Mountain Skies Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mountain Skies 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	3 5% 2 5% 4 4 5% 4 6% 8% 55%	157315
	Night	3 5% 2 5% 4 4 5% 4 6% 8% 53%	151432
Transparency	Day	26% 6% 12% 19% 21% 16%	152051
	Night	28% 7% 13% 21% 19% 11%	147703
Seeing	Day	7% 7% 20% 32% 26% 9%	17239
	Night	8% 4 16% 41% 27% 4	27769

By Month

Percentage Cloud Cover SW (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	3 8% 3 11% 7% 6% 6% 6% 9% 8% 32%	5318
	11th to 20th	Day	3 7% 4 10% 5% 6% 5% 4 6% 6% 44%	5340
	21st to end	Day	3 7% 3 6% 4 4 4 5% 6% 9% 48%	5844
February	1st to 10th	Day	6% 10% 3 9% 4 7% 7% 6% 7% 5% 36%	1890
	11th to 20th	Day	4 10% 3 10% 7% 7% 6% 6% 7% 7% 34%	1939
	21st to end	Day	5% 11% 4 8% 6% 5% 5% 4 7% 7% 38%	1730
March	1st to 10th	Day	2 10% 3 8% 6% 7% 6% 4 5% 7% 42%	4925
	11th to 20th	Day	3 8% 3 7% 5% 5% 5% 5% 8% 8% 43%	5110
	21st to end	Day	4 10% 3 10% 6% 6% 7% 6% 7% 7% 32%	5607
April	1st to 10th	Day	5% 8% 3 7% 5% 6% 5% 4 6% 7% 45%	2317
	11th to 20th	Day	5% 6% 3 7% 5% 5% 5% 5% 7% 6% 46%	2661
	21st to end	Day	5% 6% 3 6% 5% 5% 5% 4 5% 8% 49%	2730
May	1st to 10th	Day	5% 6% 2 5% 4 4 4 4 5% 7% 54%	7626
	11th to 20th	Day	4 3 1 4 3 4 5% 5% 7% 9% 55%	7596
	21st to end	Day	3 3 2 5% 4 4 5% 4 7% 8% 55%	8370
June	1st to 10th	Day	2 3 1 4 3 3 4 4 7% 10% 58%	2730
	11th to 20th	Day	4 3 1 3 3 3 3 3 4 6% 67%	2730
	21st to end	Day	2 1 1 2 3 3 4 3 4 6% 72%	2730
July	1st to 10th	Day	1 1 1 2 3 2 4 3 5% 6% 72%	7649
	11th to 20th	Day	2 2 1 3 3 4 5% 5% 7% 10% 60%	7650
	21st to end	Day	2 1 1 3 2 3 4 4 5% 8% 68%	8304
August	1st to 10th	Day	1 1 1 2 2 2 3 3 5% 7% 74%	7549
	11th to 20th	Day	1 1 1 3 3 3 5% 4 6% 7% 66%	7565
	21st to end	Day	2 2 1 4 3 4 5% 5% 7% 10% 58%	8322
September	1st to 10th	Day	3 2 1 3 2 3 4 3 5% 6% 70%	2350
	11th to 20th	Day	3 2 1 3 3 3 4 4 6% 6% 65%	2234
	21st to end	Day	4 2 1 3 2 3 2 3 5% 6% 70%	2200
October	1st to 10th	Day	4 3 1 3 2 3 3 2 4 4 70%	3448
	11th to 20th	Day	5% 4 1 4 3 3 3 4 5% 7% 62%	3250
	21st to end	Day	5% 5% 1 5% 4 3 4 3 4 6% 60%	3527
November	1st to 10th	Day	7% 4 2 6% 4 4 5% 5% 6% 7% 51%	1854
	11th to 20th	Day	6% 7% 3 6% 4 4 4 5% 7% 7% 47%	1890
	21st to end	Day	3 6% 4 9% 6% 5% 5% 5% 6% 8% 42%	1807
December	1st to 10th	Day	3 5% 2 5% 4 4 3 3 8% 12% 51%	2700
	11th to 20th	Day	4 7% 2 7% 6% 5% 6% 6% 8% 9% 41%	3214
	21st to end	Day	4 9% 3 10% 6% 7% 7% 6% 9% 8% 30%	4609
January	1st to 10th	Night	3 7% 3 9% 7% 6% 6% 6% 8% 10% 35%	7605
	11th to 20th	Night	3 9% 3 9% 5% 4 5% 5% 8% 9% 42%	7650
	21st to end	Night	2 6% 3 6% 5% 5% 5% 5% 6% 9% 48%	8370
February	1st to 10th	Night	4 8% 3 7% 6% 5% 7% 6% 8% 10% 37%	2730
	11th to 20th	Night	4 9% 3 8% 5% 6% 6% 5% 5% 8% 42%	2566
	21st to end	Night	3 9% 3 7% 5% 4 6% 5% 6% 9% 43%	2076
March	1st to 10th	Night	4 9% 3 8% 6% 7% 6% 5% 6% 7% 40%	5188
	11th to 20th	Night	3 6% 3 7% 5% 5% 5% 5% 8% 9% 44%	5110
	21st to end	Night	4 7% 3 6% 5% 5% 5% 4 7% 8% 46%	5610
April	1st to 10th	Night	6% 8% 2 6% 4 4 4 3 5% 7% 52%	2302
	11th to 20th	Night	4 5% 2 5% 5% 4 7% 5% 6% 8% 50%	1903
	21st to end	Night	6% 5% 2 5% 4 4 5% 4 7% 6% 52%	1890
May	1st to 10th	Night	4 5% 1 5% 4 4 4 4 5% 7% 57%	5321
	11th to 20th	Night	3 4 1 3 2 3 3 3 5% 8% 64%	5296
	21st to end	Night	4 4 1 4 4 3 4 3 5% 8% 59%	5837
June	1st to 10th	Night	3 4 2 3 4 4 3 3 6% 7% 60%	1890
	11th to 20th	Night	3 3 1 3 2 3 4 3 4 6% 68%	1890
	21st to end	Night	2 3 1 3 3 3 4 3 5% 7% 67%	1890
July	1st to 10th	Night	1 1 1 4 4 4 4 3 5% 5% 69%	5340
	11th to 20th	Night	2 3 1 4 4 5% 6% 5% 8% 10% 54%	5340
	21st to end	Night	1 2 1 4 3 3 4 4 6% 7% 63%	5805
August	1st to 10th	Night	2 1 1 2 2 2 4 3 5% 7% 71%	5273
	11th to 20th	Night	2 2 1 5% 3 4 4 4 6% 8% 62%	5291
	21st to end	Night	3 2 1 4 4 5% 6% 5% 7% 8% 54%	5894
September	1st to 10th	Night	3 3 1 4 2 3 2 3 4 6% 69%	2266
	11th to 20th	Night	3 3 1 3 3 3 3 3 5% 7% 66%	2233
	21st to end	Night	5% 2 2 3 2 2 3 3 3 5% 71%	2200
October	1st to 10th	Night	3 2 1 3 2 2 3 2 3 4 74%	3746
	11th to 20th	Night	5% 3 1 4 3 3 4 3 4 7% 62%	3898
	21st to end	Night	5% 5% 2 4 2 3 3 4 5% 6% 62%	4271
November	1st to 10th	Night	6% 6% 2 5% 4 3 3 3 5% 8% 53%	2665
	11th to 20th	Night	5% 5% 2 5% 4 4 5% 4 7% 8% 53%	2730
	21st to end	Night	5% 7% 2 5% 5% 4 5% 5% 7% 8% 47%	2785
December	1st to 10th	Night	3 4 2 5% 4 3 3 3 9% 10% 53%	4710
	11th to 20th	Night	5% 7% 2 6% 5% 4 5% 4 6% 8% 48%	5250
	21st to end	Night	5% 9% 3 9% 6% 7% 8% 5% 8% 8% 32%	6611

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	42% 12% 24% 16% 6%	5321
	11th to 20th	Day	38% 9% 18% 23% 9% 2	5340
	21st to end	Day	33% 9% 18% 24% 15% 1	5844
February	1st to 10th	Day	48% 9% 14% 17% 11% 1	1890
	11th to 20th	Day	45% 9% 15% 20% 11% 1	1948
	21st to end	Day	41% 8% 16% 22% 12% 1	1730
March	1st to 10th	Day	36% 9% 13% 25% 14% 4	4925
	11th to 20th	Day	35% 5% 15% 29% 14% 2	5110
	21st to end	Day	44% 9% 17% 19% 9% 1	5607
April	1st to 10th	Day	35% 7% 14% 21% 17% 6%	2317
	11th to 20th	Day	33% 6% 13% 21% 21% 5%	2664
	21st to end	Day	30% 7% 12% 24% 23% 4	2730
May	1st to 10th	Day	26% 4 11% 22% 26% 10%	7626
	11th to 20th	Day	23% 4 11% 21% 32% 9%	7599
	21st to end	Day	23% 5% 10% 20% 35% 7%	8371
June	1st to 10th	Day	20% 5% 9% 19% 34% 14%	2730
	11th to 20th	Day	18% 3 7% 14% 29% 29%	2730
	21st to end	Day	14% 2 6% 10% 28% 40%	2730
July	1st to 10th	Day	13% 1 5% 6% 16% 59%	7586
	11th to 20th	Day	17% 3 11% 13% 22% 34%	7581
	21st to end	Day	14% 3 7% 10% 22% 44%	8120
August	1st to 10th	Day	15% 3 6% 14% 22% 40%	5978
	11th to 20th	Day	19% 5% 8% 21% 27% 20%	5964
	21st to end	Day	17% 5% 9% 20% 25% 24%	6529
September	1st to 10th	Day	14% 3 8% 14% 29% 32%	2350
	11th to 20th	Day	17% 5% 9% 17% 28% 24%	2234
	21st to end	Day	16% 4 7% 19% 31% 22%	2200
October	1st to 10th	Day	18% 4 6% 13% 39% 21%	3448
	11th to 20th	Day	24% 3 9% 26% 28% 10%	3250
	21st to end	Day	25% 5% 11% 29% 26% 4	3527
November	1st to 10th	Day	31% 6% 13% 24% 20% 4	1854
	11th to 20th	Day	34% 8% 13% 28% 16% 1	1890
	21st to end	Day	37% 9% 17% 23% 13% 1	1807
December	1st to 10th	Day	28% 11% 17% 35% 8% 1	2700
	11th to 20th	Day	36% 11% 19% 26% 8%	3212
	21st to end	Day	45% 11% 16% 18% 9%	4609
January	1st to 10th	Night	40% 15% 23% 17% 6%	7605
	11th to 20th	Night	37% 13% 17% 25% 8% 1	7650
	21st to end	Night	32% 11% 18% 29% 10%	8370
February	1st to 10th	Night	40% 11% 18% 21% 9% 1	2730
	11th to 20th	Night	41% 7% 15% 23% 14% 1	2567
	21st to end	Night	37% 7% 14% 27% 13% 2	2076
March	1st to 10th	Night	41% 7% 12% 28% 12% 1	5188
	11th to 20th	Night	32% 7% 13% 29% 17% 2	5110
	21st to end	Night	33% 10% 18% 24% 13% 2	5610
April	1st to 10th	Night	33% 5% 12% 26% 19% 6%	2302
	11th to 20th	Night	30% 7% 10% 23% 26% 5%	1903
	21st to end	Night	30% 7% 13% 23% 23% 3	1890
May	1st to 10th	Night	24% 6% 11% 23% 31% 5%	5319
	11th to 20th	Night	19% 4 8% 23% 34% 11%	5297
	21st to end	Night	24% 4 8% 24% 36% 4	5838
June	1st to 10th	Night	22% 5% 9% 18% 31% 15%	1890
	11th to 20th	Night	16% 4 7% 15% 30% 27%	1890
	21st to end	Night	16% 3 6% 8% 28% 39%	1890
July	1st to 10th	Night	19% 2 6% 7% 16% 51%	5295
	11th to 20th	Night	23% 3 11% 11% 18% 33%	5286
	21st to end	Night	20% 3 8% 12% 22% 35%	5677
August	1st to 10th	Night	17% 3 7% 11% 21% 39%	4168
	11th to 20th	Night	24% 6% 9% 20% 21% 19%	4168
	21st to end	Night	23% 6% 12% 14% 22% 24%	4618
September	1st to 10th	Night	17% 3 8% 14% 25% 34%	2266
	11th to 20th	Night	19% 5% 8% 17% 27% 25%	2233
	21st to end	Night	18% 4 5% 16% 32% 25%	2200
October	1st to 10th	Night	15% 3 6% 16% 37% 23%	3746
	11th to 20th	Night	24% 4 7% 27% 29% 9%	3898
	21st to end	Night	22% 4 11% 34% 24% 4	4271
November	1st to 10th	Night	28% 6% 11% 28% 23% 5%	2665
	11th to 20th	Night	28% 7% 15% 31% 16% 3	2730
	21st to end	Night	32% 10% 15% 27% 14% 2	2785
December	1st to 10th	Night	24% 14% 24% 27% 9% 1	4710
	11th to 20th	Night	35% 8% 27% 23% 7%	5251
	21st to end	Night	46% 9% 19% 18% 7%	6611

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	14% 10% 18% 34% 16% 8%	330
	11th to 20th	Day	8% 6% 19% 36% 22% 8%	330
	21st to end	Day	10% 4 17% 40% 22% 6%	363
February	1st to 10th	Day	12% 10% 25% 33% 15% 5%	330
	11th to 20th	Day	9% 8% 27% 35% 18% 3	424
	21st to end	Day	7% 9% 26% 40% 16% 3	446
March	1st to 10th	Day	11% 9% 24% 36% 16% 5%	540
	11th to 20th	Day	7% 10% 29% 31% 19% 4	540
	21st to end	Day	13% 12% 27% 31% 15% 2	594
April	1st to 10th	Day	11% 9% 26% 29% 20% 6%	540
	11th to 20th	Day	10% 12% 28% 30% 17% 3	532
	21st to end	Day	10% 12% 25% 29% 19% 4	540
May	1st to 10th	Day	7% 9% 26% 32% 18% 8%	543
	11th to 20th	Day	8% 7% 24% 31% 23% 7%	568
	21st to end	Day	6% 7% 21% 26% 28% 12%	617
June	1st to 10th	Day	3 7% 19% 29% 32% 10%	570
	11th to 20th	Day	6% 6% 22% 35% 25% 6%	570
	21st to end	Day	2 5% 22% 35% 29% 7%	570
July	1st to 10th	Day	1 2 15% 36% 35% 10%	566
	11th to 20th	Day	4 2 12% 33% 36% 13%	564
	21st to end	Day	3 1 9% 33% 43% 12%	619
August	1st to 10th	Day	4 2 14% 34% 36% 11%	567
	11th to 20th	Day	3 3 13% 32% 36% 13%	557
	21st to end	Day	4 3 12% 30% 40% 11%	627
September	1st to 10th	Day	3 4 15% 30% 32% 16%	570
	11th to 20th	Day	4 5% 19% 27% 31% 13%	543
	21st to end	Day	5% 5% 15% 25% 35% 15%	530
October	1st to 10th	Day	5% 5% 20% 27% 26% 17%	443
	11th to 20th	Day	7% 5% 16% 32% 27% 13%	330
	21st to end	Day	6% 7% 19% 33% 27% 8%	363
November	1st to 10th	Day	11% 8% 15% 31% 24% 11%	330
	11th to 20th	Day	7% 7% 16% 38% 22% 9%	330
	21st to end	Day	10% 5% 16% 35% 25% 9%	330
December	1st to 10th	Day	7% 8% 14% 36% 25% 10%	330
	11th to 20th	Day	9% 6% 16% 42% 23% 4	330
	21st to end	Day	14% 9% 21% 33% 21% 3	363
January	1st to 10th	Night	12% 5% 15% 42% 21% 4	840
	11th to 20th	Night	11% 5% 17% 36% 26% 5%	840
	21st to end	Night	11% 5% 14% 43% 23% 4	924
February	1st to 10th	Night	10% 5% 20% 46% 15% 4	840
	11th to 20th	Night	11% 5% 19% 46% 18% 2	824
	21st to end	Night	9% 4 21% 46% 18% 1	692
March	1st to 10th	Night	10% 7% 20% 44% 17% 3	840
	11th to 20th	Night	8% 5% 18% 44% 23% 2	840
	21st to end	Night	10% 6% 21% 44% 16% 2	924
April	1st to 10th	Night	12% 7% 19% 41% 18% 3	832
	11th to 20th	Night	6% 8% 23% 48% 14% 1	623
	21st to end	Night	8% 6% 21% 40% 23% 1	630
May	1st to 10th	Night	8% 6% 22% 42% 21% 2	630
	11th to 20th	Night	6% 5% 16% 40% 30% 3	621
	21st to end	Night	8% 6% 17% 36% 29% 4	683
June	1st to 10th	Night	5% 6% 17% 40% 30% 2	630
	11th to 20th	Night	4 6% 21% 42% 26% 1	630
	21st to end	Night	3 6% 15% 42% 32% 2	630
July	1st to 10th	Night	3 1 13% 38% 40% 4	615
	11th to 20th	Night	4 1 8% 43% 39% 4	612
	21st to end	Night	4 1 7% 34% 51% 3	690
August	1st to 10th	Night	4 1 12% 41% 40% 2	627
	11th to 20th	Night	3 1 9% 44% 39% 4	624
	21st to end	Night	2 2 9% 39% 44% 3	693
September	1st to 10th	Night	4 2 11% 39% 39% 6%	796
	11th to 20th	Night	4 2 14% 37% 36% 7%	812
	21st to end	Night	7% 3 11% 33% 38% 9%	800
October	1st to 10th	Night	7% 3 15% 40% 29% 6%	800
	11th to 20th	Night	6% 4 14% 41% 30% 5%	800
	21st to end	Night	6% 4 16% 41% 29% 4	880
November	1st to 10th	Night	10% 5% 16% 36% 29% 4	820
	11th to 20th	Night	7% 4 17% 47% 20% 4	840
	21st to end	Night	9% 4 16% 41% 24% 5%	895
December	1st to 10th	Night	11% 4 15% 45% 22% 3	1050
	11th to 20th	Night	15% 3 16% 43% 19% 3	1018
	21st to end	Night	15% 6% 19% 43% 16% 2	924

Page generated at 2026-09-11 02:11:55. Version ac07

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