SIX A RANCH OBSERVATORY
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the SIX A RANCH OBSERVATORY clear sky chart from 2005-11-06 to 2026-04-17.

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	13% 9% 3 7% 5% 4 5% 4 6% 7% 37%	131175
	Night	14% 9% 3 7% 4 4 4 3 5% 7% 38%	131152
Transparency	Day	45% 3 10% 24% 16% 2	134803
	Night	46% 3 9% 23% 16% 2	135598
Seeing	Day	19% 10% 25% 24% 16% 7%	17335
	Night	19% 8% 26% 30% 16% 2	27223

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	15% 13% 6% 10% 4 4 4 3 5% 6% 30%	4691
	11th to 20th	Day	11% 11% 4 12% 6% 4 4 3 4 5% 35%	4710
	21st to end	Day	18% 13% 5% 9% 6% 5% 4 3 5% 5% 27%	5743
February	1st to 10th	Day	17% 12% 6% 8% 4 4 3 3 5% 6% 33%	2100
	11th to 20th	Day	16% 10% 4 9% 5% 4 4 3 6% 6% 35%	2048
	21st to end	Day	15% 12% 4 10% 4 4 3 3 6% 6% 32%	1730
March	1st to 10th	Day	18% 13% 4 8% 4 3 3 2 4 4 38%	4580
	11th to 20th	Day	12% 10% 3 6% 4 4 4 3 5% 7% 42%	4935
	21st to end	Day	15% 9% 3 7% 5% 5% 5% 4 6% 7% 33%	5812
April	1st to 10th	Day	15% 9% 3 6% 5% 4 5% 4 5% 7% 38%	2520
	11th to 20th	Day	16% 10% 3 7% 4 5% 4 4 5% 7% 37%	2446
	21st to end	Day	17% 7% 2 7% 5% 4 4 4 5% 6% 38%	2545
May	1st to 10th	Day	20% 9% 3 6% 6% 5% 5% 5% 7% 8% 27%	5862
	11th to 20th	Day	19% 8% 3 7% 5% 4 5% 4 5% 7% 35%	5861
	21st to end	Day	13% 8% 2 8% 6% 6% 6% 5% 8% 9% 30%	6433
June	1st to 10th	Day	11% 7% 2 6% 5% 5% 4 4 6% 7% 43%	2764
	11th to 20th	Day	7% 6% 2 7% 5% 5% 5% 5% 7% 8% 44%	2800
	21st to end	Day	9% 5% 2 6% 5% 5% 5% 5% 7% 9% 43%	2800
July	1st to 10th	Day	11% 5% 2 6% 4 4 5% 4 6% 10% 41%	6360
	11th to 20th	Day	8% 5% 2 7% 5% 6% 6% 5% 8% 10% 38%	6360
	21st to end	Day	10% 7% 3 9% 6% 7% 6% 6% 8% 9% 30%	6560
August	1st to 10th	Day	7% 5% 2 5% 4 3 4 4 6% 11% 48%	4234
	11th to 20th	Day	8% 5% 2 7% 5% 5% 6% 5% 8% 10% 40%	3919
	21st to end	Day	4 4 2 5% 4 3 4 3 6% 9% 55%	4312
September	1st to 10th	Day	11% 5% 2 7% 4 4 5% 4 5% 7% 47%	2357
	11th to 20th	Day	9% 5% 1 6% 4 4 4 3 7% 8% 49%	2200
	21st to end	Day	12% 6% 2 6% 4 4 3 3 4 6% 49%	2200
October	1st to 10th	Day	10% 5% 2 5% 3 3 4 4 5% 6% 53%	2200
	11th to 20th	Day	11% 6% 2 7% 4 3 4 3 6% 6% 48%	2085
	21st to end	Day	19% 9% 2 5% 4 4 4 3 5% 6% 40%	1980
November	1st to 10th	Day	11% 5% 2 4 4 4 4 3 5% 5% 52%	1852
	11th to 20th	Day	16% 11% 3 5% 4 3 4 3 5% 7% 39%	1890
	21st to end	Day	19% 10% 3 6% 3 4 3 2 5% 5% 38%	1890
December	1st to 10th	Day	16% 12% 3 6% 3 3 3 3 4 5% 44%	3020
	11th to 20th	Day	15% 14% 5% 10% 6% 5% 4 3 5% 6% 28%	3306
	21st to end	Day	22% 18% 6% 8% 5% 4 4 3 4 5% 20%	4070
January	1st to 10th	Night	14% 15% 5% 11% 5% 4 4 3 4 5% 29%	8232
	11th to 20th	Night	12% 12% 6% 13% 7% 5% 4 3 4 5% 29%	8280
	21st to end	Night	17% 12% 4 7% 5% 4 4 3 4 5% 34%	8428
February	1st to 10th	Night	20% 11% 5% 8% 5% 5% 4 4 5% 5% 29%	2520
	11th to 20th	Night	17% 10% 4 9% 5% 4 4 3 4 6% 34%	2458
	21st to end	Night	19% 11% 4 10% 4 3 3 3 4 6% 33%	2076
March	1st to 10th	Night	20% 14% 3 6% 4 3 3 3 4 5% 36%	5493
	11th to 20th	Night	12% 8% 2 6% 4 3 4 3 5% 6% 47%	5243
	21st to end	Night	16% 8% 3 8% 6% 5% 5% 4 5% 5% 34%	5403
April	1st to 10th	Night	13% 8% 3 6% 4 4 4 3 6% 7% 40%	2100
	11th to 20th	Night	14% 8% 3 5% 4 4 4 3 5% 6% 42%	2035
	21st to end	Night	16% 7% 3 6% 4 4 3 4 6% 9% 37%	1855
May	1st to 10th	Night	19% 10% 3 8% 4 5% 4 4 6% 8% 29%	4033
	11th to 20th	Night	16% 7% 3 7% 5% 5% 5% 4 7% 8% 34%	4032
	21st to end	Night	12% 8% 3 7% 5% 5% 6% 4 6% 9% 36%	4421
June	1st to 10th	Night	10% 6% 2 5% 5% 3 5% 3 6% 10% 46%	1631
	11th to 20th	Night	7% 5% 2 6% 4 4 5% 4 6% 9% 49%	1600
	21st to end	Night	5% 3 1 4 4 3 5% 4 9% 11% 51%	1600
July	1st to 10th	Night	8% 5% 3 7% 4 3 5% 4 5% 9% 46%	3618
	11th to 20th	Night	7% 5% 3 6% 5% 5% 5% 4 6% 10% 45%	3617
	21st to end	Night	8% 5% 2 6% 5% 5% 6% 5% 8% 12% 36%	4293
August	1st to 10th	Night	6% 4 2 6% 3 4 4 3 6% 9% 52%	2915
	11th to 20th	Night	7% 4 1 6% 4 4 5% 4 8% 10% 47%	3127
	21st to end	Night	4 3 1 3 3 3 4 3 5% 10% 62%	3586
September	1st to 10th	Night	11% 5% 2 5% 4 3 4 3 5% 6% 52%	2021
	11th to 20th	Night	7% 5% 2 5% 3 3 4 4 6% 9% 53%	2200
	21st to end	Night	13% 5% 2 6% 4 3 3 3 4 6% 51%	2200
October	1st to 10th	Night	7% 5% 2 5% 3 4 3 2 4 7% 59%	2200
	11th to 20th	Night	14% 6% 2 5% 4 4 4 2 4 5% 49%	2310
	21st to end	Night	17% 8% 3 6% 4 3 3 3 4 5% 44%	2860
November	1st to 10th	Night	13% 6% 2 5% 4 3 3 3 4 6% 49%	2666
	11th to 20th	Night	15% 8% 3 6% 4 3 3 4 5% 6% 43%	2730
	21st to end	Night	17% 11% 4 7% 4 4 3 3 4 6% 37%	2730
December	1st to 10th	Night	17% 12% 4 7% 4 3 3 3 4 5% 39%	4380
	11th to 20th	Night	18% 12% 5% 8% 4 4 4 3 5% 5% 32%	5109
	21st to end	Night	24% 17% 6% 10% 5% 5% 4 3 4 4 19%	7150

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	58% 1 10% 22% 9% 1	4692
	11th to 20th	Day	53% 2 7% 21% 16% 2	4710
	21st to end	Day	62% 2 7% 19% 5% 5%	5743
February	1st to 10th	Day	54% 2 8% 23% 12% 2	2100
	11th to 20th	Day	51% 2 9% 23% 13% 2	2058
	21st to end	Day	54% 2 8% 22% 11% 3	1730
March	1st to 10th	Day	53% 1 4 18% 17% 6%	4580
	11th to 20th	Day	44% 2 8% 28% 16% 2	4935
	21st to end	Day	50% 2 12% 22% 13% 1	5814
April	1st to 10th	Day	46% 3 12% 23% 16% 1	2520
	11th to 20th	Day	47% 3 9% 24% 15% 3	2446
	21st to end	Day	47% 3 9% 25% 12% 3	2545
May	1st to 10th	Day	54% 3 12% 20% 11%	5897
	11th to 20th	Day	51% 3 10% 23% 12%	5865
	21st to end	Day	48% 3 13% 18% 16% 1	6447
June	1st to 10th	Day	39% 3 10% 26% 18% 3	2764
	11th to 20th	Day	37% 5% 13% 24% 20% 2	2800
	21st to end	Day	36% 5% 14% 22% 18% 4	2800
July	1st to 10th	Day	39% 4 10% 24% 21% 1	6291
	11th to 20th	Day	39% 5% 13% 28% 13% 1	6285
	21st to end	Day	49% 5% 14% 21% 11% 1	6374
August	1st to 10th	Day	31% 4 13% 36% 15% 1	4221
	11th to 20th	Day	37% 4 11% 30% 17% 1	3957
	21st to end	Day	26% 4 12% 37% 18% 3	4311
September	1st to 10th	Day	37% 3 9% 25% 21% 5%	2357
	11th to 20th	Day	33% 3 9% 31% 21% 3	2200
	21st to end	Day	37% 3 8% 25% 25% 3	2200
October	1st to 10th	Day	22% 3 5% 28% 40% 2	3590
	11th to 20th	Day	29% 2 7% 29% 30% 3	3366
	21st to end	Day	36% 2 8% 26% 25% 3	3180
November	1st to 10th	Day	34% 3 9% 31% 20% 4	1852
	11th to 20th	Day	46% 2 7% 25% 17% 4	1890
	21st to end	Day	49% 2 7% 24% 16% 1	1890
December	1st to 10th	Day	45% 2 9% 28% 16%	3020
	11th to 20th	Day	59% 2 8% 21% 10% 1	3303
	21st to end	Day	68% 1 10% 13% 7% 1	4070
January	1st to 10th	Night	60% 2 9% 20% 9%	8233
	11th to 20th	Night	59% 2 6% 15% 15% 3	8280
	21st to end	Night	55% 2 9% 21% 12% 1	8428
February	1st to 10th	Night	58% 2 9% 21% 9% 1	2520
	11th to 20th	Night	53% 2 8% 23% 12% 2	2459
	21st to end	Night	54% 2 10% 20% 12% 3	2076
March	1st to 10th	Night	53% 2 5% 23% 14% 4	5493
	11th to 20th	Night	40% 2 9% 26% 22% 2	5243
	21st to end	Night	52% 2 8% 26% 11% 1	5403
April	1st to 10th	Night	43% 3 10% 27% 15% 3	2100
	11th to 20th	Night	43% 2 10% 21% 19% 4	2035
	21st to end	Night	45% 3 9% 26% 14% 3	1855
May	1st to 10th	Night	55% 3 11% 21% 8% 1	4060
	11th to 20th	Night	48% 4 12% 26% 9%	4033
	21st to end	Night	45% 3 15% 22% 13% 3	4429
June	1st to 10th	Night	34% 4 10% 27% 21% 4	1631
	11th to 20th	Night	32% 4 14% 24% 21% 5%	1600
	21st to end	Night	25% 6% 18% 24% 21% 6%	1600
July	1st to 10th	Night	35% 5% 11% 26% 21% 3	3580
	11th to 20th	Night	35% 6% 14% 29% 15% 2	3572
	21st to end	Night	39% 5% 17% 26% 12% 1	4167
August	1st to 10th	Night	30% 4 13% 38% 13% 2	2904
	11th to 20th	Night	32% 4 13% 32% 17% 1	3155
	21st to end	Night	19% 3 15% 36% 23% 3	3586
September	1st to 10th	Night	33% 3 9% 27% 21% 6%	2021
	11th to 20th	Night	29% 3 7% 29% 27% 5%	2200
	21st to end	Night	36% 2 8% 24% 26% 4	2200
October	1st to 10th	Night	20% 2 5% 20% 49% 4	3590
	11th to 20th	Night	26% 2 6% 25% 37% 4	3768
	21st to end	Night	34% 1 7% 23% 29% 6%	4610
November	1st to 10th	Night	37% 2 8% 25% 24% 4	2666
	11th to 20th	Night	42% 3 7% 23% 21% 4	2730
	21st to end	Night	50% 2 8% 23% 15% 1	2730
December	1st to 10th	Night	49% 2 9% 25% 13%	4380
	11th to 20th	Night	55% 2 10% 20% 12% 1	5111
	21st to end	Night	71% 2 7% 14% 6% 1	7150

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	25% 17% 35% 15% 6% 1	330
	11th to 20th	Day	31% 22% 32% 13% 2	330
	21st to end	Day	27% 21% 36% 13% 2	363
February	1st to 10th	Day	27% 21% 35% 14% 3	330
	11th to 20th	Day	29% 24% 33% 10% 3	330
	21st to end	Day	30% 18% 37% 12% 3	273
March	1st to 10th	Day	29% 17% 39% 12% 3	330
	11th to 20th	Day	26% 18% 35% 16% 5% 1	463
	21st to end	Day	29% 16% 34% 16% 5% 1	594
April	1st to 10th	Day	22% 16% 30% 22% 8% 1	540
	11th to 20th	Day	23% 16% 28% 19% 10% 3	516
	21st to end	Day	22% 13% 34% 20% 8% 3	500
May	1st to 10th	Day	24% 9% 30% 23% 10% 3	503
	11th to 20th	Day	22% 8% 22% 29% 14% 5%	527
	21st to end	Day	14% 5% 25% 28% 17% 11%	573
June	1st to 10th	Day	14% 4 23% 31% 19% 9%	694
	11th to 20th	Day	10% 5% 20% 34% 23% 8%	730
	21st to end	Day	11% 4 24% 30% 22% 9%	730
July	1st to 10th	Day	15% 2 13% 26% 26% 18%	721
	11th to 20th	Day	10% 3 19% 29% 28% 12%	718
	21st to end	Day	12% 2 14% 32% 26% 13%	634
August	1st to 10th	Day	10% 2 14% 29% 26% 18%	526
	11th to 20th	Day	15% 3 14% 30% 26% 12%	519
	21st to end	Day	8% 1 13% 29% 32% 17%	583
September	1st to 10th	Day	11% 3 14% 33% 29% 11%	530
	11th to 20th	Day	11% 1 11% 28% 33% 16%	530
	21st to end	Day	17% 7% 18% 24% 23% 11%	530
October	1st to 10th	Day	10% 4 22% 33% 24% 6%	530
	11th to 20th	Day	15% 12% 30% 28% 11% 5%	482
	21st to end	Day	31% 9% 35% 22% 4	363
November	1st to 10th	Day	15% 15% 34% 24% 10% 3	330
	11th to 20th	Day	25% 17% 31% 19% 7% 2	330
	21st to end	Day	29% 10% 34% 21% 6%	330
December	1st to 10th	Day	24% 17% 36% 15% 6% 1	330
	11th to 20th	Day	26% 16% 37% 15% 5% 1	330
	21st to end	Day	33% 17% 36% 11% 2 1	363
January	1st to 10th	Night	22% 15% 33% 26% 4	1050
	11th to 20th	Night	28% 14% 35% 20% 4	1050
	21st to end	Night	22% 14% 37% 25% 3	985
February	1st to 10th	Night	27% 11% 35% 24% 3	840
	11th to 20th	Night	24% 13% 37% 24% 2	823
	21st to end	Night	27% 11% 40% 21% 2	692
March	1st to 10th	Night	25% 12% 33% 26% 4	840
	11th to 20th	Night	23% 10% 37% 23% 6%	840
	21st to end	Night	24% 9% 31% 29% 7%	842
April	1st to 10th	Night	19% 10% 34% 30% 7%	630
	11th to 20th	Night	19% 9% 30% 30% 12%	611
	21st to end	Night	19% 6% 34% 31% 9% 2	600
May	1st to 10th	Night	19% 6% 29% 32% 13% 1	600
	11th to 20th	Night	15% 6% 19% 37% 19% 3	591
	21st to end	Night	10% 4 19% 36% 28% 4	650
June	1st to 10th	Night	12% 2 18% 41% 25% 3	600
	11th to 20th	Night	9% 3 15% 44% 28% 3	600
	21st to end	Night	7% 2 19% 39% 32% 2	600
July	1st to 10th	Night	12% 2 8% 31% 38% 9%	585
	11th to 20th	Night	7% 1 10% 39% 36% 7%	582
	21st to end	Night	9% 2 9% 40% 34% 7%	657
August	1st to 10th	Night	10% 2 8% 36% 40% 5%	597
	11th to 20th	Night	7% 2 9% 39% 38% 7%	594
	21st to end	Night	8% 2 8% 34% 42% 7%	660
September	1st to 10th	Night	14% 2 10% 34% 35% 5%	621
	11th to 20th	Night	9% 2 11% 34% 39% 5%	800
	21st to end	Night	15% 5% 18% 31% 28% 4	800
October	1st to 10th	Night	10% 4 19% 35% 26% 6%	800
	11th to 20th	Night	16% 6% 26% 32% 18% 2	800
	21st to end	Night	22% 8% 31% 28% 11% 1	880
November	1st to 10th	Night	17% 6% 32% 29% 14% 2	820
	11th to 20th	Night	19% 13% 29% 30% 9% 1	840
	21st to end	Night	26% 10% 27% 29% 8%	840
December	1st to 10th	Night	25% 9% 35% 25% 6%	840
	11th to 20th	Night	27% 9% 33% 24% 6%	908
	21st to end	Night	30% 12% 33% 23% 3	1155

Page generated at 2026-04-14 02:44:27. Version ac07

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