Darke Sky Ranch
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Darke Sky Ranch clear sky chart from 2005-11-06 to 2026-05-17.

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	14% 9% 4 8% 5% 5% 5% 4 5% 7% 34%	131817
	Night	14% 10% 4 8% 5% 4 4 3 5% 7% 35%	131093
Transparency	Day	48% 4 11% 22% 13% 1	135442
	Night	48% 4 10% 22% 14% 2	135538
Seeing	Day	17% 10% 26% 23% 16% 7%	17477
	Night	18% 9% 27% 29% 15% 2	27311

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	17% 14% 7% 14% 6% 5% 4 3 4 5% 21%	4691
	11th to 20th	Day	11% 14% 6% 14% 9% 7% 7% 3 4 3 21%	4855
	21st to end	Day	18% 13% 7% 13% 6% 6% 5% 3 4 5% 19%	5780
February	1st to 10th	Day	18% 15% 6% 11% 6% 4 5% 3 4 5% 24%	1974
	11th to 20th	Day	15% 12% 5% 13% 6% 6% 5% 3 5% 6% 25%	2048
	21st to end	Day	16% 13% 4 11% 6% 5% 4 2 3 6% 31%	1730
March	1st to 10th	Day	18% 14% 7% 10% 5% 4 4 3 4 5% 27%	4614
	11th to 20th	Day	11% 10% 3 8% 5% 4 4 3 5% 7% 40%	5090
	21st to end	Day	17% 11% 4 9% 5% 5% 4 3 5% 6% 30%	5586
April	1st to 10th	Day	15% 11% 3 9% 6% 5% 5% 4 5% 7% 31%	2488
	11th to 20th	Day	15% 9% 3 8% 4 4 5% 4 5% 6% 37%	2566
	21st to end	Day	18% 8% 3 7% 5% 4 5% 3 5% 8% 34%	2730
May	1st to 10th	Day	21% 9% 3 8% 5% 5% 5% 3 5% 6% 30%	5992
	11th to 20th	Day	19% 8% 3 7% 4 4 4 3 6% 8% 35%	5939
	21st to end	Day	14% 7% 2 8% 6% 5% 5% 5% 8% 9% 33%	6433
June	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 5% 5% 8% 40%	2686
	11th to 20th	Day	10% 5% 3 6% 4 4 5% 5% 6% 10% 41%	2800
	21st to end	Day	9% 5% 2 6% 6% 5% 5% 5% 8% 10% 40%	2800
July	1st to 10th	Day	9% 6% 2 7% 5% 5% 6% 4 7% 10% 39%	6360
	11th to 20th	Day	9% 6% 2 7% 5% 5% 6% 5% 7% 11% 37%	6300
	21st to end	Day	9% 7% 3 7% 6% 6% 6% 5% 7% 9% 35%	6424
August	1st to 10th	Day	8% 5% 2 6% 4 5% 5% 5% 7% 10% 44%	4234
	11th to 20th	Day	8% 5% 2 6% 4 5% 5% 4 8% 10% 43%	4174
	21st to end	Day	6% 4 2 6% 4 5% 5% 4 7% 10% 47%	4384
September	1st to 10th	Day	11% 6% 1 5% 4 4 4 4 5% 6% 50%	2285
	11th to 20th	Day	9% 5% 2 6% 4 4 4 3 6% 8% 50%	2200
	21st to end	Day	15% 6% 2 6% 5% 3 3 2 4 6% 46%	2200
October	1st to 10th	Day	12% 5% 2 6% 4 4 5% 4 5% 5% 48%	2200
	11th to 20th	Day	13% 7% 2 5% 4 4 4 3 5% 7% 46%	2021
	21st to end	Day	18% 10% 3 8% 4 3 4 3 5% 8% 33%	2026
November	1st to 10th	Day	10% 8% 3 6% 3 4 3 3 5% 7% 48%	1852
	11th to 20th	Day	15% 12% 4 9% 5% 4 3 3 4 6% 34%	1890
	21st to end	Day	18% 12% 4 9% 5% 4 4 3 5% 5% 30%	1890
December	1st to 10th	Day	16% 12% 4 8% 4 4 3 2 4 5% 36%	3020
	11th to 20th	Day	17% 15% 7% 12% 5% 5% 4 4 4 5% 23%	3463
	21st to end	Day	23% 20% 7% 10% 6% 5% 4 3 4 5% 14%	4092
January	1st to 10th	Night	13% 14% 6% 15% 8% 5% 6% 4 5% 5% 20%	8232
	11th to 20th	Night	10% 16% 9% 15% 7% 6% 5% 3 4 4 20%	8121
	21st to end	Night	18% 13% 5% 8% 6% 5% 4 3 5% 6% 28%	8401
February	1st to 10th	Night	19% 13% 5% 10% 6% 5% 4 3 4 6% 27%	2644
	11th to 20th	Night	15% 11% 4 10% 6% 5% 4 4 4 6% 31%	2458
	21st to end	Night	18% 12% 6% 10% 4 4 4 3 4 6% 29%	2076
March	1st to 10th	Night	19% 12% 4 7% 4 3 3 3 4 5% 36%	5458
	11th to 20th	Night	11% 7% 3 7% 5% 5% 5% 3 5% 7% 42%	5090
	21st to end	Night	17% 10% 4 8% 4 4 4 3 5% 7% 35%	5588
April	1st to 10th	Night	15% 9% 3 7% 4 4 5% 5% 6% 7% 35%	2147
	11th to 20th	Night	15% 8% 3 6% 5% 5% 4 4 6% 8% 38%	2001
	21st to end	Night	17% 7% 2 7% 4 5% 4 3 6% 8% 37%	1890
May	1st to 10th	Night	20% 11% 3 8% 5% 4 5% 4 5% 7% 28%	4123
	11th to 20th	Night	16% 7% 2 6% 4 4 5% 4 6% 9% 38%	4086
	21st to end	Night	12% 7% 3 7% 5% 4 4 3 6% 9% 40%	4421
June	1st to 10th	Night	10% 4 2 5% 4 4 5% 3 6% 8% 48%	1709
	11th to 20th	Night	9% 5% 2 5% 5% 4 4 4 6% 10% 46%	1600
	21st to end	Night	6% 5% 2 5% 4 4 5% 6% 7% 9% 48%	1600
July	1st to 10th	Night	9% 5% 2 6% 5% 5% 5% 3 6% 8% 46%	3618
	11th to 20th	Night	7% 5% 2 6% 4 4 4 4 7% 9% 46%	3660
	21st to end	Night	9% 6% 3 7% 4 4 5% 4 7% 10% 41%	4427
August	1st to 10th	Night	10% 6% 3 7% 4 4 5% 4 6% 9% 42%	2915
	11th to 20th	Night	6% 5% 3 7% 4 4 5% 4 7% 8% 48%	2875
	21st to end	Night	4 3 2 6% 4 5% 5% 4 7% 9% 51%	3510
September	1st to 10th	Night	12% 4 2 4 3 4 3 3 5% 8% 52%	2095
	11th to 20th	Night	8% 5% 2 4 3 3 4 4 5% 8% 56%	2200
	21st to end	Night	13% 6% 2 5% 5% 4 4 3 5% 6% 48%	2200
October	1st to 10th	Night	9% 5% 2 6% 4 3 4 3 5% 6% 53%	2200
	11th to 20th	Night	15% 7% 2 7% 5% 4 4 3 4 6% 44%	2377
	21st to end	Night	18% 8% 2 6% 4 4 4 3 4 6% 40%	2810
November	1st to 10th	Night	12% 8% 2 5% 4 3 4 3 5% 7% 47%	2666
	11th to 20th	Night	15% 9% 3 7% 5% 3 4 3 5% 7% 37%	2730
	21st to end	Night	18% 12% 5% 8% 6% 5% 5% 3 5% 6% 28%	2730
December	1st to 10th	Night	16% 11% 3 9% 5% 4 4 3 5% 6% 34%	4380
	11th to 20th	Night	18% 16% 6% 11% 4 4 4 3 5% 5% 23%	4952
	21st to end	Night	24% 18% 7% 11% 5% 4 4 3 4 4 15%	7103

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	67% 3 9% 16% 5%	4692
	11th to 20th	Day	69% 2 7% 10% 11% 1	4855
	21st to end	Day	69% 2 9% 12% 6% 1	5780
February	1st to 10th	Day	64% 3 10% 14% 8% 1	1974
	11th to 20th	Day	62% 4 9% 17% 8%	2058
	21st to end	Day	57% 4 11% 19% 9% 1	1730
March	1st to 10th	Day	61% 3 6% 19% 10% 1	4614
	11th to 20th	Day	43% 4 9% 30% 13% 1	5090
	21st to end	Day	54% 5% 11% 18% 9% 2	5586
April	1st to 10th	Day	52% 5% 11% 19% 12% 1	2488
	11th to 20th	Day	47% 5% 12% 21% 13% 3	2566
	21st to end	Day	48% 5% 11% 23% 11% 2	2730
May	1st to 10th	Day	55% 5% 11% 18% 12%	6027
	11th to 20th	Day	48% 4 12% 25% 11% 1	5943
	21st to end	Day	44% 6% 12% 25% 12% 1	6447
June	1st to 10th	Day	41% 6% 11% 23% 16% 2	2686
	11th to 20th	Day	37% 7% 13% 24% 17% 1	2800
	21st to end	Day	38% 8% 15% 22% 15% 3	2800
July	1st to 10th	Day	41% 4 9% 28% 17% 2	6291
	11th to 20th	Day	41% 3 16% 24% 15% 1	6225
	21st to end	Day	45% 4 13% 20% 15% 3	6240
August	1st to 10th	Day	34% 5% 16% 30% 14% 1	4221
	11th to 20th	Day	35% 5% 12% 32% 16%	4213
	21st to end	Day	32% 5% 13% 31% 16% 3	4383
September	1st to 10th	Day	34% 5% 11% 24% 22% 4	2285
	11th to 20th	Day	32% 5% 10% 26% 24% 3	2200
	21st to end	Day	41% 3 9% 23% 22% 2	2200
October	1st to 10th	Day	26% 3 10% 32% 28% 2	3590
	11th to 20th	Day	32% 3 10% 29% 26% 2	3274
	21st to end	Day	41% 2 9% 24% 21% 2	3250
November	1st to 10th	Day	36% 4 11% 28% 18% 3	1852
	11th to 20th	Day	52% 4 10% 21% 12% 3	1890
	21st to end	Day	55% 4 9% 21% 9% 1	1890
December	1st to 10th	Day	53% 2 12% 24% 9%	3020
	11th to 20th	Day	65% 4 10% 16% 6%	3460
	21st to end	Day	74% 4 8% 10% 4	4092
January	1st to 10th	Night	67% 3 9% 14% 6%	8233
	11th to 20th	Night	68% 3 6% 11% 10% 2	8121
	21st to end	Night	58% 2 11% 19% 8% 1	8401
February	1st to 10th	Night	60% 4 10% 17% 8% 1	2644
	11th to 20th	Night	55% 3 10% 19% 12% 1	2459
	21st to end	Night	58% 4 11% 16% 10% 2	2076
March	1st to 10th	Night	51% 3 6% 26% 11% 2	5458
	11th to 20th	Night	41% 4 12% 29% 14% 1	5090
	21st to end	Night	51% 3 8% 26% 11% 1	5588
April	1st to 10th	Night	46% 6% 11% 22% 12% 1	2147
	11th to 20th	Night	44% 6% 12% 18% 17% 3	2001
	21st to end	Night	45% 5% 12% 23% 12% 2	1890
May	1st to 10th	Night	55% 6% 9% 18% 9% 3	4150
	11th to 20th	Night	43% 7% 12% 25% 13% 1	4087
	21st to end	Night	40% 6% 13% 26% 12% 3	4429
June	1st to 10th	Night	34% 6% 10% 25% 23% 2	1709
	11th to 20th	Night	33% 7% 14% 25% 18% 4	1600
	21st to end	Night	30% 8% 16% 24% 18% 4	1600
July	1st to 10th	Night	38% 4 11% 25% 18% 4	3580
	11th to 20th	Night	35% 4 15% 29% 16% 1	3615
	21st to end	Night	38% 4 15% 22% 19% 3	4297
August	1st to 10th	Night	39% 6% 13% 28% 13% 1	2904
	11th to 20th	Night	34% 5% 11% 31% 18% 1	2902
	21st to end	Night	29% 6% 14% 28% 21% 3	3510
September	1st to 10th	Night	32% 5% 12% 26% 20% 6%	2095
	11th to 20th	Night	27% 5% 10% 27% 28% 3	2200
	21st to end	Night	37% 5% 9% 22% 24% 2	2200
October	1st to 10th	Night	23% 2 6% 32% 34% 2	3590
	11th to 20th	Night	31% 3 9% 27% 28% 2	3863
	21st to end	Night	35% 3 7% 28% 24% 4	4536
November	1st to 10th	Night	38% 4 10% 24% 21% 2	2666
	11th to 20th	Night	45% 5% 9% 23% 15% 3	2730
	21st to end	Night	57% 5% 9% 19% 9% 1	2730
December	1st to 10th	Night	53% 3 11% 23% 9% 1	4380
	11th to 20th	Night	63% 4 10% 16% 7% 1	4954
	21st to end	Night	73% 4 8% 12% 3	7103

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	23% 18% 33% 20% 6%	330
	11th to 20th	Day	24% 21% 35% 17% 3	330
	21st to end	Day	29% 18% 31% 18% 4 1	363
February	1st to 10th	Day	27% 15% 37% 14% 7%	330
	11th to 20th	Day	26% 19% 33% 15% 6%	330
	21st to end	Day	22% 15% 47% 15% 2	273
March	1st to 10th	Day	27% 18% 34% 17% 4	338
	11th to 20th	Day	25% 15% 34% 18% 6% 1	540
	21st to end	Day	28% 14% 33% 18% 6%	594
April	1st to 10th	Day	21% 18% 33% 19% 8% 2	540
	11th to 20th	Day	19% 17% 29% 22% 9% 4	532
	21st to end	Day	22% 11% 33% 22% 10% 1	540
May	1st to 10th	Day	24% 12% 31% 21% 8% 3	543
	11th to 20th	Day	21% 7% 28% 23% 13% 8%	551
	21st to end	Day	12% 6% 28% 26% 15% 12%	573
June	1st to 10th	Day	13% 6% 25% 30% 18% 8%	616
	11th to 20th	Day	12% 6% 23% 30% 22% 8%	730
	21st to end	Day	10% 6% 25% 30% 19% 10%	730
July	1st to 10th	Day	12% 3 16% 25% 26% 18%	721
	11th to 20th	Day	9% 2 20% 28% 27% 14%	697
	21st to end	Day	11% 3 13% 28% 27% 18%	576
August	1st to 10th	Day	10% 4 11% 22% 33% 19%	526
	11th to 20th	Day	11% 3 17% 27% 30% 13%	519
	21st to end	Day	9% 4 12% 26% 33% 17%	583
September	1st to 10th	Day	11% 4 14% 26% 28% 17%	530
	11th to 20th	Day	9% 2 12% 25% 33% 20%	530
	21st to end	Day	18% 9% 19% 22% 21% 10%	530
October	1st to 10th	Day	10% 6% 22% 30% 20% 12%	530
	11th to 20th	Day	16% 13% 26% 27% 14% 5%	530
	21st to end	Day	24% 13% 36% 20% 7% 1	409
November	1st to 10th	Day	14% 15% 32% 25% 9% 4	330
	11th to 20th	Day	23% 18% 28% 18% 10% 2	330
	21st to end	Day	27% 12% 39% 19% 3	330
December	1st to 10th	Day	18% 18% 38% 18% 6% 1	330
	11th to 20th	Day	24% 15% 40% 16% 5%	330
	21st to end	Day	31% 18% 30% 17% 4	363
January	1st to 10th	Night	21% 17% 35% 22% 5%	1050
	11th to 20th	Night	26% 15% 36% 19% 4	995
	21st to end	Night	23% 12% 38% 23% 3	924
February	1st to 10th	Night	24% 11% 35% 27% 3	840
	11th to 20th	Night	22% 14% 36% 25% 2	823
	21st to end	Night	25% 12% 36% 23% 3	692
March	1st to 10th	Night	23% 12% 32% 28% 5%	840
	11th to 20th	Night	18% 16% 33% 27% 6%	840
	21st to end	Night	25% 11% 31% 26% 7%	924
April	1st to 10th	Night	21% 12% 33% 27% 6% 1	677
	11th to 20th	Night	18% 12% 32% 27% 10% 1	623
	21st to end	Night	19% 6% 40% 24% 11% 1	630
May	1st to 10th	Night	22% 8% 31% 29% 9% 1	630
	11th to 20th	Night	19% 6% 24% 33% 16% 1	609
	21st to end	Night	9% 3 24% 37% 24% 3	650
June	1st to 10th	Night	12% 1 23% 41% 23% 1	600
	11th to 20th	Night	11% 3 18% 42% 25% 1	600
	21st to end	Night	8% 2 23% 38% 28% 2	600
July	1st to 10th	Night	10% 2 13% 35% 34% 6%	585
	11th to 20th	Night	8% 2 13% 40% 31% 6%	582
	21st to end	Night	11% 3 9% 40% 35% 2	657
August	1st to 10th	Night	7% 2 11% 40% 36% 4	597
	11th to 20th	Night	7% 1 11% 42% 34% 5%	594
	21st to end	Night	6% 2 12% 36% 39% 6%	660
September	1st to 10th	Night	12% 5% 14% 32% 33% 4	695
	11th to 20th	Night	9% 2 13% 34% 38% 5%	800
	21st to end	Night	16% 4 20% 33% 24% 3	800
October	1st to 10th	Night	11% 4 25% 32% 23% 5%	800
	11th to 20th	Night	18% 9% 28% 31% 13% 1	800
	21st to end	Night	22% 10% 32% 27% 9%	880
November	1st to 10th	Night	16% 9% 33% 27% 14% 1	820
	11th to 20th	Night	20% 15% 32% 27% 7% 1	840
	21st to end	Night	23% 13% 29% 28% 7%	840
December	1st to 10th	Night	25% 12% 34% 24% 5% 1	840
	11th to 20th	Night	25% 10% 35% 25% 5% 1	840
	21st to end	Night	30% 13% 31% 22% 4	1134

Page generated at 2026-05-14 01:59:01. Version ac07

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