Crystal Mountain
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Crystal Mountain clear sky chart from 2005-11-06 to 2026-05-17.

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	12% 6% 2 6% 4 4 4 4 5% 7% 45%	131203
	Night	12% 6% 2 5% 3 3 3 3 5% 6% 50%	131848
Transparency	Day	38% 5% 10% 23% 19% 5%	134809
	Night	34% 4 9% 23% 21% 7%	136319
Seeing	Day	16% 12% 25% 25% 17% 5%	17344
	Night	16% 10% 25% 30% 17% 2	27229

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	14% 10% 3 5% 4 3 3 3 4 5% 46%	5289
	11th to 20th	Day	7% 9% 3 6% 4 3 3 2 3 4 55%	5310
	21st to end	Day	20% 7% 2 6% 3 2 2 2 3 5% 49%	5812
February	1st to 10th	Day	20% 9% 2 5% 4 3 3 3 3 4 43%	1890
	11th to 20th	Day	16% 9% 4 6% 5% 3 4 3 5% 5% 42%	1949
	21st to end	Day	19% 9% 2 7% 4 4 3 3 4 5% 40%	1830
March	1st to 10th	Day	20% 10% 2 5% 4 4 3 3 4 5% 40%	5046
	11th to 20th	Day	11% 5% 2 5% 4 3 4 3 5% 6% 52%	5090
	21st to end	Day	13% 7% 3 7% 5% 4 4 3 5% 6% 42%	5583
April	1st to 10th	Day	14% 7% 2 6% 5% 3 4 4 5% 6% 44%	2488
	11th to 20th	Day	16% 7% 2 6% 4 4 3 3 4 6% 45%	2494
	21st to end	Day	16% 7% 3 6% 6% 4 5% 4 5% 6% 38%	2520
May	1st to 10th	Day	18% 8% 3 7% 4 4 4 4 5% 6% 36%	5568
	11th to 20th	Day	14% 6% 2 7% 5% 5% 5% 4 6% 6% 39%	5941
	21st to end	Day	17% 7% 3 8% 6% 5% 5% 4 6% 6% 34%	6434
June	1st to 10th	Day	9% 6% 2 7% 5% 5% 5% 4 7% 9% 41%	2600
	11th to 20th	Day	5% 4 2 6% 6% 6% 5% 6% 8% 10% 41%	2600
	21st to end	Day	4 4 1 5% 5% 6% 6% 5% 7% 10% 48%	2600
July	1st to 10th	Day	7% 6% 2 7% 6% 6% 6% 6% 8% 11% 35%	5900
	11th to 20th	Day	4 3 2 4 4 5% 5% 4 7% 9% 52%	5900
	21st to end	Day	8% 6% 3 7% 5% 5% 6% 5% 7% 9% 38%	6422
August	1st to 10th	Day	4 2 1 4 3 4 4 4 6% 8% 59%	4234
	11th to 20th	Day	10% 5% 2 7% 4 4 5% 5% 8% 9% 41%	3920
	21st to end	Day	6% 3 2 5% 4 5% 6% 5% 7% 8% 49%	4312
September	1st to 10th	Day	12% 6% 2 6% 5% 4 5% 4 5% 6% 46%	2400
	11th to 20th	Day	15% 4 2 6% 4 5% 5% 4 6% 6% 44%	2356
	21st to end	Day	11% 4 2 5% 4 4 4 3 4 6% 53%	2105
October	1st to 10th	Day	7% 4 2 5% 3 3 4 3 5% 6% 58%	2000
	11th to 20th	Day	13% 5% 2 4 2 2 2 2 3 5% 59%	2000
	21st to end	Day	17% 6% 2 4 3 2 3 2 4 5% 51%	2200
November	1st to 10th	Day	11% 6% 2 4 3 3 4 2 4 5% 56%	2058
	11th to 20th	Day	10% 5% 2 5% 3 3 3 3 5% 7% 55%	2100
	21st to end	Day	15% 9% 3 6% 3 3 3 2 4 5% 48%	1811
December	1st to 10th	Day	15% 10% 3 6% 3 3 2 3 3 5% 47%	2690
	11th to 20th	Day	13% 10% 3 8% 5% 4 3 3 3 5% 43%	3164
	21st to end	Day	17% 9% 2 7% 4 4 4 3 4 4 41%	4587
January	1st to 10th	Night	14% 8% 3 7% 4 3 3 3 4 5% 45%	7634
	11th to 20th	Night	9% 5% 1 4 2 2 2 2 3 4 66%	7680
	21st to end	Night	21% 9% 2 6% 3 3 3 2 3 4 45%	8402
February	1st to 10th	Night	19% 7% 3 6% 4 3 3 3 5% 5% 42%	2730
	11th to 20th	Night	17% 8% 3 6% 4 4 3 3 4 4 45%	2556
	21st to end	Night	16% 8% 2 4 3 3 3 2 4 5% 49%	1994
March	1st to 10th	Night	17% 9% 3 6% 3 3 3 3 4 5% 43%	5029
	11th to 20th	Night	11% 6% 2 5% 3 4 3 3 5% 5% 52%	5090
	21st to end	Night	10% 7% 3 7% 4 4 4 3 5% 8% 45%	5588
April	1st to 10th	Night	13% 5% 2 6% 4 3 3 3 5% 6% 50%	2131
	11th to 20th	Night	11% 6% 2 6% 4 4 3 3 5% 6% 50%	2075
	21st to end	Night	13% 6% 2 6% 4 4 4 3 5% 6% 47%	2100
May	1st to 10th	Night	18% 8% 3 5% 5% 4 4 4 5% 7% 38%	4532
	11th to 20th	Night	16% 8% 3 6% 4 4 4 4 6% 7% 38%	4107
	21st to end	Night	18% 8% 2 7% 4 4 4 3 5% 6% 40%	4443
June	1st to 10th	Night	6% 3 1 4 3 3 4 4 7% 10% 54%	1800
	11th to 20th	Night	4 4 1 5% 3 3 4 3 5% 9% 57%	1800
	21st to end	Night	4 2 1 3 3 3 4 4 5% 9% 62%	1800
July	1st to 10th	Night	7% 4 2 7% 5% 5% 5% 4 8% 10% 42%	4078
	11th to 20th	Night	4 2 1 3 3 3 4 4 6% 10% 61%	4077
	21st to end	Night	6% 4 2 6% 5% 4 5% 5% 7% 9% 48%	4449
August	1st to 10th	Night	3 2 1 3 3 3 3 3 5% 8% 68%	2926
	11th to 20th	Night	5% 3 1 4 3 4 4 4 6% 9% 58%	3131
	21st to end	Night	4 3 1 4 3 4 5% 4 7% 8% 57%	3586
September	1st to 10th	Night	8% 4 2 5% 3 3 4 3 5% 7% 56%	2000
	11th to 20th	Night	11% 5% 2 4 3 3 4 3 5% 7% 52%	2039
	21st to end	Night	9% 4 2 3 3 2 3 4 6% 6% 59%	2273
October	1st to 10th	Night	7% 3 2 3 3 3 4 2 4 7% 62%	2400
	11th to 20th	Night	10% 3 2 4 2 3 3 2 3 5% 62%	2400
	21st to end	Night	17% 6% 2 5% 3 3 3 2 2 4 55%	2640
November	1st to 10th	Night	11% 6% 2 4 3 2 2 2 4 6% 57%	2460
	11th to 20th	Night	10% 5% 2 4 3 3 3 3 4 6% 57%	2520
	21st to end	Night	13% 6% 2 5% 4 4 3 2 4 5% 52%	2798
December	1st to 10th	Night	13% 8% 2 6% 3 3 3 3 4 5% 51%	4720
	11th to 20th	Night	12% 8% 4 8% 5% 4 3 3 3 4 46%	5227
	21st to end	Night	16% 8% 3 6% 4 3 3 3 4 6% 44%	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	42% 3 8% 27% 18% 2	5290
	11th to 20th	Day	35% 3 6% 19% 32% 7%	5310
	21st to end	Day	41% 2 4 18% 24% 10%	5812
February	1st to 10th	Day	46% 4 8% 21% 16% 4	1890
	11th to 20th	Day	45% 4 8% 19% 19% 5%	1958
	21st to end	Day	48% 4 7% 14% 18% 10%	1830
March	1st to 10th	Day	48% 4 6% 15% 15% 12%	5046
	11th to 20th	Day	32% 5% 7% 18% 23% 14%	5090
	21st to end	Day	43% 4 9% 23% 19% 1	5585
April	1st to 10th	Day	40% 6% 9% 20% 20% 5%	2488
	11th to 20th	Day	42% 5% 9% 20% 17% 7%	2494
	21st to end	Day	45% 7% 9% 19% 18% 2	2520
May	1st to 10th	Day	47% 6% 9% 22% 13% 2	5603
	11th to 20th	Day	43% 8% 11% 22% 14% 1	5945
	21st to end	Day	47% 8% 12% 17% 14% 3	6447
June	1st to 10th	Day	36% 10% 14% 22% 16% 2	2600
	11th to 20th	Day	35% 8% 14% 24% 17% 1	2600
	21st to end	Day	29% 9% 13% 27% 19% 3	2600
July	1st to 10th	Day	40% 6% 17% 27% 9% 1	5836
	11th to 20th	Day	27% 5% 16% 36% 15% 2	5831
	21st to end	Day	40% 6% 15% 26% 13%	6240
August	1st to 10th	Day	22% 6% 15% 28% 23% 6%	4220
	11th to 20th	Day	37% 6% 14% 31% 11% 1	3957
	21st to end	Day	31% 6% 12% 30% 18% 3	4311
September	1st to 10th	Day	37% 8% 11% 20% 18% 7%	2400
	11th to 20th	Day	38% 6% 12% 18% 20% 5%	2356
	21st to end	Day	32% 6% 8% 20% 26% 8%	2105
October	1st to 10th	Day	16% 3 6% 12% 36% 27%	3260
	11th to 20th	Day	19% 2 4 19% 36% 19%	3240
	21st to end	Day	33% 3 4 29% 24% 7%	3538
November	1st to 10th	Day	31% 5% 8% 25% 23% 8%	2058
	11th to 20th	Day	30% 4 8% 23% 26% 10%	2100
	21st to end	Day	42% 3 6% 19% 25% 6%	1811
December	1st to 10th	Day	42% 3 6% 26% 20% 3	2690
	11th to 20th	Day	45% 3 5% 21% 21% 4	3161
	21st to end	Day	44% 5% 8% 23% 16% 4	4587
January	1st to 10th	Night	40% 3 7% 27% 21% 2	7635
	11th to 20th	Night	25% 3 6% 19% 36% 12%	7680
	21st to end	Night	45% 3 6% 17% 21% 8%	8402
February	1st to 10th	Night	44% 4 8% 20% 18% 5%	2730
	11th to 20th	Night	43% 3 7% 21% 20% 6%	2557
	21st to end	Night	39% 4 8% 16% 23% 10%	1994
March	1st to 10th	Night	43% 4 7% 13% 19% 13%	5029
	11th to 20th	Night	34% 4 7% 22% 18% 15%	5090
	21st to end	Night	38% 5% 9% 25% 20% 3	5588
April	1st to 10th	Night	35% 5% 10% 23% 19% 9%	2131
	11th to 20th	Night	33% 5% 10% 22% 21% 9%	2075
	21st to end	Night	37% 5% 12% 24% 19% 3	2100
May	1st to 10th	Night	46% 7% 11% 21% 12% 2	4562
	11th to 20th	Night	46% 6% 13% 23% 11% 1	4108
	21st to end	Night	44% 7% 12% 19% 13% 5%	4452
June	1st to 10th	Night	24% 9% 16% 30% 20% 2	1800
	11th to 20th	Night	24% 6% 14% 31% 23% 3	1800
	21st to end	Night	19% 7% 13% 32% 24% 5%	1800
July	1st to 10th	Night	36% 6% 17% 27% 13% 1	4035
	11th to 20th	Night	19% 5% 16% 40% 18% 2	4026
	21st to end	Night	32% 5% 16% 28% 17% 1	4320
August	1st to 10th	Night	14% 5% 17% 33% 25% 5%	2916
	11th to 20th	Night	23% 6% 18% 36% 15% 1	3160
	21st to end	Night	23% 6% 14% 30% 25% 2	3586
September	1st to 10th	Night	27% 6% 12% 25% 22% 9%	2000
	11th to 20th	Night	31% 7% 12% 20% 24% 6%	2039
	21st to end	Night	25% 5% 11% 21% 29% 9%	2273
October	1st to 10th	Night	16% 3 7% 13% 28% 32%	3920
	11th to 20th	Night	16% 3 5% 16% 37% 23%	3899
	21st to end	Night	29% 2 4 25% 28% 12%	4252
November	1st to 10th	Night	30% 4 8% 22% 26% 10%	2460
	11th to 20th	Night	30% 4 7% 23% 25% 11%	2520
	21st to end	Night	36% 4 7% 19% 27% 6%	2798
December	1st to 10th	Night	38% 3 7% 31% 18% 4	4720
	11th to 20th	Night	45% 3 5% 19% 20% 8%	5229
	21st to end	Night	42% 4 9% 25% 16% 3	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	28% 18% 32% 15% 5% 1	330
	11th to 20th	Day	23% 29% 35% 11% 2	330
	21st to end	Day	26% 28% 30% 10% 4 1	363
February	1st to 10th	Day	30% 19% 32% 14% 4 1	330
	11th to 20th	Day	31% 27% 28% 12% 1	330
	21st to end	Day	34% 27% 28% 9% 1	273
March	1st to 10th	Day	29% 20% 32% 14% 4	330
	11th to 20th	Day	31% 23% 35% 10% 1	330
	21st to end	Day	26% 24% 39% 9% 2	363
April	1st to 10th	Day	24% 20% 33% 16% 6%	508
	11th to 20th	Day	28% 18% 31% 17% 5% 1	532
	21st to end	Day	21% 19% 34% 17% 7% 2	540
May	1st to 10th	Day	22% 16% 32% 23% 7% 1	560
	11th to 20th	Day	16% 12% 30% 26% 14% 2	756
	21st to end	Day	12% 11% 29% 30% 14% 5%	789
June	1st to 10th	Day	10% 6% 22% 29% 26% 6%	730
	11th to 20th	Day	6% 5% 22% 39% 22% 6%	730
	21st to end	Day	5% 4 22% 39% 23% 7%	730
July	1st to 10th	Day	7% 3 16% 33% 32% 10%	721
	11th to 20th	Day	6% 2 16% 32% 34% 10%	718
	21st to end	Day	8% 2 13% 33% 32% 12%	794
August	1st to 10th	Day	6% 3 15% 38% 31% 7%	726
	11th to 20th	Day	12% 2 10% 33% 32% 10%	566
	21st to end	Day	12% 3 8% 31% 33% 14%	583
September	1st to 10th	Day	12% 3 12% 30% 35% 9%	530
	11th to 20th	Day	12% 5% 13% 25% 35% 11%	486
	21st to end	Day	14% 6% 20% 37% 18% 5%	330
October	1st to 10th	Day	5% 5% 31% 39% 18% 3	330
	11th to 20th	Day	15% 13% 33% 25% 12% 2	330
	21st to end	Day	28% 15% 30% 19% 7% 1	363
November	1st to 10th	Day	20% 15% 34% 18% 12% 2	330
	11th to 20th	Day	15% 17% 40% 23% 5% 1	330
	21st to end	Day	28% 18% 26% 19% 9%	330
December	1st to 10th	Day	25% 25% 27% 16% 6% 1	330
	11th to 20th	Day	24% 21% 33% 18% 5%	330
	21st to end	Day	27% 18% 38% 14% 2	363
January	1st to 10th	Night	22% 18% 30% 27% 4	1050
	11th to 20th	Night	22% 19% 32% 23% 3	1050
	21st to end	Night	22% 15% 36% 25% 3	1155
February	1st to 10th	Night	25% 16% 30% 23% 4	1050
	11th to 20th	Night	24% 20% 34% 21% 1	922
	21st to end	Night	25% 19% 33% 21% 2	610
March	1st to 10th	Night	22% 15% 30% 28% 5%	630
	11th to 20th	Night	24% 14% 31% 27% 4	630
	21st to end	Night	19% 13% 35% 28% 5%	693
April	1st to 10th	Night	17% 15% 34% 27% 6%	630
	11th to 20th	Night	15% 16% 33% 26% 10%	623
	21st to end	Night	16% 13% 36% 25% 10%	630
May	1st to 10th	Night	14% 8% 28% 40% 9% 1	630
	11th to 20th	Night	13% 10% 27% 31% 18% 1	609
	21st to end	Night	11% 6% 23% 34% 21% 5%	650
June	1st to 10th	Night	7% 2 16% 37% 34% 5%	600
	11th to 20th	Night	6% 2 15% 41% 34% 3	600
	21st to end	Night	4 1 13% 38% 38% 6%	600
July	1st to 10th	Night	4 2 10% 35% 41% 8%	585
	11th to 20th	Night	3 1 12% 32% 43% 8%	582
	21st to end	Night	5% 1 6% 38% 39% 10%	657
August	1st to 10th	Night	5% 1 9% 40% 40% 6%	597
	11th to 20th	Night	8% 2 9% 37% 37% 7%	594
	21st to end	Night	7% 1 9% 34% 42% 7%	660
September	1st to 10th	Night	9% 2 11% 32% 43% 4	600
	11th to 20th	Night	13% 1 11% 31% 39% 5%	600
	21st to end	Night	10% 3 16% 32% 34% 5%	673
October	1st to 10th	Night	9% 3 17% 38% 31% 3	800
	11th to 20th	Night	12% 7% 24% 40% 16% 1	800
	21st to end	Night	21% 9% 27% 27% 14% 1	880
November	1st to 10th	Night	13% 11% 25% 35% 14% 2	820
	11th to 20th	Night	14% 11% 32% 32% 11%	840
	21st to end	Night	19% 16% 29% 26% 11%	924
December	1st to 10th	Night	21% 12% 34% 27% 6%	1050
	11th to 20th	Night	21% 15% 30% 28% 6%	1050
	21st to end	Night	23% 17% 29% 26% 5%	1155

Page generated at 2026-05-14 00:58:58. Version ac07

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