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 2024-11-02.

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 5% 4 6% 7% 43%	101029
	Night	17% 5% 2 5% 4 4 4 3 5% 7% 44%	97901
Transparency	Day	38% 8% 12% 19% 17% 5%	100842
	Night	39% 8% 11% 18% 18% 6%	97758
Seeing	Day	18% 10% 23% 24% 17% 8%	14996
	Night	17% 8% 22% 30% 19% 4	25023

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	19% 4 1 4 3 2 3 3 5% 6% 50%	2836
	11th to 20th	Day	16% 5% 2 4 4 2 2 2 3 4 53%	2840
	21st to end	Day	27% 3 1 3 2 3 3 2 3 4 47%	3124
February	1st to 10th	Day	17% 6% 2 6% 4 3 3 3 5% 6% 45%	1859
	11th to 20th	Day	21% 6% 1 4 4 2 4 4 4 6% 44%	1848
	21st to end	Day	21% 6% 2 6% 4 4 3 3 4 4 43%	1570
March	1st to 10th	Day	28% 5% 2 4 2 3 2 2 4 5% 42%	3312
	11th to 20th	Day	14% 4 1 4 3 3 3 3 4 6% 54%	3480
	21st to end	Day	21% 5% 2 4 4 4 4 3 4 5% 43%	3815
April	1st to 10th	Day	14% 3 2 4 4 5% 4 5% 6% 7% 47%	2090
	11th to 20th	Day	15% 4 2 5% 5% 5% 4 4 5% 6% 46%	2263
	21st to end	Day	13% 4 2 5% 4 4 4 4 5% 7% 51%	2470
May	1st to 10th	Day	14% 6% 2 6% 5% 4 5% 4 6% 8% 40%	4082
	11th to 20th	Day	19% 7% 2 6% 4 4 5% 4 6% 7% 36%	4081
	21st to end	Day	10% 3 2 6% 4 5% 5% 4 8% 10% 43%	4476
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	14% 4 1 4 3 2 3 2 4 4 60%	1760
	11th to 20th	Day	16% 5% 1 3 2 3 2 3 4 5% 55%	1710
	21st to end	Day	17% 4 2 4 3 3 3 3 5% 6% 50%	1710
December	1st to 10th	Day	24% 6% 2 5% 3 3 3 2 6% 5% 39%	1710
	11th to 20th	Day	19% 4 1 4 3 2 3 3 4 6% 50%	2153
	21st to end	Day	25% 4 1 2 2 3 3 2 5% 7% 45%	3124
January	1st to 10th	Night	19% 4 1 4 3 4 3 3 4 5% 50%	4109
	11th to 20th	Night	15% 6% 2 5% 4 3 3 2 5% 5% 50%	4120
	21st to end	Night	27% 6% 2 4 3 3 3 2 3 4 44%	4532
February	1st to 10th	Night	16% 6% 2 4 3 3 4 3 4 6% 49%	2318
	11th to 20th	Night	22% 5% 2 4 4 4 2 3 5% 5% 44%	2218
	21st to end	Night	21% 5% 1 5% 4 4 4 3 4 7% 41%	1884
March	1st to 10th	Night	27% 6% 2 5% 3 3 3 2 4 5% 39%	3541
	11th to 20th	Night	11% 4 2 4 3 3 3 4 5% 7% 54%	3480
	21st to end	Night	22% 5% 2 6% 4 3 3 3 4 5% 42%	3817
April	1st to 10th	Night	14% 4 2 4 3 4 4 3 5% 5% 52%	2090
	11th to 20th	Night	14% 5% 2 5% 3 3 3 4 5% 6% 51%	1878
	21st to end	Night	12% 5% 2 5% 4 4 3 3 5% 6% 52%	1710
May	1st to 10th	Night	16% 5% 2 6% 4 4 4 4 6% 8% 40%	2813
	11th to 20th	Night	21% 5% 2 6% 4 3 4 4 4 7% 40%	2812
	21st to end	Night	10% 4 2 6% 4 5% 5% 4 7% 10% 44%	3079
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 5% 58%	2508
November	1st to 10th	Night	15% 4 1 5% 3 2 3 2 4 4 57%	2347
	11th to 20th	Night	16% 6% 2 4 3 2 3 3 3 5% 53%	2470
	21st to end	Night	18% 4 1 4 3 3 4 4 5% 6% 47%	2470
December	1st to 10th	Night	22% 6% 2 5% 4 3 3 2 4 5% 42%	2470
	11th to 20th	Night	17% 6% 2 4 2 2 2 2 4 5% 54%	3042
	21st to end	Night	25% 6% 2 5% 3 3 4 3 5% 7% 39%	4532

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	34% 5% 6% 22% 24% 9%	2837
	11th to 20th	Day	37% 3 6% 15% 29% 9%	2840
	21st to end	Day	40% 4 6% 17% 22% 10%	3124
February	1st to 10th	Day	41% 5% 8% 20% 20% 7%	1859
	11th to 20th	Day	41% 5% 8% 17% 21% 7%	1858
	21st to end	Day	45% 4 8% 14% 19% 10%	1570
March	1st to 10th	Day	45% 6% 7% 17% 15% 11%	3312
	11th to 20th	Day	33% 6% 7% 14% 27% 14%	3480
	21st to end	Day	43% 6% 7% 20% 20% 4	3815
April	1st to 10th	Day	33% 7% 11% 20% 22% 6%	2090
	11th to 20th	Day	37% 6% 8% 17% 22% 9%	2263
	21st to end	Day	32% 7% 11% 26% 20% 4	2470
May	1st to 10th	Day	40% 10% 14% 18% 15% 4	4120
	11th to 20th	Day	44% 10% 11% 20% 13% 1	4085
	21st to end	Day	32% 11% 16% 20% 20% 1	4489
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	31% 6% 10% 19% 28% 7%	2090
	11th to 20th	Day	29% 4 9% 21% 23% 13%	1908
	21st to end	Day	31% 4 6% 20% 27% 13%	2090
November	1st to 10th	Day	30% 4 7% 18% 29% 12%	1760
	11th to 20th	Day	32% 4 7% 21% 27% 9%	1710
	21st to end	Day	35% 5% 7% 19% 26% 8%	1710
December	1st to 10th	Day	47% 4 6% 18% 18% 7%	1710
	11th to 20th	Day	35% 4 7% 19% 22% 13%	2150
	21st to end	Day	38% 5% 9% 20% 24% 4	3124
January	1st to 10th	Night	36% 5% 7% 17% 27% 8%	4109
	11th to 20th	Night	35% 5% 7% 15% 27% 9%	4120
	21st to end	Night	43% 5% 5% 14% 24% 8%	4532
February	1st to 10th	Night	37% 4 9% 21% 21% 7%	2318
	11th to 20th	Night	42% 4 6% 18% 23% 7%	2219
	21st to end	Night	43% 5% 9% 15% 20% 8%	1884
March	1st to 10th	Night	46% 6% 9% 16% 15% 9%	3541
	11th to 20th	Night	29% 5% 9% 17% 25% 15%	3480
	21st to end	Night	43% 5% 7% 18% 18% 9%	3817
April	1st to 10th	Night	33% 5% 10% 21% 25% 6%	2090
	11th to 20th	Night	34% 5% 8% 21% 23% 9%	1878
	21st to end	Night	33% 6% 11% 26% 21% 3	1710
May	1st to 10th	Night	40% 10% 14% 21% 14% 2	2840
	11th to 20th	Night	42% 9% 15% 19% 14% 1	2813
	21st to end	Night	30% 12% 15% 27% 15% 1	3087
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	32% 7% 13% 15% 28% 6%	2090
	11th to 20th	Night	32% 6% 9% 21% 22% 12%	2270
	21st to end	Night	30% 4 6% 21% 28% 12%	2508
November	1st to 10th	Night	31% 5% 8% 18% 26% 12%	2347
	11th to 20th	Night	35% 4 5% 16% 28% 12%	2470
	21st to end	Night	36% 6% 6% 20% 25% 7%	2470
December	1st to 10th	Night	45% 4 7% 18% 19% 7%	2470
	11th to 20th	Night	35% 4 8% 17% 27% 9%	3044
	21st to end	Night	43% 6% 8% 20% 18% 5%	4532

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	26% 19% 37% 15% 3	270
	11th to 20th	Day	30% 20% 30% 17% 4	270
	21st to end	Day	25% 26% 35% 11% 3	297
February	1st to 10th	Day	23% 20% 39% 16% 3	270
	11th to 20th	Day	36% 19% 35% 9%	270
	21st to end	Day	34% 22% 36% 7% 1	225
March	1st to 10th	Day	23% 17% 32% 21% 7%	384
	11th to 20th	Day	22% 19% 33% 19% 5% 1	460
	21st to end	Day	23% 20% 29% 21% 6% 1	506
April	1st to 10th	Day	23% 14% 29% 23% 10% 2	460
	11th to 20th	Day	16% 16% 34% 22% 11% 2	452
	21st to end	Day	19% 10% 27% 26% 14% 5%	460
May	1st to 10th	Day	12% 14% 30% 26% 10% 7%	463
	11th to 20th	Day	20% 7% 20% 25% 20% 8%	487
	21st to end	Day	15% 7% 20% 27% 19% 12%	529
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% 16% 4	539
November	1st to 10th	Day	21% 7% 29% 26% 14% 3	352
	11th to 20th	Day	18% 19% 31% 27% 5%	270
	21st to end	Day	14% 11% 38% 24% 10% 1	270
December	1st to 10th	Day	28% 17% 32% 19% 4	270
	11th to 20th	Day	24% 14% 36% 21% 5% 1	270
	21st to end	Day	31% 15% 32% 15% 4 2	297
January	1st to 10th	Night	21% 14% 33% 26% 6%	760
	11th to 20th	Night	21% 13% 32% 28% 6%	760
	21st to end	Night	22% 14% 36% 26% 3	836
February	1st to 10th	Night	17% 13% 31% 34% 5%	760
	11th to 20th	Night	23% 16% 33% 25% 3	743
	21st to end	Night	22% 15% 34% 25% 4	628
March	1st to 10th	Night	18% 12% 31% 33% 5%	760
	11th to 20th	Night	17% 12% 34% 28% 8% 1	760
	21st to end	Night	21% 12% 29% 29% 9%	836
April	1st to 10th	Night	16% 13% 29% 32% 10%	760
	11th to 20th	Night	16% 12% 24% 33% 14% 1	616
	21st to end	Night	13% 7% 22% 36% 20% 2	570
May	1st to 10th	Night	13% 7% 24% 33% 22% 1	570
	11th to 20th	Night	17% 5% 19% 31% 23% 5%	561
	21st to end	Night	15% 2 11% 31% 32% 8%	617
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	16% 6% 20% 35% 19% 3	748
	11th to 20th	Night	18% 9% 25% 34% 13% 1	760
	21st to end	Night	19% 8% 25% 33% 13% 1	760
December	1st to 10th	Night	24% 11% 25% 32% 8% 1	760
	11th to 20th	Night	20% 8% 29% 33% 9% 1	760
	21st to end	Night	28% 10% 26% 31% 5%	836

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