Lucile Miller Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lucile Miller Observatory clear sky chart from 2005-11-06 to 2025-09-04.

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% 4 4 4 4 6% 8% 43%	126149
	Night	15% 6% 2 5% 4 4 4 3 5% 7% 44%	122584
Transparency	Day	38% 6% 12% 21% 18% 6%	129808
	Night	38% 6% 10% 19% 20% 7%	126994
Seeing	Day	17% 7% 20% 24% 20% 11%	16338
	Night	17% 6% 22% 32% 20% 3	26382

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% 5% 2 5% 3 2 2 2 3 4 58%	4056
	11th to 20th	Day	12% 6% 2 4 2 2 2 2 3 4 60%	4060
	21st to end	Day	19% 5% 2 6% 3 3 3 3 4 5% 47%	4466
February	1st to 10th	Day	16% 6% 2 5% 3 4 4 4 5% 8% 44%	1800
	11th to 20th	Day	21% 7% 2 5% 3 3 3 2 4 5% 44%	1928
	21st to end	Day	18% 7% 3 6% 4 4 3 3 5% 6% 40%	1680
March	1st to 10th	Day	20% 7% 2 6% 3 3 3 3 5% 6% 41%	4935
	11th to 20th	Day	10% 5% 2 5% 3 3 4 2 4 7% 54%	4980
	21st to end	Day	18% 7% 2 5% 4 3 3 3 4 5% 47%	5465
April	1st to 10th	Day	13% 4 2 4 3 3 4 4 6% 7% 51%	2200
	11th to 20th	Day	14% 4 2 5% 3 3 4 3 4 8% 49%	2359
	21st to end	Day	13% 6% 2 5% 4 3 5% 3 5% 7% 47%	2588
May	1st to 10th	Day	9% 7% 3 7% 5% 5% 7% 5% 7% 10% 35%	5862
	11th to 20th	Day	19% 8% 3 7% 5% 5% 5% 4 7% 9% 30%	5861
	21st to end	Day	12% 6% 3 6% 4 4 5% 4 6% 8% 43%	6433
June	1st to 10th	Day	10% 6% 3 6% 5% 4 5% 6% 7% 11% 37%	2600
	11th to 20th	Day	7% 5% 2 7% 4 6% 6% 5% 6% 11% 41%	2600
	21st to end	Day	8% 5% 2 5% 5% 5% 5% 5% 9% 11% 41%	2600
July	1st to 10th	Day	8% 5% 3 7% 5% 5% 6% 5% 8% 10% 38%	5900
	11th to 20th	Day	7% 6% 2 7% 6% 5% 7% 6% 9% 13% 33%	5900
	21st to end	Day	10% 7% 3 8% 6% 6% 6% 5% 9% 11% 29%	6424
August	1st to 10th	Day	18% 7% 2 7% 6% 6% 5% 5% 7% 9% 28%	4234
	11th to 20th	Day	10% 6% 2 6% 5% 5% 5% 5% 8% 11% 37%	4174
	21st to end	Day	6% 3 1 5% 4 4 4 4 6% 9% 54%	4416
September	1st to 10th	Day	9% 4 2 6% 5% 5% 6% 5% 8% 9% 42%	2133
	11th to 20th	Day	13% 7% 3 5% 4 4 4 4 6% 8% 42%	2090
	21st to end	Day	19% 6% 3 6% 4 4 4 4 5% 5% 40%	2090
October	1st to 10th	Day	14% 5% 2 4 3 3 3 2 4 5% 55%	1984
	11th to 20th	Day	13% 4 1 4 2 2 3 2 3 5% 58%	1900
	21st to end	Day	14% 5% 2 5% 3 3 3 2 3 4 57%	2090
November	1st to 10th	Day	14% 5% 2 5% 3 3 2 3 3 5% 54%	1957
	11th to 20th	Day	15% 5% 2 4 2 2 3 3 5% 7% 52%	1815
	21st to end	Day	15% 6% 2 4 3 3 3 3 5% 6% 50%	1800
December	1st to 10th	Day	16% 6% 2 4 3 2 2 3 4 5% 54%	2930
	11th to 20th	Day	18% 8% 2 6% 4 4 3 3 4 6% 41%	3373
	21st to end	Day	22% 5% 2 4 3 3 3 3 5% 7% 44%	4466
January	1st to 10th	Night	14% 5% 2 4 2 3 3 2 3 4 59%	5889
	11th to 20th	Night	12% 6% 2 4 3 3 3 2 3 4 57%	5900
	21st to end	Night	19% 6% 2 4 3 3 3 3 4 6% 47%	6490
February	1st to 10th	Night	16% 6% 2 5% 4 3 4 3 5% 6% 46%	2600
	11th to 20th	Night	20% 7% 2 6% 4 3 3 3 5% 5% 44%	2353
	21st to end	Night	17% 6% 2 5% 3 4 4 4 5% 6% 45%	1949
March	1st to 10th	Night	18% 6% 2 6% 3 3 3 3 4 5% 46%	4918
	11th to 20th	Night	10% 4 1 4 3 3 4 3 5% 8% 55%	4980
	21st to end	Night	16% 5% 2 4 4 4 4 3 5% 7% 47%	5467
April	1st to 10th	Night	13% 6% 2 4 3 3 4 4 5% 7% 49%	2200
	11th to 20th	Night	13% 6% 2 5% 3 3 3 2 6% 8% 50%	1989
	21st to end	Night	12% 6% 2 5% 5% 3 4 3 6% 6% 48%	1829
May	1st to 10th	Night	14% 7% 3 7% 5% 5% 4 4 6% 9% 38%	4033
	11th to 20th	Night	19% 8% 3 7% 5% 4 5% 4 5% 9% 32%	4032
	21st to end	Night	15% 5% 2 6% 4 4 4 4 5% 9% 41%	4421
June	1st to 10th	Night	10% 6% 2 6% 5% 5% 7% 5% 8% 11% 36%	1800
	11th to 20th	Night	10% 6% 3 7% 5% 5% 5% 4 7% 11% 36%	1800
	21st to end	Night	6% 6% 3 6% 6% 5% 7% 5% 9% 10% 39%	1800
July	1st to 10th	Night	10% 6% 3 8% 7% 6% 6% 5% 8% 10% 30%	4058
	11th to 20th	Night	10% 6% 3 8% 5% 6% 7% 6% 9% 13% 27%	4057
	21st to end	Night	11% 8% 4 10% 6% 6% 6% 6% 8% 12% 23%	4427
August	1st to 10th	Night	18% 9% 3 9% 6% 6% 6% 4 6% 9% 25%	2915
	11th to 20th	Night	12% 7% 3 8% 6% 6% 6% 5% 7% 10% 31%	2875
	21st to end	Night	5% 4 2 5% 5% 5% 6% 4 8% 10% 47%	3448
September	1st to 10th	Night	10% 6% 2 5% 4 6% 6% 5% 6% 8% 43%	2109
	11th to 20th	Night	16% 7% 3 7% 6% 4 5% 4 5% 7% 37%	2090
	21st to end	Night	18% 6% 3 6% 4 4 5% 2 6% 6% 39%	2090
October	1st to 10th	Night	15% 5% 2 5% 3 3 2 4 4 6% 51%	2194
	11th to 20th	Night	14% 5% 2 4 3 3 4 3 4 5% 54%	2280
	21st to end	Night	13% 4 2 4 3 2 3 3 3 4 58%	2508
November	1st to 10th	Night	15% 5% 2 5% 4 3 3 3 4 4 52%	2341
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 4 5% 53%	2580
	21st to end	Night	14% 4 1 4 3 3 4 3 6% 7% 51%	2600
December	1st to 10th	Night	16% 6% 2 4 3 2 3 2 4 5% 53%	4250
	11th to 20th	Night	22% 8% 2 6% 3 3 3 3 4 5% 42%	4822
	21st to end	Night	21% 4 2 3 3 3 3 3 4 6% 47%	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	33% 2 4 20% 33% 8%	4057
	11th to 20th	Day	30% 3 4 16% 38% 9%	4060
	21st to end	Day	41% 2 4 21% 24% 8%	4466
February	1st to 10th	Day	39% 4 10% 23% 19% 5%	1800
	11th to 20th	Day	43% 4 7% 19% 21% 6%	1938
	21st to end	Day	45% 5% 7% 15% 21% 8%	1680
March	1st to 10th	Day	44% 4 5% 15% 18% 13%	4935
	11th to 20th	Day	31% 4 6% 15% 24% 19%	4980
	21st to end	Day	41% 4 6% 24% 20% 6%	5465
April	1st to 10th	Day	32% 5% 11% 23% 23% 6%	2200
	11th to 20th	Day	35% 4 9% 22% 22% 8%	2359
	21st to end	Day	37% 6% 11% 25% 18% 4	2588
May	1st to 10th	Day	42% 8% 17% 21% 11% 2	5897
	11th to 20th	Day	50% 7% 13% 19% 11% 1	5865
	21st to end	Day	38% 7% 13% 21% 17% 4	6447
June	1st to 10th	Day	38% 8% 16% 25% 12% 1	2600
	11th to 20th	Day	35% 9% 18% 22% 14% 2	2600
	21st to end	Day	34% 10% 17% 23% 14% 2	2600
July	1st to 10th	Day	39% 9% 17% 23% 11% 1	5835
	11th to 20th	Day	40% 11% 29% 16% 4	5831
	21st to end	Day	45% 10% 17% 21% 6% 1	6240
August	1st to 10th	Day	51% 8% 17% 19% 5%	4221
	11th to 20th	Day	38% 9% 22% 18% 10% 2	4213
	21st to end	Day	27% 6% 15% 35% 14% 2	4416
September	1st to 10th	Day	35% 8% 13% 21% 19% 3	2133
	11th to 20th	Day	40% 7% 12% 19% 20% 3	2090
	21st to end	Day	46% 4 8% 20% 18% 4	2090
October	1st to 10th	Day	29% 4 8% 21% 29% 10%	3296
	11th to 20th	Day	21% 3 5% 19% 26% 26%	3140
	21st to end	Day	27% 3 6% 25% 22% 17%	3428
November	1st to 10th	Day	35% 3 6% 20% 27% 8%	1957
	11th to 20th	Day	33% 3 6% 24% 26% 7%	1815
	21st to end	Day	35% 4 7% 23% 25% 6%	1800
December	1st to 10th	Day	34% 3 7% 19% 27% 9%	2930
	11th to 20th	Day	45% 4 9% 16% 20% 7%	3370
	21st to end	Day	41% 3 9% 28% 17% 2	4466
January	1st to 10th	Night	30% 2 6% 16% 35% 10%	5889
	11th to 20th	Night	32% 3 4 16% 36% 9%	5900
	21st to end	Night	39% 2 5% 19% 25% 9%	6490
February	1st to 10th	Night	39% 5% 10% 20% 21% 5%	2600
	11th to 20th	Night	44% 4 6% 17% 22% 7%	2354
	21st to end	Night	39% 5% 8% 17% 23% 7%	1949
March	1st to 10th	Night	41% 4 6% 14% 26% 9%	4918
	11th to 20th	Night	28% 4 6% 15% 27% 19%	4980
	21st to end	Night	38% 4 6% 25% 19% 7%	5467
April	1st to 10th	Night	35% 5% 10% 20% 24% 6%	2200
	11th to 20th	Night	33% 4 8% 23% 23% 9%	1989
	21st to end	Night	37% 4 12% 22% 21% 4	1829
May	1st to 10th	Night	43% 7% 15% 21% 13% 1	4060
	11th to 20th	Night	49% 8% 12% 19% 11% 1	4033
	21st to end	Night	38% 7% 12% 25% 15% 2	4429
June	1st to 10th	Night	39% 10% 16% 21% 12% 2	1800
	11th to 20th	Night	40% 9% 18% 21% 11% 1	1800
	21st to end	Night	37% 10% 16% 19% 14% 4	1800
July	1st to 10th	Night	45% 11% 18% 17% 8% 1	4015
	11th to 20th	Night	45% 12% 25% 14% 2 1	4006
	21st to end	Night	50% 11% 17% 15% 6%	4297
August	1st to 10th	Night	54% 8% 19% 15% 5%	2904
	11th to 20th	Night	46% 10% 20% 14% 9% 1	2902
	21st to end	Night	29% 10% 14% 30% 15% 2	3448
September	1st to 10th	Night	37% 8% 15% 19% 16% 5%	2109
	11th to 20th	Night	46% 7% 11% 16% 19% 2	2090
	21st to end	Night	45% 6% 11% 17% 17% 4	2090
October	1st to 10th	Night	35% 6% 8% 18% 26% 8%	3662
	11th to 20th	Night	23% 2 5% 22% 26% 22%	3779
	21st to end	Night	27% 4 5% 22% 27% 15%	4120
November	1st to 10th	Night	36% 4 7% 19% 24% 11%	2341
	11th to 20th	Night	34% 3 7% 19% 26% 11%	2580
	21st to end	Night	33% 4 6% 24% 27% 6%	2600
December	1st to 10th	Night	35% 4 6% 21% 26% 8%	4250
	11th to 20th	Night	47% 3 7% 15% 19% 9%	4824
	21st to end	Night	39% 3 12% 22% 21% 2	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	23% 14% 35% 22% 6%	300
	11th to 20th	Day	23% 17% 33% 19% 8% 1	300
	21st to end	Day	21% 15% 40% 18% 7%	330
February	1st to 10th	Day	21% 15% 35% 21% 7% 1	300
	11th to 20th	Day	34% 16% 34% 13% 3	300
	21st to end	Day	25% 20% 32% 19% 4	279
March	1st to 10th	Day	23% 10% 31% 26% 9% 1	500
	11th to 20th	Day	19% 12% 37% 24% 8% 1	500
	21st to end	Day	23% 12% 31% 27% 7%	550
April	1st to 10th	Day	21% 13% 29% 25% 11% 2	500
	11th to 20th	Day	17% 11% 27% 26% 15% 4	492
	21st to end	Day	18% 6% 27% 26% 16% 6%	500
May	1st to 10th	Day	12% 10% 30% 26% 17% 5%	503
	11th to 20th	Day	20% 6% 15% 26% 22% 11%	527
	21st to end	Day	16% 6% 13% 23% 27% 16%	573
June	1st to 10th	Day	11% 4 14% 32% 25% 14%	530
	11th to 20th	Day	10% 3 13% 26% 27% 22%	530
	21st to end	Day	11% 2 8% 25% 34% 20%	530
July	1st to 10th	Day	11% 2 7% 19% 32% 29%	525
	11th to 20th	Day	8% 2 3 23% 33% 30%	524
	21st to end	Day	8% 2 7% 24% 33% 25%	576
August	1st to 10th	Day	19% 1 10% 22% 28% 20%	526
	11th to 20th	Day	12% 4 7% 23% 30% 24%	519
	21st to end	Day	13% 2 7% 22% 30% 26%	583
September	1st to 10th	Day	11% 2 8% 22% 31% 26%	502
	11th to 20th	Day	16% 3 11% 19% 29% 23%	490
	21st to end	Day	23% 3 14% 27% 23% 9%	490
October	1st to 10th	Day	18% 4 14% 26% 25% 13%	490
	11th to 20th	Day	12% 5% 22% 29% 22% 9%	490
	21st to end	Day	18% 6% 18% 31% 20% 7%	539
November	1st to 10th	Day	22% 5% 22% 28% 18% 5%	495
	11th to 20th	Day	18% 9% 32% 25% 13% 3	315
	21st to end	Day	9% 11% 32% 29% 14% 4	300
December	1st to 10th	Day	25% 10% 30% 26% 8%	300
	11th to 20th	Day	22% 7% 29% 29% 11% 1	300
	21st to end	Day	24% 11% 31% 22% 10% 2	330
January	1st to 10th	Night	21% 11% 32% 32% 5%	800
	11th to 20th	Night	20% 9% 33% 32% 5%	800
	21st to end	Night	19% 8% 37% 33% 3	880
February	1st to 10th	Night	19% 8% 34% 36% 3	800
	11th to 20th	Night	24% 11% 37% 26% 2	783
	21st to end	Night	20% 12% 37% 28% 3	660
March	1st to 10th	Night	18% 10% 29% 37% 5%	800
	11th to 20th	Night	18% 9% 35% 28% 9% 1	800
	21st to end	Night	21% 8% 32% 32% 7%	880
April	1st to 10th	Night	16% 11% 34% 30% 10%	800
	11th to 20th	Night	16% 10% 27% 32% 14% 1	791
	21st to end	Night	15% 5% 24% 40% 15%	629
May	1st to 10th	Night	13% 5% 27% 35% 20% 1	600
	11th to 20th	Night	18% 5% 19% 33% 24% 2	591
	21st to end	Night	17% 2 10% 32% 34% 6%	650
June	1st to 10th	Night	11% 3 13% 36% 34% 4	600
	11th to 20th	Night	13% 2 11% 29% 36% 10%	600
	21st to end	Night	7% 3 9% 32% 45% 5%	600
July	1st to 10th	Night	11% 1 7% 27% 45% 9%	585
	11th to 20th	Night	9% 2 4 26% 46% 11%	582
	21st to end	Night	8% 2 7% 31% 43% 9%	657
August	1st to 10th	Night	16% 2 6% 26% 42% 8%	597
	11th to 20th	Night	13% 3 5% 29% 41% 9%	594
	21st to end	Night	12% 2 7% 23% 47% 9%	794
September	1st to 10th	Night	12% 2 6% 31% 39% 9%	773
	11th to 20th	Night	17% 2 11% 26% 35% 9%	760
	21st to end	Night	20% 4 13% 32% 30% 3	760
October	1st to 10th	Night	16% 4 15% 31% 28% 6%	760
	11th to 20th	Night	16% 4 22% 33% 22% 3	760
	21st to end	Night	15% 5% 22% 36% 20% 3	836
November	1st to 10th	Night	18% 5% 19% 37% 18% 2	780
	11th to 20th	Night	16% 6% 26% 39% 12% 1	800
	21st to end	Night	16% 5% 28% 37% 14% 1	800
December	1st to 10th	Night	23% 7% 26% 35% 8%	800
	11th to 20th	Night	20% 5% 28% 36% 9% 1	800
	21st to end	Night	25% 6% 26% 33% 9%	880

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