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 2024-05-13.

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 5% 7% 32%	83617
	Night	15% 9% 3 8% 5% 4 4 4 6% 7% 35%	85889
Transparency	Day	50% 7% 13% 19% 10% 1	83327
	Night	48% 6% 13% 20% 11% 1	85698
Seeing	Day	24% 16% 25% 20% 11% 4	14075
	Night	21% 14% 24% 28% 11% 1	23720

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	19% 8% 3 7% 5% 6% 4 4 6% 7% 30%	2470
	11th to 20th	Day	21% 8% 2 6% 5% 5% 6% 4 4 8% 32%	2429
	21st to end	Day	14% 6% 2 7% 5% 4 5% 4 6% 8% 38%	2725
June	1st to 10th	Day	13% 7% 2 6% 6% 5% 6% 5% 6% 8% 36%	2520
	11th to 20th	Day	14% 6% 2 7% 6% 5% 6% 4 7% 9% 34%	2520
	21st to end	Day	13% 5% 2 5% 6% 6% 5% 4 7% 9% 39%	2520
July	1st to 10th	Day	10% 7% 2 7% 5% 6% 6% 5% 6% 8% 38%	2520
	11th to 20th	Day	8% 6% 3 6% 6% 6% 6% 5% 7% 9% 39%	2450
	21st to end	Day	11% 7% 2 7% 6% 6% 5% 5% 7% 8% 35%	2547
August	1st to 10th	Day	10% 5% 3 7% 6% 6% 7% 6% 8% 10% 32%	2327
	11th to 20th	Day	13% 6% 2 8% 5% 6% 6% 4 6% 8% 35%	2298
	21st to end	Day	14% 6% 2 6% 5% 5% 5% 4 7% 9% 37%	2376
September	1st to 10th	Day	18% 6% 2 6% 4 4 4 3 4 6% 42%	2160
	11th to 20th	Day	15% 6% 2 5% 3 4 4 3 6% 7% 46%	2119
	21st to end	Day	19% 7% 2 6% 4 4 5% 4 7% 7% 37%	1902
October	1st to 10th	Day	16% 8% 3 7% 4 4 5% 4 5% 7% 37%	1800
	11th to 20th	Day	18% 7% 2 8% 5% 5% 5% 3 5% 6% 36%	1800
	21st to end	Day	18% 8% 3 8% 5% 4 4 4 5% 6% 36%	1980
November	1st to 10th	Day	11% 6% 3 7% 4 4 5% 4 5% 7% 43%	1780
	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	19% 7% 3 7% 5% 4 5% 4 5% 7% 34%	1710
	11th to 20th	Night	19% 7% 2 5% 5% 4 4 4 5% 7% 37%	1514
	21st to end	Night	12% 6% 3 5% 3 4 5% 4 8% 9% 42%	1550
June	1st to 10th	Night	13% 6% 2 5% 4 5% 4 3 5% 9% 44%	1440
	11th to 20th	Night	11% 5% 2 8% 5% 4 4 5% 8% 9% 39%	1440
	21st to end	Night	10% 4 2 4 4 4 6% 5% 6% 11% 45%	1440
July	1st to 10th	Night	10% 5% 2 6% 5% 4 5% 4 6% 8% 45%	1440
	11th to 20th	Night	6% 4 1 5% 5% 4 5% 4 6% 10% 50%	1505
	21st to end	Night	9% 5% 1 6% 5% 5% 5% 5% 8% 10% 41%	1771
August	1st to 10th	Night	7% 6% 2 7% 4 4 5% 3 7% 10% 45%	1611
	11th to 20th	Night	11% 6% 3 6% 4 4 4 4 6% 7% 46%	1598
	21st to end	Night	12% 5% 2 5% 4 3 5% 4 7% 8% 44%	1966
September	1st to 10th	Night	16% 5% 2 5% 4 5% 4 3 5% 6% 45%	1800
	11th to 20th	Night	12% 5% 2 4 3 3 4 3 5% 8% 52%	1838
	21st to end	Night	18% 5% 2 5% 4 4 3 3 5% 7% 43%	2056
October	1st to 10th	Night	16% 6% 2 5% 4 3 4 4 5% 7% 43%	2160
	11th to 20th	Night	18% 6% 2 5% 4 4 4 3 5% 8% 41%	2160
	21st to end	Night	21% 7% 2 5% 4 4 4 4 5% 7% 38%	2376
November	1st to 10th	Night	11% 6% 2 6% 4 4 4 4 5% 5% 49%	2297
	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	52% 7% 11% 18% 11% 2	2470
	11th to 20th	Day	51% 7% 13% 15% 12% 3	2433
	21st to end	Day	42% 8% 17% 21% 10% 2	2725
June	1st to 10th	Day	45% 7% 12% 22% 13% 1	2520
	11th to 20th	Day	44% 8% 14% 19% 13% 1	2520
	21st to end	Day	40% 8% 14% 27% 12% 1	2520
July	1st to 10th	Day	42% 9% 13% 19% 13% 3	2450
	11th to 20th	Day	38% 9% 14% 23% 14% 1	2375
	21st to end	Day	43% 9% 12% 20% 15% 2	2405
August	1st to 10th	Day	42% 10% 16% 21% 10%	2314
	11th to 20th	Day	44% 9% 13% 22% 11% 1	2298
	21st to end	Day	42% 8% 13% 22% 13% 3	2376
September	1st to 10th	Day	43% 7% 11% 21% 15% 3	2160
	11th to 20th	Day	36% 5% 12% 27% 18% 2	2119
	21st to end	Day	45% 6% 12% 19% 16% 2	1902
October	1st to 10th	Day	46% 7% 12% 20% 14% 1	1800
	11th to 20th	Day	48% 6% 11% 24% 11%	1800
	21st to end	Day	49% 6% 13% 19% 11% 1	1980
November	1st to 10th	Day	40% 6% 16% 20% 15% 3	1780
	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	48% 6% 11% 19% 11% 4	1710
	11th to 20th	Night	46% 7% 12% 18% 14% 2	1515
	21st to end	Night	37% 9% 16% 23% 13% 2	1551
June	1st to 10th	Night	38% 7% 13% 26% 14% 2	1440
	11th to 20th	Night	38% 10% 15% 22% 14% 2	1440
	21st to end	Night	32% 10% 16% 26% 15% 1	1440
July	1st to 10th	Night	36% 10% 13% 23% 15% 4	1400
	11th to 20th	Night	29% 10% 16% 24% 19% 2	1457
	21st to end	Night	35% 10% 17% 21% 16% 2	1672
August	1st to 10th	Night	33% 10% 17% 25% 14% 1	1600
	11th to 20th	Night	36% 8% 15% 23% 15% 1	1598
	21st to end	Night	35% 8% 14% 24% 15% 3	1966
September	1st to 10th	Night	40% 6% 10% 23% 17% 4	1800
	11th to 20th	Night	30% 6% 13% 29% 19% 2	1838
	21st to end	Night	41% 5% 12% 23% 17% 1	2056
October	1st to 10th	Night	40% 7% 14% 24% 14% 1	2160
	11th to 20th	Night	42% 5% 14% 24% 13% 1	2160
	21st to end	Night	46% 5% 12% 23% 13% 1	2376
November	1st to 10th	Night	37% 5% 15% 23% 18% 3	2297
	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% 10% 3	456
	21st to end	Day	19% 11% 27% 21% 15% 6%	485
June	1st to 10th	Day	18% 17% 26% 24% 12% 3	450
	11th to 20th	Day	20% 13% 24% 28% 12% 3	450
	21st to end	Day	15% 11% 30% 27% 14% 3	450
July	1st to 10th	Day	12% 9% 26% 24% 20% 9%	445
	11th to 20th	Day	7% 10% 26% 32% 18% 7%	444
	21st to end	Day	14% 7% 28% 29% 17% 6%	488
August	1st to 10th	Day	11% 8% 26% 26% 20% 10%	446
	11th to 20th	Day	14% 8% 22% 28% 18% 10%	439
	21st to end	Day	16% 9% 25% 23% 15% 11%	495
September	1st to 10th	Day	20% 8% 17% 27% 18% 9%	450
	11th to 20th	Day	17% 6% 20% 27% 23% 8%	450
	21st to end	Day	25% 14% 22% 19% 15% 6%	450
October	1st to 10th	Day	23% 12% 18% 25% 17% 5%	450
	11th to 20th	Day	26% 16% 25% 22% 10% 1	450
	21st to end	Day	24% 20% 28% 20% 7%	495
November	1st to 10th	Day	22% 15% 24% 22% 14% 2	418
	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% 8% 24% 36% 11%	435
	21st to end	Night	15% 9% 20% 33% 21% 2	389
June	1st to 10th	Night	12% 8% 24% 38% 18%	360
	11th to 20th	Night	15% 7% 23% 35% 20% 1	360
	21st to end	Night	11% 6% 23% 44% 16% 1	360
July	1st to 10th	Night	11% 3 16% 44% 22% 3	350
	11th to 20th	Night	8% 5% 15% 40% 30% 2	413
	21st to end	Night	9% 3 18% 43% 25% 2	591
August	1st to 10th	Night	10% 2 16% 41% 29% 2	537
	11th to 20th	Night	13% 5% 13% 38% 27% 3	534
	21st to end	Night	13% 5% 16% 35% 27% 4	778
September	1st to 10th	Night	18% 4 16% 33% 26% 4	720
	11th to 20th	Night	13% 7% 16% 31% 29% 5%	720
	21st to end	Night	19% 9% 19% 30% 19% 4	720
October	1st to 10th	Night	18% 9% 21% 30% 20% 2	720
	11th to 20th	Night	22% 14% 23% 30% 10% 1	720
	21st to end	Night	26% 15% 24% 28% 7% 1	792
November	1st to 10th	Night	16% 13% 26% 34% 12% 1	740
	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 2024-05-10 12:38:58. Version ac07

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