Glass Mountains
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Glass Mountains clear sky chart from 2005-11-06 to 2025-07-21.

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	8% 5% 2 5% 3 3 4 3 5% 6% 56%	120371
	Night	8% 5% 2 4 3 3 3 3 5% 7% 58%	119022
Transparency	Day	28% 5% 10% 24% 24% 9%	124145
	Night	25% 5% 10% 24% 26% 10%	123367
Seeing	Day	10% 9% 28% 32% 18% 4	16416
	Night	10% 9% 34% 36% 11% 1	26294

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	10% 5% 2 9% 4 4 4 4 4 6% 47%	4077
	11th to 20th	Day	5% 6% 3 5% 2 3 3 3 4 6% 62%	4080
	21st to end	Day	12% 7% 3 5% 3 3 3 2 3 5% 54%	4488
February	1st to 10th	Day	8% 11% 3 7% 3 4 3 2 4 5% 51%	1940
	11th to 20th	Day	7% 7% 2 6% 4 3 3 2 3 5% 57%	1949
	21st to end	Day	7% 6% 2 5% 3 3 3 3 4 5% 58%	1650
March	1st to 10th	Day	9% 7% 2 4 2 2 2 2 3 4 61%	4689
	11th to 20th	Day	6% 6% 2 5% 3 4 4 3 5% 5% 58%	4980
	21st to end	Day	8% 6% 3 6% 4 4 4 3 5% 6% 50%	5462
April	1st to 10th	Day	8% 4 2 5% 3 2 3 3 5% 7% 56%	2215
	11th to 20th	Day	13% 6% 2 6% 4 3 4 3 5% 7% 48%	2427
	21st to end	Day	14% 5% 2 5% 3 3 4 3 5% 6% 51%	2600
May	1st to 10th	Day	17% 7% 2 5% 4 4 4 3 5% 6% 44%	5862
	11th to 20th	Day	8% 4 1 4 3 3 3 3 4 7% 59%	5862
	21st to end	Day	12% 7% 3 6% 4 4 3 4 4 5% 48%	6434
June	1st to 10th	Day	5% 4 1 4 4 4 4 4 7% 11% 53%	2600
	11th to 20th	Day	5% 3 2 4 4 4 5% 3 6% 8% 57%	2600
	21st to end	Day	5% 3 2 4 3 4 4 4 6% 8% 58%	2600
July	1st to 10th	Day	8% 5% 3 5% 4 4 4 3 5% 8% 51%	4250
	11th to 20th	Day	4 3 1 5% 3 4 4 4 6% 9% 58%	4250
	21st to end	Day	4 2 1 4 3 4 4 4 6% 9% 60%	4466
August	1st to 10th	Day	7% 3 2 5% 3 4 4 4 6% 9% 54%	4104
	11th to 20th	Day	5% 4 2 4 3 3 4 3 6% 8% 56%	4041
	21st to end	Day	4 4 1 4 3 3 3 3 5% 7% 63%	4187
September	1st to 10th	Day	8% 5% 1 4 3 4 4 3 4 6% 58%	2104
	11th to 20th	Day	10% 4 2 5% 3 4 6% 4 4 6% 52%	2090
	21st to end	Day	7% 3 1 4 3 4 3 3 5% 6% 60%	2090
October	1st to 10th	Day	9% 4 2 3 3 3 4 2 4 6% 61%	2090
	11th to 20th	Day	8% 3 1 3 2 2 2 2 3 3 70%	1914
	21st to end	Day	10% 5% 1 5% 3 2 2 3 3 4 62%	2088
November	1st to 10th	Day	7% 4 2 3 2 3 3 2 4 6% 65%	1763
	11th to 20th	Day	6% 4 2 3 2 2 4 2 4 6% 65%	1800
	21st to end	Day	9% 4 2 4 2 3 3 2 4 7% 62%	1800
December	1st to 10th	Day	4 4 1 4 3 2 2 2 4 5% 67%	2940
	11th to 20th	Day	6% 4 2 3 3 2 3 2 4 6% 64%	3391
	21st to end	Day	13% 9% 3 8% 5% 4 4 3 4 5% 44%	4488
January	1st to 10th	Night	11% 7% 2 6% 3 3 4 3 4 6% 51%	5888
	11th to 20th	Night	3 3 1 4 4 4 4 3 4 6% 63%	5900
	21st to end	Night	13% 7% 2 5% 3 2 3 2 4 5% 55%	6490
February	1st to 10th	Night	9% 9% 3 6% 3 3 2 3 5% 5% 51%	2475
	11th to 20th	Night	10% 7% 2 5% 4 3 3 3 5% 6% 53%	2338
	21st to end	Night	6% 5% 3 4 3 3 3 4 4 6% 59%	1980
March	1st to 10th	Night	5% 6% 2 5% 3 3 3 3 5% 6% 62%	5174
	11th to 20th	Night	6% 3 1 4 2 3 3 3 4 6% 65%	4980
	21st to end	Night	6% 6% 2 5% 4 5% 4 4 5% 7% 54%	5467
April	1st to 10th	Night	10% 5% 1 3 3 3 3 2 5% 8% 58%	2180
	11th to 20th	Night	12% 5% 1 5% 3 4 3 2 5% 7% 53%	1924
	21st to end	Night	13% 5% 2 3 3 3 4 3 5% 7% 51%	1800
May	1st to 10th	Night	17% 7% 2 6% 3 3 4 3 5% 5% 47%	4053
	11th to 20th	Night	7% 3 2 4 3 3 4 3 6% 8% 58%	4053
	21st to end	Night	11% 5% 2 4 3 3 3 3 5% 7% 53%	4443
June	1st to 10th	Night	6% 3 1 4 3 4 4 4 7% 10% 54%	1800
	11th to 20th	Night	5% 3 1 4 3 3 4 4 6% 11% 55%	1800
	21st to end	Night	5% 3 1 2 3 3 4 3 5% 9% 61%	1800
July	1st to 10th	Night	5% 5% 2 4 3 3 4 4 6% 10% 54%	2940
	11th to 20th	Night	3 3 1 4 3 4 4 4 5% 11% 60%	2940
	21st to end	Night	3 2 1 3 2 3 4 3 7% 10% 61%	3098
August	1st to 10th	Night	4 2 1 4 4 3 5% 5% 7% 10% 56%	2836
	11th to 20th	Night	5% 3 1 5% 4 5% 5% 4 7% 10% 51%	2798
	21st to end	Night	3 2 1 3 3 4 4 3 5% 9% 63%	3477
September	1st to 10th	Night	8% 3 1 3 3 3 3 3 4 7% 62%	2075
	11th to 20th	Night	10% 5% 2 5% 3 3 3 3 6% 6% 54%	2090
	21st to end	Night	5% 3 2 5% 3 3 4 3 4 6% 61%	2090
October	1st to 10th	Night	8% 4 1 3 3 3 3 2 5% 6% 61%	2090
	11th to 20th	Night	8% 3 1 3 2 2 2 2 4 4 69%	2245
	21st to end	Night	9% 3 1 3 3 2 2 2 3 4 66%	2508
November	1st to 10th	Night	8% 4 1 3 2 2 2 2 4 6% 65%	2534
	11th to 20th	Night	5% 4 1 4 3 3 3 3 3 6% 66%	2600
	21st to end	Night	9% 5% 1 3 2 3 2 2 3 6% 63%	2600
December	1st to 10th	Night	5% 3 1 3 2 2 2 2 4 6% 70%	4250
	11th to 20th	Night	5% 4 2 4 3 3 2 2 3 4 67%	4816
	21st to end	Night	13% 9% 2 6% 3 3 3 3 4 5% 51%	6490

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	37% 5% 10% 23% 22% 3	4078
	11th to 20th	Day	25% 3 11% 19% 33% 9%	4080
	21st to end	Day	36% 3 5% 21% 31% 5%	4488
February	1st to 10th	Day	37% 3 11% 22% 20% 6%	1940
	11th to 20th	Day	32% 4 8% 27% 22% 7%	1959
	21st to end	Day	30% 3 7% 24% 25% 12%	1650
March	1st to 10th	Day	28% 3 6% 15% 20% 28%	4689
	11th to 20th	Day	27% 5% 7% 16% 28% 17%	4980
	21st to end	Day	33% 6% 10% 31% 18% 3	5464
April	1st to 10th	Day	28% 4 10% 24% 25% 9%	2215
	11th to 20th	Day	36% 4 10% 23% 18% 9%	2427
	21st to end	Day	34% 5% 11% 26% 20% 3	2600
May	1st to 10th	Day	41% 6% 13% 16% 18% 6%	5900
	11th to 20th	Day	25% 6% 11% 24% 22% 13%	5866
	21st to end	Day	38% 6% 10% 18% 18% 10%	6447
June	1st to 10th	Day	25% 7% 19% 28% 16% 6%	2600
	11th to 20th	Day	24% 6% 13% 26% 24% 7%	2600
	21st to end	Day	22% 7% 14% 25% 24% 9%	2600
July	1st to 10th	Day	31% 6% 10% 26% 21% 6%	4186
	11th to 20th	Day	22% 6% 11% 30% 26% 5%	4181
	21st to end	Day	19% 6% 13% 30% 28% 3	4326
August	1st to 10th	Day	26% 7% 11% 28% 23% 6%	4090
	11th to 20th	Day	24% 6% 14% 30% 22% 3	4080
	21st to end	Day	20% 5% 13% 33% 25% 5%	4186
September	1st to 10th	Day	28% 5% 9% 23% 23% 12%	2104
	11th to 20th	Day	31% 6% 10% 22% 25% 6%	2090
	21st to end	Day	24% 5% 10% 23% 28% 10%	2090
October	1st to 10th	Day	16% 3 7% 16% 43% 16%	3480
	11th to 20th	Day	16% 3 6% 24% 34% 16%	3154
	21st to end	Day	21% 3 7% 26% 32% 11%	3415
November	1st to 10th	Day	21% 4 8% 24% 31% 13%	1763
	11th to 20th	Day	22% 4 8% 27% 29% 11%	1800
	21st to end	Day	25% 2 7% 29% 27% 9%	1800
December	1st to 10th	Day	21% 3 7% 39% 27% 3	2940
	11th to 20th	Day	22% 3 7% 27% 33% 7%	3389
	21st to end	Day	42% 5% 8% 24% 16% 5%	4488
January	1st to 10th	Night	34% 4 12% 28% 18% 4	5888
	11th to 20th	Night	21% 5% 11% 20% 33% 10%	5900
	21st to end	Night	33% 3 6% 18% 33% 6%	6490
February	1st to 10th	Night	36% 4 8% 24% 22% 8%	2475
	11th to 20th	Night	33% 4 10% 23% 23% 8%	2338
	21st to end	Night	26% 4 7% 21% 26% 16%	1980
March	1st to 10th	Night	23% 4 6% 18% 24% 25%	5174
	11th to 20th	Night	21% 4 7% 20% 24% 24%	4980
	21st to end	Night	29% 6% 11% 29% 22% 4	5467
April	1st to 10th	Night	26% 5% 10% 22% 25% 12%	2180
	11th to 20th	Night	31% 5% 11% 25% 18% 10%	1924
	21st to end	Night	31% 6% 11% 26% 22% 4	1800
May	1st to 10th	Night	39% 6% 12% 15% 20% 8%	4080
	11th to 20th	Night	24% 6% 12% 22% 24% 12%	4054
	21st to end	Night	30% 6% 12% 22% 19% 12%	4452
June	1st to 10th	Night	23% 8% 17% 30% 17% 5%	1800
	11th to 20th	Night	23% 6% 13% 29% 22% 7%	1800
	21st to end	Night	19% 5% 14% 29% 22% 11%	1800
July	1st to 10th	Night	25% 6% 15% 27% 24% 4	2895
	11th to 20th	Night	18% 5% 11% 28% 32% 6%	2886
	21st to end	Night	18% 5% 15% 30% 30% 3	2999
August	1st to 10th	Night	20% 7% 14% 25% 26% 8%	2826
	11th to 20th	Night	25% 6% 14% 27% 23% 5%	2825
	21st to end	Night	19% 6% 13% 36% 22% 5%	3477
September	1st to 10th	Night	23% 4 9% 25% 27% 11%	2075
	11th to 20th	Night	29% 6% 10% 20% 26% 8%	2090
	21st to end	Night	23% 6% 8% 23% 29% 11%	2090
October	1st to 10th	Night	16% 4 7% 15% 36% 22%	3480
	11th to 20th	Night	14% 3 5% 22% 37% 18%	3731
	21st to end	Night	16% 3 6% 25% 39% 12%	4120
November	1st to 10th	Night	21% 3 10% 22% 29% 15%	2534
	11th to 20th	Night	21% 4 8% 24% 31% 11%	2600
	21st to end	Night	25% 3 7% 25% 29% 12%	2600
December	1st to 10th	Night	17% 3 7% 41% 27% 4	4250
	11th to 20th	Night	23% 3 5% 27% 32% 10%	4817
	21st to end	Night	35% 5% 9% 26% 20% 6%	6490

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	12% 11% 39% 27% 10% 1	300
	11th to 20th	Day	13% 16% 37% 28% 5%	300
	21st to end	Day	9% 13% 44% 27% 6%	330
February	1st to 10th	Day	14% 16% 39% 26% 5%	300
	11th to 20th	Day	15% 16% 42% 20% 7%	300
	21st to end	Day	12% 17% 41% 25% 4 1	249
March	1st to 10th	Day	12% 16% 38% 27% 7%	300
	11th to 20th	Day	17% 19% 34% 23% 6%	300
	21st to end	Day	17% 18% 38% 23% 5%	330
April	1st to 10th	Day	16% 16% 40% 21% 7%	315
	11th to 20th	Day	13% 21% 39% 23% 3	545
	21st to end	Day	18% 16% 36% 22% 8% 1	700
May	1st to 10th	Day	14% 11% 39% 27% 9% 1	703
	11th to 20th	Day	15% 9% 29% 29% 16% 2	725
	21st to end	Day	13% 8% 32% 30% 14% 3	789
June	1st to 10th	Day	7% 5% 21% 40% 22% 5%	730
	11th to 20th	Day	5% 5% 28% 38% 21% 3	730
	21st to end	Day	4 3 27% 43% 19% 4	730
July	1st to 10th	Day	6% 2 13% 37% 32% 10%	721
	11th to 20th	Day	3 2 12% 43% 33% 8%	718
	21st to end	Day	5% 1 8% 39% 37% 10%	740
August	1st to 10th	Day	6% 1 13% 43% 32% 5%	676
	11th to 20th	Day	7% 2 14% 38% 34% 6%	665
	21st to end	Day	4 1 11% 37% 36% 10%	546
September	1st to 10th	Day	8% 1 11% 33% 37% 11%	314
	11th to 20th	Day	6% 1 21% 39% 29% 5%	300
	21st to end	Day	8% 4 21% 37% 22% 8%	300
October	1st to 10th	Day	9% 3 31% 29% 22% 6%	300
	11th to 20th	Day	7% 9% 37% 31% 13% 3	300
	21st to end	Day	18% 11% 33% 22% 13% 3	330
November	1st to 10th	Day	11% 9% 35% 32% 12% 2	300
	11th to 20th	Day	8% 15% 35% 26% 14% 2	300
	21st to end	Day	14% 14% 32% 27% 10% 3	300
December	1st to 10th	Day	8% 9% 44% 28% 10% 1	300
	11th to 20th	Day	10% 14% 36% 29% 10% 1	300
	21st to end	Day	16% 20% 34% 23% 7%	330
January	1st to 10th	Night	9% 10% 40% 35% 7%	1000
	11th to 20th	Night	11% 13% 38% 32% 6%	1000
	21st to end	Night	11% 10% 41% 32% 6%	1100
February	1st to 10th	Night	13% 12% 40% 30% 4	875
	11th to 20th	Night	13% 14% 41% 28% 4	784
	21st to end	Night	9% 14% 44% 29% 3	660
March	1st to 10th	Night	8% 14% 38% 35% 4	717
	11th to 20th	Night	16% 12% 37% 30% 6%	600
	21st to end	Night	14% 14% 42% 26% 5%	660
April	1st to 10th	Night	12% 15% 41% 26% 7%	600
	11th to 20th	Night	15% 16% 44% 22% 3	593
	21st to end	Night	16% 13% 38% 27% 5% 1	600
May	1st to 10th	Night	13% 11% 41% 30% 6%	600
	11th to 20th	Night	11% 7% 35% 34% 13% 1	591
	21st to end	Night	9% 8% 34% 36% 12% 1	650
June	1st to 10th	Night	9% 5% 29% 39% 18% 1	600
	11th to 20th	Night	6% 4 36% 45% 10%	600
	21st to end	Night	5% 3 33% 47% 12%	600
July	1st to 10th	Night	5% 1 18% 48% 25% 2	585
	11th to 20th	Night	3 1 17% 52% 26% 2	582
	21st to end	Night	5% 1 11% 53% 29% 1	624
August	1st to 10th	Night	4 1 16% 52% 26% 1	567
	11th to 20th	Night	6% 1 15% 53% 23% 1	564
	21st to end	Night	3 1 15% 46% 32% 3	627
September	1st to 10th	Night	9% 2 17% 46% 25% 2	570
	11th to 20th	Night	13% 2 25% 40% 19% 2	570
	21st to end	Night	6% 4 28% 42% 18% 1	570
October	1st to 10th	Night	10% 4 29% 37% 19% 1	570
	11th to 20th	Night	10% 8% 39% 36% 7%	725
	21st to end	Night	11% 9% 36% 30% 13%	836
November	1st to 10th	Night	10% 7% 38% 35% 10% 1	974
	11th to 20th	Night	8% 10% 40% 33% 8%	1000
	21st to end	Night	11% 12% 33% 35% 10%	1000
December	1st to 10th	Night	10% 10% 38% 35% 8%	1000
	11th to 20th	Night	13% 11% 41% 31% 5%	1000
	21st to end	Night	13% 13% 38% 32% 5%	1100

Page generated at 2025-07-18 00:10:03. Version ac07

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