Red Rock Ranch
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Red Rock Ranch clear sky chart from 2005-11-06 to 2026-06-01.

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	12% 5% 2 5% 4 3 4 3 5% 7% 50%	132549
	Night	12% 5% 2 4 3 3 3 3 5% 7% 54%	130950
Transparency	Day	34% 4 9% 22% 19% 12%	136408
	Night	31% 4 11% 21% 19% 14%	135155
Seeing	Day	12% 3 18% 33% 26% 7%	16692
	Night	13% 4 24% 43% 16% 1	26718

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	13% 5% 2 4 2 2 2 2 3 5% 59%	5289
	11th to 20th	Day	14% 5% 2 5% 2 2 2 2 4 5% 57%	5610
	21st to end	Day	22% 8% 1 5% 3 2 3 2 4 7% 44%	6468
February	1st to 10th	Day	16% 8% 3 5% 4 2 3 3 4 4 48%	2100
	11th to 20th	Day	16% 6% 2 6% 4 3 3 3 4 6% 47%	2049
	21st to end	Day	14% 6% 2 5% 3 2 2 2 5% 6% 53%	1730
March	1st to 10th	Day	14% 6% 2 4 3 3 3 3 4 6% 54%	4793
	11th to 20th	Day	15% 6% 2 5% 2 2 3 2 3 5% 54%	5090
	21st to end	Day	16% 6% 2 5% 3 3 3 2 4 6% 50%	5583
April	1st to 10th	Day	13% 6% 2 6% 4 3 4 3 4 7% 47%	2310
	11th to 20th	Day	15% 6% 2 6% 3 4 4 3 4 6% 46%	2436
	21st to end	Day	14% 7% 2 6% 5% 3 4 3 4 6% 46%	2523
May	1st to 10th	Day	19% 7% 2 7% 4 4 4 3 5% 7% 39%	5992
	11th to 20th	Day	9% 4 2 5% 4 4 4 3 6% 8% 51%	5992
	21st to end	Day	10% 6% 2 7% 5% 5% 4 4 7% 9% 40%	6577
June	1st to 10th	Day	5% 3 1 3 3 4 5% 4 7% 10% 54%	2601
	11th to 20th	Day	6% 4 2 4 4 5% 5% 4 7% 9% 52%	2600
	21st to end	Day	6% 4 2 5% 5% 6% 5% 6% 6% 9% 46%	2600
July	1st to 10th	Day	13% 8% 3 6% 4 4 4 3 5% 8% 41%	5900
	11th to 20th	Day	7% 4 2 6% 5% 5% 5% 4 6% 9% 48%	5900
	21st to end	Day	5% 4 2 6% 5% 4 5% 4 6% 10% 49%	6290
August	1st to 10th	Day	1 1 1 2 2 3 3 2 4 7% 74%	3905
	11th to 20th	Day	3 2 1 3 3 3 3 4 6% 9% 63%	3846
	21st to end	Day	5% 3 1 5% 4 4 5% 4 7% 10% 52%	4260
September	1st to 10th	Day	11% 6% 2 7% 5% 5% 6% 4 6% 7% 41%	2200
	11th to 20th	Day	10% 6% 2 7% 4 4 5% 4 6% 8% 43%	2200
	21st to end	Day	7% 3 2 5% 4 5% 5% 4 6% 7% 51%	2200
October	1st to 10th	Day	7% 4 2 5% 4 4 4 3 4 6% 58%	2200
	11th to 20th	Day	13% 4 2 6% 3 4 3 3 4 5% 53%	2190
	21st to end	Day	11% 5% 2 4 3 2 2 3 4 4 59%	2200
November	1st to 10th	Day	15% 7% 2 5% 4 3 3 3 4 6% 49%	2037
	11th to 20th	Day	14% 5% 2 4 3 3 3 3 4 6% 52%	1890
	21st to end	Day	17% 7% 3 5% 3 3 3 3 4 5% 47%	1890
December	1st to 10th	Day	29% 10% 3 5% 4 3 2 2 3 4 35%	3030
	11th to 20th	Day	17% 7% 3 6% 4 4 3 3 5% 5% 42%	3481
	21st to end	Day	12% 6% 2 4 3 3 3 2 3 4 58%	4587
January	1st to 10th	Night	14% 4 1 4 2 2 2 2 4 6% 58%	7634
	11th to 20th	Night	12% 4 1 3 2 2 4 3 3 5% 62%	7422
	21st to end	Night	25% 5% 2 5% 3 3 3 2 3 5% 44%	7746
February	1st to 10th	Night	13% 5% 2 4 2 3 4 3 4 6% 57%	2520
	11th to 20th	Night	17% 6% 2 4 3 2 2 2 3 5% 53%	2458
	21st to end	Night	13% 6% 2 5% 3 3 3 4 5% 5% 51%	2076
March	1st to 10th	Night	12% 6% 2 5% 3 4 4 3 4 7% 51%	5289
	11th to 20th	Night	13% 6% 2 5% 3 3 3 2 3 5% 57%	5090
	21st to end	Night	13% 6% 2 4 3 3 3 3 5% 6% 53%	5588
April	1st to 10th	Night	10% 5% 2 4 4 3 4 4 5% 8% 50%	2310
	11th to 20th	Night	15% 5% 1 5% 4 3 3 4 5% 6% 49%	2127
	21st to end	Night	16% 5% 2 6% 4 4 3 3 3 6% 50%	2095
May	1st to 10th	Night	20% 6% 2 6% 3 3 3 3 4 6% 44%	4143
	11th to 20th	Night	9% 4 1 4 3 3 4 4 6% 9% 53%	4143
	21st to end	Night	9% 5% 2 6% 4 4 4 4 7% 10% 46%	4542
June	1st to 10th	Night	3 2 2 3 2 3 4 3 5% 11% 63%	1800
	11th to 20th	Night	4 2 1 3 4 4 4 4 6% 9% 60%	1800
	21st to end	Night	5% 3 2 4 5% 4 4 4 6% 10% 53%	1800
July	1st to 10th	Night	8% 6% 2 7% 5% 5% 4 4 5% 9% 46%	4078
	11th to 20th	Night	3 3 2 5% 3 4 4 4 7% 10% 55%	4077
	21st to end	Night	6% 4 1 4 3 4 4 4 7% 9% 53%	4574
August	1st to 10th	Night	1 1 2 2 2 3 2 4 7% 75%	3255
	11th to 20th	Night	3 2 1 3 2 3 4 3 6% 10% 64%	3213
	21st to end	Night	4 3 1 5% 4 4 4 3 7% 10% 56%	3646
September	1st to 10th	Night	8% 5% 2 6% 5% 4 6% 4 7% 8% 45%	2200
	11th to 20th	Night	9% 6% 2 5% 4 4 4 4 6% 9% 47%	2200
	21st to end	Night	8% 4 2 5% 3 3 3 3 7% 8% 54%	2200
October	1st to 10th	Night	5% 3 2 3 3 2 3 2 4 7% 65%	2200
	11th to 20th	Night	12% 3 2 3 2 3 3 2 4 5% 59%	2200
	21st to end	Night	11% 3 1 3 3 2 2 2 3 4 66%	2628
November	1st to 10th	Night	12% 5% 2 5% 3 2 2 3 4 5% 56%	2477
	11th to 20th	Night	10% 5% 1 5% 3 3 3 3 4 6% 58%	2730
	21st to end	Night	16% 5% 2 4 3 3 3 3 5% 5% 50%	2730
December	1st to 10th	Night	31% 7% 2 5% 2 3 2 2 3 4 38%	4380
	11th to 20th	Night	19% 7% 2 5% 4 3 3 2 4 5% 47%	4946
	21st to end	Night	13% 5% 1 4 2 2 3 3 4 5% 57%	6633

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	31% 2 5% 22% 25% 15%	5290
	11th to 20th	Day	32% 1 3 13% 26% 24%	5610
	21st to end	Day	43% 2 5% 15% 25% 11%	6468
February	1st to 10th	Day	41% 3 5% 13% 18% 20%	2100
	11th to 20th	Day	40% 2 6% 17% 21% 15%	2059
	21st to end	Day	33% 3 7% 18% 22% 17%	1730
March	1st to 10th	Day	33% 2 7% 18% 16% 24%	4793
	11th to 20th	Day	35% 2 4 9% 15% 34%	5090
	21st to end	Day	36% 2 7% 20% 19% 15%	5585
April	1st to 10th	Day	37% 3 8% 18% 18% 16%	2310
	11th to 20th	Day	41% 3 7% 18% 16% 15%	2436
	21st to end	Day	39% 4 8% 19% 17% 13%	2523
May	1st to 10th	Day	45% 4 8% 21% 15% 7%	6030
	11th to 20th	Day	31% 4 11% 24% 19% 11%	5996
	21st to end	Day	38% 5% 9% 30% 17% 2	6590
June	1st to 10th	Day	23% 6% 14% 33% 20% 4	2601
	11th to 20th	Day	27% 4 12% 29% 21% 7%	2600
	21st to end	Day	32% 5% 15% 28% 16% 4	2600
July	1st to 10th	Day	41% 5% 16% 30% 8% 1	5836
	11th to 20th	Day	32% 6% 17% 31% 13% 1	5831
	21st to end	Day	29% 6% 17% 28% 18% 2	6110
August	1st to 10th	Day	11% 4 12% 43% 29% 2	3892
	11th to 20th	Day	16% 5% 21% 43% 13% 2	3882
	21st to end	Day	25% 6% 19% 26% 21% 2	4259
September	1st to 10th	Day	40% 6% 12% 21% 15% 7%	2200
	11th to 20th	Day	37% 6% 11% 23% 18% 5%	2200
	21st to end	Day	31% 4 10% 25% 17% 12%	2200
October	1st to 10th	Day	18% 3 5% 14% 22% 38%	3590
	11th to 20th	Day	30% 4 7% 17% 20% 23%	3546
	21st to end	Day	25% 3 5% 15% 36% 16%	3538
November	1st to 10th	Day	38% 3 6% 20% 20% 13%	2037
	11th to 20th	Day	34% 2 7% 19% 20% 17%	1890
	21st to end	Day	41% 3 4 19% 18% 14%	1890
December	1st to 10th	Day	56% 2 5% 11% 16% 10%	3030
	11th to 20th	Day	43% 3 7% 14% 17% 15%	3479
	21st to end	Day	31% 3 4 16% 29% 17%	4587
January	1st to 10th	Night	29% 3 7% 19% 24% 18%	7635
	11th to 20th	Night	29% 1 3 16% 26% 25%	7422
	21st to end	Night	45% 2 4 14% 22% 14%	7746
February	1st to 10th	Night	31% 3 8% 16% 19% 24%	2520
	11th to 20th	Night	36% 2 6% 20% 19% 17%	2458
	21st to end	Night	35% 4 6% 16% 22% 18%	2076
March	1st to 10th	Night	35% 3 8% 20% 15% 20%	5289
	11th to 20th	Night	33% 3 6% 14% 15% 30%	5090
	21st to end	Night	32% 3 11% 20% 19% 14%	5588
April	1st to 10th	Night	31% 4 12% 19% 18% 16%	2310
	11th to 20th	Night	36% 3 11% 17% 15% 17%	2127
	21st to end	Night	39% 3 10% 19% 16% 13%	2095
May	1st to 10th	Night	42% 4 9% 19% 16% 10%	4170
	11th to 20th	Night	28% 5% 12% 20% 20% 15%	4144
	21st to end	Night	33% 5% 15% 29% 15% 3	4551
June	1st to 10th	Night	18% 4 18% 35% 23% 3	1800
	11th to 20th	Night	20% 4 12% 31% 26% 6%	1800
	21st to end	Night	25% 8% 20% 27% 16% 4	1800
July	1st to 10th	Night	35% 7% 23% 28% 7% 1	4035
	11th to 20th	Night	23% 8% 24% 31% 14% 1	4026
	21st to end	Night	25% 7% 21% 30% 15% 2	4443
August	1st to 10th	Night	10% 4 13% 46% 25% 2	3244
	11th to 20th	Night	16% 5% 29% 40% 9% 2	3243
	21st to end	Night	23% 8% 22% 28% 17% 2	3646
September	1st to 10th	Night	34% 7% 15% 20% 16% 7%	2200
	11th to 20th	Night	33% 6% 16% 22% 18% 4	2200
	21st to end	Night	26% 5% 12% 26% 19% 13%	2200
October	1st to 10th	Night	15% 3 7% 18% 20% 36%	3590
	11th to 20th	Night	22% 3 10% 16% 23% 25%	3570
	21st to end	Night	21% 2 8% 18% 34% 18%	4240
November	1st to 10th	Night	31% 3 6% 19% 24% 16%	2477
	11th to 20th	Night	29% 3 9% 20% 21% 18%	2730
	21st to end	Night	36% 3 8% 19% 18% 16%	2730
December	1st to 10th	Night	52% 3 6% 14% 13% 13%	4380
	11th to 20th	Night	40% 3 6% 15% 19% 17%	4947
	21st to end	Night	30% 3 6% 16% 26% 20%	6633

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	20% 8% 26% 33% 13% 1	330
	11th to 20th	Day	18% 7% 23% 38% 12% 2	330
	21st to end	Day	20% 5% 31% 33% 11%	363
February	1st to 10th	Day	23% 5% 22% 28% 18% 4	330
	11th to 20th	Day	20% 6% 34% 30% 8% 2	330
	21st to end	Day	21% 5% 32% 32% 10%	273
March	1st to 10th	Day	20% 8% 33% 30% 9%	330
	11th to 20th	Day	14% 5% 38% 32% 12% 1	330
	21st to end	Day	15% 7% 34% 33% 10% 1	363
April	1st to 10th	Day	18% 5% 35% 32% 9%	330
	11th to 20th	Day	14% 8% 37% 32% 8%	474
	21st to end	Day	12% 7% 37% 34% 8%	543
May	1st to 10th	Day	18% 6% 31% 35% 8% 1	753
	11th to 20th	Day	15% 5% 29% 38% 12% 1	775
	21st to end	Day	12% 5% 24% 42% 15% 1	844
June	1st to 10th	Day	5% 1 14% 46% 28% 5%	731
	11th to 20th	Day	6% 1 14% 48% 27% 5%	730
	21st to end	Day	6% 1 9% 37% 38% 9%	730
July	1st to 10th	Day	13% 2 4 31% 39% 11%	721
	11th to 20th	Day	5% 1 3 31% 49% 10%	718
	21st to end	Day	4 2 3 31% 47% 14%	737
August	1st to 10th	Day	2 2 28% 50% 17%	527
	11th to 20th	Day	4 1 2 29% 44% 20%	517
	21st to end	Day	6% 3 2 25% 37% 28%	557
September	1st to 10th	Day	12% 2 1 19% 42% 24%	330
	11th to 20th	Day	6% 2 2 16% 45% 29%	330
	21st to end	Day	8% 1 3 18% 48% 22%	330
October	1st to 10th	Day	8% 1 7% 29% 38% 18%	330
	11th to 20th	Day	13% 1 7% 35% 37% 8%	330
	21st to end	Day	15% 5% 19% 29% 26% 6%	363
November	1st to 10th	Day	18% 2 15% 34% 23% 8%	330
	11th to 20th	Day	19% 5% 15% 28% 25% 7%	330
	21st to end	Day	23% 5% 25% 32% 12% 3	330
December	1st to 10th	Day	23% 2 18% 33% 19% 4	330
	11th to 20th	Day	22% 4 22% 32% 19% 2	330
	21st to end	Day	17% 2 24% 34% 19% 4	363
January	1st to 10th	Night	18% 4 23% 41% 13% 1	1050
	11th to 20th	Night	18% 4 26% 43% 8%	961
	21st to end	Night	19% 6% 25% 42% 7%	924
February	1st to 10th	Night	16% 7% 28% 40% 8%	840
	11th to 20th	Night	21% 5% 33% 34% 7%	824
	21st to end	Night	17% 6% 35% 39% 3	692
March	1st to 10th	Night	19% 8% 37% 31% 5%	752
	11th to 20th	Night	13% 7% 39% 36% 4	630
	21st to end	Night	12% 7% 42% 34% 5%	693
April	1st to 10th	Night	11% 8% 47% 31% 4	630
	11th to 20th	Night	16% 7% 45% 29% 3	623
	21st to end	Night	17% 8% 42% 29% 4	630
May	1st to 10th	Night	12% 8% 44% 34% 2	630
	11th to 20th	Night	13% 5% 39% 36% 7%	621
	21st to end	Night	9% 3 38% 43% 7%	683
June	1st to 10th	Night	4 2 27% 53% 15%	600
	11th to 20th	Night	3 1 33% 52% 10%	600
	21st to end	Night	5% 3 17% 56% 20% 1	600
July	1st to 10th	Night	9% 2 8% 56% 23% 2	585
	11th to 20th	Night	3 1 10% 58% 26% 2	582
	21st to end	Night	4 2 9% 58% 26% 1	657
August	1st to 10th	Night	2 1 7% 56% 34% 1	597
	11th to 20th	Night	4 2 6% 58% 27% 3	594
	21st to end	Night	5% 2 6% 46% 37% 3	660
September	1st to 10th	Night	9% 1 5% 47% 34% 4	600
	11th to 20th	Night	10% 1 5% 44% 37% 4	600
	21st to end	Night	8% 1 8% 45% 34% 4	600
October	1st to 10th	Night	6% 1 10% 50% 32% 2	600
	11th to 20th	Night	13% 2 15% 48% 21% 1	600
	21st to end	Night	12% 3 20% 40% 22% 2	868
November	1st to 10th	Night	14% 2 17% 41% 24% 1	837
	11th to 20th	Night	12% 3 19% 44% 20% 1	1050
	21st to end	Night	18% 4 21% 42% 15% 1	1050
December	1st to 10th	Night	21% 4 17% 42% 16% 1	1050
	11th to 20th	Night	22% 4 20% 40% 14%	1050
	21st to end	Night	16% 4 21% 47% 12% 1	1155

Page generated at 2026-05-29 00:53:28. Version ac07

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