The Last Stand Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Last Stand Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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	13% 6% 2 6% 5% 4 5% 4 6% 8% 42%	126825
	Night	15% 6% 2 5% 3 3 4 3 5% 7% 46%	123820
Transparency	Day	40% 4 11% 21% 15% 8%	130555
	Night	38% 4 12% 21% 16% 10%	128153
Seeing	Day	13% 6% 20% 28% 23% 11%	16105
	Night	16% 5% 22% 35% 21% 2	25976

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	20% 12% 3 7% 3 3 3 2 3 4 40%	4077
	11th to 20th	Day	17% 5% 3 6% 3 2 2 2 3 5% 51%	4080
	21st to end	Day	30% 7% 2 5% 3 2 3 3 5% 6% 35%	4623
February	1st to 10th	Day	22% 8% 2 5% 4 3 3 3 4 5% 40%	2000
	11th to 20th	Day	21% 9% 2 6% 4 4 3 2 3 5% 41%	1949
	21st to end	Day	20% 7% 2 7% 4 3 4 2 4 6% 42%	1730
March	1st to 10th	Day	13% 7% 2 7% 4 4 4 4 5% 7% 43%	4936
	11th to 20th	Day	14% 7% 3 7% 4 4 3 3 4 4 47%	4980
	21st to end	Day	18% 7% 3 6% 4 4 4 3 5% 6% 41%	5462
April	1st to 10th	Day	14% 7% 3 7% 5% 4 5% 4 5% 6% 40%	2200
	11th to 20th	Day	16% 7% 3 7% 5% 5% 5% 3 5% 5% 39%	2210
	21st to end	Day	13% 8% 2 7% 6% 5% 4 4 6% 6% 41%	2400
May	1st to 10th	Day	18% 7% 3 7% 5% 5% 5% 4 6% 8% 32%	5857
	11th to 20th	Day	11% 6% 2 6% 4 4 4 4 6% 8% 44%	5862
	21st to end	Day	9% 6% 2 7% 5% 6% 6% 5% 8% 10% 38%	6434
June	1st to 10th	Day	5% 4 2 6% 5% 4 6% 5% 8% 12% 44%	2600
	11th to 20th	Day	6% 4 2 7% 6% 5% 6% 5% 8% 10% 42%	2600
	21st to end	Day	6% 6% 3 8% 7% 6% 7% 6% 8% 10% 33%	2600
July	1st to 10th	Day	9% 6% 2 8% 6% 5% 6% 5% 8% 11% 34%	5900
	11th to 20th	Day	6% 5% 2 7% 6% 6% 7% 5% 9% 12% 34%	5900
	21st to end	Day	10% 6% 3 6% 5% 5% 5% 5% 7% 11% 37%	6422
August	1st to 10th	Day	2 1 1 3 3 4 3 4 6% 9% 64%	4234
	11th to 20th	Day	3 2 1 5% 4 5% 7% 6% 9% 12% 46%	4054
	21st to end	Day	6% 4 2 7% 6% 6% 6% 5% 8% 11% 40%	3949
September	1st to 10th	Day	8% 6% 3 7% 6% 6% 6% 4 7% 9% 38%	2200
	11th to 20th	Day	10% 6% 2 6% 6% 5% 6% 5% 6% 9% 39%	2200
	21st to end	Day	7% 4 2 5% 5% 6% 5% 4 6% 8% 47%	2200
October	1st to 10th	Day	5% 3 2 4 4 4 5% 4 6% 8% 54%	2185
	11th to 20th	Day	11% 6% 2 4 4 3 4 4 5% 6% 51%	2000
	21st to end	Day	10% 5% 2 6% 4 4 3 3 5% 6% 52%	2200
November	1st to 10th	Day	12% 6% 2 6% 5% 5% 4 4 5% 6% 45%	2058
	11th to 20th	Day	13% 6% 2 6% 4 4 3 3 5% 6% 50%	2070
	21st to end	Day	16% 7% 3 6% 5% 4 4 3 6% 5% 41%	1834
December	1st to 10th	Day	28% 10% 2 7% 4 3 2 2 4 4 33%	2940
	11th to 20th	Day	16% 8% 3 6% 4 4 4 3 5% 6% 41%	3391
	21st to end	Day	14% 7% 2 6% 3 3 3 2 4 5% 50%	4488
January	1st to 10th	Night	23% 8% 2 5% 3 3 3 2 3 5% 43%	5888
	11th to 20th	Night	15% 6% 2 5% 3 3 2 2 4 6% 52%	5900
	21st to end	Night	32% 6% 2 5% 3 3 3 2 4 6% 35%	6384
February	1st to 10th	Night	22% 8% 3 5% 3 4 3 2 5% 5% 40%	2400
	11th to 20th	Night	22% 6% 3 6% 3 2 3 2 4 6% 43%	2338
	21st to end	Night	22% 9% 3 5% 4 3 2 4 4 4 40%	1898
March	1st to 10th	Night	21% 10% 2 5% 3 4 4 3 4 5% 38%	4919
	11th to 20th	Night	19% 7% 2 6% 3 3 3 3 4 4 45%	4980
	21st to end	Night	20% 10% 3 6% 3 3 3 3 4 5% 42%	5467
April	1st to 10th	Night	15% 8% 3 6% 5% 3 5% 4 5% 6% 41%	2200
	11th to 20th	Night	18% 6% 3 5% 4 3 3 3 5% 7% 43%	2137
	21st to end	Night	17% 9% 2 5% 5% 4 4 3 5% 7% 39%	1999
May	1st to 10th	Night	27% 9% 3 7% 4 4 4 3 5% 6% 28%	4053
	11th to 20th	Night	12% 8% 2 6% 3 4 4 3 5% 8% 46%	4053
	21st to end	Night	13% 9% 3 8% 6% 5% 5% 5% 7% 10% 29%	4443
June	1st to 10th	Night	6% 4 1 5% 3 3 4 3 6% 9% 57%	1800
	11th to 20th	Night	5% 4 1 4 4 4 5% 4 6% 8% 54%	1800
	21st to end	Night	6% 3 2 5% 4 5% 5% 5% 9% 11% 45%	1800
July	1st to 10th	Night	7% 6% 2 7% 5% 5% 5% 5% 7% 10% 42%	4078
	11th to 20th	Night	5% 4 2 5% 4 4 5% 5% 7% 10% 48%	4077
	21st to end	Night	7% 6% 3 6% 4 4 5% 4 6% 10% 45%	4449
August	1st to 10th	Night	1 1 1 1 2 2 3 5% 8% 76%	2926
	11th to 20th	Night	1 2 1 3 3 3 4 4 7% 12% 59%	2985
	21st to end	Night	4 2 1 3 3 4 5% 5% 7% 12% 54%	3948
September	1st to 10th	Night	6% 4 2 6% 4 4 5% 4 7% 9% 48%	2200
	11th to 20th	Night	8% 4 2 5% 4 5% 5% 4 6% 7% 50%	2200
	21st to end	Night	7% 4 2 5% 4 3 3 4 5% 8% 56%	2200
October	1st to 10th	Night	5% 4 2 4 3 3 3 3 5% 8% 62%	2200
	11th to 20th	Night	10% 5% 2 4 2 3 3 2 4 6% 61%	2394
	21st to end	Night	10% 5% 1 3 2 3 3 3 4 4 62%	2640
November	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 5% 6% 54%	2460
	11th to 20th	Night	12% 6% 2 5% 4 3 3 4 4 5% 52%	2485
	21st to end	Night	15% 7% 3 6% 4 4 4 3 4 5% 45%	2563
December	1st to 10th	Night	28% 10% 3 7% 4 3 3 3 3 4 31%	4250
	11th to 20th	Night	16% 8% 3 7% 4 4 3 3 4 5% 45%	4816
	21st to end	Night	15% 6% 2 5% 3 3 3 2 4 5% 52%	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	51% 2 4 17% 18% 8%	4078
	11th to 20th	Day	38% 2 4 21% 22% 13%	4080
	21st to end	Day	52% 2 3 12% 19% 13%	4623
February	1st to 10th	Day	48% 3 5% 12% 17% 14%	2000
	11th to 20th	Day	48% 2 7% 15% 16% 11%	1959
	21st to end	Day	46% 3 8% 16% 16% 11%	1730
March	1st to 10th	Day	41% 4 8% 20% 12% 16%	4936
	11th to 20th	Day	42% 4 5% 9% 17% 23%	4980
	21st to end	Day	46% 3 7% 19% 18% 7%	5464
April	1st to 10th	Day	44% 4 8% 18% 15% 11%	2200
	11th to 20th	Day	46% 3 7% 16% 17% 10%	2210
	21st to end	Day	43% 4 6% 22% 17% 8%	2400
May	1st to 10th	Day	50% 4 10% 20% 12% 3	5895
	11th to 20th	Day	38% 4 11% 23% 18% 6%	5866
	21st to end	Day	38% 6% 11% 33% 11% 1	6447
June	1st to 10th	Day	31% 7% 15% 29% 17% 1	2600
	11th to 20th	Day	35% 6% 11% 27% 18% 4	2600
	21st to end	Day	45% 7% 14% 24% 9% 1	2600
July	1st to 10th	Day	45% 4 26% 21% 3	5836
	11th to 20th	Day	42% 4 23% 25% 6%	5831
	21st to end	Day	41% 4 19% 25% 8% 1	6240
August	1st to 10th	Day	18% 4 13% 48% 16%	4220
	11th to 20th	Day	28% 6% 30% 29% 6% 1	4093
	21st to end	Day	38% 7% 18% 24% 11% 1	3948
September	1st to 10th	Day	41% 9% 14% 18% 14% 5%	2200
	11th to 20th	Day	42% 7% 11% 21% 15% 4	2200
	21st to end	Day	34% 5% 11% 21% 17% 12%	2200
October	1st to 10th	Day	20% 4 6% 14% 20% 36%	3562
	11th to 20th	Day	26% 3 5% 21% 28% 18%	3240
	21st to end	Day	32% 3 4 20% 27% 14%	3538
November	1st to 10th	Day	38% 4 8% 17% 20% 11%	2058
	11th to 20th	Day	36% 4 8% 17% 21% 15%	2070
	21st to end	Day	45% 4 8% 16% 15% 12%	1834
December	1st to 10th	Day	57% 2 4 13% 19% 4	2940
	11th to 20th	Day	43% 3 7% 16% 19% 11%	3389
	21st to end	Day	39% 2 4 21% 20% 14%	4488
January	1st to 10th	Night	47% 2 5% 16% 18% 12%	5888
	11th to 20th	Night	36% 2 3 21% 23% 15%	5900
	21st to end	Night	53% 2 4 11% 18% 12%	6384
February	1st to 10th	Night	48% 3 7% 12% 16% 14%	2400
	11th to 20th	Night	45% 3 7% 16% 17% 13%	2338
	21st to end	Night	49% 3 6% 14% 17% 10%	1898
March	1st to 10th	Night	48% 4 7% 16% 15% 11%	4919
	11th to 20th	Night	44% 4 6% 12% 15% 19%	4980
	21st to end	Night	46% 3 9% 15% 17% 10%	5467
April	1st to 10th	Night	43% 4 12% 16% 14% 10%	2200
	11th to 20th	Night	42% 4 10% 16% 14% 15%	2137
	21st to end	Night	46% 4 9% 19% 14% 8%	1999
May	1st to 10th	Night	58% 4 7% 17% 10% 4	4080
	11th to 20th	Night	37% 4 11% 25% 17% 7%	4054
	21st to end	Night	49% 5% 16% 24% 6% 1	4452
June	1st to 10th	Night	22% 6% 20% 33% 18% 1	1800
	11th to 20th	Night	28% 5% 14% 32% 17% 3	1800
	21st to end	Night	31% 8% 24% 27% 10% 1	1800
July	1st to 10th	Night	37% 5% 29% 26% 3	4035
	11th to 20th	Night	31% 5% 30% 27% 6%	4026
	21st to end	Night	34% 6% 23% 25% 10%	4320
August	1st to 10th	Night	9% 3 20% 54% 13%	2916
	11th to 20th	Night	17% 6% 37% 34% 5%	3012
	21st to end	Night	23% 10% 27% 30% 10% 1	3948
September	1st to 10th	Night	31% 8% 22% 21% 13% 5%	2200
	11th to 20th	Night	32% 7% 16% 27% 15% 4	2200
	21st to end	Night	28% 5% 15% 26% 17% 10%	2200
October	1st to 10th	Night	17% 2 8% 18% 19% 36%	3590
	11th to 20th	Night	17% 2 8% 25% 31% 18%	3893
	21st to end	Night	21% 2 5% 26% 31% 16%	4252
November	1st to 10th	Night	31% 5% 11% 20% 22% 12%	2460
	11th to 20th	Night	36% 3 10% 18% 20% 14%	2485
	21st to end	Night	41% 4 8% 18% 18% 11%	2563
December	1st to 10th	Night	59% 2 5% 14% 14% 6%	4250
	11th to 20th	Night	42% 2 10% 13% 18% 15%	4817
	21st to end	Night	38% 3 4 17% 27% 12%	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	24% 6% 39% 21% 9% 1	300
	11th to 20th	Day	26% 11% 31% 24% 7% 1	300
	21st to end	Day	25% 14% 29% 22% 11%	330
February	1st to 10th	Day	26% 11% 27% 24% 10% 1	300
	11th to 20th	Day	26% 17% 30% 19% 7% 1	300
	21st to end	Day	26% 10% 40% 19% 6%	249
March	1st to 10th	Day	20% 15% 35% 24% 6%	300
	11th to 20th	Day	18% 14% 40% 20% 8%	300
	21st to end	Day	20% 14% 32% 25% 8% 1	330
April	1st to 10th	Day	18% 16% 35% 23% 8%	300
	11th to 20th	Day	20% 11% 34% 24% 9% 1	328
	21st to end	Day	18% 9% 35% 27% 10% 1	500
May	1st to 10th	Day	15% 11% 41% 24% 9% 1	698
	11th to 20th	Day	15% 4 32% 31% 14% 3	725
	21st to end	Day	11% 6% 28% 35% 17% 3	789
June	1st to 10th	Day	6% 3 17% 34% 31% 10%	730
	11th to 20th	Day	9% 1 13% 41% 29% 8%	730
	21st to end	Day	7% 3 10% 31% 31% 17%	730
July	1st to 10th	Day	8% 3 6% 31% 32% 20%	721
	11th to 20th	Day	4 1 4 31% 36% 23%	718
	21st to end	Day	6% 2 3 29% 38% 22%	794
August	1st to 10th	Day	4 1 3 27% 39% 26%	726
	11th to 20th	Day	4 1 2 28% 38% 26%	650
	21st to end	Day	9% 2 2 18% 33% 36%	363
September	1st to 10th	Day	8% 4 4 23% 34% 27%	330
	11th to 20th	Day	6% 2 5% 21% 37% 29%	330
	21st to end	Day	8% 2 4 28% 40% 18%	330
October	1st to 10th	Day	6% 1 9% 31% 38% 15%	330
	11th to 20th	Day	10% 2 11% 34% 29% 14%	330
	21st to end	Day	15% 7% 22% 28% 21% 7%	363
November	1st to 10th	Day	15% 4 19% 32% 22% 8%	330
	11th to 20th	Day	18% 4 21% 32% 21% 4	321
	21st to end	Day	21% 12% 28% 22% 13% 3	300
December	1st to 10th	Day	20% 7% 29% 29% 11% 5%	300
	11th to 20th	Day	20% 9% 30% 29% 10% 2	300
	21st to end	Day	21% 5% 37% 24% 12% 2	330
January	1st to 10th	Night	23% 8% 30% 30% 10%	1000
	11th to 20th	Night	22% 10% 27% 32% 8%	1000
	21st to end	Night	25% 7% 27% 34% 6%	1059
February	1st to 10th	Night	29% 10% 26% 31% 5%	800
	11th to 20th	Night	25% 9% 31% 30% 6%	784
	21st to end	Night	29% 9% 34% 25% 3	578
March	1st to 10th	Night	24% 8% 34% 30% 5%	600
	11th to 20th	Night	22% 8% 34% 30% 6%	600
	21st to end	Night	20% 6% 34% 33% 7%	660
April	1st to 10th	Night	18% 7% 38% 31% 6%	600
	11th to 20th	Night	22% 7% 32% 29% 9%	593
	21st to end	Night	21% 7% 35% 30% 8% 1	600
May	1st to 10th	Night	19% 6% 38% 31% 6%	600
	11th to 20th	Night	13% 2 30% 40% 14% 1	591
	21st to end	Night	11% 2 28% 42% 16% 1	650
June	1st to 10th	Night	7% 1 14% 48% 29% 1	600
	11th to 20th	Night	6% 1 17% 51% 23% 2	600
	21st to end	Night	7% 1 9% 39% 38% 6%	600
July	1st to 10th	Night	6% 2 5% 39% 41% 6%	585
	11th to 20th	Night	4 1 4 34% 52% 5%	582
	21st to end	Night	4 2 4 38% 47% 6%	657
August	1st to 10th	Night	1 1 3 41% 47% 7%	597
	11th to 20th	Night	2 1 3 37% 50% 7%	594
	21st to end	Night	5% 1 3 26% 52% 12%	660
September	1st to 10th	Night	7% 1 4 32% 47% 9%	600
	11th to 20th	Night	9% 2 3 33% 47% 6%	600
	21st to end	Night	9% 1 7% 32% 44% 7%	600
October	1st to 10th	Night	6% 1 8% 40% 42% 4	600
	11th to 20th	Night	10% 2 19% 43% 24% 2	794
	21st to end	Night	11% 5% 22% 38% 21% 2	880
November	1st to 10th	Night	12% 4 19% 44% 19% 2	820
	11th to 20th	Night	16% 4 23% 38% 17% 1	829
	21st to end	Night	19% 7% 25% 35% 13% 1	963
December	1st to 10th	Night	21% 6% 25% 32% 15% 1	1000
	11th to 20th	Night	22% 9% 28% 32% 10%	1000
	21st to end	Night	21% 7% 28% 33% 10% 1	1100

Page generated at 2025-11-14 23:55:54. Version ac07

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