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 2026-09-29.

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	14% 5% 2 5% 4 4 5% 4 6% 8% 42%	148766
	Night	16% 5% 2 5% 4 4 4 3 5% 7% 44%	142652
Transparency	Day	37% 8% 13% 19% 17% 6%	152503
	Night	37% 7% 11% 18% 19% 7%	146987
Seeing	Day	17% 10% 24% 24% 17% 8%	17539
	Night	17% 8% 22% 30% 19% 4	27527

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% 5% 2 4 2 2 2 2 3 5% 52%	5258
	11th to 20th	Day	11% 5% 2 4 3 2 2 2 2 3 65%	5280
	21st to end	Day	23% 6% 3 6% 3 3 4 2 4 4 42%	5780
February	1st to 10th	Day	16% 6% 2 5% 4 3 3 3 5% 6% 46%	2055
	11th to 20th	Day	22% 5% 1 4 4 3 3 3 5% 6% 43%	2048
	21st to end	Day	20% 6% 2 6% 3 4 3 3 4 5% 44%	1730
March	1st to 10th	Day	23% 5% 2 5% 3 4 3 3 5% 5% 41%	4862
	11th to 20th	Day	12% 4 2 4 3 3 3 3 4 6% 56%	5090
	21st to end	Day	20% 4 2 4 4 4 3 3 4 5% 47%	5586
April	1st to 10th	Day	14% 3 2 4 3 5% 4 5% 6% 7% 45%	2310
	11th to 20th	Day	14% 3 2 4 5% 5% 4 4 5% 6% 49%	2505
	21st to end	Day	13% 4 2 5% 4 4 4 4 5% 7% 49%	2730
May	1st to 10th	Day	14% 7% 3 7% 5% 4 5% 4 6% 8% 37%	7616
	11th to 20th	Day	17% 6% 2 7% 5% 4 5% 4 7% 8% 36%	7626
	21st to end	Day	20% 6% 2 8% 5% 5% 5% 4 6% 7% 32%	8391
June	1st to 10th	Day	12% 6% 2 6% 5% 5% 6% 6% 9% 10% 34%	2730
	11th to 20th	Day	7% 5% 1 5% 5% 6% 7% 6% 9% 12% 38%	2730
	21st to end	Day	8% 3 1 5% 5% 6% 7% 5% 8% 11% 38%	2730
July	1st to 10th	Day	5% 4 2 6% 5% 5% 7% 5% 9% 12% 38%	7680
	11th to 20th	Day	8% 5% 2 7% 6% 6% 7% 6% 9% 13% 30%	7680
	21st to end	Day	10% 6% 3 8% 6% 7% 7% 7% 8% 11% 27%	8340
August	1st to 10th	Day	13% 6% 2 7% 6% 6% 7% 6% 8% 11% 26%	6014
	11th to 20th	Day	7% 3 2 5% 5% 5% 6% 5% 8% 12% 44%	5951
	21st to end	Day	8% 4 2 5% 4 5% 5% 4 7% 8% 48%	5963
September	1st to 10th	Day	10% 5% 2 6% 5% 5% 6% 5% 8% 9% 40%	2310
	11th to 20th	Day	16% 5% 2 5% 5% 5% 5% 4 6% 8% 38%	2310
	21st to end	Day	17% 5% 2 5% 4 5% 4 3 5% 8% 44%	2288
October	1st to 10th	Day	16% 4 2 4 2 3 3 2 4 6% 54%	2200
	11th to 20th	Day	14% 4 1 4 3 2 2 2 4 5% 59%	2008
	21st to end	Day	16% 4 1 4 3 2 2 3 3 4 58%	2200
November	1st to 10th	Day	14% 4 1 4 3 2 3 2 4 4 59%	1937
	11th to 20th	Day	15% 5% 1 3 2 2 2 3 4 6% 56%	1890
	21st to end	Day	16% 4 2 4 3 3 3 3 5% 6% 51%	1890
December	1st to 10th	Day	20% 5% 2 4 3 3 3 2 5% 5% 50%	3020
	11th to 20th	Day	20% 4 2 5% 4 3 4 3 5% 5% 45%	3463
	21st to end	Day	25% 4 1 2 2 2 3 2 4 7% 48%	4565
January	1st to 10th	Night	18% 3 1 3 3 3 3 3 4 4 54%	7635
	11th to 20th	Night	12% 5% 2 5% 3 3 3 2 4 4 57%	7680
	21st to end	Night	21% 6% 2 4 3 3 3 2 4 5% 47%	8401
February	1st to 10th	Night	15% 6% 2 4 3 3 3 3 4 6% 49%	2562
	11th to 20th	Night	22% 6% 2 4 4 3 2 3 5% 4 44%	2458
	21st to end	Night	20% 5% 1 5% 4 4 4 3 4 7% 42%	2076
March	1st to 10th	Night	22% 5% 2 5% 3 4 4 3 5% 7% 41%	5211
	11th to 20th	Night	11% 3 1 4 3 3 3 3 5% 7% 57%	5090
	21st to end	Night	19% 4 2 5% 3 3 3 3 5% 6% 46%	5588
April	1st to 10th	Night	15% 4 2 4 4 4 4 3 5% 5% 50%	2310
	11th to 20th	Night	13% 4 1 5% 3 3 3 3 5% 6% 53%	2078
	21st to end	Night	13% 5% 2 5% 4 4 3 3 5% 6% 51%	1890
May	1st to 10th	Night	15% 6% 2 8% 5% 5% 5% 4 7% 8% 36%	5237
	11th to 20th	Night	20% 6% 2 7% 4 4 5% 4 5% 7% 37%	5234
	21st to end	Night	22% 6% 2 6% 4 5% 4 4 6% 8% 33%	5761
June	1st to 10th	Night	11% 6% 2 7% 4 6% 6% 5% 8% 8% 37%	1890
	11th to 20th	Night	9% 6% 2 5% 6% 5% 5% 5% 6% 11% 42%	1890
	21st to end	Night	8% 4 2 5% 5% 4 6% 5% 8% 12% 41%	1890
July	1st to 10th	Night	9% 6% 3 8% 6% 6% 6% 5% 8% 11% 33%	5278
	11th to 20th	Night	11% 6% 2 7% 6% 6% 7% 5% 9% 13% 29%	5277
	21st to end	Night	15% 8% 4 8% 5% 5% 5% 5% 7% 10% 28%	5736
August	1st to 10th	Night	18% 8% 3 8% 6% 5% 5% 5% 8% 9% 26%	4135
	11th to 20th	Night	12% 6% 3 7% 4 4 5% 5% 7% 11% 36%	4095
	21st to end	Night	10% 6% 3 7% 4 5% 4 4 6% 9% 42%	5110
September	1st to 10th	Night	12% 6% 3 6% 5% 5% 5% 4 7% 9% 38%	2310
	11th to 20th	Night	18% 6% 2 7% 4 5% 5% 4 5% 6% 38%	2310
	21st to end	Night	17% 5% 2 7% 5% 4 3 3 4 6% 44%	2289
October	1st to 10th	Night	15% 4 1 4 3 3 3 3 4 6% 52%	2200
	11th to 20th	Night	14% 4 2 5% 3 3 3 3 4 4 55%	2390
	21st to end	Night	16% 3 1 3 3 2 3 2 4 4 58%	2640
November	1st to 10th	Night	15% 4 2 4 3 2 3 2 3 4 56%	2576
	11th to 20th	Night	16% 6% 2 4 3 2 3 3 4 5% 54%	2730
	21st to end	Night	17% 4 1 4 3 3 4 4 5% 6% 49%	2730
December	1st to 10th	Night	18% 4 2 4 3 3 3 3 4 5% 51%	4380
	11th to 20th	Night	21% 5% 2 5% 3 3 3 3 4 4 46%	4952
	21st to end	Night	24% 5% 1 4 3 2 3 3 4 5% 45%	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	35% 4 7% 22% 25% 7%	5259
	11th to 20th	Day	27% 3 4 12% 36% 19%	5280
	21st to end	Day	44% 4 5% 15% 19% 13%	5780
February	1st to 10th	Day	40% 5% 8% 22% 20% 7%	2055
	11th to 20th	Day	42% 5% 8% 18% 21% 7%	2058
	21st to end	Day	43% 4 8% 14% 20% 10%	1730
March	1st to 10th	Day	43% 5% 7% 15% 16% 13%	4862
	11th to 20th	Day	30% 5% 6% 13% 26% 21%	5090
	21st to end	Day	40% 5% 7% 24% 19% 6%	5586
April	1st to 10th	Day	34% 7% 12% 21% 21% 6%	2310
	11th to 20th	Day	35% 6% 8% 19% 23% 9%	2505
	21st to end	Day	32% 7% 12% 26% 19% 3	2730
May	1st to 10th	Day	42% 9% 12% 22% 13% 2	7651
	11th to 20th	Day	43% 10% 15% 20% 10% 2	7630
	21st to end	Day	48% 10% 12% 14% 15% 2	8405
June	1st to 10th	Day	39% 12% 17% 22% 8% 2	2730
	11th to 20th	Day	33% 12% 20% 22% 11% 2	2730
	21st to end	Day	34% 13% 18% 20% 13% 3	2730
July	1st to 10th	Day	31% 15% 24% 23% 6%	7615
	11th to 20th	Day	38% 17% 27% 15% 2 1	7611
	21st to end	Day	44% 14% 18% 17% 6% 1	8156
August	1st to 10th	Day	44% 14% 19% 20% 4	6001
	11th to 20th	Day	27% 11% 28% 23% 10% 1	5990
	21st to end	Day	28% 11% 17% 26% 17% 2	5963
September	1st to 10th	Day	36% 11% 15% 18% 17% 3	2310
	11th to 20th	Day	41% 9% 13% 19% 16% 2	2310
	21st to end	Day	38% 7% 13% 19% 17% 6%	2288
October	1st to 10th	Day	25% 5% 8% 18% 35% 9%	3590
	11th to 20th	Day	20% 3 6% 15% 29% 26%	3248
	21st to end	Day	29% 4 6% 21% 25% 15%	3538
November	1st to 10th	Day	31% 4 7% 19% 27% 12%	1937
	11th to 20th	Day	30% 4 7% 22% 28% 9%	1890
	21st to end	Day	33% 5% 8% 19% 27% 8%	1890
December	1st to 10th	Day	38% 3 5% 17% 25% 13%	3020
	11th to 20th	Day	40% 6% 8% 15% 20% 11%	3460
	21st to end	Day	37% 5% 9% 26% 20% 3	4565
January	1st to 10th	Night	32% 5% 10% 22% 23% 9%	7636
	11th to 20th	Night	31% 4 5% 14% 33% 14%	7680
	21st to end	Night	39% 5% 6% 15% 23% 12%	8401
February	1st to 10th	Night	36% 5% 9% 22% 21% 7%	2562
	11th to 20th	Night	43% 4 6% 18% 22% 7%	2459
	21st to end	Night	41% 5% 8% 16% 21% 9%	2076
March	1st to 10th	Night	40% 6% 8% 15% 23% 8%	5211
	11th to 20th	Night	27% 5% 7% 16% 27% 19%	5090
	21st to end	Night	38% 5% 7% 22% 19% 9%	5588
April	1st to 10th	Night	34% 6% 10% 20% 24% 6%	2310
	11th to 20th	Night	31% 5% 8% 23% 24% 9%	2078
	21st to end	Night	34% 6% 12% 25% 21% 3	1890
May	1st to 10th	Night	43% 9% 12% 21% 12% 2	5264
	11th to 20th	Night	45% 8% 16% 19% 11% 1	5235
	21st to end	Night	46% 10% 12% 19% 11% 2	5771
June	1st to 10th	Night	40% 12% 16% 20% 11% 2	1890
	11th to 20th	Night	34% 10% 20% 22% 12% 1	1890
	21st to end	Night	31% 14% 19% 18% 14% 4	1890
July	1st to 10th	Night	39% 15% 24% 18% 4	5235
	11th to 20th	Night	41% 17% 25% 15% 2 1	5226
	21st to end	Night	48% 12% 18% 15% 5% 2	5606
August	1st to 10th	Night	49% 12% 18% 16% 4	4124
	11th to 20th	Night	37% 12% 23% 19% 9% 1	4122
	21st to end	Night	34% 11% 16% 24% 13% 1	5110
September	1st to 10th	Night	40% 10% 14% 20% 13% 3	2310
	11th to 20th	Night	44% 10% 13% 17% 14% 2	2310
	21st to end	Night	40% 6% 11% 18% 18% 6%	2289
October	1st to 10th	Night	26% 7% 14% 13% 32% 8%	3590
	11th to 20th	Night	20% 4 6% 17% 30% 23%	3889
	21st to end	Night	29% 4 6% 18% 27% 16%	4252
November	1st to 10th	Night	32% 5% 8% 18% 26% 11%	2576
	11th to 20th	Night	33% 4 6% 17% 29% 12%	2730
	21st to end	Night	35% 6% 6% 21% 25% 8%	2730
December	1st to 10th	Night	35% 4 6% 18% 27% 9%	4380
	11th to 20th	Night	42% 5% 9% 15% 21% 9%	4954
	21st to end	Night	40% 5% 8% 25% 19% 4	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	25% 18% 39% 14% 2	330
	11th to 20th	Day	28% 24% 28% 16% 3	330
	21st to end	Day	25% 27% 34% 11% 3	363
February	1st to 10th	Day	21% 19% 41% 16% 3	330
	11th to 20th	Day	38% 18% 34% 10%	330
	21st to end	Day	32% 22% 35% 9% 2	273
March	1st to 10th	Day	24% 17% 32% 21% 7%	456
	11th to 20th	Day	23% 19% 33% 19% 5% 1	540
	21st to end	Day	22% 20% 31% 21% 6% 1	594
April	1st to 10th	Day	23% 14% 28% 24% 10% 2	540
	11th to 20th	Day	14% 16% 33% 23% 11% 1	532
	21st to end	Day	18% 10% 27% 26% 14% 5%	540
May	1st to 10th	Day	13% 16% 31% 26% 9% 6%	543
	11th to 20th	Day	20% 8% 21% 26% 19% 7%	567
	21st to end	Day	18% 9% 20% 25% 18% 11%	617
June	1st to 10th	Day	15% 6% 18% 30% 20% 11%	570
	11th to 20th	Day	7% 5% 19% 30% 22% 16%	570
	21st to end	Day	9% 3 19% 33% 21% 15%	570
July	1st to 10th	Day	8% 4 12% 26% 31% 20%	565
	11th to 20th	Day	7% 2 11% 25% 31% 23%	564
	21st to end	Day	6% 3 12% 31% 29% 19%	620
August	1st to 10th	Day	19% 4 14% 24% 23% 16%	566
	11th to 20th	Day	10% 4 11% 28% 30% 17%	559
	21st to end	Day	14% 4 12% 25% 31% 14%	627
September	1st to 10th	Day	9% 3 16% 29% 30% 13%	570
	11th to 20th	Day	16% 4 14% 23% 33% 11%	570
	21st to end	Day	19% 7% 17% 29% 23% 7%	562
October	1st to 10th	Day	18% 5% 18% 33% 18% 8%	530
	11th to 20th	Day	14% 7% 24% 34% 18% 3	530
	21st to end	Day	19% 9% 26% 28% 15% 3	583
November	1st to 10th	Day	21% 8% 30% 25% 13% 3	415
	11th to 20th	Day	17% 18% 34% 25% 6%	330
	21st to end	Day	15% 15% 36% 24% 9% 1	330
December	1st to 10th	Day	27% 20% 33% 17% 3	330
	11th to 20th	Day	24% 14% 36% 21% 5% 1	330
	21st to end	Day	31% 15% 31% 16% 5% 2	363
January	1st to 10th	Night	21% 14% 34% 25% 6%	840
	11th to 20th	Night	20% 13% 34% 27% 5%	840
	21st to end	Night	20% 14% 38% 24% 3	924
February	1st to 10th	Night	16% 13% 32% 34% 4	840
	11th to 20th	Night	24% 16% 34% 24% 2	823
	21st to end	Night	21% 15% 36% 25% 3	692
March	1st to 10th	Night	18% 13% 32% 34% 4	840
	11th to 20th	Night	16% 12% 34% 29% 8% 1	840
	21st to end	Night	19% 12% 30% 29% 9%	924
April	1st to 10th	Night	16% 13% 28% 33% 10%	840
	11th to 20th	Night	14% 11% 24% 35% 14% 1	682
	21st to end	Night	13% 7% 21% 37% 20% 2	630
May	1st to 10th	Night	13% 7% 25% 33% 20% 1	630
	11th to 20th	Night	17% 6% 19% 32% 22% 4	621
	21st to end	Night	17% 3 13% 30% 30% 7%	683
June	1st to 10th	Night	12% 3 13% 33% 33% 6%	630
	11th to 20th	Night	9% 2 14% 30% 32% 13%	630
	21st to end	Night	8% 1 10% 32% 41% 7%	630
July	1st to 10th	Night	11% 3 7% 24% 43% 12%	615
	11th to 20th	Night	11% 3 7% 22% 42% 15%	612
	21st to end	Night	8% 2 9% 28% 42% 11%	690
August	1st to 10th	Night	19% 2 7% 24% 37% 11%	627
	11th to 20th	Night	14% 4 9% 27% 36% 10%	624
	21st to end	Night	12% 3 8% 27% 38% 11%	723
September	1st to 10th	Night	12% 3 11% 30% 36% 9%	840
	11th to 20th	Night	19% 3 13% 26% 33% 5%	840
	21st to end	Night	18% 3 14% 34% 27% 3	833
October	1st to 10th	Night	16% 5% 17% 31% 26% 6%	800
	11th to 20th	Night	14% 5% 23% 35% 20% 3	800
	21st to end	Night	16% 6% 21% 37% 17% 3	880
November	1st to 10th	Night	16% 7% 21% 35% 18% 3	820
	11th to 20th	Night	18% 9% 26% 34% 12% 1	840
	21st to end	Night	18% 9% 28% 33% 13% 1	840
December	1st to 10th	Night	24% 10% 27% 31% 7%	840
	11th to 20th	Night	20% 8% 30% 32% 9% 1	840
	21st to end	Night	27% 10% 26% 32% 5%	924

Page generated at 2026-09-26 04:18:40. Version ac07

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