Rafter M Ranch Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rafter M Ranch Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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% 6% 2 6% 5% 4 5% 4 6% 8% 41%	137917
	Night	14% 7% 2 6% 4 3 4 3 5% 7% 45%	135072
Transparency	Day	39% 7% 12% 19% 16% 7%	141592
	Night	37% 6% 11% 19% 17% 9%	139464
Seeing	Day	13% 9% 22% 27% 21% 7%	17127
	Night	15% 8% 25% 32% 17% 2	26952

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% 9% 3 7% 3 3 3 3 4 5% 44%	5289
	11th to 20th	Day	14% 6% 2 6% 4 3 3 3 3 5% 51%	5310
	21st to end	Day	27% 8% 3 8% 4 2 3 2 4 6% 34%	5812
February	1st to 10th	Day	21% 8% 4 7% 3 3 4 3 4 5% 39%	2027
	11th to 20th	Day	20% 9% 3 7% 4 4 3 3 3 5% 39%	2049
	21st to end	Day	18% 8% 2 6% 4 4 3 3 4 6% 43%	1879
March	1st to 10th	Day	15% 7% 3 8% 5% 5% 4 4 5% 7% 38%	5046
	11th to 20th	Day	13% 6% 2 6% 4 4 3 3 5% 6% 48%	5090
	21st to end	Day	17% 8% 2 6% 5% 4 4 3 4 6% 41%	5583
April	1st to 10th	Day	15% 9% 3 8% 5% 5% 5% 4 4 6% 37%	2310
	11th to 20th	Day	17% 7% 3 6% 5% 5% 4 4 6% 6% 38%	2416
	21st to end	Day	13% 7% 3 7% 5% 4 4 4 5% 6% 41%	2520
May	1st to 10th	Day	21% 8% 3 7% 5% 5% 4 4 5% 7% 32%	7375
	11th to 20th	Day	11% 6% 3 7% 5% 5% 5% 4 6% 8% 41%	7627
	21st to end	Day	11% 7% 3 8% 5% 6% 6% 5% 8% 10% 32%	8392
June	1st to 10th	Day	6% 5% 2 6% 6% 5% 6% 6% 8% 9% 41%	2730
	11th to 20th	Day	6% 3 2 5% 6% 6% 6% 6% 8% 12% 41%	2730
	21st to end	Day	5% 5% 3 8% 8% 7% 7% 6% 9% 10% 32%	2730
July	1st to 10th	Day	8% 5% 2 7% 6% 6% 6% 5% 8% 11% 35%	6030
	11th to 20th	Day	5% 5% 3 7% 6% 6% 7% 6% 10% 14% 33%	5966
	21st to end	Day	10% 7% 3 7% 5% 5% 5% 5% 8% 10% 35%	6422
August	1st to 10th	Day	1 2 1 4 4 4 5% 4 6% 10% 60%	4234
	11th to 20th	Day	2 2 1 4 4 5% 6% 6% 10% 13% 47%	4077
	21st to end	Day	5% 4 2 7% 6% 7% 6% 6% 8% 10% 39%	4131
September	1st to 10th	Day	6% 4 2 6% 6% 6% 6% 4 8% 9% 41%	2200
	11th to 20th	Day	9% 7% 3 8% 5% 5% 5% 4 7% 9% 38%	2200
	21st to end	Day	6% 5% 2 6% 5% 5% 5% 5% 6% 7% 48%	2200
October	1st to 10th	Day	6% 3 1 4 4 4 4 4 5% 6% 58%	2107
	11th to 20th	Day	10% 6% 2 5% 5% 3 3 3 4 5% 54%	2000
	21st to end	Day	13% 5% 2 5% 3 3 3 3 4 4 54%	2200
November	1st to 10th	Day	10% 7% 2 6% 5% 3 5% 3 6% 7% 47%	2058
	11th to 20th	Day	12% 6% 2 5% 3 4 4 3 6% 6% 50%	2100
	21st to end	Day	16% 7% 3 6% 5% 5% 3 3 4 5% 43%	1979
December	1st to 10th	Day	23% 8% 2 6% 4 3 3 3 4 5% 39%	3030
	11th to 20th	Day	16% 8% 3 7% 4 3 4 3 5% 6% 42%	3481
	21st to end	Day	15% 7% 3 6% 3 3 3 2 4 5% 47%	4587
January	1st to 10th	Night	17% 7% 2 5% 3 3 3 2 3 5% 49%	7634
	11th to 20th	Night	12% 6% 2 5% 3 3 3 2 4 5% 54%	7680
	21st to end	Night	29% 8% 2 6% 3 3 3 2 4 5% 35%	8402
February	1st to 10th	Night	20% 9% 3 7% 4 3 3 3 4 5% 41%	2611
	11th to 20th	Night	22% 8% 2 5% 4 3 3 2 3 4 44%	2458
	21st to end	Night	20% 8% 2 5% 4 3 3 3 4 6% 42%	1924
March	1st to 10th	Night	19% 11% 3 5% 4 3 3 3 5% 6% 38%	5029
	11th to 20th	Night	18% 7% 2 6% 4 3 3 3 4 5% 46%	5090
	21st to end	Night	19% 9% 3 7% 4 3 3 3 3 5% 41%	5588
April	1st to 10th	Night	15% 8% 2 6% 4 4 4 4 5% 7% 41%	2310
	11th to 20th	Night	14% 7% 2 6% 3 4 4 3 5% 7% 45%	2150
	21st to end	Night	15% 7% 2 6% 4 4 4 3 5% 6% 43%	2100
May	1st to 10th	Night	24% 11% 3 7% 4 3 4 3 5% 7% 28%	5491
	11th to 20th	Night	13% 8% 3 7% 4 4 5% 3 6% 8% 39%	5265
	21st to end	Night	12% 9% 3 8% 5% 4 5% 4 7% 10% 33%	5793
June	1st to 10th	Night	6% 4 2 5% 3 3 4 4 8% 10% 51%	1890
	11th to 20th	Night	4 4 2 4 3 3 4 4 6% 11% 55%	1890
	21st to end	Night	4 4 2 6% 5% 4 5% 6% 8% 11% 46%	1890
July	1st to 10th	Night	6% 5% 2 6% 5% 4 6% 5% 10% 13% 40%	4168
	11th to 20th	Night	4 4 2 6% 4 4 5% 4 6% 11% 50%	4122
	21st to end	Night	6% 6% 3 7% 5% 4 5% 4 6% 10% 45%	4449
August	1st to 10th	Night	1 1 1 3 2 2 3 2 5% 10% 70%	2926
	11th to 20th	Night	2 2 1 3 2 3 4 4 7% 13% 59%	2978
	21st to end	Night	4 3 2 6% 4 4 5% 5% 8% 13% 47%	3748
September	1st to 10th	Night	5% 5% 2 6% 4 4 5% 5% 7% 10% 48%	2200
	11th to 20th	Night	8% 7% 3 7% 5% 4 5% 4 5% 8% 45%	2200
	21st to end	Night	8% 4 2 4 3 3 4 3 6% 10% 54%	2200
October	1st to 10th	Night	3 4 2 4 3 2 3 3 4 9% 63%	2270
	11th to 20th	Night	9% 4 1 4 3 3 3 3 4 6% 61%	2400
	21st to end	Night	11% 5% 2 4 3 3 2 2 3 4 60%	2640
November	1st to 10th	Night	10% 6% 2 5% 4 3 3 2 4 6% 54%	2460
	11th to 20th	Night	11% 6% 2 5% 4 3 3 2 5% 6% 52%	2520
	21st to end	Night	17% 6% 3 6% 3 4 3 3 4 6% 45%	2637
December	1st to 10th	Night	20% 11% 3 8% 4 3 3 3 4 5% 36%	4380
	11th to 20th	Night	16% 10% 3 6% 4 3 3 2 4 4 43%	4946
	21st to end	Night	15% 7% 2 5% 3 3 3 3 3 5% 50%	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	43% 5% 7% 21% 18% 7%	5290
	11th to 20th	Day	37% 3 5% 16% 24% 14%	5310
	21st to end	Day	53% 3 4 11% 17% 12%	5812
February	1st to 10th	Day	47% 5% 5% 13% 17% 12%	2027
	11th to 20th	Day	49% 4 8% 14% 16% 9%	2059
	21st to end	Day	44% 6% 8% 15% 16% 11%	1879
March	1st to 10th	Day	43% 7% 7% 15% 16% 11%	5046
	11th to 20th	Day	37% 6% 6% 11% 19% 20%	5090
	21st to end	Day	43% 6% 8% 20% 18% 6%	5585
April	1st to 10th	Day	48% 6% 6% 15% 15% 9%	2310
	11th to 20th	Day	45% 6% 9% 14% 17% 9%	2416
	21st to end	Day	42% 6% 10% 19% 18% 6%	2520
May	1st to 10th	Day	49% 7% 10% 21% 11% 2	7410
	11th to 20th	Day	39% 7% 12% 21% 19% 2	7631
	21st to end	Day	42% 9% 15% 24% 9% 1	8405
June	1st to 10th	Day	34% 10% 17% 23% 15% 2	2730
	11th to 20th	Day	33% 11% 16% 23% 15% 3	2730
	21st to end	Day	42% 12% 15% 21% 9% 1	2730
July	1st to 10th	Day	39% 10% 29% 20% 2	5966
	11th to 20th	Day	36% 11% 24% 22% 7%	5897
	21st to end	Day	40% 10% 18% 24% 9%	6240
August	1st to 10th	Day	18% 8% 15% 39% 20% 1	4220
	11th to 20th	Day	23% 10% 32% 27% 6% 1	4116
	21st to end	Day	36% 12% 17% 21% 13% 1	4130
September	1st to 10th	Day	36% 11% 15% 18% 15% 6%	2200
	11th to 20th	Day	40% 9% 12% 19% 16% 4	2200
	21st to end	Day	31% 9% 12% 19% 16% 13%	2200
October	1st to 10th	Day	16% 4 6% 14% 21% 38%	3432
	11th to 20th	Day	22% 4 6% 16% 33% 19%	3240
	21st to end	Day	26% 5% 5% 17% 33% 14%	3538
November	1st to 10th	Day	35% 6% 10% 16% 22% 10%	2058
	11th to 20th	Day	34% 6% 10% 15% 22% 13%	2100
	21st to end	Day	43% 6% 8% 16% 18% 10%	1979
December	1st to 10th	Day	48% 4 6% 16% 21% 5%	3030
	11th to 20th	Day	42% 6% 7% 12% 22% 11%	3479
	21st to end	Day	39% 5% 6% 15% 25% 10%	4587
January	1st to 10th	Night	38% 5% 9% 19% 19% 10%	7635
	11th to 20th	Night	33% 3 5% 19% 26% 15%	7680
	21st to end	Night	53% 3 4 13% 17% 10%	8402
February	1st to 10th	Night	46% 5% 6% 12% 17% 14%	2611
	11th to 20th	Night	45% 4 7% 16% 17% 11%	2458
	21st to end	Night	45% 5% 7% 13% 16% 14%	1924
March	1st to 10th	Night	46% 6% 7% 14% 18% 9%	5029
	11th to 20th	Night	41% 6% 6% 14% 15% 19%	5090
	21st to end	Night	45% 6% 7% 16% 19% 8%	5588
April	1st to 10th	Night	42% 6% 10% 17% 15% 10%	2310
	11th to 20th	Night	39% 6% 11% 17% 15% 12%	2150
	21st to end	Night	42% 6% 9% 19% 18% 6%	2100
May	1st to 10th	Night	54% 7% 10% 18% 9% 3	5521
	11th to 20th	Night	41% 7% 12% 20% 17% 4	5266
	21st to end	Night	43% 10% 17% 24% 6% 1	5805
June	1st to 10th	Night	24% 9% 18% 30% 17% 1	1890
	11th to 20th	Night	23% 8% 20% 29% 17% 4	1890
	21st to end	Night	30% 11% 20% 27% 13% 1	1890
July	1st to 10th	Night	32% 10% 34% 21% 3	4125
	11th to 20th	Night	27% 9% 26% 30% 7%	4071
	21st to end	Night	34% 9% 19% 28% 9%	4320
August	1st to 10th	Night	11% 6% 18% 45% 20% 1	2916
	11th to 20th	Night	15% 8% 35% 33% 8% 1	3005
	21st to end	Night	27% 15% 21% 27% 9% 1	3748
September	1st to 10th	Night	28% 12% 21% 20% 13% 6%	2200
	11th to 20th	Night	36% 8% 14% 22% 15% 5%	2200
	21st to end	Night	25% 7% 16% 22% 18% 12%	2200
October	1st to 10th	Night	14% 4 8% 17% 18% 39%	3712
	11th to 20th	Night	16% 3 5% 22% 31% 22%	3899
	21st to end	Night	22% 4 5% 17% 36% 16%	4252
November	1st to 10th	Night	32% 5% 10% 20% 22% 11%	2460
	11th to 20th	Night	33% 5% 10% 18% 20% 14%	2520
	21st to end	Night	40% 5% 9% 16% 19% 11%	2637
December	1st to 10th	Night	51% 4 5% 14% 19% 6%	4380
	11th to 20th	Night	44% 5% 8% 8% 21% 14%	4947
	21st to end	Night	38% 4 6% 14% 29% 10%	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	24% 20% 34% 19% 3 1	330
	11th to 20th	Day	27% 18% 30% 22% 3 1	330
	21st to end	Day	29% 21% 29% 17% 4	363
February	1st to 10th	Day	28% 18% 34% 15% 5% 1	330
	11th to 20th	Day	32% 22% 28% 15% 3 1	330
	21st to end	Day	29% 23% 34% 11% 3	273
March	1st to 10th	Day	27% 18% 37% 14% 4	330
	11th to 20th	Day	21% 24% 35% 15% 4 1	330
	21st to end	Day	20% 17% 37% 21% 6% 1	363
April	1st to 10th	Day	19% 23% 37% 17% 4	330
	11th to 20th	Day	25% 20% 28% 20% 7% 1	454
	21st to end	Day	17% 17% 34% 22% 8% 1	540
May	1st to 10th	Day	15% 14% 39% 24% 8%	664
	11th to 20th	Day	14% 10% 30% 30% 14% 2	775
	21st to end	Day	9% 8% 30% 34% 16% 3	844
June	1st to 10th	Day	7% 2 18% 37% 28% 7%	780
	11th to 20th	Day	8% 3 17% 36% 28% 7%	780
	21st to end	Day	7% 3 13% 34% 30% 13%	780
July	1st to 10th	Day	6% 3 10% 30% 36% 16%	771
	11th to 20th	Day	4 1 6% 30% 42% 16%	744
	21st to end	Day	6% 2 6% 28% 45% 14%	794
August	1st to 10th	Day	3 1 4 27% 49% 17%	726
	11th to 20th	Day	5% 3 24% 46% 22%	660
	21st to end	Day	8% 2 6% 22% 36% 26%	480
September	1st to 10th	Day	7% 2 9% 31% 35% 15%	330
	11th to 20th	Day	6% 3 11% 28% 36% 16%	330
	21st to end	Day	7% 1 15% 34% 35% 8%	330
October	1st to 10th	Day	5% 3 18% 40% 28% 6%	330
	11th to 20th	Day	11% 6% 24% 35% 19% 5%	330
	21st to end	Day	20% 11% 24% 26% 15% 4	363
November	1st to 10th	Day	15% 8% 25% 35% 16% 2	330
	11th to 20th	Day	15% 10% 31% 33% 9% 2	330
	21st to end	Day	20% 22% 30% 19% 8% 2	330
December	1st to 10th	Day	21% 13% 37% 18% 7% 4	330
	11th to 20th	Day	25% 17% 31% 21% 6% 1	330
	21st to end	Day	19% 18% 34% 22% 5% 2	363
January	1st to 10th	Night	24% 12% 31% 25% 7%	1050
	11th to 20th	Night	21% 15% 31% 26% 6%	1050
	21st to end	Night	24% 13% 31% 27% 5%	1155
February	1st to 10th	Night	25% 15% 33% 24% 4	931
	11th to 20th	Night	28% 13% 33% 24% 3	824
	21st to end	Night	26% 16% 38% 19% 2	540
March	1st to 10th	Night	26% 14% 33% 24% 3	630
	11th to 20th	Night	20% 13% 35% 26% 6%	630
	21st to end	Night	17% 12% 42% 23% 5%	693
April	1st to 10th	Night	18% 12% 40% 27% 4	630
	11th to 20th	Night	19% 13% 32% 28% 7%	623
	21st to end	Night	18% 12% 37% 25% 8%	630
May	1st to 10th	Night	18% 13% 32% 30% 6%	630
	11th to 20th	Night	14% 8% 32% 33% 11% 2	621
	21st to end	Night	8% 6% 30% 37% 17% 1	683
June	1st to 10th	Night	7% 3 17% 44% 26% 3	630
	11th to 20th	Night	5% 2 19% 50% 22% 1	630
	21st to end	Night	6% 15% 43% 30% 6%	630
July	1st to 10th	Night	6% 1 9% 44% 36% 5%	615
	11th to 20th	Night	4 1 7% 46% 38% 4	597
	21st to end	Night	3 1 7% 49% 35% 4	657
August	1st to 10th	Night	1 1 5% 50% 39% 5%	597
	11th to 20th	Night	4 1 5% 47% 38% 5%	594
	21st to end	Night	5% 1 6% 35% 45% 8%	660
September	1st to 10th	Night	6% 1 11% 34% 41% 7%	600
	11th to 20th	Night	10% 2 10% 33% 41% 5%	600
	21st to end	Night	10% 1 12% 37% 36% 5%	600
October	1st to 10th	Night	4 2 15% 41% 33% 5%	670
	11th to 20th	Night	11% 4 25% 38% 20% 2	800
	21st to end	Night	13% 10% 27% 31% 18% 2	880
November	1st to 10th	Night	13% 6% 26% 35% 19% 2	820
	11th to 20th	Night	13% 9% 30% 34% 14% 1	840
	21st to end	Night	21% 10% 29% 27% 12% 1	957
December	1st to 10th	Night	23% 10% 28% 28% 10%	1050
	11th to 20th	Night	22% 14% 29% 27% 6% 1	1050
	21st to end	Night	23% 11% 30% 28% 7% 1	1155

Page generated at 2026-07-13 02:14:49. Version ac07

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