Mountain High observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mountain High observatory clear sky chart from 2005-11-06 to 2026-09-14.

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% 6% 2 6% 5% 5% 5% 4 6% 8% 38%	148603
	Night	16% 7% 2 5% 4 4 4 4 5% 8% 42%	142489
Transparency	Day	27% 37% 9% 12% 10% 5%	152340
	Night	25% 37% 10% 12% 11% 5%	146824
Seeing	Day	21% 14% 24% 21% 13% 6%	17463
	Night	20% 11% 21% 27% 18% 4	27452

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% 6% 1 4 2 3 3 2 4 6% 51%	5258
	11th to 20th	Day	10% 6% 2 4 2 2 2 1 2 3 64%	5280
	21st to end	Day	21% 6% 3 6% 3 3 4 3 4 6% 41%	5780
February	1st to 10th	Day	16% 8% 3 6% 4 4 3 3 5% 6% 43%	2052
	11th to 20th	Day	21% 6% 2 6% 5% 3 3 3 4 6% 40%	2048
	21st to end	Day	20% 7% 2 5% 4 3 4 3 4 6% 41%	1730
March	1st to 10th	Day	21% 7% 3 6% 4 4 4 3 5% 6% 40%	4857
	11th to 20th	Day	13% 6% 2 5% 3 3 3 3 4 6% 52%	5090
	21st to end	Day	20% 6% 2 5% 3 3 4 3 4 5% 45%	5586
April	1st to 10th	Day	14% 5% 2 7% 4 4 5% 5% 7% 8% 40%	2310
	11th to 20th	Day	15% 5% 2 5% 4 4 5% 4 5% 6% 44%	2517
	21st to end	Day	15% 5% 2 5% 4 4 4 4 6% 6% 45%	2730
May	1st to 10th	Day	16% 8% 3 8% 5% 4 5% 4 6% 7% 34%	7616
	11th to 20th	Day	17% 7% 3 7% 5% 5% 6% 5% 6% 7% 32%	7626
	21st to end	Day	19% 8% 3 8% 5% 5% 5% 4 6% 7% 29%	8391
June	1st to 10th	Day	13% 7% 3 8% 6% 6% 7% 5% 8% 9% 29%	2730
	11th to 20th	Day	8% 6% 2 6% 7% 6% 7% 6% 9% 11% 32%	2730
	21st to end	Day	8% 6% 2 7% 7% 6% 7% 6% 8% 10% 33%	2730
July	1st to 10th	Day	5% 4 2 8% 7% 7% 8% 7% 10% 11% 30%	7680
	11th to 20th	Day	10% 6% 3 9% 8% 8% 8% 6% 9% 10% 22%	7680
	21st to end	Day	12% 8% 3 10% 7% 7% 7% 6% 8% 9% 23%	8340
August	1st to 10th	Day	13% 9% 3 10% 6% 7% 7% 6% 8% 10% 20%	6014
	11th to 20th	Day	7% 4 2 5% 5% 6% 6% 5% 9% 12% 39%	5951
	21st to end	Day	7% 5% 2 7% 5% 6% 6% 5% 8% 9% 41%	5971
September	1st to 10th	Day	13% 7% 2 5% 6% 6% 7% 4 7% 7% 34%	2310
	11th to 20th	Day	16% 7% 2 7% 5% 6% 6% 4 7% 8% 32%	2233
	21st to end	Day	17% 7% 3 6% 6% 5% 5% 4 5% 7% 36%	2200
October	1st to 10th	Day	15% 6% 2 4 3 3 3 3 4 6% 50%	2200
	11th to 20th	Day	13% 4 1 4 3 3 3 3 5% 6% 56%	2009
	21st to end	Day	15% 5% 1 4 4 3 4 3 4 4 54%	2200
November	1st to 10th	Day	14% 6% 1 4 2 3 3 2 4 5% 56%	1926
	11th to 20th	Day	15% 5% 1 3 2 3 3 3 5% 6% 52%	1890
	21st to end	Day	17% 4 1 5% 3 3 4 3 5% 7% 48%	1890
December	1st to 10th	Day	18% 5% 1 5% 3 3 4 3 4 5% 48%	3020
	11th to 20th	Day	17% 8% 2 5% 3 4 5% 3 5% 5% 43%	3463
	21st to end	Day	23% 5% 1 3 3 2 3 3 5% 6% 45%	4565
January	1st to 10th	Night	18% 5% 1 4 3 3 3 3 4 6% 52%	7635
	11th to 20th	Night	12% 7% 2 5% 3 3 3 2 4 5% 55%	7680
	21st to end	Night	18% 7% 2 4 4 3 4 3 4 6% 44%	8401
February	1st to 10th	Night	15% 6% 3 5% 4 3 4 2 6% 7% 45%	2566
	11th to 20th	Night	21% 8% 3 4 4 3 4 3 4 4 42%	2458
	21st to end	Night	21% 6% 2 6% 4 4 4 3 5% 6% 39%	2076
March	1st to 10th	Night	21% 8% 2 5% 3 4 5% 5% 5% 7% 36%	5216
	11th to 20th	Night	12% 4 1 4 3 3 4 4 5% 8% 52%	5090
	21st to end	Night	18% 5% 2 5% 4 4 5% 3 4 6% 43%	5588
April	1st to 10th	Night	15% 6% 2 5% 4 3 4 4 4 6% 46%	2310
	11th to 20th	Night	14% 6% 2 4 3 3 3 2 6% 6% 50%	2069
	21st to end	Night	13% 6% 2 5% 3 3 4 3 5% 7% 48%	1890
May	1st to 10th	Night	15% 8% 2 7% 5% 5% 5% 4 8% 8% 32%	5237
	11th to 20th	Night	20% 8% 2 6% 4 4 4 4 5% 8% 34%	5234
	21st to end	Night	22% 8% 3 5% 4 4 5% 4 7% 8% 30%	5761
June	1st to 10th	Night	12% 7% 2 7% 6% 5% 5% 5% 8% 10% 33%	1890
	11th to 20th	Night	10% 6% 2 6% 4 5% 5% 4 7% 11% 40%	1890
	21st to end	Night	9% 4 2 6% 5% 5% 5% 5% 8% 11% 41%	1890
July	1st to 10th	Night	7% 6% 3 7% 5% 5% 6% 6% 9% 12% 34%	5278
	11th to 20th	Night	11% 6% 3 7% 6% 6% 6% 5% 9% 11% 29%	5277
	21st to end	Night	16% 9% 3 9% 5% 5% 5% 4 6% 10% 28%	5736
August	1st to 10th	Night	16% 8% 3 9% 5% 6% 6% 5% 7% 11% 24%	4135
	11th to 20th	Night	11% 7% 2 6% 4 4 5% 4 7% 12% 39%	4095
	21st to end	Night	8% 6% 2 5% 5% 4 5% 4 7% 10% 43%	5100
September	1st to 10th	Night	13% 6% 2 7% 5% 5% 5% 4 8% 7% 38%	2310
	11th to 20th	Night	18% 8% 2 7% 5% 5% 5% 4 6% 6% 34%	2234
	21st to end	Night	18% 6% 2 6% 4 4 4 3 5% 7% 42%	2200
October	1st to 10th	Night	16% 6% 2 5% 3 3 3 3 4 6% 50%	2200
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 4 5% 53%	2390
	21st to end	Night	16% 5% 1 3 3 3 4 2 4 5% 55%	2640
November	1st to 10th	Night	15% 6% 2 4 3 3 3 2 4 7% 52%	2588
	11th to 20th	Night	16% 6% 2 4 3 2 4 3 4 6% 51%	2730
	21st to end	Night	17% 5% 2 5% 3 3 4 3 5% 6% 46%	2730
December	1st to 10th	Night	17% 6% 2 5% 3 3 3 3 4 5% 49%	4380
	11th to 20th	Night	19% 7% 2 6% 3 3 4 3 4 6% 43%	4952
	21st to end	Night	22% 7% 2 4 3 3 4 3 4 6% 42%	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	27% 21% 7% 18% 20% 7%	5259
	11th to 20th	Day	18% 22% 3 8% 30% 20%	5280
	21st to end	Day	34% 25% 4 11% 13% 12%	5780
February	1st to 10th	Day	16% 60% 3 10% 9% 2	2052
	11th to 20th	Day	20% 59% 3 7% 8% 3	2058
	21st to end	Day	19% 60% 3 5% 9% 4	1730
March	1st to 10th	Day	33% 30% 5% 9% 12% 10%	4857
	11th to 20th	Day	19% 29% 3 10% 20% 18%	5090
	21st to end	Day	29% 29% 5% 19% 13% 5%	5586
April	1st to 10th	Day	18% 60% 6% 7% 8% 2	2310
	11th to 20th	Day	17% 60% 4 9% 7% 2	2517
	21st to end	Day	16% 60% 6% 11% 7% 1	2730
May	1st to 10th	Day	35% 28% 10% 18% 8% 1	7651
	11th to 20th	Day	35% 29% 13% 16% 6% 2	7630
	21st to end	Day	43% 27% 8% 10% 10% 1	8405
June	1st to 10th	Day	20% 61% 6% 8% 3 1	2730
	11th to 20th	Day	16% 64% 9% 7% 4	2730
	21st to end	Day	18% 61% 10% 8% 4	2730
July	1st to 10th	Day	29% 33% 19% 15% 3	7615
	11th to 20th	Day	39% 34% 19% 8%	7611
	21st to end	Day	43% 28% 13% 10% 5% 1	8156
August	1st to 10th	Day	39% 37% 13% 10% 1	6001
	11th to 20th	Day	17% 35% 24% 16% 6% 2	5990
	21st to end	Day	20% 34% 14% 23% 8% 1	5971
September	1st to 10th	Day	18% 58% 6% 9% 7% 1	2310
	11th to 20th	Day	21% 60% 7% 7% 5% 1	2233
	21st to end	Day	21% 60% 6% 8% 5%	2200
October	1st to 10th	Day	16% 37% 6% 10% 23% 8%	3590
	11th to 20th	Day	9% 36% 3 9% 17% 25%	3249
	21st to end	Day	16% 38% 5% 15% 14% 12%	3538
November	1st to 10th	Day	18% 54% 4 10% 10% 5%	1926
	11th to 20th	Day	15% 54% 3 11% 12% 5%	1890
	21st to end	Day	15% 55% 4 10% 13% 3	1890
December	1st to 10th	Day	21% 36% 5% 11% 16% 11%	3020
	11th to 20th	Day	28% 32% 5% 12% 14% 8%	3460
	21st to end	Day	27% 27% 8% 22% 15% 2	4565
January	1st to 10th	Night	25% 21% 11% 17% 16% 10%	7636
	11th to 20th	Night	22% 22% 5% 9% 25% 17%	7680
	21st to end	Night	30% 24% 7% 11% 18% 11%	8401
February	1st to 10th	Night	15% 60% 5% 11% 7% 2	2566
	11th to 20th	Night	21% 58% 4 7% 8% 2	2459
	21st to end	Night	19% 59% 3 8% 8% 3	2076
March	1st to 10th	Night	31% 32% 8% 13% 13% 4	5216
	11th to 20th	Night	18% 29% 6% 13% 17% 17%	5090
	21st to end	Night	28% 30% 8% 16% 11% 7%	5588
April	1st to 10th	Night	17% 60% 5% 7% 9% 2	2310
	11th to 20th	Night	15% 60% 4 12% 7% 2	2069
	21st to end	Night	15% 60% 5% 12% 7% 1	1890
May	1st to 10th	Night	34% 30% 10% 17% 8% 1	5264
	11th to 20th	Night	34% 29% 14% 14% 7% 1	5235
	21st to end	Night	38% 29% 12% 13% 7%	5771
June	1st to 10th	Night	19% 61% 7% 7% 4 1	1890
	11th to 20th	Night	15% 62% 10% 9% 4	1890
	21st to end	Night	13% 61% 11% 10% 5%	1890
July	1st to 10th	Night	26% 33% 25% 15% 2	5235
	11th to 20th	Night	33% 34% 23% 10% 1	5226
	21st to end	Night	40% 28% 16% 11% 3 2	5606
August	1st to 10th	Night	39% 37% 14% 10% 1	4124
	11th to 20th	Night	22% 33% 24% 15% 5% 1	4122
	21st to end	Night	18% 33% 17% 23% 7% 1	5100
September	1st to 10th	Night	16% 58% 8% 9% 7% 1	2310
	11th to 20th	Night	20% 60% 6% 8% 6%	2234
	21st to end	Night	20% 58% 7% 9% 5% 1	2200
October	1st to 10th	Night	15% 41% 9% 12% 20% 5%	3590
	11th to 20th	Night	8% 36% 4 11% 18% 23%	3889
	21st to end	Night	19% 37% 3 13% 15% 13%	4252
November	1st to 10th	Night	19% 54% 5% 10% 9% 4	2588
	11th to 20th	Night	15% 55% 4 7% 13% 6%	2730
	21st to end	Night	16% 55% 4 11% 12% 3	2730
December	1st to 10th	Night	21% 36% 6% 12% 17% 9%	4380
	11th to 20th	Night	28% 33% 5% 14% 15% 6%	4954
	21st to end	Night	31% 27% 10% 17% 14% 2	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	36% 23% 30% 9% 2	330
	11th to 20th	Day	41% 20% 26% 11% 2	330
	21st to end	Day	36% 28% 27% 7% 2	363
February	1st to 10th	Day	27% 28% 36% 8% 1	330
	11th to 20th	Day	46% 23% 26% 4	330
	21st to end	Day	41% 27% 26% 5% 1	273
March	1st to 10th	Day	32% 18% 30% 14% 5%	451
	11th to 20th	Day	32% 21% 29% 14% 3 1	540
	21st to end	Day	28% 23% 29% 15% 4 1	594
April	1st to 10th	Day	27% 16% 30% 19% 8% 1	540
	11th to 20th	Day	20% 22% 28% 20% 9% 1	532
	21st to end	Day	21% 17% 26% 21% 11% 4	540
May	1st to 10th	Day	19% 24% 26% 19% 8% 4	543
	11th to 20th	Day	22% 12% 23% 24% 12% 7%	567
	21st to end	Day	22% 12% 20% 24% 12% 10%	617
June	1st to 10th	Day	16% 9% 22% 28% 15% 10%	570
	11th to 20th	Day	10% 9% 22% 24% 21% 13%	570
	21st to end	Day	9% 7% 26% 29% 16% 12%	570
July	1st to 10th	Day	8% 4 16% 30% 26% 16%	565
	11th to 20th	Day	9% 4 17% 26% 26% 18%	564
	21st to end	Day	10% 5% 17% 28% 24% 17%	620
August	1st to 10th	Day	18% 6% 20% 24% 18% 14%	566
	11th to 20th	Day	10% 6% 16% 29% 26% 13%	559
	21st to end	Day	14% 5% 15% 25% 27% 13%	627
September	1st to 10th	Day	11% 5% 18% 26% 29% 10%	570
	11th to 20th	Day	17% 5% 17% 25% 25% 10%	542
	21st to end	Day	20% 7% 18% 29% 22% 5%	530
October	1st to 10th	Day	19% 8% 22% 28% 17% 7%	530
	11th to 20th	Day	15% 12% 28% 27% 14% 4	530
	21st to end	Day	23% 12% 25% 28% 10% 3	583
November	1st to 10th	Day	22% 13% 30% 23% 10% 2	404
	11th to 20th	Day	22% 23% 32% 19% 3	330
	21st to end	Day	23% 17% 36% 17% 5% 1	330
December	1st to 10th	Day	35% 22% 29% 11% 2 1	330
	11th to 20th	Day	29% 17% 32% 17% 4	330
	21st to end	Day	32% 20% 32% 11% 4 1	363
January	1st to 10th	Night	29% 15% 29% 21% 5%	840
	11th to 20th	Night	25% 20% 27% 22% 6%	840
	21st to end	Night	25% 17% 34% 21% 3	924
February	1st to 10th	Night	22% 16% 28% 30% 4	840
	11th to 20th	Night	29% 18% 31% 19% 2	823
	21st to end	Night	29% 20% 29% 19% 3	692
March	1st to 10th	Night	24% 15% 28% 28% 4	840
	11th to 20th	Night	24% 14% 31% 22% 7% 1	840
	21st to end	Night	23% 15% 31% 22% 8%	924
April	1st to 10th	Night	23% 17% 27% 25% 9%	840
	11th to 20th	Night	22% 12% 26% 27% 13% 1	673
	21st to end	Night	15% 11% 22% 32% 18% 2	630
May	1st to 10th	Night	16% 12% 24% 27% 19% 1	630
	11th to 20th	Night	20% 8% 18% 29% 21% 4	621
	21st to end	Night	20% 4 14% 26% 28% 8%	683
June	1st to 10th	Night	14% 4 16% 31% 29% 7%	630
	11th to 20th	Night	9% 4 17% 28% 30% 12%	630
	21st to end	Night	7% 3 14% 29% 41% 7%	630
July	1st to 10th	Night	10% 3 8% 25% 46% 9%	615
	11th to 20th	Night	12% 3 11% 23% 39% 12%	612
	21st to end	Night	9% 4 10% 26% 40% 11%	690
August	1st to 10th	Night	19% 4 10% 24% 34% 10%	627
	11th to 20th	Night	16% 5% 10% 25% 35% 10%	624
	21st to end	Night	13% 4 10% 26% 37% 11%	717
September	1st to 10th	Night	13% 4 12% 28% 34% 9%	840
	11th to 20th	Night	21% 3 12% 26% 33% 5%	813
	21st to end	Night	21% 6% 10% 33% 26% 4	800
October	1st to 10th	Night	18% 7% 18% 25% 25% 7%	800
	11th to 20th	Night	18% 11% 18% 31% 19% 3	800
	21st to end	Night	21% 9% 18% 34% 15% 3	880
November	1st to 10th	Night	18% 9% 21% 31% 18% 3	820
	11th to 20th	Night	20% 15% 22% 30% 11% 2	840
	21st to end	Night	21% 13% 21% 32% 13% 1	840
December	1st to 10th	Night	28% 15% 23% 28% 7%	840
	11th to 20th	Night	22% 14% 25% 30% 8% 1	840
	21st to end	Night	30% 13% 23% 28% 5%	924

Page generated at 2026-09-11 04:11:52. Version ac07

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