Lake Murray Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lake Murray Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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 5% 4 6% 8% 43%	120412
	Night	13% 6% 2 6% 4 4 4 4 5% 7% 45%	118720
Transparency	Day	38% 7% 11% 19% 19% 6%	124126
	Night	37% 6% 10% 18% 21% 8%	123143
Seeing	Day	15% 11% 25% 24% 18% 7%	16209
	Night	15% 9% 25% 31% 18% 2	26275

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	15% 4 1 5% 3 3 3 2 4 6% 54%	4056
	11th to 20th	Day	12% 6% 2 5% 3 2 3 2 3 5% 59%	4060
	21st to end	Day	15% 8% 3 6% 4 4 3 2 4 5% 46%	4466
February	1st to 10th	Day	14% 7% 3 6% 5% 4 4 3 6% 7% 41%	1846
	11th to 20th	Day	20% 7% 2 5% 3 3 4 3 4 6% 44%	1948
	21st to end	Day	17% 7% 2 6% 4 4 4 4 5% 5% 44%	1686
March	1st to 10th	Day	20% 8% 3 7% 4 3 3 2 4 5% 40%	4935
	11th to 20th	Day	8% 4 2 5% 4 4 4 3 5% 7% 55%	4980
	21st to end	Day	15% 7% 3 6% 4 4 4 3 4 4 46%	5465
April	1st to 10th	Day	10% 4 3 4 4 3 5% 4 7% 7% 50%	2200
	11th to 20th	Day	10% 5% 2 5% 4 4 4 3 6% 7% 49%	2323
	21st to end	Day	9% 4 2 5% 4 5% 4 4 6% 8% 50%	2560
May	1st to 10th	Day	11% 6% 3 6% 4 4 5% 4 6% 10% 40%	5862
	11th to 20th	Day	17% 7% 3 7% 5% 5% 6% 5% 7% 8% 31%	5861
	21st to end	Day	12% 5% 2 5% 4 4 5% 4 6% 8% 44%	6433
June	1st to 10th	Day	10% 6% 3 7% 5% 6% 5% 5% 8% 10% 36%	2600
	11th to 20th	Day	6% 5% 1 5% 5% 5% 7% 4 8% 11% 42%	2600
	21st to end	Day	4 5% 2 6% 5% 5% 6% 6% 10% 12% 39%	2600
July	1st to 10th	Day	13% 6% 2 8% 5% 7% 7% 6% 8% 9% 29%	4250
	11th to 20th	Day	6% 5% 2 7% 7% 7% 7% 6% 10% 14% 30%	4250
	21st to end	Day	11% 6% 3 9% 7% 7% 8% 6% 9% 12% 24%	4466
August	1st to 10th	Day	22% 6% 2 6% 6% 6% 6% 5% 7% 10% 25%	4104
	11th to 20th	Day	12% 5% 2 7% 5% 6% 6% 5% 8% 10% 35%	4044
	21st to end	Day	8% 4 2 5% 4 5% 5% 5% 7% 10% 47%	4163
September	1st to 10th	Day	7% 5% 2 8% 6% 6% 5% 5% 8% 8% 41%	2090
	11th to 20th	Day	15% 6% 3 6% 4 4 4 4 5% 8% 40%	2090
	21st to end	Day	17% 7% 2 5% 3 4 4 4 5% 6% 43%	2090
October	1st to 10th	Day	13% 7% 2 5% 4 3 2 3 4 6% 52%	2008
	11th to 20th	Day	11% 4 1 5% 3 2 3 2 4 5% 60%	1900
	21st to end	Day	11% 6% 2 5% 3 3 3 2 3 5% 57%	2090
November	1st to 10th	Day	14% 6% 2 5% 3 3 3 3 4 5% 53%	1957
	11th to 20th	Day	14% 5% 2 4 3 3 3 3 4 6% 53%	1860
	21st to end	Day	13% 5% 2 5% 4 3 4 3 4 5% 50%	1800
December	1st to 10th	Day	12% 6% 2 5% 4 3 4 3 5% 5% 51%	2930
	11th to 20th	Day	16% 5% 2 5% 4 4 4 3 4 6% 47%	3373
	21st to end	Day	20% 6% 2 6% 3 2 3 3 4 6% 45%	4466
January	1st to 10th	Night	12% 4 2 4 3 3 3 2 3 5% 57%	5889
	11th to 20th	Night	11% 6% 2 5% 4 3 4 3 5% 6% 50%	5900
	21st to end	Night	14% 9% 2 6% 3 3 3 3 4 5% 47%	6490
February	1st to 10th	Night	15% 6% 2 5% 4 4 4 3 5% 6% 46%	2550
	11th to 20th	Night	19% 7% 3 5% 4 3 3 3 4 5% 44%	2338
	21st to end	Night	15% 5% 2 5% 4 4 4 3 5% 6% 46%	1942
March	1st to 10th	Night	19% 8% 3 6% 4 3 4 3 5% 6% 40%	4918
	11th to 20th	Night	11% 3 1 4 3 3 3 3 5% 8% 55%	4980
	21st to end	Night	14% 5% 2 5% 3 4 4 4 5% 7% 48%	5467
April	1st to 10th	Night	11% 5% 2 5% 4 4 4 4 5% 7% 50%	2200
	11th to 20th	Night	11% 4 2 5% 4 3 4 2 4 6% 53%	2025
	21st to end	Night	8% 4 2 6% 4 3 4 3 6% 7% 53%	1855
May	1st to 10th	Night	10% 6% 2 7% 4 5% 5% 4 7% 10% 41%	4033
	11th to 20th	Night	18% 6% 2 6% 4 4 5% 5% 7% 8% 34%	4032
	21st to end	Night	12% 5% 2 5% 4 4 5% 5% 7% 9% 43%	4421
June	1st to 10th	Night	10% 6% 3 7% 6% 5% 5% 5% 8% 10% 34%	1800
	11th to 20th	Night	8% 6% 2 6% 5% 5% 4 5% 6% 11% 41%	1800
	21st to end	Night	5% 6% 3 7% 5% 5% 5% 7% 9% 12% 36%	1800
July	1st to 10th	Night	13% 8% 3 8% 5% 6% 6% 4 7% 10% 30%	2930
	11th to 20th	Night	9% 5% 2 7% 5% 5% 6% 6% 9% 14% 31%	2930
	21st to end	Night	12% 8% 3 9% 7% 7% 6% 6% 8% 11% 24%	3087
August	1st to 10th	Night	22% 6% 3 8% 5% 5% 6% 5% 7% 10% 23%	2825
	11th to 20th	Night	12% 6% 2 7% 5% 5% 6% 4 7% 10% 36%	2785
	21st to end	Night	6% 4 1 5% 4 4 5% 4 7% 10% 49%	3454
September	1st to 10th	Night	9% 6% 2 5% 5% 5% 6% 4 7% 9% 41%	2090
	11th to 20th	Night	13% 7% 3 7% 5% 5% 5% 4 7% 8% 38%	2090
	21st to end	Night	16% 5% 2 6% 4 4 4 4 5% 6% 44%	2090
October	1st to 10th	Night	13% 7% 2 4 3 3 3 3 4 6% 52%	2171
	11th to 20th	Night	13% 4 2 5% 3 3 3 3 4 5% 56%	2280
	21st to end	Night	10% 5% 1 5% 3 4 3 3 4 5% 57%	2508
November	1st to 10th	Night	13% 6% 2 5% 3 3 3 2 3 5% 54%	2341
	11th to 20th	Night	13% 6% 2 5% 4 2 3 3 4 6% 52%	2537
	21st to end	Night	13% 6% 2 5% 3 3 4 4 6% 6% 49%	2600
December	1st to 10th	Night	12% 5% 2 5% 4 3 3 3 4 5% 54%	4250
	11th to 20th	Night	17% 6% 3 5% 3 3 3 3 5% 6% 46%	4822
	21st to end	Night	19% 6% 2 6% 3 4 4 3 4 6% 43%	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	32% 4 5% 22% 30% 7%	4057
	11th to 20th	Day	31% 3 4 16% 36% 10%	4060
	21st to end	Day	42% 3 4 17% 23% 11%	4466
February	1st to 10th	Day	41% 4 9% 21% 19% 6%	1846
	11th to 20th	Day	44% 3 5% 18% 24% 6%	1958
	21st to end	Day	42% 5% 8% 14% 22% 9%	1686
March	1st to 10th	Day	49% 3 5% 13% 20% 10%	4935
	11th to 20th	Day	29% 5% 7% 15% 25% 19%	4980
	21st to end	Day	42% 4 6% 18% 22% 8%	5465
April	1st to 10th	Day	32% 6% 11% 20% 26% 6%	2200
	11th to 20th	Day	33% 5% 10% 20% 21% 10%	2323
	21st to end	Day	31% 7% 10% 26% 22% 4	2560
May	1st to 10th	Day	38% 8% 16% 24% 13% 2	5897
	11th to 20th	Day	48% 8% 15% 17% 11%	5865
	21st to end	Day	36% 8% 13% 20% 21% 3	6447
June	1st to 10th	Day	40% 10% 16% 22% 10% 2	2600
	11th to 20th	Day	32% 10% 17% 26% 14% 1	2600
	21st to end	Day	33% 12% 17% 22% 14% 2	2600
July	1st to 10th	Day	46% 11% 17% 18% 6% 2	4185
	11th to 20th	Day	37% 15% 25% 18% 4 1	4181
	21st to end	Day	48% 15% 18% 15% 4 1	4324
August	1st to 10th	Day	52% 10% 17% 16% 4	4091
	11th to 20th	Day	41% 10% 15% 19% 14% 1	4083
	21st to end	Day	30% 8% 15% 29% 17% 2	4163
September	1st to 10th	Day	37% 9% 14% 17% 18% 5%	2090
	11th to 20th	Day	42% 8% 10% 18% 19% 4	2090
	21st to end	Day	41% 5% 9% 20% 17% 7%	2090
October	1st to 10th	Day	31% 4 6% 18% 30% 10%	3333
	11th to 20th	Day	18% 2 5% 17% 27% 31%	3140
	21st to end	Day	26% 3 4 20% 31% 15%	3428
November	1st to 10th	Day	35% 4 6% 18% 26% 12%	1957
	11th to 20th	Day	33% 4 6% 21% 25% 11%	1860
	21st to end	Day	37% 5% 7% 19% 24% 9%	1800
December	1st to 10th	Day	35% 4 6% 17% 28% 9%	2930
	11th to 20th	Day	38% 4 10% 22% 19% 7%	3370
	21st to end	Day	41% 3 7% 23% 24% 2	4466
January	1st to 10th	Night	31% 3 4 20% 35% 7%	5889
	11th to 20th	Night	35% 3 5% 17% 32% 8%	5900
	21st to end	Night	41% 4 4 15% 28% 9%	6490
February	1st to 10th	Night	40% 4 7% 20% 21% 8%	2550
	11th to 20th	Night	43% 4 6% 18% 20% 8%	2339
	21st to end	Night	38% 5% 8% 17% 20% 12%	1942
March	1st to 10th	Night	45% 4 7% 12% 25% 7%	4918
	11th to 20th	Night	28% 4 7% 13% 29% 20%	4980
	21st to end	Night	37% 4 6% 20% 21% 12%	5467
April	1st to 10th	Night	34% 4 11% 20% 23% 8%	2200
	11th to 20th	Night	33% 4 8% 21% 23% 11%	2025
	21st to end	Night	29% 5% 10% 28% 23% 5%	1855
May	1st to 10th	Night	37% 8% 15% 26% 13% 2	4060
	11th to 20th	Night	45% 8% 19% 17% 11%	4033
	21st to end	Night	34% 8% 12% 24% 19% 3	4429
June	1st to 10th	Night	41% 10% 15% 21% 11% 2	1800
	11th to 20th	Night	35% 9% 18% 23% 13% 2	1800
	21st to end	Night	35% 13% 18% 20% 12% 2	1800
July	1st to 10th	Night	47% 10% 20% 14% 7% 2	2885
	11th to 20th	Night	39% 15% 27% 15% 3 1	2876
	21st to end	Night	53% 13% 19% 11% 4	2988
August	1st to 10th	Night	55% 11% 17% 14% 3	2814
	11th to 20th	Night	41% 10% 21% 20% 8% 1	2812
	21st to end	Night	28% 9% 15% 29% 17% 3	3454
September	1st to 10th	Night	36% 9% 15% 19% 15% 5%	2090
	11th to 20th	Night	42% 8% 11% 18% 17% 3	2090
	21st to end	Night	40% 7% 12% 18% 18% 5%	2090
October	1st to 10th	Night	33% 4 9% 18% 30% 7%	3626
	11th to 20th	Night	19% 2 5% 16% 26% 31%	3779
	21st to end	Night	25% 4 5% 19% 29% 19%	4120
November	1st to 10th	Night	34% 4 7% 16% 26% 13%	2341
	11th to 20th	Night	35% 3 6% 17% 25% 13%	2537
	21st to end	Night	35% 5% 8% 18% 25% 10%	2600
December	1st to 10th	Night	34% 4 6% 16% 31% 9%	4250
	11th to 20th	Night	40% 4 10% 18% 20% 8%	4824
	21st to end	Night	44% 4 5% 24% 20% 3	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	17% 26% 40% 14% 2	300
	11th to 20th	Day	21% 24% 35% 18% 2	300
	21st to end	Day	21% 26% 39% 12% 2	330
February	1st to 10th	Day	16% 26% 39% 15% 4	300
	11th to 20th	Day	34% 20% 33% 13% 1	300
	21st to end	Day	24% 22% 34% 15% 5%	285
March	1st to 10th	Day	20% 21% 31% 20% 8%	500
	11th to 20th	Day	19% 18% 41% 16% 5%	500
	21st to end	Day	19% 19% 38% 16% 8%	550
April	1st to 10th	Day	19% 17% 34% 21% 9%	500
	11th to 20th	Day	14% 16% 33% 22% 13% 2	492
	21st to end	Day	11% 12% 32% 26% 17% 3	500
May	1st to 10th	Day	11% 12% 34% 27% 13% 3	503
	11th to 20th	Day	17% 5% 23% 31% 16% 8%	527
	21st to end	Day	13% 8% 19% 26% 24% 11%	573
June	1st to 10th	Day	12% 5% 16% 33% 24% 9%	530
	11th to 20th	Day	9% 4 19% 26% 29% 12%	530
	21st to end	Day	6% 3 14% 33% 30% 14%	530
July	1st to 10th	Day	9% 2 10% 26% 32% 22%	525
	11th to 20th	Day	7% 2 11% 24% 38% 19%	524
	21st to end	Day	6% 2 11% 33% 35% 12%	532
August	1st to 10th	Day	14% 3 12% 26% 31% 13%	486
	11th to 20th	Day	13% 5% 11% 25% 29% 17%	479
	21st to end	Day	12% 4 10% 29% 29% 17%	539
September	1st to 10th	Day	8% 4 12% 31% 33% 13%	490
	11th to 20th	Day	18% 4 14% 26% 28% 11%	490
	21st to end	Day	20% 3 16% 28% 27% 7%	490
October	1st to 10th	Day	16% 5% 22% 31% 18% 8%	490
	11th to 20th	Day	11% 8% 25% 30% 21% 6%	490
	21st to end	Day	12% 9% 27% 28% 19% 5%	539
November	1st to 10th	Day	18% 10% 27% 25% 14% 5%	495
	11th to 20th	Day	19% 13% 38% 23% 6% 2	360
	21st to end	Day	10% 15% 40% 24% 10% 1	300
December	1st to 10th	Day	23% 20% 34% 18% 4	300
	11th to 20th	Day	22% 14% 41% 16% 6% 1	300
	21st to end	Day	27% 18% 33% 15% 4 2	330
January	1st to 10th	Night	19% 18% 33% 25% 6%	800
	11th to 20th	Night	18% 15% 34% 27% 6%	800
	21st to end	Night	19% 18% 37% 22% 3	880
February	1st to 10th	Night	19% 14% 34% 29% 5%	800
	11th to 20th	Night	23% 17% 35% 23% 2	783
	21st to end	Night	17% 17% 37% 25% 4	660
March	1st to 10th	Night	17% 14% 35% 28% 6%	800
	11th to 20th	Night	18% 13% 40% 23% 7%	800
	21st to end	Night	16% 15% 38% 24% 7%	880
April	1st to 10th	Night	15% 16% 32% 28% 9%	800
	11th to 20th	Night	13% 13% 31% 32% 11%	791
	21st to end	Night	10% 9% 32% 37% 12%	655
May	1st to 10th	Night	11% 8% 32% 37% 12%	600
	11th to 20th	Night	12% 4 30% 33% 18% 3	591
	21st to end	Night	10% 4 18% 38% 26% 4	650
June	1st to 10th	Night	11% 3 22% 38% 22% 5%	600
	11th to 20th	Night	10% 3 16% 34% 29% 9%	600
	21st to end	Night	8% 2 14% 39% 31% 6%	600
July	1st to 10th	Night	9% 2 10% 36% 37% 7%	585
	11th to 20th	Night	9% 1 7% 35% 38% 9%	582
	21st to end	Night	7% 3 10% 37% 38% 5%	624
August	1st to 10th	Night	12% 2 9% 32% 39% 6%	567
	11th to 20th	Night	15% 2 8% 34% 37% 4	564
	21st to end	Night	12% 2 11% 31% 40% 5%	767
September	1st to 10th	Night	10% 4 10% 35% 36% 5%	760
	11th to 20th	Night	15% 3 14% 28% 35% 5%	760
	21st to end	Night	18% 3 15% 33% 27% 4	760
October	1st to 10th	Night	15% 5% 18% 31% 26% 4	760
	11th to 20th	Night	15% 6% 23% 31% 22% 3	760
	21st to end	Night	11% 9% 24% 32% 22% 2	836
November	1st to 10th	Night	14% 8% 24% 31% 19% 3	780
	11th to 20th	Night	16% 11% 30% 31% 12% 1	800
	21st to end	Night	14% 11% 30% 30% 14% 1	800
December	1st to 10th	Night	18% 14% 27% 32% 9% 1	800
	11th to 20th	Night	20% 10% 27% 32% 10% 1	800
	21st to end	Night	25% 14% 26% 28% 7% 1	880

Page generated at 2025-07-18 01:09:59. Version ac07

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