Mountain Rest Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mountain Rest Observatory clear sky chart from 2005-11-06 to 2025-05-26.

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	15% 5% 2 5% 4 4 4 4 6% 7% 44%	114414
	Night	16% 5% 2 5% 4 4 4 3 5% 6% 46%	114875
Transparency	Day	36% 8% 11% 19% 19% 7%	118195
	Night	36% 7% 10% 18% 21% 8%	119233
Seeing	Day	18% 10% 24% 24% 16% 8%	15695
	Night	17% 8% 22% 30% 19% 4	25799

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% 4 1 3 2 2 2 2 4 5% 57%	4056
	11th to 20th	Day	13% 5% 2 4 3 2 2 2 2 3 62%	4060
	21st to end	Day	21% 5% 3 5% 3 3 3 2 3 4 47%	4466
February	1st to 10th	Day	17% 6% 2 5% 4 3 3 3 5% 6% 46%	1957
	11th to 20th	Day	22% 5% 1 4 4 2 3 3 4 5% 43%	1948
	21st to end	Day	20% 6% 2 6% 4 4 3 3 4 4 44%	1650
March	1st to 10th	Day	23% 5% 2 5% 3 4 3 3 5% 5% 42%	4756
	11th to 20th	Day	12% 4 2 4 3 3 3 3 4 6% 56%	4980
	21st to end	Day	20% 4 2 5% 4 4 3 3 4 5% 46%	5465
April	1st to 10th	Day	15% 3 2 4 3 5% 4 5% 6% 7% 46%	2200
	11th to 20th	Day	14% 4 2 4 5% 5% 4 4 5% 6% 48%	2384
	21st to end	Day	13% 4 2 5% 4 4 4 4 5% 7% 49%	2600
May	1st to 10th	Day	14% 6% 2 7% 5% 4 5% 4 6% 8% 39%	4212
	11th to 20th	Day	19% 7% 2 6% 4 4 5% 4 6% 7% 35%	4211
	21st to end	Day	10% 3 2 6% 4 5% 5% 4 8% 10% 43%	4541
June	1st to 10th	Day	13% 6% 2 6% 5% 5% 6% 6% 9% 10% 33%	2470
	11th to 20th	Day	7% 5% 1 5% 5% 6% 7% 6% 9% 11% 39%	2470
	21st to end	Day	9% 3 1 6% 5% 6% 7% 5% 8% 10% 38%	2470
July	1st to 10th	Day	8% 5% 2 7% 6% 5% 7% 6% 9% 12% 32%	4120
	11th to 20th	Day	9% 5% 2 7% 6% 6% 7% 6% 8% 11% 34%	4120
	21st to end	Day	11% 7% 3 9% 8% 8% 9% 7% 9% 9% 19%	4466
August	1st to 10th	Day	12% 6% 2 6% 5% 5% 6% 5% 8% 11% 33%	4104
	11th to 20th	Day	10% 4 2 6% 5% 5% 6% 5% 8% 11% 38%	4044
	21st to end	Day	8% 4 2 5% 4 5% 5% 4 7% 7% 50%	4094
September	1st to 10th	Day	11% 6% 2 7% 5% 5% 6% 4 8% 8% 39%	2090
	11th to 20th	Day	17% 5% 2 5% 5% 5% 5% 4 6% 8% 37%	2090
	21st to end	Day	18% 5% 1 5% 3 4 4 3 5% 7% 44%	2090
October	1st to 10th	Day	16% 4 2 4 2 2 3 2 4 6% 55%	2090
	11th to 20th	Day	14% 4 1 4 3 2 3 2 4 5% 58%	1908
	21st to end	Day	15% 4 1 4 3 2 2 3 3 4 58%	2090
November	1st to 10th	Day	15% 5% 1 4 3 2 2 2 4 4 58%	1843
	11th to 20th	Day	16% 5% 1 3 2 2 3 3 5% 6% 54%	1800
	21st to end	Day	16% 4 2 4 3 3 3 3 5% 6% 51%	1800
December	1st to 10th	Day	20% 4 2 4 2 3 3 2 5% 5% 51%	2930
	11th to 20th	Day	20% 4 2 5% 4 3 4 3 5% 5% 44%	3373
	21st to end	Day	26% 4 1 2 2 2 3 2 4 7% 48%	4466
January	1st to 10th	Night	17% 3 1 3 3 3 2 2 3 4 58%	5889
	11th to 20th	Night	14% 5% 2 4 3 3 3 2 4 4 56%	5900
	21st to end	Night	20% 5% 2 4 3 3 3 2 4 5% 50%	6490
February	1st to 10th	Night	15% 6% 2 4 3 3 3 3 4 6% 49%	2440
	11th to 20th	Night	23% 6% 2 4 4 3 2 3 5% 5% 44%	2338
	21st to end	Night	20% 5% 1 5% 4 4 4 3 4 7% 43%	1980
March	1st to 10th	Night	22% 5% 2 5% 3 3 4 3 5% 6% 41%	5097
	11th to 20th	Night	11% 3 1 4 3 3 3 3 5% 7% 56%	4980
	21st to end	Night	19% 4 2 5% 4 3 3 3 4 6% 46%	5467
April	1st to 10th	Night	15% 4 2 4 3 3 4 3 5% 5% 51%	2200
	11th to 20th	Night	14% 4 2 5% 3 3 3 3 5% 6% 52%	1978
	21st to end	Night	13% 5% 2 5% 3 4 3 3 5% 6% 51%	1800
May	1st to 10th	Night	16% 5% 2 6% 4 4 4 4 6% 8% 40%	2903
	11th to 20th	Night	22% 5% 2 6% 4 3 4 4 5% 7% 40%	2902
	21st to end	Night	10% 4 2 6% 4 5% 5% 4 7% 10% 44%	3130
June	1st to 10th	Night	11% 6% 2 7% 4 6% 6% 5% 8% 8% 36%	1710
	11th to 20th	Night	9% 5% 2 5% 5% 5% 5% 5% 6% 11% 42%	1710
	21st to end	Night	9% 4 2 5% 5% 4 6% 5% 8% 11% 41%	1710
July	1st to 10th	Night	13% 7% 3 9% 6% 6% 6% 5% 7% 9% 28%	2840
	11th to 20th	Night	9% 6% 2 6% 6% 6% 7% 5% 9% 13% 31%	2840
	21st to end	Night	15% 8% 4 9% 6% 6% 6% 6% 7% 9% 24%	3087
August	1st to 10th	Night	14% 6% 3 6% 5% 5% 5% 4 8% 10% 34%	2825
	11th to 20th	Night	16% 8% 4 8% 4 5% 5% 5% 7% 8% 30%	2785
	21st to end	Night	10% 5% 3 6% 5% 5% 4 4 6% 9% 44%	3524
September	1st to 10th	Night	13% 6% 3 6% 5% 5% 6% 4 7% 8% 38%	2090
	11th to 20th	Night	18% 7% 2 6% 4 5% 4 4 5% 7% 38%	2090
	21st to end	Night	17% 4 2 6% 5% 4 4 3 4 6% 45%	2090
October	1st to 10th	Night	15% 4 1 4 3 3 3 3 4 6% 52%	2090
	11th to 20th	Night	14% 3 2 5% 3 3 3 3 4 4 55%	2270
	21st to end	Night	15% 3 1 3 3 2 3 2 4 4 58%	2508
November	1st to 10th	Night	16% 4 2 4 3 2 3 2 3 5% 56%	2450
	11th to 20th	Night	17% 6% 2 4 3 2 3 2 3 5% 53%	2600
	21st to end	Night	17% 4 1 4 3 3 4 4 5% 6% 49%	2600
December	1st to 10th	Night	18% 4 2 4 3 3 3 3 4 5% 52%	4250
	11th to 20th	Night	21% 6% 2 5% 3 3 3 3 4 4 46%	4822
	21st to end	Night	25% 5% 1 4 3 2 3 2 4 5% 45%	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	31% 4 5% 19% 32% 9%	4057
	11th to 20th	Day	30% 3 4 14% 35% 13%	4060
	21st to end	Day	40% 4 5% 16% 23% 12%	4466
February	1st to 10th	Day	40% 5% 8% 21% 20% 7%	1957
	11th to 20th	Day	42% 5% 8% 17% 21% 7%	1958
	21st to end	Day	44% 4 8% 14% 21% 10%	1650
March	1st to 10th	Day	43% 5% 6% 15% 17% 13%	4756
	11th to 20th	Day	30% 5% 6% 12% 26% 21%	4980
	21st to end	Day	40% 5% 6% 23% 19% 6%	5465
April	1st to 10th	Day	34% 7% 11% 21% 21% 6%	2200
	11th to 20th	Day	36% 6% 8% 19% 22% 9%	2384
	21st to end	Day	33% 7% 12% 26% 19% 4	2600
May	1st to 10th	Day	40% 10% 14% 18% 15% 3	4250
	11th to 20th	Day	45% 10% 12% 19% 13% 1	4215
	21st to end	Day	32% 11% 16% 20% 20% 1	4554
June	1st to 10th	Day	40% 13% 17% 21% 7% 2	2470
	11th to 20th	Day	33% 12% 20% 22% 11% 2	2470
	21st to end	Day	36% 13% 16% 19% 13% 3	2470
July	1st to 10th	Day	38% 16% 21% 19% 6% 1	4055
	11th to 20th	Day	38% 14% 25% 17% 4 2	4051
	21st to end	Day	55% 15% 15% 11% 4	4324
August	1st to 10th	Day	40% 11% 16% 27% 5%	4091
	11th to 20th	Day	33% 12% 25% 17% 12% 2	4083
	21st to end	Day	29% 10% 15% 25% 20% 1	4094
September	1st to 10th	Day	38% 11% 14% 17% 17% 3	2090
	11th to 20th	Day	43% 9% 12% 17% 18% 2	2090
	21st to end	Day	38% 6% 12% 20% 17% 6%	2090
October	1st to 10th	Day	24% 4 8% 18% 36% 10%	3480
	11th to 20th	Day	21% 3 6% 15% 29% 27%	3148
	21st to end	Day	29% 4 6% 21% 25% 15%	3428
November	1st to 10th	Day	32% 4 7% 18% 27% 12%	1843
	11th to 20th	Day	32% 4 6% 21% 28% 10%	1800
	21st to end	Day	33% 5% 8% 19% 27% 8%	1800
December	1st to 10th	Day	37% 3 5% 17% 26% 13%	2930
	11th to 20th	Day	41% 6% 8% 16% 19% 11%	3370
	21st to end	Day	37% 5% 9% 26% 20% 3	4466
January	1st to 10th	Night	30% 4 7% 18% 30% 11%	5889
	11th to 20th	Night	32% 4 6% 14% 33% 11%	5900
	21st to end	Night	36% 5% 5% 15% 27% 11%	6490
February	1st to 10th	Night	36% 5% 9% 22% 21% 7%	2440
	11th to 20th	Night	43% 4 6% 18% 22% 7%	2339
	21st to end	Night	41% 4 8% 16% 21% 9%	1980
March	1st to 10th	Night	40% 6% 8% 15% 24% 8%	5097
	11th to 20th	Night	27% 5% 7% 15% 27% 19%	4980
	21st to end	Night	39% 4 6% 22% 20% 9%	5467
April	1st to 10th	Night	34% 5% 10% 20% 25% 6%	2200
	11th to 20th	Night	33% 5% 8% 22% 23% 9%	1978
	21st to end	Night	34% 6% 12% 26% 20% 3	1800
May	1st to 10th	Night	39% 10% 14% 21% 14% 2	2930
	11th to 20th	Night	43% 9% 15% 19% 14% 1	2903
	21st to end	Night	30% 12% 15% 27% 15% 1	3138
June	1st to 10th	Night	40% 12% 16% 20% 10% 2	1710
	11th to 20th	Night	34% 10% 21% 23% 12% 1	1710
	21st to end	Night	32% 15% 18% 18% 14% 4	1710
July	1st to 10th	Night	48% 14% 18% 15% 5% 1	2795
	11th to 20th	Night	38% 17% 24% 16% 4 1	2786
	21st to end	Night	54% 14% 17% 12% 4 1	2988
August	1st to 10th	Night	42% 11% 22% 20% 5%	2814
	11th to 20th	Night	46% 12% 16% 14% 10% 1	2812
	21st to end	Night	34% 10% 15% 25% 14% 2	3524
September	1st to 10th	Night	41% 9% 14% 20% 13% 4	2090
	11th to 20th	Night	44% 9% 14% 16% 15% 2	2090
	21st to end	Night	39% 6% 11% 19% 19% 6%	2090
October	1st to 10th	Night	26% 7% 14% 13% 33% 8%	3480
	11th to 20th	Night	20% 4 6% 17% 30% 24%	3769
	21st to end	Night	29% 4 6% 18% 26% 17%	4120
November	1st to 10th	Night	33% 5% 8% 18% 25% 11%	2450
	11th to 20th	Night	35% 4 5% 16% 28% 12%	2600
	21st to end	Night	35% 6% 6% 20% 25% 7%	2600
December	1st to 10th	Night	35% 4 6% 18% 28% 9%	4250
	11th to 20th	Night	42% 5% 8% 15% 21% 9%	4824
	21st to end	Night	40% 5% 8% 24% 19% 4	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% 19% 38% 15% 2	300
	11th to 20th	Day	29% 21% 29% 17% 3	300
	21st to end	Day	24% 27% 35% 11% 3	330
February	1st to 10th	Day	22% 19% 40% 16% 3	300
	11th to 20th	Day	38% 18% 35% 9%	300
	21st to end	Day	31% 22% 36% 9% 2	249
March	1st to 10th	Day	25% 17% 31% 20% 7%	420
	11th to 20th	Day	22% 19% 33% 19% 6% 1	500
	21st to end	Day	24% 20% 29% 20% 5% 1	550
April	1st to 10th	Day	24% 14% 29% 23% 9% 1	500
	11th to 20th	Day	15% 16% 34% 22% 11% 1	492
	21st to end	Day	19% 10% 26% 26% 14% 4	500
May	1st to 10th	Day	12% 15% 31% 26% 10% 6%	503
	11th to 20th	Day	21% 8% 20% 25% 19% 7%	527
	21st to end	Day	16% 7% 20% 26% 19% 12%	549
June	1st to 10th	Day	15% 5% 17% 31% 20% 12%	490
	11th to 20th	Day	8% 6% 16% 29% 23% 18%	490
	21st to end	Day	10% 3 20% 31% 22% 14%	490
July	1st to 10th	Day	9% 4 11% 25% 30% 20%	485
	11th to 20th	Day	7% 3 12% 25% 31% 22%	484
	21st to end	Day	6% 2 11% 31% 30% 20%	532
August	1st to 10th	Day	17% 4 15% 22% 25% 18%	486
	11th to 20th	Day	10% 4 12% 28% 30% 16%	479
	21st to end	Day	14% 3 10% 25% 32% 15%	539
September	1st to 10th	Day	9% 3 17% 28% 31% 12%	490
	11th to 20th	Day	18% 4 14% 21% 32% 10%	490
	21st to end	Day	20% 6% 14% 30% 23% 7%	490
October	1st to 10th	Day	19% 5% 18% 31% 19% 8%	490
	11th to 20th	Day	14% 7% 24% 33% 18% 3	490
	21st to end	Day	18% 8% 25% 29% 17% 4	539
November	1st to 10th	Day	22% 8% 29% 25% 13% 3	381
	11th to 20th	Day	18% 18% 32% 26% 6%	300
	21st to end	Day	14% 13% 37% 25% 10% 1	300
December	1st to 10th	Day	27% 18% 33% 18% 4	300
	11th to 20th	Day	25% 13% 35% 21% 5% 1	300
	21st to end	Day	32% 15% 31% 17% 5% 2	330
January	1st to 10th	Night	21% 14% 34% 25% 6%	800
	11th to 20th	Night	21% 13% 33% 27% 6%	800
	21st to end	Night	21% 14% 37% 26% 3	880
February	1st to 10th	Night	17% 13% 31% 35% 5%	800
	11th to 20th	Night	24% 16% 33% 24% 3	783
	21st to end	Night	21% 15% 35% 26% 4	660
March	1st to 10th	Night	18% 13% 32% 33% 4	800
	11th to 20th	Night	17% 12% 34% 29% 8% 1	800
	21st to end	Night	20% 12% 30% 29% 9%	880
April	1st to 10th	Night	16% 13% 28% 32% 9%	800
	11th to 20th	Night	15% 11% 24% 35% 14% 1	649
	21st to end	Night	13% 7% 21% 37% 20% 2	600
May	1st to 10th	Night	13% 7% 24% 33% 21% 1	600
	11th to 20th	Night	18% 6% 19% 31% 22% 4	591
	21st to end	Night	15% 3 12% 31% 31% 8%	634
June	1st to 10th	Night	12% 3 12% 33% 33% 7%	570
	11th to 20th	Night	9% 2 14% 29% 32% 14%	570
	21st to end	Night	9% 1 11% 33% 38% 8%	570
July	1st to 10th	Night	12% 3 8% 24% 42% 12%	555
	11th to 20th	Night	11% 2 7% 23% 41% 15%	552
	21st to end	Night	8% 2 8% 28% 43% 12%	624
August	1st to 10th	Night	16% 1 7% 25% 38% 12%	567
	11th to 20th	Night	14% 4 9% 28% 35% 10%	564
	21st to end	Night	13% 3 7% 25% 40% 12%	654
September	1st to 10th	Night	13% 3 11% 28% 36% 9%	760
	11th to 20th	Night	20% 3 13% 26% 32% 6%	760
	21st to end	Night	18% 3 14% 34% 27% 4	760
October	1st to 10th	Night	16% 5% 18% 29% 26% 7%	760
	11th to 20th	Night	15% 5% 23% 34% 20% 3	760
	21st to end	Night	16% 7% 20% 37% 18% 3	836
November	1st to 10th	Night	17% 7% 20% 35% 18% 3	780
	11th to 20th	Night	18% 9% 25% 34% 13% 1	800
	21st to end	Night	18% 9% 27% 33% 13% 1	800
December	1st to 10th	Night	24% 11% 26% 32% 8% 1	800
	11th to 20th	Night	21% 8% 29% 32% 9% 1	800
	21st to end	Night	28% 10% 25% 31% 6%	880

Page generated at 2025-05-23 09:21:04. Version ac07

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