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 2024-11-05.

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 8% 5% 5% 5% 4 6% 7% 33%	101287
	Night	16% 9% 3 8% 5% 4 4 4 6% 7% 35%	97701
Transparency	Day	48% 7% 12% 21% 11% 1	104926
	Night	46% 6% 13% 21% 12% 2	102201
Seeing	Day	24% 16% 26% 20% 11% 4	14779
	Night	21% 14% 24% 29% 11% 1	24262

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% 14% 7% 11% 6% 5% 4 3 4 5% 27%	2581
	11th to 20th	Day	11% 19% 8% 16% 7% 5% 4 3 4 5% 20%	2840
	21st to end	Day	19% 19% 7% 12% 7% 5% 4 3 3 4 17%	3124
February	1st to 10th	Day	15% 14% 5% 11% 7% 5% 4 4 4 5% 25%	1710
	11th to 20th	Day	13% 10% 4 12% 7% 5% 5% 4 7% 6% 27%	1663
	21st to end	Day	17% 12% 3 12% 7% 4 3 4 4 5% 28%	1546
March	1st to 10th	Day	20% 11% 4 8% 5% 5% 4 4 5% 6% 28%	3435
	11th to 20th	Day	12% 10% 4 9% 5% 5% 5% 4 6% 7% 34%	3480
	21st to end	Day	15% 9% 3 8% 5% 5% 4 4 4 6% 37%	3815
April	1st to 10th	Day	15% 11% 3 9% 5% 5% 4 3 6% 6% 32%	2274
	11th to 20th	Day	15% 10% 3 8% 5% 5% 5% 3 5% 6% 35%	2254
	21st to end	Day	21% 7% 3 7% 5% 5% 4 4 5% 6% 34%	2375
May	1st to 10th	Day	27% 7% 3 7% 5% 5% 4 3 5% 6% 28%	4082
	11th to 20th	Day	23% 8% 3 7% 5% 5% 5% 4 5% 7% 29%	4265
	21st to end	Day	12% 6% 2 6% 5% 5% 5% 5% 7% 9% 39%	4823
June	1st to 10th	Day	13% 7% 2 6% 6% 5% 6% 5% 6% 8% 36%	2660
	11th to 20th	Day	13% 6% 2 7% 6% 5% 5% 4 7% 9% 35%	2660
	21st to end	Day	12% 5% 2 6% 5% 6% 5% 4 7% 9% 39%	2660
July	1st to 10th	Day	7% 6% 2 8% 6% 6% 6% 5% 7% 9% 39%	4440
	11th to 20th	Day	6% 6% 3 7% 6% 6% 6% 5% 8% 10% 39%	4314
	21st to end	Day	10% 7% 3 8% 6% 5% 5% 5% 6% 7% 37%	4466
August	1st to 10th	Day	18% 6% 3 7% 5% 5% 5% 5% 7% 9% 30%	4104
	11th to 20th	Day	11% 6% 3 8% 6% 6% 7% 5% 7% 8% 35%	4025
	21st to end	Day	14% 5% 2 5% 4 5% 4 4 6% 9% 42%	4180
September	1st to 10th	Day	17% 6% 2 5% 4 4 4 3 4 6% 44%	2280
	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	10% 6% 3 7% 4 4 5% 4 5% 7% 44%	1821
	11th to 20th	Day	15% 9% 3 7% 6% 6% 5% 4 5% 7% 33%	1555
	21st to end	Day	16% 11% 3 8% 7% 5% 5% 5% 6% 5% 30%	1520
December	1st to 10th	Day	17% 14% 4 8% 7% 6% 6% 4 5% 6% 23%	1520
	11th to 20th	Day	17% 13% 5% 9% 4 4 4 3 5% 7% 27%	1910
	21st to end	Day	21% 17% 5% 11% 7% 5% 5% 3 4 5% 16%	2772
January	1st to 10th	Night	14% 11% 6% 12% 7% 5% 4 3 5% 7% 25%	4361
	11th to 20th	Night	14% 16% 7% 12% 7% 4 4 3 5% 6% 22%	4120
	21st to end	Night	23% 13% 6% 12% 6% 4 3 3 5% 4 21%	4532
February	1st to 10th	Night	15% 10% 4 10% 5% 5% 5% 3 6% 6% 31%	2470
	11th to 20th	Night	14% 10% 3 9% 6% 5% 5% 5% 7% 7% 29%	2403
	21st to end	Night	16% 10% 3 9% 6% 5% 5% 5% 5% 5% 31%	1904
March	1st to 10th	Night	22% 11% 3 6% 4 4 3 3 5% 6% 32%	3418
	11th to 20th	Night	11% 9% 5% 7% 5% 4 4 4 6% 7% 38%	3480
	21st to end	Night	14% 8% 2 7% 4 4 4 3 5% 6% 43%	3817
April	1st to 10th	Night	16% 10% 3 8% 6% 5% 5% 4 5% 6% 32%	1905
	11th to 20th	Night	15% 7% 3 6% 5% 4 5% 4 5% 7% 40%	1875
	21st to end	Night	17% 6% 2 7% 5% 4 4 4 5% 7% 39%	1821
May	1st to 10th	Night	25% 8% 3 7% 4 4 5% 4 5% 7% 29%	2813
	11th to 20th	Night	26% 9% 2 5% 4 4 4 3 6% 7% 30%	2623
	21st to end	Night	12% 5% 2 6% 4 5% 5% 4 8% 10% 38%	2732
June	1st to 10th	Night	12% 6% 2 4 5% 5% 5% 3 5% 9% 43%	1520
	11th to 20th	Night	11% 5% 2 7% 4 4 4 5% 8% 9% 41%	1520
	21st to end	Night	10% 4 2 4 4 4 6% 5% 6% 11% 44%	1520
July	1st to 10th	Night	7% 6% 3 8% 6% 5% 5% 5% 6% 9% 41%	2520
	11th to 20th	Night	5% 4 2 7% 5% 5% 6% 5% 7% 11% 44%	2641
	21st to end	Night	11% 6% 2 7% 6% 5% 5% 4 7% 7% 40%	3087
August	1st to 10th	Night	16% 7% 2 6% 4 4 5% 3 6% 9% 37%	2825
	11th to 20th	Night	13% 7% 2 6% 5% 4 4 4 6% 7% 41%	2785
	21st to end	Night	15% 4 2 5% 3 4 4 4 6% 8% 46%	3462
September	1st to 10th	Night	16% 5% 2 4 4 5% 4 3 5% 6% 46%	1900
	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	11% 6% 2 6% 4 4 4 4 5% 5% 49%	2356
	11th to 20th	Night	14% 9% 4 8% 4 4 4 4 6% 7% 36%	2622
	21st to end	Night	16% 9% 4 8% 5% 4 4 4 6% 7% 34%	2660
December	1st to 10th	Night	17% 11% 4 8% 6% 5% 5% 4 7% 7% 25%	2660
	11th to 20th	Night	16% 10% 3 9% 6% 6% 5% 4 6% 6% 30%	3285
	21st to end	Night	19% 13% 5% 10% 7% 5% 5% 4 6% 6% 20%	4884

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	61% 5% 12% 16% 4 1	2582
	11th to 20th	Day	68% 4 10% 11% 7% 1	2840
	21st to end	Day	72% 5% 9% 10% 3 1	3124
February	1st to 10th	Day	62% 5% 13% 13% 6% 1	1710
	11th to 20th	Day	55% 6% 14% 18% 6%	1672
	21st to end	Day	58% 4 12% 17% 7% 2	1546
March	1st to 10th	Day	53% 8% 13% 20% 6%	3435
	11th to 20th	Day	48% 7% 16% 20% 7% 2	3480
	21st to end	Day	46% 7% 14% 20% 11% 2	3815
April	1st to 10th	Day	52% 6% 11% 18% 11% 2	2274
	11th to 20th	Day	49% 5% 10% 18% 16% 2	2254
	21st to end	Day	50% 6% 9% 20% 12% 2	2375
May	1st to 10th	Day	55% 6% 9% 18% 10% 1	4120
	11th to 20th	Day	54% 8% 13% 16% 8% 1	4269
	21st to end	Day	38% 9% 16% 22% 13% 2	4837
June	1st to 10th	Day	44% 7% 12% 22% 14% 1	2660
	11th to 20th	Day	43% 8% 14% 20% 14% 1	2660
	21st to end	Day	39% 8% 14% 26% 13% 1	2660
July	1st to 10th	Day	40% 8% 15% 24% 11% 3	4370
	11th to 20th	Day	36% 8% 16% 25% 15% 1	4239
	21st to end	Day	43% 8% 9% 23% 16% 1	4324
August	1st to 10th	Day	48% 8% 15% 20% 9%	4091
	11th to 20th	Day	44% 8% 11% 26% 11% 1	4064
	21st to end	Day	38% 6% 13% 28% 14% 2	4180
September	1st to 10th	Day	41% 6% 10% 22% 17% 3	2280
	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	40% 6% 16% 21% 15% 3	1821
	11th to 20th	Day	51% 5% 13% 19% 9% 2	1555
	21st to end	Day	54% 6% 13% 20% 7%	1520
December	1st to 10th	Day	62% 6% 15% 12% 4	1520
	11th to 20th	Day	58% 5% 14% 19% 5%	1906
	21st to end	Day	69% 7% 10% 11% 3	2772
January	1st to 10th	Night	60% 5% 13% 16% 5% 1	4361
	11th to 20th	Night	62% 6% 14% 14% 3	4120
	21st to end	Night	67% 4 10% 14% 4 1	4532
February	1st to 10th	Night	53% 6% 14% 20% 7%	2470
	11th to 20th	Night	52% 7% 12% 20% 9%	2405
	21st to end	Night	54% 6% 10% 18% 11% 1	1904
March	1st to 10th	Night	52% 6% 12% 22% 7% 1	3418
	11th to 20th	Night	43% 7% 16% 25% 8% 1	3480
	21st to end	Night	41% 6% 11% 22% 17% 2	3817
April	1st to 10th	Night	51% 7% 10% 18% 13% 1	1905
	11th to 20th	Night	43% 6% 12% 21% 15% 3	1875
	21st to end	Night	45% 5% 10% 23% 13% 3	1821
May	1st to 10th	Night	56% 6% 9% 17% 11% 2	2840
	11th to 20th	Night	52% 9% 13% 17% 8% 1	2624
	21st to end	Night	39% 9% 17% 20% 11% 4	2739
June	1st to 10th	Night	38% 8% 13% 26% 14% 2	1520
	11th to 20th	Night	36% 10% 15% 23% 14% 2	1520
	21st to end	Night	33% 10% 16% 26% 15% 1	1520
July	1st to 10th	Night	39% 8% 13% 21% 15% 3	2480
	11th to 20th	Night	32% 10% 17% 23% 17% 1	2593
	21st to end	Night	40% 8% 13% 18% 20% 2	2988
August	1st to 10th	Night	43% 9% 15% 23% 9%	2814
	11th to 20th	Night	40% 7% 12% 27% 13% 1	2812
	21st to end	Night	35% 7% 13% 31% 13% 2	3462
September	1st to 10th	Night	39% 6% 10% 23% 18% 3	1900
	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	37% 5% 15% 23% 18% 3	2356
	11th to 20th	Night	46% 5% 13% 21% 12% 3	2622
	21st to end	Night	49% 5% 15% 22% 8% 1	2660
December	1st to 10th	Night	56% 8% 16% 15% 5%	2660
	11th to 20th	Night	54% 7% 13% 19% 6%	3288
	21st to end	Night	64% 6% 15% 12% 2	4884

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	35% 25% 27% 8% 4 1	198
	11th to 20th	Day	40% 20% 30% 9% 1	270
	21st to end	Day	32% 27% 28% 11% 2 1	297
February	1st to 10th	Day	31% 29% 25% 13% 1	270
	11th to 20th	Day	41% 27% 27% 6%	270
	21st to end	Day	37% 22% 27% 13% 1	345
March	1st to 10th	Day	32% 19% 26% 18% 4	460
	11th to 20th	Day	33% 23% 25% 14% 4 1	460
	21st to end	Day	32% 21% 26% 15% 5% 2	506
April	1st to 10th	Day	32% 20% 25% 17% 5% 1	460
	11th to 20th	Day	27% 19% 24% 18% 9% 3	452
	21st to end	Day	30% 18% 29% 14% 7% 1	460
May	1st to 10th	Day	31% 16% 25% 17% 8% 3	463
	11th to 20th	Day	27% 15% 30% 14% 11% 4	487
	21st to end	Day	18% 12% 28% 22% 15% 5%	529
June	1st to 10th	Day	19% 17% 26% 24% 11% 3	490
	11th to 20th	Day	19% 12% 24% 28% 13% 3	490
	21st to end	Day	15% 12% 31% 26% 13% 3	490
July	1st to 10th	Day	12% 9% 27% 25% 19% 8%	485
	11th to 20th	Day	8% 10% 27% 31% 17% 7%	484
	21st to end	Day	13% 8% 29% 29% 16% 6%	532
August	1st to 10th	Day	13% 8% 26% 25% 19% 9%	486
	11th to 20th	Day	13% 9% 23% 28% 17% 9%	479
	21st to end	Day	15% 9% 25% 23% 16% 11%	539
September	1st to 10th	Day	20% 9% 18% 28% 17% 9%	490
	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% 25% 23% 13% 2	435
	11th to 20th	Day	28% 23% 23% 20% 6%	215
	21st to end	Day	28% 23% 32% 14% 3	180
December	1st to 10th	Day	33% 28% 24% 8% 6% 1	180
	11th to 20th	Day	36% 17% 31% 14% 2 1	180
	21st to end	Day	34% 21% 25% 15% 5%	198
January	1st to 10th	Night	26% 21% 34% 15% 3	760
	11th to 20th	Night	30% 16% 30% 21% 2	760
	21st to end	Night	24% 23% 30% 21% 2	836
February	1st to 10th	Night	27% 18% 29% 23% 2	760
	11th to 20th	Night	27% 25% 31% 15% 1	743
	21st to end	Night	28% 18% 30% 22% 2	628
March	1st to 10th	Night	24% 19% 30% 24% 4	760
	11th to 20th	Night	28% 20% 25% 21% 6%	760
	21st to end	Night	23% 20% 30% 21% 6%	836
April	1st to 10th	Night	24% 21% 28% 22% 5%	760
	11th to 20th	Night	22% 16% 25% 28% 9%	751
	21st to end	Night	21% 14% 26% 33% 7%	681
May	1st to 10th	Night	22% 11% 23% 33% 9% 1	570
	11th to 20th	Night	20% 9% 24% 36% 11%	450
	21st to end	Night	14% 9% 20% 33% 21% 2	411
June	1st to 10th	Night	12% 7% 24% 38% 19%	380
	11th to 20th	Night	14% 7% 22% 38% 19% 1	380
	21st to end	Night	11% 6% 24% 43% 16% 1	380
July	1st to 10th	Night	10% 4 16% 45% 21% 3	370
	11th to 20th	Night	8% 5% 15% 41% 29% 2	437
	21st to end	Night	9% 3 18% 43% 24% 2	624
August	1st to 10th	Night	11% 2 15% 41% 29% 2	567
	11th to 20th	Night	13% 5% 13% 39% 26% 3	564
	21st to end	Night	13% 5% 15% 36% 27% 4	822
September	1st to 10th	Night	17% 4 17% 33% 25% 3	760
	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% 26% 34% 11% 1	760
	11th to 20th	Night	25% 18% 22% 27% 7% 1	760
	21st to end	Night	24% 19% 26% 24% 5% 1	760
December	1st to 10th	Night	30% 16% 28% 21% 6%	760
	11th to 20th	Night	26% 19% 27% 25% 3	760
	21st to end	Night	25% 19% 27% 28% 2	836

Page generated at 2024-11-03 02:11:18. Version ac07

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