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 2025-03-12.

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% 6% 2 6% 5% 4 5% 4 6% 8% 41%	109328
	Night	14% 7% 2 6% 4 3 4 3 5% 7% 46%	109874
Transparency	Day	39% 7% 11% 18% 17% 7%	113042
	Night	36% 6% 11% 19% 18% 9%	114287
Seeing	Day	13% 9% 21% 26% 22% 7%	15249
	Night	15% 8% 25% 33% 17% 2	25271

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	18% 9% 3 7% 3 3 3 3 4 4 42%	4077
	11th to 20th	Day	16% 6% 2 6% 3 3 3 3 3 5% 49%	4080
	21st to end	Day	30% 8% 2 5% 3 2 3 2 4 6% 36%	4488
February	1st to 10th	Day	22% 8% 4 7% 3 3 4 3 4 5% 38%	1930
	11th to 20th	Day	21% 9% 3 7% 4 4 3 3 3 5% 40%	1949
	21st to end	Day	19% 9% 2 6% 4 4 3 3 4 6% 41%	1792
March	1st to 10th	Day	16% 8% 3 8% 5% 5% 5% 4 6% 7% 32%	3546
	11th to 20th	Day	19% 8% 3 8% 5% 5% 4 4 6% 7% 30%	3491
	21st to end	Day	17% 8% 2 6% 4 4 4 3 4 7% 41%	3814
April	1st to 10th	Day	15% 9% 3 8% 5% 4 5% 4 4 6% 38%	2090
	11th to 20th	Day	17% 7% 3 6% 5% 4 4 4 5% 6% 39%	2183
	21st to end	Day	11% 7% 3 7% 5% 4 4 4 5% 7% 44%	2280
May	1st to 10th	Day	19% 8% 3 8% 6% 5% 5% 4 6% 7% 30%	3952
	11th to 20th	Day	14% 6% 2 6% 4 4 5% 4 6% 7% 41%	4082
	21st to end	Day	9% 6% 3 7% 6% 6% 6% 5% 8% 11% 33%	4477
June	1st to 10th	Day	6% 4 2 6% 6% 5% 5% 6% 8% 9% 43%	2470
	11th to 20th	Day	6% 3 1 5% 6% 6% 6% 6% 7% 12% 43%	2470
	21st to end	Day	5% 5% 3 8% 8% 7% 8% 6% 10% 10% 30%	2470
July	1st to 10th	Day	9% 4 2 5% 5% 6% 6% 5% 8% 11% 39%	4120
	11th to 20th	Day	6% 6% 3 7% 6% 6% 7% 6% 9% 11% 33%	4120
	21st to end	Day	13% 9% 3 8% 6% 6% 6% 5% 8% 10% 27%	4465
August	1st to 10th	Day	1 2 1 4 4 4 4 4 6% 10% 61%	4104
	11th to 20th	Day	2 2 1 4 4 5% 5% 6% 10% 13% 47%	3950
	21st to end	Day	5% 4 2 7% 6% 7% 6% 5% 8% 10% 40%	4004
September	1st to 10th	Day	6% 4 2 7% 6% 7% 6% 4 8% 9% 40%	2090
	11th to 20th	Day	10% 7% 3 8% 6% 5% 5% 4 7% 8% 37%	2090
	21st to end	Day	6% 5% 2 7% 5% 5% 5% 5% 6% 7% 47%	2090
October	1st to 10th	Day	6% 4 1 4 4 4 4 4 5% 6% 57%	2002
	11th to 20th	Day	10% 6% 2 6% 5% 3 3 3 4 5% 53%	1900
	21st to end	Day	13% 5% 2 5% 3 3 3 3 4 4 54%	2090
November	1st to 10th	Day	10% 7% 2 6% 5% 4 5% 3 6% 7% 46%	1958
	11th to 20th	Day	12% 6% 2 5% 3 3 4 3 6% 6% 50%	2000
	21st to end	Day	16% 7% 3 6% 5% 4 3 3 4 5% 44%	1885
December	1st to 10th	Day	23% 8% 2 6% 4 3 3 3 4 6% 38%	2940
	11th to 20th	Day	16% 8% 2 7% 4 3 4 3 5% 6% 42%	3391
	21st to end	Day	15% 7% 3 6% 3 3 3 2 4 5% 47%	4488
January	1st to 10th	Night	18% 7% 2 5% 3 3 3 2 4 5% 48%	5888
	11th to 20th	Night	15% 6% 2 4 3 3 3 2 3 5% 53%	5900
	21st to end	Night	30% 8% 2 4 3 3 3 2 4 5% 36%	6490
February	1st to 10th	Night	20% 9% 3 7% 4 3 3 3 4 5% 40%	2487
	11th to 20th	Night	22% 8% 2 6% 4 3 3 3 3 4 44%	2338
	21st to end	Night	21% 8% 2 6% 4 3 3 3 4 6% 40%	1835
March	1st to 10th	Night	21% 12% 3 6% 4 4 4 3 5% 6% 33%	3529
	11th to 20th	Night	23% 9% 3 8% 5% 3 4 3 5% 6% 31%	3497
	21st to end	Night	13% 7% 3 8% 5% 4 4 4 4 5% 44%	3817
April	1st to 10th	Night	14% 8% 2 6% 4 4 4 4 5% 7% 42%	2090
	11th to 20th	Night	15% 7% 2 6% 3 3 4 3 5% 7% 45%	1943
	21st to end	Night	14% 6% 2 6% 4 4 4 3 5% 6% 46%	1900
May	1st to 10th	Night	23% 9% 3 7% 4 3 4 3 5% 7% 32%	2944
	11th to 20th	Night	13% 7% 3 6% 3 4 4 3 5% 7% 44%	2823
	21st to end	Night	11% 6% 2 7% 5% 4 5% 4 7% 10% 38%	3090
June	1st to 10th	Night	6% 4 2 4 3 3 4 3 8% 10% 54%	1710
	11th to 20th	Night	3 3 2 4 3 3 4 4 6% 11% 56%	1710
	21st to end	Night	4 4 2 6% 4 5% 5% 6% 8% 11% 45%	1710
July	1st to 10th	Night	6% 4 1 5% 4 4 5% 4 9% 13% 46%	2850
	11th to 20th	Night	4 5% 2 6% 4 5% 5% 4 7% 11% 47%	2850
	21st to end	Night	9% 8% 3 8% 5% 5% 5% 4 6% 10% 37%	3098
August	1st to 10th	Night	1 1 1 3 2 2 3 2 5% 10% 70%	2836
	11th to 20th	Night	2 2 1 3 2 3 4 4 7% 13% 59%	2885
	21st to end	Night	4 3 1 5% 4 4 5% 5% 8% 13% 48%	3633
September	1st to 10th	Night	5% 5% 2 6% 4 3 5% 5% 7% 10% 48%	2090
	11th to 20th	Night	8% 7% 3 7% 5% 4 5% 3 5% 8% 45%	2090
	21st to end	Night	8% 4 1 4 3 3 3 3 6% 10% 54%	2090
October	1st to 10th	Night	3 4 2 5% 3 3 3 3 4 8% 62%	2156
	11th to 20th	Night	10% 4 1 4 3 3 3 3 4 6% 60%	2280
	21st to end	Night	12% 5% 2 4 3 3 2 2 3 4 60%	2508
November	1st to 10th	Night	10% 6% 2 6% 4 4 4 3 5% 6% 52%	2340
	11th to 20th	Night	11% 6% 2 5% 4 3 3 2 5% 6% 51%	2400
	21st to end	Night	16% 6% 3 6% 3 4 3 3 4 6% 45%	2511
December	1st to 10th	Night	20% 11% 3 8% 4 3 3 3 4 5% 36%	4250
	11th to 20th	Night	16% 10% 3 6% 4 3 3 2 4 4 43%	4816
	21st to end	Night	15% 7% 2 5% 3 3 3 2 3 5% 51%	6490

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	45% 5% 5% 19% 18% 7%	4078
	11th to 20th	Day	38% 4 5% 16% 25% 12%	4080
	21st to end	Day	52% 3 4 11% 16% 15%	4488
February	1st to 10th	Day	48% 5% 5% 13% 17% 12%	1930
	11th to 20th	Day	49% 4 7% 14% 16% 10%	1959
	21st to end	Day	46% 6% 8% 15% 15% 11%	1792
March	1st to 10th	Day	48% 8% 9% 15% 15% 5%	3546
	11th to 20th	Day	50% 8% 9% 12% 14% 6%	3491
	21st to end	Day	42% 6% 9% 18% 21% 5%	3814
April	1st to 10th	Day	47% 6% 6% 16% 15% 9%	2090
	11th to 20th	Day	45% 6% 9% 14% 17% 9%	2183
	21st to end	Day	39% 6% 9% 20% 20% 6%	2280
May	1st to 10th	Day	50% 9% 12% 15% 10% 4	3987
	11th to 20th	Day	40% 8% 10% 18% 22% 3	4086
	21st to end	Day	39% 11% 14% 27% 10% 1	4489
June	1st to 10th	Day	32% 10% 17% 24% 16% 2	2470
	11th to 20th	Day	31% 10% 16% 24% 16% 3	2470
	21st to end	Day	43% 13% 15% 20% 8% 1	2470
July	1st to 10th	Day	35% 10% 28% 24% 4	4056
	11th to 20th	Day	39% 11% 18% 21% 10%	4051
	21st to end	Day	48% 11% 18% 18% 5%	4325
August	1st to 10th	Day	17% 7% 15% 39% 21% 1	4090
	11th to 20th	Day	23% 11% 32% 27% 6% 1	3989
	21st to end	Day	36% 12% 17% 20% 13% 2	4003
September	1st to 10th	Day	36% 11% 15% 18% 14% 6%	2090
	11th to 20th	Day	42% 9% 12% 17% 16% 4	2090
	21st to end	Day	32% 9% 12% 18% 16% 13%	2090
October	1st to 10th	Day	17% 4 6% 14% 20% 39%	3327
	11th to 20th	Day	22% 4 7% 16% 33% 19%	3140
	21st to end	Day	26% 5% 5% 16% 34% 14%	3428
November	1st to 10th	Day	36% 7% 10% 16% 21% 10%	1958
	11th to 20th	Day	34% 6% 10% 14% 22% 14%	2000
	21st to end	Day	43% 6% 8% 16% 18% 11%	1885
December	1st to 10th	Day	48% 4 6% 16% 21% 5%	2940
	11th to 20th	Day	42% 6% 7% 13% 21% 11%	3389
	21st to end	Day	39% 5% 6% 15% 26% 10%	4488
January	1st to 10th	Night	38% 5% 7% 21% 20% 9%	5888
	11th to 20th	Night	35% 3 4 19% 26% 12%	5900
	21st to end	Night	51% 3 3 13% 17% 12%	6490
February	1st to 10th	Night	47% 5% 5% 12% 17% 15%	2487
	11th to 20th	Night	45% 4 7% 16% 17% 12%	2338
	21st to end	Night	47% 5% 7% 13% 15% 13%	1835
March	1st to 10th	Night	51% 6% 9% 13% 15% 6%	3529
	11th to 20th	Night	53% 8% 9% 13% 12% 6%	3497
	21st to end	Night	40% 7% 9% 15% 22% 7%	3817
April	1st to 10th	Night	41% 6% 10% 17% 15% 10%	2090
	11th to 20th	Night	39% 5% 11% 16% 16% 13%	1943
	21st to end	Night	39% 6% 9% 20% 19% 7%	1900
May	1st to 10th	Night	51% 8% 10% 17% 10% 5%	2974
	11th to 20th	Night	37% 9% 11% 18% 20% 5%	2824
	21st to end	Night	36% 11% 17% 28% 8% 1	3099
June	1st to 10th	Night	21% 9% 19% 31% 19% 1	1710
	11th to 20th	Night	21% 8% 19% 30% 18% 4	1710
	21st to end	Night	29% 11% 19% 27% 13% 1	1710
July	1st to 10th	Night	27% 11% 33% 25% 4	2805
	11th to 20th	Night	29% 11% 23% 26% 10%	2796
	21st to end	Night	42% 10% 20% 23% 5%	2999
August	1st to 10th	Night	10% 6% 18% 45% 20% 1	2826
	11th to 20th	Night	16% 8% 35% 33% 8% 1	2912
	21st to end	Night	26% 16% 21% 27% 10% 1	3633
September	1st to 10th	Night	28% 12% 21% 20% 13% 6%	2090
	11th to 20th	Night	36% 8% 14% 20% 15% 5%	2090
	21st to end	Night	26% 7% 16% 21% 19% 12%	2090
October	1st to 10th	Night	14% 4 8% 17% 17% 40%	3598
	11th to 20th	Night	16% 3 6% 21% 32% 23%	3779
	21st to end	Night	22% 4 5% 17% 36% 16%	4120
November	1st to 10th	Night	33% 5% 11% 20% 21% 10%	2340
	11th to 20th	Night	34% 5% 10% 17% 20% 14%	2400
	21st to end	Night	40% 5% 9% 16% 19% 11%	2511
December	1st to 10th	Night	51% 4 6% 15% 19% 6%	4250
	11th to 20th	Night	44% 5% 8% 8% 21% 14%	4817
	21st to end	Night	37% 4 6% 14% 30% 10%	6490

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% 20% 35% 17% 3 1	300
	11th to 20th	Day	28% 18% 28% 22% 3 1	300
	21st to end	Day	27% 21% 31% 17% 5%	330
February	1st to 10th	Day	30% 17% 33% 15% 5% 1	300
	11th to 20th	Day	33% 21% 28% 14% 3	300
	21st to end	Day	31% 22% 33% 10% 2	249
March	1st to 10th	Day	28% 18% 36% 13% 5%	300
	11th to 20th	Day	20% 25% 35% 16% 4 1	273
	21st to end	Day	21% 18% 38% 19% 4	297
April	1st to 10th	Day	17% 23% 39% 17% 4	270
	11th to 20th	Day	25% 19% 26% 21% 8% 1	381
	21st to end	Day	15% 16% 35% 25% 8% 1	460
May	1st to 10th	Day	14% 13% 38% 25% 10%	572
	11th to 20th	Day	13% 8% 30% 32% 15% 2	675
	21st to end	Day	9% 8% 29% 34% 17% 3	734
June	1st to 10th	Day	6% 2 15% 38% 31% 8%	680
	11th to 20th	Day	7% 2 17% 37% 29% 8%	680
	21st to end	Day	7% 2 12% 33% 33% 13%	680
July	1st to 10th	Day	6% 3 10% 30% 36% 16%	671
	11th to 20th	Day	4 1 5% 29% 44% 17%	668
	21st to end	Day	6% 2 5% 27% 45% 14%	739
August	1st to 10th	Day	3 3 26% 50% 17%	676
	11th to 20th	Day	5% 3 23% 47% 21%	613
	21st to end	Day	8% 2 6% 22% 36% 27%	441
September	1st to 10th	Day	7% 2 8% 28% 38% 16%	300
	11th to 20th	Day	7% 3 12% 26% 37% 16%	300
	21st to end	Day	8% 1 13% 33% 36% 8%	300
October	1st to 10th	Day	5% 4 19% 39% 27% 6%	300
	11th to 20th	Day	11% 5% 25% 35% 18% 6%	300
	21st to end	Day	21% 11% 24% 24% 16% 4	330
November	1st to 10th	Day	16% 7% 25% 35% 15% 2	300
	11th to 20th	Day	16% 10% 31% 32% 9% 2	300
	21st to end	Day	19% 23% 29% 19% 8% 2	300
December	1st to 10th	Day	20% 12% 37% 19% 8% 4	300
	11th to 20th	Day	25% 18% 31% 20% 4 1	300
	21st to end	Day	20% 18% 35% 22% 5% 1	330
January	1st to 10th	Night	25% 12% 31% 25% 7%	1000
	11th to 20th	Night	22% 15% 31% 26% 6%	1000
	21st to end	Night	23% 13% 32% 27% 5%	1100
February	1st to 10th	Night	26% 14% 33% 23% 4	887
	11th to 20th	Night	28% 12% 33% 24% 3	784
	21st to end	Night	27% 15% 37% 19% 2	515
March	1st to 10th	Night	24% 15% 33% 25% 4	600
	11th to 20th	Night	20% 12% 35% 27% 6%	575
	21st to end	Night	17% 11% 42% 24% 5%	627
April	1st to 10th	Night	17% 11% 40% 28% 4	570
	11th to 20th	Night	19% 12% 33% 29% 8%	563
	21st to end	Night	15% 11% 38% 26% 8%	570
May	1st to 10th	Night	17% 12% 33% 31% 7% 1	570
	11th to 20th	Night	13% 7% 32% 35% 12% 2	561
	21st to end	Night	8% 5% 30% 38% 17% 2	617
June	1st to 10th	Night	6% 3 15% 45% 28% 3	570
	11th to 20th	Night	4 2 17% 52% 23% 1	570
	21st to end	Night	6% 14% 43% 32% 5%	570
July	1st to 10th	Night	6% 1 8% 44% 36% 5%	555
	11th to 20th	Night	3 1 7% 45% 39% 4	552
	21st to end	Night	3 1 7% 49% 35% 4	624
August	1st to 10th	Night	1 4 51% 39% 5%	567
	11th to 20th	Night	4 1 5% 47% 38% 5%	564
	21st to end	Night	5% 1 6% 35% 45% 7%	627
September	1st to 10th	Night	6% 1 10% 33% 43% 7%	570
	11th to 20th	Night	10% 2 10% 32% 41% 5%	570
	21st to end	Night	10% 1 12% 36% 36% 5%	570
October	1st to 10th	Night	4 2 15% 41% 33% 5%	636
	11th to 20th	Night	11% 4 25% 39% 19% 2	760
	21st to end	Night	13% 9% 26% 31% 19% 2	836
November	1st to 10th	Night	13% 6% 27% 35% 18% 2	780
	11th to 20th	Night	13% 9% 29% 34% 15%	800
	21st to end	Night	20% 10% 29% 27% 13% 1	911
December	1st to 10th	Night	22% 10% 29% 29% 10%	1000
	11th to 20th	Night	23% 14% 30% 27% 6% 1	1000
	21st to end	Night	23% 10% 31% 28% 7%	1100

Page generated at 2025-03-09 07:08:42. Version ac07

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