Mt. Lion Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mt. Lion 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	15% 9% 3 9% 5% 5% 5% 4 6% 7% 32%	126139
	Night	15% 9% 4 9% 5% 5% 5% 4 5% 7% 34%	122569
Transparency	Day	49% 7% 12% 20% 11% 1	129734
	Night	47% 6% 13% 20% 11% 2	127043
Seeing	Day	24% 16% 26% 20% 11% 4	15824
	Night	21% 14% 24% 28% 11% 1	25291

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	12% 14% 7% 19% 6% 4 4 3 4 4 23%	3689
	11th to 20th	Day	10% 16% 8% 19% 9% 8% 4 3 4 4 15%	4060
	21st to end	Day	16% 17% 7% 11% 6% 4 4 3 4 5% 23%	4466
February	1st to 10th	Day	16% 14% 5% 11% 7% 5% 4 4 4 5% 25%	1800
	11th to 20th	Day	14% 10% 4 12% 6% 5% 5% 4 6% 6% 27%	1753
	21st to end	Day	16% 12% 3 13% 6% 5% 3 4 5% 6% 28%	1626
March	1st to 10th	Day	17% 12% 4 8% 5% 4 4 3 5% 5% 34%	4935
	11th to 20th	Day	12% 10% 4 8% 5% 5% 5% 4 6% 7% 35%	4980
	21st to end	Day	13% 9% 3 9% 6% 5% 5% 4 5% 6% 37%	5465
April	1st to 10th	Day	16% 11% 3 9% 5% 5% 4 3 5% 6% 31%	2394
	11th to 20th	Day	15% 10% 3 7% 5% 5% 6% 3 5% 6% 35%	2374
	21st to end	Day	20% 8% 3 7% 5% 5% 4 3 5% 6% 35%	2500
May	1st to 10th	Day	23% 7% 3 8% 5% 5% 5% 4 6% 7% 27%	5862
	11th to 20th	Day	24% 7% 2 6% 5% 5% 5% 4 5% 8% 29%	6129
	21st to end	Day	18% 6% 3 7% 5% 5% 6% 5% 7% 8% 32%	6934
June	1st to 10th	Day	13% 7% 2 6% 6% 5% 6% 5% 6% 8% 36%	2800
	11th to 20th	Day	15% 6% 2 7% 6% 5% 5% 4 7% 9% 35%	2800
	21st to end	Day	12% 5% 2 5% 5% 6% 5% 4 7% 10% 40%	2800
July	1st to 10th	Day	6% 5% 2 8% 6% 6% 6% 5% 6% 8% 40%	6360
	11th to 20th	Day	7% 6% 3 9% 7% 7% 7% 6% 8% 10% 32%	6178
	21st to end	Day	9% 7% 2 8% 5% 5% 5% 4 6% 8% 40%	6424
August	1st to 10th	Day	18% 6% 3 7% 5% 5% 6% 5% 7% 9% 30%	4234
	11th to 20th	Day	11% 5% 3 8% 6% 6% 7% 5% 7% 8% 35%	4154
	21st to end	Day	14% 5% 2 5% 4 5% 4 4 6% 9% 42%	4312
September	1st to 10th	Day	17% 5% 2 5% 4 4 4 3 4 6% 45%	2316
	11th to 20th	Day	15% 5% 2 5% 3 4 4 3 6% 7% 47%	2236
	21st to end	Day	22% 7% 2 6% 4 4 4 4 6% 6% 35%	2007
October	1st to 10th	Day	16% 8% 3 7% 4 4 5% 3 5% 7% 38%	1900
	11th to 20th	Day	17% 7% 2 8% 5% 5% 5% 3 5% 6% 37%	1900
	21st to end	Day	17% 7% 2 8% 5% 4 4 4 5% 7% 37%	2090
November	1st to 10th	Day	11% 6% 3 7% 4 4 5% 4 5% 7% 44%	1875
	11th to 20th	Day	15% 9% 3 7% 6% 6% 5% 4 5% 7% 33%	1636
	21st to end	Day	16% 11% 3 8% 7% 5% 5% 4 6% 5% 29%	1600
December	1st to 10th	Day	14% 11% 5% 14% 7% 6% 6% 3 5% 6% 23%	2600
	11th to 20th	Day	21% 14% 5% 8% 4 4 4 3 4 5% 27%	2990
	21st to end	Day	18% 17% 6% 13% 7% 6% 4 3 5% 5% 15%	3960
January	1st to 10th	Night	11% 10% 6% 21% 7% 5% 4 3 5% 6% 21%	6239
	11th to 20th	Night	11% 14% 6% 16% 7% 6% 5% 3 5% 6% 20%	5900
	21st to end	Night	17% 11% 5% 12% 6% 5% 5% 4 6% 5% 24%	6490
February	1st to 10th	Night	15% 10% 4 10% 6% 5% 5% 3 6% 6% 30%	2600
	11th to 20th	Night	15% 10% 3 9% 6% 5% 5% 5% 7% 7% 29%	2533
	21st to end	Night	15% 10% 3 10% 6% 5% 5% 5% 5% 5% 31%	2000
March	1st to 10th	Night	17% 10% 5% 9% 5% 4 4 3 5% 6% 33%	4918
	11th to 20th	Night	10% 8% 4 7% 4 4 4 3 5% 7% 44%	4980
	21st to end	Night	12% 8% 3 9% 5% 5% 5% 3 5% 6% 39%	5467
April	1st to 10th	Night	18% 10% 3 8% 5% 5% 5% 4 4 6% 32%	2005
	11th to 20th	Night	15% 8% 3 6% 5% 4 5% 4 5% 7% 39%	1975
	21st to end	Night	17% 7% 2 7% 5% 4 4 4 5% 7% 39%	1917
May	1st to 10th	Night	26% 8% 3 7% 5% 4 5% 4 5% 7% 26%	4033
	11th to 20th	Night	28% 7% 2 5% 4 3 4 3 5% 7% 32%	3759
	21st to end	Night	20% 6% 3 6% 4 5% 4 4 6% 8% 34%	3920
June	1st to 10th	Night	13% 6% 2 5% 5% 5% 5% 3 5% 9% 43%	1600
	11th to 20th	Night	12% 5% 2 7% 4 4 4 5% 7% 9% 40%	1600
	21st to end	Night	10% 4 2 5% 4 4 6% 5% 6% 11% 44%	1600
July	1st to 10th	Night	7% 6% 2 8% 5% 5% 5% 4 7% 10% 40%	3598
	11th to 20th	Night	7% 6% 3 9% 6% 6% 7% 5% 7% 10% 34%	3774
	21st to end	Night	9% 5% 2 7% 5% 4 5% 4 6% 9% 44%	4427
August	1st to 10th	Night	16% 7% 2 6% 4 4 5% 3 6% 8% 37%	2915
	11th to 20th	Night	13% 7% 2 7% 5% 4 4 4 6% 7% 41%	2875
	21st to end	Night	15% 4 2 5% 3 4 4 4 6% 8% 46%	3571
September	1st to 10th	Night	16% 5% 2 4 4 5% 4 3 5% 6% 46%	1931
	11th to 20th	Night	12% 5% 2 5% 3 3 4 3 5% 8% 52%	1941
	21st to end	Night	21% 5% 2 5% 4 4 3 3 5% 7% 41%	2171
October	1st to 10th	Night	16% 6% 2 5% 4 3 4 3 5% 7% 44%	2280
	11th to 20th	Night	17% 6% 2 5% 4 4 4 3 5% 8% 42%	2280
	21st to end	Night	20% 7% 2 5% 3 4 4 3 5% 7% 39%	2508
November	1st to 10th	Night	10% 6% 2 6% 4 4 4 4 5% 5% 50%	2421
	11th to 20th	Night	15% 10% 3 8% 4 4 4 4 6% 7% 35%	2760
	21st to end	Night	16% 9% 4 8% 5% 4 4 4 5% 7% 34%	2800
December	1st to 10th	Night	12% 10% 6% 13% 6% 5% 5% 4 6% 7% 26%	4580
	11th to 20th	Night	18% 12% 4 9% 6% 5% 4 3 5% 6% 27%	5205
	21st to end	Night	19% 14% 5% 11% 7% 5% 5% 3 5% 5% 20%	6996

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	65% 4 11% 13% 5% 1	3690
	11th to 20th	Day	72% 4 7% 9% 6% 1	4060
	21st to end	Day	65% 5% 11% 13% 6%	4466
February	1st to 10th	Day	62% 5% 13% 14% 6% 1	1800
	11th to 20th	Day	56% 6% 14% 19% 6%	1762
	21st to end	Day	57% 4 13% 17% 7% 2	1626
March	1st to 10th	Day	49% 7% 12% 20% 10% 2	4935
	11th to 20th	Day	46% 6% 15% 22% 8% 2	4980
	21st to end	Day	47% 6% 12% 20% 13% 2	5465
April	1st to 10th	Day	53% 6% 10% 17% 11% 2	2394
	11th to 20th	Day	48% 6% 10% 18% 15% 2	2374
	21st to end	Day	49% 5% 10% 20% 13% 2	2500
May	1st to 10th	Day	54% 7% 11% 15% 10% 2	5897
	11th to 20th	Day	53% 7% 11% 19% 9% 1	6133
	21st to end	Day	45% 8% 14% 20% 11% 2	6949
June	1st to 10th	Day	44% 7% 11% 23% 14% 1	2800
	11th to 20th	Day	44% 8% 14% 19% 14% 1	2800
	21st to end	Day	39% 8% 14% 26% 13% 1	2800
July	1st to 10th	Day	39% 8% 12% 26% 13% 2	6290
	11th to 20th	Day	42% 10% 16% 21% 11%	6103
	21st to end	Day	40% 7% 9% 25% 19% 1	6240
August	1st to 10th	Day	47% 8% 15% 20% 9%	4221
	11th to 20th	Day	44% 8% 12% 26% 10% 1	4193
	21st to end	Day	38% 6% 13% 28% 14% 2	4312
September	1st to 10th	Day	41% 6% 10% 23% 17% 3	2316
	11th to 20th	Day	35% 5% 12% 27% 19% 2	2236
	21st to end	Day	48% 6% 12% 18% 15% 1	2007
October	1st to 10th	Day	38% 5% 10% 25% 19% 2	3160
	11th to 20th	Day	41% 6% 9% 24% 15% 5%	3140
	21st to end	Day	32% 5% 15% 29% 19% 1	3428
November	1st to 10th	Day	39% 6% 16% 20% 16% 3	1875
	11th to 20th	Day	50% 5% 13% 20% 10% 2	1636
	21st to end	Day	55% 6% 12% 20% 7%	1600
December	1st to 10th	Day	60% 6% 14% 15% 5% 1	2600
	11th to 20th	Day	59% 4 13% 19% 5%	2986
	21st to end	Day	69% 7% 10% 9% 5%	3960
January	1st to 10th	Night	63% 5% 12% 14% 6%	6239
	11th to 20th	Night	61% 6% 12% 13% 6% 1	5900
	21st to end	Night	60% 5% 11% 16% 6% 1	6490
February	1st to 10th	Night	54% 5% 14% 20% 7%	2600
	11th to 20th	Night	52% 7% 12% 20% 9%	2535
	21st to end	Night	54% 6% 10% 18% 11% 1	2000
March	1st to 10th	Night	51% 6% 13% 20% 9% 1	4918
	11th to 20th	Night	38% 6% 18% 26% 8% 4	4980
	21st to end	Night	44% 6% 11% 19% 17% 2	5467
April	1st to 10th	Night	52% 6% 10% 18% 13% 1	2005
	11th to 20th	Night	43% 6% 12% 21% 15% 2	1975
	21st to end	Night	45% 5% 10% 24% 13% 3	1917
May	1st to 10th	Night	57% 6% 10% 14% 10% 3	4060
	11th to 20th	Night	50% 8% 12% 21% 9% 1	3760
	21st to end	Night	46% 7% 14% 20% 11% 3	3927
June	1st to 10th	Night	38% 8% 12% 25% 15% 2	1600
	11th to 20th	Night	38% 10% 15% 22% 14% 2	1600
	21st to end	Night	33% 10% 16% 26% 16% 1	1600
July	1st to 10th	Night	38% 8% 13% 24% 14% 2	3560
	11th to 20th	Night	40% 11% 17% 19% 12% 1	3729
	21st to end	Night	35% 7% 12% 23% 22% 1	4297
August	1st to 10th	Night	43% 9% 15% 23% 10% 1	2904
	11th to 20th	Night	40% 7% 12% 27% 13% 1	2902
	21st to end	Night	35% 6% 13% 31% 13% 2	3571
September	1st to 10th	Night	38% 6% 10% 24% 18% 3	1931
	11th to 20th	Night	30% 6% 13% 30% 20% 1	1941
	21st to end	Night	44% 5% 12% 22% 16% 1	2171
October	1st to 10th	Night	35% 5% 11% 27% 21% 1	3800
	11th to 20th	Night	32% 5% 13% 25% 19% 6%	3779
	21st to end	Night	32% 4 14% 28% 21% 2	4120
November	1st to 10th	Night	36% 5% 15% 23% 18% 3	2421
	11th to 20th	Night	47% 5% 13% 21% 11% 3	2760
	21st to end	Night	50% 5% 15% 21% 8% 1	2800
December	1st to 10th	Night	53% 8% 18% 17% 4	4580
	11th to 20th	Night	58% 6% 15% 17% 5%	5208
	21st to end	Night	64% 6% 12% 12% 6%	6996

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	36% 25% 26% 8% 4	220
	11th to 20th	Day	42% 19% 30% 8% 1	300
	21st to end	Day	32% 28% 27% 10% 2 1	330
February	1st to 10th	Day	34% 29% 24% 12% 1	300
	11th to 20th	Day	43% 26% 26% 5%	300
	21st to end	Day	35% 23% 29% 13% 1	376
March	1st to 10th	Day	33% 19% 27% 17% 4	500
	11th to 20th	Day	33% 22% 25% 14% 4 1	500
	21st to end	Day	32% 21% 27% 14% 4 2	550
April	1st to 10th	Day	34% 20% 25% 16% 4 1	500
	11th to 20th	Day	28% 19% 24% 17% 9% 3	492
	21st to end	Day	30% 19% 29% 14% 7% 1	500
May	1st to 10th	Day	31% 17% 26% 17% 8% 3	503
	11th to 20th	Day	29% 14% 29% 14% 10% 3	527
	21st to end	Day	21% 12% 28% 21% 14% 5%	573
June	1st to 10th	Day	19% 17% 26% 24% 11% 3	530
	11th to 20th	Day	20% 12% 25% 27% 12% 3	530
	21st to end	Day	15% 11% 30% 28% 13% 3	530
July	1st to 10th	Day	11% 9% 27% 26% 19% 8%	525
	11th to 20th	Day	8% 10% 27% 31% 18% 7%	524
	21st to end	Day	13% 8% 28% 29% 16% 6%	576
August	1st to 10th	Day	12% 8% 26% 27% 18% 9%	526
	11th to 20th	Day	14% 9% 23% 28% 18% 9%	519
	21st to end	Day	15% 10% 25% 23% 16% 11%	583
September	1st to 10th	Day	19% 8% 17% 28% 18% 9%	502
	11th to 20th	Day	17% 6% 19% 27% 23% 8%	490
	21st to end	Day	29% 12% 21% 18% 13% 6%	490
October	1st to 10th	Day	23% 12% 20% 24% 16% 5%	490
	11th to 20th	Day	25% 17% 26% 21% 10% 2	490
	21st to end	Day	22% 20% 29% 21% 8%	539
November	1st to 10th	Day	22% 16% 26% 22% 13% 2	453
	11th to 20th	Day	27% 23% 25% 19% 5%	236
	21st to end	Day	31% 23% 31% 13% 3	200
December	1st to 10th	Day	33% 30% 23% 9% 6% 1	200
	11th to 20th	Day	36% 17% 31% 14% 2 1	200
	21st to end	Day	33% 21% 26% 15% 5%	220
January	1st to 10th	Night	26% 23% 33% 15% 3	800
	11th to 20th	Night	30% 17% 30% 21% 2	800
	21st to end	Night	24% 24% 30% 20% 2	880
February	1st to 10th	Night	28% 19% 29% 23% 2	800
	11th to 20th	Night	29% 25% 31% 15% 1	783
	21st to end	Night	27% 19% 31% 22% 2	660
March	1st to 10th	Night	25% 19% 30% 23% 4	800
	11th to 20th	Night	27% 20% 26% 21% 6%	800
	21st to end	Night	23% 19% 31% 20% 6%	880
April	1st to 10th	Night	26% 21% 28% 21% 5%	800
	11th to 20th	Night	22% 17% 25% 28% 8%	791
	21st to end	Night	22% 13% 26% 33% 6%	717
May	1st to 10th	Night	23% 11% 22% 33% 9% 1	600
	11th to 20th	Night	22% 8% 24% 36% 11%	474
	21st to end	Night	15% 9% 21% 32% 20% 2	433
June	1st to 10th	Night	13% 7% 24% 38% 18%	400
	11th to 20th	Night	15% 7% 23% 37% 18% 1	400
	21st to end	Night	11% 5% 23% 42% 18% 1	400
July	1st to 10th	Night	10% 4 16% 45% 21% 3	390
	11th to 20th	Night	7% 5% 15% 41% 30% 2	461
	21st to end	Night	9% 3 17% 44% 25% 2	657
August	1st to 10th	Night	11% 3 15% 41% 29% 2	597
	11th to 20th	Night	13% 6% 13% 39% 26% 3	594
	21st to end	Night	13% 5% 16% 36% 26% 3	865
September	1st to 10th	Night	17% 4 17% 32% 26% 3	773
	11th to 20th	Night	13% 6% 15% 31% 30% 5%	760
	21st to end	Night	22% 9% 18% 29% 18% 4	760
October	1st to 10th	Night	18% 9% 21% 29% 20% 2	760
	11th to 20th	Night	22% 14% 23% 30% 11% 1	760
	21st to end	Night	25% 14% 24% 30% 7%	836
November	1st to 10th	Night	15% 13% 27% 34% 11% 1	780
	11th to 20th	Night	26% 18% 22% 27% 7% 1	800
	21st to end	Night	25% 20% 26% 24% 5% 1	800
December	1st to 10th	Night	29% 17% 28% 20% 5%	800
	11th to 20th	Night	26% 19% 27% 25% 3	800
	21st to end	Night	25% 18% 27% 28% 2	880

Page generated at 2025-09-01 01:51:03. Version ac07

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