26% 28% 12% 3 1	460
	21st to end	Day	30% 25% 29% 11% 4 1	506
April	1st to 10th	Day	28% 24% 30% 13% 4 1	460
	11th to 20th	Day	25% 23% 27% 16% 6% 2	452
	21st to end	Day	29% 18% 34% 14% 3 2	460
May	1st to 10th	Day	31% 20% 27% 15% 5% 2	463
	11th to 20th	Day	25% 14% 32% 20% 7% 2	487
	21st to end	Day	18% 10% 28% 24% 17% 2	529
June	1st to 10th	Day	18% 13% 30% 26% 11% 2	490
	11th to 20th	Day	17% 10% 31% 26% 14% 2	490
	21st to end	Day	13% 10% 34% 26% 13% 4	490
July	1st to 10th	Day	13% 4 24% 33% 20% 6%	485
	11th to 20th	Day	9% 5% 25% 36% 21% 4	484
	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	22% 13% 31% 21% 10% 3	374
	11th to 20th	Day	23% 20% 32% 19% 6% 1	370
	21st to end	Day	23% 23% 32% 18% 5%	222
December	1st to 10th	Day	32% 26% 28% 9% 6%	180
	11th to 20th	Day	34% 23% 28% 12% 3	180
	21st to end	Day	29% 23% 27% 13% 8% 1	198
January	1st to 10th	Night	21% 23% 36% 17% 2	760
	11th to 20th	Night	27% 18% 31% 22% 3	760
	21st to end	Night	19% 22% 38% 18% 2	836
February	1st to 10th	Night	22% 20% 37% 20% 1	760
	11th to 20th	Night	28% 25% 34% 13% 1	743
	21st to end	Night	26% 20% 36% 15% 2	628
March	1st to 10th	Night	23% 23% 31% 20% 3	760
	11th to 20th	Night	24% 23% 31% 17% 5%	760
	21st to end	Night	21% 17% 40% 17% 4	836
April	1st to 10th	Night	21% 23% 35% 17% 4	760
	11th to 20th	Night	20% 14% 38% 23% 6%	751
	21st to end	Night	22% 16% 32% 24% 7%	760
May	1st to 10th	Night	24% 12% 34% 24% 7%	611
	11th to 20th	Night	20% 8% 36% 29% 7%	374
	21st to end	Night	15% 7% 26% 38% 12% 1	411
June	1st to 10th	Night	14% 6% 30% 38% 11%	380
	11th to 20th	Night	10% 8% 28% 43% 11%	380
	21st to end	Night	10% 4 27% 46% 13% 1	380
July	1st to 10th	Night	11% 5% 22% 42% 18% 1	370
	11th to 20th	Night	7% 2 22% 41% 26% 1	368
	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	755
	11th to 20th	Night	23% 17% 26% 26% 8%	760
	21st to end	Night	18% 17% 35% 22% 7% 1	760
December	1st to 10th	Night	26% 19% 30% 19% 6% 1	760
	11th to 20th	Night	21% 18% 34% 22% 5%	760
	21st to end	Night	22% 19% 33% 24% 2	836

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